Design of electrical parameter measurement system based on single chip microcomputer

In automatic measurement and control systems, it is often necessary to measure and display relevant electrical parameters. At present, most measurement systems still use transformer-type voltage and current transformers. Due to the non-ideality of the transformer, there are large errors in the ratio and phase measurement, which often require hardware or software compensation, thereby increasing the complexity of the system. The use of Hall detection technology can overcome these shortcomings of the transformer, can measure AC signals of various shapes from DC to hundreds of kilohertz, and achieve undistorted transmission between the primary and secondary sides, while achieving isolation between the main circuit loop and the electronic control circuit. The output of the Hall sensor can be directly interfaced with the single-chip microcomputer. Therefore, Hall sensors have been widely used in microcomputer measurement and control systems and intelligent instruments, and are a new generation of products to replace transformers. Here, the use of Hall sensors to measure electrical parameters, especially high voltage and high current parameters, is proposed.

　　1 Measurement principle

　　1.1 Hall Effect Principle

　　As shown in Figure 1, an N-type semiconductor sheet, with a length of L, a width of S, and a thickness of d, is applied with a magnetic field of magnetic induction intensity B in the direction perpendicular to the plane of the semiconductor sheet. If a current Ic is passed in the length direction, the moving charges are acted upon by the Lorentz force, and the positive and negative charges will move toward the two ends of the conductor in the direction perpendicular to the magnetic field and the current, respectively, and form a stable electromotive force UH at both ends of the conductor, which is the Hall electromotive force (or Hall voltage). This phenomenon is called the Hall effect. The magnitude of the Hall voltage UH=RIB/d=KHICB, where R is the Hall constant; KH is the product sensitivity of the Hall element.

　　1.2 Principle of measuring electrical parameters using Hall sensors

　　From the Hall voltage formula, we know that for a formed Hall sensor, the product sensitivity KH is a constant value, then UH∝ICB. As long as the size of UH is measured by the measuring circuit, if one of the two parameters B and IC is known, the other can be calculated. Therefore, any unknown quantity that can be converted into B or J can be measured using the Hall element, and any unknown quantity converted into the product of B and I can also be measured. The measurement of electrical parameters is based on this principle. If the control current IC is a constant, the magnetic induction intensity B is proportional to the measured current, and a Hall current sensor can be made to measure the current. If the magnetic induction intensity B is a constant, IC is proportional to the measured voltage, and a voltage sensor can be made to measure the voltage. The Hall voltage and current sensors can be used to measure the power factor, electric power and frequency of the AC.

　　From UH=KICB, we know that if IC is DC and the current IO that generates the magnetic field B is AC, UH is AC; if IO is also DC, the output is also DC. When IC is AC and IO is also DC, the output is AC with the same frequency as IC and its amplitude is proportional to the measured DC IO. When the direction of the measured current IO is changed, the polarity of the output voltage UH changes accordingly. Therefore, the Hall sensor can be used to measure both DC and AC quantities.

　　2 System structure diagram

　　The structure of the detection system is shown in Figure 2. The measured value is converted into a voltage signal by the Hall sensor, selected by the signal conditioning circuit and the multi-way conversion switch, and sent to the single-chip microcomputer through the A/D converter. The single-chip microcomputer uses 89C51, which is the main controller of the system. The keyboard uses a 2×4 keyboard to select the type of measured value, and a digital tube or liquid crystal is used to display the size of the measured value.

　　3 Measurement methods of electrical parameters

　　3.1 Measurement of voltage and current signals

　　The current can be measured using a magnetic balance Hall current sensor (also called a zero flux Hall sensor) as shown in FIG3 .

