CN104020346A - Micro-current signal extraction system - Google Patents

Abstract

The invention relates to a micro-current signal extraction system, which adopts high-precision resistance sampling signal, converts the current signal into a voltage signal and sends it to an analog signal pre-conditioning circuit, and then passes through a primary amplification circuit, an analog filter circuit and a secondary amplification circuit in sequence to complete the analog signal. Pre-conditioning, and then output the signal after the analog signal pre-conditioning to the digital signal pre-conditioning circuit, and then pass through the A/D conversion circuit, range conversion processing and digital filter circuit in turn, and output the required micro-current signal after digital signal conditioning. Combining analog signal and digital signal conditioning, the preamplifier of the system chooses a chopper-stabilized integrated operational amplifier with high gain, high input impedance, and high common-mode rejection ratio, and a dual-integral AD analog-to-digital conversion chip, which suppresses the power frequency of 50Hz Interference, while 4-wire serial communication greatly saves I/O interface, efficient and reliable data transmission. Accurate measurement of micro-current can be realized, which can be used for insulation monitoring of electrical equipment.

Description

Microcurrent signal extraction system

technical field

The invention relates to a signal processing technology, in particular to a microcurrent signal extraction system.

Background technique

Micro-current signal extraction has a wide demand in the online monitoring of electrical equipment in power systems. However, in the power system, the operating environment of electrical equipment is complex, and there are various noise interferences, mainly including power line noise, electrical equipment noise, radio frequency noise, and mechanical origin noise. Power line noise is mainly caused by power frequency electromagnetic fields, peak pulses caused by high-power switch on and off, and grid voltage fluctuations; electrical equipment noise mainly includes power frequency electromagnetic fields generated by equipment, corona discharge spark discharge, arc discharge, etc. Noise interference; radio frequency noise is mainly interference caused by communication tools such as TV, radar, and wireless communication in daily life, mostly modulated electromagnetic waves; mechanical noise is mainly mechanical vibration converted into electrical noise, such as piezoelectric effect noise, microphonic effect noise, etc. Therefore, it is particularly important to condition the current signal in the extraction of micro-current signals. Only by performing necessary and appropriate filtering on the signal can the required signal be extracted and the operating status of the equipment can be correctly judged.

At present, the technical level of the research and development of the weak current acquisition system at home and abroad has become increasingly mature. The highest current resolution of the foreign advanced potentiostat can reach 30fA, but its price is expensive. There is still a gap between domestic product quality and technical indicators compared with foreign countries, and further research is needed. In view of the above situation, this patent designs a circuit capable of measuring tiny currents, thereby realizing the monitoring of the insulation state of electrical equipment.

Contents of the invention

The invention aims at the problem that the accuracy of the weak current acquisition system lags behind foreign countries, and proposes a micro-current signal extraction system to realize the precise measurement of the micro-current.

The technical solution of the present invention is: a micro-current signal extraction system, including signal acquisition, analog signal pre-conditioning circuit and digital signal pre-conditioning circuit, adopts high-precision resistance sampling signal, converts current signal into voltage signal and sends analog signal pre-conditioning Circuit, the voltage signal passes through the primary amplification circuit, the analog filter circuit and the secondary amplification circuit in turn to complete the analog signal pre-conditioning, the analog filter adopts the fifth-order Butterworth low-pass filter to filter out all signals higher than 3HZ; the analog signal The pre-conditioned signal is output to the digital signal pre-conditioning circuit, and then passes through the A/D conversion circuit, range conversion processing and digital filter circuit in turn, and outputs the required micro-current signal after the digital signal conditioning is completed.

The voltage signal output by the signal acquisition is output to the analog signal preconditioning circuit after being isolated by a linear optocoupler and a transient suppression tube connected in parallel.

The primary amplifying circuit selects the chopper-stabilised integrated operational amplifier ICL7650, and the ICL7650 is connected as an inverting amplifier. Diode protection circuit, ICL7650 operational amplifier input and output capacitors in parallel to suppress high-frequency oscillation.

The analog filter circuit uses a fifth-order Butterworth switched capacitor filter, which can be a MAX7410 filter from MAXIM Company.

The A/D conversion circuit selects a double-integral A/D chip capable of suppressing 50HZ power frequency interference, and can adopt a MAX132-18-bit double-integral AD chip of Maxim Corporation.

