WO1986000145A1

WO1986000145A1 - Improvements in speed measurement

Info

Publication number: WO1986000145A1
Application number: PCT/GB1985/000262
Authority: WO
Inventors: Phillip James Mcfarlane
Original assignee: AVIATRICS Ltd
Current assignee: AVIATRICS Ltd
Priority date: 1984-06-12
Filing date: 1985-06-12
Publication date: 1986-01-03
Anticipated expiration: 1986-12-12
Also published as: GB8414975D0; AU4491285A; EP0217797A1

Abstract

A device suitable for use in measuring the speed of relative movement between two bodies, for example the speed of an agricultural vehicle relative to the ground. The device includes a transmitter transducer (2) mounted on the vehicle and arranged to transmit radiation of predetermined frequency, suitable ultrasonic radiation, forwardly and downwardly towards the ground. Radiation reflected from the ground includes a relevant component whose frequency is shifted from the predetermined frequency due to the Doppler effect. The reflected radiation is received by a receiver transducer (3) and an electrical signal from the transducer is applied to a single conditioning circuit (6, 7, 8). This circuit generates an electrical signal whose frequency changes in accordance with slow changes in the relevant component of the received radiation but is substantially uninfluenced by modulation in amplitude or rapid changes in frequency or phase. A signal representing the frequency of the transmitted radiation and a signal from the signal conditioning circuit are applied to a subtractor circuit (9), whose output has an electrical parameter representing a difference in frequency between the two signals applied thereto, and hence representative of the speed of travel of the vehicle. The subtractor (9) circuit does not produce an output in response to changes in the sense of relative movement which do not persist, for example changes resulting from a crop waving in the wind.

Description

IMPROVEMENTS IN SPEED MEASUREMENT

This invention relates to measurements of the speed of relative movements between two bodies.

It is often useful or necessary to measure the speed of relative movement between two bodies, or the distance traversed by one body relative to another, without providing a physical contact, such as a wheel, between the bodies. For example, in measuring the ground speed of a motor car it is inconvenient to provide an extra ground wheel or to make a connection to the existing ground wheels. In the case of a tractor, wheel slip over the ground makes measurements obtained from any device including a ground wheel inaccurate. In the case of a person walking or running or a vehicle running on skids or air cushions, a wheel is not practicable. The same is also true of devices for use in certain industrial processes and machines, for example devices for measuring the speed of travel of paper in a paper mill.

To overcome these problems, it has been proposed to provide a transmitter on one body, to direct radiation from the transmitter towards the other body, and to detect radiation reflected back from the other body towards the transmitter. A beat detector is then used to detect the Doppler shift between the frequencies of the transmitted and reflected radiation. Devices employing radio (radar) and sound (ultrasonic) radiation have been proposed. The proposed devices usually give inaccurate results. For example, devices for measuring ground speed rely for reflected radiation upon irregularities in the surface of the ground, and the amplitude of the reflected signal therefore varies. Since a conventional beat detector is very sensitive to signal amplitude, changes in. the surface of the ground cause errors in the Doppler frequency which is detected. These errors manifest themselves as variations in Doppler frequency against speed over different surfaces and as variations in Doppler frequency against speed itself ie. the devices are non-linear.

In general, ultrasonic devices would be preferred for use on vehicles since radar devices are more expensive and mechanically fragile. However, there is a further problem with ultrasonic devices, namely, direct noise pick-up due to mechanical vibration of the vehicle or to wind blowing on the transmitter and receiver transducers. With a poor reflecting surface such noise can swamp the true reflected signal.

