Patent GB-1446801-A (GB)
Title: Time interval phase detection in distance measuring apparatus
IPC: G01S17/32
Filed: 1974-02-06
Abstract: 1446801 Optical distance measurement; digital phase meter HEWLETT-PACKARD CO 6 Feb 1974 [9 May 1973] 05533/74 Headings H4D and H3H In the optical distance measurement equipment as described in Specification 1,281,838, the phase comparision circuitry is substituted by digital phase comparison circuitry and the measurements are automatically calculated and displayed. The mechanical features of the invention and the I.F. stage are substantially as described with reference to Figs. 1 and 2 (not shown) of the above Specification. However, instead of varying the modulation of electroluminescent diode (17) in decade increments, one of two modulating frequencies 75 kHz or 15 MHz are selected by arithmetic unit 200, Fig. 2, and measurements are made solely in the metric system. The digital circuitry essentially comprises: a first digital circuit for taking a multiplicity of samples of the phase difference existing between reference signal 34 and received signal 36 which are time multiplexed at the chopper signal B1-B2 frequency; and a second digital circuit for averaging the results of said multiplicity of samples to determine the true phase difference within a specified degree of accuracy, which is specified in terms of a value of the statistical variance of the results from the particular number of sets of samples of phase difference calculated. A complete measurement cycle is performed first at the "coarse measurement frequency", 75 kHz, having a modulation wavelength 4 km. and measurement wavelength 2 kin., which is the maximum measurement distance. The result from this measurement is made with a resolution of 0À2 m. The system, as controlled by unit 200 changes to a modulation frequency of 15 MHz, having a wavelength 10 m. A phase shift of 360 degrees, therefore occurs at each multiple of 10 m. from instrument to target, but the measurement results in a figure between 0 m. and 10 m. having a resolution 0À001 m. The two measurements are then combined to give the required distance measurement and may be digitally displayed having been corrected by en-  vironmental sensors 222 (as in the abovementioned Specification). The display data may further be available for output to peripheral data storage and computing units. Fig. 2 illustrates in block diagrams the digital circuitry used to measure, by digital time interval means, the phase separation between the group of signals D 1-2  outputted by the limiter (65) which alternate, at the chopper signal frequency F 1-2  (5 Hz), between the received and the reference signal, which intervals are detected by edge detector 208. When a received or target path is detected, the TWED flag is set in coincidence with the positive edge of the chopper signal (Fig. 3, not shown). Similarly the RWED flag is set in coincidence with the negative edge. These flags are set in response to positive and negative edge enable signals from unit 200. Limiter output signals D 1-2  are applied, along with the 3À75 kHz reference frequency from phase shifter 202 to flip-flop phase detector 204, and an AND gate phase detector 206. The phase shifter 202 is enabled, and shifts the reference frequency signal by 180 degrees, when, on an initial cycling of the instrument, a small phase difference is detected, thus improving noise immunity. The output from flip-flop phase detector 204 in response to an enable signal from unit 200 is a logic level pulse waveform, the pulse width of which is a linear function of the phase difference between its input signals from 0 to 360 degrees. The AND gate phase detector 206 provides, in response to an enable signal, an output signal which is proportional to the phase signal which is proportional to the phase difference between its input signals, and has a duty cycle which is a maximum (i.e. 50%) at zero phase difference, and a minimum (i.e. zero) at a phase difference of 180 degrees. These output signals are always related to an integral number of phase comparisons, and are applied to separate inputs of OR gate 210. The resulting output is applied, along with 15 MHz modulation signal G 1-2 , through AND gate 212, as a clock signal, to accumulator 214, which comprises a chain of six binary coded decimal decade counters arranged in ripple-through configuration. Pulses occurring at the output of OR gate 210 are counted by counter 216, which detects the 250th pulse of a sequence, and provides the phase detector flag, for unit 200 which then reads the accumulator output. Digital control and arithmetic unit 200 contains a digital representation of the sequential steps necessary to effect a distance measurement, in accordance with a detailed flow chart (Figs. 4A, 4F, not shown). The control functions are performed by selecting, in a programmed manner, various ones of enable lines 218 in response to the information observed on flag return lines 220. Unit 200 also performs various mathematical operations upon the data received from the accumulator 214, and an environmental sensor 222, required in calculating the final result of the distance measurement. These operations include an iterative process in which unit 200 stores samples of the detected phase difference from accumulator 214 during a period of reference path signal, and subtracts the average of these from the average of the samples taken during the following target path signal. The resulting quantity is proportional to the distance to be measured, subject to a known multiplicative constant. Each such process constitutes a system sample and the value of each system sample is stored. The unweighted arithmetical average and the statistical variance of any N system samples is calculated and subjected to tests, when N is a multiple of 4, to compare these calculated values with preprogrammed values of variance. If the obtained values repeatedly fail this test up to the value of N = 64 samples, the system switches off and indicates that the measurement conditions are too noisy. Otherwise, when the test has been passed, the values so obtained are further processed to yield a signal which operates display unit 224 with the calculated value of the distance within the preprogrammed specified degree of accuracy.