The beneficial effects of the present invention are: the micro-current signal extraction system of the present invention combines analog signal and digital signal conditioning, and the pre-amplification of the system selects the ICL7650 chopping and zero-stabilizing integrated operation with high gain, high input impedance, and high common mode rejection ratio Amplifier, 18-bit double-integral AD analog-to-digital conversion chip, suppresses 50Hz power frequency interference, and at the same time, the serial communication of MAX132 greatly saves I/O interface, efficient and reliable data transmission. Accurate measurement of micro-current can be used for insulation monitoring of electrical equipment, such as high-voltage cable insulation and leakage current measurement of sheath, and good results have been obtained.

Description of drawings

Fig. 1 is a block diagram of the microcurrent signal extraction system of the present invention;

Fig. 2 is a microcurrent signal extraction system sampling and amplifying circuit diagram of the present invention;

Fig. 3 is the fifth-order Butterworth analog filter circuit diagram of the microcurrent signal extraction system of the present invention;

Fig. 4 is the 18-bit AD conversion circuit diagram of the micro-current signal extraction system of the present invention;

Fig. 5 is a general flowchart of the micro-current signal extraction system of the present invention.

Detailed ways

The system analyzes the measured signal components, and the micro-current signal extraction module uses high-precision resistance sampling signals to convert current signals into voltage signals for easy extraction. Perform primary amplification, analog filtering and secondary amplification on the converted signal to complete the analog signal pre-conditioning. Considering that the signal contains many noise signals, a 5th-order Butterworth low-pass filter is used to filter out all signals higher than 3HZ. This system chooses MAX7410 chip to realize its function very well. After the filtered signal is amplified twice, the output is A/D converted, and the analog signal is converted into a digital signal, which is convenient for subsequent data processing. This system uses the MAX132 chip, which can realize 18-bit high-precision A/D conversion. Suppress 50HZ power frequency interference, and then go through range processing and MCU digital signal processing, mainly digital filtering, to get the desired data signal and complete digital signal conditioning. Finally, the signal is displayed through LCD12864, as shown in Figure 1, the system block diagram. In order to prevent the input signal from being too large and causing damage to the subsequent measurement circuit, the input signal is protected and isolated. It first passes the signal through the parallel transient suppression tube. When the signal exceeds its threshold, the voltage remains at the transient suppression The voltage value of the tube, so as to ensure that the input voltage does not exceed the maximum value that the subsequent measurement circuit can withstand. In order to prevent the breakdown of the sampling resistor due to improper measurement, which will cause the ground potential to rise and cause damage to the measurement circuit, a linear optocoupler is used for isolation.

Sampling and amplification circuit diagram as shown in Figure 2: ICL7650 is a chopper-stabilized zero-integrated operational amplifier, which has the characteristics of ultra-low offset voltage and ultra-low drift, high gain, high input impedance, high common-mode rejection ratio, etc., and its performance is extremely superior and stable . It is often used as a preamplifier in circuits that measure weak signals. In Figure 2, the ICL7650 is connected as an inverting amplifier. R8 is a 10K high-precision sampling resistor, which converts the flowing current into a voltage Vin; then the voltage signal is added to the two input terminals of the ICL7650, and the weak signal is amplified and output from pin 10 of the chip. In order to protect the ICL7650 from burning out due to the large input signal amplitude, 4 diodes are connected at the input end of the signal to protect the circuit. The capacitor C1 added to the input and output terminals of the ICL7650 operational amplifier is to prevent the circuit from generating high-frequency oscillation. Because of its small capacity, it has very little influence on the amplification factor of the signal.

As shown in Figure 3, the fifth-order Butterworth analog filter circuit diagram uses MAXIM's MAX7410 filter, which is a fifth-order Butterworth, switched capacitor filter (SCFS). The circuit is powered by 5V, has low power consumption, and a low-pass filter with a bandwidth of 15KHZ. The circuit's cutoff frequency is determined by the circuit's clock signal frequency (see Equation 2). The clock signal of the circuit can be provided in two ways, one is to connect a capacitor directly to the 8-pin, and realize oscillation with the chip to generate a clock signal, and its clock frequency depends on the size of the external capacitor (see formula 1), and the other is Input the external clock directly from pin 8, this way can change the frequency of the external clock signal at any time, so as to conveniently change the cut-off frequency of the filter circuit. According to the requirements of the signal, select C _OSC as 0.1uF (here refers to C11 in the figure), and the cut-off frequency is 3Hz. (Formula 1)

(Formula 2)

The function of the A/D converter is to convert the input analog signal into digital form, so that the CPU can process the analog signal received from the circuit under test. As shown in Figure 4, the 18-bit AD conversion circuit diagram, the A/D conversion chip of this system is the MAX132-18-bit dual-integral AD chip of Maxim, which has high conversion accuracy and can suppress 50HZ power frequency interference. And has the following advantages:

(1) High measurement accuracy and short conversion cycle: MAX132 internally adopts a multi-slope integration method to provide high-resolution conversion that is much faster than the standard AD conversion time, and the conversion times are greater than 100 times/s. at full scale for In the case of 512mV, the input signal resolution can reach 2 V/LSB.

