[A-Current Signal Detection Device] Shaanxi Province First Prize_Topic A
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This current signal detection device is composed of a self-wound manganese-zinc magnetic ring, an I/V conversion circuit and an AD conditioning circuit. The power amplifier circuit uses TI's high-current OPA548 operational amplifier chip, and is based on TI's high-performance dual-core, 200MHz TMS320F28379D floating-point microcontroller to perform FFT Fourier transform and display it in real time on the LCD screen. OPA548 can stably output a current of 10mA to 1A peak-to-peak. The I/V conversion composed of a high-permeability manganese-zinc magnetic ring coil and OPA209 is conditioned by the AD pre-stage, and the current signal frequency, current peak-to-peak value and the amplitude of each harmonic component are obtained through 1024-point FFT analysis and displayed in real time. After four days and three nights of hard work, this competition made us deeply aware of the importance of considering non-ideal situations, including the need to take into account the impedance of instruments and lines, as well as the depth of understanding of program modules, so that we can solve problems more specifically when we encounter them. In short, our engineering literacy, time awareness, and cooperation ability have been greatly improved. Current signal detection device Abstract: This current signal detection device is composed of a self-wound manganese-zinc magnetic ring, an I/V conversion circuit and an AD conditioning circuit. The power amplifier circuit uses TI's high-current OPA548 operational amplifier chip, and is based on TI's high-performance dual-core, 200MHz TMS320F28379D floating-point microcontroller to perform FFT Fourier transform and display it in real time on the LCD screen. OPA548 can stably output currents from 10mA to 1A peak-to-peak. It consists of a high-permeability manganese-zinc magnetic ring coil, After the I/V conversion of OPA209 is conditioned by the AD pre-stage, the current signal frequency, current peak-to-peak value and the amplitude of each harmonic component are obtained through 1024-point FFT analysis and displayed in real time. After testing, the system fully meets the accuracy and measurement range required by the question, and some indicators far exceed the requirements of the question, including frequency error less than 0.01%, peak-to-peak measurement error less than 1%, and harmonic amplitude error less than 2%. Keywords: Current signal detection; Mn-Zn magnetic ring; I/V conversion; FFT; Power amplifier circuit 1. System solution 1. Scheme demonstration and comparison [Scheme 1] The power amplifier circuit uses a current feedback operational amplifier, and the current detection circuit uses a current monitor chip. The power amplifier in this solution uses the THS6214 dual-port current feedback op amp, with an output current of 400mA per channel, a BW of 160MHz, and a slew rate of 3800V/us. It is more suitable for high-frequency signals and must be connected in parallel to meet the requirements of this question. Considering that current feedback is prone to phase shift during the negative feedback process, which turns negative feedback into positive feedback, it is very easy to produce self-excitation when the feedback resistor is not selected properly, which increases the complexity of debugging. Using INA282's 50x gain can measure very small current signals, and high-precision differential amplification can be used to sample the voltage drop on the resistor. In the current detection circuit, since the current induced by the inductor coil in the front stage is bipolar, it needs to be level-shifted before it can be input into INA282, and the circuit structure is relatively complex. In addition, INA282 has a fixed gain, which is not convenient for AD front-stage conditioning. At the same time, it was found during debugging that the frequency response flatness was not high. In summary, this solution is not used for this problem. [Solution 2] The power amplifier circuit uses a voltage feedback op amp, and the current detection circuit uses a manganese core magnetic ring and an I/V converter to detect the induced current. By comparing several voltage feedback op amps (BUF634, OPA548), we can choose the OPA548 op amp with a continuous output current of up to 3A and a stronger load capacity. Its internal over-temperature shutdown can greatly enhance the stability of the circuit. At the same time, a heat sink and a fan are installed to meet the requirements of the current peak-to-peak value of not less than 1A and the current signal is not distorted. Compared with the current feedback op amp, this solution has obvious advantages. I/V conversion can use the current-voltage conversion of the OPA209 precision op amp to convert the induced current into a voltage signal. Its high input impedance can be used to isolate the front-stage inductor coil. During debugging, it was found to have extremely low distortion and stable frequency response. After the bias network, the bipolar signal can be conditioned to unipolar. It only needs to adjust the dynamic range to be within the ADC range. Therefore, this solution is suitable for this problem. 