US2716026A

US2716026A - Web guiding device

Info

Publication number: US2716026A
Application number: US253019A
Authority: US
Inventors: Charles G Axworthy
Original assignee: Union Wadding Co
Current assignee: Union Wadding Co
Priority date: 1951-10-25
Filing date: 1951-10-25
Publication date: 1955-08-23
Anticipated expiration: 1972-08-23

Description

Aug- 23, 1955 C. G. AxwoRTHY 2,716,026

WEB GUIDING DEVICE Filed Oct. 25, 1.951 5 Sheets-Sheet l I PHOTO-CELL AMF: n-c

" RecTlFlaR TRA RME

INVENTOR. ('af/es .x wo/r7gy ATTORNEYS,

Aug. 23, 1955 c. G. AxwoRTHY WEB GUIDING DEVICE:

5 Sheets-Sheet 2 Filed OCT.. 25, 1951 INVENTOR. (aKZ//es G? fx wor/ey A T TORNEYS Aug. 23, 1955 C. G. AXWORTHY WEB GUIDING DEVICE Filed oct. 25, 1951 5 Sheets-Sheet 3 Puo-ro CELL AM Pl.. D-C REcnFlEn Wn d faf/es 6: ik wo/uf/ PHOTO CELL ANPI.. D.C

REC TIFIER 6.V TRANS- IN VEN TOR.

AT TOR N EYS.

Aug. 23, 1955 c. G. AxwoRTHY 2,716,026

WEB GUIDING DEVICE Filed Oct. 25, 1951 5 Sheets-Sheet 4 l `PHOTO CELL AM PL D. C. .RECTIFIER Gv. TRANSFORMER Jag?. 9. ""6

INVENTOR.

ATTORNEYS.

Aug. 23, 1955 c. G. AxwoRTHY WEB GUIDING DEVICE 5 Sheets-Sheet 5 Filed OC. 25 1951 INVENTOR. 6'. 7x wol/ffy ,w n. v. o s 2 @kar/e5 ATTOF? N EYS United States Patent 2,716,026 Patented Aug. 23, 1955 WEB GUIDING DEVICE Charles G. Axworthy, Pawtucket, R. I., assigner to Union Wadding Company, a corporation of Rhode Island Application October 25, 1951, Serial No. 253,019

Claims. (Ci. 271-2.6)

This application is a continuation in part of my copendiug application Serial No. 158,163, filed April 26, 1950, now abandoned.

This invention relates to an apparatus for guiding a web of fibrous material in such a manner as to prevent the web from deviating more than a prescribed distance either side of an established line taken either through the center of the web or a line along which one edge of the web is designed to follow.

In the guiding of webs the edge of the web is frequently used to manipulate some feeler for control of the guiding means by pressing upon the feeler. In the use Of webs of unspun bers of a delicate construction, the edge of the material is physically insucient to cause the actua-tion of the 'feeler iinger and the usual device which is provided Vcannot be used for such guiding. Further, in some webs the edge of the work or web is in a wavy path and such that it would be impractical to utilize pressure means for control of the web as a whole.

One of the objects of this invention is to provide a control for the web which will function without physical contact of the web with any feeler means.

More specifically, an object of the invention is to use a source of light which will be directed upon some light responsive means which the web will control, dependent upon its position to vary the amount of light which is transmitted from the source to the light responsive means and thus control the guiding device.

Another object of this invention is to provide for certain ilexibility of movement of the work between minimums or certain tolerances between which no action will occur until the error is of a greater amount than the minimum out of the course desired.

Another object of this invention is to minimize the socalled hunting action which may occur in movement of the guiding device in order that the work may conform to a certain path of travel.

Another object of this invention in order to minimize the hunting action is to cause the means which govern the operation of the guider to travel with the work as it moves.

With these and other objects in View, the invention consists of certain novel features of construction, as will be more fully described and particularly pointed out in the appended claims.

