Development of non-contact high-voltage electrometer based on single-chip microcomputer

Design concept and overall plan

1 Design concept of non-contact electroscope

The electrical tester developed this time is mainly used for monitoring medium and high voltage transmission lines in railways. Its design requirement is to monitor the operation of medium and high voltage lines in railways. It is required to correctly judge whether the high voltage line is energized, and display the power or no power signal through the light-emitting diode, and at the same time send out voice prompt information of the corresponding status.

In addition, the electroscope is usually installed outdoors, so the device must also meet the requirements of outdoor work.

2 Overall design scheme and block diagram

This electrical tester uses the SPE61A single-chip microcomputer as the detection and control core. The device uses a copper sheet sensor to monitor the electric field near the high-voltage line, converts the electric field signal into a voltage signal and sends it to the signal processing circuit. The signal processing circuit converts the analog signal into a DC power-on or power-off state signal and sends it to the single-chip microcomputer. The single-chip microcomputer compares the received signal with the previous signal. If the signal collected this time is the same as the last collected signal, it will continue to collect the next signal. If they are different, it will send a new signal to the voice module, indication module and signal transmission module, and latch the signal collected this time to compare the signal collected next time. After receiving the signal, the voice module will issue a voice prompt of power or power off; after receiving the signal, the indication module will issue a power or power off indication; after receiving the signal, the signal transmission module will issue a power or power off state signal and send it to the indoor monitoring computer. The hardware structure diagram is shown in Figure 1.

Figure 1 Hardware structure diagram

System hardware design

1. Microcontroller

This system uses the Lingyang SPE61A microcontroller as the detection and control core. SPCE061A is another 16-bit microcontroller launched by Lingyang Technology Company after the μ'nSP series products SPCE500A. It is designed with two 16-bit IO ports. The controller can work in a wide power supply voltage range (2.6~5.5V) and system clock frequency range (0.375~24.576 MHz). In addition to the increase of the data bus to 16 bits to increase the working speed, the SPE61A 16-bit microcontroller integrates more system peripheral resources. Among them are large-capacity ROM and static RAM, infrared communication interface, RS-232 universal asynchronous full-duplex serial interface, 10-bit A/D and D/A conversion, built-in microphone input channel with automatic gain control, 32768Hz real-time clock and low voltage reset/low voltage monitoring system. In addition, SPE61A also embeds LCD control drive and dual-tone multi-frequency signaling (DTMF) generator functions.

The biggest feature of SPE61A is that it has a built-in 7-channel 10-bit voltage analog-to-digital converter (ADC) and a single-channel sound analog-to-digital converter. The sound analog-to-digital converter input channel has a built-in microphone amplifier and automatic gain control (AGC) function. This enables SPE61A to have preliminary voice playback and recognition functions.

In this system, the SPE61A microcontroller mainly monitors and processes the input signal. After receiving the signal from the signal processing module, the microcontroller will make corresponding judgments and send the processing results to the subsequent circuits.

2 Sensors

The sensor is used to read the electric field signal and is the signal source of the entire device. The sensor is installed directly below the high-voltage line at a distance of 0.8m and is used to read and convert the electric field signal near the high-voltage line. The sensor in this device adopts a copper sheet capacitor design, with two wires drawn from both ends of the capacitor, one of which is used as the positive pole of the signal output, and the other is grounded as the ground wire.

In addition, for safety reasons, an additional ground wire (called the detection ground wire) is drawn out from the ground wire end of the sensor as the ground wire of the indication module, voice prompt module and signal transmission module in the circuit, which is used to determine whether the sensor transmission line is intact. At this time, if the signal transmission line is disconnected, the ground wire is also disconnected, and then the indication module, voice prompt module and signal transmission module cannot work, the signal indicator light cannot light up, the voice module can send voice information, and the power and no-power signals transmitted to the computer are also disconnected at the same time, and no error signals will be sent, which improves the reliability of the system and avoids the error of no-power signals when the signal line is disconnected, causing accidents.

The sensor adopts a closed hemispherical smooth shell design, which is completely waterproof and meets outdoor working requirements.

