Wireless transmitters, receivers, and transceivers are becoming more common in metering systems. Smart meters involve some unique design challenges, as this article explains.
The notion of using technology and intelligent systems to enable the efficient use of energy and other resources has become a familiar 21st-century theme. The term “smart metering” jumps from the headlines of both mainstream and technical media every day. The general public often associates smart metering with intelligent electricity meters used to enable the Smart Grid. In reality, smart metering is also used to monitor other forms of energy usage such as natural gas and heat (i.e., thermal energy), as well as water, a vital resource around the world.
Metering information from residential, commercial, and industrial facilities is typically sampled at regular intervals and aggregated by a common metering collector before being sent to the service provider. Unlike electricity meters, gas, water, and heat meters are powered by batteries and have service life expectations of up to 20 years. This creates unique challenges for metering system designers who have to balance the limitations of current energy storage technology with the ever growing power consumption requirements of these more complex systems. New system architectures and power management strategies are evolving to meet these changing requirements.
There are three distinct categories of metering systems, each with its own unique requirements. The most common type is the electricity meter, which quantifies the consumption of electrical energy. The second most common is a meter that measures consumption of fluids such as water, natural gas, or fuel oil. The third category — heat meters or heat cost allocators — quantifies the consumption of thermal energy.
Electricity metering systems comprise two functional areas: metrology (or measurement) and the communications subsystem. Metrology requirements vary by region and meter type (residential versus industrial). Key variables include the number of phases being measured, measurement accuracy, the requirement for different rates depending on time of use and the level of security required at the communication layer.
Electricity meters measure the electrical power consumed by a customer, the power factor of the load and the time of electricity consumption to support multi-rate metering. These measurements rely on various sensor technologies that match the number of electrical phases in the system. Consumer meters are typically single phase while commercial and industrial customers often use multiphase meters. These meters usually derive power from the mains but require an alternate supply such as a battery or super-capacitor to maintain a state in disconnect or disruption of service conditions.
Gas and water meters (Figure 1) are generally battery-powered systems that include a microcontroller (MCU) that interfaces to a metering sensor, display, and communications block — typically a wireless transmitter or transceiver. These systems often use positive displacement flow meters to measure the number of times a unit volume of the fluid moves through the meter. For viscous fluids, volume is measured by a magnet or shaft that rotates. Each revolution is converted to an electrical signal and accumulated by the MCU. Less viscous fluids, such as natural gas, might rely on ultrasonic transducers to measure mass flow. Regardless of the material that is measured, low-power consumption is a critical design requirement in these metering systems, which typically are not wired to an electricity source.
Figure 1: Example of a smart gas/water meter system based on the Si10xx wireless microcontroller, which includes a high-performance sub-GHz wireless transceiver.
Thermal energy meters (heat meters and heat cost allocators) are typically installed in multi-tenant buildings that rely on centralized heating systems. These meters measure the amount of heat being delivered to a location in a given period of time. Like gas/water meters, thermal energy meters are battery-operated systems optimized for the lowest overall system power. They also contain an MCU that measures the flow and temperature of the heating fluid and incorporates a display and communications block. Heat is billed by the power delivered to the location measured by the heating fluid flow and the input and exit fluid temperatures over a given time period. This information appears on a display either integrated into the meter or remotely located and is transmitted over a wireless link to a collector, where it is aggregated and communicated to the service provider.
Metering functions and requirements
Each type of meter must provide one or more of the following functions:
- Quantitative measurement: The primary metering function is to accurately measure a quantity of energy or fluids. Measurement systems span a range of topologies and components including temperature sensors, flow sensors, shunt resistors, isolation transformers, current transformers, and time-keeping systems.
- Control and calibration: These systems are used to compensate for small variations in the measurement system and to handle functions such as tamper resistance and interruption of service.
- Communications: Wired or wireless connections can be used to configure the meter’s parameters and transfer stored data to a host, as well as to update metering firmware or other operational characteristics.
- Power management: Low-power and system robustness are essential in the event of power loss. In battery-powered metering systems, power management is critical to minimize power consumption and maximize battery life.
- Display: Low-cost, low-power LCD and LED displays in seven segment, alphanumeric, or matrix format are common user interfaces. Regulatory requirements often stipulate that customers must be able to view the billable quantity directly from the meter.
- Synchronization: Timing synchronization is critical for the reliable transmission of data to a central hub or other collector system to support functions such as data analysis and accurate billing. This is essential for a wireless network that has an unpredictable or asynchronous communication protocol.
In some applications and markets, meters are subject to stringent low-power requirements. For example, the service interval for an underground water meter is 20 years or more. For these applications, specialized lithium battery chemistries (such as lithium thionyl chloride or Li-SOCl2) with very low self-discharge rates are needed to meet the longevity requirement. These battery chemistries can be quite costly compared to mainstream battery types.
Another key requirement for smart meters is the use of high-performance yet low-power MCUs. Most metering systems require MCUs that consume very little power while offering an array of integrated features such as a real-time clock, analog-to-digital converter, and communications interface. More advanced features such as integrated LCD controllers, a cyclic redundancy check block, or an encryption engine can reduce the MCU’s processing burden, enabling it to reside in low-power modes for long periods of time and reducing overall system power.
Figure 2: Silicon Labs’ Si10xx wireless MCU provides a control and connectivity solution that combines an ultra-low-power MCU core with a sub-GHz wireless transceiver.
Wireless transmitters, receivers and transceivers are becoming more common in metering systems. Key features include high levels of integration, very low-power operation, fast start-up from low-power states, high receiver sensitivity (greater than -118 dBm) and high transmit powers without external power amplifiers (up to 20 dBm). More advanced features include automatic packet handling, integrated FIFO and variable frequency and modulation schemes.
Wireless MCUs (see Figure 2) that combine the MCU with a wireless transceiver are also available for smart meter applications. These highly integrated single-chip devices can help reduce BOM and system cost while providing a low-power embedded control solution capable of high-performance wireless connectivity.
Other key technologies for next-generation metering systems are wired access products such as modems for line-based data communication, timing solutions for network synchronization and CMOS-based digital isolation products for safety and compliance of electrical meters.
As more embedded intelligence is integrated into smart meters, we’ll see an explosion of innovative applications and additional opportunities to harness the advanced capabilities these intelligent systems will bring to energy and resource consumers everywhere.
