Design and development of intelligent resistance, capacitance and inductance measuring instrument

0 Introduction
    There are currently three methods for implementing RLC measurement. 1) Bridge method, which has high measurement accuracy and is widely used. Now many types have been derived. However, the bridge method measurement requires repeated balance adjustment, the measurement time is long, and it is difficult to achieve fast automatic measurement. 2) Resonance method, which requires a higher frequency excitation signal, is generally not easy to meet the requirements of high accuracy. Since the test frequency is not fixed, the test speed is also difficult to improve. 3) Volt-ampere method, which is the most classic method. Its measurement principle comes from the definition of impedance. Obviously, pure resistance can be divided by DC voltage, but for impedance and capacitive reactance, a higher frequency AC must be used. The circuit is relatively complex, making this solution unrecognized. This system uses the volt-ampere method, which relatively simplifies the circuit and has better human-computer interaction.

1 System solution implementation
    The overall design idea is to add an excitation signal to one end of the network device to be tested and add a sampling resistor to the ground at the other end. Through the automatic switching of the frequency, the AD reads different sampling voltages. We can judge the properties of the components to be tested according to the AD sampling voltage corresponding to the excitation signal, and further switch the sampling resistor to accurately measure the size of the components to be tested. This series of operations are all completed automatically. The system principle implementation block diagram is shown in Figure 1.

2 Hardware Implementation
2.1 Hardware Circuit Diagram
    The system hardware implementation circuit is shown in Figure 2. Considering the internal resistance of the analog switch, we choose relays as the gear switch. In order to measure accurately, this paper uses multiple voltage followers to prevent excessive current from dividing the voltage at the signal source end.

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2.2 True RMS circuit
    system hardware implementation circuit is shown in Figure 3. Considering the internal resistance of the analog switch, we choose relays as the gear switch. In order to measure accurately, this paper uses multiple voltage followers to prevent excessive current from dividing the voltage at the signal source end.
2.3 Self-made test signal source circuit
    According to the needs, take low-pass passive filters with cut-off frequencies of 1kHz, 10kHz, and 100kHz, and shape the PWM or square wave output by the microcontroller (because the MSP430 microcontroller cannot output PWM with too high a frequency, we directly output 10kHz and 10kHz square waves, and filter out the second harmonic and above components through a low-pass filter to obtain its fundamental component) into a sine wave, use relays to switch different filters to obtain different signals, and connect a first-level op amp after filtering each frequency point; amplify to the same amplitude, in order to meet the gain bandwidth product and slew rate of the amplified 100kHz signal, the op amp uses TL084. Through testing, it is found that the passive filter resistor is gradually increased and the capacitor is gradually reduced, which has the best filtering effect. Therefore, the parameters of the filter amplifier circuit diagram shown in Figure 4 are obtained through simulation.

3 Software Implementation
3.1 Algorithm Mathematical Description
    Resistance measurement can be directly obtained by a DC voltage divider, and its formula is:
     R = (V / Vad-1) * R0 (1)
    Capacitance measurement can be obtained by a moderate low frequency f. At this time, the impedance of the capacitor is large. Because there is a -90° phase shift for the capacitor, we take the overall modulus and simplify it to obtain the calculation formula of the capacitance:
   

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3.2 Software Flowchart
    According to the above algorithm analysis, the software flow chart of this paper is shown in Figure 5:

4 Experimental results and analysis
    After the circuit design is completed, this paper gives three sets of experimental test data, which are shown in Table 1, Table 2 and Table 3, respectively. Table 1 is the test data of the resistor network, Table 2 is the test data of the capacitor network, and Table 3 is the test data of the inductor network. The experimental data show that except for the relatively large error of inductance measurement, other measurements can accurately reflect the properties and size of the components to be tested, which can meet general practical needs.

5 Conclusion
    This paper designs an intelligent resistance, inductance and capacitance measuring instrument based on digital control. After the circuit design is completed, it can be seen from the actual measurement data that, except for the relatively large error in inductance measurement, other measurements can more accurately reflect the properties and sizes of the components to be measured; by consulting the data, it is found that the size of the inductance is different at different frequency points, that is, the size of the inductance is related to the corresponding measured frequency point. The design of this system only takes three frequency points, and the maximum frequency is 100kHz, so the error is large. We can reduce the error by increasing the number of frequency points and the maximum frequency and increasing the sampling resistance.