GB1579900A - Sheet transport apparatus - Google Patents

Description

(54) SHEET TRANSPORT APPARATUS (71) We, INTERNATIONAL BUSINESS MACHINES CORPORATION, a Corporation organized and existing under the laws of the State of New York in the United States of America, of Armonk, New York 10504, United States of America do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates to apparatus for transporting flexible sheets.

According to the invention, apparatus for transporting flexible sheets comprises means to transport a sheet, including a surface to receive a sheet, vacuum means for attracting the leading edge portion of the sheet to the surface, ionization means for electrostatically charging the sheet to cause the sheet to adhere temporarily to the surface by electrostatic attraction; and fluid pressure means for applying fluid under pressure to the leading edge portion of a sheet to detach the leading edge portion of the sheet from the surface.

The scope of the invention is defined by the appended claims; and how it can be carried into effect is hereinafter particularly described with reference to the accompanying drawings, in which: Figure 1 is an isometric view of sheet transport apparatus according to the present invention; Figure 2 is a side elevation view of the apparatus of Figure 1; Figure 3 is a partially cutaway side elevation view of the apparatus of Figure 1; Figure 4 is a broken front elevation view of a drum forming part of the apparatus of Figure 1; Figure 5 is a partially cutaway side elevation view of the drum of Figure 4; Figure 6 is a sectional view of an end portion of the drum of Figure 5, its mounting and controls therefor; Figure 7 is a diagrammatic illustration of the velocity profile of the drum of the apparatus of Figure 1; Figure 8 is a schematic diagram of the drive circuitry for the apparatus of Figure 1; Figure 9 is a schematic diagram of servo circuits of Figure 8; Figure 10 is a diagrammatic illustration of the operation of the apparatus of Figure 1 during sheet loading and; Figure 11 is a diagrammatic illustration of the operation of the apparatus of Figure 1 during repeat cycle sheet unloading and loading.

In an embodiment of sheet transport apparatus according to the invention, a low inertia rotary drum 10 (Fig. 1) is fed single flexible sheets 11 from a bin 12 by conveying belts 13. Processed sheets are fed by the same belts 13 from the drum 10 to an output bin 14.

The conveyor belts 13 are entrained around a drive roll 20 and an idler roll 21. Within the belts 13 is a vacuum plenum chamber 22 connected by a conduit 23 to a source of vacuum. The sheets 11 in the bin 12 are biased upwards against a feed drive roller 16 (Fig. 2) by a motor driven elevator mechanism 18. The individual sheets 11 are fed from the bin 12 by the feed drive roller 16 directly towards and perpendicular to the belts 13, which are driven by the drive roll 20 in the direction of arrow 25. As the edge of a fed sheet 11 contacts the belts 13, the motion in the direction of arrow 25 causes the sheet to be deflected downward and to gradually change direction 90C and to come into full contact with the belts 13. Each sheet 11 is held against the belts 13 by vacuum from the vacuum plenum chamber 22 and adheres temporarily thereto.

Entry guides 26 and 27 are located between the idler roll 21 and the drum 10. There is vacuum in the idler roll 21, so that as the belts 13 move round with the idler roll, the sheet 11 tends to continue in the original direction, entering a slot between the guides 26 and 27. The guides redirect the sheet in a direction tangential to the circumference of drum 10.

A solenoid operated mechanical gate 28 (Fig. 3) is normally positioned in the sheet path between the guides 26 and 27 to stop any sheet from proceeding toward the drum 10. Energisation of the solenoid causes the gate 28 to be moved out of the guide path so that the belts 13 drive a sheet 11 along the guide path to the drum 10.

The drum 10 has two pressure conduits 30 and 31 for the transmission of vacuum or pressurized air to respective longitudinal slots 48 and 49 in the surface of the drum.

