GB1470294A - Optical digitising system - Google Patents

Abstract

1470294 Position determination CETEC SYSTEMS Ltd 25 Sept 1974 [18 July 1973] 41651/74 Heading H4D The distance between a reflective cursor 10, Fig. 1, and each of two amplitude modulated radiation transmitter/receiver units 17, 17A and found by measuring the phase difference between the modulations of the beams transmitted and received by each unit after reflection at the cursor. Transmitter/receiver units.-Each unit emits visible, U.V., I.R., or microwave radiation which is guided to the cursor by mirrors 18, 18A ; the modulation of the beam reflected by the cursor is compared in phase with the emitted beam by counting clock pulses to give the cursor distance from the unit in a digital manner, and the digital measurements from each unit are combined at 115 to give the co-ordinates of the cursor for display or machine control at 116. One unit is shown in Fig. 3, and has a GaAs emitter 20 whose radiation in the I.R. region is passed via lens 121, splitter 122 and filter 32 to two reflectors on the cursor 10. The reflected beam is directed to detector 26. The emitter 20 is modulated at 300 MHz. A further GaAs emitter 31, modulated at 299.985 MHz, acts as a local oscillator and its output is mixed with the reflected beam. The local oscillator output also passes to detector 28 as does some of the output from emitter 20; the filters 32 prevent the local oscillator beam passing to the cursor however. As a result each detector produces signals at 300 MHz, 299.985 MHz, 599.985 MHz and 15 kHz, the latter being selected for processing. Circuitry.-At 300 MHz the modulation wavelength is 1 m. ; thus to find the distance of the cursor from the unit it is necessary to know the number M of complete modulation wavelengths as well as the phase difference # between the two beams. In Fig. 4 the 15 kHz signal from detectors 26, 28 in a unit are fed via amplifiers 46 to threshold devices 47. When the detector signals exceed a given level the devices 47 open or close gate 48 so that clock pulses are counted at 49. The moments at which the devices 47 operate, and hence the count at 49, depends on the phase difference between the two signals. The 300 MHz drive for emitter 20 is derived from a 10 MHz oscillator 40 via multiplier 41, doubler 42, amplifier 43, controlled divider 44 and amplifier 45, This drive also serves as the above mentioned clock. The 299.985 MHz drive for emitter 31 comes from a V.C.O. 52, doubler 53, controlled divider 54 and amplifier 55. To maintain the 15 kHz frequency difference, the drive for emitter 20 is divided by 20,000 at 50 (k give a 15 kHz signal) and this is compared at 51 with the signal from detector 28; any phase difference here is used to control the V.C.O. To find the value of M, the modulation wavelength is increased by operating the dividers 44, 54 to reduce the modulation frequency. Suppose the cursor-unit separation will not exceed 10 m.; then the dividers are set to divide by 10 so that the modulation at emitter 20 is 30 MHz (modulation wavelength 10 m.) and at emitter 31 is 29.985 MHz. Counter 49 gives one pulse to logic 56 for every 2000 input pulses; the number of pulses received by the logic 56 is the value of M required when 300 MHz is used. Thus to find M, a division factor is chosen so that the cursorunit distance is less than one modulation "wavelength; the phase difference is found and is appropriately divided to give M. The value of M is then combined with the phase difference at 300 MHz in adder 56 to give the digital output. Cursor and tracking.-The cursor is a transparent body with a graticule at one end; its outer surface, can have a reflective V groove, or can be a reflective cylinder or part sphere, Fig. 2 (not shown); in the latter case the body has a refractive index of 2, and its outer surface is 30% reflecting. The radiation is directed to the cursor by mirrors 18, 18A whose positions are controlled by motors M in Fig. 1. Two split detectors 67, 67A receive the reflected beams via splitters 64, 64A and control the motors so that the beams remain centered on these detectors. If either beam is blocked, the detectors make the mirrors rotate rapidly until the cursor is found again, and during this time the circuitry of Fig. 4 is disabled. Modifications.-A He-Ne laser with external modulation by a Pockels cell can replace the emitter 20, Fig. 6 (not shown); this laser serves as the transmitter source for both units 17 and 17A, and a local oscillator is not used. Though Fig. 1 shows units 17, 17A as fixed, with the cursor being reflective, the two units may be mounted on the cursor for use with mirrors mounted on two edges of a platen, as in Fig. 5. Here the cursor co-ordinates are measured directly by the two units. In Fig. 1 the tracking mirrors may be replaced by revolving scanning mirrors, the Fig. 4 circuitry being gated on whenever the transmitted beam strikes the cursor.