US2661208A - Feeding of magnetic sheet material - Google Patents

Description

1953 A. FOWLER FEEDING 0E MAGNETIC SHEET MATERIAL 4 Sheets-Sheet 1 Filed May 16. 1951 INVENTOR. 14L EXANDI? 1 01 1 45? BY Z z ATTOEA EYJ 174 vfAl Dec. 1, 1953 Filed May 16. 1951 A. FOWLER FEEDING OF MAGNETIC SHEET MATERIAL L/NE 1 LINE 2 ONE REVOLUTIONF CLUTCH 4 Sheets-Sheet 2 INVENTOR. ALEXANDER FOWLEQ ATTQQA/EYS Dec. 1, 1953 FOWLER 2,661,208

FEEDING OF MAGNETIC SHEET MATERIAL Filed May 16. 1951 4 Sheets-Sheet 3 LINE 1 L/NE 2 IN V EN TOR. flLEX/M/DE/Z FOWLER Dec. 1, 1953 A. FOWLER 2,661,208

FEEDING OF MAGNETIC SHEET MATERIAL Filed May 16. 1951 4 Sheets-Sheet 4 42 IN V EN TOR.

380 Al E'XANDER FOWLER Patentecl Dec. 1, 1953 UNITED STATES PATENT OFFICE FEEDING'OF MAGNETIC SHEET MATERIAL Alexander Fowler, Brooklyn, N. Y., assignor to Hercules Electric & Mfg; a., Inc., Brooklyn, N. Y., a corporation of New York Application May 16; 1951, Serial No. 226,656

28 Claims. 1

This invention relates to the feeding of sheet material, and more particularly to the automatic separation and feeding of single sheets in succession from a stack of ferromagnetic sheets.

The primary object of the present invention is to generally improve magnetic sheet separation and feeding apparatus. A more particular object is to produce a wide separation of the sheets magnetically.

A further object of the invention is to guard against the possibility of more than one sheet adhering to the feed roller. In accordance with my invention thisobject of insuring the feeding ofonly a single sheet at a time may be fulfilled by energizing the main induction coil with a slowly growing current (or one tooth of a low speed saw-tooth wave). In such case the topmost sheet begins to separate ahead of the next subjacent sheet, and so on, and there is a relatively wide and individual separation of the sheets as the topmost sheet reaches the feed roller. When the top most sheet reaches the feed roller the energization of the main induction coil is terminated, and the topmost sheet is promptly fed from the stack.

A further object of the present invention is to eliminate the need for an elevator table by affording so substantial a lift of the topmost sheet as to accommodate the reduced height of the stack as it is consumed.

Still another object of the present invention is-to provide electronic circuits and apparatus for supplying the desired saw-tooth or growth current for energization of the magnetic field. The term growth current is more apt than sawtooth current because for the present purpose the important thing is that the current grow from a relatively low value to a relatively high value, but there is no need for the increase to be a linear one.

Still another object of the present invention is to provide electronic circuits which will operate on a single phase power supply, and on a three phase power supply.

In many cases the sheets of the stack tend to adhere tightly because of an oil film thereb'=- tween. This is particularly true in winter, for although the separating means may be located in a building, the sheets are more usually stored outdoors or. inan unheated storage shed from which they are brought to the separating means without a long warmmp period in which they might gradually acquire the building temperature; Further objects or the present invention are to provide apparatuswhich will help overcome adhesion between the sheets, and which.

will prevent repeated or renewed adhesion of the sheets while in the sheet separating apparatus.

For certain purposes it may be desired to feed.

sheet, and a further feature and object of the present invention is to provide means for this purpose.

To accomplish the foregoing generalobjects, and. other more particular objects which. will hereinafter appear, my invention resides in the sheet separating and feeding elements, and their relation one to another, as are hereinafter more particularly described in the following specification. The specification is accompanied by drawings in which:

Fig. 1 is a plan view of sheet separating and feeding apparatus embodying features of my invention;

Fig. 2 is a section taken in elevation approximately in the plane of the line 2-4! of Fig. 1;

Fig. 3 is a schematic wiring diagram explanatory of one form of my invention;

Fig. 4 is explanatory of a current growth curve or saw-tooth wave used in my invention;

Fig. 5 is explanatory of the operation of a modified electronic apparatus shown in Fig. 7

Fig. 6 is. a current growth curve generally similar to that shown in Fig. 4 but explanatory of a modification;

Fig; '7 is a schematic wiring diagram for an electronic form of my apparatus energized from asingle phase power supply; and

Fig; 8 is a schematic wiring diagram of electronic apparatus energized from a three. phase power supply.

The present application is a continuation-inpart of my prior application of like title, Serial No. 129,662, filed November 26, 1949, and some of the foregoing objects, as well as the subject matter of Figs. 1 through 4 hereinafter described,

have been tak n from that application, while paratus further includes feed means generally designated 2E3 for feeding the uppermost sheet 21'; through the coil it and away from the stack 54. The feed means 25 preferably includes an electro-magnetic roller 24 capable of feeding sheet 22 with the latter disposed beneath the roller. The roller is preferably intermittently driven, as by means of a magnetic clutch 2% disposed between the roller and a constantly rotat ing motor 28. The latter may rotate one or more sets of later feed rollers, indicated schematically at 25, through a drive 21.

The sheets may be stacked between guide plates 30, supported within side plates 32, preferably by means of adjusting screws 32, so that sheets of different width may be accommodated. The sheets are aligned at their forward edge by means of front stops such as the posts 35, which are preferably made of non-magnetic material. The table 12, on the other hand, may be made of ferrous or magnetic material in order to repel and raise even the last sheet of the stack.

Before referring to the wiring diagram, attention is directed to the slip rings 33 (Fig. l) and brushes ta which supply magnetizing current to the coils 42 of the feed roller Also the feeler M (Fig. 2) controlling switch contacts i When the topmost sheet 22 reaches the feed roller 2d the feeler is moved from a lower down-position to the position t4 shown, thus closing the normally open contacts at. It will be understood that the showing in Fig. 2 is merely schematic, and that in practice a fully enclosed and sensitive microswitch may be used for the intended purpose. Moreover, other means to discriminate be-- tween the absence or presence of a sheet may be used.

It is desirable to delay re-energization of the main induction coil until the trailing edge of a preceding sheet has safely left the magnetic feed roller 2 1. This avoids any possibility of the next top sheet being caused to fly upward prematureh in which case the forward portion of said sheet may strike the bottom of the trailing portion of the preceding sheet, thus holding it back both physically and magnetically, or, in the alternative, causing the second sheet to be prematurely pulled forward by the first sheet. i'his result may be accomplished by using a time delay relay, or by using a second feeler, and the latter ar rangement is schematically shown in Fig. 2 in which it will be seen that a second feeler is provided outside the stack, said feeler serving to close contacts Ell when engaged by a sheet passing through feed rollers 25. This is described further as an incident to the description of Fig. 3 which follows.

Mo'tm' driven growth curve circuit Referring now to Fig. 3, the main feature of this arrangement is that the induction coil it is energized by a gradually increasing current which starts at zero and rises to a desired value. Thus the current may follow an intermittent saw-tooth wave such as that illustrated in Fig. 4. In Fig. 4 it will be seen that the current rises from the point 5% to a maximum at 5B, whereupon it is cut off and remains off until the point 58. The interval between points 56 and 5t corresponds to the time during which the sheet is away from the stack by the feed roller. When i main induction coil it is re energlzed beginning at point 5%. the current again increases to a maximum as indicated at lit and is again out If the induction coil is energized in this manner the topmost sheet tends to be raised from the next subjacent sheet before the latter is raised at all, whereupon both rise before the third sheet rises, so that the fanning out or divergence or separation of the sheets is maintained as the induction coil is further energized. In any event the topmost sheet alone reaches the feed roller, and the induction coil may then be deenergized without danger of the feed roller holding more than one sheet. Indeed, by promptly tie-energizing the induction coil when the topmost sheet reaches the feed roller it is found that the feed roller may be left at full energize.- tion, and further that it may be kept under constant rotation. Thus the apparatus of Fig. 3 is more complicated in introducing suitable means to apply a growth current or saw-tooth current to the induction coil, but may be simplified in other respects as by eliminating the electrically operated clutch for the feed roller shown in Fig. 1.

