Design of inductive displacement sensor circuit system

With the mature development of sensor technology, sensors have been widely used in various measuring devices. In many geometric measurement devices, displacement sensors are an indispensable component. For example, the 891EA gear measuring center produced by Mahr is an early electronically developed measuring center. The probe it uses is a lateral displacement probe, and the sensor of this probe is a one-dimensional inductive displacement sensor. Due to hardware limitations, the original probe circuit system has a small linear measurement range and low accuracy, which can no longer meet the measurement needs of 891EA.

In this paper, a probe circuit system is designed based on the characteristics of the sensor. It can not only meet the mechanical action of the measurement center, but also improve the measurement accuracy on the basis of completing the measurement function of the original circuit, expand the detection range of the probe, and improve the safety of measurement; at the same time, the probe circuit system is connected with the A/D card to realize the automatic detection function under the control of the computer.

This paper introduces a circuit system of inductive displacement sensor. The system uses an AD698 chip as the core of the signal adjustment circuit, converts the displacement output signal into the corresponding DC voltage value, and combines with a series of other circuit modules to realize the displacement measurement of the probe. The calibration test of the probe proves that the system has high accuracy and large linear measurement range.

1 System structure and working principle

Figure 1 Block diagram of the probe circuit system

Figure 1 is a block diagram of the probe circuit system. The probe circuit system mainly consists of five parts: signal conversion circuit, operational amplifier circuit, filter output circuit, range switching circuit and window voltage comparison circuit. The sensor outputs an AC voltage signal, the voltage value is proportional to the position of the sensor core, and it is converted into a corresponding DC voltage signal through the signal conversion circuit.

The operational amplifier circuit amplifies the DC voltage signal to meet the voltage requirements of the subsequent circuit; the amplified DC signal is output to the A/D card through the filter output circuit, and automatic detection is achieved under computer control; at the same time, the filtered signal is displayed through the range switching circuit to display the DC voltage signal as a displacement value corresponding to different ranges of the meter, thereby providing intuitive measurement results; the filtered signal can detect the displacement state of the probe through the window voltage comparison circuit, and display the output in the form of detection, installation, alarm and other states, ensuring the safety of the installation and detection process.

2 Main functional modules

2.1 Signal conversion circuit

The function of the signal conversion circuit is to convert the AC voltage signal output by the sensor into a corresponding DC voltage signal. The design of the signal conversion circuit directly affects the measurement accuracy of the entire probe circuit system and is the core part of the probe circuit system.

The sensor is a linear differential displacement sensor, whose input is the mechanical displacement of the magnetic core, and whose output is an AC voltage signal proportional to the position of the magnetic core. The primary coil of the sensor is excited by an external reference sine wave signal source, and the two secondary coils are connected in series in reverse. The movement of the magnetic core can change the coupling flux between the primary coils, thereby generating two AC voltage signals with different amplitudes.

Figure 2 AD698 working principle diagram [page]

2.2 Operational amplifier circuit and filter output circuit

According to the characteristics of the sensor output signal, a signal conversion circuit based on the AD698 chip is designed. AD698 is a high-precision linear differential displacement transformer (LVDT) dedicated signal adjustment circuit that can simultaneously receive two differential AC voltage signals. Figure 2 is a working principle diagram of using AD698 to convert the sensor signal.

AD698 uses a sine wave function oscillator and power amplifier to drive the primary coil of the sensor, and uses two synchronous demodulators to decode the primary and secondary voltages, and finally outputs a DC voltage signal after filtering and amplification; AD698 can obtain the output voltage value of the secondary coil and the reference voltage value of the primary coil output through synchronous demodulation, and can achieve zero-position voltage drift compensation by calculating the A/B ratio and setting the external component parameters accordingly, thereby meeting the measurement accuracy requirements; AD698 has a wide input voltage and output voltage range (both positive and negative voltage input and output can be achieved), which can realize the measurement of the probe from zero position to maximum displacement and from maximum displacement back to zero position, expanding the measurement range.

The operational amplifier circuit and the filter output circuit further adjust the different AC voltages of the converted signal, which can well match the AD698 chip and meet the processing requirements of the next level circuit. The filter output circuit is composed of a second-order "Π" type filter circuit, which can ensure a shorter delay time while meeting the filter accuracy requirements.

2.3 Window Voltage Comparison Circuit

The function of the window voltage comparison circuit is to display the motion state of the probe in real time. It is composed of multiple comparison circuit units with different parameters, and compares the voltage signal after filtering output with the reference voltage value of each comparison unit, thereby reflecting the different motion states of the probe in real time.

Combined with the external signal light and buzzer, the working state of the probe can be judged and the probe can be controlled accordingly, thereby preventing the danger caused by the probe's over-range movement and improving measurement safety. The probe movement states set here are installation state, detection state and alarm state.

2.4 Range switching circuit

The function of the range switching circuit is to display the voltage signal as the displacement value corresponding to the different ranges of the meter, thereby providing intuitive measurement results. The range switching circuit is composed of three amplifier circuits with different magnifications, and an external meter (displayed as the displacement corresponding to the voltage value) is connected to realize the display of the output voltage value corresponding to the displacement of the sensor. The magnification of the range switching circuit corresponds to the different ranges of the meter. By setting the circuit resistance parameters, the three ranges of 500, 150, and 30μm can be selected for the probe.

3 Experimental verification

The left and right tooth surface displacement measurement of the probe is calibrated using a first-class ring gauge calibration device and a digital voltmeter. The calibration method adopts the static characteristic calibration method. The calibration test records are shown in Figures 3 and 4. In the figure, k1 and k2 represent the sensitivity of the probe when it moves from zero position to maximum displacement and from maximum displacement back to zero position, respectively.

The calibration formula is as follows:

Where: U is the output voltage value of the probe circuit system, V; k is the probe sensitivity, mV/mm; S is the probe displacement, mm; b is the zero position voltage value offset, mV.

Figure 3 Left tooth surface measurement data curve

Figure 4 Right tooth surface measurement data curve

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From the calibration records we can get:

1) Probe sensitivity

Left tooth surface: - 010186 mV /mm;

Right tooth surface: 010183 mV /mm.

2) Probe displacement linear measurement range

Left tooth surface: 312~51112μm;

Right tooth surface: 411~50717μm.

3) Zero voltage value offset

Left tooth surface: 41412 mV;

Right tooth surface: 51027 mV.

Experimental data show that the probe circuit system has high measurement accuracy and a large linear measurement range (±500 μm), which meets the measurement requirements of the 891EA gear measurement center.

4 Conclusion

The circuit system can convert the fine displacement detected by the probe into a corresponding DC voltage signal through a series of signal adjustments, filtering and amplification. The probe calibration test has proved that the system has high measurement accuracy, large linear measurement range and good measurement safety performance, meeting the measurement requirements of the 891EA gear measurement center.