US3704373A

US3704373A - Scanning head arrangement having servo motor armature secured to shaft on which photocell is carried

Info

Publication number: US3704373A
Application number: US185282A
Authority: US
Inventors: Francis G Bardwell; John W Dobrauz; Frank L Sobchak
Original assignee: Stewart Warner Corp
Current assignee: Stewart Warner Corp
Priority date: 1971-09-30
Filing date: 1971-09-30
Publication date: 1972-11-28
Anticipated expiration: 1989-11-28

Abstract

The following specification describes a tracing scanning head having a photocell for sensing the presence of a pattern. The cell is supported on one shaft of a pair of coaxial shafts for movement transverse to the shaft axes. A pinion carried by the other shaft engages a rack carried by the one shaft for moving the photocell in response to rotation of the other shaft to compensate for the kerf or tool cutting width. A servo motor armature is fixed to the one shaft and rotates the one shaft and photocell to maintain the photocell angularly aligned with the line. A pair of easily readable knobs are provided at the upper end of the shafts to permit the offset to be easily determined. A preamplifier is carried by the one shaft to amplify low level photocell signals before they are transmitted over slip rings and brushes, and a neon bulb shielded by a metal ring casting lights the drawing pattern. A transparent shield depending from the casting protects the photocell lens. A pair of lamps cast indicia on the paper to enable the direction of scanning head travel to be easily ascertained.

Description

United States Patent HAVING SERVO MOTOR ARMATURE SECURED TO SHAFT N I r PHOTOCELL 1s CARRIED 1151 3,704,373 Bardwell et a1. 1 1' Nov. 28, 1972 [54] SCANNINGHEADARRANGEMENT [57] ABSTRACT The following specification describes a tracing scanning head having a photocell for sensing the presence of a pattern. The cell is supported on one [72] Inventors: Francis G. Bardwell, Elmhurst; John v shaft of a P of coaxial shafts for movement w, m- Frank so I verse to the shaft axes. A pinion carried by the other both f Chicago, all f L shaft engages a rack carried by the one shaft for movv H ing the photocell in response to rotation of the other 3] Asslgnee? y m- Corporation, shaft to compensate for the kerf or tool cutting width.

Chlcago, A servo motor armature is fixed to the one shaft and [22] Filed: Sept 30, 1971 rotates the one shaft and photocell to maintain the .photocell angularly aligned with the line. A pair of [2 PP No.: 185,282 easily readable knobs are provided at the upper end of g the shafts to permit the offset to be easily determined.

52 us. (:1 .250/202 318/577 Preamplifier is carried by shaft amplify 51 Int. Cl. G05b 19/36- 066k 11/02 level Signals befm they are "ammitted 58 Field of sesi'ch I 250/202 318/577 Sl'iP rings and brushes 3 bulb shielded v by a metal ring casting lights the drawing pattern. A 5 6] References Cited transparent shield depending from the casting protects the photocell lens. A pair Of lamps cast indicia on the UNITED STATES PATENTS paper to enable the direction of scanning head travel I t b 'l rtai d. 3,322,952 5/1967 Jewell ..250/202- e y me 13 Claims, 8 Drawing Figures Primary Examiner.lames W. Lawrence Assistant Examiner-T. N. Grigsby Attorney-Augustus G. Douvas et al.

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PATENTEDnuvze m2 3. 704,373

SHEET UF 7 INVENTORS Francis 6. Bardwell John W. Dobrouz Frank L. Sobchuk PATENTED W I973 3 704,373

' saw u ur 7 us 9 2| I INVENTORS Francis (5. BordweH John W. Dobrouz Frank L. Sobchuk SHEET 5 [IF 7 FIG.6

INVENTO RS Francis G. Bordwell John W. Dobrouz Frank L. Sobchok PATENTEUnuvza m sums 0F 7 INVENTORS 7 Francis 6. Bardwell John W. Dobruuz Frank L. Sobchuk PATENTEDmvzs I972 SHEET 7 BF 7 I NV E N TO R S Francis 6. BurdweH John W. Dobrouz Frank L. Sobchuk Attorney v SERVO MOTOR ARMATURE SECURED TO SHAFT ON WHICH PHOTOCELL IS CARRIED BACKGROUND OF THE INVENTION arrangement. These elements include a servo motor which rotates a photocell through an idler gear to maintain the photocell in the proper angular position relative the line or edge in responset'o an appropriate signal as, for-example, described in 11.5. Pat. Nos. 3,213,282 or 3,423,589. This gearing introduces bulk and the problems of wear and backlash, while a load is placed on the resolver gear used to control the coordinate drive or servo motors. In addition, space for mounting a synchro 'to control a torch rotator, for example, is difficult to provide, as are the kerf adjustment, electrical wiring and components, together with general operator visibility and control.

