Smart meters and the use of different architectures (and different regulatory requirements) in the global market are rapidly evolving. Because they are in the process of being promoted to utility customers by hundreds of millions, they have great interest-and greatly reward-the successful smart meter design in its most basic form, the energy and power measurement provided by the meter, and the data transmission. Real-time clock maintenance and data display on the front panel of the instrument.
The key design requirements of smart meters include the following: (1) They should work with low energy, so that they can run on battery power for a long time, (2) They must include security features that can protect the content of communication and secure stored data. Basically, it also provides one-way communication, so that the power supplier can automatically and remotely read the meter using different communication solutions, including RF wireless, power line carrier, and general packet radio system (GPRS) data communication.
Smart meters and advanced metering infrastructure (AMI) architecture provide two-way communication, and provide improved reliability and accuracy, and monitor interruptions, and provide the ability to remotely disconnect and increase the variable tariff full load to encourage consumers to shift peak load options benefit. Smart meters can also communicate directly with other meters and with internal display units to allow the two utility companies and their customers to better manage energy consumption.
As the implementation and architecture become more and more complex, electricity meters require more processing power and more flash software stacks, communication protocols and firmware updates. The meter also has a communication interface. In the United States, many companies have chosen ZigBee radio stations as the link's utility, while in Europe, some public utility groups have agreed to use power line communication nodes. MCU requirements Low power consumption is the basic requirement of smart watt-hour meters, which, in turn, enables the MCU to be used for sensing/measurement of power. Low power consumption is also advantageous because even if the meters are powered by a power source, they must be able to use battery power if the power source loses the real-time clock (RTC) to keep running. Smart meter applications of microcontrollers require high-resolution A/D converters for current and voltage measurement; usually 16-bit or 24-bit A/D conversion speed is not a problem, so converters can be used. Dual-position A/D usually requires simultaneous measurement, and may require temperature measurement and intrusion detection. The third A/D-the meter must be prevented from tampering. Data transmission will most likely need to be encrypted using AES, DES, RSA, ECC or SHA-256. High EMC suppression IC reduces the need for external components. And EEPROM may require data recording and storage of calibration data. The measurement may be one, two, or three-phase electric energy meter measurement. Single-phase electric meters are common in most residential applications. This typically has one voltage and one current being measured, and it supports low to medium loads. Two-phase electric meters, which are not common worldwide, and are mainly used in Japan, with two voltages and two currents for measurement. Each stage is closed 180 degrees, it is usually a large and medium-sized load.
Finally, three-phase measurements are commonly used in large office spaces and industrial applications. But there are also three different phases that are 120 out of phase with each other. Three voltages and three currents need to be measured, so at least six instantaneous snapshots of the energy consumption and power factor obtained by the ADC are required. The inclusiveness of each A/D programmable gain stage in the candidate MCU is a great aid to sensor interface. In the energy metering service, an MCU may need to handle many things.
Figure 1 is a functional block diagram showing the processor in the center, as well as various peripheral processors, which may need to be handled in a good smart meter design.
Smart meter block diagram
Figure 1: Block diagram of a typical smart meter.
So, now that we have defined the MCU for the smart meter service, where do we find the requirements for such a thing?
There are several possibilities here. The NXP EM773FHN33 of 32-bit energy metering IC is an ARM-based Cortex-M0, low-cost, 32-bit, energy metering IC. It runs at 48 MHz and is equipped with a nested vector interrupt controller, serial wire debug, 32 KB of flash memory and 8 bytes of SRAM. In addition, in its peripheral supplement, the MCU includes an I²C bus interface, an RS-485 / EIA-485 UART, with SSP function, an SPI interface, three general-purpose counters/timers, and up to 25 general-purpose I/O terminals. The pin and a "metering engine" are designed to collect the voltage and current input of a load to calculate active power, reactive power, apparent power and power factor. There are two current inputs and one voltage input, and some have a measurement accuracy of 1 percent. It has a 0.85mm HVQFN plastic thermally enhanced, low profile quad flat package with 33 terminals. The energy metering IC is accurate with retractable input sources up to 230 V / 50 Hz / 16 A and 110 V / 60 Hz / 20 1%. 16-bit MCU with high-resolution ADC Texas Instruments MSP430AFE253IPW low-power 16-bit MCU is targeted for utility metering applications with a single-phase metering analog front end that supports more than 2,400 0.1% accuracy: 1 dynamic range. The MSP430AFE253IPW has three 24-bit A/D converters and up to 16 bytes of flash memory, 512 bytes of RAM, and temperature measurement. There is also an MCU, which is faster and 10-bit A/D. For the FS of the accuracy index given by the 24-bit A/D, the maximum offset error is 0.2%-this makes about a 19-bit converter. The active mode power supply current is only 220A for 1 MHz, 2.2 V and 0.5A for standby. It runs at -40°C to 85°C. Among them, A/D can be used for anti-tampering function.