In the accompanying drawings:

Figure 1 is a top plan view of the guiding rolls with the control means for the rolls located along one edge of the work;

Figure 2 is a section on line 2-2 of Figure l;

Figure 3 is a section on line 3-3 of Figure l;

Figure 4 is a sectional view of the control means such as shown in Figure 1;

Figure 5 is a section on line 5--5 of Figure 4;

Figure 6 is a top plan view of a pair of rolls with a modied arrangement of control but showing the control also from one edge of the work;

Figure 7 is a View similar to Figure 6 showing a control from both sides of the work that an average between the opposite edges may be had and the work guided along its center line;

Figure 8 is a plan View similar to Figure 7 but showing a modified arrangement of control in that there is utilized an adjustable potentiometer and a fixed source of light at one edge of the work;

Figure 9 is a view similar to Figure 8 but illustrating the opposite edges of the work as being controlled and the guiding along a center line through the work established;

Figure 10 is a diagrammatic view showing the relationship of the light source, photo tube, and web;

Figure ll is a diagrammatic view showing the same relationship as in Figure l0 but of a modiiied arrangement;

Figure l2 is-a circuit diagram of the arrangement of Figures 6 and 8;

Figure 13 is a circuit diagram of the movable light source and light responsive cell as shown in Figure 7.

In proceeding with this invention I have provided a pair of rolls mounted in a carriage so that a plane through the axis of the rolls is at right angles to a pivotal axis upon which the carriage is mounted. Mechanical means are provided for varying the position of the can riage about the pivotal axis so that the rolls when at an angle to the axis of the web will direct a web which passes between them in one direction or the other. The carriage is moved by an electric motor and this electric motor is controlled by an electric circuit which is actuated by means of a light source directed upon a light responsive cell such as a photo tube which controls a circuit so that depending upon the amount of light which is picked up by the photo tube the electric motor will be driven in one direction or the other to move the guiding rolls. The amount of light that is directed onto the photo tube is governed by the position of the web of material which may be between the light source and the photo tube so that as its position varies, varying amount of light will be transmitted thereto. In some instances this light source and photo tube will move with the guiding means in order that hunting action may be reduced, while in other instances the light source may be fixed and the movement of the rolls may actuate a potentiometer which provides the variable in the electric circuit and causes movement of the rolls so that hunting action may be minimized. In some instances the guiding will be with reference to a line along which the edge of the fabric should move, while in other instances a control is provided at both edges of the fabric so that their average may be taken into account and the path of travel of the center line of the fabric thus controlled.

From this it will be apparent that the invention has two branches comprising movement and non-movement of the controlling light and also two branches comprising the control from either one or both edges of the web which is to be guided. Electrically, as will be observed from the similarity of circuits in Figures l2 and 13 these branches are essentially the same as will hereinafter appear. The electrical value of the armature 61 is so chosen that voltage of a certain magnitude will be required before its operation so that a neutral zone is formed and hunting is minimized.

With reference to the drawings, and particularly Figures l to 3, the web of material or work to be guided is designated W and is directed between a pair of guide rolls 10 and 11 which are mounted in a carriage designated generally 12vwhich comprises end frames 13 connected by a bar 14. The bar is pivoted at its center upon post 15 which extends at right angles to a plane passing through axes of the rolls 10 and 11 and is mounted on a Vertical standard 16 having a base 17 for resting upon a iloor or some support. The web w travels in the direcltion of the arrow shown in Figure l and passes over roll 11 and beneath roll 10. The rolls will guide the web to pass beyond the rolls at right angles to the axes of rotation thereof. If the guide rolls were adjusted to a position exactly at right angles to the direction of movement of the web w, the alignment of the web w would not be changed. However, when the rolls are positioned at an angle to the direction of movement of the web w, for instance by turning the carriage 12 about the pivot 15 in a clockwise direction as seen in Figure l, the movement of the web w will be deliected towards the left. Moving the carriage 12 about the pivot 1S in a counter-clockwise direction will ycause the movement of the web w to be deiiected towards the right. Thus, by changing the position of the axes of rotation of the rolls 10, 11 relative to a predetermined line, the direction of movement of the web w can be controlled so that the portion thereof passing beyond the guide rolls follows a predetermined path notwithstanding lateral variations in the position of the portion in advance of the rolls. Y

Referring now to Figure 4, a control unit 19 is mounted on standard 2t) and comprises a framework 21 having a pair of

bearings

22 and 23 in which a rod 24 is slidably mounted. Along one edge of this rod there is formed a rack 25 which is driven from an electric motor 26 mounted in the frame by means of a bracket 27 and having a reduction gear casing 28 from which there is driven a pinion 29 which through gears 30, shaft 31, and

pinion 32 drives the rack 25 to move the rod 24 in one n direction or the other dependent upon the direction ofv rotation of the motor 26. The rod 24 is connected to a stiff wire 33 extending through a housing 34 to be connected to a collar34 (Figs. l and 3) on the bar 14 at a point spaced from the pivot 15 so as to swing the carriage about the pivot 15 in one direction or the other depending upon the movement of the reciprocating or sliding wire 33.

