GB1478835A - Velocity measuring device - Google Patents

Abstract

1478835 Velocity measurement acoustic reflectivity STIFTELSEN INSTITUTET FOR MIKROVAGSTEKNIK 31 May 1974 [1 June 1973] 24198/74 Heading G1G [Also in Division H4] To enable an object to determine its speed over a reference surface definable by a spacial waveform function e.g. its brightness variation, the object is provided with a plurality, e.g. 2, of sensors, spaced in the direction of the velocity to be measured, said sensors detecting the value of said spacial waveform function in their area of view. The values S11, S21, Fig. 1 of the spacial waveform function at a first sampling instance produced by respective two spaced sensors are stored and a linear interpolation dotted line, made therebetween. At a later instance during which the object has travelled a distance VT, a second sample S12 of the spacial waveform function is made by one of the sensors and the value S12 is compared with the interpolation. The value of the velocity of said object is determined from the position on the interpolation that the value S12 becomes equal to the interpolation. If the spacial waveform function did vary linearly (as the interpolation) the velocity V would be given by the simple equation. In practice however the inter-sample time T is replaced by the time from the instance of first sampling to the 'instance' that the second sample value S12 becomes equal to the interpolation. A first embodiment, Fig. 4, switches G1, F1 close at the first sampling instance to store the sample values S11, S21 in respective capacitors C12, C22. Prior to the next closure of switches G1 and F1, switches G2 and F2 close whereby sample values S11 and S21 are transferred to storage capacitors C11 and C21. The second sample, after time T, is then made by switches G1 and F1 closing again, the sample value S12 then being stored in capacitor C12 and passed through amplifier 1 to appear at one input of a comparator 11. The first sample values S 11 and S21 are supplied to the positive inputs of respective operational high impedance amplifiers 8 and 9 feeding respective ends of linear potentiometer 46. The negative inputs to the operational amplifier are fed by tappings from the potentiometer such that at the first tapping point O a voltage representing the sample S11 is produced and at the second tapping point 1 the sample value S21 appears, the voltage between two tapping points and also between the two outputs of the amplifiers varying in a linear fashion thereby to produce the required interpolation. The 'wiper' of the potentiometer then moves down the potentiometer to produce a linearly varying voltage PT which is compared at 11 with the second sample value S12 to give a state transition signal UC at a time t1 when equality is detected. In practice the potentiometer 46 will comprise a plurality of discrete resistances and the 'wiper' will comprise a switching network actuated at a frequency fo. A pulse K of duration Tl commencing at the start of the potentiometer wiper' movement and terminating at the instance of equality can be produced using logic combinations of the signal UC, a signal P representing the sign difference between the sample S11, S21, and a reset signal R which pulse K when logically combined with a pulse A representing the time for the 'wiper' to sweep through the first tapping point O, produces a pulse of duration Tv proportional to the velocity of the object. The velocity may be indicated in terms of the number of pulses at frequency fo. The above system is most accurate when VT is approximately equal to L, i.e. the second sample S12 is taken at approximately the same position in the spacial waveform function as the sample S21. Such a desirable state however will only be maintained for a very limited range of velocities. To overcome this the counting frequency fo may be automatically controlled such that V = VO where VO = <SP>L</SP>/T. At velocities below a certain velocity Vmin, VO is allowed to remain constant at Vmin and the duration T1 of pulse K, Fig. 5, is accordingly allowed to reduce. Fig. 6 shows a circuit for automatically varying fo wherein the difference between the duration T1 of pulse K and the duration of pulse K for a case of V = VO is determined in terms of the number of fo pulses by means of a differential integrator 12. The output U of said integrator is fed to a non-linear circuit 13, the exponentially varying output I of which is fed to an analog to frequency converter 14 the output of which is the desired varying frequency fo. Fig. 7 (not shown), shows a modification of the circuit of Fig. 4 for a case where the spacing L between the sample sensors is very small and substitutes a pulse K* for the pulse K. To enable the spacing L to be small compared with the smallest wave length in the spacial waveform function it is desirable that the areas viewed by the two sensors should overlap. In the case of optical embodiments, Figs. 9 and 10, the two photo-electric sensors 21 and 22 may view overlapping areas 21' 22' either by means of a semi-transparent mirror 33, Fig. 9, or by laterally offsetting the detectors, Fig. 10. In a micro-wave embodiment Fig. 11 (not shown), three sensors may be used, the output of the middle sensor being combined with that of each of the two outer sensors to produce an equivalent overlap. In an acoustic embodiment for determining the speed of a ship over the sea bed, the sensors may alternatively be switched from a transmitting to a receiving mode Fig. 13 (not shown). To enable the system to work down to zero velocity and then into negative velocity, the areas being viewed by the two sensors may be additionally scanned, e.g. for an optical embodiment, Figs. 17, 18 (not shown) the overlapping sampled areas may be scanned across the surface by means of a rotating prism. This scanning may be alternatively produced by having a plurality, e.g. 10 of sensors Fig. 19, switches SW1, SW2 enabling the samples S1, S2 to be sequentially obtained from successive pairs of the sensors. An acoustic equivalent of the optical system of Fig. 19 is described Figs. 20, 21 (not shown). Instead of producing the two samples S11, S21 simultaneously, they may be produced sequentially, Fig. 22, therefore shows an arrangement comprised by a 5 element sensor, samples from the 5 elements 1 to 5 are stored cyclically at a first sampling occasion in respective capacitors A2, A3, A4, A5 and are then compared with sample values obtained from the same elements at a second sampling occasion. The sample value U21 obtained from sensor element 2 at the first sampling occasion and stored in capacitor A2 is compared in comparator 1 with the interpolated signal between sample values U22 and U12 obtained at the second sampling instance to give the transition signal UC, as in the arrangement of Fig. 4. A polarity signal P is also obtained as before together with signals enabling the automatic variation of the frequency FO.