Design of street lamp cable anti-theft alarm based on power line carrier

introduction

This paper proposes a street light cable anti-theft alarm solution based on power line carrier technology and analog-to-digital converter TLC3548, which can monitor the cable connection and disconnection status in real time 24 hours a day, measure the broken cable location more accurately, and report the alarm in time.

1 System Working Principle

The street light cable anti-theft alarm consists of two parts: the anti-theft host at the front end of the cable and the anti-theft slave at the end of the cable. Since the system alarm signal is collected for the power cable, which is itself a good carrier of the signal, the cable communication technology between the host and the slave adopts power carrier communication technology. Power line communication PLC (Power Line Communication) is a unique communication method for the power system. It refers to the technology of using existing power lines to transmit analog or digital signals at high speed through carriers. The biggest feature of power line communication is that there is no need to re-establish the network. As long as there is a cable, data can be transmitted.

The power supply cables for street lamps are all three-phase four-wire. Since the three phases must be cut when the cable is cut, the system only needs to monitor one phase. The host is placed at the transformer end, and the slave is placed at the end of the line (the last lamp pole of the street lamp or the street lamp control box). When someone steals or cuts the line or an accident occurs, the system will obtain the necessary alarm information. When the street lamp is turned on at night, if there is a cable break, the system can detect the cable fault by judging the significant changes in current data and lighting rate; when the street lamp is turned off during the day, the lighting cable has no electricity, and the power carrier technology must be used to detect the cable fault. The structural diagram of the street lamp cable anti-theft alarm system based on power carrier and TLC3548 is shown in Figure 1.

As shown in Figure 1, each street lamp (1, 2...n) is connected in parallel with a power resistor (RT1, RT2, ... RTn). If the line is broken or cut during a power outage, the host alarm terminal sends a carrier signal. Since the cable is broken in the middle, the power line carrier communication channel is blocked, and the end slave lacks power and enters the receiving state. The alarm terminal host sends a group of carrier signals every 5 seconds. If it does not receive a response signal from the end slave, it can be considered that the power line between the host and the slave is broken.

When the cable is broken, the number of loads at the street lamp terminal changes, and the impedance of the entire cable under test changes accordingly. TLC3548 and its peripheral circuits can accurately measure the change value. By comparing the single-chip microcomputer with the standard impedance value, the position of the cable break from the street lamp control distribution cabinet (transformer end) can be measured more accurately. The alarm host reports the alarm to the street lamp controller via RS485, and then the street lamp controller reports the alarm information to the management server of the street lamp control center via the GPRS communication module. After receiving the alarm signal, the staff immediately notifies the engineering repair vehicle for repair, and can work with the 110 alarm center if necessary.

2 Hardware Design

The cable anti-theft alarm host can detect 6 lines at the same time, and each line needs to be equipped with a slave. The host is divided into two parts in hardware: the display board and the control board. The display board mainly realizes human-computer dialogue, including buttons such as setting, adding and subtracting, viewing, and confirming, as well as 8-bit LED digital tubes and light-emitting diodes for displaying cable status information, thereby realizing the input and output functions of the host. The control board of the host includes hardware resources such as single-chip microcomputer, power carrier module, analog-to-digital converter TLC3548, clock circuit, RS485 communication interface, relay output control, power supply module and backup rechargeable power supply, local sound and light alarm, etc. The hardware principle block diagram is shown in Figure 2, which shows the situation of measuring one cable.

The host microcontroller uses Microchip's PIC18F2520, which integrates 256 bytes of EEPROM and has an enhanced UART interface with an external RS485, which can well meet the hardware control requirements of the host. After the host's microcontroller receives the light-off command from the street light controller through RS485, it controls TLC3548 to select the cable under test, and then drives the relay control circuit. The two relays in Figure 2 connect the measured cable. At the same time, the microcontroller controls the power carrier module to transmit a carrier signal to the cable, receives the response signal from the slave, and inquires the cable impedance value measured by TLC3548 for comparison with the calibrated impedance standard value, and stores the data. If the host does not receive the response signal from the slave, the microcontroller immediately reports the alarm and the calculated cable break distance to the street light controller through RS 485, and drives the local sound and light alarm device.