In some cases instead of the motor 26 transmitting movement tothe carriage, as shown in Figures l to 5, the motor 26 in Figures 6 to 9 transmits motion to the gear box 28 from which there extends a shaft 35 having a worm 36 thereon meshing with the worm gear 37 with its center co-axial with the pivot i5 and fixed to the carriage 12 to rotate the carriage about the pivot 15 directly.

As shown in Figures 8 and 9, shaft 35 is extended as at so as to drive pinion 38 and in turn the gear 39 to rotate a wiper arm or movable contact of the potentiometer 40 and eliect a control of the electrical system, as will be more fully hereinafter described.

In Figures 4 and 5 light source 41 is mounted upon a bracket 42 which is secured to the rod 24 so as to move with this rod as the motor causes its movement in one direction or the other. At the end of the rod 24 there is an arm 42 which extends upwardly and supports a light responsive source 43 which may be a photo tube and which will pick up light from the source 41 thru a window 44. This photo tube is indicated in the other figures. The amount of light which will be picked up by the photo tube will be dependent upon the position of the work W, shown in dot and dash lines in Figure 4, to control the amount that the work moves with reference to the window. If the work extends to the right to the full depth of slot 45 in Figure 4, the amount of light transmittedfrom the light source or lamp 41 to the photo tube 43 will be obscured; whereas, if the work is entirely out of the slot 45, much more light from the light source 41 will be transmitted to the photo tube. When the work is traveling in substantially the desired path the edge of the work will be at the position shown in Figure 4 and one-half ofthe light of the light source will be transmitted to the photo tube. The photo tube is placed in an electric circuit, to be hereinafter described, which operates a motor controller circuit so as to effect rotation of motor 26 in one direction when the work moves over the slot a predetermined distance inwardly of the normal path of travel of the work and eiects rotation of the motor in the other direction when the work moves over the slot a like predetermined distance outwardly Vof said normal path of travel of said work. Thespacing between these distances provides a neutral band in which the work may shift and not aiect operation of the motor. The neutral band is made of a length about one third of the range of the activity of the photo tube from maximum to a point where it is in complete shadow.

In some cases instead of the light source and the light responsive means being mounted directly upon the rod such as 24, these two units may be mounted upon a slide f. 46, as shown in Figures 6 and 7 movable in a guide 47 and reciprocated in response to theV movement of the carriage by a link 48 and an arm 49 which is secured to the worm wheel 37 so as to move therewith.

ln case two edges of the fabric are to be averaged so that the center line may be moved along a desired path, the photo tube and light source are duplicated at the other edge of the web as shown at 43' in Figures 7 and 9. A rod 50 will connect the

slides

46 and 46 in the

guides

47 and 47 to move together by thel link 48 as in Figure 7, the operation being otherwise the same. It will, of course, be understood that where the potentiometer is provided the light and the light responsive means or photo tube will remain xed and will not shift with the rolls.

In Figure l2 l show an electric circuit for Figures 1, 6, and 8, the negative terminal of all supply voltages being returned to ground.l The photo tube ,43, heretofore mentioned, has a constant supply voltage 89 connected to it.

Triode amplifier tubes

53 and 54 are connected to the photo tube 43 through the

resistances

51, 51', 52, and 52. These amplifiers are biased by the variable cathode resistors 90 and 91. The potentiometer 40 has its variable arm 57 arranged in this system which controls` the voltage impressed upon the grids of the triode tubes and 56 which are connected to the coil of relay 61 and to the ground as shown.

rl`he normal running position of the web would cause light to fall on the photo tube 43 illuminating one half of the cathode with the other half being in the shadow. This would be the balanced condition, and the correcting rolls would be in a position perpendicular to the length of the web with the potentiometer variable arm 57 in a near central position. Under this condition the cathode of the photo tube develops a voltage of a certain amplitude across resistors 51 and 51'. The grid of tube 54 has a constant voltage amplitude with referenceV to the cathode which will be adjusted to be equal to the amplitude across

resistors

51 and 51 and is represented as Ez. The grid of tube 53 is at a potential appearing across part of resistor 51 and all of resistor 51 which is shown in the drawing as being designated E1. Thetubes 53 and 54 are biased by variable resistors 90 and 91 and are adjusted so that the voltage appearing between points 64 l and 62 is equal and opposite to the voltage appearing between the points and 62. The voltage appearing between the grids of

tubes

55 and 56 and the ground will therefore be equal, producing zero voltage across the coil of relay 61 which will be de-energized, the relay contacts assuming a neutral position.