Another problem, particulary with measurements on agricultural vehicles, is that bodies moving in the beams of transmitted and reflected radiation can cause false outputs. Thus, crops blowing in the wind cause an output even if the vehicle itself is stationary. Ultrasonic devices also encounter problems due to moving boundary layers in the air. On the one hand the devices need high sensitivity in order to operate with weak reflected signals, yet on the other hand moving bodies can cause a Doppler output signal even if there is a stronger reflected signal from a static body. According to the present invention there is provided a device suitable for use in measuring the speed of relative movement between two bodies, comprising means for transmitting radiation having a predetermined frequency from one body towards the other body, means for receiving radiation transmitted from the transmitting means and reflected by the other body and generating an electrical signal including a component whose frequency is shifted from the predetermined frequency as a result of relative movement between the bodies, signal conditioning means coupled to an output of the receiving means and adapted to generate an electrical signal of frequency representative of the frequency of the said component, the frequency of the electrical signal generated by the conditioning means changing in accordance with slow changes in frequency of the said component of the received radiation but being substantially uninfluenced by modulation in amplitude or rapid changes in frequency or phase of the received radiation, subtractor means, and means for applying the electrical signal from the conditioning means and an electrical signal of frequency representing the predetermined frequency of the transmitted radiation to the subtractor means, whereby the subtractor means generate an electrical output signal having an electrical parameter which represents a difference between the frequencies of respective electrical signals applied thereto, the said output signal being representative of the speed of the relative movement between the two bodies.

Suitably, the signal conditioning means are adapted to continue generating an electrical signal even during a short interval of time in which no electrical signal is generated by the receiving means.

The conditioning means may comprise a phase comparator, a variable frequency oscillator having a control input and an output, means for applying the electrical signal from the receiving means and an electrical signal from the output of the oscillator to the phase comparator, whereby the phase comparator generates an electrical output signal having an electrical parameter representing a difference in phase or frequency between the two electrical signals applied thereto, and means for applying the electrical output signal from the phase comparator to the control input of the oscillator, whereby the frequency of the electrical signal at the output of the oscillator moves towards the frequency of the said component of the electrical signal from the receiving means.

Alternatively, the conditioning means may comprise a filter adapted to transmit only electrical signals whose frequency lies within a narrow range of frequencies, the centre of which narrow range is variable throughout a predetermined range including the predetermined frequency in accordance with a voltage applied to a control input of the filter, and a frequency/voltage converter coupled between an output of the filter and the control input and adapted to generate a voltage representative of the frequency of the electrical signal at the output of the filter.

According to the invention there is also provided a device suitable for use in measuring the speed of relative movement between two bodies, comprising means for transmitting radiation having a predetermined frequency from one body towards the other body means for receiving radiation transmitted from the transmitting means and reflected by the other body and generating an electrical signal including a component whose frequency is shifted from the predetermined frequency as a result of relative movement between the bodies, subtractor means, and means for applying the electrical signal from the receiving means and an electrical signal of frequency representing the predetermined frequency of the transmitted radiation to the subtractor means, the subtractor means being adapted to generate an electrical output signal having an electrical parameter which represents a difference between the frequencies of respective electrical signals applied thereto, and hence the speed of relative movement between the two bodies, as long as the sense of the said difference remains constant, but the subtractor means being further adapted to produce no output signal in response to changes in the sense of relative movement between the two bodies which do not persist.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a block diagram of the device according to the invention for measuring the ground speed of a vehicle; and

Figures 2, 3, 4 and 5 are modifications of the device shown in Figure 1.

Figure 1 of the drawings shows a device according to the invention which is suitable for use in measuring the ground speed of an agricultural vehicle. Measurement is effected by transmitting radiation downwardly and forwardly of the vehicle, detecting radiation reflected back from the ground or bodies on the ground, and determining the Doppler shift between the frequencies of the transmitted and reflected radiation. The radiation generated by the device may be ultrasonic radiation of about 50KHz, radar of 10GHz to 24GHz, laser radiation modulated at 3 or 4GHz or unmodulated laser radiation of about 800 urn.

Referring now to Figure 1, the present device includes an oscillator 1 for generating an electrical driving signal for a transmitter transducer 2. The transducer 2 is, in use, mounted at the front of the vehicle and provides a beam of radiation which is directed downwardly and forwardly to a location a short distance in front of the vehicle.

