Patent GB-1391779-A (GB)
Title: Optical systems
IPC: G01P3/36
Filed: 1971-10-05
Abstract: 1391779 Optical velocity measurement PILKINGTON PE Ltd 22 Dec 1972 [5 Oct 1971] 46212/71 Heading H4D [Also in Division G1] In a system for measuring displacement or velocity relative to a surface by modulating radiation from the surface by means of a grating to generate an intensity modulation frequency component representative of the rate of displacement, low frequency components produced by large scale variations in the surface brightness are eliminated by arranging the grating to divide the radiation into two beams in which the displacement frequency components are out of phase, and taking the difference between the signals produced from these beams. As shown, Fig. 2, radiation from the surface G is focused by a telecentric lens 7 on to a grating 8 in the form of a circular disc with a radial pattern of transparent and reflective bands. The transmitted and reflected radiation is focused by lenses 9 and 11 on to photodiodes 10 and 12 the outputs of which are applied via amplifiers 13 and 15 to a differential amplifier 14 which produces the difference signal. The system is moved relative to the surface G (e.g. by being mounted on a vehicle) and small irregularities in the surface cooperate with the grating 8 to produce out of phase displacement frequency components in the detector signals. The grating 8 is also rotated by a motor 18 and this increases the apparent frequency of the desired components. The speed of motor 18 is controlled by a servo system to maintain the frequency at the output of amplifier 14 constant. This is achieved by comparing this output with a reference frequency from oscillator 14 by means of a frequency comparitor 16 which generates an error signal to control the motor speed. This arrangement results in the grating being rotated at such a speed that pulses due to the grating bands alone have a frequency equal to the difference between the frequency of oscillator 24 and the frequency component which would be produced by the surface G with the grating stationary. This difference frequency is picked up by means of a LED source 20 and a photodiode 21 and applied together with the oscillator 24 output to a bidirectional counter 23 the instantaneous counter of which is therefore proportional to the displacement. The surface may be illuminated by a near IR region source, e.g. a filtered incandescent lamp, and Ge photo-diodes may be used as the detectors. The grating may also be rotated at constant velocity in which case the outputs of amplifiers 14 and 20 are applied to the bidirectional counter to produce the displacement measurement. The grating may also be cylindrical or conical or may be in the form of a flat belt. The image of the surface may be scanned across a stationary grating by a rotating mirror or prism. In a simplified system the surface image is focused  directly on to a stationary grating the output of the difference amplifier then having a frequency proportional to the surface velocity. A "flywheel" effect may be included to reduce errors due to a temporary loss of signal, e.g. when crossing a dark area of the surface. In the embodiment described above, this effect is achieved by the inertia of the motor and grating disc but it is possible to incorporate a "flywheel oscillator" into the circuit. The telecentric lens serves to eliminate the effects of variations in the height of the measuring system above the surface but instead of this two similar systems may be spaced vertically by a fixed distance and their outputs applied to a computer to eliminate variations due to height to give a measure of the speed. Alternatively the computer may eliminate variations due to speed resulting in a measurement of the height above the surface and this could be used to measure the height of the surface of a flowing liquid. Orientational information from an external source may be used to calculate ground displacement or to stabilize the direction of travel of the system. Also two systems arranged at right angles may be used for movement over an area. The system may be mounted so that it is held vertically to give a measure of the true horizontal displacement over inclined terrain. Alternatively the system may be held at right angles to the surface and correction made by a computer fed with angle of climb information.