Over the past few years, the electromechanical energy meter market has undergone a rapid overhaul, turning to electronics to meet the demands of both providers and consumers. The adoption of electronic components enables energy providers to take advantage of new technologies to implement sophisticated communication capabilities, and provide multi-rate billing and smart card prepayments. Electronic meters provide better accuracy, higher reliability and less risk of field failures. In addition, electronic meters can communicate via a network, enabling utilities to detect power outages, identify the problem and expedite a solution immediately.
In order to meet the market’s requirements of higher accuracy and lower costs, Texas Instruments has introduced the industry’s most advanced family of energy metering microcontroller units (MCUs) – the MSP430FE42x. This single-chip approach simplifies the diverse demands of electricity meter designs by offering high performance analogue peripherals, low cost digital logic and an embedded Flash microprocessor as a complete system-on-chip. The embedded Flash microprocessor enables data logging and automated meter reading (AMR) configurations. Original end-equipment manufacturers (OEMs) of electronic meters can now differentiate their devices in the marketplace, using one integrated circuit (IC) to lower manufacturing cost, shorten design cycles, extend flexibility, and increase accuracy.
FIRST GENERAION ELECTRONIC SOLUTIONS
An electronic meter requires circuitry with precision voltage and current conversion over a wide dynamic range, a programmable Flash microprocessor, non-volatile storage, a time-of-day function, and flexible display and communication features. First generation electronic energy meters used a combination of analogue devices and digital signal processing-based microcontrollers, as shown in Figure 1.
While the multi-chip solution provides improved results over electromechanical meters, long-term reliability is still a concern; the existence of several devices increases the risk of failures in the field. OEMs need to balance cost optimisation with strict reference designs, and have often invested in customised application-specific integrated circuits (ASIC) and fixed-function analogue front-end (AFE) chips in an attempt to do this. Employing custom ASICs meant expensive non-recurring engineering charges and risky schedules. OEMs required a solution that increased accuracy, reduced hardware and software development costs and time to market, and eased implementation – all at the same cost or less.
The highly flexible MSP430FE42x (see Figure 2) uniquely reduces chip count from five to one when compared to first generation electronic meters. Combining an energy measurement module with high performance analogue-to-digital converters (ADC) and a microcontroller, a single chip dramatically simplifies hardware design and lowers the bill-of-material cost. The mixed signal IC integrates the metrology function as a ready-to-use embedded signal processor (ESP) to eliminate the need for basic power calculations using software development. Instead, the OEM can spend time on developing software for feature-rich AMR communication and various user controls.
EMBEDDED SIGNAL PROCESSOR
The 16-bit ESP is a fixed function peripheral specifically tailored for energy meter applications. With a dedicated digital signal processor (DSP) multiplying current and voltage to derive power consumption, the ESP provides all the metrology functions as a peripheral (see Figure 3). The ESP features three independent 16-bit sigma-delta ADC with programmable gain amplifier (PGA) pairs with simultaneous sample and hold. One ADC is used for current measurement, one for voltage and a third for line-to-neutral current comparisons to enable tamper detection.
With the 16-bit ESP autonomously amplifying, converting, sampling and computing instantaneous and average power, the main central processing unit (CPU) gains full access to 32kB of in-system programmable (ISP) Flash and 1kB RAM to develop unique application features. External non-volatile memory, typically used for calibration and unit identification serial number, can be eliminated with ISP and instead saved in normal Flash. Consumption data can also be logged in the ISP Flash (see Figure 4).
The 16-bit ESP is preprogrammed and uses an optimised CPU, hardware multiplier, random access memory (RAM) and random operating memory (ROM). A precision voltage reference and a temperature sensor are also integrated to reduce external components, making the system less vulnerable to extreme environments. The ESP is simply enabled initialisation, with no application software required. Communication with the main 16-bit CPU is performed over the existing data address busses via readable and writeable mailbox registers. Digital calibration is performed during manufacturing, and no additional software support is needed. Furthermore, similar tests can be conducted in the field to confirm a meter’s accuracy.
ACCURACY AND RELIABILITY
The single-chip approach also extends long-term reliability. One of the most common failures found in an electromechanical environment is the crystal, which is in essence a mechanical element. For this reason, the MSP430FE42x requires only a single low-frequency 32kHz crystal for real time clock generation and to enable an ultra-low-power standby mode in a power outage situation. The embedded high-speed clock system is generated digitally independent of the crystal, and in the event of an external crystal failure, it provides a failsafe mode to allow the MCU to continue functioning exclusively from the internal oscillator.
To meet stringent accuracy needs, the metrology function can provide 0.1% accurate energy measurement over a dynamic current range of 1000 to 1, greatly surpassing the 2% accuracy common in mechanical meters. With a high sampling rate of 4096 per second, the ESP digitally removes harmonics of the 20th order from 50 or 60Hz, providing even greater accuracy, especially in an environment with a fast switching transient load. Critical energy calculations are completely managed by the ESP, freeing the microprocessor so that users can implement differentiated features such as display AMR.
EASE OF IMPLEMENTATION
With fewer components and less time required for hardware and software development, designers can now concentrate on implementing advanced features such as automatic meter reading, smart card prepayment, and multi-tariff billing. The MSP430FE42x is flexible and able to support a wide range of AMR communication protocols such as data over power line or radio frequency (RF) transmission. The single-chip approach will significantly reduce manufacturing investments while increasing measurement accuracy and reliability – a marriage of benefits that goes well beyond the traditional mechanical energy meter design.