Application of TDC-GP2 measurement chip in ultrasonic flowmeter

Preface

Compared with flow meters using traditional measurement methods, ultrasonic flow meters have many advantages: they do not change the flow state of the fluid and do not generate additional resistance to the fluid; they can adapt to the measurement of fluids of various pipe diameters and will not increase the instrument cost due to different pipe diameters; their transducers can be designed to be clamped and can be used for mobility measurement. As a high-precision time measurement chip, TDC-GP2 not only integrates the time measurement function, but also provides ultrasonic transducer drive pulses and temperature measurement functions for the application of ultrasonic flow meters and heat meters. Compared with ultrasonic flow meter solutions using discrete components or FPGAs, the solution using TDC-GP2 greatly simplifies the hardware circuit design and significantly reduces the power consumption of the entire machine, making it the ultrasonic flow meter solution with the simplest circuit and the lowest power consumption.

The measurement principle of ultrasonic flowmeter

Taking the time-difference ultrasonic flowmeter, which is widely used, as an example, by measuring the propagation time of ultrasonic waves in the fluid upstream and downstream respectively, the real-time flow rate of the fluid is calculated using the linear relationship between the fluid flow rate and the difference in the propagation time of ultrasonic waves downstream and upstream, and then the corresponding flow value is obtained.

As shown in Figure 1, the propagation speed of ultrasound in a stationary fluid is represented by C, and the propagation time of the downstream and upstream flow is:

This includes the response time of the transducer, the delay caused by the circuit elements, etc. Due to the consistency of the upstream and downstream paths, the upstream and downstream are the same. The time difference between the upstream and downstream propagation is:

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High-precision time measurement principle of TDC-GP2

The key to time-difference ultrasonic flow measurement is the measurement of ultrasonic propagation time. The time-to-digital conversion chip TDC-GP2 from ACAM of Germany provides a typical time resolution of 65ps and a measurement range from 0 to 4ms.

As shown in Figure 2, the TDC core measurement unit measures the time interval between the START and STOP pulses. The typical value of the transmission delay of each gate circuit is 65ps. The TDC core measurement unit obtains the time interval between the START and STOP signals by counting the number of gate circuits that the START signal passes through before the STOP pulse arrives. The TDC-GP2 chip uses special design and wiring methods to ensure that the time delay of each gate circuit is strictly consistent, but this time delay will vary with the power supply voltage and temperature. Therefore, TDC-GP2 has designed a reference clock to calibrate the delay of the gate circuit. At the same time, this reference clock will also participate in the time measurement when the measured time is long.

Since the TDC core measurement unit counts the number of gates that the electrical signal passes through, its time measurement range is limited by the counter capacity, and can be measured up to 1.8us. For applications where the measured time exceeds this range, TDC-GP2 combines reference clock measurement with TDC core measurement unit. As shown in Figure 3, the TDC core measurement unit only measures TFC1 and TFC2, while TCC completes the measurement by counting the number of reference clock cycles. The measured time TSS can be obtained by the following calculation:

After each measurement, TDC-GP2 can automatically calibrate the gate circuit delay, such as Cal1 and Cal2 in Figure 3. The TDC core measurement unit measures the reference clock period, and the reference clock period is known, so the accurate gate circuit delay can be inferred from the measurement results. The above calculations and corrections are all automatically completed by TDC-GP2, and the final corrected measurement results will be given in units of reference clock periods to facilitate user calculations.

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Low power consumption of TDC-GP2

The innovative measurement mechanism of TDC-GP2 determines its low power consumption. As can be seen from Figure 3, when TDC-GP2 is measuring time, its power-hungry core measurement unit is not always working. It is only used to measure the time from the rising edge of the START signal to the next reference clock rising edge (TFC1), and the time from the rising edge of the STOP signal to the next reference clock rising edge (TFC2), and a large amount of time measurement in the middle is completed by counting the number of reference clock cycles. The power consumption of the TDC core measurement unit is 15mA when working, and less than 150nA when not working. Since the working time of the TDC core measurement unit accounts for a very small proportion of the time in a measurement, and the time of each measurement in pipeline flow measurement is generally in the microsecond level, the average power consumption of TDC-GP2 can reach an extremely low level. Taking two measurements per second as an example, the average power consumption can be less than 2uA.

TDC-GP2 Pulse Generator

TDC-GP2 not only has an ultra-low power consumption time measurement unit, but also integrates a pulse generator for driving ultrasonic transducers. The frequency and phase of the generated pulses can be controlled by setting the registers, and up to 15 consecutive pulses can be generated at a time. The pulse generator has two output pins, FIRE1 and FIRE2. These two output pins have a driving capacity of 48mA respectively. If they are used in parallel, the driving capacity can be increased to 96mA. For small diameter flow measurement, there is no need for a front-end amplifier circuit, and the FIRE output pulse can be used directly to drive the ultrasonic transducer.

Application of TDC-GP2 in Ultrasonic Flow Meter

TDC-GP2 has a high-precision time measurement function with a resolution of 65ps, which provides basic measurement guarantee for the application of time difference flowmeter; TDC-GP2's pulse generator can directly drive the ultrasonic transducer in small diameter flow measurement without the need for additional driver chips; TDC-GP2's low power consumption characteristics greatly reduce the overall power consumption of the flowmeter, providing an excellent solution for battery-powered equipment.

Compared with ultrasonic flowmeter solutions using discrete components or FPGA, the ultrasonic flowmeter solution using TDC-GP2 greatly simplifies the hardware circuit design. It only needs to match MCU and simple comparator and analog switch components to complete the design of control and time measurement circuits. This solution simplifies the circuit design and greatly reduces the PCB area of ​​the equipment, making the production and maintenance of the equipment more convenient and easy.

TDC-GP2 also has two-way temperature measurement function, which can be directly connected to PT1000 or PT500 thermal resistor for temperature measurement, which provides an integrated solution for the application of heat meters.

Conclusion

Ultrasonic flowmeters will be the future development direction of flowmeters. TDC-GP2 provides ultrasonic flowmeters with the highest integration, highest measurement accuracy and lowest power consumption. The household ultrasonic heat meter solution based on the TDC-GP2 flow measurement principle has been widely promoted by ACAM's China general agent, Shiqiang Telecom. TDC-GP2 has been widely used in ultrasonic heat meters. Shiqiang Telecom can provide you with an easy-to-use evaluation kit. For more details, please call Shiqiang Telecom.