Either one of two abnormal conditions can occur, one when the web moves to a position to cause a shadow to fall over the cathode of photo ytube 43 and the second condition is when the web moves so as to allow the cathode to be fully illuminated.

In the iirst abnormal condition due to the web shadow covering the cathode of the photo tube 43, the voltage E1 decreases and the voltage E2 remains the same. causes decreased current in tube 53 with the current in tube 54 remaining the same. Therefore, the voltage between

points

64 and 62 decreases, and the voltage between This Y points 65 and 62 remains the same. This results in the voltage of the grid of tube S5 decreasing due to a decrease in the voltage across

points

64 and 62, while the voltage of the grid of tube 56 remains constant. This upsets the balance between

tubes

55 and 56 and current flows in the coil of relay 6l in one direction, causing the relay to close points 92 which causes motor armature 93 to turn in the preferred direction to move the correcting rolls to restore the web travel to normal position.

Should the second condition occur Where the tube is fully illuminated, then the current would increase voltage E1 while the voltage E2 would remain the same, causing more current to ow through tube 53 than tube 54 and more current to flow in tube 55 than in tube 56 which would result in upsetting in the other direction the balance between the tubes to cause the relay 61 to move in the opposite direction and cause points 94 to be closed which will cause the motor armature 93 to turn in the opposite direction to restore web travel to normal position.

The operation of all the circuit elements up to the relay is instantaneous and proportional to signals received from the cathode of the photo tube so that no delays are caused by any of these circuit elements. lt is desirable to prevent hunting in the mechanism which is accomplished in Figures l and 6 by moving the photo tube and the light source with the correcting rolls. This has the effect of anticipating the return of the web to normal running position and of preventing the web edge from going beyond normal running position in the opposite direction.

With the web displaced from normal towards the left in Figure l and towards the right in Figure 6, photo tube cathode would be between 50% and 100% obscured by the web shadow depending upon how great the displacement happened to be, and how rapidly the web is displaced.

This condition of the cathode causes a decrease in voltage E1 with respect to E2 and leads to movement of correcting rolls as described above, and of the connected photo tube cathode toward the displaced web edge. As the cathode approaches the web edge it becomes less obscured, voltage E1 rises and approaches Ez in value. When E1 equals E2 approximately, motor driving correcting rolls stop and as Ei exceeds E2 slightly, motor drives correcting rolls in opposite directions. This reduces corrective eiect, which tapers oi to zero as web reaches normal running position. Result is return of both web and photo tube to normal.

Web edges are often of wavy form, and in order to minimize movement, the resistance of the relay 61 is so chosen that it will not operate until there is a certain predetermined amount of current iiowing through its coil which is responsive to the amount of light cast upon the photo tube 43. Thus l provide a dead spot which may be adjusted by means of the variable resistance 90-91 so that the desired band of inactivity of the relay is provided for. I usually choose this band of inactivity to be about 1/3 of the range of activity of the photo tube from maximum to the point where it is in complete shadow.

In this circuit the potentiometer is shown in the diagram as existing as a unit as shown in Figure 8.

Figures l and 6 illustrate a photo tube and light source at the edge of the fabric and, therefore, would use a circuit such as shown in Figure l2 where a single photo tube is provided. In Figures l and 6, however, the photo tube and light source move rather than the arm 57 of the potentiometer and there is no variable potentiometer shown in either of these iigures. The arm 57 will be hand adjusted in these figures and remain fixed in this adjusted position. Electrically, however, the shifting of the photo tube and light source is the same as the shifting of the arm of the potentiometer with the web.

Phototube and exciter lamp source may be located centrally over the normal running position of the web and the potentiometer 40 varied by mechanical means from the correcting rolls as shown in Figure 8. Potentiometer arm 57 would be in centre for normal running position of web and to right or left depending upon corrective direction of rolls.

Assume web runs on" normal so as to obscure light reaching photo tube cathode. This results in decrease of voltage E1 with respect to E2 held constant as the current in the photo tube circuit has decreased. This causes the voltage across 64, 62 to decrease with respect to voltage across 65, 62. Point 62 represents movable potentiometer arm attached to correcting rolls and arranged to move in a direction to cause voltage 64, `62 to increase with respect to

voltage

65, 62 in proportion to displacement of rolls.

