Application of ZigBee Technology in Temperature Control System

Application of ZigBee technology in temperature control system

0 Introduction     At present, short-range wireless communication technology has become an important branch of wireless communication technology. In reality, the data transmitted by many systems are usually a small amount of burst signals. ZigBee technology adopted here is a short-range wireless communication technology with unified standards, which is specially used for the transmission of such information and works in the 2.4 GHz fee free and application free frequency band, The transmission rate is 250 KB / s. This wireless communication technology has the following characteristics: long transmission distance, about 30 m indoor, low power consumption. In the working mode, ZigBee technology has low transmission rate and small amount of transmission data, so the signal transmission and reception time is very short. Secondly, in the non working mode, ZigBee node is in sleep mode.     ZigBee technology is applied to the temperature control system here. Many of the existing temperature control systems use wired mode. When signals are transmitted by wire, the wiring cost is expensive, the maintenance is difficult, and it is difficult to expand. In some cases, the wiring can not even succeed. This is a great difficulty, and the survivability of the system is relatively poor. Once the main control chip has a problem, the whole system will collapse. After applying ZigBee technology, the temperature control system can change from wired mode to wireless mode.

1. Characteristics of ZigBee Technology     ZigBee's characteristics of low rate, low power consumption and short distance transmission make it very suitable for supporting simple devices. ZigBee defines two devices: full function device (FFD) and simplified function device (RFD). For fully functional devices, it is required to support all 49 basic parameters. For simplified functional devices, only 38 basic parameters are required in the minimum configuration. A fully functional device can communicate with simplified functional devices and other fully functional devices, and can work in three ways: personal area network coordinator, coordinator or device. The simplified functional device can only talk to the full functional device, which is only suitable for very simple applications.

2 characteristics of mesh network in peer-to-peer network     Peer to peer networks are all connected by complete functional devices (FFD), which can provide higher reliability, as shown in Figure 1.

To sum up, ZigBee technology is used to realize network transmission, and mesh network supported by ZigBee is used to realize this function. Peer to peer networks are all connected by complete functional devices (FFD), which can provide higher reliability.

The network has a main control node to start the whole network. The main control node is powered by a special power supply. In addition, there are control nodes specially responsible for routing information to provide necessary routes for information transmission. They can also participate in temperature measurement. There are also some terminal nodes that can be specially used to measure the temperature of endpoints, In addition, when necessary, if the network fails, they start the corresponding command, which can act as the function of routing.     This wireless network has strong survivability. If a node fails, the routing node can choose another route to complete the transmission.

3. Establishment process of mesh network     All stations here are FFD (full function device), and a network can be established by the following method.     Each large ZigBee mesh network can be composed of a central coordinator (Pan) and a router. Here, the following primitives are used:

The primitive is generated by the application layer of the device with ZigBee coordinator capability and sent to its network layer management entity to request to initialize the device to become the coordinator of a new network.

This primitive returns the execution result of a request to initialize a ZigBee coordinator in the network. If the primitive is executed successfully, the status parameter is set to success.     After the ZigBee coordinator is set, the nlme-per-mit-joining.request primitive is used to allow other devices to connect to its network.

The valid values of the parameters in permitduration range from 0x00 to 0xff. 0x00 and 0xff respectively indicate whether the connection is valid. There is no definite time limit. 0xff is selected here.     return:     NLME-PERMIT-JOINING.confirm(     Status                      Corresponding to request status     )    After receiving the primitive, the upper layer of the initialized device will know the execution result of allowing other devices to connect to the network request primitive. The result received here should be 0xff.     In ZigBee network, a router should also be configured. The primitive is as follows:

Because the mesh network is adopted, the parameter of the join asrouter here is set to true.     After a new device is successfully connected to the network, it sends the nlme-join.independence primitive. Its syntax is as follows:

So far, a complete ZigBee network has been established successfully.     This network has good survivability. In the mesh network, when a link or a device fails, the upper management device will reorganize the routing information. If the upper device lacks routing capacity or other reasons, the router device will send a routing error instruction frame to the main coordinator, which contains the reasons for the failure. If necessary In this case, you can even reorganize a new mesh network from a node.     There is another difference between ZigBee wireless ad hoc network and ad hoc (wireless ad hoc multi hop network). The latter re selects a new route after a certain time, while ZigBee

When a chip fails, it will automatically search for the next route, so that the transmission of information will not be affected; when necessary, it can even select a chip to restart the whole network system; moreover, in this way, the relay transmission between chips can maximize the network coverage. If a star network is adopted, it is in the terminal node Not FFD, but RFD. If FFD fails, the whole system will not work, and its scope of action will be limited.

4 Introduction to DS18820 chip     The digital temperature sensor DS1820 of Dallas Semiconductor Company of the United States is the first temperature sensor supporting "one wire bus" interface in the world. There are three key steps in the whole process:     (1) Search for the serial number of DS18820.     (2) For example, the host controls DS18820 to complete the temperature conversion process.     (3) Set DS18820 to start temperature conversion and read the value of temperature.     According to the communication protocol of DS18820: reset the DS18820 before each read and write. After the reset is successful, send a ROM instruction and finally send a ram instruction, so as to carry out the predetermined operation on the DS18820.     DS18820 is divided into two parts: read program and write program. The read sequence is divided into read 0 sequence and read 1 sequence. For DS18820, the read time slot is that after the single bus is pulled down from the host, the single bus must be released within 15 s to allow DS18820 to transmit data to the single bus. DS18820 needs at least 60 seconds to complete a read sequence process μ S. the write timing of the DS18820 is still divided into two processes: write 0 timing and write 1 timing. The requirements for write 0 timing and write 1 timing of the DS18820 are different. When writing 0 timing, the single bus must be pulled down by at least 60 μ s. Ensure that DS18820 can operate within 15 45 μ S can correctly sample the "0" level on the I / O bus; when the 1 sequence is to be written, after the single bus is pulled down, it is at 15 μ The single bus must be released within s.

5 system connection diagram     The system connection diagram is shown in Figure 2.

There are two ways to connect DS18820 to MCU in hardware. One is VCC connected to external power supply, GND grounded, and I / O pin connected to MCU I / O line; the other is powered by parasitic power supply. At this time, VCC, GND grounded, and I / O connected to MCU I / O pin. Whether internal parasitic power supply or external power supply, when sensor DS18820 is in write memory operation and temperature a / D conversion operation During operation, there must be strong pull-up on the bus, and the maximum pull-up opening time is 10 μ s. Such processing can effectively reduce power consumption.

6 Conclusion     To sum up, in the modern temperature control system, the ZigBee technology of wireless communication can be used to easily establish the network. The mesh network supported by ZigBee can maximize the survivability of the network, and the chip relay transmission can maximize the coverage of the network. The DS18820 chip can also be used to collect temperature data and complete the collection Writing different control programs to ZigBee chip can control the time and duration of heater.

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