GROUND VELOCITY SENSOR WITH DROP-OUT DETECTION

Abstract: A transmitter (22) emtis a signal at the frequency of an oscillator (32). A reference phase-locked loop (34) provides a corresponding reference velocity voltage (Vt). The reflected signal is received by a receiver (24) and processed by a second phase-locked loop (48) which provides a Doppler velocity votlage (Vr) which is corrected by subtracting (58) the reference voltage (Vt). In order to avoid spurious response by the second PLL (48) a drop-out detector (46) provides a drop-out signal (D/O) when the amplitude of the received signal (fr) is below a threshold (V th ). The drop-out signal causes a low-pass filter (52) in the second PLL (48) to hold its output unchanged for the duration of the drop-out. A digitally implemented embodiment is also disclosed.

Description: This invention relates to a sensor for sensing the relative velocity between one object moving relative to another, such as the ground velocity of a vehicle, such as an agricultural vehicle. On most agricultural and off-highway equipment in use today, vehicle speed or velocity is sensed by a magnetic pickup which senses the wheel speed. However, there are problems with this measurement technique. For example, the rear or driving wheels can slip relative to the ground, therefore producing erroneous ground speed readings. When sensing ground speed from the front wheels, the front wheels may be off the ground at times, thus not rotating at true ground speed. As a result of steerage, the front wheels may not track ground speed because of wheel skidding. Because of the inaccuracies introduced by this type of ground speed measurement, it cannot be effectively used as an input parameter for closed loop control systems such as spraying, planting, and other implement or vehicle controls. To solve these problems, Doppler-type ground speed sensing systems have been proposed. The velocity, Vg, can be determined from the frequency shift between the received and transmitted signals,Δf, by a modified Doppler equation: (1) Vg = (c Δf) ÷ (ft 2 Cos a ) where a is the angle of the signal transmission axis from horizontal, where c is the speed of sound, and where ft is the transmitted signal frequency. A proposed Doppler-type ultrasonic ground velocity sensing system is described in U.S. Patent Application, Ser. No. 609,626 (12676-US) filed 14 May 1984 and assigned to the assignee of the present application. This and other speed sensing systems utilize phase-locked-loops (PLL) in the signal processing circuitry. In such systems, as also disclosed in European Patent Application EP-A-0162646 which claims priority from U.S. Patent Application Ser. No. 609626, the magnitude of the received signal can fluctuate considerably in amplitude due to variations in the reflectivity of the ground and due to momentary destructive interference among wave fronts from the various reflecting areas of the ground. Large fluctuations in the receive signal magnitude can cause periods of "drop-out" during which the received frequency is not detectable because the received signal magnitude is too low. These periods of signal drop-out can cause such systems to produce an erroneous or biased velocity output signal. To be more specific, the low magnitude receive signal can cause the PLL to drop out of its locked state, with the result that the output voltage from the PLL goes to zero. Thus, in operation, the PLL output signal will fluctuate between zero and a correct indication of ground speed. When this output signal is averaged or filtered, the result will be an erroneously low indication of ground speed. U.S. Patent Ser. No. 4,359,734 discloses a Doppler velocity detector employing a phase locked loop 41 at the receiver. This phase locked loop is used to track at a frequency which is intermediate of the transmitted and received frequencies. International Application No. WO 83/01120 discloses a dropout compensation circuit for a Doppler velocity sensor in which it is the Doppler shift which is tracked. According to the disclosure of this document, a switch 52 operates in a dropout to disconnect a comparator output to an integrator. A dropout is detected as the absence of pulses on an input. Summary of the Invention An object of the present invention is to provide a speed sensing system with a simple and reliable system for compensating for fluctuations in the amplitude of the reflected or received signal. Another object of the present invention is to provide a Doppler-type speed sensing system which is insensitive to transmit frequency and component drift. Another object of the present invention is to provide a Doppler-type speed sensing system with a signal drop-out detecting capability which does not have timing problems. These and other objects are achieved by the present invention as defined in claim 1 which includes an transmitter and receiver and a signal processing circuit. The signal processing circuit includes a pair of phase-locked-loop (PLL) circuits, the first receiving the transmit frequency, the second receiving the reflected frequency. A drop-out detect circuit has a comparator which receives the reflected signal and compares its rectified and averaged amplitude to a threshold. When this rectified and averaged amplitude drops out or falls below the threshold, the comparator turns on and it opens a switch in the second PLL so that the output of the second PLL remains substantially unchanged for the duration of the drop-out condition. An output signal which represents the ground velocity (speed and direction) is derived from the difference between the