Detailed overview of the principle of ultrasonic ranging sensor and the circuit design of the ultrasonic ranging transducer

Due to the strong directivity of ultrasonic waves, slow energy consumption, and long distances in the medium, ultrasonic waves are often used for distance measurement. For example, ultrasonic rangefinders and level measuring instruments can be realized by ultrasonic waves. Ultrasonic testing is often quick, convenient, simple to calculate, easy to achieve real-time control, and can meet industrial practical requirements in terms of measurement accuracy, so it has also been widely used in the development of mobile robots. In order for a mobile robot to automatically avoid obstacles and walk, it must be equipped with a distance measuring system so that it can obtain the distance information (distance and direction) from the obstacle in time. The three-direction (front, left, and right) ultrasonic distance measurement system introduced in this article is to provide a movement distance information for the robot to understand its front, left and right environment.

Second, the principle of ultrasonic distance transducer

1. Ultrasonic generator

In order to study and use ultrasound, many ultrasound generators have been designed and made. Generally speaking, ultrasonic generators can be divided into two categories: one is to generate ultrasonic waves electrically, and the other is to generate ultrasonic waves mechanically. Electrical methods include piezoelectric, magnetostrictive, and electric, etc.; mechanical methods include flute, liquid whistle and air whistle. The frequency, power and sonic characteristics of the ultrasonic waves they produce are different, so their uses are also different. At present, the piezoelectric ultrasonic generator is more commonly used.

2. Principle of piezoelectric ultrasonic generator

The piezoelectric ultrasonic generator actually uses the resonance of a piezoelectric crystal to work. The internal structure of the ultrasonic generator is shown . It has two piezoelectric wafers and a resonance plate. When a pulse signal is applied to its two poles, the frequency of which is equal to the natural oscillation frequency of the piezoelectric wafer, the piezoelectric wafer will resonate and drive the resonance plate to vibrate to generate ultrasonic waves. On the contrary, if no voltage is applied between the two electrodes, when the resonance plate receives ultrasonic waves, it will press the piezoelectric chip to vibrate and convert the mechanical energy into electrical signals, and then it will become an ultrasonic receiver.

3. The principle of ultrasonic transducer for distance measurement

The ultrasonic transmitter emits ultrasonic waves in a certain direction and starts timing at the same time as the launch time. The ultrasonic waves propagate in the air and return immediately when encountering obstacles on the way. The ultrasonic receiver stops timing immediately after receiving the reflected waves. The propagation speed of the ultrasonic wave in the air is 340m/s. According to the time t recorded by the timer, the distance between the launch point and the obstacle (s) can be calculated, namely: s=340t/2

Figure 1 Ultrasonic sensor structure

This is the so-called time difference ranging method.

Third, the circuit design of the ultrasonic ranging transducer

The characteristic of this system is the use of a single-chip microcomputer to control the transmission of ultrasonic waves and the timing of the round-trip time of ultrasonic waves from transmission to reception. The single-chip selection is economical and easy to use, and there is a 4K ROM on-chip for easy programming. The circuit schematic diagram is shown . Only the wiring diagram of the front ranging circuit is drawn, and the left and right ranging circuits are the same as the front ranging circuit.

1. 40kHz pulse generation and ultrasonic emission

The ultrasonic sensor in the distance measuring system adopts the piezoelectric ceramic sensor of UCM40, and its working voltage is a pulse signal of 40kHz, which is generated by the single-chip computer executing the following program.

The input terminal of the front ranging circuit is connected to the P1.0 port of the single-chip microcomputer. After the single-chip microcomputer executes the above program, it outputs a 40kHz pulse signal at the P1.0 port, which is amplified by the transistor T, drives the ultrasonic transmitter UCM40T, and sends out 40kHz pulsed ultrasonic waves. And continue to transmit 200ms. The input ends of the right and left ranging circuits are connected to the P1.1 and P1.2 ports respectively, and the working principle is the same as that of the front ranging circuit.

2. Ultrasonic reception and processing

The receiving head adopts the UCM40R paired with the transmitting head, ultrasonic transducer sensor converts the ultrasonic modulated pulse into an alternating voltage signal, which is amplified by the operational amplifiers IC1A and IC1B and then added to IC2. IC2 is an audio decoding integrated block LM567 with a locked loop. The center frequency of the internal voltage-controlled oscillator is f0=1/1.1R8C3, and the capacitor C4 determines its locking bandwidth. Adjusting R8 on the transmitting carrier frequency, the LM567 input signal is greater than 25mV, and the output pin 8 changes from high level to low level, which is used as an interrupt request signal and sent to the microcontroller for processing.

The output terminal of the front ranging circuit is connected to the MCU INT0 port, the interrupt priority is the highest, the output of the left and right ranging circuit is connected to the MCU INT1 port through the output of the AND gate IC3A, while the MCU P1.3 and P1.4 are connected to the input of IC3A At the end, the identification of the interrupt source is handled by the program query, and the interrupt priority is right first and then left.