Wireless lighting energy saving mechanism based on sleep-wake strategy

　　With the continuous development of radio technology, wireless communication has gradually been integrated into all aspects of life, and home control has continued to develop in the direction of intelligence, automation and networking. For traditional wireless lighting control systems, even when the wireless device is in idle state, its wireless receiving part is still active, waiting for the system wireless control signal. Long-term uninterrupted operation will cause a lot of energy waste. In terms of power consumption sources, for wireless sensor network node SoC, the following working states can be designed: normal mode, shallow sleep mode, and deep sleep mode. Combining the characteristics of ZigBee technology, this paper proposes a dormant energy-saving strategy, which enables wireless devices to enter an extremely low power consumption state without performing any operations, thereby improving energy utilization.

　　1 ZigBee Technology

　　ZigBee is a short-range, low-power wireless communication technology based on IEEE 802.15.4. Its network can accommodate a large number of nodes, with a maximum point-to-point transmission distance of 75 m. Nodes can communicate with each other within the transmission range, and it supports a variety of self-organizing network topologies.

　　Compared with traditional wireless communication technologies, ZigBee has the following features: Power saving: two AA batteries can work for up to 2 years; Reliable: CSMA/CA is used to avoid data conflicts; High capacity: the network can accommodate up to 65,000 nodes; Low cost; Low rate: the transmission rate is 250 Kb/s; High security: AES-128 encryption is supported. Therefore, ZigBee is mostly used in situations with cost and power consumption requirements, low transmission rate, and small data volume.

　　2 System Planning

　　As shown in Figure 1, the system consists of an embedded controller, lighting control nodes, switch nodes, and routing nodes.

　　The embedded controller centrally monitors and controls the status of the lighting system. Users can view the status of all lighting devices in the system through the embedded controller and control them through the touch screen. As a secondary control unit, the switch node can send switch signals to the lighting node to control its switch status. However, the lighting node is the execution device in the system, receiving control commands and performing corresponding actions. Each switch node can be bound to multiple lighting nodes.

　　2.1 Network Topology

　　In a ZigBee network, there are generally three functional devices: network coordinator (with network establishment and data forwarding functions), router (with data forwarding function) and terminal device (without data forwarding function). This system adopts the mesh topology shown in Figure 1. It is a network structure with high reliability and large network capacity. Several special routers are placed in the network, which are responsible for data forwarding. Generally, only the coordinator and router are active in the network, and the terminal device enters sleep mode.

　　2.2 Node Configuration

　　According to the functional requirements of each node in the system, the embedded controller can centrally control the network and is configured as a coordinator as the builder of the network; the routing node is a special node that only acts as a data aggregation point for data forwarding and does not perform other operations; and the switch node is only awakened after a manual switch operation, is active in the network for a short time, does not need to forward data, and is configured as a terminal device.

　　3. Implementation of network node energy saving solution

　　The low-power design of network nodes is one of the hot spots in the development of wireless sensor network applications. Therefore, it is necessary to propose and summarize the low-power design methods of nodes from two aspects: hardware design and software design. In the common ZigBee SoC solution, the node consists of a processor (MCU), a wireless transceiver (RF), peripherals and a power supply. Among them, the processor, as the core unit of the node, is responsible for data processing and the allocation of internal chip resources; the wireless transceiver sends and receives data packets to realize network communication functions.

　　For SoC architecture, a single-component wireless sensor sleep model can be used for analysis. According to the reference, the wireless transceiver is the main source of node power consumption. Generally, the data transmission volume of the ZigBee network is small, and most nodes are idle. In order to reduce the energy consumption of the network, the various sleep modes provided by the ZigBee node can be used to turn off the wireless transceiver of the idle node and put the processor into sleep mode.

　　3.1 Event-driven

　　The function of the switch node is to detect the operation of the switch panel and send the switch information to the corresponding lighting node without actively participating in wireless communication. The switch node adopts the deep sleep mode with the lowest energy consumption, shuts down the digital regulator, high-speed RC oscillator and all crystal oscillators, and can only be awakened by external interrupts. Its sleep and wake-up process is shown in Figure 2.

　　3.2 Scheduled wake-up

　　As the execution part of the system, the lighting node mainly receives control signals and performs corresponding operations. Since it needs to wait for wireless control signals to trigger services, it cannot be awakened by external interrupts. The shallow sleep mode provides a timer wake-up function. In this mode, the digital regulator, high-speed RC oscillator and high-speed crystal oscillator are turned off, and only the low-speed crystal oscillator is retained to provide the clock. The MCU can be awakened regularly through the sleep timer.

　　As shown in Figure 3, the sleep timer wakes up the node with a period tperiod. The entire wake-up process is the same as the switch node, and its average power is:

　　As the application execution part of the wireless lighting system, the lighting node is the component that directly provides services to users. After the sleep mechanism is implemented, the device will be in sleep state most of the time, and only wake up periodically to send and receive data or detect the status of the channel. If the sleep time is too long, it will affect the device's response speed to the control signal, and even cause the control signal transmission to fail. Therefore, the sleep time needs to be experimentally evaluated in the application to avoid users waiting too long or operation failure.

　　4 Data Analysis

　　This system uses CC2430 as the wireless communication chip, high-performance 8051 as the core, and integrates ZigBee RF transceiver. As mentioned above, the wireless node adopts two different sleep and wake-up mechanisms to achieve energy-saving strategies. According to the reference, the data analysis is shown in Figure 4 and Figure 5.

　　As shown in Figure 4, the factors that affect the power of the switch node are the running time trun and the number of switches n. Among them, trun is related to the communication process. The more target nodes the control information has, the larger trun is; while the number of switches n is determined by the usage habits, and the average power increases with the frequency of switching. If a certain switch information needs to control two lighting nodes at the same time (trun=30 ms), the switch is switched 20 times a day, and the average power is about 0.5 mW; if it controls three nodes and switches 10 times a day, the average power is 0.31 mW. As shown in Figure 5, the average power of the lighting node is determined by the running time trun and the wake-up period tperiod. Among them, trun is related to the circuit design and the execution device; the wake-up period is related to the network response speed. The larger the tperiod, the longer the network response time. In the control of lighting, the real-time requirements of the system are not large. At the same time, considering the requirements of energy saving and user operation, the wake-up period is between 250 and 400 ms, and the power of the lighting node can be controlled below 10mW.

　　5 Conclusion

　　The sleep strategy of the wireless lighting system in this paper can be applied not only in ZigBee networks, but also in multi-component wireless nodes composed of processors and wireless transceivers. The research results show that the sleep wake-up strategy for each component of the wireless node can effectively control its power consumption and improve energy utilization. It will have a good reference value under the development trend of home automation and energy conservation and environmental protection.