3 Signal Processing Module

The signal processing consists of a signal following circuit, a filtering circuit, a voltage doubling rectifier circuit, a subtraction circuit, an amplifying circuit, a Schmitt trigger circuit, a signal indication and a monitoring signal sending circuit, and its schematic diagram is shown in Figure 2. The input signal comes from the industrial frequency AC voltage signal sent by the sensor. The signal is followed by the signal following circuit and filtered after forward biasing. Then the AC signal is voltage doubled and rectified into a DC signal. The subtractor subtracts the interference signal between adjacent lines. The amplifier amplifies the signal as necessary and sends it to the Schmitt trigger. The trigger sends two signals, one with power and one without power, to the microcontroller according to the size of the input signal.

The functional structure and principle of each module of the device have been discussed in detail in the relevant literature, so this article will not repeat them again.

Figure 2 Functional block diagram of signal processing circuit

4 Voice Module

The voice prompt information processing is completed by the voice module of SPE61A. In this system, the power amplifier circuit and speaker are directly connected to the module, and the voice prompt information is sent out through the external speaker.

5 Indicator module

The indicator module consists of two light-emitting diodes, red and green. Red indicates power, and green indicates no power. Since the SPE61A port has sufficient driving capability, the light-emitting diodes are directly connected to the microcontroller port in this system.

Figure 3 Signal transmission

6 Signal transmission module

This electroscope not only displays whether the high-voltage line is energized in the indication module, but also sends the signal to the indoor monitoring computer for monitoring. If the signal is sent to the computer through the electroscope device, a common ground line problem will be encountered. In order for the computer to correctly identify the signal sent by the electroscope, the computer and the electroscope must share a ground line. Otherwise, the computer will recognize the signal incorrectly due to the long transmission line and the lack of a common ground line. The signal transmission method of this device is shown in Figure 3. The computer sends a signal, and the electroscope controls the conduction or disconnection of the energized or non-energized line through a relay switch. The computer receives the two signals sent by itself through the relay switch. If the energized line sends a high-level signal, the non-energized line sends a low-level signal, and the high-voltage line is energized; if the non-energized line sends a high-level signal, the energized line sends a low-level signal, and the high-voltage line is not energized. If both lines send high-level signals or low-level signals, the circuit is wrong.

The power amplifier circuit in the voice module, the light-emitting diode in the indication module and the control circuit in the signal sending module all use the detection ground wire as the ground wire, so that the sensor transmission line can be judged to avoid the disconnection of the sensor line and the issuance of erroneous indication information.

In addition, since all the hardware must work outdoors, the entire hardware equipment is installed in a specially designed waterproof metal casing.

System software design

Figure 4 Software Flowchart

The system software design is shown in Figure 4. When the system is powered on for the first time, it is initialized to the powered state and the result is displayed in the indicator module of the signal port. The voice subroutine is called to output the voice prompt information. After a delay of 1s, the state is latched. The system first scans the input port, and then compares the scan result with the data in the latch. If they are the same, the watchdog is directly cleared and the port scan is returned to enter the next cycle. If they are not the same, the scan result (powered or not) is output to the indicator module in the port to display the result, and then the voice subroutine is called to output the voice prompt information, and then the watchdog is cleared, and the state signal collected this time is latched for comparison when the signal is collected next time, and then the next cycle is entered. In order to ensure the normal operation of the system, a watchdog clearing program is set in both branch loops.

The entire working process of the system is as follows: when it is powered on for the first time or after restart, it will be displayed as powered and a voice prompt will be given. If the indication status does not change after a delay of 1 second, it indicates that the line has power. Otherwise, the indication status will change after the delay and there will be a voice prompt indicating that the line has no power.

In addition, in the voice output, the voice prompts are two sentences, namely "The line is powered on, please pay attention to safety" and "The line is powered off".

Conclusion

The non-contact tester does not need to be in direct contact with the high-voltage line when testing electricity, so it is safe and convenient to use. The tester designed in this paper uses a single-chip microcomputer as the detection and control core, and has both visual and voice prompts when testing electricity. The system signal is stable and has high reliability.