US2250209A

US2250209A - Control system

Info

Publication number: US2250209A
Application number: US304996A
Authority: US
Inventors: David R Shoults; William D Cockrell
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1939-11-17
Filing date: 1939-11-17
Publication date: 1941-07-22
Anticipated expiration: 1958-07-22

Description

i y D. R. SHOULTS ET AL, 2,250,209

CONIROLSYSTEM Filed Nov. 17, 1939 2 Sheets-Sheet 1 Fig.1.

Inventor:

David R Shoults,

b William D. Coc reH,

His Attorne July 22, 1941. D. R. SHOULTS ET AL 2,250,209

CONTROL SYSTEM Filed Nov. 17, 1939 2 Sheets-Sheet 2 IT'IVGT'WLOT: David R. Shoults, William D. Co'ckrell,

His Attorneg- Patented July 22, 1941 CONTROL SYSTEM David a. Shoults and William D. Coolu'ell, Schenectady, N. Y., asslgnors to General Electric Company, a corporation of New York Application November 17. 1939, Serial No. 304,996

17 Claims.

This invention relates to control systems, more particularly to systems for controlling the relative positions of a length of moving material and an element operating thereon so that the element performs its operations in register with marks, designs, or other matter which recur at predetermined intervals on the length of material, and an object of the invention is the provision of a reliable and improved system of this character.

Heretofore register control systems have been used in which register marks or indicia have been printed on a strip of material at intervals of from several inches to several feet, and in which these marks were scanned by a light sensitive scanning head as the strip of material was .fed to the operating element. In such systems, when a register mark passes the scanning head, at such a time as to indicate an error in register between the strip and the operating element, a correction is made in the register of a predetermined provision of a. register control system having much greater accuracy than the register control systems used heretofore.

In carrying the invention into effect in one form thereof, light sensitive means are provided for responding .to reference marks or indicia on a length of moving material to produce an electrical impulse in accordance with the position of thematerial, and means are provided for producing an electrical impulse in accordance with the position of the element which is to operate on the material. Means responsive to both the im pulses produced by the material and by the operating element are provided which correct any error in the register of the operating element with respect to the length of moving material by consmall amount, such for example, as of aninch. During the time this correction is being made the equipment is not responsive to any further control impulses. As it requires approximately one-half second in most cases, with contactor control to accomplish any substantial correcting movement of the strip of material, this means that the correcting speed is approximately of an inch per second or approximately 4 inches per minute. If the accuracy to be held is closer than g of an inch, then the correction that is made in response to a register mark must be made smaller and this requires that the cork 'recting speed must be correspondinglyreduced.

There is then, a more or less fundamental limitation in the correcting speed which can be obtained with the type of register control apparatus used heretofore.

In the control system of the present invention, the register marks or indicia are spaced closely together on the strip so that the number of impulses per second is much greater than the number per second in the equipment used heretofore. For example, the spacing of the register marks in the'system of the present invention may be such that the number of impulses will be greater than 40 per second, and the correction that is being applied at any instant will depend upon a summation of past indications of error and also upon the corrections, which have been applied. Thus instead of operating on an intermittent basis as the systems heretofore used have operated, the control system of the present invention will in efiect be a continuously functioning device, and a further object of the invention is the tinuously varying their relative positions, as long as any error exists.

In order to produce this continuous correction, a suitable electrical energy storage devicev is provided. When an error exists in the register of the operating element with respect to the length of material, the electrical impulses produced by the length of material and by the element produce a variation in the amount of electrical energy stored in the energy storage device, and means responsive to the amount of energy stored in the device are provided for correctin the error in register.

In illustrating the invention in one form. thereof, it is shown as embodied in a system for controlling the register between ainoving length of material on which the faces of postage stamps have been printed and a perforating element and an element for severing sheets of stamps from the advancing length after perforation thereof. It will be understood, however, that the invention has other applications.

For a better and more complete understanding of the invention, reference should now be had to the following specification and to the accompanying drawings of which Fig. l is a simple, diagrammatical sketch of an embodiment of the invention and Fig. 2 is a simple, diagrammatical view in perspective of certain elements of the invention and certain elements of a stamp perforating machine, and serves to illustrate the application of the invention to the perforating ma chine.

