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Ultrasonic Ground Speed Sensor
This project, an ultrasonic ground speed sensor from
1987, offers an unusual usage of the the ultrasonic transmit receive transducers.
Instead of the usual distance measuring scenario this design uses the Transmit and Receive Transducers pointed (at an angle) to the ground allowing detection of a receive signal from ground reflections. By arranging subtraction of the transmitted signal it is used as a dopler radar where the increase or decrease in difference frequency is equivalent to the ground speed. In addition the output also indicates the direction of travel.
Executive Summary of the Ultrasonic Ground Speed Sensor
An ultrasonic ground speed sensor has a vehicle-mounted transmitter and a receiver for receiving signals reflected from the transmitter to the receiver by terrain over which the vehicle travels. An attenuated and phase-shifted transmit frequency is superimposed upon an output of the receiver to cancel cross talk, thus providing a substantially pure reflected frequency. A phase-locked loop converts the reflected frequency to a voltage which represents the direction and speed of the vehicle.
Background of the Ultrasonic Ground Speed Sensor
This design relates to a sensor for sensing the ground speed of a vehicle, such as an agricultural vehicle.
Doppler-type ground speed sensing systems are well known. The velocity, v, can be determined from frequency shift, Δf, by a modified Doppler equation:
where c is the signal or wave propagation speed, where α 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.
Summary of the Ultrasonic Ground Speed Sensor
An object of the present design is to provide a speed sensing system which can sense both the direction and magnitude of the speed of a vehicle.
Another object is to provide a speed sensing system 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.
These and other objects are achieved by the present vehicle speed sensing system which includes an ultrasonic transmitter and an ultrasonic receiver for receiving a signal reflected from terrain over which the vehicle travels. At zero speed, the receiver output frequency is equal to the transmit frequency.
Because the reflected frequency is directly detected, the zero speed situation can be distinguished from the signal dropout situation where there is no receiver output. A signal processing circuit superimposes an attenuated, phase-shifted transmit frequency upon the output of the receiver to cancel out cross talk between the transmitter and receiver, thus providing a substantially pure, unmixed reflected frequency.
The signal processing circuit also includes 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 a difference derived by subtracting a constant frequency from the reflected frequency.
The output voltage thus provides an indication of the speed and direction (forwards or backwards) of the vehicle motion.
Description of the Ultrasonic Ground Speed Sensor
The ground speed sensing system 10 for mounting on a vehicle lower frame member 11 includes a conventional ultrasonic (generally 20-200 kilohertz (kHz) transmitter 12, such as which continuously transmits an ultrasonic signal, T, at a preferred transmit frequency, f(t), of 40 kHz. The system also includes a conventional ultrasonic receiver 14.
The receiver 14 receives reflected ultrasonic signals, R, with frequency f(r), which are reflected from the terrain over which the vehicle travels.
Figure 1 : Is a simplified schematic of the present invention for the ultrasonic ground speed sensor
If the transmitter 12 and receiver 14 are aimed towards the ground in the forward direction with respect to vehicle motion (FIG. 1), then the reflected signal, R, 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.
Figure 2a : Is a schematic diagram of the signal processing circuit of fig. 1 for the ultrasonic ground speed sensor
FIG. 2A and 2B show the schematic of the ground speed measurement system. In FIG. 2A, 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 phase-shift circuit 18. Circuit 18 includes resistors R4 and R5, capacitor C2, an op-amp, IC2, feedback capacitor C3, variable phase-adjust 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 R11 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 or modified signal at 24 which is an amplified signal, R', having a substantially pure reflected frequency, f(r).
This reflected frequency f(r) is "unmixed" in the communications technology sense of that term in that the reflected frequency f(r) is produced by circuit 20 which has no homodyne or heterodyne mixer.
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, IC5, and resistors R16 and R17. This is essentially a "squaring" circuit with hysteresis to eliminate low level noise.
Figure 2b : Is a schematic diagram of the signal processing circuit of fig. 1 for the ultrasonic ground speed sensor
The output of circuit 28 is received by a phase-locked-loop (PLL) circuit 30 (FIG. 2B). PLL circuit 30 includes a resistor R21, a pair of diodes D1 and D2, a phase comparator (PC) 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:
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), less a constant. Preferably, the voltage, Vg, is conditioned by a buffer amplifier 40 which includes a capacitor C8, resistors R23 and R24 and an differential op-amp IC6. Finally, the voltage, Vg is conditioned by a noise-limiting, 2-pole, low pass filter circuit 42.
Circuit 42 includes resistors R25 and R26, capacitors C9 and C10 and an op-amp IC7. Preferably, filter circuit 42 has a break frequency of 10 Hz and a damping factor of 0.71. This provides, at the output of IC7, 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 50 identical to PLL 30. This reference PLL 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.
The differential op-amp IC6 generates at its output a voltage which is proportional to the difference between the signals applied to its (+) and (-) inputs. 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:
C1 .0015 MF
C2, C7 .001 MF
C3, C5 100 pF
C4, C10 1 MF
C8 47 pF
C9 3 MF
C6 .01 MF
R1, R2, R3 1 M
R8, R21 10k
R10, R16 2.2k
R11, R26 22k
R23, R26 68k
R6, R9 Variable 0-100k
R16, R13 25k potentiometer
R22 Variable 0-10k
D1, D2 IN914
IC1-5 LF 356 (Motorola or equiv.)
PL(32), VCO(36) CD 4046 (RCA or equiv.)
IC6, 7 MC 34001 (Motorola)
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