Development of micro-capacitance measurement module based on RISC-SOC

0Introduction

Capacitive sensors are widely used in industry, military and other fields. Therefore, accurate measurement of capacitance, especially small capacitance, has always been a very important content. At present, most measurement methods have low integration level and some have low precision. The bridge method uses the bridge balance principle to measure capacitance, and the measurement result is greatly affected by the performance of the bridge arm capacitance. The oscillation method has a simple circuit structure, but when the capacitance to be measured is below 100PF, the internal capacitance between the plates often contaminates the measurement result; in addition, the oscillation method has poor anti-interference ability to measure capacitance. The capacitive reactance matching method proposed in this paper is to connect the capacitor to be measured to the capacitive reactance matching circuit. The capacitor to be measured presents a fixed capacitive reactance under high-quality AC excitation. Through the capacitive reactance-voltage conversion circuit, a voltage value proportional to the capacitance can be obtained. After ADC sampling, the capacitance value can be calculated. Experimental results show that this method can ensure measurement accuracy and has strong anti-interference ability.

1 Basic Principle of Micro-Capacitance Measurement Module

The principle block diagram of the micro-capacitance measurement module is shown in Figure 1, and the appearance is shown in Figure 2.

The module includes a lead capacitance suppression circuit, a capacitive reactance voltage conversion circuit, an integrated RISC-SOC mixed signal processor, a 485 interface, an LCD display, etc. The working principle of the module is as follows: the RISC-SOC mixed signal processor chip uses the DDS direct digital frequency synthesis method according to the CPU instructions to generate a stable sine wave with a stability better than 1/1000 and a distortion less than 1/1000 through 12-bit D/AC as the excitation source for measurement. Under the action of this AC excitation source, the capacitor to be measured presents a fixed capacitive reactance Z.

It can be seen from formula (1) that the size of 1/Z is proportional to the size of the capacitance value. A voltage signal proportional to the capacitance value can be obtained through a 1/Z-voltage conversion circuit, and the capacitance value can be calculated based on this. The sine wave table ROM contains a 64-point sine wave table. Keeping the DDS frequency unchanged, the peak value of the wave table is increased by 10 times, and the range can be reduced by 10 times; changing the number of points of the sine wave table to 640 points can obtain a sine wave with a frequency of 1OOHZ, and the range is expanded by 10 times. Therefore, there is no need for a hardware switch, and the range can be switched by a pure software method. The 485 interface circuit mainly transmits data according to the Modbus-RTU protocol; the LCD performs real-time online display. The lead capacitance suppression circuit is used to eliminate the influence of the lead capacitance itself on the measurement and shield external interference. [page]

2 Microcapacitance measurement module circuit design

The microcapacitance measurement design is shown in Figure 3.

2.1 Mixed Signal Processor

We chose TI's MSP430F4270 microcontroller as the mixed signal processor. The MSP430F4270 microcontroller integrates a large number of CPU external resources such as a 16-bit RISC processor, a 12-bit DAC, a 16-bit ADC, a 32K code memory, a liquid crystal driver, a liquid crystal bias generator, and a watchdog. Using the 12-bit DAC of the 4270 microcontroller, a 1KHZ sine wave is generated in the interrupt in the form of a table lookup DDS as the excitation source of the measurement circuit. The measurement result is sampled by the 16-bit ADC of the 4270 microcontroller and converted into a capacitance value. The final measurement result is displayed on the segment code LCD. Since the internal integrated liquid crystal controller and bias generation circuit, the hardware only needs to directly connect the liquid crystal and the segment code control pin. The conversion result can also be communicated with the outside world through the 485 interface. The 4270 microcontroller does not have a serial port, and the serial port can only be realized by software simulation. The 4270 manual recommends using a 32K external crystal and multiplying the frequency to get the CPU clock. However, considering that the frequency lock loop (FLL) of the internal frequency multiplication has frequency jitter and interferes with the sine wave, the multiplication rate should be as low as possible. This system selects a 4MHz external crystal and doubles the frequency (the lowest multiplication value of the FLL) to obtain an 8MHz system clock.

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2.2 Capacitance Measurement Circuit

The capacitance measurement circuit is shown in Figure 4. The sine wave generated by DDS is a sine wave with a voltage range of 0-1.2V, that is, a sine wave with a DC voltage of 0.6V and a peak-to-peak voltage of 1.2V. First, the DC component is isolated by C1, and then amplified by the op amp U3A by 10 times to become a sine wave with positive and negative symmetry and a peak-to-peak voltage of 12V. The sine wave generated by DDS still has residual high-frequency components. C2 and R2 form a low-pass filter with a transition frequency of 3KHz, which retains the excitation sine wave and filters out the residual high-frequency components. The amplified sinusoidal voltage is applied to the measured capacitor Cx. Under this excitation, the current flowing through Cx is converted into a voltage value by U3B. U3B outputs a sinusoidal voltage with an input amplitude proportional to the size of Cx and has nothing to do with the length of the shielded lead. C3 is used to further filter out the residual high frequency. U3C and U3D form a precision detection circuit to convert the output AC voltage of U3B into a DC voltage. The output of the op amp cannot be directly connected to the Sigma-Delta ADC inside the 4270. It is necessary to add an RC filter before sampling.

3 Lead capacitance suppression

Since the wire itself has capacitance, it will interfere with the capacitance measurement, so measures should be taken to reduce or eliminate the lead capacitance. As shown in Figure 5, Cy1 and Cy2 are equivalent lead capacitances. Cy1 is between the excitation source and the ground, and is connected in parallel with Cx. Since the excitation source has a very low impedance, the current between a and c has almost no effect on the voltage between a and b. Therefore, Cy1 will not affect the measurement. Cy2 is between point b and the ground. Point b uses an op amp to give a virtual ground, so there is no voltage difference between b and c. And the capacity of the distributed capacitance Cy2 between b and c is relatively small (tens of pF), and the impedance is high, so the voltage between b and c is almost 0, and no current passes. It can be seen that Cy1 and Cy2 will not affect the measurement. The dotted line in the figure is the shielding layer, and the shielding layer of the cable completely shields the influence of the external interference electric field on the measurement.

Figure 5 Lead suppression circuit

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4 Communication interface

4.1 Interface between microcapacitance measurement module and PC

As shown in Figure 6, the measurement module can be connected to a PC via a 485-232 adapter. Running the CapMonitor software on the PC allows for remote control and viewing, as well as calibration and setup.

4.2 Interface between microcapacitance measurement module and MCU

The IO port of the microcontroller is TTL level, so a MAX485 or similar 485 level transceiver chip is required. The reference circuit is shown in Figure 7:

If the IO port of the microcontroller is similar to the 51 microcontroller and has a weak pull-up, R3 may not be needed.

5 Conclusion: The capacitance measurement module designed in this paper has a high degree of integration and low power consumption. The capacitance measurement error is ≤±1% within 200PF and ≤±0.5% above 200PF. The range can reach 0-20000PF. It can be made into a portable capacitance measuring instrument and can also be used as a component of some measurement systems.