A practical solution for high-precision measurement of displacement devices using microcontrollers and FPGAs

Displacement sensors are widely used in industrial and control fields, such as process detection, physical measurement and automatic control. Due to its low measurement accuracy, it often cannot meet social needs and also limits the application of sensors. Therefore, a displacement measurement device based on microcontroller and FPGA is designed here , which can also achieve high linearity, replace manual work in various harsh environments, achieve high-precision measurement, and has certain practical value.

1 Overall design scheme and implementation block diagram

The overall block diagram of the system is shown in Figure 1, which consists of a signal generation part, a differential amplifier part, a transformer coupling part, a signal processing part, a data sampling part, and a processing and display part. The signal generated by DDS technology is sent to the differential transformer after differential amplification by THS4503. The signal output by the differential transformer is sent to MAXl97 for sampling after amplification, rectification and filtering. The sampled data is processed by the processing unit and the measured displacement is displayed on the LCD .

2 Analysis of data processing methods

The differential transformer is an open magnetic circuit. The mutual inductance between the primary and secondary sides changes accordingly with the movement of the magnetic core, causing the voltage of the two secondary coils to change accordingly. The change in displacement is converted into a change in the output voltage. After rectification, data is collected and processed to obtain the d value. Figure 2 shows the table lookup method used for differential transformer data processing: First, a vernier caliper is used to measure several groups of displacement values. The number of groups measured is determined according to the measurement range and the measurement results, and the corresponding d value is recorded and drawn into a table. In actual measurement, the displacement range is determined by table lookup based on the measured d value, and the segmented broken line method is used within this range to obtain the accurate displacement value. The table lookup method can accurately locate the displacement range, and the data obtained has a small error and high accuracy.

3 Differential amplifier circuit

THS4503 is selected as the differential amplifier circuit. Since the frequency of the excitation signal is fixed at 100 kHz, a capacitor is added to the feedback resistor of the differential amplifier to achieve the effect of filtering and avoiding self-excitation. The signal output from the low-pass filter needs to be amplified by the differential amplifier and then output a pair of differential signals to provide voltage for the primary coil of the transformer. The power supply adopts ±5V dual power supply, and the two output terminals of THS4 503 are connected to the two ends of the primary coil of the transformer through two isolation resistors with a resistance of 12Ω. The specific circuit diagram is shown in Figure 3.

4 Test content and results

During the circuit debugging process, each module was debugged separately, and the debugging results were good. The debugging data table is omitted. When debugging the whole machine, the signal waveforms at points A, B, and C were measured. The signal waveform at point C is good and undistorted. The DC signal output at points A and B has no ripple jitter and the value is accurate. For displacement measurement, the experimental data are shown in Tables 1 and 2. By analyzing the displacement measurement results, it can be seen that the displacement measurement has a high accuracy, the maximum error is 0.5 mm, and the measurement range is -20 to 20 mm. In general, the completion of the entire system is good.

5 Conclusion

The system's displacement measurement range is extended to -20 to 20 mm, and the actual measured approximate linear range of the homemade differential transformer is about -24 to 24 mm. It can achieve higher-precision measurements and a good dynamic range, but the linearity is not very good. This is mainly due to the limitations of factors such as the non-ideal uniformity and symmetry of the coil winding and the non-ideal core specifications. However, software correction can greatly improve the accuracy of displacement measurement, and the stability of the linearity will also be improved.