SUMMARY OF THE INVENTION The present invention incorporates a servo motor ar-' mature coaxial with and secured to the shaft carrying v the photocell to thereby eliminate the need for gearing between the shaft and motor armature. The elimination of the gearing avoids the problem of backlash and wear while the space savings permit the incorporation of a synchro in addition to a resolver in the headhousing with each coupled to the photocell shaft through a respective gear under relatively no load.

The photocell is supported on the. photocell shaft for movement transverseto the shaft axis by a rack, which in turn is engaged by a pinion on one end of a shaft coaxial with the photocell shaft to offset the photocell in accordance with the size of the tool cut or kerf. Both shafts are provided with easily accessible knobs at their upper end exterior to the housing and marked with indicia for facilely indicating the position of the photocell.

A preamplifier carried on the photocell shaft amplifies the photocell signals before the signals are transmitted through any slip ring connections to thereby reduce noise effects on the low level photocell signal while the leads to the photocell and amplifier are extended through milled slots in the photocell shaft to provide shielding for the leads-which are encircled by bearings and the servo motor armature. v

Additionally, lamps carried with the photocell have their filaments arranged to cast an indicia on the drawing for indicating the direction in which the scanning head is moving, while a metal ring casting which supports a neon scanning lamp or bulb shields the photocell from radiation and rapid voltage changes. A

protects the photocell lens and diffuses the lightwhile permitting the drawing beneath the head to be seen.

transparent plastic hood depending from the casting BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an illustration of a typical line tracing coordinate drive metal cutting installation;

FIG. 2 is an isometric view of the scanning head with portions of the internal apparatus seen through the housing;

FIG. 3 illustrates a portion of the'drawing and the lamp reflectance pattern thereon for indicating direction of travel;

FIG. 4 is an isometric view of the scanning head with portions of the internal apparatus seen through the housing which is partially broken away to reveal the inner apparatus in greater detail;

FIG. 5 is an exploded isometric view taken in one direction of the photocell assembly;

FIG. 6 is an exploded isometric view taken in the opposite direction from FIG. 5 of the photocell assembly;

FIG. 7 is an exploded isometric view of the scanning lamp and shield assembly; and

'FIG. 8 is a sectional view taken partially along the line 8-8 in FIG. 2 and looking in the direction of the arrows.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1 a typical line tracing coordinate drive installation is indicated by the reference character 10. The installation 10 includes a table 12 on which a paper 14 or the like is secured. The paper has a pattern or drawing 16 thereon, which is followed by a scanning head 18 incorporating the principles of the present invention.

The scanning head 18 has a housing 20 which is secured to a carriage 22, in turn movably supported for translatory movement in one coordinate or the direction X by guideways or ways 24. A rack 26 is attached to carriage 22 for moving the same in the direction X under control of a gear 28 driven by a coordinate drive or servo motor indicated at 30 through appropriate gearing. Rack 26 also drives a plurality of cutting torches fed over a gas line 34, or cutting tools indicated at 36 36X in the X direction for cutting in a corresponding direction a sheet metal plate 38 fixedly carried by a plurality of supporting rails 40.

The rack 26, together with the associated apparatus including carriage 22, torches 36 36X and the drive motor 30 are driven in another coordinate or Y direction perpendicular to the X direction by a second coordinate drive or servo motor indicated at 42 and associated gears to enable the scanning head 18 and torches 36 36X to completely follow the drawing 16 and align with pattern 16 in the event of scanning head displacement. Electrical connections between the scanning head 18 and the

respective servo motors

30 and 42 and a control panel 44' are established by

cables

46 and 48.

Referring now to FIGS. 2, 4 and 8, it will be seen that the scanning head housing 20 is ring-shaped and has a generally circular annular lower portion 50 with an enlarged generally rectangularly shaped annular upper 'portion 52 separated by a generally horizontal integrally formed wall 54. A rectangular plate'56 closes the upper end of portion 52 and a shaft 58 extends therethrough for receiving a knob 60 having a radial pointer indicia and a peripheral opening 62. A second or kerf adjustment shaft 64 is rotatably supported coaxially in shaft 60 and against axial movement by a pair of axially spaced bearings 66. The shaft 64 at the upper end. extends beyond shaft 60 and has a knob. 68 seated in a recess of knob 60. Knob 68 is marked with indicia 69 both on its upper and peripheral surface corresponding to' respective tool cutting diameters on either side of a zero line, and these are seen either by components and terminals forconnection to the conmounted, on wall 54 together ,with a synchro 76 and a resolver 78 which are electrically connected over cable 48 to appropriate components.

