Design of Remote Street Light Monitoring System Based on GPRS and PLC

1 Introduction to GPRS Technology
GPRS (General Packet Radio Service) is a wireless packet switching technology based on the second-generation mobile communication system GSM. It is particularly suitable for intermittent, sudden or frequent, small-scale data transmission, and is also suitable for occasional large-scale data transmission. The transmission rate of GPRS can reach up to 171.2 kbps, and the average rate in actual applications is as high as 53.6 kbps. GPRS provides a connection between mobile users and data networks, and provides mobile users with high-speed wireless interfaces and X.25 services. GPRS uses data packet switching technology. Each user can occupy multiple wireless channels at the same time, and the same wireless channel can be shared by multiple users, so resources are effectively utilized. Users are always online and charged according to traffic, which reduces service costs.
Using GPRS for data transmission has the following advantages:
① Wide access range. GPRS is an upgrade on the existing GSM network, which can make full use of the national telecommunications network and provide remote access network deployment for user data terminals conveniently, quickly and at low cost.
② High transmission rate. The theoretical value can reach up to 171.2 kbps, which is more than ten times the speed of the circuit data exchange service in the current GSM network. The speed of the next generation GPRS service can even reach 384 kbps, which can fully meet the application needs of users.
③ Fast login. The waiting time for GPRS access is short, and the connection can be established quickly, with an average time of 2 seconds.
④ Always online, providing real-time online function. "Real-time online" or "always online" means that users are always connected to the network. Even if there is no data transmission, the terminal is always connected to the network, which will make access to services very simple and fast.
⑤ Billing by traffic. Users only occupy wireless resources during the period of sending or receiving data, and are charged according to the number of data packets received and sent by the user. When there is no data traffic, users will not be charged even if they are online.
⑥ Switching freely. When users are transmitting data, it does not affect the reception of voice signals. There are two ways to switch between data services and voice services: automatic and manual. The specific form depends on different terminals.

2 System Overall Structure
According to the design requirements and functions to be achieved of the street light remote measurement and control system, the system is roughly divided into a three-layer network structure consisting of a central control room, a centralized controller, and a street light controller. The overall structure of the system is shown in Figure 1.

The first layer of the central control room is a PC server, which is responsible for monitoring the street lights of the entire city; the second layer of the centralized controller is responsible for controlling all street lights on a street; the third layer of the street light controller is responsible for all lamps on the same lamp pole. Among them, the GPRS wireless communication network is used between the first layer and the second layer. The distance between the two layers is far, and although the communication cost is high, there are few communication members. The second and third layers apply narrowband power line carrier communication technology, which uses the existing power lines to transmit signals. There is no need to lay additional cables, and there is almost no operating cost. It is particularly suitable for situations with many communication objects. In addition
, this design also has functions such as electricity metering. The voltage transformer and current transformer collect the power parameters of the street lights in each section in real time, analyze and store the collected data, or transmit the working parameters of each section on site (including voltage and current switch quantities, etc.) back to the monitoring center through the inspection of the monitoring center. The monitoring terminal can automatically detect emergencies such as tripping, circuit breaker, voltage abnormality, power supply failure, abnormal switch control, and upload the alarm data to the monitoring center in time so that the on-duty personnel of the monitoring center can understand the situation and deal with it in time. GPRS communication network is the data transmission channel between the monitoring center and the wireless data acquisition monitoring terminal. The fixed IP address method is used to actively and timely upload the collected working parameters to the monitoring center through the GPRS network.

3 System Hardware Design
3.1 GPRS Sending Module Circuit Design
The GPRS module mainly realizes the function of wireless Internet access. There are some mature products on the market, such as Sony/Eircsson's M47c, Simens' MC35, etc. Here, Cello's CMS91 is selected. It is a dual-band GSM/GPRS10-level module. Its main advantages are low power consumption, simple interface, complete AT command function, support for GPRS CLASS10, development of multimedia applications, and low price. At the same time, it also provides SMS (short message service) and voice functions. The GPRS module provides an RS232 interface, which can be used to control the module, such as dialing and switching modes. Once the module is connected to the Internet, the collected data can be sent to any host with a public network IP address using TCP/IP transmission, thereby realizing wireless transmission of collected data. FIG. 2 shows the circuit schematic diagram of the GPRS sending module composed of CMS91.

