US3391337A - Scanning-type moisture detection system with sequential solid-state switching and synchronous material marking means - Google Patents

Description

July 2, 1968 F. K. PREIKSCHAT SCANNING-TYPE MOISTURE DETECTION sYsTEM WITH SEQUENTIAL SOLID-STATE SWITCHING AND SYNCHRONOUS MATERIAL MARKING MEANS Filed June 21, 1965 2 Sheets-Sheet X INVENTOR. FRITZ K ME/Ks'c/MT drrakmsv SCANNING-TYPE MOISTURE DETECTION SYSTEM WITH SEQUENTIAL SOLID-STATE SWITCHING AND SYNCHRONOUS MATERIAL MARKING MEANS Filed June 21, 1965 2 Sheets-Sheet 2 y 1968 F. K. PREIKSCHAT 3,391,337

I @1 6 /6 34 L +loov F uh an m u mm r n l9 19 W 3. ATTORNEY United States Patent SCANNING-TYPE MOISTURE DETECTION SYS- TEM WITH SEQUENTIAL SOLID-STATE SWITCH- ING AND SYNCHRONOUS MATERIAL MARK- ING MEANS Fritz K. Preikscliat, Bellevue, Wash, assignor to Laucks Laboratories, Inc., Bellevue, Wash., a corporation of Washington Filed June 21, 1965, Ser. No. 465,330 4 Claims. (Cl. 324--61) ABSTRACT OF THE DISCLOSURE Recurrent sequential scanning of successive zones across an advancing sheet of wood veneer or other material in order to detect moisture content in the material is performed rapidly with consistent accuracy by arrayed electrodes and respectively associated solid-state switches normally back-biased against conduction so as to isolate the electrodes from the bridge-type detection circuit. The switches are momentarily forward-biased in sequential order so as to connect the electrodes with the detection circuit by suitable timing means such as a rotary mechanical switch in the bias circuit. Operated synchronously with the switches is a separate sequentially operated selector switch means arranged to connect individual zone markers to the detection circuit as each zone of test material is being subjected to the measurement function and thereby to mark the material selectively in those zones wherein moisture content deviates excessively from a given norm or range. Normal back-bias of the solid-state switches affords virtually complete electrical isolation of the detection circuit from the inactive electrodes so as to avoid admittance loading of the detection circuit. However, use of low-impedance noncritical timing means for the solid-state switches operated synchronously with the sequential timing device controlling marker selection is permitted without interference with the critical highimpedance electrode and detection circuits, or creating problems with switch noise, drift and erratic timing, such as proved to be unavoidable with attempts to use electromechanical switching of the scanned electrodes.

This invention relates to mechanism for detecting in sheet material, such as veneer, paper, etc., particular locations in which the moisture content exceeds a desired value by more than a predetermined amount. Such apparatus can be used to advantage in a plywood plant or in a paper mill, for example.

In the past devices have been available for detecting moisture content in sheet material in excess of a predetermined value for fairly large areas, such as for a strip of substantial width extending across a sheet of such material. In general such indication has been accomplished by eflecting relative movement between the detecting apparatus and the sheet material in a direction lengthwise of the sheet. Where, at some location across the width of the sheet, the moisture content has been excessive or the aggregate moisture content in a strip across the width of the sheet has been excessive, the edge of the sheet has been marked at such excessive moisture location. It has been difiicult, however, to detect with accuracy a particular moisture spot as distinguished from a band extending across such a sheet.

A principal object of the present invention is to pro- 3,391,337 Patented July 2, 1968 vide a reliable moisture detector for sheet material which can scan the entire surface of the sheet automatically at closely spaced locations, and, if desired, of marking specific locations of excess or insufficient moisture, while the moisture-detecting device and the sheet are moving relatively at high speed.

A further object hereof is to provide in association with an electrode assembly electronic scanning circuit means operable to connect individual electrode units of the assembly into a common measurement system recurringly in successive order through connections having consistently uniform and equal impedances and without introducing admittance loading into the essentially highimpedance input of the measurement system.

A further object is to provide such moisture-detecting and marking apparatus which will require few moving parts.

A further object is to provide such moisture-detecting apparatus which can detect the moisture content accurately over a considerable range of moisture variation.

It is also an object to provide such moisture-detecting apparatus which will be compact and can be incorporated conveniently in conjunction with sheet-moving means with a minimum of modification of such sheet-conveying means.

FIGURE 1 is a top perspective of exemplary sheetconveying means with which the moisture detector of the present invention is associated, parts being broken away.

FIGURE 2 is a section through the moisture-detecting electrode box installation on an enlarged scale.

FIGURE 3 is an electric circuit diagram of the moisture-detecting apparatus.

FIGURE 4 is a graph illustrating a moisture-detecting characteristic of the apparatus.

