A new LED energy-saving street light control solution

With the continuous improvement of my country's urbanization level, the urban lighting system is constantly expanding. How to save energy and improve the management level of street lamp systems is an urgent issue to be solved. At present, the street lamp control systems in major cities generally have the problem of low energy utilization. Especially after midnight, there are few vehicles and pedestrians on the streets. It is not necessary to turn on all street lamps. Some street lamps can be turned off appropriately. The manual control method used in the past is a waste of manpower, cumbersome operation, and the opening and closing time of street lamps every morning and evening is inaccurate, and the human factor has a great influence. Later, some towns set up photoelectric control circuits to control the street lamps to automatically light up after dark at night and automatically turn off after dawn in the morning by controlling the changes of photoresistor devices. This method has low reliability and is susceptible to interference. The street lamps are too bright in the second half of the night, which wastes electricity and cannot effectively control the street lamps in energy saving. The technology of using Zig2Bee wireless network to control street lamps is quietly emerging. The word ZigBee comes from the communication method used by bee colonies for survival and development. Bees transmit information such as the location, distance and direction of newly discovered food sources by dancing ZigZag-shaped dances. ZigBee is a group of technologies related to networking, security and application software developed based on the IEEE802.15.4 wireless standard. IEEE802.15.4 only handles the MAC layer and physical layer protocols. The ZigBee Alliance has standardized its network layer protocols and APIs.

ZigBee is an emerging short-distance, low-power, low-data-rate (10-250 kB/S), low-cost, and low-complexity wireless network technology. Its network capacity is large and can accommodate 65,000 devices.

It provides data integrity check and authentication functions, and adopts AES2128 encryption algorithm. The frequency bands used are 2.4GHz, 868MHz (Europe) and 915MHz (USA), all of which are license-free (free) frequency bands.

ZigBee's network topology structure has mesh topology and tree topology formats. The network mainly contains three types of devices, including network coordinator, full function device (FFD), and reduced function device (RFD). Its schematic diagram is shown in Figure 1. Only the network coordinator can form a network connection with other FFDs or reduced function devices (RFDs). FFD has the function of a router and can provide two-way transmission of information. RFD can only transmit information to or receive information from FFD.

1 System composition

The LED energy-saving street light control system based on ZigBee consists of the following three parts, and the system working diagram is shown in Figure 2.

(1) A microprocessor control system (including routing nodes and leaf nodes) installed on a street light pole;

(2) Server monitoring system of the control center (including coordinator and PC);

(3) Implement a ZigBee wireless network that connects the microprocessor control system on the street light pole with the control center.

The microprocessor control system installed on the street light pole is a real-time control system composed of a light intensity sensor, a street light dimming controller (LED lights are controlled by PWM), a communication control system, a DC power supply system, etc. In the ZigBee network, it can be a router (FFD) or a reduced function node (RFD).

The system adopts a multi-path tree network topology structure. There is only one network coordinator (FFD) in the system, which is the monitoring system of the control center, several FFD routing nodes and several RFD nodes.

The routing node can be installed beside the road and connected in a string as a relay controller for wireless nodes to achieve the purpose of remote control. At the same time, it can also be used as a user node to control LED lights.

The RFD user node can only receive signals and control its own LED lights according to the received signals, and has no relay or routing function.

2 Software Design

The system software design mainly includes four parts: PC server background software design, coordinator software design, router software design and leaf node software design.

The PC server uses VB to write programs and communicates with the coordinator through the serial port to control the entire system. It needs a good human-machine interface. When working, the PC server first sends read and write (a certain node) control commands to the coordinator, and then the coordinator controls the entire network. The coordinator then returns the data information obtained from the Zigbee network to the PC server. This requires the PC to have a database to store system parameters and working operation data.

After the coordinator is powered on and initialized, it first actively establishes a network, and then waits for routing nodes and leaf nodes to join. If a node is found to apply to join the network, a network number is assigned to the node. At the same time, the coordinator also monitors the command data sent from the serial port of the PC server, and controls a certain node according to the corresponding command, such as setting node lighting control information, setting node time period control information, reading node lighting control information, reading node time period control information, etc. If a certain node is to read relevant information, the read data information must also be sent to the PC through the serial port. The coordinator software workflow is shown in Figure 3.

When the router is powered on and initialized, it needs to first apply to join the network established by the coordinator. If it fails to join, it needs to repeatedly apply to join the network, otherwise the node cannot be controlled by the system network. After successfully joining the network, the routing node works in the monitoring state. First, monitor whether there are other routing nodes or leaf nodes applying to join the network. If so, the node also needs to be assigned a network address. Second, determine whether there is a command sent from the coordinator. If it is a setting command, the node needs to be set according to the command parameters; if it is a read status command, the corresponding node working status needs to be sent to the coordinator according to the command. Third, according to the preset value of brightness control, the ambient brightness information is continuously collected and compared with it. Through the comparison results, it is determined whether the street lights should be turned on or off, and whether the PWM duty cycle of the LED lights should be increased or reduced. The program flow of the router is shown in Figure 4.

The program flow of leaf nodes is similar to that of routing nodes, but leaf nodes are terminal nodes and do not support the addition of other nodes. The flow is shown in Figure 5.

3 Conclusion

(1) Applying ZigBee wireless technology to the management of street lights solves the problem of low energy utilization of street light systems;

(2) Using high-power LEDs to develop new LED lamps, providing people with a new way of lighting;

(3) LED lamps have the advantage of low power consumption (a 10W LED lamp is equivalent to the illumination of a 60W fluorescent lamp), which makes the system have the advantages of low operating cost, high reliability, and small size. It can greatly save energy and will have a very broad application space in the street lamp wireless monitoring system;

(4) By combining with various other new sensors and power controllers, remote intelligent control of street lamps can be realized, achieving the goal of saving electricity and energy over a large area and a wide range.