The timing of the opening of gate 28 is such that the leading edge of a sheet 11 contacts the drum 10 to overlie the slot connected to conduit 30. Vacuum is applied to the conduit 30 to attract and hold the leading edge of the sheet to the drum so that it adheres thereto temporarily. As the drum rotates in the counterclockwise direction of arrow 32 (Fig. 1), the sheet 11 is drawn from the guide path between the guides 26 and 27. The belts 13 are operated at a slightly slower linear velocity than the linear velocity of the surface of the drum 10 to prevent buckling of the sheet during loading and to keep it taut.

The drum 10 is coated with a dielectric so that at least the surface contacting the sheet is nonconductive. An activated ioniztng corona wire 34 with shield 35 ionizes the surrounding air and directs the ions toward the drum 10. This causes the surface of the drum to become charged. As a sheet 11 is interposed between the corona and drum, the sheet being electrically insulative is charged on the side facing the corona at the same polarity as the drum. The side of sheet 11 facing the drum is thus charged to the opposite polarity and is thereby attracted to the drum 10 and adheres thereto temporarily.

As the drum rotates nearly a complete revolution, the trailing portion of the sheet 11 is wrapped around the drum and the trailing edge of the sheet overlays the slot connected to conduit 31, to which vacuum is applied, to hold the trailing edge of the sheet tightly against the drum.

The sheet on drum 10 is thus tightly affixed at the leading and trailing edges by the applied vacuum and the intermediate portions of the sheet are attracted to the drum by means of the applied electro static charge. The drum 10 may rotate one or many times to enable the sheet adhering thereto to be processed, for example by printing.

An exit guide 38 is located between drum 10 and the idler roll 21. At the completion of processing, and as the leading edge of the sheet 11 approaches the guide 38, the leading edge vacuum is shut off and pressurized air is applied to the conduit 30 to lift the leadig edge of the sheet 11 from the drum 10. This is called a "puff" or "puffing". As the leading edge of the sheet is raised from the surface of drum 10 in this manner, it contacts the guide 38 which strips the sheet from the drum and guides it into contact with the belts 13 on the roller 21. The electrostatic force continues to hold the remainder of the sheet to the drum until the sheet is stripped off. As before, vacuum in the plenum chamber 22 draws the sheet 11 into firm contact with belts 13 for transport in the direction of arrow 39.

Vacuum is shut off from the conduit 31 before the trailing edge is stripped from the drum. As the sheet is drawn upwards, it passes one or more discharge electrodes 37, which are connected to electrical earth, and which discharge the static electrical charges from the sheet 11. As the sheet reaches the drive roll 20, in which there is no vacuum, the sheet continues up in its original direction to a guide 40, which directs the processed sheet towards the bin 14, into which it is deposited.

The drum 10 (Figs. 4, 5 and 6) is designed to have an extremely low inertia, and includes a hollow cylindrical shell 45 having the two longitudinal slots 48 and 49 cut through the surface. The cylindrical shell 45 is preferably made of a strong conductive material, such as a metal. The internal part of the cylinder is occupied by a drum baffle 50 of a lightweight, nonporous material. Longitudinal grooves 46 and 47 are formed in the surface of the baffle 50 aligned with the respective slots 48 and 49. Conduits 30 and 31 are formed in an end cap 56 which includes a spindle 58 and are aligned with the grooves 46 and 47 respectively, in the baffle surface. The drum also has an end cap 55 including a spindle 57. The spindle 58 is supported for drum rotation in bearings 61 and 62 and the spindle 57 is connected to a servo motor 60.

The spindle 58 includes ports 64 and 65 which communicate, respectively, with the conduits 30 and 31, and to a leading edge vacuum and air source 66 and to a trailing edge vacuum source 67.

The drum 10 can be driven at two speeds by the motor 60. Starting from rest 73 (Figure 7) the drum is accelerated to a low load speed 70 of 30 inches per second drum surface velocity. An initial sheet 11 is loaded on to the drum 10 at this speed in 0 3 seconds.

The motor increases the speed of the drum by rapid acceleration 74 to a high processing speed 72 of 300 inches per second. Processing, for example printing by ink jet, takes place during the high speed 72. This is followed by rapid deceleration 75 to low speed 71, during which the initial sheet is unloaded and a fresh loaded. The rapid acceleration and deceleration can be accomplished most advantageously by having a low inertia drum 10, lightweight materials, and keeping the drum diameter to a minimum.