outputs of the first and second PLLs. The result is that the output signal is substantially unaffected by the reflected signal drop-out condition. These and other objects are also achieved by a preferred embodiment of the present invention which includes a master clock from which all timing is derived, a transmit amplifier and transducer, a receive amplifier and transducer, a digital frequency detector and a signal dropout sense and hold circuit. The master clock is divided down to the transmit frequency (40 kHz is preferred) to drive the transmit amplifier and transducer. The received echo is amplified, squared, and sent to the frequency detector, where the difference between its period and that of the transmit signal is measured. This difference is proportional to speed. The frequency detector contains two down counters. One is driven by the receive signal, the other is driven by a signal whose frequency is derived from the master clock. The receive counter initializes and turns on and off the clock counter. During each measurement interval, the clock counter measures the number of clock cycles that occur during a particular number (1280) of receive cycles. Since the transmit frequency is also derived from the master clock, each count of the clock counter represents a fixed fraction of a transmit cycle. Therefore, the final count represents the difference between the transmit and receive periods in terms of the transmit period , not in terms of an independent reference. This makes the detector output insensitive to drifts in the master clock frequency or to changes in the characteristics of circuit components. At the end of each measurement interval, the final count is latched for output and the counting process begins again. The drop-out detector monitors the amplitude of the receive signal. Thus, in both embodiments, the variable amplitude reflected signal is received by an input of a drop-out detection circuit. Drop-out is considered to occur when this amplitude drops below the minimum level for which true Doppler shift can reliably be detected. When the drop-out condition occurs in the preferred embodiment, the clock and receive counters are inhibited and the frequency detector waits until the drop-out condition ceases. At this time, counting resumes. The present invention is distinguished from the prior art U.S. Patent Ser. No. 4,359,734 and International Application No. WO 83/01120 at least by the features that the phase locked loop operates at the frequency of the received signal itself and in that the drop-out detector is responsive to the level of the received signal to detect drop-outs when the signal level falls below a predetermined threshold. Brief Description of the Drawings Fig. 1 is a simplified schematic of a speed sensing system. Fig. 2 is a schematic block diagram of one embodiment of the signal processing circuit of Fig. 1. Figs. 3 - 7 are detailed circuit diagrams of various portions of Fig. 2. Detailed Description The ground speed sensing system 10 includes a continuous ultrasonic transmitter and receiver unit 12 for mounting on a vehicle lower frame member 14. The unit 12 has transmit and receive horns 16 and 18, preferably aimed in a forward direction at an angle α of approximately 37 degrees from horizontal. A signal processing unit 20 provides an ultrasonic transmit signal T to the unit 14 and receives a signal from the unit 14. The horns could be conventional ultrasonic horns, but preferably would be similar to the horn assembly described in U.S. Patent application, Ser. No. 660,819, filed 15 October 1984 (12817), and assigned to the assignee of the present case. The horns 16 and 18 are coupled to a corresponding conventional ultrasonic transmitter 22 and receiver 24. The transmitter 22 preferably transmits continuously at a transmit frequency ft of 40 kHz. The output of the receiver 24 will be a reflected signal R with frequency fr. Cross talk between the transmitter and receiver is preferably eliminated by using cork to accoustically isolate the horns 16 and 18 from each other or by suspending the horns via known rubber "shock mounts" by placing a sonic baffle or partition between the horns and by using cork to accoustically enclose portions of the horns from which ultrasonic energy could otherwise escape. If the horns 16 and 18 are aimed towards the ground in the forward direction with respect to vehicle motion, then the reflected signal, R, will have a reflected frequency, fr, which will be higher than ft if the vehicle and system 10 are moving forward. Conversely, fr will be lower than ft if the vehicle and system 10 are moving in reverse. As best seen in Fig. 2, the signal processing circuit 20 includes an oscillator 32 which provides the 40 kHz frequency. Circuit 20 also receives the output R of the receiver 24. The transmit signal T is sent to the transmitter 22 and to a reference phase-locked-loop (PLL) circuit 34. The reference PLL 34 includes a phase comparator 36, a low pass filter 38 and a voltage controlled oscillator 40, connected as shown. The reference PLL 40 provides a reference voltage, Vt, at 42 which is proportional to the transmit frequency ft, less 40,000 Hz. The output (R) of receiver 24 is received by an input amplifier 43. The output of input amplifier 43 is coupled to a squaring amplifier 44 and by an amplitude comparator or "drop-out" detector circuit 46. The circuit 46 rectifies the R signal and compares the average amplitude of the rectified R signal to a threshold voltage Vth, such as 2 volts, for example, which is slightly above the amplitude at which the PLL drops out of lock. The