Control and connectivity considerations in the design of smart meters by Jonathan Page, MSC Gleichmann - Part 3
The first two articles in this series covered aspects of smart meter design from the perspective of their operation within the consumer premises i.e. requirements for local displays and controls (part one) and sensing and measurement technology (part two). To be fair, given the topic is ‘smart meters’, much of what we’ve considered so far has been more about the transition from traditional power/gas/water metering technology, with electro-mechanical sensors and clunky mechanical dial- or odometer-type displays, to modern all-electronic meters, with solid-state sensors and LCD readouts conveniently located within the home.Now,
Part 3 – Communication is the name of the game
As previously noted, the various types of meter have differing requirements depending on what they are measuring and where they are located. In turn this has a bearing on the form of communication technology possible. For example, an electrical power meter or an in-home energy control/monitoring unit connected to the mains supply can use power line communication signalling to send data both within the home and across the power distribution network to the utility company. By contrast, meters for gas, water and heat (measuring water temperature and flow) are rarely located with convenient access to an electrical supply so must rely on wireless communication technology. In addition, such meters will typically be powered by batteries, which have to provide an operating lifetime of 10 years or more. This makes the energy consumption of the microcontroller and radio circuits a critical design consideration.
Before proceeding to understand the various communications options and potential design solutions, it is worth reviewing some of the microcontrollers targeted at metering applications to appreciate the features they offer and what differentiates them. Energy consumption is clearly important but we must remember that energy is the product of power and time. Hence, opting for a lower performance MCU, just because it appears to consume less power, may be a false economy – a more powerful processor that can complete a task quickly and then spend longer in a deep sleep mode may be more efficient. Equally, as described in the last article, an MCU like Atmel’s AVR series AT32UC3L, which has on-chip peripherals that operate independently of the core CPU, can contribute to an even more efficient solution, especially in battery-powered equipment.
Most MCU vendors offer a range of devices at different performance levels. Atmel’s solution for smart metering is illustrated in figure 1 while the recent introduced SAM4SP32A is the first Cortex-M4 SoC to comply with the PRIME (Power Line Intelligent Metering Evolution) specification.
Figure 1: Atmel addresses smart metering with a range of devices
Renesas is another chip manufacturer with a strong focus on metering applications. Within its comprehensive product line are two new devices that are particularly relevant: Its 32-bit RX21A MCU series features a 24-bit Delta-sigma A/D converter, offering superior measurement accuracy compared to previous 16-bit ADCs. The RX21A also integrates other key smart meter functions such as an encryption engine, real-time clock and temperature and anti-tamper sensors. Renesas’ RL-78/L12 and /L13 series 16-bit MCUs feature an on-chip LCD controller and are designed specifically for ultra low power use in battery-powered devices, including smart meter applications.
As a specialist distributor of high technology products and developer of embedded computer boards, MSC Gleichmann has the expertise to advise on the best choice of MCU for a given application. It can help designers consider not just the aspects of performance, power consumption and package size discussed above but also the selection of other features within the different manufacturers’ ranges e.g. what integrated peripherals might be useful or how much on-chip Flash memory should be specified.
In terms of communications ports, SPI, SDIO, UART, IrDA are pretty much ubiquitous technology across general purpose MCUs and provide the means to interface to more specific wired or wireless communications devices. Where required, these ports also support external hardware security devices for data encryption, data-logging non-volatile memories and utility pre-payment modules.
For wired communications, technologies such as Ethernet and Hi-Speed USB, commonly found in higher-end MCUs, will be more applicable to energy monitors or the data concentrators within smart metering networks rather than in metering devices themselves. More specific to electrical power metering is Power Line Communication, a technology that, as its name implies, transmits data over the power line by modulating the supply with an RF signal. The modem and associated coupling circuits, which isolate the electronics from the supply voltage, may be implemented as an external module although there is a growing trend for the PLC modem to be integrated in dedicated MCUs, such as Renesas’ M16C/6S1 or Atmel’s SAM4SP32A. The Atmel device, as noted earlier, is the first in its class to integrate a PRIME-compliant PLC modem (the PRIME Alliance is an industry body whose aim is standardization across chipsets, metering devices and the associated hardware and software technologies to ensure regulatory conformance and non-proprietary open system architecture).
The wireless data communication technologies particularly applicable to metering, especially water and gas meters that don’t have a wired connection and are dependent on battery power, are those used in 2G and 3G cellular telephone networks and industrial wireless standards such as ZigBee.
GPRS is used more than other 2G or 3G technologies because of better signal coverage, lower power consumption and the ready availability of low-cost, fully certified (CE, FCC, etc.) modules from manufacturers like Quectel.
Figure 2. Quectel’s M10 quad-band GPRS module
The current drawn by a GPRS module will depend on its class and mode of operation. The class defines the number of slots available for uploading data so for more data Class 12 offering 4 slots may be required but for smaller data packages Class 8 may be sufficient. Different paging modes (DRX1 – DRX9) define to how often (in multiples of 0.236 seconds) the module wakes up to transmit data; clearly slower rates consume less power.
ZigBee is a standard for low-power data communication in the ISM radio bands operating at 868MHz in Europe, 915MHz in the USA and, more globally, at 2.4GHz. Unfortunately high attenuation at 2.4GHz may limit its range of operation or even prevent its use altogether for water meters that are buried in the ground. An alternate European standard specifically developed for metering is Wireless M-Bus; this also operates in the ISM bands. Wireless M-Bus supports operation at lower frequency carriers e.g. 433MHz allowing narrowband communication over longer distances e.g. several hundred metres between meter and concentrator. It also uses a lean protocol stack, which reduces the demand on MCU performance and improves energy efficiency. Despite these M-Bus advantages, ZigBee is clearly the more universal standard and, being a mesh rather than tree network, is not so vulnerable to a single point of failure.
ZigBee and Wireless M-Bus modules are available from a number of manufacturers with MSC Gleichmann offering a range of transceivers from Panasonic that cover the various standards and frequencies. Once again Atmel is present in this market with its ‘ZigBit’ AT86RF212 (700/800/900MHz) and AT86RF23x (2.4GHz) series RF transceivers along with an ATmega MCU that integrates a 2.4GHz radio on chip.
This article was intended to close out a series on control and connectivity in the design of smart meters. While the focus of this piece was intended to be the technology required for remote data communication it has been impossible to avoid some consideration of the microcontrollers that will be needed at the heart of any design. And, as we’ve seen, the requirements and range of devices available mean this is not a trivial selection process. This is where the expertise of a specialist distributor like MSC Gleichmann can really help. All the more so, when that distributor also has experience and knowledge of the communications technology and other products relevant to smart meter designs.