For receiving radiation reflected back from the ground, a receiver transducer 3 is mounted on the vehicle adjacent to the transducer 2. An output of the receiver transducer 3 is connected to a signal conditioning circuit formed of a band pass filter 4, a limiting amplifier 5 connected to an output of the filter, and a phaselocked loop connected to an output of the amplifier. The phase-locked loop contains a phase comparator 6, a low pass filter 7 and a voltage controlled oscillator 8. As shown in Figure 1, the phase comparator 6 has two inputs, one connected to the output of the amplifier 5 and another connected to an output of the oscillator 8. An output of the comparator 6 is connected via the filter 7 to a control input of the oscillator 8.

A subtractor 9, which could be a simple phase detector followed by a low pass filter, has one input connected to the output of the oscillator 9 and another input connected to the output of the oscillator 1.

In use of the present device, the transducer 2 transmits radiation of frequency equal to the predetermined frequency of the driving signal from the oscillator 1. As described above, this radiation forms a beam which is directed downwardly and forwardly towards the ground.

Radiation which is reflected from the ground and then impinges upon the receiver transducer 3 includes a component which is shifted from the predetermined frequency as a result of relative movement between the vehicle and the ground. Electrical noise and frequencies outside the range of interest are also present. The transducer 3 generates an alternating electrical signal having frequency components corresponding to the frequency components in the incoming radiation. The electrical signal from the transducer 3 is applied to the band pass filter 4, which removes the electrical noise and frequency components outside the range of frequencies of interest, and the output of the filter is applied to the limiting amplifier 5. At the output of the amplifier 5 there is a signal whose amplitude remains constant as long as the level of the incoming radiation remains above a predetermined minimum value .

The phase comparator 6 receives the output signal from the amplifier 5 and an output signal from the oscillator 8 and produces a control signal whose d.c level at any instant represents any difference in phase between these two output signals. The control signal from the phase comparator 6 is applied via the low pass filter 7 to the control input of the oscillator 8.

Assuming the frequencies of the output signals from the amplifier 5 and the oscillator 8 are different, the d.c. level of the control signal from the comparator 6 varies in accordance with the varying phase difference between the two output signals. The control signal from the comparator 6 is then a beat signal. When applied to the oscillator 8 this beat signal causes the output signal from the oscillator to vary until its frequency becomes equal to the frequency of the output signal from the amplifier 5. The control signal is then maintained at a steady d.c. level which represents the constant phase difference between the output signals from the amplifier 5 and the oscillator 8.

The subtractor 9 receives the electrical driving signal from the oscillator 1, which has a frequency equal to the frequency of the transmitted radiation, and the output signal from the oscillator 8, which has a frequency equal to the frequency of the relevant component of the incoming reflected radiation (ie. the component whose frequency is shifted from the predetermined frequency). At the output of the subtractor 9 there is a signal having an electrical parameter, ie. frequency, which represents the difference between the frequencies of the two signals received thereby. From this difference signal, the speed of movement of the vehicle over the ground is calculated, using well known equations defining the Doppler shift in frequencies between relatively moving bodies.

It will be appreciated that the effect of the signal conditioning circuit including the phase locked loop is to produce at the output of the oscillator 8 an output signal having substantially the same frequency as the relevant component of the output signal from the amplifier 5« However, the low pass filter 7 does not pass high frequency signals and so noise signals have little effect on the oscillator 8. In fact, the phase locked loop has the ability to track an incoming signal of constant or slowly changing frequency in the presence of considerable amounts of random noise. The signal conditioning circuit as a whole is substantially uninfluenced by modulation in amplitude or rapid changes in phase or frequency of the received radiation. Even if the incoming signal disappears for a short time, the output signal from the loop remains constant until the signal reappears. The noise immunity allows good results to be achieved with low-cost ultrasonic equipment.

Typically, the signal conditioning circuit can respond to changes in the frequency of incoming radiation of, say, 50 to 55KHz which take place in about 200 m.s. More rapid changes have no effect.

Figure 2 of the drawings shows a modification of the device shown in Figure 1. In the device of Figure 2 a phase comparator 21 , corresponding to the comparator 6 of Figure 1, has one input connected to an output of a voltage controlled oscillator (not shown) and a second input connected to a limiting amplifier (also not shown) via a variable phase shifter 22. An output of the comparator 21 is connected to an integrator 24, corresponding to the filter 7 of Figure 1, via a low pass filter 23. A feedback connection extends from an output of the filter 23 to a second input to the phase shifter 22.