A plurality of brushes 80 spaced axially of shaft 64 extend from block 74 for engaging respective slip rings 82 spaced axially of shaft 64 and secured thereto for rotation therewith by a common insulator ring 84. The rings 82 are connected with a respective terminal such as '85 at the upper end of ring 84, and a respective insulted wire therefrom passes through a respective milled slot 86 extending axially of the shaft 58 and through a bearing 88 which rotatably supports the shaft 60 in wall 54 and which holds the shaft 60 against movement upwardly of wall 56. A spring 90 biases the bearing 88 upwardly from a gear 92 fixed to shaft 60. Gear 92 meshes with gears 94 and 96 for driving synchro 76 and resolver 78, respectively.

Secured to shaft 58 directly below the gear 92 are the commutator segments and armature 98 of a photocell rotational servo motor 100 whose field magnets 102 are fixed to the internal surface of annular portion 52. A bearing 104 rotatably supports the shaft 58 in a radial plate 106 below motor 100 and prevents axial movement downwardly of shaft 60. Plate 106 is secured to wall 54 by means such as stud 108, and its upward movement is limited by an internal peripheral shoulder 109 in portion52. Electrical power for the servo motor 100 is provided by electrical connections from cable 48 through wall 54 to brushes 110.

Secured to an annular member 111 on shaft 58 and below plate 106 is a preamplifier 112 on a printed circuit board 113 carrying a connector. The electrical connections to the preamplifier on board 112 are extended through the milled slots 86 and electrical connections from the preamplifier and the milled slots 86 are extended from the connector on board 113 to a photocell and kerf assembly 114.

The photocell assembly 114, as best seen in FIGS.

and 6, comprises a plate 116 formed on the lower end of annular member 111 from which a second plate 118 is suspended by means of a pair of springs 120 sup- I ported by washers and nuts threaded on screws 121 slots 122 and across the axis of shaft 58 for engagement with pinion on the end of shaft 64 to move plate 118 radially relative the shaft axes for kerf adjustment.

The plate 118 has a central opening 125 across which the rack 124 extends and in which an insulator carrying a photocell 126 is secured below the rack 124 with the electrical connections therefor extended from connector on board 112. The photocell 126 is dual cell type as, for example, described in the aforementioned patents.

A conically shaped flanged lens tube 128 is secured to the lower side of plate 118 in encircling relationship to the photocell 126, together with a pair of insulator blocks 130 on opposite sides of the photocell 126 and tube 128. The blocks 130 are located on a radial line perpendicular to slots 122 and each carries a

respective lamp

131 and 132 together with electrical terminals for connection to the connector on board 113. A pair of lens 134 carried bya radial flange on tube 128 are spaced from and aligned with a

respective lamp

131 and 132 to form an image of the lamp filaments on the paper 14 to determine the direction of head travel, as will be explained. The tube 128 also has a lens 136 at the lower end in axial alignment with the photocell 126 to focus the image of the pattern or drawing 16 on the photocell 126.

A lower metal ring 138 having a recessed annular flange 139 intermediate its ends, as seen in FIG. 7, receiving a split ring neon bulb 140 encircling the tube 128, is secured to the lower end of housing portion 50. The bulb 140 is spaced axially just below the photocell 126 and

lamps

131 and 132, and is secured to a radial flange 139 by a pluralityof spring tabs 142. Electrical connections for the bulb 140 are extended from cable 48 and conductors 144 external to housing 20, as best seen in FIG. 4, to a female socket 146 adjacent the lower edge of portion 50. Socket 146 is engaged with a male socket 148, best seen in FIG. 7, carried by ring 138 and connected to lamp 140 through respective resistors. In operation, the bulb 140 is pulsed to enable the photocell to follow the drawing, as described in U.S. Pat. No. 3,423,589, instead of relying on the shutter arrangement described in U.S. Pat. No. 3,213,282. In addition, a transparent annular plastic shield 150 is secured to the lower end of ring 138 and extends to a position just above the paper 14 for protecting the lens tube, lamps and associated apparatus while permitting the drawing to be viewed.