In this design, the CMS91 module is equivalent to an application system for a wireless modem user. It needs to connect to the operator's Internet access server through PPP (LCP/PAP/IPCP) before using TCP/IP/UDP or higher-level application layer programs (such as HTTP, FTP, etc.) for communication. The module has integrated an antenna receiver module, which needs to be connected to the SIM card socket when actually used. The GPRS terminal communicates with the device through the RS232 interface, and the level conversion chip MAX232 is used to realize the conversion between the microprocessor's TTL level and RS232 level. MAX232 can meet the requirements of TIA/EIA-232-F and 1TU v. 28 standards. Its working power supply voltage is 3~5.5 V, with 1 driver and 1 receiver. The data rate can reach up to 250 kbps. The chip has electrostatic protection function and automatic disconnection.

3.2 Design of power line carrier module
The power line interface module consists of a line driver and a line interface. Its main functions are:
① In the sending mode, it is used to amplify and filter the transmission signal (AT0) sent by ST7537;
② In the receiving mode, it provides the receiving signal from the power line to the receiving port of ST7537;
③ It has a protection circuit to resist spike pulses and overload.

The block diagram of the power line interface module is shown in Figure 3. The line driver amplifies the output signal (AT0) of ST7537. In order to make the line driver suitable for the power line, a line interface is used. A transformer is used in the line interface, and its functions are:
◆Isolate other circuits from the power line;
◆Send the transmission signal to the power line;
◆Extract the receiving signal from the power line;
◆Filter out the harmonics in the transmission signal.
The circuit schematic diagram of the power line interface module is shown in Figure 4.

The composite transistors Q1, Q2, Q3, and Q4 form a push-pull amplifier. The resistors R1 and R2 enable the amplifier to achieve optimal performance. When (receiving mode), the ST7537 outputs a signal PABC=1 and turns off the bipolar transistors Q1 and Q5, cutting off the power supply of the power amplifier and making the power amplifier inoperative.
The transformer consists of 1 main winding and 2 secondary windings. The winding ratio is 4:1:1, and its parameters are: 9.4μH for the main winding, 140μH for the secondary winding, and C1=2.2 nF. In order to prevent nonlinear distortion, the linearity of C2 must be very good, and C3 filters out the 50/60 Hz signal from the power line and has a short-circuit protection function. When the phase is unknown, an additional capacitor C4 is added to C3 to form a discharge circuit to avoid the risk of electric shock.
In order to avoid the damage of the circuit by the spike signal, a bidirectional voltage regulator is used. When the voltage value is greater than or equal to the voltage of the Zener diode, the Zener diode will be shorted to the ground to protect the interface circuit from being burned out.
In addition, the system uses the DS1302 trickle charging clock chip from Dallas Semiconductor. This chip is a programmable I2C serial interface clock chip and also provides 31 bytes of non-volatile SRAM for data storage. The advantage is that the circuit structure is simple and can be used as SCL and SDA signal lines through any I/O port of the microcontroller. The programming is simple and the cost is low.

4 System software design
The system mainly uses the wireless modem CMS91 for remote transmission of historical data, real-time data and report information. The AT command of the microcontroller is used to set up CMS91 before surfing the Internet and transmit data. When the correct feedback response from CMS91 is received, a physical channel is established between CMS91 and the GPRS network. The microcontroller controls the operation of the modem by sending different AT commands to it.
After CMS91 is powered on, the application needs to operate the ON/OFF control bit of CMS91 through the P0 port. The formal startup process of CMS91 takes about 3 to 5 seconds. If CMS91 is connected to a valid SIM card, CMS91 will attach to the GPRS network. The serial port read and write operations of CMS91 are still implemented by the interrupt service program. After resetting and powering on, the program first sets the parameters such as the working frequency, and then performs dial-up and PPP negotiation. After the PPP negotiation is successful, the system local IP will be obtained. Once the system obtains its own IP, it is actually connected to the Internet. However, to communicate with another IP terminal connected to the Internet, it is necessary to establish an end-to-end TCP connection with the other IP terminal. After the TCP connection is successful, the entire program will maintain this connection state. After entering the TCP connection state, data sent by another IP terminal of the TCP connection may be received. After layer-by-layer unpacking processing, various application layer data above the TCP layer can be obtained. If you want to send data to the other party, you must first send an interrupt request, and then wait until the TCP connection is established before sending. This part of the TCP/IP protocol processing is completed by the CMS91 embedded single chip microcomputer.
The GPRS module sending subroutine and receiving subroutine flow is shown in Figure 5.

5 Conclusion
Compared with the previous clock and photoelectric control street lights, the remote street light monitoring system based on GPRS and PLC designed in this paper can effectively monitor the street light lines, realize remote control, telemetry and telesignaling functions, and run stably and reliably. This design uses GPRS and PLC for communication, without the need to re-lay cables and build a new communication network, with very low operating costs and good application and promotion value.