Abstract:
An obstacle detecting system for vehicles has an ultrasonic vibrator having a vibration plate and mounted on a vehicle to transmit and receive an ultrasonic wave. An obstacle is detected in response to an output signal of the ultrasonic vibrator. When the temperature around the ultrasonic vibrator is below a predetermined temperature corresponding to snowfall, the system calculates a ratio of a period of reverberating vibration of the ultrasonic vibrator relative to a predetermined time period following an ultrasonic wave transmission, and determines accumulation of snow when the calculated ratio exceeds a predetermined time ratio. The system notifies an abnormality of the ultrasonic vibrator arising from the accumulation of snow.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-263797 filed Aug. 31, 2000. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an obstacle detecting system, which detects obstacles against a vehicle by the use of an ultrasonic wave. 
     Obstacle detecting systems for vehicles use an ultrasonic vibrator, which transmits an ultrasonic wave and receives the ultrasonic wave reflected by an obstacle. The ultrasonic vibrator has a casing, a piezoelectric device provided in the casing, and a vibration plate made of an aluminum film and provided in the opening of the casing. 
     The piezoelectric vibrator vibrates at a frequency of the drive voltage applied thereto. The vibration of the piezoelectric vibrator is transmitted to the vibration plate, so that the casing resonates with the vibration. Thus an ultrasonic wave is transmitted from the vibration plate. The casing continues to resonate by inertia vibration after the piezoelectric device stops the vibration, resulting in reverberating vibration. Thus, the ultrasonic vibrator transmits the ultrasonic wave in response to the vibration of the piezoelectric device and the reverberating vibration. 
     The ultrasonic vibrator is mounted on a vehicle in such a manner that the vibration plate is exposed to the exterior of the vehicle. As a result, snow tends to stick to or accumulate on the vibration plate of the ultrasonic vibrator in winter, thereby increasing the weight of the vibration plate. The casing therefore continues the reverberating vibration for a longer period at a frequency, which is slightly deviated from its resonant frequency. 
     Under this condition, the ultrasonic vibrator is likely to continue to transmit the ultrasonic wave even after starting to receive a reflected ultrasonic wave. It is difficult to distinguish whether the output signal of the ultrasonic vibrator results from the reflected wave or the reverberating vibration. Thus, the obstacle detection system tends to detect that an obstacle exists close to the vehicle although it exists remote from the vehicle. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an obstacle detecting system, which is capable of detecting and notifying accumulation of snow on an ultrasonic vibrator. 
     According to the present invention, an obstacle detecting system for vehicles comprises an ultrasonic vibrator having a vibration plate and mounted on a vehicle to transmit and receive an ultrasonic wave, and an obstacle is detected in response to an output signal of the ultrasonic vibrator. The system checks whether the vibration plate has accumulation of snow based on a period of reverberating vibration of the vibration plate, and notifies an abnormality of the ultrasonic vibrator arising from the accumulation of snow. 
     Preferably, the system executes a snow accumulation check operation only when the temperature around the ultrasonic vibrator is below a predetermined temperature corresponding to snowfall. The system calculates a ratio of the period of reverberating vibration of the ultrasonic vibrator relative to a predetermined time period following an ultrasonic wave transmission, and determines the accumulation of snow when the calculated ratio exceeds a predetermined time ratio. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
     FIG. 1 is a block diagram showing an obstacle detecting system according to an embodiment of the present invention; 
     FIG. 2 is a flow diagram showing a control process of a microcomputer used in the embodiment shown in FIG. 1; 
     FIG. 3 is a signal diagram showing an output of an amplifier used in the embodiment when an ultrasonic vibrator has no accumulation of snow; and 
     FIG. 4 is a signal diagram showing an output of the amplifier when the ultrasonic vibrator has accumulation of snow. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, an obstacle detecting system is constructed as a rear sonar for a vehicle. It has an ultrasonic vibrator  10 . This ultrasonic vibrator  10  is mounted on a rear bumper of the vehicle at a central position in the right-left direction of the vehicle for detecting any obstacle existing in the rear direction of the vehicle. This ultrasonic vibrator  10  comprises a casing  11 , a vibration plate  12  attached to the casing  11  for transmitting and receiving ultrasonic waves, and a piezoelectric device (not shown) provided in the casing  11  for vibrating the vibration plate  12 . The vibration plate  12  is exposed to the rear outside of the vehicle so that the ultrasonic waves are radiated in the rearward direction. 
