By Lv Haitang and Lin Dusheng
The rapid development of modern electronics and network technologies is contributing to the continuous improvement of various AMR solutions, among which power line communication (PLC), RS 485, and radio frequency (RF) are the most commonly used technologies.
PLC was the first AMR technology applied in China. There are two main PLC technologies – spread spectrum communication (SSC) and orthogonal frequency division multiplexing (OFDM), of which SSC is the more widely used. The SSC16 series chips have been developed domestically based on SSC technology. The chips feature the following advantages:
- Strong disturbance immunity, which ensures reliable data transmission under the poor communication conditions of low voltage power lines
- Ability to realise code division multiple access (CDMA) so that different users can communicate at the same time through the low voltage distribution system
- Very low and masked power spectral density (PSD) which is not easily captured.
PLC technology has both advantages and disadvantages. The main advantages are:
- No need to lay new wires as power lines are widely built
- Low workload and low cost
- Little maintenance is needed using power lines as the communication channel.
The main disadvantages are:
- Different types of power lines will show different impedances and attenuations to signals
- Power load changes constantly and different loads have different characteristics
- The various electromagnetic disturbances in the power grid can affect the communication quality.
Based on the above characteristics, PLC technology is recommended mainly for use in rural AMR applications, in which a concentrator is installed under a transformer. The concentrator reads data from PLC meters and transmits the data to the utilities via modem. With its advantage of low cost, PLC technology can also be applied to prepayment meters to realise remote monitoring and closed loop management of energy.
RS485 adopts differential signal negative logic. +0.2 V - +6 V is indicated as ‘0’ and -6 V - -0.2 V is indicated as ‘1’. There are two-wire and four-wire connection modes. In an AMR system, two-wire is used. This connection is of bus topology structure, in which a maximum 256 nodes can be connected to the same bus. In an RS485 network, master/slave communication is generally adopted, with one master PC managing a number of slave PCs. In most circumstances, when connecting RS485 communication links, only a twisted pair is used to connect the ‘A’ and ‘B’ terminals of each interface. In theory, the maximum communication distance of RS485 can extend up to 1,200 metres.
The main advantages of RS485 are:
- Logic ‘0’ and ‘1’ are indicated by the voltage differences between the two wires +0.2-6 V and -0.2-6 V respectively. The interface signal level is lower than that of RS-232-C, so the interface circuit chip is not easily damaged. Also the level is compatible with TTL level, which facilitates the connection with a TTL circuit
- Maximum data transmission speed is 10 Mbps
- The RS485 interface adopts the role of balancing driver and differential receiver so that the common mode disturbance immunity is improved, i.e. it has good noise immunity
- Maximum 256 transceivers are allowed to connect to the bus. With this multi-station capability, a device network can be conveniently established via a single RS485 interface
- Normally only two wires, a shielded twisted pair (STP), are needed in the half duplex network formed by the RS485 interface.
The main disadvantages are:
- In the whole network only one node can transmit data to the bus at any one moment and all other nodes must be in receiving status. If there are two or more nodes transmitting data to the bus at the same time, all transmission fails
- A special communication cable is needed. The maximum communication distance depends on communication cable type.
RS485 technology is recommended for use in new intelligent buildings, where the RS485 twisted pairs are laid in advance and meters with RS485 interface are installed. When there are a large number of meters, data can be transmitted to the concentrator via collectors and repeaters, and then further transmitted to the utilities via modem. Considering the technology’s advantages of reliable communication and good real time capability, a switching function can be added to the meters to realise remote monitoring.
RF covers the range from very low frequency (VLF: 10-30 kHz) to extremely high frequency (EHF: 30-300 GHz). RF technologies can work freely under a variety of conditions to make reliable data integration without the need for outside interference. The main RF technologies applied in AMR include GSM, GPRS, and ZigBee. GSM (Global System for Mobile Communication) is a development from cellular telephony. GSM adopts digital modulation and the key technology is time division multiple access (TDMA).
GPRS (General Packet Radio Service) is a wireless packet data technology based on the GSM system. It provides wide area end-to-end wireless IP connection. Compared to the dial-up data exchange of GSM, the packet data technology adopted by GPRS is a high speed data processing technology.
ZigBee is a two-way wireless communication technology featuring short distance, low complexity, low power consumption, low data speed, and low cost. It is used mainly in data exchange between low power electronic devices within a short range. There can be as many as 65,000 wireless communication modules in a ZigBee network, which is very similar to telecommunication networks like CDMA or GSM. Each ZigBee module is like a telecommunication station, and the modules can communicate with each other within the whole network. The communication distances between the nodes can range from the standard 75 metres to hundreds of metres and even several kilometres. The ZigBee network can also be connected to other networks.
ADVANTAGES AND DISADVANTAGES OF RF TECHNOLOGIES
The advantages and disadvantages of the various RF technologies are as follows. The main advantage of GSM, with its good anti-piracy ability, large network capacity, high reliability and high sensitivity, is that it can provide many data transmission services. Its main disadvantage is high cost. The main advantages of GPRS are that it ensures relatively stable network access; it is cost effective to develop as it is developed based on the existing GSM network; and its high speed which reaches 115 kbps. In addition, GPRS has the feature of being always on, so that users can connect to the network at any time. However, like GSM, the main disadvantage of GPRS is also high cost.
The main advantages of ZigBee are:
- Power saving, as a result of the short working period, low power consumption of communication, and standby mode
- Reliability: Collision avoidance is adopted, with a special time slot allocated for those communications that need fixed bandwidth so that competition and conflict are avoided when transmitting data. The MAC layer adopts completely confirmed data transmission, that is, every data packet sent must wait for the confirmation from the receiver
- Low cost of the modules, and the ZigBee protocol is patent fee free
- Short time delay, typically 30 ms for device searching, 15 ms for standby to activation, and 15 ms for channel access of active devices
- Large network capacity: One ZigBee network contains one master device and maximum 254 slave devices. There can be as many as 100 ZigBee networks within one area
- Safety: ZigBee provides a data integrity check and authentication function. AES-128 is adopted and at the same time each application can flexibly determine its safety property.
The main disadvantages of ZigBee include short range, low complexity, and low data speed. Due to their high cost, GSM and GPRS are normally used in concentrators to transmit data to the main station, or in high end multi-function meters. ZigBee is used mainly in the concentrators, data collectors, repeaters, and meters installed in the urban distribution AMR systems and prepayment systems. Because of the good real time capability of RF, meters are often equipped with a remote control function.
AMR systems utilising PLC, RS485, and RF have all been successfully deployed in China. In particular RF technology is developing rapidly. For example, the AMR system with ZigBee as its core technology developed by Holley Metering Ltd. provides a brand new meter reading solution to customers.
AMR systems are generally built with one technology as the core, and a combination of other technologies to solve problems caused by system compatibility and/or special field conditions. For example, in a PLC system, there might be some meters unable to access the system. In this case, ZigBee meters can be used and a ZigBee/PLC converting module can be provided at the receiving end to solve the problem and ensure that the system works properly.