The device shown in Figure 2 is designed to deal with the problem of sudden phase changes in the incoming radiation, caused by changes in the locations from which radiation is reflected to the receiver transducer as the vehicle moves forwardly over the ground. With the device of Figure 2, such sudden phase changes are translated into corresponding changes in the phase of the electrical signal applied from the limiting amplifier and the phase shifter to the second input to the phase comparator 21 and changes in the output signals applied from the comparator and the filter 23. A feedback signal is then applied from the filter 23 to the second input to the phase shifter 22, which causes the signal applied from the phase shifter 22 to change in a sense corresponding to a removal of the previous change in the output signal from the comparator 21. As long as there is a phase difference between the signals applied to respective inputs to the phase comparator 21 there is an output signal from the comparator to the filter 23 and the integrator 24. This output signal is effectively integrated by the integrator 24, which gives an output proportional to the time for which a phase difference has lasted. If a phase change lasts for only a short time, however, its effect is rapidly removed by the feedback connection and there is but a small effect on the output from the integrator 24 to the voltage controlled oscillator.

Figure 3 shows an alternative to the low pass filter 7 of Figure 1. The filter of Figure 3 has two R.C. networks, R1-C1 and R2-C2, which have different time constants. It is used with a conventional multiplying phase comparator (balanced modulator).

Figure 4 of the drawings shows a digital circuit which can be used as the subtractor 9 in the device of Figure 1.

The circuit of Figure 4 includes an up-down counter 41 having a clock input CK, a control input U/D and an output C. When clock pulses are applied to the input CK the counter counts upwards if there is a logic 1 signal at the input U/D and downwards if there is a logic O at the input U/D. The output C is normally at a logic O and is only switched to a logic 1 if the count recorded by the counter moves upwards to a predetermined maximim value in a positive direction or to a predetermined maximum value in a negative direction. The output C is returned to a logic O if the signal at the input U/D is switched to place the counter in a condition for counting in the reverse direction. Assuming there is a reverse count, the output C remains at a logic O when the signal at the input U/D is switched back to its previous value.

The control input U/D of the counter 41 is connected to a first input (c) of the subtractor, which is in turn connected to the oscillator 1 in the device of Figure 1. This means that the counter 41 is in a condition to count upwardly or downwardly according to whether the driving signal from the oscillator 1 is on a positive or negative half cycle.

The input ( c) of the subtractor is also connected to a divider circuit 2, whose output switches from a logic O to a logic 1 condition on every fourth occasion that the signal at the input (c) is switched from a logic 1 to a logic O ie. the circuit operates as a 'divide-by-four' circuit. Connected to the output of the divider circuit 2 is a differentiator 5 for converting each transition from a logic O to a logic 1 into a short, positive pulse. The output of the differentiator 5 is connected to a first input to an OR gate 7, whose whose output is connected to a first input to an AND gate 8. An output of the gate 8 is connected to the clock input CK of the counter 41.

A second input (b) of the subtractor is connected to the output of the voltage controlled oscillator Vco in the device of Figure 1, and hence via the phase locked loop, the limiting amplifier 5 and the filter 4 to the receiver transducer 3. This means that the input (b) is at a logic 1 or a logic O according to whether the electrical signal generated by the transducer 3 is on a positive or negative half cycle.

Within the subtractor, the input (b) is connected to a divider circuit 3 which is a divide-by-four circuit operating in the same manner as the divider circuit 2. An output of the circuit 3 is connected to a control input d of a bistable circuit 4, whose clock input is connected to the input (c) of the subtractor. The bistable 4 has an output e which assumes a logic level the same as the logic level at the input d whenever there is a transition from a logic O to a logic 1 condition at the clock input. At all other times the output e remains unchanged.

The output e of the bistable 4 is connected to a differentiator 6, of similar construction and operation to the differentiator 5, and an output of the differentiator 6 is connected to a second input to the OR gate 7.

The output C of the up-down counter 4 is connected via an inverter 9 to a second input of the AND gate 8. Also connected to the output C is a first input of an AND gate 10, which has a second input thereof connected to the output of the OR gate 7. An output of the gate 10 forms an output of the device of Figure 1.