When decrease in voltage across 64, 62 caused by obscuring photo tube 43 is equalled by increase in voltage across 64, 62 with respect to 65, 62 due to movement of potentiometer arm in the proper direction by correcting rolls, a balanced condition between voltage across 64, 62 and 65, 62 prevents further movement of correcting rolls. Iniiuenced by correcting rolls, the web returns to normal track, and slightly beyond, thus causing less than 50% of cathode to be obscured and resulting in increase of voltage across 64, 62 over voltage across 65, 62 causing correcting rolls to return to normal. This leaves potentiometer 40 with arms 57 in centre and in balance. Photo tube 43 is in light balance, and the system comes to rest, thus maintaining normal web edge travel.

In Figure 13 I show an electric circuit for Figure 7 where two photo tubes are used with like parts of the circuit designated the same as in Figure 12.

With the circuit in operation and balanced, the web running in a central position, the correcting rolls in a position perpendicular to the length of the web, and the photo tubes centrally located above the web, the anode current of

tubes

53 and 54 will be equal and will result in equal and opposite polarity across the two halves of potentiometer 67. There is then no direct voltage since neither tube is conducting. Relay 61 is in the open or released condition.

If we now assume that the web for some reason moves in translation, there will be an increase of light on one photo tube and a decrease of light on the other, since the photo tubes are located at opposite edges of the web. This will increase the current in one photo tube circuit and decrease it in the other. Assume for the sake of illustration that the current of photo tube 43 decreases and that of photo tube 43 increases. This will result in increased anode current ilow in tube 53 and decreased ow in tube 54. The net result of these changes will be to make point 64 more negative than before and point 65 less negative than before. This means that the voltage will be the same polarity as in circuit above described but that it will be of twice the magnitude since the circuit is of the push-pull variety. The polarity will be such as to make the grid of triode tube 5S more negative and the grid of triode tube 56 more positive. Tube 56 will then start to conduct, closing relay 61 and causing the motor to run in such direction as to start the web moving back to its proper position.

As the motor rotates the correcting rolls, it also moves the position of the photo tubes laterally in such direction as to equalize the light incident on the two tubes and so equalizes the voltage drop across the two halves of potentiometer 67. The potential at

points

64 and 65 become equal and since they are of opposite polarity the voltage across

tubes

55 and 56 becomes zero and relay 61 releases and the motor stops running. It thus leaves the rolls displaced by an amount proportional to the displacement of the web.

As the web begins to return to its original position the anode current of photo tube 43 decreases and that of photo tube 43 increases and a voltage of opposite polarity appears across

tubes

55 and 56. This causes triode tubev 55 to start conducting and relay 61 to oppositely-move, causing the motor to run in the opposite direction or'V toward thel center position of the rolls. When the web has-returned to its central position and the correcting rolls. are centered the original conditions of balance obtained and the system is at rest.

While the showing in Figure l3 indicates the circuit for Figure 7 where there are photo tubes and light sources at both edges of the fabric and both of Vthese move with no variable potentiometer, the circuit would also apply to Figure 9 where there are photo tubes and light sources at both edges of the fabric which are stationary and a potentiometer for varying the potential as applied to the

tubes

55 and 56. In the case of Figure 9, the contact point 57 would be moved in response to the movement of` the rolls for varying the electrical conditions of the circuit.

In Figure 4 I have illustrated switches at 71 and 72 against which the arm 73 of the bracket 42 may contact to break the circuit from the electric motor so that when the carriage reaches such limit of travel in either direction, the circuits will be broken. The switches 71 and 72 are omitted in both of the diagrammatic views Figures 12 and 13.

In the showing in Figures l to 9 the light source is on one side of the plane of the web of work while the photo tube is on the other side of the plane of the web in such a position as to receive the direct rays of light from the light source. However, in Figure l0 the light source 80 although on one side of the plane of the web of work W does not throw its rays 81 directly against the photo tube 82 but rather the rays are reflected from the surface 83 against this photo tube 82 although the circuits which are heretofore described will be the same.

In Figure ll a still diierent arrangement is provided in which both the light source 80 and the photo tube 82 are located on the same side of the plane of the web of work and the Work serves as a reflecting surface for the light. In this case, the light is directed from the source 80 through a lens 84 for condensing its rays into an area designated generally 85 which will be black and non-reilecting. However, should the work move into this area, then there will be reflection and this reilection of light will be picked up through the lens 86 to be impinged upon the photo tube 82. In some cases, however, instead of the surface 85 being relatively non-reflective with reference to the work, the area 85 might be more highly reflective than the work and the work serve to diminish the reflection which could be utilized in a circuit of the type described.

