GROUND SPEED SENSOR

Abstract: The sensor comprises an ultrasonic transmitter and re­ ceiver and a processing circuit in which a phase-locked loop tracks the received frequency in order to indicate the speed of a vehicle over the ground. A circuit (18) provides an at­ tenuated and phase-shifted replica of the transmitted signal. This is combined in a circuit (20) with the received signal (14) to cancel out a cross-talk component in the received signal. A low pass filter (26) and squaring circuit (28) provide a signal (30) for the phase locked loop which is substantially pure at the reflected and received signal frequency. It is therefore possible to compare this with a reference frequency corre­ sponding to zero speed to provide an output signal with sign and magnitude indicating respectively the direction (for­ wards or reverse) and speed of movement of the vehicle.

Description: GROUND SPEED SENSOR This invention relates to a sensor for sensing the ground speed of a vehicle, such as an agricultural vehicle, as set forth in the introductory part of claim 1. Doppler-type ground speed sensing systems are well known. The velocity, v, can be determined from frequency shift, Af, by a modified Doppler equation: v = c( 6 f/2f(t))sec ce where C is the signal or wave propagation speed, wheres is the angle of the transmission axis from horizontal, and where f(t) is the transmitted signal frequency. Conventional Doppler radar systems include a homodyne detector which mixes the transmitted signal with the received signal, thereby producing a frequency which is proportional to the absolute value of the difference between the transmit and receive frequencies. With such systems, it is impossible to distinguish between forward and backward motion. Furthermore, at low speeds, fluctuations due to waving of grass or vehicle pitching can cause the average indicated speed to be greater than the average actual speed. Another problem with homodyne-type systems is that the response time of the output is proportional to ground speed (frequency shift). At low vehicle speeds, this slow response time may cause inaccuracies. Finally, such a system may yield ambiguous information because a zero frequency output signal from the receiver may be the result of zero ground speed, or it could be the result of a signal "dropout" condition. An object of the present invention is to provide' a speed sensing system which can sense both the direction and magnitude of the speed of a vehicle and which has no ambiguity with respect to zero speed and signal dropout. A further object is to provide a speed sensing system which has a response time which is independent of vehicle speed. The sensor according to the present invention is characterised in the manner set forth in claim 1. At zero speed, the receiver output frequency is equal to the transmit frequency. Because the reflected frequency is directly detected, the ero speed situation can be distinguished from the signal dropout situation where there is no receiver output. The signal processing circuit can include a phase-locked loop circuit which receives the pure reflected frequency and which generates an output voltage with an amplitude and sign corresponding to the amplitude and sign of the difference derived by subtracting the constant frequency from the reflected frequency. The output voltage thus provides an indication of the speed and direction (forwards or backwards) of the vehicle emotion. The invention will be described in more detail, by way of example, with reference to the accompanying drawings, in which: Fig 1 is a schematic diagram of a Doppler ground speed sensor. Figs 2a and 2b are schematic diagrams of the signal processing circuit of an embodiment of the invention. The ground speed sensing system 10 for mounting on a vehicle lower frame member 11 includes a conventional ultrasonic (generally 20 - 200 kHz) transmitter 12, such as which continuously transmits an ultrasonic signal, T, at a preferred transmit frequency, f(t), of 40 kEi. The system also includes a conventional ultrasonic receiver 14. One such transmitter and receiver is available from the Southwest Research Institnte and is described in SAE Paper 790510 by Gorfs and Benison. However, an equivalent transmitter and receiver would also suffice. The receiver 14 receives reflected ultrasonic signals, R, with frequency f(r), which are reflected from the terrain over which the vehicle travels. If the transmitter 12 and receiver 14 are aimed towards the ground in the forward direction with respect to vehicle motion, then the reflected signal, X, will have a reflected frequency, f(r), which will be higher than f(t) if the vehicle and system 10 are moving forward. Conversely, f(r) will be lower than f(t).if the vehicle and system 10 are moving in reverse. In actuality, the output of the receiver will contain an f(r) component and a "cross talk" component "C" with frequency f(t) due to cross talk between the transmitter 12 and the receiver 14. A signal processing circuit 15 includes a buffer 16 which receives the transmit signal, T, from a pre-amp (not shown) of the transmitter 12. Buffer 16 includes a capacitor C1, three identical resistors, R1, R2 and R3 and an integrated circuit operational amplifier IC1. The output of buffer 16 is coupled to an attenuation and phaseshift circuit 18. Circuit 18 includes resistors R4 and R5, capacitor C2, an op-amp, IC2, feedback capacitor C3, variable phaseadjust resistor R6 and voltage divider resistors R7 and R8. Circuit 18 attenuates and phase shifts the transmit signal, T, and provides a phase-shifted and attenuated output, T'. The T'signal is applied to a variable (amplitude adjusting) resistor R9 of summing circuit 20. Summing circuit 20 also includes resistors R10 and Ril and op-amp IC3. Resistor R10 receives the output (R + C) of receiver 14 so that the receiver output can be superimposed on the T' signal. Preferably, the phase and amplitude of T' is such that it just cancels out the cross talk component C of the receiver output, leaving a voltage at 24 which is an amplified signal, R', having a susbtantially pure reflected frequency, f(r). This signal R' with the pure reflected frequency, f(r), is received by a low pass filter and amplifier circuit 26. Circuit 26 includes capacitors C4 and C5, a comparator input, bias adjust potentiometer R13, resistors R12, R14 and R15 and an op-amp IC4. The output of circuit 26 is received by a comparator circuit 28 which includes an op-amp, ICS, and resistors R16 and R17. This is essentially a "squaring" circuit with hysteresis to eliminate low level noise. The output of circuit 28 is received by a phase-locked-loop (PLL) circuit 30. PLL circuit 30 includes a resistor R21, a pair of diodes D1 and D2, a phase comparator (PC3 32, a low pass filter 34, (including resistor R18 and a capacitor C6) and a voltage controlled oscillator (VCO) 36. The VCO 36 preferably includes resistors R19 and R20, variable resistor R22 and capacitor C7 and is powered by +8 and -8 volts. The PLL 30 provides a voltage, Vg, at 38, which has a magnitude and sign corresponding to the magnitude and sign of the difference: f(r) - f constant, where f constant will be equal to f(t) neglecting drift in f(t) and in other frequency-determining components of the PLL. As an alternative to A PLL, a slope detector with a low pass filter characteristic could be used instead. This difference is, in turn, proportional to the ground speed of the vehicle (not shown). Preferably, the voltage, Vg, is conditioned by a buffer amplifier 40 which includes a capacitor C8, resistors B23 and R24 and an op-amp 1C6. Finally, the voltage, Vg, is conditioned by a noise-limiting, 2-pole, low pass filter circuit 42 Circuit 42 includes resistors B25 and R26, capacitors c9 and ClO and an op-asp IC7. Preferably, filter circuit 42 has a break frequency of 10 Ez and a damping factor of .71. This provides, at 44, an output voltage, Vv, with a magnitude representing the speed of the vehicle and with a sign representing the direction of vehicle travel (forward or reverse). Of course, by providing an offset level, then the direction information could be represented by whether the output voltage is above or below the offset level, not by whether it is positive or negative. An optional feature would be to provide an additional or reference PLL 5Q identical to PEL 30. This reference P$L 50 would receive the transmit signal T from transmitter 12 and would provide a reference output voltage Vt which would be proportional to f(t), less a constant. This reference voltage Vt would be coupled to the (-) input of IC6 via a resistor R26. In this manner, drift in the transmit frequency or in the PLLs 30 and 50 would be cancelled and the voltage Vv would be relatively uneffected by component drift. Suggested component values for the circuit 15 are listed in the following table: Capacitors C1 .0015 mF C2, C7 .001 mF C3, C5 100 pF C4, C10 1 mF C8 47 pF C9 3 mF C6 .01 mF Resistors (Ohm) Diodes R1, R2, R3 1M D1, D2 IN914 R4 3.9 k R5 75 k R7,R14,R17 100 k R8, R21 10 k R10, R16 2.2 k R11, R26 22 k R12 2 k R15 39 k R18 6.8 k R19 390 k R20 62 k R23, R26 68 k R24 18 k R25 4.7 k R6, R9 Variable O - 100 k R16, R13 25 k potentiometer R22 Variable O - 10 k ICs IC1 - 5 LF 356 (Motorola or equiv.) PL(32),VC0(36) CD 4046 (RCA or equiv.) 1C6, 7 MC 34001 (Motorola).