Referring to Fig. 3, a main power switch 62 is closed to supply power from an ordinary source indicated by lines I and 2. This supplies power through conductors t3 and it to feed roller 24, the return being through conductors i and 16 to line 2. The power supply to the induction coil It is varied by of suitable variable impedancc 8t. If the power supply is alternating current this be either a conventional Varies or a rheostat. Variac a trade name of the General Radio Company for a variable voltage (disclosed in Patent #2,6G9,013). It a device which increases or decreases voltage by mechanical rotation of a contact engaging the windings of a transformer. If the power supply is direct current the variable impedance would, of course, be a rheostat. The power to the 'Variac fill is from line i through conductors 68, "H2 and iii, the return being through conductors 84, it, and 76, and back to line 2.

The Variac is driven through a one-revolution clutch and reduction gearing Bl by means of a motor at energized from the power line through conductors 9i) and E52. One-revolution clutch 81 is electrically operated, and is initially engaged because of a supply of power from line I through conductor Qt, normally closed contacts 96 of relay and a conductor lull. The return is through a conductor It? to line 2. The resulting rotation of the Variac causes a growth in output current from zero until the one-revolution clutch disengages. The power available in this way for the main induction cell It could grow to a value greater than needed to raise the first sheet into engagement with the feed roller, but the power supply is interrupted by suitable means as soon as the sheet reaches the feed roller. The mechanical rotation of the Variac is continued under drive of motor 88, and is terminated after one revolution by the operation of the one-revolution clutch, thus preparing the apparatus for the separation and delivery of the next sheet.

In the meantime the growing current is supplied frcm the Variac through conductor Hi l, the normally closed contacts Hit of relay and conductor Hi8 to the induction coil id, the return being through conductor i i In the present apparatus the complete passage of a sheet is determined by the use of two feelers as shown in Fig. 2, instead of a single feeler combined with a time delay relay, although either arrangement might be used. The first and second feelers or microswitches are inace 1,2108:

dice-ted at. 4,4 and 48'. The contacts are normally open. When. the topmost sheet flies upward and reaches the feed roller it closes, the switch 44, thus energizing. relay coil H2 and so opening the contacts I06 and thereby interrupts.

ing the energization of the induction coil. This;

drops all sheets but the uppermost. sheet, the latter being. held by the fully magnetized. feed roller, which may be constantly rotated and which prommptly begins. feeding the sheetout of the stack. In this case the feed roller may be permanently magnetized, as by making thesame of Alnico. or other highly magnetic material. However, as here shown itis magnetized electrically, with a constant current supply.

Energization of relay 93. opensthe contacts 98 and thus de-energizes the electrically controlled.

clutch 86 so that when itdisengages at the end of a single revolution it will not again engage unless and until the preceding sheet has been fed entirely out of the apparatus.

When the trailing edge of the sheet leaves the feeler 44 the relay coil I I2 is not yet de-energized because of the action of the feeler 48, and therefore the main induction coil is not yet re-energized. However, when the trailing edge of the sheet leaves the second feeler t8 the relay coil H2 is de-energized, the contacts 96 and 1&6 are again closed, and the induction coil is again energized to raise another sheet to the feed roller.

In Fig. 3 the drive motor for feed roller 24 is not shown. It maybe any motor connected to the available power. supply. It may run continuously, and the feed roller may be energized continuosly, with no disadvantage except a slight waste of electric power. Continuous feed of sheets one after another is contemplated. However, by additions to the circuit one may provide intermittentv energization of the motor or/and the roller, and manual or treadle controlled feed. Thus a clutch '26 is shown in Fig. 1, for intermittent operation of the feed roller while permitting continuous run of the motor 28. of these features are described later.

It will be seen that the described apparatus provides a gradually increasing current in the main winding It. The term gradually is to be understood in a relative sense, for if the apparatus is operated at high speed to feed a sheet every few seconds the entire growth of the current may occur in a very short time, yet that time is long compared to an instantaneous-application of full current. With a growth current the first sheet will be raised from the second, following which the second will be raised from the third, while the first will. tend to maintain its distance from the second, and so on, thus causing all the sheets to separate from one another and to fan out over asubstantial distance. This eliminates the need for critical adjustment of the circuit elements and current values. It provides a large tolerance in the. operation of the apparatus, so that if one sheet varies somewhat from another in thickness or weight, etc., the operation will nevertheless be unhampered.

Even the refinement of an elevator table has been found unnecessary, and in a particularcase I have located the feed roller 16" above the table, making it possible to feed sheets from a stack 14;" high. The stack may be fed down to a thickness of, say 2", so that in effect, a 12" stack may be fed, and then another 12" stack added to the residual sheets.

The: residual stack is preferably left on the table because it acts as a priming stack and Some Single phase electronic circuit The system of Fig. 3 is an electro -mechanical system because of the motor drive employed to. produce the growth current. However, a substantially wholly electronic system is shown in.

Fig. "l, and eliminates the need for the motor,

clutch, and Variac shown in Fig. 3'. The specific,

system employs a thyratron, and firesthe thyratron by means of a trigger wave. The main induction coil of the apparatus receives the D. C. or rectification increments. The trigger wave isshifted, and this provides a growth current because of the increasing increments of plate current as the firing point of the thyratron changes.

The plate and grid waves of the thyratron may be of like frequency and each of uniform magnitude, but relatively shiftable in phase, With the waves out of phase no current will fiow, and

with the waves in phase a maximum current will flow. At intermediate values the current. supplied by the thyratron may be varied by shifting the phase of the trigger wave supplied to the grid. On the other hand the trigger wave and the plate wave may be given. a phase difference which is maintained constant, and the axis of the trigger wave may be raised or lowered by chang ing the magnitude of a bias superimposed thereon. This also has the effect of varying the plate current, and because either method or a com bination or both may be employed I prefer to use the broad term shift in respect to the trigger wave, in order to include either a phaseshift or a bias shift, or both.

Referring to Fig. 7, the main coil is shown, at it, and the magnetic feed roller at 24, the latter being driven by a motor 28. Single phase power is supplied at lines I and 2. The apparatus of Figs. 1 and 2 applies to this circuit, except that the clutch 25 of Fig. 1, and the second trip switch, or microswitch 48 of Fig. 2, are omitted. The. first trip switch it is employed.

When the main switch I It is thrown on, the

transformer l I 2 is energized, heating the filament of thyratron tube ti l, through secondary 6..

After the tube is warmed up a circuit breaker, l 1.8

is closed, and then the operating switch I20 is closed, which starts the cycle as follows:

Relay I22 is energized from line i through the normally closed contacts of relay 535, which s still ale-energized because the trip switch 54 is open. Actuation of relay iii-2 opens the com tacts I28. When contacts 12s of relay 22 open.

the growth current starts and the sheets begin to separate. The growth current is obtained from the transformer secondary int, and is controlled by thyratrcn H 3. A trigger wave is su plied from transformer secondary its to the grid;

The resistor I32 and capacitor ltd provide a phase shifting circuit for the trigger wave. The series resistor it limits the grid current. Resistor I32 of the phase shifting circuit is val-is able to permit line control of the phase relation of trigger wave or grid voltage to plate voltage. sired the manual operation explained latter.