An off line switch 152 and a rheostat 154, respectively secured in opposite walls of the enlarged rectangular portion 52, are provided for enabling the operator to receive a signal when the head 18 is aligned with pattern 16 and to control the gain of the amplified photocell signals. The entire scanning head 18 is moved into alignment with the pattern 16 through operation of

motors

30 and 42 under manual control to position the light from bulb 140 centrally over the pattern 16, after operation of appropriate switches at the control panel 44 enables the apparatus. The bulb 140 is pulsed to enable the photocell 126 to steer the head as explained in aforementioned U.S. Pat. No. 3,423,589. The knob 60 is rotated until the photocell 126 assumes a predetermined angular position relative the pattern, at which time the operator is signalled and the servo motor 100, under control of the photocell will maintain I gularlyaligned with-pattern 16. The tools 36 36X, of

course, are simultaneously positioned in accordance with the positionof head 18.

The operator also rotates the kerf adjustment knob 68, which he may observe from either above or through opening 62, to a position, as marked thereon, corresponding'to the tool diameter or the size of the cut with the direction of rotation determined by-which side of pattern 16, the cut is to be made. Rotation of knob 68 rotates the pinion Win a respective direction to move the rack 124, plate 118, photocell-126, lamps'l31 and 132 and lens .134 and 136 ina corresponding direction. This directionis perpendicular ,to the drawing line. Since the photocell 126controls the drive motors and servo motor 100 to align the photocell properly with the pattern, the shaft 58 and torches 36 36X are offset from the pattern to compensate for the cut, when the photocell is aligned with pattern 16.

The speed at which the drive motor 30, forexample, drives the head 18 in one coordinate direction is selected at panel 44 and the torches 36 36X prepared for operation. The

drive motors

30 and 42 are then placed under automatic operation and motor 30 drives the head 18 and the torches 36 36X in the desired direction along the pattern 16.

As described in the aforementioned patents, the head 18 moves along the line in the X direction, for example, under control of the drive or servo motor 30. The photocellmaintains itself in proper angularalignment with the line 16 through operation of servo motor 100, to in turn rotate gears 90, 94 and 96 for controlling the synchro 76 and resolver 78 accordingly. The resolver 7.8 in turn controls the drive or servo motor 42, for example, inaccordance with a displacementsignal from the photocell and the speedof motor 30 to maintain the photocell'126 properly positioned in the Y direction relative the line. a I

As the photocell senses a curve, such as 160 for example, it transmits a so-called difference signal to operate the servo motor 100 for rotating the photocell accordingly to maintain the photocell at the predetermined angle relative the line, while the resolver 78 controls the coordinate servo motor in accordance with the displacement signal and the rotation of the shaft 58 to maintain the photocellposition over the pattern. Rotation of shaft 58 simply rotates the rack 124 about the pinion 70 without moving the rack transversely. The synchro 76, which is also adjusted in accordance with the rotation of shaft 58, is used in the event torch rotators are provided for use in rotating the torches 36 36X about their longitudinal axis for a bevel cut of the plate 38. In that event the synchro 76 controls the rotation of the torch rotators to ensure the bevel is properly out along the curve 160. When the head 18 operatesin the Y direction, the servo motor 42 continuously drive the head 18 as the photocell orients the head 18 tofol low the line 16.

The transparent plastichood 150 permits'the operator to view the drawing 16 and also serves to diffuse the neon lamp light and protectthe lens tube 128 from striking any objects on the table 12. t

The foregoing constitutes an improved scanning head arrangement whose inventive concepts are believed set forth inthe above and the accompanying claims.

What is claimed is: v

' 1.'A scanning head having a photocell for sensing the presence of a pattern and a bulb for illuminating said pattern with said photocell providing a signal indicative of the direction of movement of said head relative said pattern, the improvement comprising one shaft, means for rotatably supporting said shaft in said head,'means carrying said photocell on said one shaft at any one of a plurality of positions lying on a line transverse to the axis of said-shaft, and a servo motor having an armature secured to'said shaft for rotating said one shaft for maintaining said photocell oriented at a predetermined angle relative said pattern in response to a signal from said photocell. g

2. A scanning head having a photocell for sensing the presence of a pattern and a bulb for illuminating said pattern with said photocell providing a signal indicative of the direction of movement of said head relative said pattern, the improvement comprising a photocell shaft, means for rotatably supporting said photocell shaft in said head, a second shaft, means for rotatably supporting said second shaft coaxially in said photocell shaft, means for carrying said photocell on said photocell shaft for movement transverse to the axis of said shafts, means for moving said photocell transverse'to said axis in response to the rotation of said second shaft relative said photocell shaft, a lens carried by said photocell carrying means at a position spaced from said photocell, and a servo motor having an armature secured to said photocell shaft for rotating said photocell shaft for maintaining said photocell oriented at a predetermined angle relative said pattern in response to a signal from said photocell.

