By Wu Yue and Lv Yong
ZigBee wireless communication technology provides a good solution to the above issues. By applying ZigBee technology to AMR, and using smart terminals in the system, data can be collected, transmitted, and managed easily. In addition to enabling the utilities to monitor and analyse energy consumption status, the system also provides value-added functions such as remote control, diagnostics and upgrading, which has significant importance in areas such as enhancing utilities’ working efficiency, realising automatic load monitoring and control of power systems, and improving the power utilisation factor.
The ZigBee-based AMR system is designed based on factors including the network scope and cost (Figure 1).
The concentrator is the coordinator of the network and the logic centre of the whole ZigBee AMR system. It gathers data from all the meters within the network, and then processes, stores and delivers the data outside the ZigBee network and background system.
The data collector is the FFD node in the ZigBee network. It performs the functions of router and repeater, and also connects meters via RS485 bus or in ZigBee manner. The collector collects and manages data from the meters, and is a major device of the system.
The ZigBee meter is an energy metering unit with ZigBee communication capability. It uploads data to the concentrator, automatically or by request, and receives and executes commands from the concentrator. Most ZigBee meters are the RFD nodes in the ZigBee network, and communicate with the concentrator through other FFD nodes. Some ZigBee meters can also be configured as FFD nodes, depending on the field communication conditions, and provide routing and transmittal services for surrounding nodes.
Hand held unit (HHU)
The HHU is a meter reading device enabling the user to collect data in a mobile manner within a certain area. It is also indispensable for the engineers to install and test the ZigBee AMR system.
SYSTEM OPERATION MODES
Based on the above architecture, Holley Metering Ltd designed their ZigBee AMR system using a platform provided by Ember. The system operation modes are as follows.
The concentrator is the logic centre of the system and plays a vital role. According to the definition set by ZigBee PRO standard, every FFD in the network is equivalent to a coordinator after the network is formed. That is to say, in the ZigBee AMR system theoretically each and every collector can be a concentrator. Therefore, the physical concentrator needs to claim its logic status as the concentrator to all the other nodes so that the data collected by each collector will be delivered to it. At regular time intervals, the concentrator broadcasts an advertisement to all nodes in the network, which includes information such as the address of the concentrator and network synchronisation time. The nodes that receive the advertisement will get three items of important information:
- Concentrator address
- Routing information for connecting to the concentrator
- Synchronisation time of the network.
The nodes that receive the advertisement need to reply to the concentrator with information such as the node address, neighbouring nodes, and status of the managed meters. The concentrator then sorts and processes the responses to get information on the whole system’s scale, structure and status.
Automatic acquisition of meter information
In general, the collector connects meters via RS485 bus. It follows the communication protocol that the meters support, and receives and manages data from each meter. By using DLT/645 protocol that is widely adopted in the domestic meter industry, the collector can automatically obtain information such as number of meters connected, meter ID, and meter status.
After getting the meter information, the collector will store the information in non-volatile media (e.g. E2PROM, Flash, etc.). Thus, in case of power failure, the information will not be lost.
Freezing data by each collector
Normally in an AMR system, when the collector receives the meter reading command, it needs to forward the command to the meters it manages and then receive back the data from the meters. As the meter’s communication speed is very slow (1,200 to 9,600 baudrate), and the process is carried out meter by meter, there will be a long time lapse, especially when there are large numbers of meters and meter reading items. In addition, frequent communication will cause a jam and affect the stability of the ZigBee network.
In order to improve the real time capability of meter reading, and improve network communication efficiency, in the ZigBee AMR system the collector reads data from the meters regularly and stores the data as well as the time stamp when the data is collected. This behaviour of storing data regularly is known as “data freezing.” Thus, when the collector receives the meter reading command, it does not need to forward the command to the meters. Instead, it can send the frozen data to the concentrator directly in a certain form.
Concentrator collecting all frozen data in the network
When the concentrator receives a meter reading command, it will broadcast the command to all nodes in the ZigBee AMR system to send back their frozen data. The concentrator processes the data and then uploads it to the higher software. The concentrator can also broadcast the meter reading command by itself without receiving a command from the higher software. By doing so, the concentrator finishes the second round of data freezing and can deliver the data to the management software immediately after receiving the command, so that the response time is shortened.
Concentrator collecting frozen data from specific nodes
After the concentrator sends the meter reading command, some nodes may fail to send back the frozen data within a set time. In this case, the concentrator will give a command to these non-responding nodes. If the concentrator does not receive frozen data from a node after sending the command several times, it will stop trying and report an alarm message so that the problem can be solved timeously.
Special or real time commands to specific meters
The ZigBee AMR system may include different types of meters. Although the meters support a similar communication protocol, there will be certain meters that may have special functions (e.g. TOU), and need special commands to operate. Also in some cases, the AMR system will send real time commands to some meters (e.g. remote switch-on/off) and the meters need to give a real time response and finish the work. Under these circumstances, it is obvious that being able to read frozen data only cannot meet the requirements of a ZigBee AMR system.
In order to be compatible with the communication protocols of all meters, as well as to meet real time requirements, the concentrator can send special or real time commands to specific meters. These commands are essentially to use the ZigBee communication protocol nested with meter communication protocol, that is, to use meter communication data packets as data loads of ZigBee communication data packets, and send to the ZigBee communication nodes. The nodes that receive such real time commands do not process the data load (i.e. meter communication data packets) but forward it directly to their managing meters. Similarly the nodes do not process the data returned by the meters and send it back directly to the concentrator.
HHU reading frozen data
As a meter reading device, the HHU can also read frozen data, but reads data only from the nodes within range. By performing the meter reading tasks repeatedly in different areas, it can collect all frozen data from widespread nodes.
A PILOT ZIGBEE AMR SYSTEM
Holley Metering Ltd established the first domestic pilot ZigBee AMR system in Guizhou Province of China. This pilot system contains a ZigBee concentrator/gateway, ZigBee data collector/ repeater, ZigBee meters, ZigBee HHU, RS485 meters, and system management and testing software. This comprises one concentrator (coordinator), 127 collectors (each connecting to 16 meters via RS485), and 65 ZigBee meters (all FFDs). That is, the pilot ZigBee AMR system is composed of 193 ZigBee FFDs and 350 meters. The pilot system has operated for over 5 months, and turns out to be stable. The main testing parameters are shown in Table 1.
From the above results, it can be seen that the ZigBee AMR system has clear advantages in the key technical indices such as network form time, meter reading, real time remote control, and power consumption. Subsequently Holley Metering Ltd has finished remote upgrading of the ZigBee network to meet wider application needs.