Secondary its of transformer t in con- To summarise, in the special case where time junction with full-wave rectifier Hill and potendelay de-energization is wanted, in addition to tiometer I42, provides a positive D. C. grid pcmagnetization, the thyratron serves tential or bias which raises the axis of the tin purposes in one, and is trouble-free in wave and so increases the amount or plate cur-- maintenance. rent fed through the main sheet separatin coil lilnergization of relay opens contacts I24, I6. This bias may set for the lnaxirnt dedeenergizing relay and permitting contacts sired amount of thyratron plate current. T ans- 1238 to again close. This bucks the trigger Wave former secondary and rectifier supply a 10 elevating potential of thyratron lit, and so colnegative bias to lower the or the trigger lapses (either completely or partially) the field wave. The thyratrcn plate current is in alter in the main coil It. The field in the main coil hate or positive half waves, as suggested is collapsed because the trigger Wave axis is low- 5. The trigger wave from secondary toi has ered by rectifier supply Mil, M8, thus pulling its phase displaced by the resistor and capac the A. C. trigger wave down to the point Where itor I32, Ital, and has its horizontal axis raised the thyratron does not fire (01' tires at desired by the bias supply its, the amount being minimum),

adjustably controlled by potentiometer no At the same time relay I also closes its conthe circuit goes into operation, however, the cl tacts and i s. Contacts ItZ initiate the cycle feet of the rectifier circuit Mt, its is to pull the so of the upper thyratron tu'oe ill? with the aid axis of the wave down again. it is initi of secondaries and tilt; of transformer ITO. pulled down to a point where none, or i' Secondary -aes the trigger wave, and sired, only a small chip or plate curl I apacitor Ilt provide phase as shown at in 5. The amount wave, which controls the is pulled down then. decreases by reason. ratron and so controls the ole- R. C. decay circuit Hi2, 7). of the A. C, grid voltage Wave rise firing of the thyratron keeps rncvi to the left, thus causing larger of plate current to flow through as shown at I56 and IE8 in Fig. eter I60 (Fig. '7) is an additional or 8: 25 oi" the gh rectifier Hit and po- '1 relay contacts 562 leadand resistors Hit, i238, potential to thyratron whatever plate current is aeitor .W, its, thereby trol which may or may not he is a. required to ener ize the magnetic roller at its particular application. It controls how lower stage of magnetization. A. C. Wave is brought down. 3') Contacts i" itiate operation of a time .4 around tuhe I94. Tuhe I554 a decay circuit to act as a timer .-,y rotation of the feed roller after the the roller (as indicated by of the trip switch 2 The time delay mplished through grid rectification by or resistor E96 and capacitor E98. The resistor 2% is used to limit grid current.

Because of grid root ation the capacitor ap- 45 pl a negative potential on the grid of the tube, and plate current does not flow. However, when the contacts Wt close the charge stored in capacitor dur 1g tie grid rectification period the discharges through the grid circuit 52222, and when the d charge has reached a certain point the tube wi It is this time required for that results in the desired time delay action of the circuit.

When the tube i relay 9 is energized, cl .2; contact- 1 and Contacts 264 inin the sense that if there is 1So With, then less drop would be neeJe. a certain plate current value. In a s fore the duplicate controls may not he 11 eded, and in a simpler circuit one the other may be employed alone.

Leaving the lower part or the diagram produces the growth current, the one of the main coil It causes the sheet to it reaches the rol r and switch a l, closing the latter relay IEE, which functions to rnagnetxze roller 24 and to start the motor 533. particular circuit here own the n of the feed roller thyratron tuhe it? controlled r "c two widely different or u.

of magnetization of the l. roll, as Ve-ll crease the throo h magnetic roller 24 complete cut-off when the feed roll not in by supplying more potential than preuse. A relatively small magn t 1 :iously ap; ied through contacts 462 of relay I25 to the grid of thyratrcn For this purpose the on 15 cent .ctentiometer 208 is made higher than that of pot ometer i822, At the oa..'l th closing of clay contacts 2555 enert feed motor to through switch 2 iii, rectifier It, and 2m.

so far described the operation would be wholly automatic, he delivery of one sheet follo ing another, but in many applications a or p al. control. is Wanted, and for this purpose it is merely necessary to add any convenient form. lanual or pedal switch 22a in quate to hold the toprnos' shoot while the coil I 6 is being tie-energized to per t do of the othe* sheets. A d gree of magnetization employed to actu feed the sheet.

It may be explained that it find it to employ a thyratron as switch difficulty with oxidation or contac' of ordi mechanical relays. The thyratron far '1 as an energy controlling thus el ing the need for a power rheostat with dissipation of energy to bring the feed down to desired cnergization. .oreover, we r incorporate in the tuhe circuit switch 220 13F" lly open switch. Thus to provide a time delay action before vy or the diagr. shows two arrangements in one, for the magnetic roller current, which delay dewith automatic operation switch 2H) would be .486. or omitted, and the closed and there would be no manual or foot treadle switch, but with switch 2I0 open it becomes necessary for an operator to close a manual or foot treadle switch 220 to initiate the operation. The motor is a shunt motor symbolized at 28. but electrically the terminals 2I6 represent the connections to the field of the motor, while the terminals we represent the connections to the armature of the motor. A shunt motor is used in order to give a wide range of speed control, and also because it can be started and stopped more readily than a single phase induction motor.

When the sheet leaves the feed roller 24 the trip switch 44 opens and relay I 26 is de-energized. This opens contacts I62 and I64, and permits closing of contacts I24. The opening of contacts I6 cuts off the current flow through tube I94, deenergizing relay H32, and opening contacts 204 and 206. The opening 0? contacts 206 stops the motor. The opening of contacts 204, in conjunction with the opening of contacts I62 of relay I26, completely tie-energizes the roller after elapse of a time interval caused by the delay circuit I84, I86 and [88. The closing of contacts I24 operates relay I22, thereby opening contacts I28, and so again initiating the growth current in the main sheet separating coil I6. This causes the cycle to repeat.

The described circuit is designed for apparatus in which not only the feed of an entire sheet, but also the partial feed of a sheet, say only one foot at a time to a shear press, may be controlled manually. In such case a single sheet lift is followed by a whole series of sheet feed movements to the press. For this purpose the feed roller might be left energized continuously, but that is not altogether desirable because of waste of energy, and possible overheating of the feed roller. Instead the feed roller preferably is energized only during its feed action, and is then reduced in energization to a value sufficient to hold the sheet up against the roller, although not enough to feed the sheet, In accordance with the present circuit, instead of the full energization taking place strictly in time with the operation of the feed motor, the time delay circuit I84, I86 prolongs the full energization for a brief interval, say one-half or one second, to make sure that the sheet does not break away from the feed roller because of its own inertia when the roller stops abruptly.

Thus in a simpler form the circuit of Fig. '7 might provide for continuous fully automatic feed of sheets, one immediately after the other, with continuous energization of the feed. motor 28 and the feed roller 24. In slightly more complex form the feed roller may be de-energized between feed operations. In still more complex form the feed motor may be de-energized between feed operations. In still more complex form the feed roller may be energized with two degrees of magnetization, a lesser one for holding but not feeding a sheet, and a stronger one for feeding the sheet. In the present circuit there is the still further refinement of prolonging the full energization or magnetization of the roller beyond the rotation of the motor, a feature particularly useful for partial or incremental sheet feed.

Circuit breaker I I8 is used in this circuit to protect the thyratron I I4, and the main coil, and to act as a cut-off device if the tube is operated at maximum load for too long a time. The circuit breaker includes a thermal time delay mechanism to take care of the possibility of a sheet not being raised. It will be recalled that the raising of a sheet is what cuts off the energization of the main coil I6, and if no sheet is raised to the trip switch 44 something may be assumed to be wrong, and instead of letting the main coil be subjected to full energization indefinitely, it is cut off after, say four seconds, which is beyond any reasonable time needed to raise a sheet.

Secondary II6 of transformer H2 is the filament heating source for thyratron H4. Transformer secondary 222 of transformer I16 is used to heat the cathode of tube I94. The plate circuit of this tube is supplied directly from the line, rather than from a transformer secondary. Secondary 224 of transformer I10 is used to heat the cathode of thyratron I12. Secondary 226 of transformer I19 is used to supply plate current to thyratron I12, and consequently to magnetize the feed roller 24.

The rectifier 239 across the main coil I6 is used to give increased life to thyratron H4, and incidentally add energy to the main coil I6, The rectifier 230 is faced in direction opposite to the rectifier action of the tube, and functions during the negative plate cycle or rectifier cycle of the tube to discharge some of the magnetic energy of the main coil. It should be kept in mind that we deal here with a heavily inductive circuit. The rectifier 230 may be termed a back rectifier.

An alternate idea which might be used but is thought less preferable is the use of a shunt circuit 232, 234 across the thyratron II I, this being a series R. C. circuit of very small current value to just effect the proper commutation time of the tube. In either case the idea is to help lessen the problem caused by tearing of electron surface material from the cathode of the tube because of the interrupted or pulse nature of the operation.

The use of a rectifier 23!! has an important advantage over the use of a shunt circuit and that is in restoring or making useful some energy which would otherwise be wasted. This is symbolized in Fig. 5 by the parts 232, 234, 236 added to the normal increments I56, I58, I58, the parts 232, 234 and 236 corresponding to the energy fed back from the main coil. This is apart from the improvement in commutation. This same idea has here been employed also in connection with the feed roller circuit and even in connection with one of the relay circuits. Specifically the back rectifier- 238 serves the purpose of improving commutation of thyratron I12 and at the same time adding energy to the circuit. The back rectifier 240 is used across the motor circuit to add energy to the circuit. The back rectifier of tube I94 is indicated at 242 and provides additional energy in the non-conducting cycle of tube I94, the additional energy going to relay I92. In all there are four back rectifiers used across inductive loads for the same principle as was explained above, except that in the case of one of them (rectifier 248) there is no question of commutation at a cathode, but in all of them there is some restoration of current from inductive load.