In FIGURE 1 sheet-transporting belts 1 and 2 carried and driven by rollers 3 and 4 respectively are adapted to transport the sheet material S in the direction of the arrows. Rollers 3 and 4 are spaced apart sufiiciently to accommodate between them the elongated moisture detection unit housing 5 positioned beneath and closely proximate to the path of movement of the sheet material S. Bridging platforms 6 at opposite sides of the housing face span the spaces between it and the belts 1 and 2 so that the sheet material will be backed properly between the belts. The sheet material is held firmly by hold-down belts 7 against the housing face and thereby against an electrode array incorporated therein, as hereinafter described, such belts being carried by pulleys 8 mounted on the shafts 9. These belts are driven in synchronism with the belts 1 and 2.

As shown, the electrode assembly comprises the series array of individual electrode strips or segments 10 extending in a row transversely to the direction of movement of the sheet material S. The size, spacing and number of electrode strips in a given array may vary by choice, depending upon the span of the material to be scanned for moisture content, together with the size and the spacing interval between individual zones or areas in which the sheet material is to be tested. For example, in some applications an electrode array spanning thirteen feet is necessary, in which test intervals measuring six inches across the width of the material provide sufiicient resolution for test purposes. In that case twenty-five six-inch electrode strips, successively spaced apart by a small fraction of an inch, will make up the necessary thirteen foot span.

As shown best in FIGURE 2, the electrode array strips are mounted in an elongated aperture formed in the conductive housing Wall 34. The aperture rim extending the length of the housing at opposite sides of the array serves as the opposing or ground electrode common to all the strips. This aperture, together with the individual electrode strips, are covered and sealed by an insulation sheet 11 joined to the aperture rim. The electrode strips are connected separately through individual resistances 12 to respectively different switch contacts 13 of a rotary switch 14. In addition, each electrode strip is connected within the electrode housing to a diode 15 and to a trimming condenser 16 associated therewith. Each such trimming condenser may have a slotted adjustment screw 16 which is accessible through an opening 16" in the enclosure side wall so as to permit adjusting the capacitance value. All of the diodes 15 are connected by a common conductor 15' to the common measurement circuit to be described hereinafter.

Located adjacent to the discharge side of the hold-down belts 7 are sheet material marker means, shown in FIG- URE 1 as including marker dye supply conduits 17 and 17 Each conduit supports and feeds a succession of nozzle units 18 located at intervals along its length corresponding substantially to the locations of the individual electrode strips widthwise of the material sheet S. These nozzle units are individually controlled by solenoid valves (not shown) in response to control signals from the common measurement circuit to be described hereinafter. In FIGURE 3 the solenoid valves are designated as markers 19. For example, red dye can be supplied through conduit 17 and green dye through conduit 17, with the individual markers being arranged for actuation so that if an excessively wet spot is detected in the field of one electrode strip 10 the corresponding marker nozzle solenoid associated with conduit 17 will be actuated momentarily to apply a red spot to the material, whereas if an excessive dry spot is detected in the field of an electrode segment 10 the appropriate marker nozzle solenoid associated with conduit 17 will be momentarily energized to apply a green mark to the sheet at the appropriate location. Obviously, equivalent alternative electrode-controlled means for selective marking of the sheet may be used if desired.

Switch 14 is preferably operated continuously during movement of the sheet material on the conveyor system, such that the individual electrode strips 10 are connected successively into the sensing and control circuit in recurring cycles. The test interval lengthwise of the moving sheet is therefore determined by the relationship between sheet speed and rotational speed of switch 14. For example, if the sheet material is traveling 20 feet per second and the rotary switch 14 is turning twenty revolutions per second, the distance between the centers of locations being tested along the length of the web will be one foot. Alternatively, if the sheet is moving at a speed of four feet per second and switch 14 is turning at a scanning speed of sixteen cycles per second, the test interval lengthwise of the sheet will be three inches. In that event, if the width of the electrode segments parallel to the direction of sheet travel, as shown in FIGURE 2 for example, is three inches, the entire area of the sheet will be effectively checked for moisture content by the electrode array at this scanning speed with a resolution area of three by six inches.

In order to relate the sensing function of individual electrodes with the marking function of individual markers, a second multiple contact rotary switch 14' is provided which is rotated synchronously with switch 14 and which has contacts connected to the individual markers 19 and positionally related to the corresponding contacts 13 of switch 14. Each marker 19 is thus connected to the circuit when the correspondingly located electrode segment is being connected to the sensing and control circuit.