2. Overall design Figure 1 System overall plan The system is mainly composed of OPA548 power amplifier circuit, self-wound manganese-zinc magnetic ring coil, I/V converter, and DC bias network. Finally, the FFT result is obtained through TMS320F28379D and the data is displayed on the serial port screen. The input signal can output 10mA to 1A current through the fixed gain power amplifier circuit. The high magnetic permeability manganese-zinc magnetic ring coil performs current mutual induction. The induced current is converted into a voltage signal through the I/V converter composed of OPA209 precision operational amplifier and isolates the front and rear stages. After being translated by the resistor network, it is input into the 12-bit ADC of the microcontroller for sampling and analysis, and the peak-to-peak value and frequency of the current signal are measured in real time. The amplitude of the fundamental wave and each harmonic component is obtained through the 1024-point FFT Fourier transform analysis. 2. Theoretical Analysis and Parameter Calculation 1. Fixed Gain Amplifier Circuit According to the requirements of the question, when the frequency range of the input sinusoidal signal is 50Hz-1000Hz, the peak-to-peak current flowing through the 10Ω load resistor is required to be no less than 1A, and the current signal is required to be distortion-free. OPA548 can continuously output 3A current with a peak value of 5A, which meets the requirements of the question. And the gain-bandwidth product is 1M and the slew rate is 10V/us, which can fully meet the frequency range required by the question. Using the inverting proportional amplifier built by this circuit, the peak-to-peak value of the signal source can be made 0.1 times of the peak-to-peak value of the current. Take R2=R1=1KΩ, then That is, the peak-to-peak value of the loop current is 2. Current detection circuit In order to meet the measurement range and accuracy of the current to be measured of 10mA-1A, the induction coil needs to increase the mutual inductance as much as possible to improve the detection sensitivity of small signals. The mutual inductance depends on the number of coil turns, core material, core size, wire diameter and mechanical strength, winding method, etc. By comparing cores of different materials and different magnetic permeabilities (ferrite, microcrystalline magnetic ring, manganese zinc, etc.), it is finally found that excessive permeability and excessive number of turns will lead to coil saturation. It is necessary to ensure that the induced current waveform is not distorted within the dynamic range of the signal to be measured before it can be restored. From the Ampere loop theorem, Induced electromotive force in the coil Where N is the number of coil turns, S is the cross-sectional area of the coil, μ is the magnetic permeability of the magnetic ring, and R is the coil radius. It can be seen from the formula that the internal resistance of the coil can be reduced, the magnetic permeability can be increased, and the cross-sectional area of the coil can be increased by increasing the wire diameter. Appropriately reducing the coil radius can improve the detection sensitivity and avoid coil saturation. In order for the single-chip microcomputer to sense the current signal, it is necessary to convert the current on the induction coil into a voltage signal. The I/V converter composed of the OPA209 precision op amp can convert the tiny current signal into a voltage signal. The voltage signal can be expressed as: Where i is the induced current, Rf is the potentiometer resistance. Adjusting Rf can make the voltage signal fully utilize the ADC range to improve the current measurement resolution. Using FFT to measure frequency and peak-to-peak value has extremely high accuracy for tiny signals without relying on external circuits. 