In using the subtractor circuit of Figure 4, there is a pulse at the output of the differentiator 5 each time the output of the divider circuit 2 switches from a logic O to a logic 1 condition. Since this switch at the output of circuit 2 occurs when the signal at the input (c) to the subtractor swtiches from a logic 1 to a logic O, the control input U/D of the counter 41 is then at a logic O and the counter is in a condition for counting down.

Assuming the count recorded by the counter 41 has not fallen below the predetermined negative maximum value, the output C of the counter is at a logic O, the output of the gate 9 is at a logic 1, and the gate 8 is therefore enabled. The pulse from the differentiator 5 is therefore able to pass through the gate 8 to the clock input CK of the counter 41, where it causes the counter to count down by '1'.

If the count recorded by the counter 41 has reached the predetermined negative maximum value the output C is at a logic 1 condition. The output of the inverter 9 is then at a logic O, which means that the gate 8 is closed but the gate 10 is enabled. A pulse from the differentiator 5 and the gate 7 is then passed via the gate 10 to the output of the subtractor.

As regards the signal at the input (b) to the subtractor, the output e of the bistable 4 only changes state when the signal at the input (c), which is applied to the clock input d of the bistable, switches from a low to a high condition. Assuming the frequencies of the signals at inputs (b) and (c) are approximately the same, then since the signal at input (b) has passed via the divider circuit 3. there is a change from a logic O to a logic 1 at the output e of the bistable 4 every fourth time the signal at the clock input switches from a low to a high condition. This change at the output of the bistable 4 causes a positive pulse at the output of the differentiator 6, which is applied via the gate 7 to the first input of the gate 8. At this time the counter 41 will be set to count upwards, since the signal from the input (c) at its control input U/D is in logic 1 condition. Accordingly, assuming the count recorded by the counter 41 has not reached the predetermined positive maximum value, so that the output of the counter is in a logic 0 condition and the gate 8 is enabled, the positive pulse is applied to the clock input CK of the counter and the recorded count increases by '1'. If the count recorded by the counter 41 has reached the positive maximum value, the pulse from the differentiator 6 is applied via the gate 10 to the output of the subtractor. In the result, each pulse derived from the signal at the input (c) causes the counter 41 to count down until the count reaches the predetermined negative maximum value . Further pulses are then applied to the output of the 3ubtractor via the gate 10. Each pulse derived from the signal at the input (b) causes the counter 41 to count upwards until the predetermined positive maximum count is reached. Further pulses are then likewise applied to the output of the subtractor.

When the vehicle carrying the present device is moving forwardly over the ground, the frequency of the pulses derived from the input (b) is greater than the frequency of pulses derived from input (c). The count recorded by the counter 41 eventually reaches a predetermined positive maximum value. Most of the time, the pulses derived from the input (b) alternate with pulses, from the input (c). Pulses from the input (c) reduce the count below the positive maximum value and succeeding pulses from the input (b) return the count back to the positive maximum value again. Owing to the higher frequency of pulses derived from the input (b), there is every so often an extra pulse from input (b). At the time when this pulse arrives the output of the counter 41 is at a logic 1, the gate 8 is closed, and the pulse is therefore directed via the open gate 10 to the output of the subtractor. The rate at which pulses appear at the output, which is equal to one quarter of the difference between the frequencies of the signals at the inputs (b) and (c) is representative of the speed of forwards movement of the vehicle.

If the vehicle is reversed, pulses from the input (c) have a higher frequency than the pulses from input (b). The count recorded by the counter 41 then oscillates between the predetermined maximum value and a value just above this.

The output of gate 10 can be connected to an indicating device configured to measure frequency. This frequency, scaled by an appropriate factor, indicates the speed at which the vehicle is travelling.

Alternatively, the indicating device can be configured to count the total number of pulses received, scaled (multiplied) by an appropriate factor, and this will indicate the distance travelled, or area covered.

The output can also be fed to a device such as a wheel slip indicator, or application rate meter, which requires a speed input in the form of a pulse train.