Referring now to the drawings, a length of material in having 'the faces (not shown) of postage stamps previously printed thereon is unwound from a roll (not shown) and fed by means of feed rolls H in the direction of arrow i2 to the perforating element I3 and to the sheet severing element i4. The feeding element H comprising top and bottom roll which grip the material tightly is driven by suitable driving means, such for example, as the electric motor I5 which is supplied from a suitable source (not shown). It will be understood that any suitable means may be provided for varying the speed of motor l5 so that the perforating machine may be operated at a. desirable speed.

The perforating cylinders 13 are connected to motor 15 through a mechanical differential device is, bevel gearing i1, and shaft IS. The ring gear 169. of the differential device is driven by a small direct current motor i9 to which it is connected by means of worm gearing 20. The ratio of the gearing between the drive motor l5 and perforator cylinders I3 is such that when the ring gear IE8 is stationary, the perforator rolls run 1% (or other selected value) faster than the feeding element ll and when the ring gear is being driven at full speed in the proper direction, the perforator rolls run 1% (or other selected value) slower than the feeding element.

The motor i9 receives armature power from a small direct current generator 2i driven from the machine drive shaft so that the output of this generator and the consequent speed of the correcting motor is is proportional to the value of the field excitation of the generator and also to the speed of the machine.

As shown in Fig. 1, the field winding Zia of generator 222 is supplied from the direct current bus 22 through a plurality of high vacuum ampliier valves 23, 2i, and 25. The circuit is traced as follows: from the top line of bus :22, through conductor 23, field winding 2 is, switch it? (in the right-hand position thereof), through the anodecathcde circuits of

valves

23, 24, and 35 in parallel, and thence by conductors 28, 29, and to the mid or grounded side of bus 22.

The control grids of these amplifier valves are in turn energized from a voltage amplifying tube 3; which is controlled by register conditions of the perforating element i3 with respect to web ID.

The control system is provided with a scanning head which includes a source of light 329., a light sensitive device 32b such as a photoelectric cell, and an amplifier tube 33. This scanning head detects the passage of indicia or register marks 34 printed on the web. A second scanning head 35 having two light sensitive devices such as photoelectric cells 36 and 37, and amplifier tubes 38 and 39, respectively, observes the motion of a disc 40 which is attached to the perforating cylinder l3. This disc has a series of slots cut in it, in exact relationship with the register marks on the length of material I0 when the material is being fed into the machine under proper conditions for correct perforation. A lamp 4| projects a beam of light through each slot on disc 40 as it passes between the lamp and scanning head 35. Scanning head 35 is provided with two slits 36a and 31a and also with two prisms 36b and 31b. Prisms 36b and 31s are so arranged that light beams from source 4| entering slits 36a and 31s are directed on photoelectric cells 33 and 31, respectively. The slits 36,. and 31s are so positioned that under conditions of correct register light falls on one of the photo tubes in the double scanning head just before and on the other photo tube just after the instant that a register mark 34 passes beneath scanning head 32 and darkens the photo cell 32%. Preferably the slits 36; and 31s are so positioned that the image of one of the slots of disc 40 as magnified by lens 42 is slightly narrower than the space between slits 38. and 31.. Thus there is a very brief interval or "dead zone" at the correct time of arrival of a register mark beneath scanning head 32, this interval being equal to the time required for the register mark 34 to pass beneath the scanning head 32.

Preferably the register marks 34 on material I0 occur very close together, e. g., at intervals of approximately one inch, and the number of slots on disc 43 is such that one slot passes between source 4| and scanning head 35 each time a register mark 34 passes beneath scanning head 32. The control is so arranged that if the image of a slot in the disc 40 falls between the slits 36a and 31- during the instant that a register mark 34 passes beneath scanning head 32 and darkens photo cell 32b, no correcting impulse will be passed to the correcting mechanism.

