Error and precautions of ultrasonic distance sensor
Publish Time: 2018-09-21 Origin: Site
In the industry, the typical application of ultrasonic distance transducers are non-destructive testing of metals and ultrasonic thickness measurement. In the past, many technologies were hindered by inability detecting the inside of objects, and the emergence of ultrasonic sensing technology changed this situation. Of course, more ultrasonic sensors are fixedly mounted on the different devices to snap the signals people need. In the future applications, ultrasound will be combined with information technology and new material technologies, and more intelligent and highly sensitive ultrasonic sensors will appear.
The remote control ultrasonic remote control can control household appliances and lighting. Small ultrasonic sensor (Φ12-Φ16), working frequency is 40KHZ, remote control distance is about 10 meters. Remote control transmission, this is an oscillator composed of 555 time base circuit,to adjust 10K potentiometer, making the oscillation frequency 40KHZ sensor is connected to the 3rd foot, when the button is pressed, the ultrasonic wave is sent out and the circuit is received. The power supply is stepped down by 220V, rectified, filtered, and regulated to obtain a 12V operating voltage. Since it is a non-isolated power supply, the entire circuit should be packaged in the plastic case to prevent electric shock (also pay attention when debugging). The signal is received by the ultrasonic receiver and amplified by Q1 and Q2 (the L and C resonant tanks are tuned at 40 kHz). The amplified signal triggers a bistable circuit composed of Q3 and Q4, and Q5 and LED are used as trigger isolation, and can be illuminated. Since the bistable state is random at startup, a clear button is added.
The trigger signal of the Q5 output turns on the triac and the load is turned on. To load an open circuit, press the send button once. Liquid level indication and controller.Because the ultrasonic wave has a certain attenuation in the air, the signal of ultrasonic distance sensor transducer sent to the liquid surface and reflected back from the liquid surface is related to the liquid level. The higher is liquid level position, the larger the signal; the lower is the liquid level.The signal is small.The received signal is amplified by BG1 and BG2, and rectified into DC voltage by D1 and D2. When the voltage is 4.7KΩwhich exceeds the turn-on voltage of BG3, a current flows through BG3, and the ammeter indicates that the current is related to the liquid level. When the liquid level is lower than the set value, the comparator output is low. BG does not conduct. If the liquid level rises to the specified position, the comparator flips and outputs a high level. BG is turned on, J is sucked, and the infusion switch can be turned off by a solenoid valve to achieve the purpose of control .
The liquid level test is the basic principle of ultrasonic measurement of liquid level: the ultrasonic pulse signal emitted by the ultrasonic distance measurement sensor propagates in the gas,which is reflected after the interface between air and liquid, and receives the echo signal after the echo signal is received. Time, you can convert the distance or liquid level. Ultrasonic measurement methods have many advantages unmatched by other methods: (1) without any mechanical transmission components, nor with the liquid to be tested, it is non-contact measurement, which is not afraid of electromagnetic interference and strong corrosive liquids such as acid and alkali, so performance Stable, high reliability and long life; (2) Its short response time makes it easy to realize real-time measurement without hysteresis.
The distance measuring transducer sensor is used in the system operation at a frequency of about 40 kHz. The ultrasonic pulse is emitted by the transmitting sensor, and the liquid surface is reflected and returned to the receiving sensor to measure the time for the ultrasonic pulse to be transmitted from the receiver to the receiving. According to the speed of sound in the medium, the distance from the sensor to the liquid surface can be obtained. To determine the liquid level. Taking into account the influence of the ambient temperature on the ultrasonic propagation speed, the propagation speed is correct by the temperature compensation method to improve the measurement accuracy. The calculation formula is: V=331.5+0.607T .Where: V is the propagation speed of the ultrasonic wave in the air; T is the ambient temperature.S=V ×t/2=V×(t1-t0)/2 Where: S is the measuring distance; t is the time difference between transmitting the ultrasonic pulse and receiving its echo; t1 is the ultrasonic echo receiving time; t0 is the ultrasonic pulse transmitting time. Using the capture function of the MCU, it is convenient to measure the time t0 and the time t1. According to the above formula, the measuring distance S can be obtained by software programming. Since the MCU of the system selects a mixed signal processor with SOC characteristics and integrates a temperature sensor therein, the temperature compensation of the sensor can be conveniently realized by using software.
Precautions:
1: To ensure reliability and long service life,which do not use the sensor outdoors or above the rated temperature.
2: Since the ultrasonic sensor uses air as the transmission medium, reflection and refraction at the boundary may cause malfunction when the local temperature is different, and the detection distance may also change when the wind is blown. Therefore, sensors should not be used next to devices such as forced fans.
3: Jets ejected from air nozzles have multiple frequencies and therefore affect the sensor and should not be used near the sensor.
4: Water droplets on the sensor surface shorten the detection distance.
5: Materials such as fine powder and cotton yarn cannot be detected when is absorbing sound (reflective sensor).
6: Sensors cannot be used in the vacuum or explosion-proof areas.
7: Do not use the sensor in areas with steam; the atmosphere in this area is uneven.which will produce a temperature gradient that will cause measurement errors。
Exposure problem:
The application of the working of ultrasonic distance sensor is simple, convenient, and low cost. However, current ultrasonic sensors have some disadvantages, such as reflection problems, noise, and crossover problems. The problem with reflection is that if the object being detected is always at the right angle, the ultrasonic sensor will get the correct angle. But unfortunately, in the actual use, few detecting objects can be detected correctly.
There may be several errors in the transducer:
When the measuring object is at an angle to the sensor, the detecting distance and the actual distance has a triangular error.
This problem is the same as the reflection of light in high physics. At a certain angle, the emitted sound waves are specularly reflected by the smooth object, so the echo cannot be generated and the distance reading cannot be produced. At this time, the ultrasonic sensor will ignore the existence of this object.
This phenomenon is common when detecting corners or objects of similar structure. The sound wave is received by the sensor after multiple rebounds, so the actual detecting value is not the true distance value. These problems can be solved by using multiple ultrasonic coils which arranged at an angle. By detecting the return value of multiple ultrasonic waves, it is used to screen out the correct reading.
4. Noise
Although most distance measuring ultrasonic transducer operates frequencies of 40-45 Khz, they are much higher than the frequencies humans can hear. However, the surrounding environment also produces noise of similar frequency. For example, the motor generates a certain high frequency during the rotation process, the high frequency noises generate by the friction of the wheel on the hard ground, it is the vibration of the robot itself, and even when there are multiple robots, the sound waves are emitted by other robot ultrasonic sensors. These will cause the sensor to receive an erroneous signal. This problem can be solved by encoding the transmitted ultrasonic waves, such as transmitting a set of sound waves of different lengths, and calculating the distance only when the probe detects the same combined sound wave. This can effectively avoid misreading caused by environmental noise.
5. Cross problem
The crossover problem is caused when a plurality of ultrasonic sensors are mounted on the robot at an angle. The sound waves emitted by the ultrasonic X are specularly reflected and obtained by the sensors Z and Y. At this time, Z and Y calculate the distance value based on this signal, so that the correct measurement cannot be obtained. The solution is to encode the signal from each sensor. Let each ultrasonic sensor listen to only its own voice.