Electronic Utility Metering: A Simple Approach
Rodger Richey, Applications Manager, Advanced Microcontroller Architecture Division, Microchip Technology Inc.
Electronic utility meters offer several advantages, when compared to the traditional mechanical and/or electromechanical solutions in use today. Whether it is Gas, Water, Heat or Energy being measured, some or all of these features may apply:
• improved accuracy
• ease of calibration
• anti-tampering protection
• automated meter reading
• advanced billing (time of use, pre-pay...)
Implementing an electronic meter does not have to be a complex endeavor. The examples shown in this article reduce each of the meter types down to a pulse counter on a microcontroller (MCU). Figure 1 shows a block diagram of a typical counter found on a MCU.
Meters are classified in terms of the accuracy of their measurements. It is typical, for example, for a mechanical energy meter to have an accuracy of better than 2 percent. By comparison, an average electronic meter may have accuracy down to 0.2 percent. The virtue of having a MCU in the design is that the accuracy is a function of the software parameters. A single hardware platform can be developed to support various levels of accuracy, and allows for a streamlined production and economies of scale for both the manufacturer of the meter and the utility company that deploys the meter into the field.
Ease of Calibration
The typical mechanical meter has many moving parts. Over time, these parts may need to be re-aligned or adjusted to bring the meter back into compliance. This would usually involve removing the meter and returning it for calibration. The equivalent electronic meter may also require calibration due to aging of components. However, the use of non-volatile memory (EEPROM or Flash) in MCUs provides a convenient method of storing and updating calibration information, and can even be designed to self calibrate.
One of the largest problems, globally, with utility metering is theft. In many cases, a meter is tampered with to alter the measurement. Theft is mostly found in energy meters and can take many forms. Depending on the type of meter, some meters can be inserted backwards causing the energy counter to decrement rather than increment. Also, older meters with rotating disks made of steel were susceptible to magnets slowing down the rotation speed, thereby causing incorrect energy measurement.
Electronic meters can use several simple methods to detect tampering and theft. Particularly in the case of energy meters, it is possible to detect a number of “typical” conditions such as:
• asymmetrical loads (closing the loop with the ground to avoid metering)
• temporary meter disconnect (or bypassed)
• use of permanent magnets to saturate current transformers or stop the counter
Once tampering has been detected, several actions can be taken by the meter. If the meter controls the application of power, it can remove power to the load. Another alternative is to note that a tamper condition has occurred and to light an indicator or send a message to the utility company, if the meter has a communication mechanism.
Automated Meter Reading
One of the biggest benefits of electronic meters is the addition of automated meter reading (AMR). Considerable savings can be realized by removing the human element from the process of recoding usage. This process is labor intensive, prone to human error (or even bribery) and a source of inconvenience for both the customer and the meter reader, depending on the location of the meter at the site.
Several technologies are currently employed for AMR of electronic meters or to retrofit existing mechanical/electromechanical meters. Electronic meters can read and communicate automatically through mechanisms such as:
• Infrared – short-range Infrared LED through the faceplate of the meter
• Radio Frequency (RF) – short and long range, such as ZigBeeд protocol or cellular networks
• Data modem via a telephone line
• Power Line Carrier (PLC) – short to medium range
• Serial Port (RS-485)
Some AMR advantages can be obtained simply by communicating with a handheld device (via IrDA™ protocol or RF up to several hundred feet away). While this does not eliminate the need for an operator to visit each location, it ensures that the readings are accurate and speeds up the process considerably. Additionally, the ZigBee Alliance is developing a metering profile that is intended to make it easy for manufacturers of water, gas, heat and energy meters to interoperate and send usage data through one communication medium.
As metering process automation increases, so does the need for secure data storage and communication technology. Privacy and integrity for the data being collected by the utilities is of great importance. This can be achieved via internal data EEPROM on the MCU itself or by using an encryption algorithm to store the data externally. The second concern is secure communication of the usage data. Here again, there are several encryption algorithms and handshaking that can be used to ensure the secure transfer.
Electronic meters have been billing based on Time of Use (TOU). TOU sets up periods during the day that are identified as on-peak (high usage) and off-peak (low usage). There are several benefits of TOU billing. First, customers can be offered lower pricing when usage occurs in the off-peak periods. Second, TOU billing will naturally shift more usage away from peak periods, due to the higher costs charged to customers during that period. The amount of investment to provide new utility distribution infrastructure is quite high. TOU billing helps to shift demand away from the peak periods and maintain a consistent capacity for an increasing demand. To implement TOU billing, the meter needs to have a Real Time Clock and Calendar (RTCC) to track the usage throughout the day. Electronic meters can easily implement the RTCC functionality in software or use an external device.
The newest addition in billing is the ability to pre-pay for energy usage. This is primarily implemented through energy meters. The customer will purchase finite units of energy ahead of time on magnetic cards. These cards are placed in the meter, which enables the meter to supply energy for the specified load and time period. Pre-payment reduces the cost of billing and collections for the utility company and also helps some customers to plan their monthly costs.