The times given in Fig. 7 are exemplary and may vary according to the nature of the processing operation. The ratio between the load speed and the processing speed of one to ten is not critical, but should be sufficiently high to warrant use of a low speed paper handling mechanism at the expense of a low inertia drum and attendant high speed servo.

The sheet transport apparatus has drive circuitry (Fig. 8) for performing the function of driving drum 10 at a slow load speed and at a high processing speed and accelerating and decelerating between the two speeds.

A start switch 80 is actuated to supply a signal on line 81 to activate a power supply for making power available to the various components of the apparatus, such as the sheet feed roll 16 (Fig. 2), the conveyor belt mechanism for driving belts 13, the solenoid to close gate 28, the charge corona 34, the vacuum source for the vacuum plenum chamber 22, and the vacuum and air pressure sources of the leading edge vacuum and air source 66 (Fig. 6) and the trailing edge vacuum source 67. The feed roll 16 (Fig. 2) supplies a sheet 11 to be conveyed by belts 13 in the direction of arrow 25 to gate 28.

A new sheet sensor 82 detects the presence of a sheet at gate 28. Feed roll 16 will not be operated again until such time as the sensor 82 indicates that the prior sheet has been fed past opened gate 28, such that no part of the sheet remains present at the sensor.

Sensing mechanisms of this type and their arrangement with feeding mechanisms are well known.

Start switch 80 also supplies a signal on line 82A to delay circuit 83 and to low speed acceleration circuit 84. Circuit 84 generates a specialized acceleration waveform to drive the motor 60 from a stop to the load speed.

The output of circuit 84 is supplied to switch 90. The switch is operated by load speed detector circuit 91 to the "1" condition as will be described. The output of circuit 84 is therefore transmitted by switch 90 to power amplifier 92. Amplifier 92 converts the voltage input signal to a drive current to drive motor 60 accordingly. Motor 60 thus accelerates the drum 10 from a stop to the load speed in accordance with the signal from circuit 84.

Motor 60 is connected to tachometer 95 which supplies a tach signal to load speed detector circuit 91 and to load speed servo circuitry 96. Load speed detector circuit 91 is switched into operation when the pulse rate from tachometer 95 is within a specified percentage of the desired load speed. An example of such a circuit may be a pretuned filter. When the pulse rate enters the desired frequency band, the circuit provides a signal on line 98 to operate switch 90 from condition "1" to condition "2". When in condition "1", switch 90 connects input "1" to the output, and when in condition "2", switch 90 connects input "2" to the output. In the absence of a signal on line 98, switch 90 reverts back to the "1" condition.

In the "2" condition, switch 90 supplies the output of load speed servo 96 to power amplifier 92. Load speed servo 96 may be any type of fine tuning or servoing circuitry for maintaining the speed of motor 60 and drum 10 within defined limits. An exemplary servo circuit will be described hereinafter.

After a sufficient time for motor 60 to reach the load speed, delay circuit 83 supplies a signal on line 99 to AND circuit 100. The other input to AND circuit 100 is an output of sensor 82 on line 101. This output indicates that a new sheet is in position at gate 28.

Assuming a new sheet is in position, AND circuit 100 supplies a signal via OR circuit 104 and line 105, to a sequencer 106, and on line 107 which is connected to input 108 (Fig. 6) of latch 109.

In response to the signal at input 108, latch 109 provides a signal on line 110 to the leading edge of vacuum and air source 66.

This signal operates the source to supply a vacuum, via port 64, to the leading edge conduit 30.

The sequencer 106 also receives inputs from a high frequency clock 115 and an index output on line 116 from the tachometer 95.

The frequency of clock 115 is several hundred times the revolution speed of drum 10. The index output signal on line 116 occurs once per drum revolution and indicates a specific rotational position of the drum 10. The index pulse is used by sequencer 106 to set the proper phase of the sequencer as it counts out clock pulses received from clock 115.

