introduction
For most of the 20th century, power meter readers recorded electricity usage by looking at the electromechanical counters of the meter. Then go to the office and enter the recorded meter readings into the power company's user charge record. Today, with advances in technology, this labor-intensive manual meter reading operation has gradually been replaced by an automatic meter reading (AMR) system.
In the AMR market in the early 1990s, power companies recognized the commercial value of AMR because it reduced the number of employees who read the meter to save money. At that time, the power company only needed to make a meter reading for residents/industrial users every month. Therefore, the early AMR system used a close-range solution and still needed to go to the meter for on-site meter reading. The data stream is unidirectional: from the meter to the meter reader terminal held by the power company staff.
Today, the requirements for AMR have far exceeded the cost of saving meter reading. Now, in order to manage their resources more effectively, power companies are looking for technical solutions that provide instant two-way communication between electricity meters and home offices. This is why AMR is popular. AMR has new advantages in user relationship management (CRM) and avoiding loss of revenue, and AMR's electronic challenge-response capabilities can achieve these advantages.
Power line communication to achieve AMR
To meet the communication needs of today's AMR, power companies are looking for power line communication (PLC) technology. PLC is an industry standard method for transmitting data (audio, video, control signals, etc.) over power lines. Because the PLC has the following technical features, the power company is very interested in it:
Reduce infrastructure costs. The PLC system relies on existing power lines and does not require the construction of a new network infrastructure.
Coverage in remote areas is broader than RF solutions.
Total control can be achieved by the management center, reducing the total cost of management.
Prevent loss of income. The PLC can monitor remotely and detect the tampering behavior of the meter. This helps control losses (ie, anti-stealing) and maintenance in the event of a system failure or crash.
Can integrate a variety of different services and achieve "smart" appliance control.
Remote monitoring of other sensors (fire, temperature, inlet, outlet, etc.).
Record daily electricity consumption and shorten the time from “watch to charge†to improve financial income.
Prepaid and billing accounts are automatically managed.
A pay-per-use program is implemented.
Maxim's PLC/AMR Reference Design
Maxim offers an automated PLC/AMR solution that is similar to the common Ethernet networking approach from an application perspective. This reference design is based on Orthogonal Frequency Division Multiplexing (OFDM) technology, which enables high reliability and maximum transmission rate of 14 Mbps.
For OFDM systems, the requirements of the Meter Monitoring System (AMR) are relatively easy to meet. Once a month, tens to hundreds of bytes of AMR messages must be transmitted. Most of the rest of the time, the communication channel is idle. Therefore, neither data rate nor data response time is a particularly important factor in AMR systems. On the contrary, reliability and a robust network are essential, and OFDM has proven to be the best solution to overcome the shortcomings of the inherent noise of AC lines.
In the Maxim reference design, the MAXQ3120 fuel-measuring microcontroller communicates with the MAX2986 PLC baseband chip over an asynchronous serial link at a data rate of 1200 bps. A slight modification to the firmware of the MAX2986 allows communication through its internal UART and identifies the frame format that conforms to the DL/T645 meter protocol.
One problem that this reference design must address is the relationship between the MAC layer address and the DL/T645 network address. Since DL/T645 does not have a mechanism to resolve the MAC layer address (ARP in the IP stack has this function), there are two solutions: let the host track the MAC address of the network and each meter, or between the MAC address and the network address. Have a fixed relationship. This reference design uses the latter method.
In this design, a meter with a blank network address area will challenge the relevant PLC chipset and be assigned an address. In response, the PLC chipset sends the MAC address itself as a payload, sending a standard DL/T645 address setup message. In this way, each meter has a unique network address (as long as the MAC address is unique). Note that this is the only time the meter chip (MAXQ3120) actively sends a message.
Communication proceeds as follows:
The host PC generates a request and sends the request to the external PLC Modem.
The PLC Modem analyzes the request and converts the meter address in the DL/T645 header to an Ethernet MAC.
The Ethernet packet with the new MAC is sent out through the power line.
The meter receives the packet and determines that the message was sent to itself. The source MAC address is stored for use in the response packet.
The PLC MAC extracts the payload and sends it to the MAXQ3120.
The MAXQ3120 responds to the request and sends a packet back to the PLC MAC chip.
The MAC controller constructs and sends an Ethernet packet with the stored request packet source address as the destination address of the response packet.
The host PLC Modem receives the response packet, extracts the payload, and sends it to the host PC.
Using this method, the PLC/AMR meter integrates the PLC protocol and the DL/T645 protocol. This approach avoids the need to establish network layer addresses and MAC addresses at the same time in the meter.
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