3. In the scanning head claimed in claim 2 a metal ring for carrying said bulb with said bulb being a split ring neon bulb encircling said photocell and spaced between said line and photocell with said metal ring having a radial metal portion between said bulb and photocell.

4. In the scanning head claimed in claim 2 a preamplifier carried by said photocell shaft for amplifying a signal from said photocell.

5. In the scanning head claimed in claim 4, a plurality of axially extending slots in said photocell shaft for receiving leads extending to said preamplifier.

6. The scanning head claimed in claim 2 in which said means for moving said photocell comprises a rack movably carried by said photocell shaft, and a pinion engaged with said rack and rotatable with said second shaft for moving said rack.

7. The scanning head claimed in claim 6 in which said means carrying said photocell comprises a plate having a pair of radially extending longitudinally aligned slots, means for biasing said plate axially in one direction of said shaft, and means passing through a respective slot and secured to said photocell shaft for holding a respective biasing means against said plate.

8, In the scanning head claimed in claim 7, a pair of lamps carried by said plate on opposite sides of said second shaft axis and along a radialline perpendicular to the longitudinal axis of said slots, with said lamps having a respective filament for indicating the direction of travel of said head.

. 9. In the scanning head claimed in claim 2 a knob at one end of said photocell shaft-having a pointer and an opening circumferentially aligned with said pointer, a knob on one end of said second shaft seated in said photocell shaft knob and having indicia visible on one radial surface of said second shaft knob and through said opening for circumferentially adjusting the position of said second shaft relative said pointer.

10. A scanning head having a photocell for sensing the presence of a pattern and abulb for illuminating said pattern with said photocell providing a signal indicative of the direction of movement .of said head relative said pattern to control a servo motor for rotating said head in correspondence with a curve in said pattern, the improvement comprising a pair of coaxial shafts, means for rotatably supporting one of said shafts in said head and for rotatably supporting the other shaft in said one shaft, means for carrying said photocell on said one shaft for movement transverse to the axis of said shafts, a preamplifier carried by said one shaft for amplifying signals derived from said photocell, a rack carried by saidphotocell carrying means, a gear carried by said other shaft and engaged with said rack for moving said photocell carrying means and photocell transverse to said axis, a lens carried by said photocell carrying means at a position spaced from said photocell, said one shaft having a plurality of circumferentially spaced slots through which connections are extended to said preamplifier, means at one end of each of said shafts having indicia for indicating the transverse position of said photocell relative the axis of said one shaft, and an armature for said servo motor secured coaxially to said one shaft for rotating said one shaft for enabling said photocell to follow a curved pattern in response to a signal from said photocell.

11. In the scanning head claimed in claim 10, a pair of spaced bulbs carried by said photocell carrying means for providing indicia adjacent said pattern corresponding to the direction of travel of said scanning head, and a transparent annular shield carried by said head for encircling said lens to protect said lens while enabling said indicia and pattern to be viewed.

12. A scanning head having a photocell for sensing the presence of a pattern and a neon bulb for illuminating the pattern with said photocell providing a signal indication of the direction of movement of said head to control a pair of drive motors having coordinate drives for'moving the head in the direction of said pattern, the improvement comprising a pair of coaxial shafts, means for rotatably supporting one of said shafts in said head and for rotatably supporting the other shaft in said one shaft, means for carrying said photocell adjacent one end of said shaft for movement transverse to the axis of said shafts, a preamplifier carried by said one shaft adjacent said one end of said shaft, a rack carried by said photocell carrying means, a gear carried by said other shaft and engaged with said rack for moving said photocell transverse to said axis, a lens carried by said photocell carrying means at a position spaced from said photocell, a pair of lamps spaced on opposite sides of said photocell and carried by said photocell carrying means, metal means for carrying said neon bulb at a position spaced axially between said photocell and lens and encircling said shafts with said metal means shielding said photocell, said one shaft having a plurality of milled slots through which connections are extended to said preamplifier and lamps, a respective knob at the opposite end of each of said shafts for indicating the relative positions of said shafts and the transverse position of said photocell relative said one shaft axis, and a servo motor having an armature secured to said one shaft for rotating said one shaft to follow a curved pattern in response to a signal from said photocell.

