Principle, working process and structure analysis of indirect measurement method using optical signal synchronization

In the power relay protection system, phase measurement is a regular item. From the traditional “zero-crossing” method, to measure the phase angle of two AC signals, the usual practice is to amplify the two AC signals , shaping, become a square wave that changes at the zero-crossing point, and at the same time, it is compared in a loop, and then the main parameter of the phase difference (Δtx) of the same frequency signal is measured. However, there are often many signals that need to be connected to the field measurement, which can easily lead to wiring errors.In addition, there are multiple loop signals connected to the device when the phase measurement of the line is carried out, in the event of a wiring error in the field, or the internal

introduction

In the power relay protection system, phase measurement is a regular item. From the traditional “zero-crossing” method, to measure the phase angle of two AC signals, the usual practice is to amplify the two AC signals , shaping, become a square wave that changes at the zero-crossing point, and at the same time, it is compared in a loop, and then the main parameter of the phase difference (Δtx) of the same frequency signal is measured. However, there are often many signals that need to be connected to the field measurement, which can easily lead to wiring errors. In addition, there are multiple loop signals connected to the equipment when the phase measurement of the line is performed. If there is a wiring error in the field, or there is a problem with the isolation between the internal channels of the instrument, it is easy to cause a short circuit between the loops, resulting in an accident.

Based on the above situation, the traditional measurement method must be changed in principle to meet the needs of the testing process.

Principle, working process and structure analysis of indirect measurement method using optical signal synchronization

Indirect measurement method and structure synchronized with optical signal

This design adopts an indirect measurement method, which does not need to introduce two on-site AC signals into the same device, that is, the measurement process is carried out independently in the loops of each signal. The condition of this indirect measurement method is that there must be a synchronization signal as the measurement reference, so that the correlation between the measurement loops of each independent loop can be established, so that Δtx and T0 can be finally measured. The method of synchronizing phase measurement using infrared optical signal is adopted here. The optical signal is used as the source of the synchronizing signal, and the synchronization can be performed without the connection relationship on the circuit. At the same time, it can also be used as the carrier of data communication.

The system includes a host and several measurement components. The host is the core part of the system, and the number of measuring components depends on the actual measurement needs (for example, when measuring a hexagonal diagram, it should be 6 measuring components). The peripheral circuit is relatively simple. It mainly relies on an optical transmitter and an optical receiver to form a communication interface. The output end of the single-chip microcomputer is driven by an inverter to control the optical transmitter to send out modulated optical signals to the measuring components. The input of the single-chip microcomputer is directly connected to the optical receiver, and the optical receiver demodulates the modulated optical signal sent by the measuring component, and the single-chip computer can identify the encoded signal through the program. The optical transmitter is mainly used to start the measurement process, while the optical receiver realizes the data communication between the host and the measurement components.

The principle of indirect measurement

On the one hand, the host controls the measurement process, and sends an infrared light synchronization signal to each measuring component to start the measurement. On the other hand, after each component completes the measurement, it summarizes the measurement data of each component to the host through infrared light communication, and then calculates to determine the measured parameters. , that is, the indirect measurement method that introduces three-dimensional variables replaces the direct measurement method. This indirect measurement method no longer needs to directly measure the time difference, but only needs to establish the time relationship between each parameter and the optical synchronization signal, and then calculate the time difference. The loop no longer needs to be connected on the circuit, and only relying on an optical synchronization signal can indirectly measure the phase relationship between multiple measurement loop parameters.