The sequencer 106 may comprise a sequential shift register responding to the clock pulses from clock 115 to shift one position for each clock pulse. Specific ones of the shift register outputs will hold until ANDed with an index pulse before sequencing to the next position for phase adjustment.

Sequencer 106 first supplies an output on line 120 to operate the solenoid of gate 28 to open the gate. The sheet 11 held by gate 28 is thus fed towards drum 10 in a precise relationship with the position of the drum while rotating such that the leading edge of the sheet 11 contacts and is held in place by the vacuum in leading edge conduit 30.

Drum 10 continues to rotate, pulling sheet 11 from between the guides 26 and 27, and wraps the sheet about the drum as the drum rotates. During this time, the ionization from corona 34 creates a charge on the insulated sheet to hold the sheet against drum 10. At a later time, sequencer 106 supplies a signal on line 124 to input 125 (Fig. 6) of latch 126.

This signal sets the latch 126 so that it supplies a signal on line 127 to operate the trailing edge vacuum source 67. Vacuum source 67 then supplies a vacuum, via port 65, to trailing edge conduit 31. This vacuum draws in and holds the trailing edge of sheet 11. Sequencer 106 then supplies a signal on line 130 to circuit 131. Circuit 131 generates a specific waveform to accelerate motor 60 and drum 10 from the load speed to the high speed within a relatively short time span.

This waveform is supplied on line 132 to switch 134. The controlling input to switch 134 is supplied by high speed detector circuit 138 on line 139, via AND circuit 141. As no signal is present on line 145, inverter 142 supplies a gating signal to the AND circuit 141.

High speed detector 138 is similar to low speed detector circuit 91, except that it operates at a significantly higher frequency.

So long as motor 60 and drum 10 are not at the high speed, no signal is supplied on line 139 and switch 134 is in the "1" condition.

In that condition, the switch connects the line 132 to power amplifier 92. Power amplifier 92 thus responds to the waveform from circuit 131 to drive motor 60 to accelerate from the load speed to the high speed. Upon reaching the approximate high speed, circuit 138 supplies a signal on line 139, gated by AND 141, to switch 134. The switch then disconnects input "1" and connects input "2" to power amplifier 92. Input "2" is connected to high speed servo 140.

High speed servo 140 may be any suitable servo circuit for maintaining the speed of motor 60 and drum 10 within the requirements of the processing required. Upon completion of the sequencing for the processing step, sequencer 106 supplies an output signal on line 145 to circuit 146. As motor 60 and drum 10 are not operating at the load speed, detector 91 is providing no signal on line 98, and switch 90 is therefore in the "1" state. The output of circuit 146, which is a deceleration waveform, is provided by the switch 90 to power amplifier 92. The signal on line 145 is also inverted by inverter 142 to block AND circuit 141 so that no signal is applied from the detector circuit 138 to switch 134. Switch 134 thus switches back to the "1" state and terminates application of the servo drive signal to power amplifier 92. Power amplifier 92 thus responds to the output of circuit 146 by decelerating motor 60 and drum 10 to the load speed. The load speed detector 91 and load speed servo 96 thus take over the drive of motor 60.

Sequencer 106 then applies a signal on line 150 to input 151 (Fig. 6) of latch 109.

This signal turns off latch 109 and terminates the signals therefrom on line 110 to leading edge vacuum source 66. The result of the operation is to terminate the supply of vacuum to port 64 and to conduit 30.

Next, sequencer 106 supplies a signal on line 152 to input 153 (Fig. 6) of latch 154.

This operates leading edge vacuum and air source 66 to supply air under pressure via port 64 and conduit 30 to lift the leading edge of the processed sheet 11 from the surface of the drum 10. As the leading edge of the sheet is raised from the surface of the drum, guide 38 intercepts the leading edge of the sheet and strips the sheet from the drum as the drum rotates. The electrostatic charge holds the sheet to the drum as the stripping occurs, keeping the sheet from flying off.