　　When the measured current IIN flows through the primary loop, a magnetic field HIN is generated around the wire. This magnetic field is concentrated by the magnetic ring and induced to the Hall device, causing it to output a signal UH. This signal is amplified by the amplifier A and input into the power amplifier Q1 and Q2. At this time, the corresponding power tube is turned on, thereby obtaining a compensation current IO. Since the magnetic field HO generated by this current passing through the multi-turn winding is opposite to the magnetic field HIN generated by the primary loop current, the original magnetic field is compensated, so that the output voltage UH of the Hall device gradually decreases. Finally, when the magnetic field generated by IO multiplied by the number of turns N2IO is equal to the magnetic field generated by the primary N1IIN, IO no longer increases, and the Hall device achieves zero flux detection. The time to establish this balance is within 1 μs. This is a dynamic balance process, that is, any change in the primary loop current IIN will destroy this balanced magnetic field. Once the magnetic field loses balance, the Hall element will output a signal. After amplification, the corresponding current will immediately flow through the secondary coil for compensation. Therefore, from a macroscopic point of view, the ampere-turns of the secondary compensation current are always equal to the ampere-turns of the primary current, that is, N1IIN=N2IO, so IIN=N2I2／N1 (IIN is the measured current, that is, the current in the primary winding in the magnetic core, N1 is the number of turns of the primary winding; IO is the current in the compensation winding; N2 is the number of turns of the compensation winding). From the number of turns of the primary and secondary sides, it can be known that as long as the current IO of the compensation coil is measured, the primary current IIN can be known. If the primary side is a wire through-the-core type, then N1=l, IIN=N2IO. The same principle can be used to measure voltage. Just connect a resistor R1 in series in the primary coil loop to convert the primary current IIN into the measured voltage UIN. That is, UIN=(R1+RIN)IIN=(R1+RIN)N2IO／N1, RIN is the internal resistance of the primary winding (generally very small and negligible). To measure the UHV AC voltage, it is necessary to first reduce the voltage through an isolation transformer, measure the reduced voltage, and then multiply the measured data by the reduction factor to get the voltage being measured. The measured output signal is in the form of current IO. If an appropriate resistor R0 is connected in series between the output circuit of the Hall current sensor and the zero point of the power supply, and the voltage is taken across the resistor, a voltage output is formed. The output voltage is obtained through a voltage adjustment circuit, a linear amplifier circuit, an unequal position compensation circuit, an emitter-collector follower, etc. to obtain the required voltage, which is convenient for measurement and display. [page]

　　3.2 Measurement of electrical parameters such as power, power factor, and frequency

　　From the definition of sinusoidal AC active power P=UIcosψ, as long as U, I and the current and voltage phase difference ψ are accurately measured, P and cosψ can be calculated. When using traditional electromagnetic voltage and current transformers for measurement, due to the non-ideality of the transformer, in addition to the ratio error, there is a large phase error, which makes the measured ψ value unable to truly reflect the nature of the load. If Hall voltage, current sensors and true RMS converters (such as AD637) are used, the measurement accuracy of power and power factor can be greatly improved. In addition, Hall sensors can also measure AC quantities of arbitrary waveforms from DC to 100 kHz, thus overcoming the disadvantage of electromagnetic transformers having specific rated frequencies. The true RMS converter can convert AC quantities of sinusoidal waveforms or arbitrary waveforms into DC quantities. The output DC is proportional to the effective value of the AC quantity, and the conversion accuracy is high, so the measurement is relatively accurate.

　　The measurement principle is shown in Figure 4. After the AC and DC voltages and currents are isolated and converted by the Hall current sensor and Hall voltage sensor, the corresponding voltage signals are obtained, which are then converted into DC by a true RMS converter (DC does not need to be converted). The magnitude of the DC voltage is proportional to the effective value of the AC current. The DC voltage (or the converted DC voltage) is sent to the microcontroller after A/D conversion, which collects the magnitudes of U and I.

　　In addition, the electrical signals U1 and U2 output from the secondary side of the sensor are passed through zero-level comparators 1 and 2 respectively. When the signal changes from negative to positive and passes through the zero point, a pulse is generated and added to the input of the gate control circuit. Assuming that U1 is ahead of U2, the former is used as an open signal and the latter is used as a close signal. The gate control circuit generates a rectangular pulse with a pulse width corresponding to the phase difference between the two signals. This pulse is sent to the T1 port of the timer/counter of the single-chip microcomputer. The single-chip microcomputer measures the time interval t between the leading edges of two adjacent rectangular pulses, which is the period Tx of the measured signal (frequency fx=1/Tx). The other way is sent to the AND gate circuit, and the counting AND gate is turned on. During this period, the time-stamp signal Ts is counted via the AND gate to the TO port of the timer/counter of the single-chip microcomputer. The design value is N, and the phase difference between U1 and U2 is △ψ=Ts/TxN×360°. The power factor cosψ is calculated by the single chip microcomputer, and the active power P = UIcosψ is further calculated, and the measured parameters U, I, P, cosψ, ψx, etc. are sent to the display circuit for display. If the total power of the three-phase circuit is to be measured, the power of each phase is measured separately, and then the power of the three phases is added. In addition, the system can also measure electrical parameters such as reactive power and apparent power.

　　4 Conclusion

　　The electrical parameter detection system based on the Hall sensor has good linearity, accuracy and good response time. The temperature drift is small. The temperature coefficient of the Hall voltage is only 0.03% to 0.04% in the temperature range of -40 to +45°C. The measurement method introduced here achieves the purpose of high-precision isolation transmission and accurate detection of electrical parameters, and is particularly suitable for the measurement of high-voltage and high-current electrical parameters. This lays a good foundation for the development of a new electrical parameter measurement instrument and has certain application value in engineering. The disadvantage is that the Hall element is affected by unequal potentials, which requires compensation circuit correction.