Resistor 244 is used in the circuit to connect the cathode and screen grid to the plate circuit to aid in grid rectification for the timing circuit.

In the description of the circuit of Fig. '7 I mentioned the possibility of only partially instead of wholly collapsing the field of the main induction coil I6. This may be explained with reference to Fig. 6 of the drawing, in which it Will be seen that the growth curve or saw-tooth wave is much like that shown in Fig. l, except that a minimum current value is provided somewhat above the zero current axis 2%. This is a refinement which may be resorted. to when dealing with sheets which tend to repeatedly adhere together, as, for example, if coated. with a tacky oil film. The partial energization level 253 may be made such to hold the sheets in slightly fanned or separated condition, so that they will not re-adhere each time they descend. Thus the growth current causes a progressive fanning of the sheets until the uppermost sheet reaches the feed roller, at which time the up=permost sheet is held by the reed roller and the remaining sheets are permitted to descend almost all the way, but not quite far enough to rest firmly on one another with the possibility of readhering. To accomplish this result it is merely necessary to so adjust the lowering of the trig ger wave of the thyratron Ht (as determined by the inter-related potentiometer adjust? its 142 and I53) as to provide email, unli'o cly main-- tained increments of plate current instead of complete cut-off.

To summarize the operation of the circuit of Fig. 7, thyratron I it acts as a valve to control the main sheet lifting coil Iii. The transformer sec ondary I44 supplies plate current and the trans" former secondary 13d supp-lies a trigger wave to the grid. The phase of the trigger wave is shit by the phase shifting circuit I32, i3 1. The height of the axis of the t wave determined by adding bias pot. ial supplied from a transformer secondary and bridge rectifier and potentiometer. In the present case there are two such bias circuits, one of which Hit, 142) elevates, and the other of which (hit, I33, I60) depresses the trigger wave, the latter being effective when the coil is not functioning, and the former being effective when it is. The growth of the plate current takes place because the trigger wave axis is raised by discharge of potential from the decay circuit 152, I54.

Thyratron I12 acts as a valve to control the magnetization of the feed roller 29. Trans former secondary 22d supplies plate current, and transformer secondary I65 supplies a trigger wave to the grid. The phase of the trigger wave shifted by phase shifting means Ht, lit. The height of the axis of the trigger wave is estab lished by bias potential supplied. by transformer secondary HEB, rectifier 183 and potentiometer I82. This is adjusted to supply the needed current for the sheet holding The magnet-- ization is increased during the sheet feeding stage by raising the trigger wave this being done by sending current through resistor 208, the current being supplied by relay I92 controlled by a third thyratron ice. The relay I"? also supplies current to operate the motor If it be desired to delay the start of the sheet feeding operation until after ample time for collapse of the main field and the fanned sheets, such delay is provided by a time delay circuit associated with tube Hi3 using C. circuit I96, I98. The delay in firing of tube Hi l delays both rotation and full magnetization of the feed roller.

If it be desired to briefly continue iuil energization of the roller after motor stops. in order to compensate for inertia of the moving sheet, such prolongation is provided. by the time delay or decay circuit iitl, llf semiautomatic sheet feed or fractional sheet feed is desired a suitable normally open manual. or

treadle switch -220'is employed, the switch 2H1 being opened or eliminated, but for fully auto matic feed of successive sheets the switch 213 kept closed.

Three phase electronic circuit Three phase power supply is available in most industrial plants, and is preferred because it makes more efficient use of the power available.

A three phase circuit is shown in 8. The main power lines are marked Ll, and L3, with a neutral return N. The three phases are set up in Y form, but the entire circuit may equally well be set up in delta form, without the neu tral return, in those areas of the country where that is the common practice. The circuit also can be adapted for two phase power supply.

Closing pushbutton 300 energizes contactor 302 which remains on through the interlock circuit switch 304. The closing of contacts 32 I, 322, 323 of the contactor 302 connects the three lines Ll, L2 and L3 to the three transformers 324, 326 and 328, respectively, and also energizes the transformer 330. The transformers heat the filaments of the tubes. Contactor 302 sticks by reason of a circuit from Ll through contacts 306 through normally closed contacts 308 of a relay 310, through the coil of contactor 302, and conductors 312, M4, 316, and thence back to the neutral N of the line. After the tubes have been warmed up switch 332 in series with line Ll is closed, thereby energizing relay 334 by way of conductors 336 and 338. This opens the normally closed contacts 340 of relay 334, which initiates operation of the three phase circuit shown in the upper part of the diagram.

This three phase circuit is similar in operation to the previously described single phase circuit, except that there are three circuits, one for each phase, each of the three circuits being the same as that previously described, but with a single R. C. circuit to control the current growth. The R. C. circuit which lets the trigger wave rise is shown at 342 and 344.

Taking phase 3 or the supply from line L3 as typical, the operation is as follows: Secondary 346 of transformer 328 heats the filament of the thyratron 348. The plate is supplied directly from the line L3 rather than from a transformer secondary. Secondary 350 provides the trigger wave, and its phase is shifted by the circuit of condenser 352 and resistor 354. The resistor 353 is used to limit the grid current of. the thyratron 343. The resistor 354 is adjustable to determine the phase shift. Secondary 358 together with rectifier 360, supplies the phase number three portion of the current which provides the posltive o. 0. potential to the grid of the thyratron 343. This positive potential, which is adjustable through potentiometer 352, determines the extent to which the trigger wave axis is raised. This is the maximum rise of the trigger wave, when not pulled down to in point, which may be either complete shut-off or a residual value for slight sheet se aration before explained.

The potentiometer 362 which establish blas or height to which the trigger wave iis ig raised, works in common for all three tubes. For blas purposes D. C. is wanted, and with a three phase clrcuit the bias producing current is obtamed from all three phases with three rectifiers combined. In Fig. 7 a full-wave rectifier circuit Was p yed, but in a three phase circuit it is enough to use simple half-wave rectifiers,

itial or rest.

as shown, because :of the overlapping of the three phases.

Secondary 364 of'transf-ornier szs provides .the negative bias used to lower the trigger wave to cut off or reduce the current flowing through thyratron t lt. Here again there are three secondaries and three rectifiers for the three phases. The potentiometer 368 controls the degree to which the trigger wave is lowered; andhere again the currents from all three are combined in a single potentiometer to obtain D. C. applied to all three tubes. The bias potential thusobtained is passed through the combination capacitor are and rheostat 3 so that when contacts sec of relay 334 open the potential will decay through this combination 342, 34 i, and the time factor of this R. C. circuit determines the rate of growth of the current in the plate circuit of thyratron 348.

Similar description applies to the thyratron 310 of phase 2 and thyr tron 3'52 oi phase I. A fuse 314 is used as a protection in the plate cir cult of thyratron 348, and similar fuses are used for the other tubes.

In passing it may be mentioned that while a common control is used to raise the tri ger wave, and a common control is used to lower the trigger wave, three independent controls are used for the phase shift of the trigger'wave at each tube. This is shown at 352, 35$ for thyratron and similar or duplicate controls are shown for thy ratrons 3'59 and 312, respectively. This is desir able because each trigger wave must be shifted in phase relative to the plate wave of its own tube, and also to fire alike, because although the tubes come within manufacturers specification limits they may nevertheless have slight differences in firing potential, and the independent adjustment here provided makes it poss.....e to bring them into uniform operation.

The combined outputs of the three thyratrons is a D. C. power supply for the main induction coil or sheet separating coil It, and it is a growth current because of increasing increments, all as previously described in connection with Fig. 5, except that there are increments in three displaced phases which are combined in the common main coil It. The lower terminal of the coil It is connected to the neutral return N, and the upper terminal has all three cathodes connected to it in common.