Next, sequencer 106 supplies an output on line 158 to input 108 (Fig. 6) of latch 109, and to input 159 of latch 154. This signal turns off latch 154 and turns on latch 109.

The resultant presence of a signal on line 110 and absence of a signal on line 155, causes the leading edge vacuum and air source 66 to switch from supplying air pressure to supplying a vacuum to port 64 and to conduit 30. With the leading edge vacuum thus applied, a subsequent sheet may be gated onto the drum.

Sequencer 106 then supplies a signal on line 160 to operate sensor 161 (Fig. 2) to detect the presence of the leading edge of sheet 11 on guide 38. Should sensor 161 fail to detect the presence of a sheet, a failure will have occurred. Any of various possible failure modes may then be initiated, ranging from a simple retry of the leading edge puff to a power shutdown.

Assuming no failure indication, sequencer 106 then provides a signal on line 170 to input 171 (Fig. 6) of latch 126. This causes the latch 126 to turn off and to discontinue the signal on line 127, thereby terminating operation of the trailing edge vacuum source 67. Vacuum is thus no longer applied via port 65 and conduit 31 to hold the trailing edge against drum 10. This frees the trailing edge of the sheet 11 from the drum and allows the sheet to be drawn away from the drum by belts 13 of the conveyor system.

Sequencer 106 then supplies a signal on line 175 so that the sequencer may act similarly to a ring circuit and recycle. Line 175 comprises one of the inputs to AND circuit 103. The other inputs to the AND circuit comprise an output on line 102 from sensor 82 which indicates a new sheet is present at the gate 28, and on line 176 which is connected to the output of sensor 161, which indicates that the previous sheet was detached from the drum.

Again assuming that a failure mode was not entered, the detach sensor supplies an output signal on line 176, and assuming a new sheet is present, the new sheet sensor supplies an output at input 102 of AND circuit 103. In this situation, the recycling signal on line 175 is transmitted by the AND circuit 103 and OR circuit 104 to input 105 of the sequencer 106, thereby initiating another load and processing sequence. The loading and processing and unloading of sheets may continue in this fashion until a new sheet is no longer available at gate 28.

The system will then idle until such time as start switch 80 is again operated. Assuming that the motor and drum remain idling at the load speed and that the power remains on, operation of start switch 80 results in generation of the waveform of circuit 84 but which will be blocked by switch 90 due to its activation by the signal on line 98 to the "2" condition. After appropriate delay by circuit 83, however, the start signal is applied on line 99 to AND circuit 101 in conjunction with an indication at input 101 of the presence of a new sheet, loading and processing of a new sheet will begin.

A servo system (Fig. 9) may be employed as high speed servo 140 and as load speed servo 96. The tachometer 95 comprises an index wheel 190, a sensor 191, and an amplifier 192. The resultant tachometer pulses are supplied to input 193 of phase comparator 194. Input 195 of the phase comparator is connected to a clock 196.

Clock 196 is arranged to provide pulses at precisely the pulse rate which would be derived by sensor 191 if motor 60 were running at precisely the correct rotational velocity. Phase comparator 194 thus produces a wave form of interspersed positive clock pulses 197 and negative tachometer pulses 198. The waveform is run through a low path and phase correction filter 199. The resultant voltage signal indicates by its polarity, the direction, and by its amplitude, the amount, of error adjustment to be made. This signal is supplied to gain compensation circuit 200 which supplies a compensated adjustment signal via the appropriate switch 90 or 134 to power amplifier 92.

The sequence of various operations and the angular positions of the drum at the moment of such operation after the motor and drum have been accelerated to the load speed are shown in Fig. 10. First, the index mark is sensed and employed to synchronize sequencer 106. Second, sequencer 106 provides a signal on line 120 to open gate 28.

Third, the leading edge of the gated sheet 11 will be attracted and held to the surface of drum 10 by the leading edge vacuum applied as the result of the signal on line 107 by the start switch 80. Fourth, sequencer 106 supplies a signal on line 124 to actuate the traiing edge vacuum during the second revolution of the drum from the index point.

