Patent GB-1513882-A (GB)
Title: Optical detecting system
IPC: G01H9/00
Filed: 1975-05-30
Abstract: 1513882 Photo-electric detector of movement DECCA Ltd 7 May 1976 [30 May 1975] 23640/75 Heading G1A [Also in Division H4] A remote movement director comprises means for directing a coherent light beam at an object, means for mixing light reflected from the object with light from the source to derive a Doppler signal which represents movement of the object, means responsive to control signals to provide a reproducible scan of the light beam over a region of the object, means to detect a local maximum or minimum of the Doppler signal, and means operable in response to the detector to determine the values of the control signals associated with the local maximum or minimum. The main aim of the invention is the determination of regions of the object of relatively high or low reflectivity. In a particular embodiment, a coherent light beam from a laser 2 is directed through an optical system 4 via a focusing system at an object 5. The optical system 4 which produces the Doppler signal includes means for modulating one or other of the light beams that are combined so as to produce a relative frequency shift sufficient to make the Doppler frequency associated with zero movement of the detector relative to the object a non-zero positive frequency. The focusing system comprises a lens 7 mounted to allow movement in a plane normal to the path of the light beam, Fig. 1 (not shown), also a lens 10 mounted for movement along the path of the light beam and a fixed lens 12. The focusing system enables the point of focus of the light beam to be changed from near infinity to very close to the lens 12 by a relatively slight movement of only the lens 10. Scanning of the light beam over the surface of the object 5 is affected by means of two orthogonal coils 13, 14 driven by amplifiers 15, 16 while, if desired, a slight movement such as a circular movement may be superimposed on the scanning movement by means of quadrature signals applied from amplifiers (17, 18, Fig. 1), to lines which pass along a cantilever (9) supporting the lens 7 to Piezo-electric drivers e.g. biomorphs spaced apart at 90 degree intervals around the periphery of the lens 7. The Doppler signal is fed via a detector 26 to one input of a comparator 28 which also receives a signal corresponding to that stored in a sample and hold store 30. When the stored value is exceeded by the output of detector 26, the comparator 28 fires a monostable circuit 29 which feeds a control signal for enabling the store 30 to receive the relatively high output of the detector 26. A signal denoting the maximum received amplitude is stored. Although not normally required, the minimum received amplitude could be stored by a similar circuit. The light beam is scanned with the aid of a sawtooth waveform generator 21 feeding the amplifier 15 via a switch 23 and a staircase waveform generator 22 feeding the amplifier 16 via a switch 25. The outputs of the generators 21, 22 are received by sample and hold stores 31, 32 which are opened simultaneously with the opening of the sample and hold store 30. Accordingly, whenever a new high value of Doppler signal is stored, the stores, 31, 32 effectively store signals representing the co-ordinates of the region which yields that value. At the end of the scanning, the switches 23, 25 which are preferably ganged together are changed over so that the position of the lens 7 is controlled by the stored co-ordinate signals. Accordingly, the stores 31, 32 feed the amplifiers 15, 16 and thereby the coils 13, 14 which deflect the lens 7 so as to shift beam and redirect it to the region yielding the maximum returned signal.