Doppler-effect type ground speed sensing apparatus

Abstract: Vehicle anti-lock brake control apparatus having wheel speed sensor (20-26) for detecting actual speed of a vehicle wheel, solenoid-operated valve (30) for regulating the wheel braking pressure, and controller (40) for controlling the valve, on the basis of the detected actual speed, and vehicle ground-speed detected by ground-speed sensor (8). The controller controls the valve at a controlled duty cycle, which is determined on the basis of at least a wheel speed error and a time derivative of this error. The error is obtained as a deviation of the detected actual wheel speed from a target wheel speed obtained on the basis of the detected vehicle ground-speed.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a Doppler-effect type ground-speed sensing apparatus employing a technique for improving the accuracy of ground-speed measurement. Discussion of the prior art A conventional ground-speed sensor is disclosed in Japanese Laid-Open Publication No. 63-64861 and includes (a) a transmitter attached to a body of the vehicle and transmitting a detecting wave toward the road surface in a predetermined direction of propagation, (b) a receiver attached to the vehicle body and receiving a portion of the wave which is irregularly reflected by the road surface, and (c) a signal processor which provides an output representative of the ground-speed of the vehicle on the basis of the frequency of the wave transmitted by the transmitter and the frequency of the portion of the wave received by the receiver. The optical axis of the known Doppler-effect type ground speed sensor is inclined in one of the opposite running directions of the vehicle, namely, either in the forward running direction or in the reverse or backward running direction of the vehicle. Moreover, document DE-A-3 825 639 discloses an anti-slip system for vehicles, in which a Doppler-effect type sensor is employed which is directed to the ground in an oblique manner. A control unit determines from the Doppler frequency and by support of signals derived from further vehicle running condition related sensors the sideward slipping, the wheel slip etc. of the vehicle and feeds corresponding countereffecting signals to a servo system. The known ground sensors of this kind are arranged such that the optical axes of both transmitter and receiver are aligned with each other and are parallel to the direction perpendicular to the running directions of the vehicle. These sensors suffer from a large output signal variation when the actual vehicle ground speed is higher than about 100 km/h resulting from a low intensity of the wave received by the receiver at such vehicle speeds and leading to a reduced accuracy of detection of the ground speed of the vehicle. The above drawback of the prior art sensors is derived from the fact that the velocity of an air stream between the transmitter and the road surface increases with an increase in the running speed of the vehicle, and that the tendency of scattering of the wave propagating through the air stream increases with an increase in the air flow velocity, whereby the intensity of the wave received by the receiver decreases with the vehicle running speed. Further, the above drawback arises also from the fact that the movement of the receiver with the vehicle causes a decrease in the intensity of the received wave when the sensor axis is inclined in the backward or reverse running direction. SUMMARY OF THE INVENTION It is therefore an object of the invention to provide a Doppler effect type ground speed sensing apparatus which can accurately detect the ground speed of a vehicle substantially free from the influence of the velocity of the air stream between the sensor and the road surface and the influence of the vehicle speed. According to the invention, this object is accomplished by the features of Doppler-effect type ground speed sensing apparatus as defined in claim 1. Alternatively, this object is accomplished by the features of the Doppler-effect type ground speed sensing apparatus as defined in claim 2. Thus according to the present invention, the receiver of the Doppler-effect ground speed sensor is spaced apart from the transmitter thereof in one of the opposite running directions of the vehicle in which the direction of propagation of the detecting wave from the transmitter toward the road surface is inclined with respect to the downward direction perpendicular to the running directions. That is, if the direction of propagation of the wave is inclined in the forward running direction, the receiver is spaced apart from the transmitter in the forward running direction. If the direction of the wave propagation is inclined in the backward running direction, the receiver is spaced apart from the transmitter in the backward running direction. The above arrangement of the Doppler-effect ground-speed sensor according to the invention is advantageous over the prior art sensors in that the irregularly reflected wave received by the receiver is more similar to the wave regularly reflected by the road surface, than in the known sensor. In other words, the intensity of the irregularly reflected wave received by the receiver of the sensor according to the invention is higher than that received by the receiver of the known sensor. Thus, the frequency of the wave received by the receiver of the sensor according to the invention more accurately reflects the running