The sequence of operations for unloading a processed sheet of paper from the drum 10 and loading of the next sheet together with the angular position of the drum are illustrated in Fig. 11. First, sequencer 106 supplies a signal on line 150 to drop the leading edge vacuum. Second, the index point is sensed and a signal supplied on line 116 to synchronize the sequencer 106. Third, sequencer 106 supplies a signal on line 152 to apply the puff of air pressure. Between the time that the leading edge vacuum has been turned off, and the air puff has been provided, the motor 60 and drum 10 will have decelerated from the high speed to the load speed. Fourth, the sequencer 106 supplies a signal on line 158 to discontinue the air pressure puff and to turn the leading edge vacuum on as the leading edge of the sheet 11 will have engaged guide 38. Fifth, sequencer 106 supplies a signal on line 160 to test the detach sensor 161. Sixth, assuming that a new sheet is available to be loaded and the detach sensor does not indicate a failure, sequencer 106 supplies a recycle signal on line 175 and operation of the sequencer is reinitiated to supply a signal on line 120 to open the gate 28. Seventh, the leading edge of the new sheet 11 is loaded by the leading edge vacuum to the surface of the drum 10. Removal of the trailing edge vacuum as in Fig. 10 is optional depending upon the ability of guide 38 to strip off the trailing edge of the document.

Although the positions are shown very precisely, variations of the angles of rotation may be used for various circumstances. For example, the third or puff step of Fig. 11 may take place at a much earlier point if the "lift" response time of the leading edge of the sheet is slow.

It will be appreciated that many modifications of apparatus within the scope of the invention may be made. The low transport speed and high processing speed are not essential. The sheet need not be attached by leading and trailing edge vacuum, but other means for temporarily adhering the sheet may be used, such as electrostatic charge means. A drum need not be used, but, for example, a set of belts instead. There may be several exit points for a sheet and the moment of application of fluid pressure to the leading edge of a sheet selected to ensure exit of a sheet at a particular point.

This may be achieved by arranging the exit guides around a drum similar to the drum 10 and timing the application of fluid pressure to a conduit similar to the conduit 30. It could be achieved by providing a movable puffer member between sheet transport belts, or by a series of stationary puffer members selectively operable. The basic feature is that the leading edge of a sheet is detached by the application of fluid pressure from sheet transport means to which it is temporarily adhered.

WHAT WE CLAIM IS: 1. Apparatus for transporting flexible sheets, comprising means to transport a sheet, including a surface to receive a sheet, vacuum means for attracting the leading edge portion of the sheet to the surface, ionization means for electrostatically charging the sheet to cause the sheet to adhere temporarily to the surface by electrostatic

