Optical Detecting System

This invention relates to an optical detecting system in which a beam of coherent light is directed at an object in order to detect the presence or movement thereof. The invention is particularly intended for use in a system in which the vibration of the object is detected by mixing light from the source with light reflected back from the object in order to derive a doppler signal of which the frequency represents the movement of the object.

The amplitude of a signal which is reflected from an object varies greatly by virtue of irregularities in the reflectivity of the object. One object of the present invention is the provision of a scanning system by means of which an area of the surface of the object can be scanned to determine a region of relatively high or low reflectivity. In a specific form of the invention representation of the position of a region of relatively high reflectivity may be stored and, thereafter, used to control the beam so that it is redirected at that region for, for example, further examination of the object of the movement thereof. For this purpose the beam may be scanned in two directions and representations of signals controlling this scanning may be stored when the amplitude of the received doppler signal, or a signal representing that amplitude, exceeds a stored signal value. It is thereby possible to store a signal representing the maximum amplitude in the doppler signal and the coordinates of the region which yields the maximum amplitude.

FIGS. 1 and 2 illustrate one embodiment of an optical detector of vibration .

FIG. 1 illustrates the principal parts of a detector of remote movement in which a coherent light beam is directed at an object. The detector is contained within a housing 1 and includes a source of coherent light constituted by a laser 2 which provides a light beam 3 which is directed through an optical system 4 and a focussing system, to be described, at an object 5. The optical system 4, of which the particular construction is not important, is in this particular embodiment arranged to combine the light from the laser with light reflected back from the object in order to form a doppler signal which is fed to a processing system 6. The system 4 may include means for modulating either the light from the laser or the light which is directed at the object in order to introduce between the light beams that are combined a relative frequency shift so as to make the doppler frequency associated with zero movement of the detector relative to the laser a non-zero positive frequency.

The focussing system comprises a lens 7 which is mounted in a canister 8 itself supported on a cantilever 9 extending from a fixed block 19. The purpose of this arrangement is to allow the lens 7 to vibrate or otherwise move in a plane normal to the path of the light beam without moving along that path so that the beam can be scanned over the surface of the object 5 without substantially altering the forcussing of the beam. The optical system further includes a lens 10 which is mounted on a carriage 11 for movement to and fro along the path of the light beam. The optical system finally includes a lens 12 which is fixed in position and preferably forms part of the housing 1. The lens 10 is much weaker than the lens 7, having for example a focal length at least five times longer than that of the lens 7, and is preferably weaker than the lens 12. The focussing system enables the point of focus of the light beam from the laser to be changed from near infinity to very close to the lens 12 by a relatively slight movement of only the lens 10. Thus the mounting of the lens 7 does not have to accommodate movement of the lens 7 along the path of the light beam and the mounting of the lens 12 as part of the wall of the closure is greatly simplified.

The lens 7 may be moved in a plane normal to the path of the light beam by two orthogonal coils 13 and 14. These are diagramatically shown; their axes would usually be in the plane of the paper and into the plane of the paper. The coils 13 and 14 are driven by amplifiers 15 and 16 in order, in this embodiment of the invention, to provide a substantial scanning movement of the light beam over the surface of the object 5. It may be desired to superimpose on the scanning movement of the beam a slight movement such as a circular movement. For this purpose quadrature signals may be applied from amplifiers 17 and 18 to lines which pass along the cantilever 9 to piezo electric drivers, preferably in the form of bimorphs which are spaced apart at 90.degree. intervals around the periphery of the lens 7 and extend in the canister 8 lengthwise thereof. Energisation of each pair of diametrically opposed bimorphs will move the lens relative to the canister 8 in a plane normal to the axis of the light beam.

FIG. 2 repeats part of the system of FIG. 1 and principally describes one form of electrical processing circuit. The optical system 4 in this apparatus includes a photodiode 20 at which the doppler signal is developed. It is fed to a detector circuit comprising a series diode and a shunt capacitor and resistor. Across the resistor is developed a rectified and smoothed signal representing the instanteous doppler frequency. This voltage signal is monitored by a voltmeter 27.

The signal from the detector 26 is compared with a stored signal. For this purpose the output of the detector 26 is fed to one input of a comparator 28 which also receives a signal corresponding to the signal stored in a sample and hold store 30. Normally, on setting up the apparatus the sample and hold store would be fed with a zero input. When the stored value is exceeded by the output of the detector 26 the comparator enables the sample and hold store to receive the relatively high output from the detector 26. For this purpose the comparator 28 fires a monostable circuit 29 which feeds a control signal for enabling the sample and hold store to change its stored value. The sample and hold store is enabled for a time such as 100 microseconds.

A signal denoting the maximum received amplitude is stored. It would be possible though not normally required to determine the minimum received amplitude by a similar circuit; in such a case the sample and hold store would initially store a signal denoting a high amplitude for the received doppler signal and the store would be enabled when the received amplitude of the doppler signal fell below that denoted by the stored signal.

In order to aid the determination of the region that yields, in this embodiment, the highest amplitude in the doppler signal, the beam is scanned with the aid of two waveform generators 21 and 22. The generator 21 provides a sawtooth waveform which is fed to the amplifier 15 via a switch 23. The frequency of the sawtooth may be for example of the order of 10 Hz. The purpose of this is to develop a repetitive scan back and forth in one direction over an area of the object. The scan is displaced in a transverse direction by means of a staircase waveform generator 22 which feeds the amplifier 16 and thereby the coil 14. The steps of the staircase which would normally be followed by a flyback to a value corresponding to that at the beginning of the staircase waveform and would be such as to provide for example 100 steps of movement in 10 seconds.

The outputs of the sawtooth waveform generator and the staircase waveform generator are received by stores constituted by sample and hold stores 31 and 32 at the time when the amplitude of the received doppler signal exceeds the value stored in the sample and hold store 30. The monostable circuit 29 opens the sample and hold stores 31 and 32 simultaneously with opening the sample and hold store 30. Accordingly, when a new high value of doppler signal is stored, the sample and hold stores 31 and 32 effectively store signals representing the coordinates of the region which yields that value. The storage process is repeated for any new maximum occurring during the scan, the coordinates stored in the sample and hold stores 31 and 32 being changed each time a new high value is stored in the sample and hold store 30.

At the end of the scanning of the area of the target, the switches 23 and 25, which are preferably ganged together, are changed over so that the position of the lens 7 is controlled by the coordinate values that have been stored. Accordingly the sample and hold stores 31 and 32 feed the amplifiers 15 and 16 and thereby the coils 13 and 14 which deflect the canister 8 for the lens 7 so as to shift the light beam and redirect it to the region yielding maximum returned signal.

It may happen that the region of maximum yield is inconveniently positioned at the edge of the area which is scanned. It is possible to obtain indications not only of the maximum value but of the second highest value by the provision of further sample and hold stores as described and to disable one or more of them at some predetermined point in the scan. However such an arrangement would be substantially more complicated than that which has been described.