Patent GB-1281838-A (GB)
Title: Distance measuring apparatus
IPC: G01S17/32
Filed: 1970-09-17
Abstract: 1281838 Distance measuring HEWLETTPACKARD CO 17 Sept 1970 [24 Nov 1969] 44374/70 Heading H4D General.-In optical distance measuring equipment, a beam of amplitude modulated infra-red light is alternately transmitted to a remote reflector and fed through a reference path, the light received back from the reflector having its modulation phase compared with the reference beam received at the end of the reference path. The transmitted and reference beams are in the form of within-pulse amplitude modulated pulses producing corresponding bursts of modulation signals in the equipment photodetectors. Continuous waves of the modulation signals, retaining the same respective phases are produced from the respective pulses for the phase comparison. The modulation frequency is varied in decade steps so that the distance can be given in decimal, non- ambiguous form. Mechanical Fig. 1.-The Ga-As electroluminescent diode 17 is fed with the pulsed modulation signal A1-2 to produce the amplitude modulated infra-red beam. This is split at semi-mirror 19 into a transmitted beam 21 and a reference beam 23. A rotating disc shutter 29 has sequential parts alternate ones 25 of which are bent up, so that the transmitted and reference beams are alternately passed/blocked in antiphase. The reference beam bursts are fed through a light pipe 37 of known length and focused on to mirror 15 by lens 35. The transmitted beam bursts are reflected from mirror 15 and collimated by lens 9 on to the remote reflector 13. The reflected bursts are received and focused by coaxial annular lens 11 on to mirror 15, via mirror 41. Mirror 15 reflects the sequential reference and received reflected bursts on to a photo diode 43. A light source and detector 40, 42 produce a chopper signal B1-2 synchronous with the chopping of the light beams. If stage Fig. 2.-The modulation signal FM is produced by synthesizer 45 at frequencies which are decade multiples of 24À58244 KHz for feet measurement and at 14À98545 KHz for metric measurement under the control of function selector 99. The signal FM is fed to the diode 17  via amplifier 49. The synthesizer 45 also produces a reference or IF signal FR at 24À58244 KHz. A local voltage controlled oscillator 53 nominally produces a signal at frequency FM + FR. This signal is mixed at 51 with the modulation signal FM to give a signal nominally at frequency FR. This is compared with the reference signal in detector 55 and any output therefrom is used to adjust the oscillator 53. The local signal FM+FR passes through detector photo diode 43 and mixes with the detected reference and reflected bursts 34, 36 to give the IF signals shown at frequency FR. Channel separator Fig. 3.-The sequential IF signal is fed to a phase detector 67 the output of which, with switch, 69 in the shown position, is fed via an integrator 71 to the control input of a phase shifter 75. The reference signal FR, frequency halved at 83 is phase shifted by 75 and then returned to frequency FR in multiplier 85. Antiphase outputs are selected at switch 109 and fed, via switch 97 in the shown position to the second input of phase detector 67. The phase locked loop so formed gives a continuous output H3-4 on line 63 at frequency FR and in phase with the reference signal part of the IF signal. Switches 69 and 97 are switched by the chopper signal, so that the channel 73, 77, 87, 91, 95 gives a continuous output I3-4 on line 61 at frequency FR and in phase with the reflected part of the IF signal. Converter Fig. 4.-The two continuous signals I3-4 and H3-4, exhibiting the distancedependent phase differences as shown, are fed, via Schmitt triggers 74, 72, to a bi-stable circuit 76 which produces a pulsed output, the pulses having widths equal to the phase difference. These are fed to a gate 82 to pass current from source 80 to integrating means 78 such that a current appears at the output of means 78 proportional to the phase difference. The relation between the phase difference # and the distance R is of the form such that if the modulator wavelength # is initially made equal to 2 feet, the phase difference will give the fraction of a foot part of the distance and if the wavelength is then multiplied by ten, the phase difierence wil give the 0 to 10 feet part of the distance and so on. A function selector 99, Fig. 2, thus successively steps up the modulators wavelength by ratios of ten by control of the synthesizers 45 and local oscillator 53. Assuming that the shortest wavelength is being used, switch 101 is activated and is set in any one of ten positions to give corresponding increasing current values 0 to 9. These values are of opposite sign to the phase difference dependant output current and when two currents are equal, zero reading appears on meter 90. Switch 101 is then stopped in its position to indicate the fractional foot part of the distance. Switches 103, 105, 107 are similarly successively set to give the units, tens and nundreds parts of the distance. Gate 86 and filter 88 receive the signals H3-4 and I3-4 and produce the triangular response shown to the phase difference. The phase difference regions #=0 and #360 degrees are inacurate. A level detector 92 therefore detects when the response is below a detection line indicating these regions and produces a current which drives the needle of meter 90 off scale. The positions 0, 1, 2 and 8, 9 for switches 101/107 also indicate that the inaccurate regions are near and a signal G3-4 is then produced which changes over switch 109 so that the signal H3-4 is phase shifted by 180 degrees. The phase difference also moves into the more accurate middle region.