output of circuit 46 is normally low, but goes high during periods when the output of receiver 24 is low or "drops out". The output of squaring amplifier 44 is sent to another or receive phase-locked-loop detector 48. The receive PLL detector 48 includes a phase comparator 50, a filter circuit 52 with a hold switch controlled by the output of circuit 46, and a voltage controlled oscillator 54. Thus, PLL 48 provides at 56 a voltage Vr which is proportional to the frequency fr of the receive signal R, less 40,000 Hz. The voltages Vt and Vr are coupled to a difference amplifier 58 which generates an output voltage Vo which is proportional to f = (fr - ft) , which, in turn, is proportional to the vehicle ground speed Vg. Preferably, the reference PLL 34 will be mounted near the receive PLL 48 and will have the same type of temperature sensitive components. Since reference PLL 34 is directly driven by the transmit signal T and since outputs of PLLs 34 and 48 are subtracted from each other, this will eliminate drift in the output signal Vo due to temperature induced drift in the oscillator 32 or in components coin to PLLs 34 and 48. For further details relating to the circuit of Fig. 2, the reader is referred to Figs. 3 - 7. Beyond this, it should be pointed out, as best seen in Fig. 7, that the receive PLL 48 includes an integrating amplifier IC10 (with feedback capacitor C4). The output of phase comparator 50 is coupled to an input of the integrating amp IC10 via voltage control switch IC1 and to a proportional amplifier (IC7). Switch IC1 operates so that when the output from circuit 46 is low (when no signal drop-out condition exists) then the input of IC10 is coupled to the output of phase comparator 50 so that the output Vr of IC10 will continuously vary in magnitude proportional to changes in the frequency of received signal R. However, when the magnitude of R "drops out", then the output of drop-out detector cirucit 46 goes high, causing switch IC1 to disconnect the output of phase comparator 50 from the input of IC10. Thus, when received signal drop-out occurs, the output Vr of IC10 will remain unchanged until the drop-out condition ceases. The output of proportional amp IC7 and the output Vr of IC10 are summed together by inverter and summing amp IC8. The output of IC8 is coupled to an input of VCO 54 to complete the phase locked loop circuit 48.

Claims: A Doppler velocity sensor comprising a transmitter (22) for transmitting a signal of a transmission frequency (ft) to an object moving relative thereto, means (32, 36, 38, 40) providing a first signal (Vt) proportional to the difference between a predetermined frequency and the transmitted frequency (ft), a receiver (24) for receiving a Doppler-shifted signal (R) reflected by the object, and a circuit (48) including a phase locked loop (50, 52, 54) responsive to the received signal (R) from the receiver (24) to provide a second signal (Vr) related to the frequency (fr) of the received signal (R) and means (58) for combining the first and second signals to generate an output signal (Vo) representative of the velocity of relative movement (Vg) corresponding to the Doppler shift (Δf), characterized in that the phase locked loop (50, 52, 54) locks to the received frequency (fr) itself to provide the second signal (Vr) proportional to the difference between the predetermined frequency and the received frequency (fr), the second signal (Vr) being the control voltage to a voltage controlled oscillator (54) of the phase locked loop (50, 52, 54), a drop-out detector (46) is provided which is responsive to the level of the received signal (R) to detect dropouts in this signal when the signal level falls below a predetermined threshold (Vth), and the phase locked loop (50, 52, 54) is controlled by the drop-out detector (46) to hold the output signal (Vr) unchanged during a drop-out. A Doppler velocity sensor according to claim 1, characterized in that the drop-out detector comprises a rectifier (D1, D2) for rectifying the reflected signal (R), smoothing means (IC6, C9) for smoothing the rectified signal, and a comparator (IC11) for comparing the rectified and smoothed signal with the threshold (Vth). A Doppler velocity sensor according to claim 1 or 2, characterized in that the phase-locked loop (48) comprises a voltage controlled oscillator (54), a phase comparator (50) comparing the output of the oscillator (54) with the reflected signal (R) an integrator (IC10, C4) integrating the output of the comparator (50) to develop a control voltage for the oscillator (54), and a switch (IC1) controlled by the drop-out detector (46) to disconnect the output of the comparator (50) from the integrator (IC10, C4) during the presence of a drop-out. A Doppler velocity sensor according to claim 3, characterized in that the integrator (IC10, C4) provides the output signal (Vr) with a magnitude corresponding to the speed of relative movement. A Doppler velocity sensor according to claim 3 or 4, characterized by a summing amplifier (IC8) which adds a signal proportional to the output of the phase comparator (50) to the output from the integrator (IC10, C4) to provide the control voltage for the voltage controlled oscillator (54). A Doppler velocity sensor according to any of claims 1 to 5, characterized by a second phase-locked loop (34) locked to the frequency of the transmitted signal (T) and providing the reference signal (Vt).

Inventors: FERGUSON, LARRY WAYNE, PENFOLD, GARN FARLEY, PHELAN, JAMES JOSEPH
Applicants: DEERE & COMPANY
Application year: 1985
Main classification: G01S15
IPC codes: G01S13, G01S15