13. In the head claimed in claim 12, a synchro, a resolver, and means gearing said one shaft to both said synchro and resolver adjusting the position of both said synchro and resolver in accordance with the rotational position of said one shaft.

Claims

1. A scanning head having a photocell for sensing the presence of a pattern and a bulb for illuminating said pattern with said photocell providing a signal indicative of the direction of movement of said head relative said pattern, the improvement comprising one shaft, means for rotatably supporting said shaft in said head, means carrying said photocell on said one shaft at any one of a plurality of positions lying on a line transverse to the axis of said shaft, and a servo motor having an armature secured to said shaft for rotating said one shaft for maintaining said photocell oriented at a predetermined angle relative said pattern in response to a signal from said photocell.

2. A scanning head having a photocell for sensing the presence of a pattern and a bulb for illuminating said pattern with said photocell providing a signal indicative of the direction of movement of said head relative said pattern, the improvement comprising a photocell shaft, means for rotatably supporting said photocell shaft in said head, a second shaft, means for rotatably supporting said second shaft coaxially in said photocell shaft, means for carrying said photocell on said photocell shaft for movement transverse to the axis of said shafts, means for moving said photocell transverse to said axis in response to the rotation of said second shaft relative said photocell shaft, a lens carried by said photocell carrying means at a position spaced from said photocell, and a servo motor having an armature secured to said photocell shaft for rotating said photocell shaft for maintaining sAid photocell oriented at a predetermined angle relative said pattern in response to a signal from said photocell.

8. In the scanning head claimed in claim 7, a pair of lamps carried by said plate on opposite sides of said second shaft axis and along a radial line perpendicular to the longitudinal axis of said slots, with said lamps having a respective filament for indicating the direction of travel of said head.

9. In the scanning head claimed in claim 2 a knob at one end of said photocell shaft having a pointer and an opening circumferentially aligned with said pointer, a knob on one end of said second shaft seated in said photocell shaft knob and having indicia visible on one radial surface of said second shaft knob and through said opening for circumferentially adjusting the position of said second shaft relative said pointer.

10. A scanning head having a photocell for sensing the presence of a pattern and a bulb for illuminating said pattern with said photocell providing a signal indicative of the direction of movement of said head relative said pattern to control a servo motor for rotating said head in correspondence with a curve in said pattern, the improvement comprising a pair of coaxial shafts, means for rotatably supporting one of said shafts in said head and for rotatably supporting the other shaft in said one shaft, means for carrying said photocell on said one shaft for movement transverse to the axis of said shafts, a preamplifier carried by said one shaft for amplifying signals derived from said photocell, a rack carried by said photocell carrying means, a gear carried by said other shaft and engaged with said rack for moving said photocell carrying means and photocell transverse to said axis, a lens carried by said photocell carrying means at a position spaced from said photocell, said one shaft having a plurality of circumferentially spaced slots through which connections are extended to said preamplifier, means at one end of each of said shafts having indicia for indicating the transverse position of said photocell relative the axis of said one shaft, and an armature for said servo motor secured coaxially to said one shaft for rotating said one shaft for enabling said photocell to follow a curved pattern in response to a signal from said photocell.

12. A scanning head having a photocell for sensing the presence of a pattern and a neon bulb for illuminating the pattern with said photocell providing a Signal indication of the direction of movement of said head to control a pair of drive motors having coordinate drives for moving the head in the direction of said pattern, the improvement comprising a pair of coaxial shafts, means for rotatably supporting one of said shafts in said head and for rotatably supporting the other shaft in said one shaft, means for carrying said photocell adjacent one end of said shaft for movement transverse to the axis of said shafts, a preamplifier carried by said one shaft adjacent said one end of said shaft, a rack carried by said photocell carrying means, a gear carried by said other shaft and engaged with said rack for moving said photocell transverse to said axis, a lens carried by said photocell carrying means at a position spaced from said photocell, a pair of lamps spaced on opposite sides of said photocell and carried by said photocell carrying means, metal means for carrying said neon bulb at a position spaced axially between said photocell and lens and encircling said shafts with said metal means shielding said photocell, said one shaft having a plurality of milled slots through which connections are extended to said preamplifier and lamps, a respective knob at the opposite end of each of said shafts for indicating the relative positions of said shafts and the transverse position of said photocell relative said one shaft axis, and a servo motor having an armature secured to said one shaft for rotating said one shaft to follow a curved pattern in response to a signal from said photocell.