The ferrous sheet rises until it reaches the feed roller and trips the trip switch 3&0, shown at the bottom of the diagram. When trip switch 3% is closed it energizes relay 3B2, thereby open ing contacts 384, and so deenergizing relay St l and thus permitting contacts S lt to again close which provides the negative bias potential necessary to pull the trigger wave down and so collapse the main coil field, because when the full negative potential is applied to the trigger wave it cuts off triggering of the thyratrons (or re duces the output to a small residual).

Besides elimination of the main field, the other things accomplished by the trip switch are to energize the feed roller and to cause its rotation. A thyratron tube may be employed to control the supply of energy to the feed roller, and preferably dccs so in variable increments determined by shifting a trigger wave, because in that case the thyratron acts both as a switch and as a current controlling means, with no difficulty of dirtying of contacts, etc. Three phase energization of the roller might be employed by using three thyratrons, but in the present circuit the 114 apparatus has been simplified by using only a single phase supply for the feed'roller.

The trip switch 3%, by actuating relay 382, closes contacts 386 and 388. The latter supplies a trigger wave from transformer secondary 392 to the grid of thyratron 399, tl'irough a phase shift circuit comprising a resistor 3% and capacitor Series resistor 323 is used to limit the grid current. The plate of tube 3% is supplied directly from the line L2, instead of from a secondary. The secondary see of transformer in conjunction with rectifier cs2, provides a positive bias potential to raise the trigger wave. This causes the thyratron to supply current to the magnetic roller 2%, for a partial or reduced en- .sation for merely holding but not feeding ti sheet. The rise of the trigger wave axis which determines the amount of current flowing through magnetic roller 2 is controlled by adjusting a potentiometer set. In this apparatus, as in '7, the feed roller is operated in three the first being zero energization during initial lifting of the sheet. second is the present partial energisation, sufficient to hold the sheet but not feed the sheet. This is applied the instant the sheet operates the trip switch, and before the roller rotates. The current flowing to the magnetic roller is taken from the lines L: and Li, and is controlled by the thy-ratron tube while resistors toe and @538 complete the grid circuit.

With the closing of contacts tilt of relay 382 a time delay circuit centering .on tube H2 is initiated. This delays action of relay Mil to provide a delay before starting and putting on the full energization of the roller, and among other things, affords time for the sheets, other than the top sheet, to descend. Here again grid rectification takes place through resistor tit and capacitor did, while resistor ii?! limits the grid current. The potentiometer y controls to some degree the time of the delay. The main control is by adjustment of resistor Aid as part of the decay circuit using capacitor ME. The use of a time delay circuit at this point is a refinement, and in many cases will prove neither essential nor even desirable.

It may be mentioned that in this circuit the action of single relay simultaneously applies full energization to magnetize the feed roller and to start the motor which rotates the feed roller. (In the alternative, .it might engage the clutch of a continuously running motor.) The primary reason for the time delay is to delay rotation of the feed roller, but it then is also advantageous to delay full energization because it saves power, and avoids any possibility of tending to feed two sheets forward because of sticking at the back ends of the sheets if the sheets have not been given adequate time to descend fully (or to slightly fanned position). In many installations it will be found that full energization and rotation of the feed roller can be started instantly after top sheet reaches the feed roller, but this part of the circuit included in an excess of can to make delay available should it be found desirable.

After the time delay of tube ii??? runs out it fires and relay sill is energized, closing contacts 422. This energizes relay li l through normally closed switch or, if the operation. is to be senn-automatic, through a normally open inanual or foot switch 28, in which case switch 625 is to be assumed left open or omitted. When relay is energized, contacts 239 close, and provide additional bias potential to the grid of thyratron 15 3'90, thereby increasing the plate current through the magnetic roller 524. The controlled by a potentiometer the ad itiona posltive bias shunts or bypasses the C. cuit 434, 436, Me.

When relay 42d is energized contacts and 442 also are closed, thereby supplying on) rent to the feed motor (shown symbolically at 28) through rectifiers 44:3, .3, and rheostat 450. The armature terminals are indicated 452, and the field terminals are iidicated at F? the motor being an ordinary shunt motor, wi. 'l advantages previously stated. lit receives the combined rectified current from the three pha e lines Ll, L2 and L3. Rheostat may be to adjust the speed of the motor. The motor return goes back through conductors and 345 to the neutral return N.

If a foot switch till; or other manually or ated switch should be used feed, a time delay is prefeiz l collapsing the magnetic roller 24. This prevent. the sheet from the rolls of the sheet when in root, is here obtained by the ac which delays the fall of the tr thyratron 398. The capacitor resistor 408, tends to absorb applied, thereby slowing up the ger wave at the tube as the charge through the R. C. circuit.

When the sheet has been who the trip switch and contacts 338 open, there? ,r timing circuit of tube ill. and relay contacts 422 then ope opens the circuit of relay contacts 438, 440 and 442, which stops the fee motor 28. At the same time contactrelay 424 reopen, which would energize the magnetic roller were same time wholly ole-energized because contacts 388 are opened when relay 332 is de-energ ced and cut off entirely the flow of current thr L the thyratron 3st] and the magnetic roller.

In addition the de-energization of relay permits contacts 384 to re-close, wh energizes relay 33 1, thereby closing ntacts and opening contacts 34s. The former a time delay cut-off or non-shect-liit .d or netio possihle tails of the circuit are next described. One is the provision of a non sheett safety means to cut off energization of the n coil event that a sheet is not raised within a reasonable time. in the present this is done by the provision of a time delay circuit centering about the tube It will be recalled that by closing the operating switch 332 the was energized, which in turn opened the contacts S ill to initiate the growth current, and at the same time closed the contacts liiii. The latter contacts initiate operation of t me delay circuit, which in turn limits total ti ie the growth current circuit may remain in its on condition. This may he, say four seconds, for if a sheet is not delivered ""ithin some such maximum reasonable time something may be assumed to be wrong, and th power supply to the main coil l 6 is cut oii.

The time delay circuit operates as follows:

Grid rectification takes place through resistor 462 and capacitor Mal, and causes a negative potential to be applied to the grid of tube 568 as a result of the charge built up on capacitor 464. Grid resistor is used to limit the grid current. Wh n contacts 453 of relay 334 close, the charge decays through resistors iii-l2 and silt, and when it falls low enough the tube fires, thereby actuating the relay 3m and opening its contacts 3%, which shuts 0c the power to the entire system. This is so because contacts 308 are in the stick circuit or the main contactor 392, and let the contactor open, which opens contests 32!, and and so opens the main lines Ll, Lil ant. L l. Resistor t is sod to tie the cathode into the plate to aid in g rectification. During all normal operation. sheet is lifted and reaches the roller long before the tube lial fires. The tub fires only if for some reason a sheet does not rise to the magnetic roller.

The foregoing descr n assumed the use of a foot switch lat after lug semi-automatic and even partial sheet feed, as for example not employed for partial sheet feed (switch 426 closed) then the sheets are fed one after another by reason of the last that when the tail end of the sheet being fed leaves the trip switch 380, trip switch opens and so initiates the next whole sheet lifting cycle. The growth current or sheet fanning system provides some delay anyway, an:- there no need for a special time delay to insure a between successive sheets. However, a s longer delay were wanted such a time delay relay could be employed. The til delay circuit here shown for prolonging the igization oi the feed roller beyond its rotation, namel the decay circuit 435, 435, is not needed when the foot switch is omitted, and the parts ill-"l, it-ii simply be eliminated. The identical circuit shown with the parts 434, 136 eliminated would let the full stage energiration of the feed roller terminate on termination of rotation.

In this circuit, as in Fig. 7, it is advantageous to employ back rectifiers. The back rectifier 4TB helps commutation in the thyratrons 343, 310 and illfi, prolongs their life, and provides a more efficient circuit, considering that it is heavily inductive. Back rectifier ll?! is used across the relay Sill. Rect er l'lll is a back or con nected across the elay 4H3. Rectifier ill is used as the back rect' er for thyratron and feed roller aiding commutation and increasing the circuit efficiency.

Resistor dill is used to tie the cathode of tube 412 to the plate, aiding in grid rectification. Transformer secondary is for heating the cathode of thyratron. 3H secondary is for heating the cathode of tube use, and secondary its is for heating the cathode of tube :ll2. In the case of tubes lilil and M2 the secondaries are connected across the filaments and across the potentiometers and 129, that is, the filaments, the secondaries and the potentiometers all in shunt re ation. Potentiometer 4% is used to to adjust the range of the time delay caused by the circuit of tube 466, the primary adjustment being th resistor 3552.