If a vehicle on which the present device is mounted is moving forwardly towards a stationary crop, the pulses from input (b) have a higher frequency than the pulses from input (c). As described above, the count recorded by the counter 41 soon reaches the predetermined positive maximum value. Pulses at a frequency representing the difference between the frequencies of pulses at the inputs (b) and (c) are applied to the output of the subtractor.

If the crop now moves forwardly and rearwardly in the wind at a speed greater than the speed of forwards movement of the vehicle, the sense of the relative movement is reversed during part of each forwards movement of the crop. At that time, the frequency of pulses from input (c) is greater than the frequency of pulses from input (b) and the count recorded by the counter 41 moves down below the positive maximum value. No pulses then appear at the output of the subtractor circuit. When the relative movement between the vehicle and the crop returns to its initial sense, the count returns to the positive maximum value, and pulses are then again applied to the output of the subtractor. The length of the counter is so chosen that during the reversal inthe sense of relative movement, the negative maximum value of the count is not reached.

If the vehicle is stopped and then driven in the reverse direction (ie. there is a change in the sense of relative movement which does persist), the count recorded by the counter 41 moves from the positive maximum value to the negative maximum. Pulses are then again applied to the output of the subtractor circuit.

It will be appreciated that mechanical vibration of the vehicle can cause a reversal in the sense of relative movement in similar manner to the reversal caused by a waving crop. This other reversal is likewise ignored by the device.

It will be appreciated that the transducers 2 and 3, above, can be replaced by a single transducer which serves both to transmit radiation and to receive radiation reflected from the ground or other objects.

In the device of Figure 1, modified to include a subtractor 9 as shown in Figure 4, the transmitter transducer 2 is preferably an ultrasonic transducer which is driven by an electrical driving signal of 50KHz. Assuming the vehicle upon which the device is mounted travels forwardly at 35 miles per hour, the relevant component of the radiation received by the transducer 3 is approximately 55KHz. Output pulses from the subtractor 9 then have a pulse repetition frequency of 1.25KHz, owing to the inclusion of the divide-by-four circuits 2 and

3.

This means that a pulse is generated at the output of the subtractor 9 once during each movement of the vehicle through a distance of about 25 mms. In the counter 7 there are sixty-four counts between the positive and negative maximum counts. Accordingly, crop moving in the wind would need to move through a distance of 1.6 metres before it would cause the subtractor to provide an output pulse representing relative movement in the reverse sense .

It will be appreciated that any pitching motion of the vehicle will alter the effective angle of beam depression and hence the Doppler frequency. In the case of a tractor or other agricultural machine in which the chassis is normally unsprung, any pitching motion would be of a very small order, however in the case where a full tank of liquid is carried, normally behind the rear wheels, the angle of the beam relative to the ground, could change significantly.

For such an application, two transducers (or sets of transducers) would be used, one pointing forwards and downwards, the other rearwards and downwards. If the two Doppler frequencies are then added electronically, the sum will be proportional (to a very close approximation) to the true Doppler frequency and true speed. This is because any pitching movement will increase the angle of one transducer and reduce the other by the same angle.

However, since the horizontal component of the beam is proportional to the cosine of the angle of depression, and such a function is not linearly related to the angular change, a slight error would occur. To take a typical example; if both beams were projected at an angle of 45°to the horizontal, a pitch up of 5° would make the front angle 40° and the rear angle 50°. In this instance the figures would be as follows:-

First Beam Cosine Rear Beam Cosine

45° 0. 707 45° 0. 707 40º 0.766 50º 0.643

Change +0. 059 Change -0. 064

Difference = -0.005

= a change -1- 4%

The maximum pitch angle will rarely exceed 5 though 7° might be momentarily reached under violent acceleration. If therefore the front beam is set at an angle 5° greater than the rear beam, then the effect of 5° pitching will be to change the angles around exactly. (See Figure 2). Angles A1 and B1 become A2 and B2. Since A1 = B2 and A2 = B1, the two sums A1 + B1 and A2 + B2 are equal so that the pitching error is zero.