I claim:

l. A guider for a travelling web of material comprising a pair of parallel rollers between and in contact with which the web travels, a carriage for said rollers, means for pivoting said carriage to swing about an axis at right angles to the plane of the axes of said pair of rollers, mechanical means for shifting said carriage about its pivot, an electric circuit, a source of light in said circuit, a light responsive means in said circuit positioned to receive the light transmitted from the source dependent upon its position relative to the web, said electrcal circuit being responsive to the amount of light received by said light responsive means to actuate the mechanical means to shift the carriage about its pivot, a variable in said circuit connected to said carriage and movable therewith to control movement of the rolls and minimize huntingaction of said rolls, said circuit including an inactive area midway of the travel of said variable.

2. A guider as in claim 1 wherein the variable in said circuit changes the electrical'output thereof.

3. A guider as in claim l wherein said electrical circuit is balanced when the web is travelling in the desired position and said circut includes a normally balanced Cil 8 potentiometer and the variable in said circuit changes the balance thereof.

4. A guider as in claim 1 wherein said electrical circuit is oaianced when the web is travelling in the desired position and said circuit includes a normally balanced potentiometer having an arm mechanically connected to the carriage to move therewith to serve as the variable in said circuit and control the balance thereof.

5. A guider as in claim l wherein said light source and light responsive means is located at one'edge of the web.

6. A guider as in claim l wherein said light source and light responsive means is located one at each edge of the web.

7. A guider for a travelling web of material comprising a carriage for a pair of parallel rollers between and in contact with which the web travels, means for pivoting said carriage to swing about an axis at right angles to the plane of the axes of said pair of rollers, mechanical means for shifting said carriage about its pivot, a source of light on one side of the plane of the web and a single light responsive means on the other side of the web with a neutral area so arranged that the edge of the web may intercept varying portions of the light transmitted from the source to the light responsive meansl dependent upon its position relative to the web and an electrical circuit responsive to the amount of light received by said light responsive means on either side of a neutral area of substantial extent to 'actuate the'mechanical means to shift the carriage about its pivot, said electrical circuit having a balanced potentiometer, the arm of said potentiometer being mechanically coupled to said carriage for movement therewith to minimize hunting action of the rollers.

8. A guider as in claim 7 wherein said light source and light responsive means is located one at each edge of the web.

9. A guider for a travelling web of material comprising a pair of parallel rollers between and in contact with which the web travels, a carriage for said rollers, means for pivoting said carriage to swing about an axis at right angles to the plane of the axes of said pair of rollers, mechanical means for shifting said carriage about its pivot, a normally balanced electric circuit, a source of light, a light responsive means coupled to said circuit and positioned to receive an amount of light from said source dependent upon its position relative to the web,

said mechanical means being responsive to changes in the amount of light received by the light responsive means unbalancing said circuit, said mechanical means being responsive to said unbalance to shift the carriage about its pivot, a normal balanced potentiometer in said circuit across the output of said light responsive means,

said light source and said light responsive means being connected to said carriage and movable therewith to serve as a variable in said circuit and rebalance said potentiometer to minimize hunting action of said rolls, said circuit including an inactive area midway of the travel of said variable.

l0. A guider for a travelling web of material comprising a pair of parallel rollers between and in contact with which the web travels, a carriage for said rollers, means for pivoting said carriage to swing about an axis at right angles to the plane of the axes of said pair of Y rollers, mechanical means for shifting said carriageV about its pivot, a normally balanced electric circuit, a source of light in said circuit, a light responsive means coupled to said circuit and positioned to receive an amount of light from said source dependent upon its position relative to the web, the balance Vof said `electrical circuit being responsive to changes in the amount of light re-v serve as a variable in said circuit to control movement of the roller and minimize hunting action of said rolls, said circuit including an inactive area midway of the travel of said variable.

References Cited in the file of this patent UNITED STATES PATENTS 1,634,984 Dickhaut July 5, 1927 10 King Apr. 27, 1937 Gulliksen July 27, 1937 Friedmann May 17, 1938 Kelling June 29, 1948 Bishop Sept. 4, 1951 FOREIGN PATENTS Great Britain May 28, 1934