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. start switch 80 is again operated. Assuming that the motor and drum remain idling at the load speed and that the power remains on, operation of start switch 80 results in generation of the waveform of circuit 84 but which will be blocked by switch 90 due to its activation by the signal on line 98 to the "2" condition. After appropriate delay by circuit 83, however, the start signal is applied on line 99 to AND circuit 101 in conjunction with an indication at input 101 of the presence of a new sheet, loading and processing of a new sheet will begin. A servo system (Fig. 9) may be employed as high speed servo 140 and as load speed servo 96. The tachometer 95 comprises an index wheel 190, a sensor 191, and an amplifier 192. The resultant tachometer pulses are supplied to input 193 of phase comparator 194. Input 195 of the phase comparator is connected to a clock 196. Clock 196 is arranged to provide pulses at precisely the pulse rate which would be derived by sensor 191 if motor 60 were running at precisely the correct rotational velocity. Phase comparator 194 thus produces a wave form of interspersed positive clock pulses 197 and negative tachometer pulses 198. The waveform is run through a low path and phase correction filter 199. The resultant voltage signal indicates by its polarity, the direction, and by its amplitude, the amount, of error adjustment to be made. This signal is supplied to gain compensation circuit 200 which supplies a compensated adjustment signal via the appropriate switch 90 or 134 to power amplifier 92. The sequence of various operations and the angular positions of the drum at the moment of such operation after the motor and drum have been accelerated to the load speed are shown in Fig. 10. First, the index mark is sensed and employed to synchronize sequencer 106. Second, sequencer 106 provides a signal on line 120 to open gate 28. Third, the leading edge of the gated sheet 11 will be attracted and held to the surface of drum 10 by the leading edge vacuum applied as the result of the signal on line 107 by the start switch 80. Fourth, sequencer 106 supplies a signal on line 124 to actuate the traiing edge vacuum during the second revolution of the drum from the index point. The sequence of operations for unloading a processed sheet of paper from the drum 10 and loading of the next sheet together with the angular position of the drum are illustrated in Fig. 11. First, sequencer 106 supplies a signal on line 150 to drop the leading edge vacuum. Second, the index point is sensed and a signal supplied on line 116 to synchronize the sequencer 106. Third, sequencer 106 supplies a signal on line 152 to apply the puff of air pressure. Between the time that the leading edge vacuum has been turned off, and the air puff has been provided, the motor 60 and drum 10 will have decelerated from the high speed to the load speed. Fourth, the sequencer 106 supplies a signal on line 158 to discontinue the air pressure puff and to turn the leading edge vacuum on as the leading edge of the sheet 11 will have engaged guide 38. Fifth, sequencer 106 supplies a signal on line 160 to test the detach sensor 161. Sixth, assuming that a new sheet is available to be loaded and the detach sensor does not indicate a failure, sequencer 106 supplies a recycle signal on line 175 and operation of the sequencer is reinitiated to supply a signal on line 120 to open the gate 28. Seventh, the leading edge of the new sheet 11 is loaded by the leading edge vacuum to the surface of the drum 10. Removal of the trailing edge vacuum as in Fig. 10 is optional depending upon the ability of guide 38 to strip off the trailing edge of the document. Although the positions are shown very precisely, variations of the angles of rotation may be used for various circumstances. For example, the third or puff step of Fig. 11 may take place at a much earlier point if the "lift" response time of the leading edge of the sheet is slow. It will be appreciated that many modifications of apparatus within the scope of the invention may be made. The low transport speed and high processing speed are not essential. The sheet need not be attached by leading and trailing edge vacuum, but other means for temporarily adhering the sheet may be used, such as electrostatic charge means. A drum need not be used, but, for example, a set of belts instead. There may be several exit points for a sheet and the moment of application of fluid pressure to the leading edge of a sheet selected to ensure exit of a sheet at a particular point. This may be achieved by arranging the exit guides around a drum similar to the drum 10 and timing the application of fluid pressure to a conduit similar to the conduit 30. It could be achieved by providing a movable puffer member between sheet transport belts, or by a series of stationary puffer members selectively operable. The basic feature is that the leading edge of a sheet is detached by the application of fluid pressure from sheet transport means to which it is temporarily adhered. WHAT WE CLAIM IS:

1. Apparatus for transporting flexible sheets, comprising means to transport a sheet, including a surface to receive a sheet, vacuum means for attracting the leading edge portion of the sheet to the surface, ionization means for electrostatically charging the sheet to cause the sheet to adhere temporarily to the surface by electrostatic

attraction, and fluid pressure means for applying fluid under pressure to the leading edge portion of a sheet to detach the leading edge portion of the sheet from the surface.

2. Apparatus according to Claim 1, in which the transport means comprising a rotatable drum.

3. Apparatus according to Claim 3, including means for controlling the sequence and timing of the operation of the vacuum means and the fluid pressure means.

4. Apparatus according to Claim 3, in which the vacuum means and fluid pressure means are effective upon the sheet through a common conduit.

5. Apparatus according to Claim 2, 3 or 4, in which the attachment means includes additional vacuum means for attaching the trailing edge portion of the sheet to the surface of the drum.

6. Apparatus according to any preceding claim, in which the ionization means comprises an ionizing corona for creating ionization and a shield directing the ionization towards the surface of the transport means upon a sheet interposed the corona and the surface.

7. Sheet transport apparatus substantially as hereinbefore particularly described with reference to the accompanying drawings.