The Research of Ultrasonic Detection Method for Car Back Obstruction

Ultrasonic, used in security technology such as car collision avoidance and distance measurement, also can be used in detecting obstruction behind the car when backing up. In this paper, we analyze the interference of ultrasonic signal when transmitting and receiving, and then resolve it by software. There is a blind area and distance limitation in ultrasonic distance measurement .The cause of distance limitation is that the amplitude value of received signal is at least larger than the specified threshold value. However the requirement is low, the minimum amplitude value of received signal must be lager than that of the noise, otherwise it is difficult to distinguish the signal we need from the noise. The software signal resolvent mainly are: system error compensation, transmitted pulse modulation technique, delayed receiving technique, digital filter technique and time delay killing transmitting vibration. The result of experiment shows that the system’s dead band is less than eight centimetres; the distance measurement is farther than ten meters; the three-pointdetection-overlay zone is wide; measurement accuracy is high, relative error is less than 2%. Obviously, when all-around ultrasonic software signal resolvent is applied, low-cost ultrasonic sensors is used widely and ability of system is strengthened to adapt to the environment Introduction: Presently, the detection technique of laser, radar, infrared ray and ultrasonic have been widely applied at the aspects of safety technique of car collision avoidance and distance measurement. At the aspect of collision avoidance laser, radar and infrared ray are commonly applied to measure the control range between two cars and the range which should be measured behind the car. At the aspect of distance measurement the technique of ultrasonic is applied to measure the detection range when a car change the driveway and to detect the obstruction behind the car when backing up or parking|. Because of the expensive price, the distance measurement system of backing up with the technique of laser and radar is only set on the minority of slap-up cars, so the research of the distance measurement system of backing up with high ratio of capability and price for the medium cars and the low-end cars is an important task of auto-electron industry. The automatic distance measurement system of backing up introduced in this paper can automatically measure the distance between the trail of the car and detect the obstruction behind the car, further more it can show the distance and give a sound-light alarm in real time, so it can ensure the car to run safely and reduce the accident ratio. The driver does not need to intermeddle in or manipulate this system. This system will have a prosperous application prospect. It will cut a way through the market of the medium cars and the low-end cars and provide a new research method for the car collision avoidance. The automatic distance measurement system of backing up is an electro-mechanical integrative instrument, which adopts the technique of ultrasonic sensor and SCM. When the electric signal is imported into the emitter, the emitter transmits ultrasonic, the receiver receives the reflected wave, the sound wave transmitting time and the distance are in direct ratio, so obtain the function of distance measurement. The important research content of this system is: the research of method about ultrasonic distance measurement and the system about ultrasonic distance measurement. This part includes how to choose the model of distance measurement sensors and design the detection system, the research and the application of ultrasonic, transmitted and received technique. The research and the design of processing system about measuring signal channels and calculating signal .The important part is the hardware and the software design about the SCM signal processing system, the compensation and the emendation of the system error and the random error of signal measured, design of the interface about distance display system and the sound-light alarm system and the drivers, the research of the anti-jamming and reliability of the automatic distance measurement system of backing up. Results: The ultrasonic distance measurement is an untouchable detection mode. Compared with else detection modes, it does not influenced by ray, temperature and colour etc, and it has the great capability to adapt to the circumstance. The ultrasonic sensor adopts transmission mode or reflection mode on the physical configuration, the reflection mode has the single explore-head and the double explorehead, his system adopts double explore-head reflection mode, it is that the sending signal and the receiving signal are on the same side. The double explore-head mode has a unique advantage, which is that there is much less blind area than single explore-head mode. Because the sending explore-head is separated from the receiving explore-head, the sending explore-head does not have sending voltage directly, theoretically, it does not have blind area. But the receiving circuit is influenced by the sending circuit more or less, and the ultrasonic from the sending explore-head possibly move around to the receiving explore-head directly. So there should be existed some blind area, but in fact more less than the blind area of single explore-head . It is the ultrasonic pulse echo technique (sonar technique) that is most widely used in the continuous distance measurement technique. The working principle is: the sender sends ultrasonic pulse, then the sound wave transmits in the medium. The sound wave is reflected when it comes up against the objects. Recording the time from sending to receiving, we can calculate the distance from the sensor to reflection point according to the velocity of sound in the medium. If the vertical distance from the sensor to the object detected is L, the time from sending to receiving is t, the velocity of sound is c, then the distance measured L=(c*t)/2.For the sending and the receiving reflection mode ,if the distance between the centers of the two sensors is 2a,the diagonal path from the sensor to the object detected is s, then s=(c*t) /2,Generally speaking ,the sending sensor and the receiving sensor are put close, when s>>a, L≈s. The ultrasonic detection distance relates with size, figure, material and position of the object. The bigger the reflector is, the better the reflectance is, and the stronger the reflection signal is. Generally speaking, the detection distance is farther if the facade of the reflector is smooth and flat than that if the facade of the reflector is coarse, the detection distance of the reflector with solidity facade is farther than that with loose facade, the detection distance is farther if the reflector facade is vertical to the sending signal than that if there is a obliquity between the reflector facade and the sending signal (the reflected ultrasonic does not return to the explore-head along the primary path, so the reflected signal received by the receiving explored-head will be greatly weakened). The figure 1 illustrates the principle of the hardware circuit of the system, which includes three parts: ultrasonic sending and receiving circuit; signal process and SCM control circuit; display and alarm circuit. The working principle of the system hardware is: the control signal A from SCM use conversion multiplexer and the driver to control the three couples of sensors to work in time-sharing (B is control signal from one group of sender), transmit 40KHZ ultrasonic, after the receiver receives the return ultrasonic, transformed into rectangle wave C through the magnifying and modulation circuit, delivered to the signal process circuit through conversion multiplexer (D is the synchronous control signal from SCM to signal process circuit).The signal process circuit changes the time signal measured into width of level signal E, and outputs it to the SCM to be processed. The SCM displays F after being processed, and gives the adjustable-volume alarm G according to the distance . C F A B