Doppler ultrasonic flowmeter is the use of Doppler frequency shift principle to measure liquid flow rate. Therefore, the Doppler ultrasonic flowmeter is suitable for measuring fluids containing solid particles or bubbles, and is not suitable for measuring pure water.

I am currently making a Doppler ultrasonic flowmeter, and I will write the process here to remember the bumps in it:

First of all, the ultrasonic flowmeter needs two ultrasonic probes, one is used to transmit signals, and the other is used to receive signals, then the circuit is divided into two parts: one is used in the transmitting probe drive signal, and the other is used in the receiving signal processing part;

The first is the drive circuit: First, the square wave signal is generated by crystal oscillator, and then the square wave signal is built by the op amp circuit to build a phase shifter, and then the two signals are amplified through the op amp circuit to drive the probe as the drive signal. The final debugging drive signal is shown as follows:

The point of attention here is that the amplitude and peak-to-peak value of the drive signal can not be too small, and it is generally recommended that it is greater than 5V.

The above part is divided into three circuits, one is the phase shift circuit built by the op amp, the other is the follower to improve the load capacity, and the last is the signal amplification circuit. The signal can be successfully driven by driving the probe.

The second part is the processing part of the received signal, similar to the processing of the detector signal, the first signal is isolated processing, the purpose of isolation is to do not affect the back-end signal of the front detector signal, after isolation, the signal is sent to the bandpass filter, the center frequency and quality factor of the bandpass filter should be designed, the center frequency is set according to the parameters of the probe, The probe is different, the center frequency is different.

The output signal of the bandpass filter is sent to the signal amplifier, and the general amplification ratio is set according to actual needs, of course, it is also related to the supply voltage of the operational amplifier, and the amplified signal cannot be saturated, which will affect the final test results; Then add the bias voltage, because the detector signal has positive and negative, belongs to the AC signal, so to add the bias voltage, and then the signal is sent to the multiplier, the multiplier's reference signal uses the crystal output signal mentioned above, do difference frequency processing, and the final multiplier output signal is sent to the low-pass filter, the low-pass filter cutoff frequency depends on the actual need, The output signal of the low-pass filter is sent to 555 timer, which is converted into a pulse signal, and the pulse signal is sent to the single chip microcomputer for processing. Using the input capture function of the single chip microcomputer, the identification result of the single chip microcomputer is calibrated with the actual flow value, and a series of conversion operations are carried out to finally get the flow value we want, and the current output of 4-20mA can be added in the later stage. More suitable for long-distance transmission. The following figure shows the driving circuit and the amplifying circuit respectively.