On the other hand, if during the time the register mark 34 darkens the photo tube 32s, the light image of a slot of disc 40 overlaps one of the slits 359. or 315 of scanning head 35 by a minute amount, e. g., an amount corresponding to .001 inch movement of the Web, an electrical impulse will be passed through one or the other of

mixing valves

44 and 46 to an energy storage oevice 46, illustrated as comprising a condenser 41 of relatively high capacitance and a condenser 48 of relatively low capacitance connected in series relationship. Condenser 41 is shunted by a high resistance resistor 49 and therefore its circuit has a relatively long time constant, and condenser 48 is shunted by a relatively low resistance resistor 49:. and therefore its circuit has a short time constant.

Alternating current power is supplied from the alternating current line to the primaries of transformers 5G and Si which supply lamps, heaters and rectifier supply. Direct current for operation of the amplifier valves is obtained through rectifiers, and this direct current is filtered by reactor 53 and capacitors 34. The transformer and rectifier may be designed to produce any suitable value of direct current voltage. For example, this voltage may be 350 volts and may be applied to a suitable voltage divider comprising resistor sections 55, 56, and 51. The point between resistor sections 55 and 56 may be grounded as indicated at 53 so that conductor 38 is a. desired voltage above ground potential, e. g. 200 volts, and conductor 30 is a desired value below ground potential, e. g. volts.

The anode of amplifier tube 33 is connected through resistor 6| to conductor 59 and the connection point 3311. between the anode and resistor is connected through a condenser 52 to the grids 4a and 45. of mixer valves 44 and 43.

Similarly, the anode of amplifier valve 33 is connected through a. resistor 63 to conductor 33, and the connection point 33. is connected through condenser 64 to the grid 44s of mixing valve 44. Likewise the anode of amplifier valve 39 is connected to the supply conductor 39 through resistor 35 and its connection point is connected through condenser 65 to the grid 45b of mixing valve 45.

The voltages applied to the grids of amplifier valve 33 are such that the valve normally conducts current when no register mark is beneath scanning head 32. When a register mark passes beneath scanning head 32 and darkens photo cell 32s, the voltage applied to grid 33b decreases momentarily and the current flowing in the anode-cathode circuit is reduced abruptly. As a result the IR drop across resistor 6| decreases and the voltage of point 33. rises sharply, and as soon as the register mark has passed, the voltage oi point 33. falls abruptly to its former value. Consequently, positive voltage impulse is transmitted through condenser 82 to grids 64a and 45-. of mixing valves 44 and 45.

, Likewise, the voltages applied to the grids of valves 38 and 33 are such that the valves are conducting when no light from source 4! enters either slit 38. or 3h. In this connection it will be noted that photo cells 36 and 31 are connected oppositely in circuit withgrids 33p and 39p of amplifier valves 33 and 33 as compared with the connection of photocell 32b and the grid 33 of amplifier valve 33. Consequently when light from source 4| enters slit 38s or slit 31a and falls upon photo cell 36 or upon photo cell 31', the current flowing in the anode-cathode circuit of amplifier valve 38 or amplifier valve 39 decreases abruptly and the voltage of the point 38s. or point 39:. rises abruptly. When light from the source 4i ceases to fall upon the cells 36 and 31, the voltages oi points 388 and 33 decrease rapidly to their normal values. Consequently an electrical impulse is transmitted through condenser 34 or condenser 63 to the grids 44s or 45b of mixing valves 44 or 45.

These two valves are pentodes arranged for suppressor grid control so that if either the suppressor grid or the control grid is sufilciently negative no current will flow in the anode circuit. Therefore, it is necessary that light from source 4| enter one of the slits 3'52. or 31s and that simultaneously one of the register marks 34 should pass under scanning head 32 so as to darken photo cell 32b in order that either of these mixing valves may be rendered conducting.

In order that the mixing valve 44 may cause a positive impulse of current to be supplied to energy storage device 48, the connection point .e oi its anode with resistor 61 is connected to the control grid of a valve 68 which is connected as an inverter valve to invert the negative impulse from valve 44 and apply a positive impulse to energy storage device 48. The voltages applied to the grids of valve 38 are such that the valve is normally conducting, i. e., current is flowing in its anode circuit.

With the foregoing understanding of the elements and their organization in the complete control system, the operation of the system itseli will readily be understood from the following description.

