People often ask that the received signal is very stable when the wind speed is below 20 m/s, but when the wind speed exceeds 20 m/s, the anemometer received signal starts to shake, and the measurement time difference is hard to handle, etc.
First, let’s analyze several situations that cause signal instability.
①Sound wave direction shift
Anemometer transducers are opposite, or reflect on the same surface. Because the wind speed is propagating, the probe emits in one direction, and the wind has one direction. When the two are superimposed, there will be an offset. As a result, the emitted sound waves were deflected. The received signal starts to jitter.
② Caused by frequency deviation
The ultrasonic wave propagates in the flow field, and there is a Doppler effect. The greater the wind speed, the greater the frequency deviation. Each transducer has a frequency response curve. The transmission may be 200Khz, and the receiving end may receive 210 KHz or other frequencies. If the frequency changes, the received signal will be weak.
③ The radiation efficiency becomes worse
The larger the gas velocity, the smaller the density, the smaller the gas acoustic impedance, the smaller the radiation impedance, and the smaller the output signal. The lower the pressure, the smaller the signal.
The factors that cause instability are mentioned above, and the solutions on the circuit are listed below:
The first: do automatic gain on the receiving circuit, and do different amplification for different received signals, all of them are amplified to the same amplitude, AD sampling is performed, and then the algorithm is processed.
For example, if the received signal is 20mV, it needs to be amplified by 50 times to 1000mV, and then AD sampling is performed. If the received signal is 2mV, it needs to be amplified by 500 times to 1000mV, and then AD sampling is performed.
The second: double beam detection method
It is generally considered that the flow velocity of the gas passing through the middle of the probe is the same for the 2 series of pulses emitted next to each other. After the first series of pulses are transmitted, the signal received by the receiving circuit is amplified according to the previous amplification factor. For example, it is 50 times before, if the received signal is 5 mV, then the amplified signal is 250 mV. That is, the received peak-to-peak value is detected, and the amplification factor of the receiving circuit is determined according to the amplified signal. For example, if it needs to be amplified to 1000 mV, the actual amplification factor is: 1000 mV ÷ 250 mV × 50 times=200 times. Then a series of the same pulses are transmitted, and the receiving circuit adjusts the amplification factor of the latter series of pulses to 200 times according to the amplitude of the previous received signal.
The above two methods have specific applications in actual products. There is a prerequisite for using the above methods, that is, the noise of the circuit body should be low enough.
To make an ultrasonic anemometer, the noise of the circuit body must be controlled within 2mV. If it is an ultrasonic gas flow meter, the noise of the circuit itself should preferably be within 0.3mV.
If the noise of the circuit itself is relatively large, for example: 10mV. Then to achieve a signal-to-noise ratio of 10db, it means that the voltage of the transmitter must be very high, so that the receiving circuit can work normally.