Ultrasonic sensor Ranging principle

Ultrasonic Ultrasonic (Waves):

The frequency at which human ears can hear is 20hz~20khz. When the vibration frequency of sound waves is greater than 20KHz or less than 20Hz, we can not hear. Therefore, we refer to sound waves that are above 20000 Hz as "ultrasound". Because of its good directivity, penetrating ability, easy to obtain a more concentrated sound energy, in the water spread far away, can be used for ranging, speed, cleaning, welding, gravel, sterilization and disinfection. There are many applications in medicine, military, industry and agriculture. such as ultrasonic cleaning machine, ultrasonic humidifier, medical examination b Super, color super, ultrasonic flaw detector and so on.

Sound is generated by vibration, the device that can produce ultrasonic is ultrasonic sensor, which is used to be called ultrasonic transducer or ultrasonic transducer. Ultrasonic probes are mainly composed of piezoelectric wafers, which can either emit ultrasound or receive ultrasound. The materials that make up the wafer can be of many kinds. The size of the wafer, such as diameter and thickness, varies, so the performance of each transducer is different, and it must be pre-understood before use.

Commonly used is piezoelectric ultrasonic generator, is the use of piezoelectric crystal resonance to work. The ultrasonic sensor probe has two piezoelectric wafers and a resonant plate. When the polarity of the pulse signal, the frequency is equal to the intrinsic oscillation frequency of the piezoelectric wafer, the piezoelectric wafer will be resonant, and drive the resonance plate vibration, will produce ultrasonic. Conversely, if the two electrodes do not have an external voltage, when the resonance plate received ultrasonic wave, will be compressed piezoelectric chip for vibration, the mechanical energy into an electrical signal, then it becomes an ultrasonic receiver. Ultrasonic sensor is the use of the principle of the piezoelectric effect of electric energy and ultrasonic transformation, that is, when the ultrasonic wave, the electrical energy into the ultrasonic wave, and in the reception, the ultrasonic vibration converted into electrical signals.

Ultrasonic Distance Measurement principle:

The most commonly used ultrasonic ranging method is the Echo detection method, as shown below, the ultrasonic launcher to a certain direction of the emission of ultrasound, at the time of the launch of the counter start timing, ultrasonic waves in the air, the way to meet obstacles in the surface immediately reflected back, ultrasonic receiver received reflection of the ultrasound immediately stop the timing. Ultrasonic waves in the air propagation speed of 340m/s, according to the time t recorded by the timer, you can calculate the transmission point distance from the obstacle surface S, namely: S=340T/2

Ultrasonic emission circuit: 555 timer generated 40KHZ pulse signal, added to the pin of the ultrasonic probe, so that the internal piezoelectric wafer resonance, outward emission ultrasonic.

Ultrasonic receiving circuit: Because the ultrasonic receiving probe produces the electrical signal is very weak, need to be amplified processing, the following figure, by the transistor and op amp LM324 constitute amplification circuit, to receive signal amplification, drive relay.


Generally, an integrated signal amplifier chip is used to amplify the signal. CX20106 is a dedicated integrated preamplifier for Sony, consisting of preamplifier, limiting amplifier, bandpass filter, detector, integrator and integral circuit. The preamplifier has an automatic gain control function to ensure that the amplifier has a higher gain when the ultrasonic sensor receives a faint voltage from the far reflected signal, and that the amplifier does not overload when the near-distance input signal is strong.

Ultrasound is also a kind of sound wave, its sound velocity v is related to temperature. When used, if the propagation medium temperature change is not small, it can be approximated that the ultrasonic velocity in the process of transmission is basically unchanged. If the precision of distance measurement is very high, the result should be corrected numerically by means of temperature compensation. V = 331. 4 + 0. 607T, in the formula, T is the actual temperature unit of ℃,v for ultrasonic wave in the medium of transmission speed Unit is m/s

The actual measurement due to the sensor and the measured object angle is different, the surface of the object being measured may not be flat, resulting in several special cases, will lead to errors in the measurement results, as shown below, can be resolved by rotating the angle of the probe multiple measurements.

The main performance indicators of ultrasonic sensors include:

(1) Operating frequency. The operating frequency is the resonant frequency of the piezoelectric element. When the frequency of AC voltage added to it is equal to the resonant frequency of the wafer, the output energy is the largest and the sensitivity is the highest.
(2) Operating temperature. Because the piezoelectric material of the Curie point is generally relatively high, especially when the use of ultrasound ultrasonic probe power is small, so the operating temperature is relatively low, can work for a long time without failure. Medical ultrasound probes are of high temperature and require separate refrigeration equipment.
(3) sensitivity. Mainly depends on the manufacture of the wafer itself. Large electromechanical coupling coefficient and high sensitivity.

Ultrasonic distance Measurement module: There are a lot of well-done measuring modules in the market, the price performance varies.

The HC-SR04 ultrasonic ranging module provides 2cm-400cm non-contact distance sensing, ranging accuracy up to 3mm, and the module includes an ultrasonic transmitter, receiver and control circuitry. Basic working principle:

(1) using the IO port trig trigger ranging, to at least 10us high-level signal;

(2) The module automatically sends 8 40khz square waves, automatically detects if there is a signal return;

(3) There is a signal return, through the IO port echo output a high level, the duration of the high level is the ultrasonic wave from the launch to the time of return.

Test distance = (high-normal * sound velocity (340m/s))/2;