The hardware design of the burglar alarm slave is relatively simple, mainly including single chip microcomputer, power carrier module, relay control circuit, power module and backup rechargeable power supply and other hardware resources. When the slave receives the power carrier signal from the host, it immediately sends back a "response" signal to the alarm terminal to indicate that the tested line is normal.

2.1 Design of power carrier communication control circuit

The power carrier communication module uses KQ100F produced by Sichuan Keqiang Company. This module can send and receive data at the zero point of the fundamental wave of the mains sine wave with less interference. It has high receiving sensitivity and significant remote transmission effect.

The principle of the power carrier communication control circuit is shown in Figure 3. The control end of KQ100F consists of three ports: RX, TX, and R/T. The output signals are all TTL level. TX is connected to the RB4 port of PIC18F2520 to send data; RX is connected to the RB5 port of PIC18F2520 to receive data; R/T is the receive/transmit control end. When R/T is high, the module is in the receiving state, and when R/T is low, it is in the sending state, which is controlled by the RB3 port of PIC18F2520.

The VAA terminal is the power supply for transmission. The current is about 300 mA during transmission. In order to increase the transmission distance of the power carrier, a DC regulated 15 V power supply is used. The two AC terminals of KQ100F are connected to the live wire L and the neutral wire N of the mains through the optocoupler and relay control circuit. The optocoupler TIL113 is controlled by the RA0 and RA1 ports of PIC18F2520. Both the live wire and the neutral wire are connected with protection diodes.

2.2 TLC3548 analog data acquisition circuit design

TLC3548 is a 14-bit high-resolution serial analog-to-digital converter from TI. It uses an SPI serial input structure and has the characteristics of 8 channels, fast conversion speed, and low power consumption. The conversion time of TLC3548 is only 2.895μs within the operating temperature range, the sampling rate is up to 200 ksps, and the maximum linear error is ±1LSB. TLC3548 has an on-chip 8-channel multiplexer, which can be set to 8 unipolar ADCs or 4 bipolar ADCs. 

The analog data acquisition circuit of TLC3548 is shown in Figure 4. RC2, RC3, RC4 and RC5 of PIC18F2520 form an SPI interface to communicate with TLC3548. When the conversion result of TLC3548 is finished, the EOC output terminal becomes high level to indicate that the conversion is completed. The RB0 port of PIC18F2520 can detect this signal and output an interrupt signal to TLC3548. TLC3548 uses a 5 V reference voltage, and a 0.1 μF internal bandwidth compensation capacitor is connected in parallel between its BGAP pin and the analog ground. A0~A5 of TLC3548 are respectively connected to 6 channels of cable street lamp resistance load change value acquisition channels. Figure 4 shows the first channel. The voltage value measured by its front-end analog signal input channel AIN1 must have a matching high-precision winding resistor R1, and its resistance value must be calibrated at the street lamp cable application site and re-measured as the cable street lamp load changes. The power distribution cabinet where the host of the street light cable anti-theft alarm is located is equipped with a current sensor. The analog channel AIN7 of TLC3548 can collect the 0-5V signal output by the current sensor to determine whether there is current in the street light cable.

3 Software Design

3.1 Anti-theft alarm host software design

The master and slave software of the anti-theft alarm adopts mixed programming of C language and assembly language. The design of the software program adopts modular programming ideas, mainly including key processing program, display program, main program, serial terminal program, etc. The key processing program of the host display panel mainly includes functions such as standard impedance value setting, channel selection, working mode, carrier transmission cycle, clock setting, etc. The host software flow chart is shown in Figure 5.

As shown in Figure 5, the control program flow of the microcontroller PIC18F2520 is as follows:

First, initialize the function modules in the alarm host at the starting end, and when the street light is turned on, control relays J1 and J2 respectively to disconnect the live wire L and neutral wire N of the cable.

