As meter manufacturers embrace the automated meter reading (AMR) era, energy meters (e-meter) will evolve from electromechanical Ferraris counters to fully electronic meters. By turning to electronic meters, manufacturers will significantly increase system accuracy and performance, while keeping mechanical wear and tear to a minimum. With the adoption of electronic meters, new technologies with advanced communication capabilities can provide features such as multi-rate billing, smart card prepayment and remote data management over network.
At the very center of e-meters are embedded mixed signal integrated circuits (IC) that contain circuitry with high performance analog and low cost digital logic. Early e-metering inventions often contained multiple devices such
as an analog front end or an application-specific integrated circuit (ASIC), a basic microcontroller, non volatile memory, a real-time-clock function, multiple crystals and a display driver. These ICs handle the interface to the analog world, digitization of information coming in, data management and return of the measured or stored information to utility providers. Many meter manufacturers use ASICs with customized features to differentiate their end products in the market. Yet such custom solutions can be costly and significantly increase the risks of delayed product launches.
Meter manufacturers are migrating toward application specific standard products (ASSP) that can provide a balance of high performance analog, low cost digital and reduced time to market in metering systems. In order to meet the demanding needs of the e-metering industry, Texas Instruments introduced the MSP430FE42x family of ultra-low-power metering microcontrollers (MCU) for single-phase electronic e-meters.
Compared to IC solutions in early meters, the MSP430FE42x reduces chip count from five to one. It incorporates onto a single chip a Flash microcontroller unit (MCU), high performance analog to digital converters (ADC), a fixed function digital signal processor and communication functions. By having the essential energy processing function embedded in hardware, designers of e-meters can now redirect their attention to implementing differentiated features such as display and automated meter reading (AMR).
Flexible sensor interface The MSP430FE IC interfaces directly with current and voltage sensors to measure the current and voltage flow necessary for perform energy calculations. Two forms of current sensors can be used: shunt and current transformers (CT). For low cost meters, the programmable gain amplifier (PGA) allows for small, low-cost shunts to be used. For meters requiring line isolation, CTs with load resistors are commonly used (
Figure 1). The MSP430FE42x provides phase calibration correction for phase shifts introduced by the CT.
The CT enables electrical network isolation and provides an accurate measuring variable (voltage signal) for the primary current. In selecting the CT, direct current (DC) tolerance must be accounted for in order for the CT not to go into saturation. Phase shifts introduced in the CT can be corrected automatically using the MSP430-FE42x. Proper CT selection depends on voltage signal variation that must meet the requirements defined in various technical standards for the respective device accuracy class. In Asia, the standard is defined by IEC 1036.
A complete packageThe MEP430FE427 comes with a 16-bit MCU and additional peripherals for displays and AMR communication. To optimize the energy processing, the MSP430 FE42x incorporates the metrology function as an embedded signal processor (ESP) called the ESP430CE1. With the ESP430 CE1 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 Flash and 1kB RAM to develop unique application features.
The CPU is capable of providing 8 million instructions per second (MIPs) performance. By using in-system programmable (ISP) Flash, external non-volatile memory typically used for calibration and unit identification serial numbers can be eliminated and alternatively saved in normal Flash. Period-ically consumption data can also be logged in the ISP Flash.
In addition, advanced communi-cation features such as infra-red and radio frequency can be implemented using two industry standard 16-bit timers available on chip. An enhanced watchdog timer is available for fail-safe mode, switching from crystal to the internal high speed clock when the crystal fails. A USART serial communication interface and a 128 segment LCD driver are also available. By using industry standard peripherals, common software libraries can be reused. Refer to
Figure 2 for block diagram.
Embedded signal processorThe ESP430CE1 in simplified terms is essentially a fixed function peripheral specifically designed for e-metering application (
Figure 3).
It includes three independent fully differential 2nd order 16-bit sigma delta analog-to-digital converters (ADC) with PGAs sampling at a rate of 1MHz and an internal voltage reference. One channel is used for current measurement, one for voltage, and the third for line to neutral current comparisons, typically used for tamper detection in 3-wire single phased systems. In addition to the internal voltage reference, an external 1.25V reference can also be used.
Three independent sigma delta ADCs enable synchronous measure-ment of current and voltage values. These values are multiplied to give power. By summing the total power used over time, energy consumed in kW/h is obtained.
