Embedded power meters, or submeters, provide energy-monitoring features to individual products such as smart plugs, home appliances, and other power-consuming units. For product designers, the challenge in providing these submetering capabilities can often depend more on ease of design as much as required accuracy of power measurement. In building effective submetering solutions, engineers can achieve the required balance of simplicity and sophistication using a broad range of integrated devices from manufacturers including Analog Devices, Atmel, Freescale Semiconductor, Maxim Integrated, STMicroelectronics, and Texas Instruments, among others.
Trends such as the Internet of Things (IoT) promise to dramatically sharpen the resolution of electronic sensing and control with a proliferation of smart devices reaching into nearly every facet of life. Submeters offer much the same sort of refinement, providing homeowners and business operators with fine-grain information on power consumed by individual products. Instead of the aggregate power consumption measured by power-utility meters, these devices help individuals gain a better understanding of the sources and reasons for energy consumption in their homes and facilities.
Fundamental to all energy meters, however, reliable measurement of voltage and current is critical for calculation of a wide variety of important parameters including active, reactive, and apparent energy; active, reactive, and apparent power; power factor; peak current and voltage; and RMS current and voltage. Beyond those universal capabilities, smart-energy meter designs continue to evolve in complexity as developers find growing requirements for connectivity, security, and monitoring capabilities (Figure 1).
Figure 1: Energy-meter complexity rises with greater demand for accuracy, connectivity, security, and additional functionality. (Courtesy of Texas Instruments)
In contrast, submeters maintain more modest requirements and strive for sufficient capability at the lowest possible cost. In many cases, submeter designs can pare requirements down to basic energy measurements, taking advantage of connectivity, security, and other features incorporated in the host application that embeds submeter functionality. Furthermore, the accuracy of power measurement and of various derived parameters varies significantly with the type of energy meter. Power meters used by utility companies for billing purposes typically require 0.1 percent accuracy (Class 0.1 meters). In contrast, energy meters embedded in appliances and smart plugs have significantly more relaxed accuracy requirements in providing users with suitable power-consumption information.
Although energy calculations can be quite complex and extensive in high-end meters, all energy-metering designs begin with measurement of instantaneous voltage and current. Consequently, in its most basic form, an energy-measurement design requires transducers to sense voltage and current values, an analog front end (AFE) to capture data from the sensors, a processing unit to perform energy-measurement calculations, and some mechanism to display those results or transmit data to higher level applications (Figure 2). In submeters, simple resistor dividers can be used as voltage sensors. For current sensors, designers can typically use shunt resistors – or incorporate current transformers or Rogowski coils when isolation is required.
Figure 2: Simple requirements ease design complexity for submeters and other non-billing energy-metering applications, even eliminating the need for a separate analog front-end when an MCU with integrated analog peripherals can provide sufficient accuracy. (Courtesy of Freescale Semiconductor)
For submetering applications with less stringent accuracy requirements, MCUs with integrated analog-digital converters (ADCs) and other on-chip peripherals are well- suited for building simple yet effective submeter designs. For example, the Texas Instruments MSP430AFE253 ultra-low-power mixed-signal MCU integrates three independent 24-bit sigma-delta A/D converters with differential PGA inputs, a 16-bit timer, a 16-bit hardware multiplier, USART communication interface, watchdog timer, and 11 I/O pins.
Freescale Semiconductor bills its own MCF51EM256 MCU as a smart-meter-on-chip. With its integrated 32-bit ColdFire MCU, 16-bit ADC, and metrology-specific peripherals, the MCF51EM256 is optimized for energy metering applications – even including an embedded LCD controller for local display of measurements. With these integrated MCUs, energy metering uses the on-chip analog processing capabilities for measurement and processor core for energy calculations. As a result, engineers using these devices need only add a few external components to build a complete energy-measurement design (Figure 3).