The common measurement circuit which performs the sensing and control functions of the system can be of different types but preferably comprises a normally balanced bridge circuit 20, together with bias-controlled switching diodes 15 to connect the electrode units into the bridge circuit momentarily in sequence. In the illustration this bridge circuit and its associated components are in transistorized form. The bridge is energized by highfrequency oscillator 21 which is capacity-coupled to the wiper of potentiometer R2 serving as one corner of the bridge. Adjustment of the position of this wiper permits compensating for any unbalance between resistances R1 and R3 connected in the adjacent arms of the bridge serially with the respective resistance sections of the poten tiometer winding. The opposite corner of the bridge is connected to ground. One intermediate corner E is periodically connected to ground potential through the individual test electrode segments 10 and opposing ground electrode comprising the housing wall, such coupling being by way of common conductor 15' extending to the electrode switching diodes 15. The opposite intermediate bridge corner D is continuously connected to ground potential through the parallel-connected condensers 23, one of which comprises a trim condenser adjustable to balance the bridge with the electrode units empty or confronting sheet material of predetermined dryness. In effect condensers 23 constitute a dummy electrode, although in this example ordinary condensers are used which do not provide the advantage of electrode temperature compensation afforded in the preferred embodiment later to be described herein.

Output from bridge 20 is derived between junction points D and E and comprises a high-frequency voltage related in value to the admittance effect of moisture in the sheet material S. This output voltage is capacitance coupled through the respective condensers C1 and C2 to the control electrodes of field-effect transistors T1 and T2. Transistor bias voltage from regulated power supply 22 (energized by alternating current through terminals 23) is applied through resistances R4 and R5 respectively, whereas the related ground return circuits are completed through the respective resistors R6 and R7. In odrer to further amplify the output signal from bridge circuit 20 for delivery through a relatively high-current, low-impedance source such signal is fed successively through transistor amplifiers T3, T4, and T5. The output electrode of transistor T2 is connected through condenser C4, the winding of potentiometer R9 and resistance R11 to a positive bias terminal of power supply 22. The corresponding electrode of transistor T1 is connected through condenser C3 and the series resistances R8 and R10 to ground. One input electrode of transistor T3 is connected to the junction between resistances R8 and R10, whereas its opposing electrode is capacitance-coupled through condenser C5 to the base electrode of transistor T4. Bias to transistor T3 is supplied by power supply 22 through the resistance R12. The opposing input-side electrode of transistor T4 is grounded through the parallel-connected resistance R14 and capacitor C6 while its output electrode is connected to the base of transistor T5. The latter is positively biased, as is the output electrode of transistor T4, through the common bias resistance R16. The collector of transistor T5 is connected directly to a point of positive potential, whereas the emitter of such transistor is ground-returned through resistance R15 and serves as the output of this amplifier. Such output is capacitancecoupled through condenser C7 to the input of amplifier 30.

Amplifier 30 delivers a high-frequency bridge unbalance signal to the primary of transformer 31 whose secondary has its end terminals connected to opposite corners of a diode-resistance ring demodulator 26. The intermediate corners of ring demodulator 26 are energized from the end terminals of the secondary of transformer 28. The primary of transformer 28 is energized by oscillator 21 through reference signal amplifier 27. With the center tap of transformer 23 grounded, the center tap of transformer 21 delivers a direct-voltage output signal.

This output signal is proportional in magnitude to the prevailing unbalance of the bridge circuit, hence to the moisture loading of the individual test electrode segment which is then connected in the bridge circuit. This direct-voltage signal may be recorded in the unit 32. as shown and may also be applied to the level selector 3 3 whose function will be described.

For reasons previously mentioned, bias-controlled switching diodes are used to connect the individual electrodes 10' into the common measurement bridge circuit momentarily in sequence and to isolate them from the circuit at all other times. Thus, each electrode segment 10 is connected to conductor 15 through an individual diode 15 which is normally back-biased against conduction by connecting its anode to ground through a resistance 12 and a second resistance in series therewith, and by connecting its cathode through conductor 15' to a source of positive bias. This positive bias potential is delivered for convenience with bridge junction E. An adjustable trim condenser 16 is connected between each individual electrode segment and ground in order to equalize its normal admittance with those of all other electrodes. As will be noted, the anode" of each diode is connected through resistance 12 to a different stationary contact 13 of switch 14. The power-driven rotor of this switch 14 is connected through conductor 35 to a point of high potential in the output of power supply 22, such potential being suificiently high that its application to the anode of a diode 15 will produce conductivity in such diode and thereby connect the associated electrode segment 10 to the bridge circiut input junction E. Thus, as the switch 14 rotates (which it normally does at constant speed during system operation) the electrodes are successively connected by their respective diodes 15 into the bridge circuit and at all other times are isolated therefrom by the applied back-bias. Capacitance coupling of the oscillator 21 to the bridge circuit and capacitance coupling of the bridge terminals D and E to the amplifiers T1 and T2 serves to permit voltage controlled switching of diodes 10' without disturbing voltages on transistors T1 and T2 and without permitting operating bias in the bridge circuit from affecting the diodes.