It is assumed that the motor I is energized and is driving the feed rolls H, the perforating cylinders l3, and cutofl cylinders I4- The perforating cylinders andthe cutoif cylinders are driven at a speed which would be 1% faster than the speed ofthe material I0 if the correcting member I6; of the differential device were moti0nless.. Switch 21 is closed to the right to connect the field 2's. of generator 2| to the anode circuits of valves 23, 24 and 25. a

The movable contact 69 is connected through resistors 49 and 49a to the grid oi? the amplifier valve 3| which controls the valves 23, 24, and 23 which supply the excitation current to the generator 2| from which correcting motor I. is supplied. Consequently the position of movable contact 63 on the section 51 of the voltage divider controls the speed of the correcting motor i9. Movable contact 69 is moved to a. po-

sition on section 51 of the voltage divider so that the correcting motor drives the correcting element i3; at a speed such that the speed of the perforating cylinders I3 is equal to the speed 01 material III with normal conditions of web tension and printing. In other words, the motor l3 operates at approximately half speed.

I! the perforating cylinders l3 are perforating in correct register with the stamp face designs on material ill, the light image of each slot on disc 40 lies between the two slits 36a and 31; at the instant that one of the register marks "34 is in position beneath scanning head 32 to darken the photo cell 32b. Consequently, the electrical impulse transmitted by photo cell 32o through amplifier 33 and condenser 52 to the grids 44s and 5a of mixing valves 44 and 45 in response to a spot 34 is not applied to the grids simultaneously with the electrical impulses transmitted by photo cells 38 and 31, through amplifier valves 38 and 39 and through condensers B4 and 33 to the grids 44b and 5b. Since the impulses received on the grids of each mixing valve are not received simultaneously, these mixing valves remain non-conducting and no electrical impulse is transmitted to the electrical energy storage device 43. As long as this condition of correct register obtains, the grid voltage of amplifier valve 3! remains unchanged and the speed of the correcting motor i3 which is ultimately controlled by amplifier valve 3! re mains unchanged.

However, ii the perforating cylinders it should become out of register with the stamp faces on material ill by a minute amount, e. g., .001 in., the impulses applied to the grids of one or the other of the mixing valves 44 or 45 will be simultaneous; one or more electrical impulses of duration proportional to the amount of misreg ister will be transmitted to the electrical energy storage device 43, and the speed of the correcting motor will be changed.

For example, assume that for some reason such as shrinkage of the material ID, the periorator rolls i3 become slow with respect to the design on material [0 so that an out of register condition arises. As soon as an extremely small amount of this out oi register condition, e. g., .001 in., is present, some light from source 4i will pass through a slot on disc 40 and enter slit 36a and fall upon photo cell 36 at the exact instant that a register mark 34 passes beneath scanning head 32 and darkens photo cell 32b. Consequently the electrical impulses transmitted by photo cells 32a and 36 will be received simultaneously on the grids 4a. and 44b of mixing valve 44. As a result of the simultaneous application of control voltages to its grids, valve 44 becomes conducting. The current increases abruptly in the anode circuit of valve 44 and also ceases abruptly after a slight movement of the register mark 34 or the slot in the disc 40 and thus results in an impulse of current of extremely brief duration. This current impulse lowers the voltage of connection point 44c which lowers the potential of grid 68a of inverter valve 88. As a result the current normally flowing in the anode circuit of valve 68 is abruptly reduced and then abruptly increased when the impulse of current through valve 33 ceases. The decrease oi current flo wing' in the anode circuit of valve 38 raises the potential of connection point 685 for the duration of the impulse of voltage on the grid, and a positive impulse of current is transmitted through condenser I2, rectifier 13, resistor 14, and conductor 15- to the terminal 483 of condenser 48.

and H are charged so that the voltage of term-' Condensers l8 inal 48a is increased positively. This increase of positive voltage raises the grid voltage of amplifier valve H, which results in increasing the anode current of valve 31 flowing through resistor l8 and lowers the grid voltage of valves 23, 24, and 25. This results in decreasing the voltage of generator 2i and decreasing the speed of correcting motor is which increases the speed of perforating rolls 23. The speed of the correcting motor, and therefore the amount of the correction, is dependent on the polarity and. magnitude of the charge on condensers l1 and t.

