GB1416541A - Method and apparatus for following the motion of a datum mark - Google Patents

Abstract

1416541 Photo-electric mark followers H W WALLACE & CO Ltd 21 Dec 1973 [6 Nov 1972] 51100/72 Heading G1A One dimensional movements of a datum mark are followed by a carriage having a lamp to illuminate the mark, a photo-electric cell having a pair of light-sensitive electrodes on to which the image of the mark is projected, and a motor responding to a difference signal from the photoelectric cell to move the carriage. The boundary between the electrodes of the photo-electric cell is arranged perpendicular to the direction of movement. The invention is described with reference to a tensile testing apparatus in which a sample provided with two bench marks is stretched, two carriages being provided to follow the movements of the marks. The distance between the carriages is measured, e.g. by means of a wire rotating a drum driving a potentiometer on one carriage or by a toothed strip on one carriage operating a switch on the other, to determine the extension of the sample. The digital measuring system may also utilize a rotary digital encoder or a toothed strip may operate a mechanical counter. The apparatus may be arranged to operate a graphical plotting machine. The apparatus comprises a vertical pillar 1 on which the carriages are supported by wheels 3, 5. A lamp assembly 21 illuminates the specimen 26 and the reflected light is imaged by lens 30 on to a differential photo-electric cell having two sensitive electrodes. The electrodes are connected to a differential amplifier to produce an error signal which is filtered and amplified and applied to a motor 14 which drives the wheel 3 to cause the carriage to follow the bench mark. The gain of the servo loop is controlled by means of an FET connected in a feedback path of the differential amplifier. Coarse loop gain is provided by feeding the total light signal from the differential photo-cell to a current generator which provides the bias for the FET. Loop gain is increased when an error signal is produced by subtracting the modulus of the error signal from the total light signal. Fine loop gain control is provided by detecting the noise component of the error signal and adding this to the coarse control signal to reduce the loop gain. If the sample is of a highly extensible material, e.g. rubber, the bench marks may have an exponential taper on both edges so that as the mark is stretched at least some part is of suitable width to produce an error signal when displaced.