Diodes 15 are of the solid-state type (i.e., germanium, etc). As such, they introduce only a very small amount of capacitance into the circiut and while, as shown, they may be controlled by a mechanical switch, they exhibit substantially unvarying conduction impedance as compared with mechanical switch contacts. These attributes are of considerable importance to consistently reliable and uniform moisture measurements in all areas of the sheet material when it is realized that the range of moisture content in such materials changes the electrode capacitance by only a few hundredths of a picofarad. As described in co-jending application Ser. No. 531,207 filed Mar. 2, 1966, a thermistor 24, connected via a shielded conductor 25 to the output of the bridge circuit may be used to compensate for the elfects of temperature change in the material itself which otherwise also greatly affects the relationship between moisture content and apparent electrode admittance.

As previously mentioned, switch 14' rotates synchronously with switch 14 so that as each individual electrode strip 10 is being connected momentarily into the bridge circuit, the associated marker 19 is being connected momentarily by switch 14' to the output of level selector 33. This level selector comprises a threshold circuit of any suitable or known type which permits actuation of an individual marker 19 when the level of the output signal from ring demodulator 26 exceeds a certain value. Thus minor and inconsequential fluctuations in the balance of bridge circuit 20 do not actuate the markers. It will be evident that a separate set of markers or a separate and additional rotary switch equivalent to switch 14 may be employed if desired, such that unbalance of the bridge in the opposite sense may also be detected and utilized to apply marks to the sheet material. Thus, the specific arrangement shown in FIG- URE 3 is capable of marking the sheet material in the case of excess moisture, for example, whereas by the provision of an additional level selector energized by the same or an additional ring demodulator and feeding a separate sequencing switch equivalent to switch 14' the sheet material may also be marked for areas of insufiicient moisture content.

FIGURE 4 illustrates a typical response characteristic of the circuit in terms of output voltage as a function of material moisture content. From this graph it will be recognized that the circuit may be designed and adjusted so that the output voltage becomes positive when moisture exceeds a preselected level such as 5 percent, and negative when it drops below that moisture level. Initial adjustment of the bridge circuit to determine the zero condition may be achieved by appropriate setting of the trim condense-r in the dummy electrode unit 23.

I claim as my invention:

1. Material scanning type moisture detection apparatus comprising an array of similar tes-t electrodes and associated electrode means arranged in electrical relationship with the test material, a capacitive impedance measurement circuit common to said electrodes and electrode means, and means for connecting the individual test electrodes and associated electrode means in capacitive impedance measurement relationship successively to the measurement circuit, including bias-controlled solid-state switches individual to the respective test electrodes and commonly housed for uniformity of thermal environment, means normally back-biasing said switches against conductivity, and means for momentarily forward-biasing said switches in sequence to render the same conductive and thereby connect said test electrodes and associated electrode means in capacitive impedance measurement relationship to said circuit sequentially to scan the test material.

2. The apparatus defined in claim 1, wherein the means for connecting the individual electrodes comprising a rotary mechanical switching means having contacts successively connected to forward-bias the solid-state switches in sequential recurring cycles.

3. The apparatus defined in claim 2, and a plurality of indicating means corresponding to the respective electrode-s, and indicating switch means operable synchronously with the mechanical switching means and arranged to responsively connect the individual indicator means to the measurement circuit.

4. Material scanning type moisture detection apparatus comprising an array of similar test electrodes and associated electrode means arranged in electrical relationship with the test material, a capacitive impedance measurement circuit common to said electrodes and electrode means, means for connecting the individual electrodes and associated electrode means in capacitive impedance measurement relationship successively to the measurement circuit, including bias-controlled solid-state switches individual to the respective test electrodes and commonly housed for uniformity of thermal environment, means normally back-biasing said switches against conductivity, means for momentarily forward-biasing said.

switches in sequence to render the same conductive and thereby connect said test electrodes and associated electrode means in capacitive impedance measurement relationship to said circuit sequentially to scan the test material, a plurality of markers operatively associated with the respective test electrodes and individually operable to mark the test material selectively at locations thereon electrically related to the respective test electrodes, means for operating said markers in response to predetermined measurement circuit response including selector switch means for operatively connecting said measurement circuit to the individual markers sequentially in timed relation with the sequential momentary forward-biasing of markers.

References Cited 736,464 9/1955 Great Britain. UNITED STATES PATENTS FritZinger X 5 Primary Examiner. 2/ 1957 Dallas 32461 ARCHIE R. BORCHELT, Examiner. 4/1960 Gootherts 340-176 X 6/1960 Shawhan XR E. E. KUBASIEWICZ, Asszstant Exammer. 12/1960 Shawhan 32461 XR FOREIGN PATENTS 673,684 11/1963 Canada.

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