There are also nine versions of the MSP430AFE2xx device family (Figure 2), and all have SPI and UART interfaces, LCD controller, 16-bit timer/PWM, watchdog and hardware multiplier. These chips do not have a real-time clock or data encryption.
TI's MSP430AFE2xx family
Figure 2: TI's MSP430AFE2xx series provides SPI and UART interfaces and an LCD controller.
8-bit or 32-bit choice
The 8-bit Freescale MC9S08GW MCU (Figure 3) has a dedicated differential amplifier and two 16-bit A/D converters with up to 16 channels. The device has 64 KB of flash memory, an RTC with tamper protection, LCD controller up to 288 segments, and CRC data check. It runs up to 3.6 V at 20 MHz at 2.15 V and up to 10 MHz at 1.8 V. The chip is available in a 1010 mm or 1414 mm LQFP package.
Another possibility for Freescale is their K30 Cortex-M4 32-bit MCU and low-power segment LCD controller for driving up to 320 segments (Figure 3). The PK30X256VLQ100 has a single 6-bit A/D converter, 256 KB flash memory, an RTC, interrupt controller, and CRC data checksum.
Freescale K30 block diagram
Figure 3: Freescale K30 block diagram.
Microcontroller and LCD driver and low power consumption mode
Microchip's PIC18F87K90 is a good choice for measurement, although its 24-channel A/D conversion is limited to 12-bit resolution. It has a real-time clock, 128 bytes of flash and 1 byte of EEPROM, plus LCD driver for 192 pixels and 4 external interrupts. In the power-saving mode, the IC's power supply current is 600 nA at a maximum of 60°C. The RTC requires up to 4.6A at 3.3 V and 60°C. The linearity error of the A/D integral is &1 LSB typical, but 6.0 LSB (maximum)-quite spread. The specified differential linearity error is 1 typical and +3.0 / -1.0 maximum. This is in the industrial temperature range. No encryption or tamper proofing is provided. The SOC is close. A different approach is taken by ADI. Its ADE7880 is not a real MCU but is more SOC and adjusts the electronic meter application "calculation function block". It is an adaptive real-time monitoring harmonic engine designed for three-phase electric energy metering and functions. The ADE7880 device uses a second-order sigma-delta analog-to-digital converter (ADC), digital integrator, reference circuit, and all the total (fundamental and harmonic) active and apparent energy measurement, effective value calculation, And the fundamental wave only power and the signal processing IC required for reactive energy measurement can monitor three user-selectable harmonics, in addition to the fundamental. It automatically tracks the fundamental frequency and provides real-time harmonic measurement updates. Harmonic analysis includes current RMS, voltage RMS, active, reactive and apparent power, power factor, and harmonic distortion, total harmonic distortion plus noise (THD + N) calculations. The ADE7880 uses a 7-second order A/D converter, a digital integrator, a reference circuit, and all the required signal processing capabilities. It supports IEC 62053-21, IEC 62053-22, IEC 62053-23, EN 50470-1, EN 50470-3, ANSI C12.20 and IEC 61000-4-7 standards and requires approximately 25 mA to work.
The smart meter applications discussed here, such as MCU, are very capable and form the focus of a smart meter system. Although some MCUs have integrated AFEs, in other cases, signal capture and conversion requirements can lead to the use of separate analog front-end chips. In an electric meter, the AFE senses current and voltage, the sensed value is converted into a digital form, and then the digital value is sent to the microcontroller. In all cases, other components for fully smart meter operation will be required. A device that is an essential peripheral device for a smart meter, such as an EEPROM chip, and provides a line isolation optocoupler. And, of course, software is needed to perform various data processing functions, including the calculation of the amount of electricity used, and the processing of the customer's energy costs. That is to say, all the mentioned MCUs are available now, and a complete smart meter function with one or two external integrated circuits can be achieved-at very low power consumption.