All of the aforementioned billing options depend on the base utility meter. It may seem that the development time could be increased because of two developments: the basic meter and the new features like anti-tampering, AMR, security and billing. Later, this article explains how the basic meter design is reduced to a simple pulse counter and most of the development effort is spent on the user interface. Most MCUs have the capability to clock an internal timer through an I/O pin. Some of these MCUs can clock the timer while the device is in a low-power mode and wake up when the timer overflows. This provides the best flexibility because gas, water and heat meters may not have a local source of power, instead relying on battery power.
Gas and Water Meters
Gas and water meters are the simplest meters to design. Because of the mechanics involved in measuring the flow of gas or water, the outputs are usually a rotating shaft (as found in gas meters) or a spinning magnet (in water meters). Figure 2 shows the block diagram for a gas meter. The gas meter has a slotted disc on the output shaft and a corresponding opto-reflector that outputs a stream of pulses. Each pulse represents a known quantity of gas. Water meters tend to have rotating magnets inside the meter and hall-effect sensors are used to output pulses every time the magnet passes. Both of these pulse streams can be connected to the clock input of a counter inside the MCU.
One of the challenges associated with gas and water meters is that there are typically no line-power sources close to the meter. This means the meter has to run off battery or solar power. Solar cells are quite expensive and create additional mechanical costs to mount. The solution is to use a low power MCU that can count pulses, periodically record the data to non-volatile memory and upload the information once a month for billing. The example shown in Figure 2 is one of the PIC16F9xx devices from Microchip Technology. These devices have 4 - 8Kbytes of Flash program memory, up to 336 bytes of RAM, 256 bytes of data EEPROM, an internal 8MHz oscillator, 10-bit A/D, I2Cд port, SPIд port, USART, 28 - 64 pins and can drive up to 168 pixels (4 COMs x 46 SEGs). These features coupled with low power operation (0.5 uA typical @ Sleep, 190 uA typical @ 1 MHz) provide an ideal MCU for battery-powered gas and water meters.
Depending on the area and country you are living in, your heating may come from hot water being pumped through a radiator. A heat meter contains a little more complexity than gas or water because of the thermodynamics involved with calculating energy usage from temperature and flow. A heat meter measures the temperature of the water both on the entrance to the radiator and the exit from the radiator. It also measures the flow rate of the water through the radiator. Based on these measurements, the MCU calculates energy usage via thermodynamic formulas. Figure 3 shows an example heat meter.
To keep costs low on heat meters, a MCU can be used to calibrate and signal condition the temperature sensors. These are typically RTDs or similar devices that are designed to withstand immersion in liquids. The calibration table to convert the analog output of the sensor into a linear temperature can be stored on the MCU. The flow meters used in heat meters are similar to those found in water meters and have pulse outputs.
Heat meters have an additional challenge not faced by gas or water meters. Heat meters are located inside the customer’s residence, not outside like gas and water. Without AMR, someone must be home to let the meter reader in to record the energy usage. An MCU-based heat meter can easily implement some type of RF to allow meter reading without the customer at home. The example shown in Figure 3 also uses the PIC16F9XX devices because of the low-power operation and integrated LCD module.
Perhaps most of the focus on electronic meters has been with energy meters. Theft in developing countries has been, and continues to be, the most prevalent force driving electronic meters. Not only is the meter tampered with to reduce the displayed energy usage, but the meter readers are susceptible to bribes to record less energy. A completely electronic meter with some sort of automated meter reading can save significant amounts or revenue for the utility company.
The largest challenge with energy meters is the requirement needed to accurately record energy usage. As mentioned before, some manufacturers require down to 0.2 percent accuracy. The meter also needs to handle large inductive loads like those found in refrigerators, HVAC, washers and dryers. For these reasons, MCU- or discrete-based solutions provide the best solution for designers. Fortunately, several manufacturers offer both types of devices. In the interest of keeping it simple, the discrete device provides the interface to the load and power source, the engine to measure current and voltage and calculate energy usage, and the simple pulse output. Figure 4 shows an example using the PIC16F9XX device for the MCU and the MCP3905 from Microchip Technology to measure energy usage. The MCP3905 offers 0.1 percent typical accuracy, negative power indication and support for shunt resistors on current measurement. The energy output is designed to drive 2-phase stepper motors found in mechanical displays, but can also easily drives a MCU counter input.
Compared to their mechanical brethren, electronic utility meters offer small, robust, more accurate solutions that provide anti-tamper circuits and methods to increase revenue for utility companies and lower costs to their customers. Using electronic solutions, such as those from Microchip Technology, can help reduce the complexity of the meter design, down to counting pulses. This allows designers to focus on the features that facilitate data collection and billing. For solutions to gas, water, heat and energy meters, visit Microchip’s online Metering Design Center.
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