Secondly, PIC18F2520 receives the standard impedance value A of the cable through the GPRS communication module, updates the value, and then determines whether it has received the light-off command issued by the remote control center; if not, the program returns to the initialization completion point and waits for the light-off command to be received; if PIC18F2520 receives the light-off command, it drives TLC3548 to collect the current sensor to measure the current data of the cable to determine whether there is current in the cable.

If PIC18F2520 determines that there is current on the cable, the standard impedance value B of the cable at this time is calculated through TLC3548, and the standard impedance value B at this time and the cable's unbroken state are uploaded to the remote control center via the GPRS communication module; if PIC18F2520 determines that there is no current on the cable, the relays J1 and J2 are controlled to be energized, and the power carrier communication module is controlled to send a power carrier signal to the live wire L and the neutral wire N of the cable.

If PIC18F2520 receives the power carrier response signal from the slave alarm device at the end of the cable through the power carrier communication module, it means that the cable is normal. TLC3548 calculates the standard impedance value C of the cable at this time and updates the value. The updated standard impedance value and the cable's no-cable-break status are uploaded to the remote control center through the GPRS communication module; if PIC18F2520 does not receive the response signal, it is considered that the cable is broken. At this time, PIC18F2520 controls the power carrier communication module to continue to transmit the power carrier signal, and drives TLC3548 to measure the AIN1 channel voltage, and converts the obtained voltage data into the actual impedance value D of the cable at this time, and calculates the difference with the standard impedance value A of the cable when it is normal received in advance through the GPRS communication module. PIC18F2520 converts the calculated difference through the table lookup program, that is, the accurate distance from the cable break to the starting end of the cable is obtained, and then uploads this distance value and alarm information to the remote control center through the GPRS communication module, and drives the sound and light alarm device to alarm on the spot at the location of the alarm host at the starting end. After receiving the alarm information, the management personnel can immediately organize relevant personnel to go to the scene to check, repair or report to the police.

3.2 Anti-theft alarm slave software design

The software design of the burglar alarm slave is relatively simple. Its main function is to cyclically check the carrier signal sent by the host, send a response code signal to the host every 5 seconds, and start the timer.

In addition, the transmission distance of the power carrier signal on the cable is about 3 km. If the distance of the street lamp cable exceeds this limit, a relay anti-theft alarm is required. At this time, the relay anti-theft alarm has both the carrier transmission function of the host and the function of receiving the carrier signal from the slave.

4 Experimental Results

The street light cable anti-theft alarm designed in this paper is applied to the remote street light management system of Changle County, Weifang City. Taking Gushan Street in Changle County as an example, the entire street is 3.12 km long, with 396 street lights in both directions, and the measured normal impedance value A is 6.87 kΩ. Taking 6 street lights in Gushan Street as an example, the cables are cut at the locations of street lights #1, #50, #100, #200, #300 and #396 respectively. The experimental data are listed in Table 1.

It can be seen from Table 1 that the closer the street lamp is to the alarm host (distribution cabinet), the greater the abnormal impedance value caused by the cable break and the voltage value collected by the TLC3548 analog channel AIN1. The breakpoint distance measured by the street lamp cable anti-theft alarm can be accurate to 1 m, so the cable break position can be determined more accurately.

Conclusion

The street light cable anti-theft alarm based on power carrier technology and TLC3548 has achieved the expected results in the experiment, but there are also some problems that need to be improved. As we all know, the laying structures of street light cables in different cities and streets are diverse, and some environments are extremely complex. After the cable is powered on, there are many situations such as capacitive reactance, inductive reactance, and leakage, which will cause errors in the impedance calculation of TLC3548. Therefore, it is particularly important to conduct on-site inspections of the application environment of lighting cables, combine actual design technology with construction plans, and add anti-theft relay alarms when necessary to improve the reliability of the system.