GB890887A - Improvements in or relating to automatic line tracers - Google Patents

Abstract

890,887. Electric copying machines. CANADIAN WESTINGHOUSE CO. Ltd. Nov. 17, 1958, No. 36909/58. Class 38 (6). [Also in Group XL (b)] A photo-electric line follower for controlling a gas torch or milling head, uses a circular scan to produce an A.C. signal which is (a) used to orientate the scanning head so that the line being followed is tangential to a datum line on the head and (b) is resolved into x and y coordinate error signals to control x and y servomotors keeping the head on the line. A spotlight 21 illuminates the line 20, the light from which is focused on to a photo-cell 23 via a mirror 24 which is rotated by a synchronous motor 25. The mirror is inclined at a small angle to the motor axis to provide the circular scan. The complete assembly is mounted in a casing 13 which is rotatable by a motor 26 relative to a platform 10 about the axis O-O. The platform 10 is movable by x and y coordinate servomotors (not shown). The thickness of the line being followed is such that the output of the photo-cell is substantially a sine wave of twice synchronous frequency and this signal after amplification at 34, Fig. 3, is applied to a phase comparator 36 together with a reference signal derived from the A.C. mains, L, N through a frequency doubler 37. Any error signal drives the motor 26 to rotate the whole scanning assembly 13 &c until the cell output is in phase with the reference signal. In rotating the assembly 13 &c. it also rotates the rotor of a resolver 28. The stator of the resolver is energized by a preset velocity signal tapped off a potential divider 38 and by the output of a low-pass filter 35 energized by the output of the photo-cell 23. If the scanning head is not correctly aligned, then the amplitude of the A.C. signal varies at synchronous frequency and this component only is passed by the filter to the resolver, which then energizes the x and y co-ordinate servomotors through a motor control 40 to correct the error. The position of the spot-light 21 is manually adjustable to allow for tool offset. In a modification, the scanning head includes a second photocell 39 ahead of the main cell 23, the output of which acts on the control 40 to slow the motors on curves. In a second modification, the resolver 28 is of the D.C. type, and the filter 35 is replaced by a phase-sensitive detector. In a third modification, the reference signal is derived from a tachometer driven by the motor 25. In a fourth modification the motor 26 and the resolver 28 are mounted separately from the head and a second resolver is provided coupled to the motor so that all three rotors move together. The second resolver is then used to maintain the system in phase synchronism either by driving the motor 25 or by controlling the reference frequency. In a fifth modification, the photo-cell is rotated instead of the mirror. In a sixth modification, the lens 22 is mounted eccentrically and rotated.