Nowadays, reversing radar has been widely used in daily life, but some reversing radar has only alarm function, or there are some blind areas that cannot be detected, which leads to the owner can not completely rely on radar. Objects lower than the reversing radar have the opportunity to avoid detection. For example, low stones, iron pillars or snowdrifts may avoid sound waves and silence the radar. If the vehicle continues to reverse at this time, it is likely to encounter obstacles. Radar with poor response time will not give an alarm until a certain reaction time. Therefore, reversing radar makes people’s life faster to some extent, but it does not show comprehensive and accurate characteristics. We can use the principle of ultrasonic ranging to solve a few problems in the practical application of reversing radar, using ultrasonic sensors and single-chip microcomputer design reversing radar has the advantages of simple design, high reliability, small error.

The principle of ultrasonic ranging is that, due to the slow speed of the car when reversing, the analysis of relative speed can be considered that the car is stationary, so the influence of doppler effect can be ignored in the system. Specific frequency signal generated by a single chip microcomputer, and then use
ultrasonic launcher, encounter obstacles to return after ultrasonic receiver, after tested device for processing of received signal, and then calculated by the single chip microcomputer, ultrasonic transmitter to the distance between the obstacle, the calculation results by the display, and drive buzzer hint according to distance from different frequency.

Let’s first analyze several factors that may lead to ranging error. This error may be affected by the fluctuation of echo envelope. Due to the difference in the size the distance from the obstacle, ultrasonic incidence Angle, reflective medium material and other aspects, the difference in the echo amplitude obtained by the receiving transducer is greater than expected. For example, for the same obstacle, the distance from the receiving probe is 0.2m and 2m, and the difference in the echo envelope amplitude is 100 times. Besides, there are obstacles of the same distance and material, because the effective reflection area is different, the echo amplitude will vary large. When using a single level threshold, if the threshold is set too high, it may cause weak echo leakage to trigger. If the threshold is too low, measurement errors may occur.

After analyzing the above reasons, in order to overcome the adverse effect of echo envelope fluctuation, we can use the method of variable threshold or normalized echo envelope to solve the problem. First, an ultrasonic wave is sent, then the amplitude of the echo envelope is measured, and the maximum peak value of the echo envelope is recorded. Then, an ultrasonic wave is sent, and then the echo is received. After multiplying the echo peak recorded in the previous step by a proportionality coefficient, the threshold value of the trigger level of this
measurement is determined. This ensures that the triggering time of each
measurement is at the same position of the echo. Or the automatic gain of the
receiving amplifier circuit can be controlled by using the echo peak recorded
in the first time in the process of receiving the second wave

Increasing the sensitivity of the receiver circuit can increase the detection range, but the improvement of sensitivity is limited by environmental noise. If the
sensitivity is too high, then environmental noise and various noises of the car
itself will cause the receiver circuit to fail to work properly. Increasing transmission power can increase detection distance, but the increase of
transmission power is limited by ultrasonic probe and cost. When the obstacle
is close, the large transmitting power makes the ultrasonic wave reflect back
and forth between the car and the obstacle for many times, which not only makes
the nearby obstacle impossible to measure, but also interferes with the
measurement of other channels, in addition to increasing the blind area at the
detection near end.

Because ultrasonic ranging is only applicable to close range, when the distance is
relatively far, the attenuation is more serious, the reflected signal is relatively weak, so it is best to set up an amplification circuit at the receiving end, and then demodulate its output signal through the detection circuit, and finally compare the detection output signal shaping.

Ultrasonic receiving circuit needs to consider the following aspects: first, in ultrasonic ranging, sound velocity is the benchmark of measurement. For the air medium, sound velocity is affected by temperature, humidity, atmospheric pressure and other factors, among which temperature has the largest impact. The formula to compensate this impact is c=331.5+ 0.60t. In addition, there are environmental noise interference. Because the ultrasonic echo signal is very weak and contains noise, but the signal noise is relatively small. In order to increase
the stability of the receiving circuit and improve the SNR, the receiving circuit
should use the second-order filter amplifier, and the filter amplifier should
use the second-order band-pass filter amplifier.

The designed ultrasonic ranging reversing radar system can meet the design requirements and guarantee the safety of the vehicle reversing to a great extent because the ultrasonic probe can be used to detect the obstacle behind the vehicle when reversing. Also, the distance is calculated by high-speed single-chip
microcomputer, and the temperature compensation of the ultrasonic ranging system is realized by high-precision sensor, which improves the accuracy of distance calculation.