29 June 2011
Power metering reference design centres on ARM based mcu
The need to monitor energy consumption is throwing up a range of solutions, not only for electricity meters themselves, but also for devices that allow consumers to monitor their power consumption on a regular basis.
Looking to improve the UK's energy consumption and its greenhouse gas emissions, the Department for Energy and Climate Change hopes to see 47million smart meters installed in 26m properties by 2020. By using smart meters, it believes, consumers could reduce their bills by up to £100 a year.
Freescale has recently unveiled a reference design for a single phase electricity meter targeted at domestic applications. The design, which features the company's ARM Cortex-M4 based Kinetis microcontroller, offers the ability to communicate using ZigBee. In this way, the device can communicate with the consumer and with utility companies over the emerging 'smart grid'.
Called the MK30X single phase electricity meter reference design, the approach is said to address the need for accuracy and low cost by providing efficient a/d converters, combined with an embedded programmable gain amplifier to increase the accuracy of energy measurement. It also featuers a shunt sensing circuit implementation.
The device selected for the reference design is the Kinetis MK30X256, which features 256kbyte of flash, 256kbyte of FlexNVM, 4kbyte of FlexRAM and 64kbyte of sram. "Our Kinetis line is the most scalable portfolio of ARM Cortex-M4 mcus in the industry," said Bruno Baylac, director and general manager of Freescale's Metering, Medical and Connectivity Solutions business. "Our MK30X single phase electricity meter reference design further demonstrates the versatility of the Kinetis line."
Support for 32bit math operation allows the design to run fast Fourier transform (FFT) based metering algorithms, which calculate metering quantities from elementary voltage and current harmonics. The design also includes a 128bit unique identification, while the 32k FlexMemory not only provides every meter with an individual identifier, but also tamper detection. Firmware, based on the MQX real time operating system, enables designers to customise the reference design to meet their particular requirements.
According to Freescale, the reference design is suitable for meters measuring currents of up to 120A. Capable of working with voltages ranging from 85 to 264V ac, the device is accurate to within 1% when measuring active and reactive energy.
The Kinetis mcu features an lcd driver which can address up to 320 segments. In this case, the design drives a 4 x 40 segment lcd. The designer can choose any measured value – including V, A, W, kWhr, time and date – simply by pushing one of the device's built in buttons.
The designer can also choose from a range of communication interfaces for remote collection of data. One option is Freescale's MC1322x ZigBee platform. This comprises low power, 2.4GHz transceiver, a 32bit ARM7 based mcu, hardware acceleration and a set of mcu peripherals.
Typically, this module draws 29mA in transmit mode with the mcu active. The module communicates with the host Kinetis mcu using the i2c interface. With a data rate of 250kbit/s, the MC1322x platform has a typical range of 300m.
Author
Graham Pitcher
Supporting Information
Downloads
34893\P41.pdf
Websites
http://www.freescale.com/smartenergy
Companies
Freescale Semiconductor UK Ltd
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