speed of the vehicle. That is, the output of the receiver is less likely to be subject to the drawback that the intensity of the received wave decreases with an increase in the vehicle speed. Accordingly, the error in the output of the receiver is reduced, whereby the error or variation in the frequency output of the signal processor is consequently reduced, assuring a reduced amount of error in the ground-speed of the vehicle as detected by the ground-speed sensor. The spaced-apart relation of the receiver relative to the transmitter according to the Doppler-effect ground-speed sensor according to the invention is thus substantially free from the influence of the velocity of the air stream between the sensor and the road surface and the influence of the vehicle speed. BRIEF DESCRIPTION OF THE DRAWINGS The above and optional objects, features and advantages of the present invention will be better understood by reading the following detailed description of a presently preferred embodiment of the invention, when considered in connection with the accompanying drawings, in which: Fig. 1 is a side elevational view schematically showing an ultrasonic vehicle ground-speed sensor according to an embodiment of the present invention; Fig. 2 is a side elevational view indicating the relative position between the transmitter and receiver of the ground-speed sensor of Fig. 1; Fig. 3 is a graph indicating amounts of variation in the vehicle ground-speeds detected by the sensor of Fig. 1 and a prior art sensor; and DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 shows a ground-speed sensor 8 which is used to detect the ground-speed U of the vehicle, namely a running speed of the vehicle relative to the road surface, in the running direction of the vehicle. Preferably, the ground-speed sensor 8 is of a Doppler-effect type which utilizes the Doppler effect of an ultrasonic wave. For improved accuracy of detection of the vehicle ground-speed U, the ground-speed sensor 8 is desirably a Doppler-effect type constructed as shown in Figs. 1 and 2. The Doppler-effect ground-speed sensor 8 of Figs. 1 and 2 includes a transmitter 110 which transmits a ultrasonic wave toward the road surface at a predetermined angle of incidence relative to the road surface as indicated in Fig. 2, and a receiver 112 which receives a portion of the ultrasonic wave irregularly reflected by the road surface and generates an output signal in the form of a sine wave representative of a change in the sound pressure of the received wave. The transmitter 110 and the receiver 112 are attached to a rear end portion of the underside of the body of the vehicle, such that the receiver 112 is spaced a distance L apart from the transmitter 110 in the backward running direction of the vehicle, as indicated in Fig. 2. The optical axes of the transmitter and receiver 110, 112 are inclined in the backward direction of the vehicle, at the same angle  with respect to the road surface. The Doppler-effect ground-speed sensor 8 further includes a signal processor 114 electrically connected to the receiver 112. The signal processor 114 receives the output of the receiver 112, and calculates the frequency of the wave received by the receiver 112, from the received output of the receiver 112. Described in detail, the signal processor 114 produces pulses corresponding to those of the peaks of the sine wave represented by the output of the receiver 112, which exceed a threshold level. The frequency of the wave received by the receiver 112 is calculated on the basis of the number of the pulses per unit time. The signal processor 114 further operates to calculate the ground-speed U of the vehicle, according to the following equation, on the assumption that the angle of incidence of the ultrasonic wave from the transmitter 110 upon the road surface, and the angle of propagation of the reflected wave from the road surface with respect to the road surface are both almost equal to the angle of inclination  of the optical axes of the transmitter and receiver 110, 112. U = C·(Ft - Fr)/{(Ft + Fr)·cos} where, C: Sound velocity of the ultrasonic wave Ft: Frequency of the transmitted ultrasonic wave Fr: Frequency of the received ultrasonic wave A known Doppler-effect ground-speed sensor suffers from a considerable increase in the error or variation in the detected ground-speed U, when the actual ground-speed U exceeds about 100km/h, as indicated by solid line in the graph of Fig. 3. This increase in the output variation appears to result from a decrease in the intensity of the wave received by the receiver 112 as the actual ground-speed U increases. That is, when the actual ground-speed U is relatively high, the number of the peaks of the sine wave represented by the output of the receiver 112, which peaks exceed the threshold, tends to decrease leading to the reduction in the number of the corresponding pulses from which the frequency Fr of the wave received by the receiver 112 is calculated. Accordingly, the frequency Fr as calculated by the signal processor 114 tends to be lower than the actual frequency of the received wave. In the Doppler-effect ground-speed sensor 8, the receiver 112 is spaced the predetermined distance L apart from the transmitter 110 in the backward running direction of the vehicle, so that the irregularly reflected wave received by the receiver 112 is more similar to the wave regularly reflected by the road surface than in the known sensor. As shown by dashed