As long as the error in register exists, electrical impulses will continue to be applied simultaneously to the grids Na. and b of valve H, and positive current impulses will continue to be transmitted to the condensers H and 48 in rapid succession, i. e., each time a register mark on the material passes beneath the scanning head. The magnitude of the impulse which results from the combined action of the two scanning heads and which is applied to charge the condensers ll and 48 varies with the magnitude of the error in register. This is because the length of time the indication from the register marl; overlaps the indication from the slit varies with the magnitude of the error in register. This is a very desirable feature because it contributes to the stability of operation of the correcting system. At any instant the correcting motor runs at a speed dependent upon the charge on the condensers. In other words, the correction which is being applied at any instant depends upon the past history of the indications of error. Thus, instead of working on an intermittent basis, the correcting equipment operates continuously as long as any error in register exists.

When impulses are transmitted to the condensers 41 and 48 as described in the foregoing,

a high resultant voltage exists across the con densers for a short time after the charge is placed on the condensers. the relatively low time constant of condenser 48 and resistor 49;, the condenser 48 discharges rapidly and its voltage decays rapidly. Consequently the resultant voltage of both condensers decreases rapidly to a value near the value which existed before the charge was put on but slightly increased with respect thereto. The result of this action is that at first the speed of the correcting element 95;; of the differential is changed a relatively large amount for a relatively short time, and thereafter its speed is returned to a However, because of I value which is only slightly changed from its original value and at which it continues to run until a subsequent correction becomes necessary. Thus, there will result the action of a temporary speed increase or decrease for a position correction combined with a follow up or velocity correction. This method of correcting is very effective in preventing hunting.

A rectifier valve 11 has its anode connected to negative supply conductor and its cathode connected to the anode of rectifier valve 13.

After an impulse has been transmitted through If the opposite error in register should occur, 1. e., if the perforator rolls should become fast with respect to material In, impulses from both scanning heads will be simultaneously received on the grids l5. and 45a of mixing valve 45. Each of these impulses will reduce the potential of connection point 45. below the potential of terminal 48 of condenser 48 and current will flow during each impulse from terminal 48- through conductor 15, rectifier valve '18 and condenser 18 and valve 45 to ground 58. In consequence, negative impulses are transmitted to condensers 41 and 48 to reduce the charge thereon or possibly reverse the polarity, and ultimately to increase the speed of correcting motor I! and correcting element 6a driven thereby. This will decrease the speed of the perforating rolls and bring them into correct register with the stamp face markings as explained in the foregoing.

It should be noted that the cutting cylinders H are connected through bevel gearing 19, shaft 80, and bevel gearing 8| to the perforating cylinders and is therefore driven synchronously with the perforating cylinders l3. Consequently, when the perforating cylinders are in correct register with the stamp faces on the strip, the cutting cylinders out of! sheets of stamps in correct register with the stamp faces. When the perforating cylinders are out of register, the cutting cylinders are likewise out oi register and the operation of the register control system to restore correct register oi the perforating cylinders results in restoring correct register of the cutting cylinders.

Although in accordance with the provisions of the patent statutes, this invention is described as embodied in concrete form and the principle of the invention has been explained together with the best mode in which it is now contemplated applying that principle, it will be understood that the elements and connections shown are merely illustrative and that the invention is not limited thereto since alterations and modifications will readily suggest themselves to persons skilled in the art without departing from the true spirit of this invention or from the scope of the annexed claims.

What we claim as new and desire to secure by Letters Patent of the United States, is:

1. A control system for apparatus having an element operating on a length of moving material comprising light sensitive means actuated by indicia on said material for producing current variations, a second light sensitive means for producing current variations in accordance with the position of said element, means responsive to the substantially simultaneous occurrence of said current variations for correcting the relative positions of said material and said element and electric valve means provided with a pair of control grids and requiring the substantially simultaneous variation of the voltages of both said grids to change the conductivity of said valve means and electrical connections from said first light sensitive means to one of said grids and from said second light sensitive means to the other of said grids.