To summarise the operation of the circuit of 8, thyratrons Mil, and ill. act valves for controlling the field of the main sheet lifting coil it. The lines Ll, L2 and L3 supply plate current. and the three transformer secondaries like secondary 358 supply trigger waves to the grids of the thyratrons. The phase of each trigger wave is shifted relative to its corresponding plate supply by the three phase shifting circuits like the circuit 352, 354, these being independently adjustable to establish a uniform firing point. The maximum height of the axes of all three trigger waves is determined by adding positive bias potential supplied by three transformer secondaries like secondary 2.53 and associated three rectifiers like rectifier Iitil, the combined rectified currents being passed through a single potentiometer 382 which affords adjustment of the height. The trigger wave may be depressed by the addition of negative bias potential supplied by three transformer secondaries like the secondary and three rectifiers like its associated rectifier 3%. The combined rectified currents are combined in a single potentiometer 3&3 which serves to adjust the drop. The desired growth of the current sup plied to the drain coil id is obtained by the use of a decay circuit 3 32, 3 2 5 which reduces the negative bias taken from potentiometer 368.

The thyratron 3% acts as a valve for controlling the magnetization of the feed roller M. Plate current is supplied directly from the line, and a transformer secondary 39?. supplies a trigger wave to the grid. The phase of the trigger wave is shifted by phase shifting means 3%, 396. The 1 height of the axis of the trigger wave to supply the needed current for the sheet holding stage is established by bias potential supplied by a transformer secondary are, rectifier tilt, and potentiometer Mi l. The magnetization is then increased during the sheet feeding stage by raising the trigger wave axis, this being done by sending current through bias potentiometer 32. The current to operate the motor 28 is taken directly from lines Li, L2 and L3 with a neutral return N, the three currents being rectified and combined when using a series motor as here shown. The supply of current to the motor is put on or cut off by a relay 424.

If it be desired to delay the start of the sheet be desired to briefly continue full energization r of the roller after the motor stops in order to compensate for the inertia of the moving sheet, such prolongation may be provided by the decay circuits 534, 336, 468.

General There may be difiiculty in separating the sheets due to oil film adhesion. Even when the apparatus is used indoors it often receives a stack of sheets fresh from outdoors storage, and because of the cold temperature in winter, and the long time which it would take for the sheets to acquire room temperature, the sheets stick very tightly because the oil film is congealed. A first improvement is to not let the growth current come down to zero. Instead the minimum current (as shown in Fig. 6) keeps at least some slight separation of the sheets, and this prevents them from again adhering and avoids the need to completely break the seal each time. An-

18 other step found useful in a serious case is to limit the height of the stack to only a few inches so that the separation of the sheets is a maximum during each cycle. Still another step is the retention of a residuary stack beneath the stack being fed.

It is believed that the construction and operation, as well as the advantages of my improved apparatus for separating and feeding sheets, will be apparent from the foregoing detailed description thereof. Although single phase power is used for the feed roller, three phase power may be used. The advantage of the single phase circuit shown is economy in construction and apparatus, with some sacrifice in electrical efiiciency. On the other hand, by accepting a greater apparatus cost for a three phase thyratron supply to the feed roller, one may gain slightly better electrical efliciency. This is merely a matter of engineering judgment for any one particular installation. In general the feed roller uses much less power than the main induction coil, which explains why in the present case a three phase supply is used for the main induction coil, while using only a single phase supply for the feed roller. Incidentally, there is arguably some ad vantage in single phase supply for the feed roller because it may supply a degree of slight pulsation and vibration of the sheet, which may facilitate sheet separation at the lower rear end of the sheet, despite adhesive oil film.

The circuits of Figs. 7 and 8 are almost fully electronic in that even for time delay a tube and circuit are employed. An electronic time delay relay is more trouble-free than an electromechanical time delay relay, but the latter may be used. There is no important change, for nowadays one can buy a complete electronic time delay relay assembly, just as one can buy an electro-mechanical time delay relay, to use in a circuit at a desired point.

The sheets may be fed one after another automatically with or without time delay therebetween. The sheets may also be fed semi-automatically, requiring a manual or pedal switch operation, and the sheets may be fed incrementally as when working with a shear press or the like. The feed roll may be energized continuously or may be deenergized when not in use, and if desired, may be energized in two different stages, one for holding and one for feeding. The feed motor may be operated continuously or only when in use. When feeding sheets incrementally the full energization of the feed roller may be prolonged slightly beyond the operation of the motor to overcome inertia of the moving sheet. Safety means may be provided to guard against prolonged energization of the main coil in the event that no sheet is lifted within a reasonably short time. Back rect'ifiers may be provided to increase the efilciency of the circuit. Re-adhesion of oil film coated sheets may be prevented by maintaining a small holding current through the main coil instead of cutting off the current entirely. Many of these features are, of course, optional.

It will be understood that to obtain. the benefit of the growth current idea it is not at all essential to simultaneously employ the large coil surrounding the stack, which invention constitutes the subject matter of my aforesaid copending application Serial No. 129,662, filed November 26, 1949, of which the present application is a continuation-in-part. In other words, while I have shown my invention applied to a main coil which entirely surrounds the stack of sheets, the growth current idea is equally well applicable to numerous other possible forms of magnet employed. to magnetize the sheets and to thereby cause them to mutually repel.

It will also be understood that while I have shown the sheets disposed and fed horizontally, the sheets and the surrounding apparatus may be oriented differently. For example, the sheets may be stacked in nearly upright position and repelled at their upper edges, the sheets then turning away from the group about their lower edges until they engage feed means.

It will therefore be apparent that while I have shown and described the invention in several preferred forms, changes may be made in the structures disclosed without departing from the scope of the invention as sought to be defined in the following claims. In many claims the reference to a top sheet and the raising of a top sheet to a holding means above the stack, etc. is to be understood primarily in a relative sense, because the sheets need not necessarily be initially stacked in horizontal position.

I claim:

1. The method of separating the end ferromagnetic sheet from a group of ferromagnetic sheets by magnetic repulsion between sheets, which includes passing a very feeble magnetic field through the entire group of sheets and relatively gradually increasing the strength of the magnetic field passing through the entire group of sheets as a function of time until the end sheet has been moved away a desired distance.

2. The method of separating the top ferromagnetic sheet from a stack of ferromagnetic sheets by magnetic repulsion between sheets, which includes passing a very feeble magnetic field through the stack of sheets and relatively gradually increasing the strength of the magnetic field as a function of time until the top sheet has been raised to a desired height, holding the top sheet, then cutting off the magnetic field to permit the remaining sheets to fall, and then removing the held top sheet.

3. Apparatus for separating the end ferromagnetic sheet from a group of ferromagnetic sheets by magnetic repulsion between sheets, said apparatus including an electromagnet so positioned as to pass a magnetic field through the entire group of sheets, a source of current to energize the electromagnet, and means for relatively gradually increasing the strength of the current in order to increase the strength of the magnetic field passing through the entire group of sheets as a function of time until the end sheet has been moved away a desired distance.

4. Apparatus for separating the top ferromagnetic sheet from a stack of ferromagnetic sheets by magnetic repulsion between sheets, said apparatus including an electromagnet so positioned as to pass a magnetic field through the stack of sheets, a source of current for energizing said electromagnet, a sheet holding means disposed above the stack, means to relatively gradually increase the strength of the current and consequently the strength of the magnetic field as a function of time until the top sheet has been raised to the sheet holding means, means to sharply reduce the magnetic field to permit sheets other than the top sheet to descend, and means to cause feeding of the held top sheet away from the stack.

5. Apparatus for separating the top ferromagnetic sheet from a stack of ferromagnetic sheets, said apparatus including an electroma net so positioned as to pass a magnetic field through the stack of sheets, a source of current to energize the electromagnet, and means for relatively gradually increasing the strength of the current in order to increase the strength of the magnetic field as a function of time until the top sheet has been raised to a desired height, said electromagnet comprising a coil extending entirely around the stack With the axis of the coil extending generally in the direction of the plane of the sheets.