Figure 3: With integrated ADCs, MCUs such as the Texas Instruments MSP430AFE253 provide a simple but effective solution for embedded-energy metering. (Courtesy of Texas Instruments)
When more accurate measurements are required, integrated MCUs such as the Freescale Kinetis MKM33 metering MCUs can provide power calculations with 0.1 percent accuracy. Based on the 32-bit ARM Cortex-M0+ core, the MDK33 MCU integrates a high-precision 24-bit sigma-delta ADC, 12-channel 16-bit SAR ADC, programmable gain amplifier, and high-precision voltage reference.
The emergence of multicore MCUs offers an attractive solution for more sophisticated single-chip submeter designs. With multicore devices, designers can complete execution of sophisticated applications without compromising energy measurement. In this approach, one core can be used as a real-time processor for energy measurement while another core can focus solely on high-level application processing. For example, the Atmel SAM4CMS8, SAM4CMS16 and SAM4CMS32 members of the company’s SAM4CM family combine a pair of ARM Cortex-M4 32-bit cores with an on-chip energy-metering AFE module, as well as extensive embedded flash, SRAM and on-chip cache. The on-chip energy-metering module includes multiple high-resolution sigma-delta ADCs, voltage reference, temperature sensor, and low-noise programmable gain amplifier able to accommodate a wide variety of current and voltage sensors.
While the use of on-chip ADCs and associated peripherals can meet many submetering requirements, applications requiring even greater measurement accuracy can take advantage of dedicated metering ICs designed to work as intelligent AFEs for MCU-based designs (see Figure 2). Unlike conventional AFEs for general data acquisition and signal conditioning, these specialized devices integrate analog peripherals and digital-signal processing capabilities that enable them to calculate and deliver key energy measurements to a host processor.
For example, the Analog Devices ADE7763 (Figure 4) energy-metering IC integrates two second-order, 16-bit Σ-Δ ADCs, a digital integrator, reference circuitry, a temperature sensor, and all the signal-processing capabilities needed to generate active and apparent energy measurements, line-voltage period measurements, and rms calculation on the voltage and current channels. The device includes a selectable on-chip digital integrator that provides a direct interface to di/dt current sensors such as Rogowski coils, eliminating the need for an external analog integrator while providing precise phase matching between the current and the voltage channels.
Figure 4: Dedicated energy-measurement ICs such as the Analog Devices ADE7763 augment traditional analog front-end capabilities with specialized functionality for performing complex energy calculations without intervention of the host processor. (Courtesy of Analog Devices)
The Maxim Integrated 71M6541D metering SoC integrates a 5 MHz 8051-compatible MPU core, a 32-bit computation engine, analog peripherals, flash memory, RAM, RTC, LCD driver, and SPI interface, among other capabilities. At the heart of its analog-processing capabilities the device uses Maxim Integrated’s Single Converter Technology, comprising a 22-bit delta-sigma ADC, multiple analog inputs, digital-temperature compensation, and precision-voltage reference. In combination with the 32-bit computation engine, these on-chip analog blocks allow the device to support a wide range of metering applications with very few external components.
Similarly, the STMicroelectronics STPM01 energy-metering IC combines sigma-delta ADC blocks, voltage reference, voltage regulator, and a fixed-function DSP to provide active, reactive, and apparent energy, as well as RMS and instantaneous values of voltage and current. The device performs parallel A/D conversions on two independent channels. In turn, the converted data is supplied to the internal DSP, which filters and integrates those signals as needed to improve resolution and compute required measurement values.
Submetering designs can present significantly simpler requirements compared to high-end utility meters used for billing. Embedded in smart plugs, appliances, and other power-hungry products, submeter designs can draw on a variety of available ICs to meet a very broad range of accuracy requirements. While MCUs with integrated-analog capabilities can address more relaxed accuracy requirements, the combination of MCUs and dedicated energy-metering ICs provides a solution for much greater accuracy. Using available MCUs and metering ICs, engineers can find the required balance between design simplicity and measurement capabilities required to embed energy measurement in any target application.
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