2. A register control system for apparatus having an element operating on a length of moving material comprising an electric valve provided with a pair of control members, said valve being normally non-conducting and being rendered conducting by the substantially simultaneous application or voltage to said control members, light sensitive means responsive to said material for applying voltage to one of said control members, light sensitive means actuated by said element for applying voltage to the other of said members, and means controlled by said electric valve for correcting the relative positions of said material andelement.

3. A register control system for apparatus having an element operating on a length of moving material comprising in combination, an electric valve having a pair of control members, said valve requiring the substantially simultaneous variation of the voltage of said control members to eifect a change in its conductivity, light sensitive means actuated by indicia orr said material for applying a voltage to one of said control members, a second light sensitive means actuated by said element for applying a voltage to the other of said control members, the arrangement of said light sensitive means being such that both said light sensitive means apply vol tages to said control members substantially simul taneously when an error exists in the relationship of said element and material, and means responsiveto a change in the conductivity oi said valve for correcting said error.

4. A register control system for apparatus having an element operating on a length of moving material comprising a pair of electric valves each having a pair of control members, each of said valves requiring the substantially simultaneous application of voltage to both of its said control members to effect a change in its conductivity, light sensitive means responsive to indicia on said material for applying a voltage to one control member of each of said valves, light sensitive means actuated by said element for applying voltage to the other control member of each of said valves, said light sensitive means being so arranged that when an error exists in the relative positions of said element and said length of material, voltage is applied simultaneously to both control members 01' one of said valves depending on the direction of said error, and means responsive to a change in the conductivity oi said valves for correcting said error.

5. A register control system for apparatus having an element operating on a length of moving material comprising an electric valve provided with a control grid and a suppressor grid, said valve being normally nonconducting and being rendered conducting in response to the substantially simultaneous applicationof control voltages to both said grids, light sensitive means responsive to indicia on said material for applying a control voltage to one of said grids, light sensitive means actuated by said element for applying a control voltage to the other of said grids, said light sensitive means being arranged so that said control voltages are supplied substantially simultaneously only when an error exists in the relative position." or? said element and said material, and means responsive to the passage 01 current of said valve for correcting said error.

6. A register control system for apparatus having an element operating on a length of moving material comprising an electrical energy storage device, means responsive to relative positions of said material and element for controlling the amount or electrical energy stored in said energy storage device, and means responsive to the amount of energy stored in said energy storage device for correcting the relative positions of said length of material and said element.

7. A register control system for apparatus having an element operating on a length of moving material comprising a condenser, light sensitive means responsive to the relative positions of said element and material for controlling the charge on said condenser, and means responsive to the charge on said condenser for correct i ing an error in the relative positions 01' said element and said length of material.

8. A register control system for apparatus having an element operating on a length 01 moving material comprising an electrical energy storage device, means responsive to the relative positions of said material and element for causing an amount of electrical energy varying with the magnitude of the error in register between said material and element to be stored in said energy storage device, and means responsive to the amount of energy stored in said energy storage device for correcting the relative positions of said length of material and said element.

9. A register control system for apparatus having an element operating on a length of moving material comprising in combination, a condenser, light sensitive means responsive to the relative positions of said element and length of material for increasing the charge on said condenser in response to an error in one direction in the relative positions of said element and length of material and for decreasing the charge on said condenser in response to an error in the opposite direction in the relative positions of said element and material, and means responsive to the charge on said condenser for correcting said errors.

10. A register control system for apparatus having an element operating on a length 01' moving material comprising in combination, an energy storage device, light sensitive means responsive to the relative positions of said material and said element for controlling the amount of energy stored in said energy storage device, means responsive to the amount of energy stored in said device for correcting an error in the relative positions of said element and material, said energy storage device comprising a portion having a relatively short time constant-and a second portion having a relatively long time constant thereby to provide a relatively large temporary correction and a small relatively permanent correction inthe relative positions 01' said element and said material.

11. A register control system for apparatus having an element operating on a length 01' moving material comprising in combination, an electric valve provided with a pair of control members, said valve requiring the substantially simultaneous application of control voltages to said control members to effect a change in the conductivity of said valve, light sensitive means responsive to indic'ia on said material for applying control voltage to one oi said control members, means responsive to operation of said element for applying a control voltage to the other of said control members, said light sensitive means and element responsive means being arranged so that said control voltages are applied substantially simultaneously to said control members when an error exists in the relative positions of said element and said material thereby to effect a change in the current flowing through said valve, and means responsive to said change in current for correcting said error.