The advantage of this method is that each measurement loop no longer needs the connection of the reference point, the loop is relatively independent, and the time difference between the zero-crossing moment of the respective AC signal and the optical synchronization signal is measured separately as the basic parameter of phase measurement. The association between them is not by the connection in the form of a circuit, but by an optical signal, so that the short circuit between the circuits can be prevented, and the connection of the instrument can be reduced. In addition to being used as a synchronization signal, the optical signal is also used as a data transmission channel. Each measurement loop transmits the measurement data through light, and concentrates on the host part to finally complete the numerical Display of the parameters.

work process

At the beginning of a measurement cycle, the host controls the optical transmitter to send out a synchronous infrared light signal, and the optical receivers of the measurement components can receive this signal at the same time, and the single-chip microcomputers of each measurement component will start their own measurement process at the same time. After the measurement process is completed, the single-chip microcomputer of each component sends the measurement data back to the host in turn. The host single-chip microcomputer receives the data of each measurement component in turn through the optical receiver and summarizes these basic data. Finally, the host will display the corresponding number after calculation. value, so far one measurement cycle is completed.

host part

In the first stage, the host optical transmitter sends out a synchronous optical signal, and starts each measurement component to enter the measurement state at the same time. At this time, the P3.4/T0 pin of the single-chip microcomputer is set to the output state, and a modulation signal is generated when it is working. The device 74LS04 drives the photoelectric transmitter. According to the agreement of the program, this signal is an optical signal indicating “start”, that is, a synchronization signal for starting measurement is transmitted to each measurement component through this optical signal.

In the second stage, each measurement component enters the measurement at the same time, and after the measurement is completed, each component transmits the measurement data back to the host in turn. The host computer measures and recognizes the pulse of the P3.3/INT1 pin, determines the signal sent by the measuring unit through decoding, and completes the work of “retrieving data”.

Measurement part

The circuit structure of each measurement component is shown in Figure 1. The main part of which is UA1 (OP07) is the signal amplifier. For example, when the clamp current is used as the measurement of the current signal, the input electrical signal is generally relatively small and must pass through Amplify processing. The main part of UA2 (LM331) is the zero-crossing comparison circuit, which is mainly used to convert the signal into a square wave with zero-crossing change. The rising edge of this square wave represents the zero-crossing point of the AC signal. Also included in Figure 1 is the optocoupler SA1 (TIL117), which on the one hand isolates the circuit, and at the same time converts the square wave signal to TTL level for measurement on P3.2 (INT0) of the microcontroller, this pin is set For the input state, the software can easily measure the rising (or falling) edge of the square wave signal. Compared with the existing circuit, the measurement part is much simplified. The traditional circuit is to process the AC signal of the two loops—that is, the zero-crossing points of the two signals are directly compared in one device to determine the phase difference (Δtx). The circuit is no longer based on direct comparison between the two signals, and the measurement method has also undergone great changes. It uses a common optical pulse as the measurement synchronization signal.

After the measurement is completed, the switch signal is output from the P3.4/T0 pin of the single-chip microcomputer of the measurement unit, and the photoelectric transmitter is driven by the inverter 74LS04, and then the measurement data of each measurement unit is transmitted to the host through the optical signal. Since each measurement part is numbered, the working program of each measurement part will send data to the host in turn according to its own serial number.

Principle, working process and structure analysis of indirect measurement method using optical signal synchronization 

Working timing

Figure 2 describes the timing relationship of data communication. When the optical receiver output signal has a falling edge (that is, Ps=0), it means that the signal from the host is received, and the timing starts when the rising edge arrives, and the subsequent data transmission is also based on this The rising edge is the reference standard. The measurement times Txi+T0i and Txj+T0j are not more than 40ms. For the first measurement unit, the data can be transmitted after the synchronization signal starts the measurement and then delays TM1≥Txi +T0i. In order to be reliable, this design takes TM1=50ms as the delay time of the measurement process. Assuming that the time of each data transmission is TN, then the delay of the second measurement component transmitting data is the first delay time plus TN, that is: TM2=TM1+TN, and the calculation of the subsequent delay TM is analogous.

According to this process, the host will sequentially save the measurement data of each part in its internal storage area for subsequent calculation and display.

Epilogue

This indirect measurement method is an improvement on the basis of the traditional measurement method. The optical signal is introduced into the measurement process as a reference quantity, and the final parameter data is obtained by the control, storage, calculation and processing functions of the computer. Since this method relies on optical signals for synchronization and data transmission, multiple measurement loops no longer require a direct connection on the circuit, but are carried out independently, which is very useful for solving practical problems.

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