Claims: CLAIMS 1. A DopZler ground speed sensor for a vehicle, comprising a transmitte (12) for transmitting a signal with a given transmit frequency, a receiver (14) for receiving a reflected signal having a shifted frequency dependent on the speed and direction of movement of the vehicle, and a processing circuit (30) responsive to the received signal to provide a signal (Vv) representing the speed of the vehicle, characterised by a circuit (18) for producing an attenuated and phase-shifted replica of the transmitted signal, means (R9, RIO) for combining the received signal and the replica signal so as to cancel a transmitted signal cross-talk component in the received signal, thereby to provide an unmixed signal with substantially the pure reflected frequency, and in that the processing circuit (15) generates an output signal (Vv) with an amplitude proportional to the difference between the reflected frequency of the umaixed signal and a predetermined constant frequency, hereby the output signal indicates both the speed and the direction of movement of the vehicle. 2. A speed sensor according to claim 1, characterised in that the transmitter (12) continuously transmits the transmit frequency. 3. A speed sensor according to claim I or 2, characterised in that the output signal (Vv) has a sign and magnitude corresponding to the direction and speed respectively of vehicle movement. 4. A speed sensor according to claim 1, 2 or 3, the combining means comprises a humming amplifier (IC3), a first input resistor (R9) for the replica signal and a second input resistor (R10) for the received signal, the first input resistor being a variable resistor for amplitude adjustment of the replica signal. 5. A speed sensor according to claim 4, characterised by a lowpass filter (26) having an input coupled to the output of the gumming amplifier (it3), a squaring circuit (28) having an input coupled to the output of the low pass filter and an output coupled to the processing circuit (30). 6. A speed sensor according to any of claims 1 to 5, characterised by a low pass filter (42) receiving the output signal generated by the processing circuit (30) for eliminating noise from the output signal. 7. A speed sensor according to any of claims 1 to 6, characterised in that the processing circuit (30) comprises a phase-locked loop. 8. A speed sensor according to any of claims 1 to 7, characterised in that the transmitted and reflected signals are ultrasonic signals.

Inventors: PHELAN, JAMES JOSEPH
Applicants: DEERE & COMPANY
Application year: 1985
Main classification: G01S13
IPC codes: G01S15, G01S13