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Indirect measurement method based on optical signal

In the power relay protection system, phase measurement is a frequent item. From the traditional "zero-crossing" method of measurement, to measure the phase angle of two AC signals, the usual method is to amplify the two AC signals , Shaping, becoming a square wave that changes at the zero-crossing point. At the same time, it is also compared in a loop to measure the main parameter of the phase difference (Δtx) of the same frequency signal. However, there are often more signals that need to be connected for field measurement, which can easily cause wiring errors. In addition, there are multiple loop signals connected to the equipment during the phase measurement of the line. If there is a wiring error in the field, or the isolation between the internal channels of the instrument is faulty, it is easy to cause a short circuit between the loops and cause an accident.

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

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This design adopts an indirect measurement method, no need to introduce 2 field AC signals into the same device, that is, the measurement process is carried out independently in the loop of each signal. The condition of this indirect measurement method is that there must be a synchronization signal as the measurement reference, so that a correlation can be established between the measurement loops of each independent loop in order to measure Δtx and T0. The method used here is the infrared light signal for synchronization phase measurement. The light signal is used as the synchronization signal source, and synchronization can be performed without the connection relationship on the circuit. At the same time, it can also be used as a data communication carrier.

This system includes a host and several measuring components. The host is the part of the system, and the number of measurement components depends on the actual measurement needs (for example, when measuring the hexagonal graph, there should be 6 measurement components). The host is the main part of the MCS-51 series AT89C51 single-chip microcomputer. The circuit is relatively simple. It mainly relies on an optical transmitter and an optical receiver to form a communication interface. The output terminal of the single-chip microcomputer is driven by an inverter to control the optical transmitter to send a modulated light signal to the measuring component. The input of the single-chip microcomputer is directly connected with the optical receiver, and the optical receiver demodulates the modulated optical signal sent by the measuring component, and the single-chip microcomputer can identify the coded signal through a program. The optical transmitter is mainly used to start the measurement process, and the optical receiver realizes the data communication between the host and the measurement component.

Principle of indirect measurement method

On the one hand, the host controls the measurement process and sends infrared light synchronization signals to each measurement component to start the measurement. On the other hand, after each component completes the measurement, the measurement data of each component is summarized to the host through infrared optical communication, and then the calculation is performed to determine the measured parameter , That is, the indirect measurement method of three-dimensional variables is introduced to replace the direct measurement method. This indirect measurement method no longer needs to directly measure the time difference. It 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 to the circuit, and the phase relationship between multiple measurement loop parameters can be measured indirectly by only relying on an optical synchronization signal.

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 time of the respective AC signal and the optical synchronization signal is measured separately as the basic parameter of phase measurement. The connection between them does not rely on the connection of the circuit, but on the optical signal, so that the short circuit between the circuits can be prevented, and the connection of the instrument can also 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 to 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 synchronized infrared light signal, and the optical receiver of the measurement component can receive this signal at the same time, and the single-chip microcomputer of each measurement component will start its 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 through the optical receiving head and summarizes these basic data. After the calculation, the host displays the corresponding digital value. , At this point, a measurement cycle is completed.

host part

In the stage, the host optical transmitter sends out a synchronous light 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, which passes through the inverter 74LS04 Drive the photoelectric transmitter, according to the agreement of the program, this signal is a light signal indicating "start", that is, a synchronization signal for starting measurement is transmitted to each measuring component through the light signal.

In the second stage, each measurement component enters the measurement at the same time, and after the measurement is completed, each component sends the measurement data back to the host in turn. The host computer measures the pulse of the P3.3/INT1 pin and recognizes the program. After decoding, the signal sent by the measuring component is determined, and the work of "retrieving data" is completed.

measurement part

The circuit structure of each measurement component is shown in Figure 1. The main part of 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 be passed through Enlargement 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 changes. The rising edge of this square wave represents the zero-crossing point of the AC signal. Figure 1 also contains the photocoupler SA1 (TIL117), which isolates the circuit on the one hand, and also converts the square wave signal to TTL level for measurement on the P3.2 (INT0) of the microcontroller. This pin is set In the input state, it is easy to use software to measure the rising (or falling) edge of the square wave signal. Compared with the existing circuit, the measurement part is simplified a lot. The traditional circuit processes the AC signals of the two loops—that is, directly compares the zero-crossing points of the two signals 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 light pulse as the measurement synchronization signal.

After the measurement is completed, the P3.4/T0 pin of the single-chip microcomputer of the measurement unit outputs the switch signal, drives the photoelectric transmitter through the inverter 74LS04, and then transmits the measurement data of each measurement unit to the host through the optical signal. Since each measuring component is numbered, the working program of each measuring component will send data to the host in turn according to its serial number.

work sequence

Figure 2 describes the timing relationship for data communication. When the output signal of the optical receiver has a falling edge (ie Ps=0), it means that the signal from the host is received. 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 time Txi +T0i and Txj+T0j is not more than 40ms. For a measurement component, after the synchronization signal starts the measurement, the data can be transmitted after delaying TM1≥Txi +T0i. For reliability, this design takes TM1=50ms as the delay time of the measurement process. Suppose the time of each data transmission is TN, then the delay time of the second measurement component to transmit data is the delay time plus TN, that is: TM2 = TM1 + TN, and the calculation of the subsequent delay TM is analogous to this.

The host will save the measurement data of each part in its internal storage area in turn according to this process for later calculation and display.

Conclusion

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 relying on the functions of computer control, storage, calculation and processing. Because this method relies on optical signals for synchronization and data transmission, multiple measurement loops no longer need to be directly connected to the circuit, but are performed independently, which is very useful for solving practical problems.

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