6. Apparatus for separating the top ferromagneti sheet from a stack of ferromagnetic sheets, said apparatus including an electromagnet so positioned as to pass a magnetic field through the stack of sheets, a source of current for energizing said electromagnet, a sheet holding means disposed above the stack, means to relatively gradually increase the strength of the current and consequently the strength of the magnetic field as a function of time until the top sheet has been raised to the sheet holding means, means to sharply reduce the magnetic field to a small amount which permits sheets other than the top sheet to descend but which is adequate to keep the next sheets slightly separated to prevent ire-adhesion, and means to cause feeding of the held top sheet away from the stack.

7. Apparatus for separating the top ferromagnetic sheet from a stack of ferromagnetic sheets, said apparatus including an electroma net so positioned as to pass a magnetic field through the stack of sheets, a source of current to energize the electromagnet, a thyratron to control the flow of current to the electromagnet, and means to apply a trigger wave to the grid of the thyratron in shifting bias or phase relation so as to supply gradually increasing increments of plate current to the magnet in order to increase the strength of the magnetic field as a function or" time until the top sheet has been raised to desired height.

8. Apparatus for separating the top ferromagnetic sheet from a stack of ferromagnetic sheets, said apparatus including an electromagnet so positioned as to pass a magnetic field through the stack of sheets, a source of current to energize the electromagnet, a sheet holding means disposed above the stack, a thyratron to control the flow of current to the electromagnet, means to apply a trigger wave to the grid of the thyratron in shifting bias or phase relation so as to supply gradually increasing increments of plate current to the magnet in order to increase the strength of the magnetic field as a function of time until the top sheet has been raised to the sheet holding means, means to abruptly shift the trigger wave to sharply reduce the magnetic field to permit sheets other than the top sheet to descend, and means to cause feeding of the held top sheet away from the stack.

9. Apparatus for separating the top ferromagnetic sheet from a stack of ferromagnetic sheets, said apparatus including an electromagnet so positioned as to pass a magnetic field through the stack of sheets, a source of current to energize the electromagnet, a sheet holding means disposed above the stack, a thyratron to control the flow of current to the electromagnet, means to apply a trigger wave to the grid of the thyratron in shifting bias or phase relation so as to supply gradually increasing increments of plate current to the magnet in order to increase the strength of the magnetic field as a function of time until the top sheet has been raised to the sheet holding means, means to abruptly shift the trigger wave to sharply reduce the magnetic field to a small amount which permits sheets other than the top sheet to descend but which is adequate to keep the next sheet slightly sep arated to prevent re-adhesion, and means to cause feeding of the held top sheet away from the stack.

10. Apparatus for separating the top ferromagnetic sheet from a stack of ferromagnetic sheets, said apparatus including a main electromagnet so positioned as to pass a magnetic field through the stack of sheets, a source of current for energizing said electromagnet, means to relatively gradually increase the strength of the currrent and consequently the strength of the magnetic field as a function of time until the top sheet has been raised to the sheet holding means, an electromagnetic sheet feed roller disposed above the stack, means to sharply reduce the main magnetic field to permit sheets other than the top sheet to descend, a thyratron for controlling the supply of current from the source to the roller, means to supply a trigger Wave to fire the thyratron in order to supply increments of current to the roller, and means to shift the trigger wave to different bias or phase positions.

11. Apparatus for separating the top ferromagnetic sheet from a stack of ferromagnetic sheets, said apparatus including a main electromagnet so positioned as to pass a magnetic field through the stack of sheets, a source of current for energizing said electromagnetic, means to relatively gradually increase the strength of the current and consequently the strength of the magnetic field as a function of time until the top sheet has been raised to the sheet holding means, an electromagnetic sheet feed roller disposed above the stack, means to sharply reduce the main magnetic field to permit sheets other than the top sheet to descend, a thyratron for controlling the supply of current from the source to the roller, means to supply a trigger wave to fire the thyratron in order to supply increments of current to the roller, and means to shift the trigger Wave to three different bias or phase positions corresponding to complete cut-off, a holding current sufficient to hold the top sheet but inadequate to feed the sheet, and a feed current sumcient to grip the sheet tightly for feeding the same.

12. Apparatus for separating the top ferromagnetic sheet from a stack of ferromagnetic sheets, said apparatus including a main electro magnet so positioned as to pass a magnetic field through the stack of sheets, a source of current to energize the electromagnet, an electromagnetic sheet feed roller disposed above the stack, a first thyratron to control the flow of current to the electromagnet, means to apply a trigger Wave to the grid of the thyratron in shifting bias or phase relation so as to supply gradually increasing increments of plate current to the magnet in order to increase the strength of the magnetic field as a function of time until the top sheet has been raised to the roller, means to abruptly shift the trigger wave to sharply reduce the magnetic field to permit sheets other than the top sheet to descend, a second thyratron for controlling the supply of current from the source to the roller, means to supply a trigger wave to fire the thyratron to supply increments of ourrent to the roller, and means to shift the trigger wave to different bias or phase positions in properly timed relation to the operation of the main magnet.

13. Apparatus for separating the top ferromagnetic sheet from a stack of ferromagnetic sheets, said apparatus including a main electromagnet so positioned as to pass a magnetic field through the stack of sheets, a source of current to energize the electromagnet, an electromagnetic sheet feed roller disposed above the stack, a first thyratron to control the flow of current to the electromagnet, means to apply a trigger wave to the grid of the first thyratron in shifting bias or phase relation so as to supply gradually increasing increments of plate current to the magnet in order to increase the strength of the magnetic field as a function of time until the top sheet has been raised to the roller, means to abruptly shift the trigger Wave to sharply reduce the magnetic field to permit sheets other than the top sheet to descend, a second thyratron for controlling the supply of current from the source to the roller, means to supply a trigger Wave to fire the second thyratron to supply increments of current to the roller, and means to shift the trigger Wave to three different bias or phase positions corresponding to complete cut-off, a holding current sufficient to hold the top sheet but inadequate to feed the sheet, and a feed current sufficient to grip the sheet tightly for feeding the same.

14. Apparatus for separating the top ferromagnetic sheet from a stack of ferromagnetic sheets, said apparatus including an electromagnet so positioned as to pass a magnetic field. through the stack of sheets, a three-phase source of current to energize the electromagnet, three thyratrons to control the flow of current to the electromagnet from the three phases respectively, means to apply trigger Waves to the grids of the thyratrons so as to supply increments of plate current to the magnet, and a single means to simultaneously shift the three trigger Waves in order to supply gradually increasing increments in order to increase the strength of the magnetic field as a function of time until the top sheet has been raised to desired height.

15. Apparatus for separating the top ferromagnetic sheet from a stack of ferromagnetic sheets, said apparatus including an electromagnet so positioned as to pass a magnetic field through the stack of sheets, a source of current to energize the electromagnet, means for relatively gradually increasing the strength of the current in order to increase the strength of the magnetic field as a function of time until the top sheet has been raised to a desired height, and time delay relay means to cut off the supply of current to the electromagnet if a sheet is not raised in response to energization of the magnet.

16. Apparatus for separating the top ferromagnetic sheet from a stack of ferromagnetic sheets, said apparatus including an electromagnet so positioned as to pass a magnetic field through the stack of sheets, a source of current for energizing said electromagnet, a sheet holding means asposed above the stack, means to relatively gradually increase the strength of the current and consequently the strength of the magnetic field as a function of time until the tcp sheet has been raised to the sheet hclding means, means to sharply reduce the magnetic field to permit sheets 23 other than the top sheet to descend, means to cause feeding of the held top sheet away from the staclz, and a time delay relay to delay iceding of the top sheet until after the other sheets have had time to descend.

17. Apparatus for separating and successively feeding ferromagnetic sheets from a stack of sheets, said apparatus comprising a support for the stacl: of sheets, an induction coil extending entirely around the stack of sheets, with the axis of the coil. extending generally in the direction of the plane of the sheets and generally perpendicular to the desired direction of separation of the sheets, means to energize the coil in order to raise and separate the uppermost sheets of the stack, and feed means to receive and feed the uppermost sheet away from the coil and stack, said means to sue gize the induction coil including current varying circuit elements arranged and operated to energize the induction coil with a current of relatively gradually increasing magnitude in order to help insure the separation of the topmost sheet from the immediately suh- J'acent sheet.