The maximum error in this case would occur at the halfway point, ie. 2 1/2 degrees of pitch-up and the figures would be as follows:-

Front Beam Cosine Sum of Rear Beam Cosine

Cosines

50º 0. 642788 1 . 349895 45° 0. 7071 07

47.5° 0.67559 1 .351 1 8 47.5º 0.67559

Difference of Cosines = 0. 001 285

- Change of 0.09% In the case of a vehicle pitching downwards as under severe braking, a change of pitch 2° downwards would produce an error of 0.2%, and 3° would produce an error of 0.36%. To sum up, a total pitching movement from +7° to -2° would result in a maximum error of only 0.2%, in addition of course, such conditions are normally transitory so that these small errors become even less significant.

Figure 5 shows an alternative form of signal conditioning circuit. The circuit is an adaptive filter including a band-pass filter 30 having a narrow pass band whose centre frequency can be varied by varying a voltage applied to a control input thereof. Coupled between an output of the filter 20 and the control input is a frequency/voltage converter 31.

When the circuit of Figure 5 is operating, an incoming signal from the transducer 3 of Figure 3 causes an electrical signal of the same frequency to appear at the output of the filter 30. An appropriate voltage to tune the filter 30 to a narrow range of frequencies centred on that frequency is then applied from the converter 31 to the control input of the filter.

If there is a gradual change in the frequency of the incoming signal, there are corresponding changes in the frequency of the output signal and in the voltage from the converter 31. The filter 30 therefore tracks the incoming frequency.

Because of the narrow pass band of the filter 30, the relevant incoming frequency is passed by the filter whilst sideband frequencies due to noise are heavily attenuated.

If the relevant incoming frequency disappears for a short time, the resonant nature of the filter 30 causes the output signal to be sustained.

As described above, the devices shown in the drawings use ultrasonic radiation, at a frequency of about 50KHz. Because the speed of sound in air varies with air temperature, compensation is used to keep accuracy. This temperature compensation operates by increasing the transmitter frequency as the temperature increases.

The transmitter and receiver transducers are both electrostatic types manufactured by the Polaroid Corporation of Massachusetts, USA.

Claims

CLAIMS :

1. A device suitable for use in measuring the speed of relative movement between two bodies, comprising means for transmitting radiation having a predetermined frequency from one body towards the other body, means for receiving radiation transmitted from the transmitting means and reflected by the other body and generating an electrical signal including a component whose frequency is shifted from the predetermined frequency as a result of relative movement between the bodies, signal conditioning means coupled to an output of the receiving means and adapted to generate an electrical signal of frequency representative of the frequency of the said component, the frequency of the electrical signal generated by the conditioning means changing in accordance with slow changes in frequency of the said component of the received radiation but being substantially uninfluenced by modulation in amplitude or rapid changes in frequency or phase of the received radiation, subtractor means, and means for applying the electrical signal from the conditioning means and an electrical signal of frequency representing the predetermined frequency of the transmitted radiation to the subtractor means, whereby the subtractor means generate an electrical output signal having an electrical parameter which represents a difference between the frequencies of respective electrical signals applied thereto, the said output signal being representative of the speed of the relative movement between the two bodies.

2. A device as claimed in claim 1, wherein the signal conditioning means are adapted to continue generating an electrical signal even during a short interval of time in which no electrical signal is generated by the receiving means.

3» A device as claimed in claim 1 or 2, wherein the conditioning means comprise a phase comparator, a variable frequency oscillator having a control input and an output, means for applying the electrical signal from the receiving means and an electrical signal from the output of the oscillator to the phase comparator, whereby the phase comparator generates an electrical output signal having an electrical parameter representing a difference in phase or frequency between the two electrical signals applied thereto, and means for applying the electrical output signal from the phase comparator to the control input of the oscillator, whereby the frequency of the electrical signal at the output of the oscillator moves towards the frequency of the said component of the electrical signal from the receiving means.

