Design of intelligent anti-electricity theft system based on LPC2132

With the continuous development of computer technology and communication technology, the Internet and embedded intelligent instruments are becoming more and more widely used. Its emergence provides strong technical support for distributed control systems to achieve real-time and reliable data communication between nodes. It is a serial communication network that effectively supports distributed control or real-time control . This paper analyzes the design of an intelligent anti-electricity theft system based on LPC2132.

1 Overall hardware design

According to the above functional planning of the device, the device design can be divided into two parts. The intelligent monitoring system consists of two parts: the upper computer and the lower computer. The upper computer is implemented by a PC. The lower computer includes a main control microcontroller composed of MSP430F149, a 4×4 human-machine interface keyboard, a relay control circuit , and an acoustic, photoelectric , and alarm circuit.

1.1 Sensor Selection

1.1.1 Electric energy Metering chip selection

The power consumption collection part mainly uses the power metering chip AD7755 to collect and store power. A day can be divided into 48 time periods, that is, one time period every half an hour. The microcontroller sends the number of pulses collected in half an hour and the maximum power value collected in half an hour to the database server, and then the database server realizes real-time data update and publishes it in the form of a dynamic web page. The power sampling circuit is the key circuit for power metering, as shown in Figure 1.

Figure 1 Power sampling circuit

1.1.2 Selection of oil temperature sensor

Platinum resistance is used as the sensor for transformer oil temperature measurement , and its temperature measurement circuit is shown in Figure 2. The oil temperature sensor is used to measure the oil temperature of the transformer. When the oil temperature is higher than the set value, an alarm is issued and the transformer switch is cut off .

Figure 2 Temperature measurement circuit

Its parameters are as follows:

① Measuring range: -200℃～+850℃; ② Allowable deviation value △℃: Class A ±(0.15＋0.002│t│), Class B ±(0.30＋0.005│t│); ③ Minimum insertion depth: The minimum insertion depth of the thermal resistor is ≥200 mm; ④ Allowable current ≤5 mA.

1.1.3 Selection of oil level sensor

UZF2 (side mounted) remote transmission type column control level gauge is selected. The magnetic column level gauge is a device specially developed for liquid level measurement based on the principle of magnetism, Archimedes (law of buoyancy) and other principles, which is cleverly combined with the characteristics of mechanical transmission. The control type adds a magnetic control switch on the basis of the column level gauge. While monitoring the liquid level, the magnetic control switch signal can be used to control or alarm the liquid level; the remote transmission type adds a 4~20mA transmitter sensor on the basis of the column level gauge. While monitoring the liquid level on site, the change of the liquid level is transmitted to the control room through the transmitter sensor, cables and instruments to achieve remote monitoring and control.

1.1.4 Selection of anti-electricity theft signal

After the whole system is powered on, the main control system and microcontroller start self-checking. After initialization, if no command is received from the remote management center via GPRS, the main measurement and control system sequentially reads the current and monitoring data of the meter, measures the oil temperature and oil level of the transformer and processes the data, determines whether the meter box door is normal, receives the measurement and monitoring data sent by the measurement and control subsystem at regular intervals, and processes the data and outputs the control. At the same time, the measurement and control subsystem sequentially measures the current and the oil temperature in the high-voltage magnetic sleeve, and monitors the state and oil level of the positioning electromagnet in the high-voltage magnetic sleeve. When the main measurement and control system receives the command sent by the remote management center via GPRS, it determines whether the command is a closing or disconnecting command for the load switch inside the transformer, or a positioning or release command for the positioning electromagnet in the magnetic sleeve and a positioning or release command for the positioning electromagnet in the meter box door. The main measurement and control system sends the corresponding command content to the measurement and control subsystem and outputs the control to complete various functions. In addition, the main measurement and control system forms two curves on the LCD in real time for the measured current value and the current value read from the meter and stores them. The control circuit mainly realizes the control of the positioning electromagnet in the metering box. The positioning electromagnet mainly realizes the locking control of the metering box door. When the positioning electromagnet coil is energized in the positive direction, the positioning bolt of the electromagnet locks the metering box door. When the power is energized in the reverse direction, the positioning bolt of the electromagnet is released and the metering box door can be opened. Figure 3 is the circuit diagram of the output control electromagnet.

When PTC3 and PTF6 are both high level, the positive direction of the electromagnet coil is energized, locking the metering box door and preventing it from being opened. When it needs to be opened, make PTF7 and PTC0 both high level, the electromagnet is released, and the metering box door can be opened.

Figure 3 Output control circuit of electromagnet

1.2 MCU Selection

The design uses the PHILIPS LPC2132 chip based on ARM architecture as the MCU of the data acquisition terminal . The LP C2132 microcontroller is based on a 32-bit ARM7TDMI-S CPU that supports real-time simulation and embedded tracing, and has 64 kB of high-speed Flash memory.

The 128-bit memory interface and unique acceleration structure enable 32-bit code to run at the maximum clock rate. The remote data acquisition system is connected to the Ethernet through the network interface module . The remote computer sends task requests or the acquisition system actively transmits data to the remote monitoring computer to achieve the interconnection between the device and the Internet.

1.3 Selection of network interface chip

The design uses the 28-pin independent Ethernet controller ENC28J60 launched by Microchip Technology (US Microchip Technology Company); the EIA/568A wiring standard of the RJ45 connector network cable is adopted, the transmission medium uses an eight-core twisted pair cable, and the RJ45 connector is used as the interface.

The design of the network interface circuit is based on the functions, timing and logic levels of LPC2132 and ENC28J60. The network interface circuit is shown in Figure 4.

1.4 Serial Bus Interface

LP C2132 has two UART serial interface modules, which can be easily connected to serial devices. In the design, UART0 is used as the RS232 interface and UART1 is used as the RS485 interface. RS232 is mainly used for system debugging and can also be connected to devices that use RS232 communication. RS485 is used as the input and output channel of the gateway.

Figure 4 Network interface circuit

1.4.1 RS-232 conversion circuit

The design uses the SP3232E chip. It can convert the input level to a standard TTL level (for the MCU) and can also convert the input TTL level to a negative logic level (for the PC). Its working circuit is shown in Figure 5.

Figure 5 SP3232E circuit

1.4.2 RS485 interface circuit

The design uses Sipex's low-voltage RS485 interface chip SP3481, which only needs a +3.3 V power supply to generate the 1.5 V voltage difference required for differential output. SP3481 works in half-duplex mode. The chip includes a driver and a receiver, and has 8 pins on the outside.

The R/D signal output by LPC2132 directly controls the transmitter/receiver enable of the SP3481 chip: when the R/D signal is "1", the transmitter of the SP3481 chip is enabled and the receiver is disabled. At this time, the LPC2132 can send data bytes to the RS485 bus; when the R/D signal is "0", the transmitter of the SP3481 chip is disabled and the receiver is enabled. At this time, the LPC2132 can receive data bytes from the RS-485 bus. As shown in Figure 6. In this circuit, only one of the "receiver" and "transmitter" in the SP3481 chip can be in working state at any time.

Figure 6 RS-485 circuit connection

2 Conclusion

With the widespread application of electronic technology, intelligent anti-electricity theft control technology will surely become a development trend. This paper proposes to use gateways and new sensors to test transformer parameters . The debugging results show that the system is highly reliable and easy to use. The anti-electricity theft intelligent control system established by the above method has the characteristics of simple wiring, stable system controller, and high data transmission reliability. It has achieved good results in actual use and has certain promotion significance.