3. Harmonic component measurement According to the requirements of the topic, the amplitude and frequency of the harmonics are measured. The fundamental frequency is 50-200Hz, and the harmonic measurement does not exceed 1kHz. The single-chip microcomputer uses FFT Fourier transform to perform harmonic analysis on the sampled signal. The ADC sampling frequency fs=2k, the number of points is 1024, and the frequency resolution is A=fs/1024≈2Hz, which meets the harmonic measurement range and accuracy required by the topic. The fundamental frequency is determined by comparing the maximum amplitude of each point in the frequency domain, and then the amplitude at each harmonic frequency is obtained According to the Fourier analysis formula [font=微软雅黑, The harmonic amplitude can be obtained by FFT Fourier analysis. The microcontroller obtains the specific data of the fundamental wave amplitude and each harmonic amplitude by calculation and compares them with the MATLAB standard results. III. Circuit and software design 1. Power amplifier circuit The OPA548 voltage feedback op amp is used to form an inverting proportional amplifier. The OPA548 used has high output current, adjustable current limit, high slew rate, low quiescent current and other excellent characteristics, which can meet the current requirements of output current range of 10mA-1A without distortion. The OPA548 achieves control gain by controlling the variable resistor. When R2=R1=1K, the peak-to-peak value of the signal source can be 0.1 times the peak-to-peak value of the current. Figure 3 OPA548 power amplifier circuit 2. I/V conversion circuit The I/V converter composed of OPA209 precision operational amplifier can convert tiny current signals into voltage signals. Where Rf is an adjustable resistor, which converts the actual induced current signal in the test into a voltage within the ADC range. Rf is a 1kΩ potentiometer, which can adjust the voltage signal to the ADC range to improve the current measurement resolution. Figure 4 I/V conversion circuit 3. Software Design Figure 5 Basic software flow chart The 12-bit ADC voltage resolution of TMS320F28379D is 0.8mV. The voltage signal output by the I/V converter is collected by the induction current, and FFT analysis is performed in real time to obtain the fundamental frequency, which is the frequency of the sine wave. The current signal is calibrated by fitting the fundamental amplitude to obtain the precise peak-to-peak value of the current. At the same time, the harmonic amplitude is obtained based on the signal at each point in the frequency domain calculated by FFT. The sampling frequency fs=2k, the number of points is 1024, which can meet the harmonic measurement below 1K required by the question. 4. Test plan and test results 1. Test Instrument List The test list is shown in Table 1. Table1 Test Instrument List 3.Test results Input frequency/Hz | | | | | | | | | | [/td][td =52] | | [align= center]99.7 | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
Table2 Current Peak-to-Peak ValueTestFrequency Response Stability Vpp=1V Result analysis: By fixing the peak-to-peak value Vpp of the input signal of the signal source and changing the frequency of the input signal, the measurement error caused by the frequency change is determined. The frequency response of the self-wound manganese-zinc magnetic ring is stable within the range required by the question. The test found that the use of I/V change circuit to detect induced current has a higher accuracy. Input voltage / V | | | | | | | | | | | [/td][t d=46] | | [ align=right] 301 [/td ][td=46] | [/td][td=46 ] | | [align =center]800 | | | [ align=right] 1% | | | | | | | | | | | | | | | | |
结果分析：在固定频率下，改变输入信号的幅值来改变环路电流，确定10mA-1A范围内的测量精度。可见使用FFT测量峰峰值具有较高的精确性。
结果分析：使用FFT测量频率对大小信号都有极高的准确性，完全达到频率精度要求。FFT分解出基波信号后，通过对基波频率测量得出准确频率值，而不受信号幅值大小的影响。
Result analysis: The use of FFT has a high accuracy in decomposing low-order harmonics. The 1st to 7th harmonics of the non-sinusoidal signal current are within the error range after analysis with the MatLab theoretical calculation results, meeting the accuracy required by the question. For higher harmonics, they have been optimized through further calibration. V. Summary This system combines the power amplifier circuit made of OPA548, manganese core magnetic ring coil, I/V conversion circuit, and TMS320F28379D microcontroller as the terminal sampling display. The peak-to-peak value, frequency and harmonic amplitude of the current signal are accurately measured by FFT. The frequency error is less than 0.01%, the peak-to-peak measurement error is less than 1%, and the harmonic amplitude error is less than 2%. After the final cascade and debugging, the system has stable performance and can fully meet the requirements of the topic. Some indicators such as frequency, peak-to-peak value, and harmonic amplitude exceed the accuracy requirements of the topic.
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