4. A device as claimed in claim 3, wherein the means for applying the electrical signal from the receiving means to the phase comparator comprise a phase shifter, the means for applying the electrical output signal from the phase comparator to the control input of the oscillator comprise a low pass filter, and means are provided for connecting an output of the filter to a control input of the phase shifter, whereby the connecting means, upon a sudden change in phase of the electrical signal from the receiving means causing a sudden change in the electrical output signal from the phase comparator, apply a feedback signal to the phase shifter so that the phase of the signal applied from the phase shifter to the phase comparator changes in a sense such as to reduce the said change in the output signal from the phase comparator.

5. A device as claimed in claim 1, wherein the conditioning means comprise a filter adapted to transmit only electrical signals whose frequency lies within a narrow range of frequencies, the centre of which narrow range is variable throughout a predetermined range including the predetermined frequency in accordance with a voltage applied to a control input of the filter, and a frequency/voltage converter coupled between an output of the filter and the control input and adapted to generate a voltage representative of the frequency of the electrical signal at the output of the filter.

6. A device as claimed in any one of the preceding claims, wherein the subtractor means are adapted to generate an electrical output signal having an electrical parameter which represents a difference between the frequencies of respective electrical signals applied thereto, and hence the speed of relative movement between the two bodies, as long as the sense of the said difference remains constant, but the subtractor means being further adapted to produce no output signal in response to changes in the sense of relative movement between the two bodies which do not persist.

7. A device as claimed in any one of the preceding claims, wherein the subtractor comprises means for generating a train of pulses whose repetition frequency represents the predetermined frequency of the transmitted radiation, second means for generating a train of pulses whose repetition frequency represents the frequency of the electrical signal from the conditioning means, and means for applying each train of pulses to a counter, the counter being adapted to count in one sense upon receipt of a pulse from one train and to count in an opposite sense upon receipt of a pulse from the other train.

8. A device as claimed in claim 7, wherein the counter is adapted to provide a predetermined output signal if the nett count recorded thereby has reached a predetermined maximum value.

9. A device as claimed in claim 7, wherein the counter is arranged so that when the nett count recorded thereby has reached a first predetermined maximum value in one sense the application thereto of a pulse which would otherwise cause an increase in the nett recorded count produces no change in the nett recorded count, whereas the application thereto of a pulse which corresponds to a decrease in the nett recorded count causes the nett recorded count to fall below the first predetermined maximum value , whereby a change in the sense of relative movement which does not persist does not result in an output signal from the counter.

10. A device suitable for use in measuring the speed of relative movement between two bodies, comprising means for transmitting radiation having a predetermined frequency from one body towards the other body, means for receiving radiation transmitted from the transmitting means and reflected by the other body and generating an electrical signal including a component whose frequency is shifted from the predetermined frequency as a result of relative movement between the bodies, subtractor means, and means for applying the electrical signal from the receiving means and an electrical signal of frequency representing the predetermined frequency of the transmitted radiation to the subtractor means, the subtractor means being adapted to generate an electrical output signal having an electrical parameter which represents a difference between the frequencies of respective electrical signals applied thereto, and hence the speed of relative movement between the two bodies, as long as the sense of the said difference remains constant, but the subtractor means being further adapted to produce no output signal in response to changes in the sense of relative movement between the two bodies which do not persist.

11. A device as claimed in claim 10, wherein the subtractor comprises means for generating a train of pulses whose repetition frequency represents the predetermined frequency of the transmitted radiation, second means for generating a train of pulses whose repetition frequency represents the frequency of the electrical signal from the conditioning means, and means for applying each train of pulses to a counter, the counter being adapted to count in one sense upon receipt of a pulse from one train and to count in an opposite sense upon receipt of a pulse from the other train.

12. A device as claimed in claim 11, wherein the counter is adapted to provide a predetermined output signal if the nett count recorded thereby has reached a predetermined maximum value.

13. A device as claimed in claim 11 , wherein the counter is arranged so that when the nett count recorded thereby has reached a first predetermined maximum value in one sense the application thereto of a pulse which would otherwise cause an increase in the nett recorded count produces no change in the nett recorded count, whereas the application thereto of a pulse which corresponds to a decrease in the nett recorded count causes the nett recorded count to fall below the first predetermined maximum value, whereby a change in the sense of relative movement which does not persist does not result in an output signal from the counter.