(2) Low power consumption: MAX132 uses CMOs circuit to ensure that its typical operating current is only 60uA; in sleep mode, it even only needs 1uA current to be in the monitoring working state.

(3) Adopt 4-wire serial interface: MAX132 adopts a unique 4-wire interface mode, only need four wires: chip select (CS), clock (SCLK), data input (DIN), data output (DOUT) The I/O interface with any general-purpose microprocessor can be realized conveniently, and the time of the whole communication process is guaranteed at the millisecond level.

(4) 4-bit programmable parallel output: MAX132 also has a 4-bit programmable digital output that can be set by serial commands, which can be used to control the gain of the input signal or set the working environment, and can even be directly used to expand the parallel interface.

MAX872 is a precision micro-power 2.5V voltage chip, which provides 2.5V high-speed reference voltage for AD chips, with ultra-low quiescent current and 200mV low dropout voltage.

System overall process design: After the system is powered on, first initialize the entire system. The initialization program includes initializing and clearing various parameters and chips, and setting some port attributes. Then judge the range, and then call the A/D conversion subroutine for data acquisition, and perform A/D analog-to-digital conversion on the input signal. After the conversion, in order to improve the accuracy of the measured signal, the system continuously collects the input voltage 10 times and performs sliding average filtering processing, and the result is used as the corresponding voltage value. Then convert the value, convert the obtained value into the required display quantity, call the display subroutine of LCD12864, display the interface of the panel, and finally call the communication subroutine to realize serial communication. The overall flow chart of the system is shown in Figure 5:

The article expounds the characteristics of the weak signal, which contains various complex noises. In order to filter out the noise signal, the system adopts the method of combining analog filtering and digital filtering. The preamplifier of the system chooses the ICL7650 chopper-stabilized integrated operational amplifier with high gain, high input impedance, and high common-mode rejection ratio, and the 18-bit double-integral AD analog-to-digital conversion chip, which suppresses the 50Hz power frequency interference. Line communication greatly saves I/O interface, efficient and reliable data transmission. The small current signal extraction system developed according to this paper can realize accurate measurement of micro current, and can be used for insulation monitoring of electrical equipment, such as high voltage cable insulation, leakage current measurement of sheath, and has achieved good results.

Claims (5)

1. A micro-current signal extraction system, characterized in that it includes signal acquisition, an analog signal pre-conditioning circuit and a digital signal pre-conditioning circuit, adopts high-precision resistance sampling signals, converts current signals into voltage signals and sends analog signal pre-conditioning circuits , the voltage signal passes through the primary amplifier circuit, the analog filter circuit and the secondary amplifier circuit in turn to complete the analog signal pre-conditioning. The conditioned signal is output to the digital signal pre-conditioning circuit, and then passes through the A/D conversion circuit, range conversion processing and digital filter circuit in turn, and outputs the required micro-current signal after the digital signal conditioning is completed. 2. The micro-current signal extraction system according to claim 1, wherein the voltage signal output by the signal acquisition is output to the analog signal pre-conditioning circuit after being isolated by a linear optocoupler and a transient suppression tube connected in parallel. 3. according to the described micro-current signal extracting system of claim 2, it is characterized in that, described primary amplifying circuit selects chopping wave stable zero integrated operational amplifier ICL7650 for use, ICL7650 is connected into reverse amplifier, 10K high-precision sampling resistor two ends connect ICL7650 respectively The input terminal, two positive and negative diode protection circuits are respectively connected between the two input terminals and the ground, and the input and output terminals of the ICL7650 operational amplifier are connected in parallel with capacitors to suppress high-frequency oscillation. 4. according to the described microcurrent signal extraction system of claim 2, it is characterized in that, described analog filtering circuit selects the fifth-order Butterworth switched capacitor filter for use, can adopt the MAX7410 filter of MAXIM Company. 5. according to the described micro-current signal extraction system of claim 2, it is characterized in that, described A/D conversion circuit selects the dual-integral A/D chip that can suppress 50HZ power frequency interference for use, can adopt the MAX132-18 double integral A/D chip of Maxim Company. Integral AD chip.