line in Fig. 3, the above arrangement of the transmitter and receiver 110, 112 assures a significant decrease in the amount of increase in the output variation when the actual ground-speed U exceeds about 100km/h. As a result, the overall output variation of the sensor over the entire range of the vehicle speed is significantly reduced. In other words, the range of the vehicle ground-speed U that can be accurately detected by the sensor is widened. The curve indicated by the dashed line in the graph of Fig. 3 was obtained under the following condition or parameters: Angle of inclination  40° Height of the transmitter and receiver 110, 112 from the road surface 300mm Sound velocity C 350m/s Distance L 80mm Although the distance L is 80mm in the illustrated embodiment, this distance L is equal to a distance of forward movement of the vehicle body (and therefore of the receiver 112) at the vehicle speed of about 100km/h while the ultrasonic wave generated by the transmitter 110 is reflected by the road surface and is received by the receiver 112. The distance L of 80mm by which the receiver 112 is spaced rearwardly of the transmitter 110 permits the receiver 112 to receive the wave which is irregularly reflected by the road surface, at the same angle as the irregularly reflected wave which is to be received by the receiver of the known ground-speed sensor when the vehicle is stopped. In other words, the distance L permits the receiver 112 to receive the irregularly reflected wave whose intensity is the same as the irregularly reflected wave received by the receiver of the known sensor when the vehicle is stopped. Thus, the distance L permits reduction in the amount of increase in the output variation or error of the sensor, when the actual vehicle speed is higher than about 100km/h. The Doppler-effect ground-speed sensor shown in Figs. 1 and 2 has a further advantage, when the output of the sensor obtained in a given period is averaged before it is used for example in an anti-lock pressure control operation. That is, the reduced output variation of the ground-speed sensor makes it possible to shorten the time necessary to average the output, and thereby improve the response of the averaged sensor output to the change in the actual ground-speed of the vehicle, whereby the response of an anti-lock brake control apparatus is accordingly enhanced. In the illustrated Doppler-effect ground-speed sensor of Figs. 1 and 2, the signal processor 114 is adapted to calculate the ground-speed U of the vehicle on the assumption that the angle of incidence of the transmitted ultrasonic wave upon the road surface and the angle of propagation of the reflected wave from the road surface toward the receiver 112 are almost equal to the inclination angle  of the optical axes of the transmitter and receiver 110, 112, as described above. However, the angle of incidence of the transmitted wave and the angle of propagation of the reflected wave may be actually detected, so that the ground-speed U is calculated on the basis of the detected angles. If the optical axes of the transmitter and receiver 110, 112 are inclined in the forward running direction of the vehicle with respect to the vertical direction (direction perpendicular to the running directions), the receiver 112 is spaced apart from the transmitter 112 in the forward running direction.

Claims: A Doppler-effect type ground-speed sensing apparatus arranged in a body of a vehicle, including a ground-speed sensor which comprises a transmitter (110) attached to said body of the vehicle and transmitting a wave toward a road surface in a predetermined direction of propagation, a receiver (112) attached to said body and receiving a portion of said wave which is irregularly reflected by said road surface, and a signal processor (114) which provides an output representative of a ground speed of the vehicle on the basis of frequencies of said wave transmitted by said transmitter and said portion of the wave received by said receiver, characterized in that said direction of propagation of said wave transmitted by said transmitter (110) is inclined in-a forward running direction of the vehicle, and said receiver (112) is spaced apart from said transmitter in said forward running direction. A Doppler-effect type ground-speed sensing apparatus arranged in a body of a vehicle, including a ground-speed sensor which comprises a transmitter (110) attached to said body of the vehicle and transmitting a wave toward a road surface in a predetermined direction of propagation, a receiver (112) attached to said body and receiving a portion of said wave which is irregularly reflected by said road surface, and a signal processor (114) which provides an output representative of a ground speed of the vehicle on the basis of frequencies of said wave transmitted by said transmitter and said portion of the wave received by said receiver, characterized in that said direction of propagation of said wave transmitted by said transmitter (110) is inclined in a backward running direction of the vehicle, and said receiver (112) is spaced apart from said transmitter in said backward running direction. An apparatus according to claim 1 or 2, characterized in that said transmitter (110) generates an ultrasonic wave.

Inventors: IKEDA SHINJI, MIZUKOSHI MASAHI, IKEDA, SHINJI, MIZUKOSHI, MASASHI, MIZUKOSHI, MASAHI
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1992
Main classification: B60T8
IPC codes: B60T8