     In the ultrasonic vibrator  10  the piezoelectric device vibrates when driven with a driving voltage. This vibration is transmitted to the vibration plate  12  to vibrate the vibration plate  12 . The ultrasonic vibrator  10  transmits ultrasonic waves outward (rearward of the vehicle) by the vibration of the vibration plate  12 . The ultrasonic vibrator  10  receives at the vibration plate  12  ultrasonic waves reflected by obstacles, and vibrates at a frequency of the reflected wave. The vibration plate  12  transmits this vibration to the piezoelectric device, which in turn generates a piezoelectric voltage as a reception signal. This reception signal has a magnitude proportional to the received reflected wave. 
     The drive voltage for the piezoelectric device is set to 40 kHz so that the ultrasonic wave is transmitted at this frequency. The casing  11  is constructed to resonate at this frequency. As a result, the casing  11  starts to resonate when the vibration of the piezoelectric device is transmitted to the vibration plate  12 . The casing  11  continues to resonate by inertia, as reverberating vibration, even after the transmission of vibration of the piezoelectric device stops. 
     The obstacle detecting system also has a driver circuit  20 , a band pass filter circuit (BPF)  30 , an amplifier circuit  40 , an envelope detector circuit  50 , a temperature sensor  60 , a microcomputer  70 , a display  80 , a buzzer  100 , and driver circuits  90  and  110 . 
     The driver circuit  20  is controlled by the microcomputer  70  to generate oscillation pulse signal at a frequency of 40 kHz to drive the piezoelectric device of the ultrasonic vibrator  10 . Specifically, the driver circuit  20  generates the oscillation pulse signal, which lasts 250 μs each time 150 ms elapses after the previous oscillation period (250 μs). This 150 ms is set as a transmission stop period. 
     The BPF  30  filters out noise signal from the reception signal produced from the ultrasonic vibrator  10  to produce a filter signal. The amplifier circuit  40  amplifies the filter signal (FIGS. 3 and 4) to produce an amplified signal. The envelope detector circuit  50  detects an envelope of the amplified signal. The temperature sensor  60  detects temperature around the ultrasonic vibrator  10 . 
     The microcomputer  70  is programmed to execute a control process shown in FIG. 2 to calculate a distance of the vehicle to an obstacle in response to the envelop-detected signal. The display  80  and the buzzer  100  are mounted on an instrument panel of the vehicle. The display  80  and the buzzer  100 , driven by the driver circuits  90  and  110 , respectively notify visually and audibly the calculated distance and existence of the obstacle. 
     Operation of the obstacle detecting system, particularly operation of the microcomputer  70 , is described with reference to FIG.  2 . The microcomputer  70  causes the driver circuit  20  to generate the oscillation pulse signal at 40 kHz at step  200 . The piezoelectric device of the ultrasonic vibrator  10  is driven with this drive voltage to transmit the ultrasonic wave from the vibration plate  12 . The microcomputer  70  stops generation of the oscillation pulse signal of the driver circuit  20  at step  210  after the oscillation period of 250 μs. Thus, the vibration of the piezoelectric device stops. When the piezoelectric device stops vibration, the ultrasonic vibrator is switched from the transmission mode to the reception mode. 