12. A register control system for apparatus having an element operating on a length of moving material comprising a pair of electric valves each provided with a pair of control grids, each said valves requiring the substantially simultaneous application of control voltages to both said grids to effect a change in the conductivity of said valve, light sensitive means responsive to indicia on said material for applying control voltages to one grid oi each of said valves, means responsive to said element for applying control voltages to the other grid of each of said valves, said light sensitive means and said element responsive means being arranged so that when an error exists in the relative positions of said material and said element said control voltages are applied substantially simultaneously to the grids of one or the other of said valves depending on the direction of said error thereby to effect a change in the current iiowing through said valve, and means responsive to said change in current for correcting said error.

13. A register control system for apparatus having an element operating on a length of moving material comprising in combination, an electric valve having a pair of control grids, said valve requiring the substantially simultaneous application of control voltages to said grids to effect a change in the conductivity of said valve, means responsive to an error in the relative positions of said material and element ior applying control voltages to both said grids simultaneously thereby to effect a change in the current conducted by said valve, and means responsive to said change in current for varying the relative speeds of said element and material to correct said error comprising a generator having its voltages controlled in response to said change in current and a motor supplied from said generator.

14. A register control system for apparatus having an element operating on a length of material comprising an electric valve having a pair of control grids, said valve requiring the substantially simultaneous application of control voltages to said grids to effect a change in the conductivity of said valve, means responsive to an error in the relative positions of said element and material for applying substantially simultaneous control voltages to said grids to effect a change in the current conducted by said valve, an electrical energy storage device connected to said valve so that its stored energy is varied in response to said current change, and means for correcting said error comprising a generator connected to have its field excitation varied in accordance with the amount of energy stored in said energy storage device and a motor supplied irom said generator.

15. A register control system for apparatus having an element operating on a length material comprising in combination, means for feeding said material to said element, means for driving said element and feeding means at predetermined relative speeds, an electric valve having a pair of control grids, said valve requiring the substantially simultaneous application of control voltages to said grids to render said valve conducting, means responsive to an error in the relative positions of said element and material for applying control voltages to said grids sub stantially simultaneously, a condenser connected to said valve so that its charge is varied by the current conducted by said valve, and means responsive to the variation of the charge on said condenser for correcting said error at a rate dependent on said charge comprising a generator having its excitation varied in accordance with said charge and a motor supplied from said-generator and mechanically connected to said driv ing means.

16. A register control system for apparatus having an element operating on a length of material comprising means ior driving said element and material at predetermined relative speeds, an electric valve having a pair of control grids, said valve requiring the substantially simultaneous application of control voltages to said grids to render said valve conducting, means responsive to an error in the relative positions of said element and material (or substantially simultaneously applying control voltages to said grids to render said valve conducting, a pair of condensers connected in series relationship and connected to have their charge varied in response to the current conducted by said valve, and means responsive to the variation in said charge for varying the relative speeds of said element and material to correct said error at a rate dependent upon the magnitude of said charge, one of said condensers having a relatively large time constant and the other having a relatively small time constant thereby to provide a temporary relatively large speed correction, and a small relatively permanent speed correction.

17. A register control system ior apparatus having an element operating on a length of material comprising means for driving said element and material at predetermined relative speeds, a pair 01 electric valves each having a pair of control grids, each of said valves being normally nonconducting and being rendered conducting by applying control voltages to both its grids, light sensitive means responsive to an error in the relative positions of said element and material for substantially simultaneously applying control voltages to the grids of one of said valves to render one or the other 0! said valves conducting depending on the direction of said error, a pair of condensers connected in series relationship and connected with said valves so that the charge on said condensers is increased in response to the current conducted by one oi said valves and decreased in response to the current conducted by the other of said valves, and means responsive to a variation in the charge on said condensers for correcting said error, one of said condensers having a relatively large time constant and the other having a relatively small time constant thereby to produce a relatively large temporary correction and a small relatively permanent correction.

DAVID R. SHOULTS. WILLIAM D. 'COCKRELL.