Each PGA has gains of 1 to 32, which enables the use of shunt resistors down to 250 to 300uOhm.The internal offset of each PGA and the sigma delta ADC signal can be measured and removed by shorting the differential inputs during the initialization phase of the module. Since the internal offset is not dependant on CPU, this process is automatic.
Automated digital calibration is also advantageous for long-term stability since no longer is there need to rely on mechanical gears that wear out over time. Calibration is automatic after the gain, offset and divider constants have been predetermined, and are stored in the MCU's memory.
As the converted line voltage and current signals are digitally processed, peak voltage and current, energy and line period measurement and negative power flow indication are returned. The ESP430CE1 performs the necessary calculations to supply accurate active energy, apparent energy, RMS voltage, current and waveform samples with precise phase matching between current and voltage channels. These values are important to utility providers since most household loads are either more capacitive or inductive. In the past, mechanical counter meters could not accurately measure such loads, resulting in lost revenue to utility providers.
The ESP430CE1 comes pre-programmed and uses an optimized CPU, hardware multiplier, RAM and ROM. To further reduce external components, an integrated temp-erature sensor vital for additional compensation in harsh conditions is available. The ESP430CE1 is simply enabled without any additional software algorithms. The 16-bit CPU communicates to the ESP430CE1 via existing mailbox registers. Calibration is completed during manufacturing.
AccuracyHaving 16-bit resolution on the sigma delta ADC allows for large dynamic current range measurement. A wide dynamic current range is crucial since in any given meter, the current consumed can lie anywhere between several milli-amperes to tens of amperes. For this very reason, the MSP430FE42x contains a metrology function that can provide up to 0.1% accurate energy measurement over a dynamic current range of 1000 to 1, surpassing the 2% accuracy required in typical meters.
In order for the waveform to achieve the accuracy defined in IEC1036 the MSP430FE42x, with a high sampling rate of 4096 per second, is capable of measuring harmonics of up to the 20th order, in either 50 or 60Hz mains, particularly in an environment with fast switching transient load. With critical energy calculations completely handled by the ESP430CE1, designers gain full control of the MCU to implement other communication features.
Anti-tamperingThe ESP430CE1 offers two different energy measurement modes: the energy-only and energy-with-tamper-detect. In the energy-only mode, only a single current sensor from an ADC input is measured while the other ADC input is used for on-chip temperature measurement. In this mode, the device detects tampering when
the mains of the voltage are disconnected or when there is reverse polarity on either the voltage or the current connections of the electricity meter.
In the energy-with-tamper-detect mode, both I1 and I2 current are connected to current sensors. Energy-only is measured on I1 and the second independent current measurement channel is used for monitoring the neutral line. The ESP430CE1 measures the meter current at both current inputs and compares the two currents. The larger of the two RMS values is used for calculations. If the difference in the RMS values is lower than a programmable threshold current set, tampering check is not performed. If the difference in RMS value is larger than the threshold set, then the tampering flag is set. In addition to the current comparison, tampering is also monitored when the mains are disconnected or when reverse connections exist.
DependabilityIn addition to providing tamper detection, the single-chip e-meter IC also extends long term meter reliability. E-meters must be dependable in extreme conditions and have a long operating life. Operating at 3V, the MSP430FE42x, requires only a single low-frequency 32kHz watch crystal for time-stamping and enables an ultra-low power mode in power outages, reducing the need for multiple crystals previously needed in meters. The integrated temperature sensor and voltage reference also enable the ESP to maintain accuracy under extreme conditions.
During a power outage, the device will operate in a standby mode, consuming only 1.1uA with real-time clock still active. Independent of the crystal, the high speed system clock is digitally generated, and in the event of an external crystal failure, it provides a failsafe mode to allow the MCU to continue working at a minimum level.
The MSP430FE42x's combina-tion of high analog performance circuitry and MCU offers a great deal of potential for meter manufacturers to develop flexible, modern and cost sensitive e-meters. Designers can now focus on implementing advanced AMR technology and differentiating their meter products in the marketplace and expect shorter design cycles, lower manufacturing costs and higher accuracy.
About the authorMay Ann Choo is a product marketing engineer for Texas Instruments' MSP430 product line. She earned a Bachelor of Science in Electrical Engineering degree from University of Arkansas at Fayetteville.