     The microcomputer  70  checks at step  220  whether the temperature of the ultrasonic vibrator  10  detected by the temperature sensor  60  is equal to or higher than 0° C. If the check result is YES indicating no sticking nor accumulation of snow, the microcomputer  70  inputs the envelope detection signal from the envelope detector  50 . This envelope detection signal corresponds to the ultrasonic wave reflected by the obstacle and received by the ultrasonic vibrator  10 . 
     The microcomputer  70  calculates a distance between the vehicle and an obstacle at step  240  by using the amplified signal or the envelope detection signal. Specifically, the distance is calculated as follows. The amplified signals are shown in FIGS. 3 and 4 in case of no snow accumulation and snow accumulation, respectively. First, a time period T in which the amplitude of the ultrasonic wave X, that is, the amplitude V of the amplified signal, exceeds a threshold Vth as shown in FIG. 3, is measured in a predetermined period T 3  (1.4 ms) following a predetermined period T 1  (1.6 ms) from the start of transmission of the ultrasonic wave of the ultrasonic vibrator  10 . The distance is calculated as a product of the propagation speed of the ultrasonic wave and one half of the measured time period T. The predetermined period T 1  is set to correspond to a period from the start of ultrasonic wave transmission to the end of reverberation in the normal (no snow) condition, thus covering a transmission period TR (oscillation pulse generation period 0.25 ms) and a reverberation period T 4 . 
     The microcomputer  70  causes at step  250  the driver circuit  90  to drive the display  80  so that the calculated distance is displayed visually. At the same time, the microcomputer  70  causes the driver circuit  110  to drive the buzzer if the calculated distance is less than a predetermined distance. 
     If the check result at step  220  is NO indicating snow, the microcomputer  70  calculates, as the reverberating period, the time period T 4  or T 5 , in which the magnitude V of the amplified signal exceeds the threshold Vth after the transmission of the ultrasonic wave. 
     The microcomputer  70  checks at step  270  whether the ultrasonic vibrator  10  has an accumulation of snow thereon. The microcomputer  70  determines accumulation of snow if the calculated period of reverberating vibration is more than 70% of a predetermined time period T 6  (2.5 ms), which is after 1.2 ms from the start of ultrasonic wave transmission. 
     If no snow accumulates on the ultrasonic vibrator  10 , the reverberating vibration continues for the period T 4  as shown in FIG.  3 . However if snow accumulates on the ultrasonic vibrator  10 , the reverberating vibration continues for the period T 5  as shown in FIG.  4 . The period T 5  is longer than the period T 4 . It was confirmed in the experiments that accumulation of snow can be detected by checking whether the time period T 4  or T 5  is in excess of a predetermined ratio (70%) of the predetermined time period T 6  under the condition that the detected temperature is 0° C. 
     If the check result at step  270  is NO indicating no snow accumulation, the microcomputer  70  executes steps  230 - 250 . If the check result is YES indicating snow accumulation, the microcomputer  70  causes at step  271  the driver circuit  90  to drive the display  80  so that abnormality of the ultrasonic vibrator  10  may be displayed. With this display, a driver of the vehicle is enabled to recognize that the ultrasonic vibrator  10  and hence the obstacle detecting system cannot operate normally. 
     The microcomputer  70  then checks at step  260  whether the predetermined transmission stop period (150 ms) has elapsed. If this check result is NO indicating that the system is still in the transmission stop period, the microcomputer  70  repeats the above process from step  220 . If the check result is YES indicating that the system is now not in the transmission stop period, the microcomputer  70  repeats the whole process from step  200 . 
     The present invention should not be limited to the above embodiment, but may be implemented in various ways. For instance, the ultrasonic vibrator  10  may be driven at a different frequency other than 40 kHz. The snow accumulation on the ultrasonic vibrator  10  may be notified by other devices such as voice or light. The reference temperature 0° C. for checking snowfall may be set to other temperatures. The ultrasonic vibrator  10  may be mounted on other locations of a vehicle such as outer peripheral wall parts of a vehicle so that this system may be used as clearance sonar. The accumulation of snow may be detected by comparing the reverberation period with a predetermined reference period.