18. Apparatus for separating and successively feeding ferromagnetic sheets from a stack of sheets, said apparatus comprising a support for the stack of sheets, an induction coil extending entirely around the stack of sheets, a sheet feeding means disposed near the leading end of the stack, drive means for intermittently driving the feed means, means to energize the induction coil in order to raise and separate the uppermost sheet of the stacl; and to bring it to the feed means, and appropriate switch means for the induction coil interlocked with the drive means whereby the induction coil is energized while the feed means is inoperative and is de energized while the feed means is operative, said circuit ele-' ments energize the induction coil including current varying circuit elements arranged and operated to energize the induction coil with a current of relatively gradually increasing magnitucle in order to help insure the separation of the topmost sheet from the immediately subjacent sheet.

19, Apparatus for separating and successively feeding ferromagnetic sheets from a stack of sheets, apparatus comprising a support for the stack of sheets, a main induction winding extending entirely around the stack of sheets, a magnetic roller having a magnetizing coil, intermittently operable drive means for intermittently rotating the roller, means to energize the main induction winding in order to raise and separate the uppermost sheet of the stack and to bring it to the roller, means for supplying current to said coil, means to energize the roller coil to only a limited extent sufficient to hold the uppermost sheet when the feed roller is not rotating, means to more strongly energize the coil in order to feed the sheet without appreciable slippage when the roller is rotated, and. appropriate switch means to energize the main induction winding when the feed roller is stationary, and to de-energize the main induction winding when the feed roller is rotating, said means to energize the main induction winding including current varying circuit elements arranged and operated to energize the main induction winding by a current of relatively gradually increasing magnitude in order to help insure the separation of the topmost sheet from. the immediately suhjacent sheet.

20. Apparatus for separating and successively feeding ferromagnetic sheets from an upright stack of generally horizontal. sheets, said apparatus comprising a support for the stack. of sheets, an induction coil disposed in an u; ght plane and having each of its windings extending entirely around the stack of sheets, with the top of the coil higher than the top of the to provide substantial clearance therehetween, with the aXis of the coil extending generally horizontally in the direction in which the sheets are to he fed, means to energize the coil in order to raise and separate the uppermost sheets the said means including current varying circuit elements arranged to supply the induction co' with a current of relatively gradually iucreas "lg intensity in order to help insure the sep ration of the topmost sheet from the irnmedi: y suhiacent sheet, feed means to receive the uppermost sheet when raised and to feed it in a generally horizontal direction from the mil and stack.

21. Apparatus for separating and successively feeding ferromagnetic sheets upright stack of generally horizontal sheets, said laratus comprising a support for the stack of sheets, an induction coil disposed in an upright plane and having each of its windings extending Q; ely around the stack of sheets, with the top of the coil higher than the top of the stack to provide substantial clearance therehetween, and with the axis of the coil extending generally horizontally in the direction in which the sheets are to be fed, means to energize the coil in order to raise and separate the uppermost sheets or the stack, means including current vary circuit elements arranged to supply the induction coil with a current of relatively gradually increasing intensity in order to help insure the separation 01": the topmost sheet from the immediately suhjacent sheet, and a magnetic feed roller having a horizontal axis extending across the direction of feed and disposed at a point higher than the stack hut lower than the top of the induction coil, said magnetic feed roller serving to recei e uppermost sheet when raised and to feed it in generally horizontal direction away from. the coil and stack.

22. Apparatus for separating successively feeding ferromagnetic shee s from a stack of sheets, said apparatus comprising a support for the stack of sheets, an induction coil extending entirely around the stack. of sheets, with the a; s of the coil extending generally in the direction of the plane of the sheets and generally perpenclicular to the desired direction of separation of the sheets, means to energize the coil in order to raise and separate the uppermost sheets of the stack, and feed means to receive and feed the uppermost sheet away from the c ll and stack, means to energize the induction coil including a variable impedance driven by a motor nd a onerevolution clutch, whereby the induction coil. is energized by a current of gradually increasing intensity in order to help insure the separation of the topmost sheet from the immediately subjacent sheet.

23. Apparatus for separating and successively feeding ferromagnetic sheets from a stack of sheets, said apparatus comprising a support for the stack of sheets, an induction coil extending entirely around the stack of sheets, a sheet feeding means disposed near the leading end of the stack, drive means for intermittently driving the feed means, means to energize the induction coil in order to raise and separate the uppermost sheet of the stack and to bring it to the feed means, and appropriate switch means for the induction coil interlocked with the drive means whereby the induction coil is energized while the feed means is inoperative and is de-energized while the feed means is operative, said means to energize the induction coil including a variable impedance driven by a motor and a one-revo1ution clutch, whereby the induction coil is energized by a current of gradually increasing intensity in order to help insure the separation of the topmost sheet from the immediately subjacent sheet.

24. Apparatus for separating and successively feeding ferromagnetic sheets from a stack of sheets, said apparatus comprising a support for the stack of sheets, a main induction winding extending entirely around the stack of sheets, a magnetic roller having a magnetizing coil, intermittently operable drive means for intermittently rotating the roller, means to energize the main induction winding in order to raise and separate the uppermost sheet of the stack and to bring it to the roller, means for supplying current to said coil, means to energize the coil to only a limited extent sufiicient to hold the uppermost sheet when the feed roller is not rotating, means to more strongly energize the coil in order to feed the sheet Without appreciable slippage when the roller is rotated, and appropriate switch means to energize the main induction winding when the feed roller is stationary, and to de-energize the main induction winding when the feed roller is rotating, said means to energize the main induction winding including a variable impedance driven by a motor and a one-revolution clutch, whereby the main induction winding is energized by a current of gradually increasing intensity in order to help insure the separation of the topmost sheet from the immediately subjacent sheet.

25. Apparatus for separating ferromagnetic sheets from a stack of such sheets, said apparatus comprising a support for the stack of sheets, an induction coil extending entirely around the stack of sheets, with the axis of the coil extending generally in the direction of the plane of the sheets and generall perpendicular to the desired direction of separation of the sheets, a source of power to energize the coil in order to raise and separate the uppermost sheets of the stack, and current varying circuit elements between said source and said coil arranged and operated each time a sheet is to be separated to energize the induction coil with a current of relatively gradually increasing magnitude in order to help insure the separation of the topmost sheet from the immediately subjacent sheet.

26. Apparatus for separating ferromagnetic sheets from a stack of such sheets, said apparatus comprising a support for the stack of sheets, an induction coil disposed with the axis of the coil extending generally in the direction of the plane of the sheets and generally perpendicular to the desired direction of separation of the sheets, said coil being so large in diametral dimension that the sheets may be moved through the induction c the field of the coil runs through the stack, with the stack acting as a core for the field and coil, a source of power to energize the coil in order to raise and separate the uppermost sheets of the stack, said current varying circuit elements between said source and said coil arranged and operated each time a sheet is to be separated to energize the induction coil with a current of relatively gradually increasing magnitude in order to help insure the separation of the topmost sheet from the immediately subjacent sheet.

27. Apparatus for separating ferromagnetic sheets for removal from an upright stack of generally horizontal sheets, said apparatus comprising a support for the stack of sheets, an induction coil disposed in an upright plane and having each of its windings extending entirely around the stack of sheets, with the top of the coil higher than the top of the stack to provide substantial clearance therebetween, and with the axis of the coil exending generally horizontally in the direction in which the sheets are to be removed and generally perpendicular to the desired direction of separation of the sheets, a source of power to energize the coil in order to raise and separate the uppermost sheets of the stack, and current varying circuit elements between said source and said coil arranged and operated each time a sheet is to be separated to energize the induction coil with a current of relativel gradually increasing magnitude in order to help insure the separation of the topmost sheet from the immediately subjacent sheet.

28. Apparatus for separating ferromagnetic sheets for removal from an upright stack of generally horizontal sheets, said apparatus comprising a support for the stack of sheets, an induction coil disposed in an upright plane with the top of the coil higher than the top of the stack to provide substantial difference in elevation therebetween, and with the axis of the coil extending generally horizontally in the direction in which the sheets are to be removed, said coil being so large in diametral dimension that the sheets may be moved through the coil in the direction of the axis of the coil, and said coil being disposed so close to the stack that the field of the coil runs through the stack, with the stack acting as a core for the field and the coil, a source of power to energize the coil in order to raise and separate the uppermost sheets of the stack, said current varying circuit elements between said source and said coil arranged and operated each time a sheet is to be separated to energize the induction coil with a current of relatively gradually increasing magnitude in order to help insure the separation of the topmost sheet from the immediately subjacent sheet.

ALEXANDER FOWLER.

References Cited in the file of this patent UNITED STATES PATENTS Number

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