A simple solution for electronic utility meters

introduction

Electronic utility meters offer many advantages over traditional mechanical and electromechanical meter solutions currently in use. Whether it is a gas, water, heat, or electricity meter, electronic meters can take advantage of some or all of the following features:

Higher
accuracy Easy calibration
Anti-tamper protection
Automatic meter reading
Good security
Advanced billing methods (time-of-use billing and prepaid, etc.)
The design of electronic meters does not necessarily have to be complex. In the examples introduced in this article, the implementation of various meters can be greatly simplified by using pulse counters based on single-chip microcomputers (MCUs). Figure 1 shows the block diagram of a typical counter based on an MCU.

Figure 1 MCU 8-bit or 16-bit timer with external clock input

Higher accuracy

Meters are classified according to their measurement accuracy. For example, a mechanical meter has a typical accuracy of about 2%. In contrast, a common electronic meter can achieve a measurement accuracy of 0.2%. If an MCU is used in the meter design, the measurement accuracy can be adjusted by changing software parameters. In this way, only one hardware platform needs to be developed to support multiple levels of measurement accuracy, thus simplifying the production process for meter manufacturers and bringing good economies of scale to the utility companies that install the meters.
Easy calibration

Conventional mechanical instruments contain many moving parts. As the instrument is used for a long time, these parts may need to be recalibrated to restore it to normal state. When adjusting, the instrument usually needs to be disassembled and returned to the manufacturer for calibration, which is very inconvenient. However, by using the non-volatile memory (EEPROM or Flash) in the MCU, it is very convenient to store or update the calibration information, and it can even be designed to adopt automatic calibration.

Figure 2 Gas meter structure diagram

Tamper protection

One of the biggest problems with utility meters in general is theft. In many cases, the meter is tampered with in order to change the measurement. Theft often occurs with electric meters and can take many forms. Depending on the type of meter, some meters may be wired backwards and count down instead of up. Also, older meters with steel spinning disks are susceptible to magnets that slow down the rotation and cause erroneous measurements. There are several simple methods that can be used to detect tampering and theft of electronic meters. For electric meters in particular, there are several "typical" situations that can be detected, such as:

Asymmetric load (circuit is grounded, resulting in no metering of electricity consumption)

The meter is temporarily disconnected (or bypassed)

Use permanent magnets to saturate current transformers or stop counters

·Malicious damage[page]

Once tampering is detected, there are several things that can be done to the meter. If the meter controls the power supply, it can disconnect the power to the load. Alternatively, if the meter has a communication mechanism, it can indicate that tampering has occurred by lighting a light or sending an alarm message to the utility company.

Electronic meter reading

One of the biggest advantages of electronic meters is the addition of the automatic meter reading (AMR) function. This eliminates the need to send a dedicated person to the site to register usage data, which can greatly save costs. Manual meter reading is a labor-intensive job that is prone to human error (even bribery). Since meters are installed in different places, manual meter reading is very inconvenient for both users and meter readers.

There are currently a variety of technologies that can implement the AMR function of electronic meters, or improve existing mechanical/electromechanical meters. Automatic meter reading and communication of electronic meters can be achieved in the following ways:

Infrared - short-range infrared LED transmission through the meter panel;
Radio Frequency (RF) - short-range or long-range communication, such as ZigBeeTM protocol or cellular network;
Data modulation and demodulation over telephone lines;
Power Line Carrier (PLC) - short-range to medium-range transmission
Serial (RS-485)
In some cases, the benefits of AMR functionality can be achieved simply by communicating with a handheld device (via IrDATM protocol or RF, with a maximum communication distance of several hundred feet). Although this method still requires meter readers to visit each meter installation site, this method ensures that the data reading is accurate and can greatly speed up the meter reading process. In addition, the ZigBee Alliance is developing a measurement solution that will enable manufacturers of water, gas, heat and electricity meters to cooperate with each other to send usage data through a common communication medium.

Good safety

As the automation of measurement processes increases, the need for secure data storage and communication technologies is also increasing. It is very important to ensure the confidentiality and integrity of the data collected by the utility. This can be achieved by storing the data externally to the meter using the MCU's own internal data EEPROM or using encryption algorithms. Another concern is the secure communication of the meter's usage data. Again, several encryption algorithms and handshake protocols can be used to ensure secure data transmission.

Figure 3 Heat meter structure diagram
Advanced billing method

Electronic meters already have the ability to charge based on Time of Use (TOU). TOU sets peak (higher usage) and off-peak (lower usage) periods. TOU charging has several benefits. First, if users use during off-peak periods, they can enjoy lower prices. Second, since users during peak periods pay higher usage prices, TOU charging naturally reduces peak usage to a large extent. The investment in laying new utility infrastructure is quite high. TOU charging helps to divert peak usage demand and maintain a stable capacity as user demand continues to grow. To implement TOU charging, a Real Time Clock and Calendar (RTCC) needs to be set up inside the meter to track user usage throughout the day. Electronic meters can easily implement RTCC functions through software or using external devices.

The latest billing method is prepayment. This function is mainly implemented in the electricity meter. Users can use a magnetic card to purchase a certain amount of electricity in advance, and then insert the magnetic card into the meter to make the meter supply power to the specified load within a certain period of time. Prepayment reduces the cost of billing and meter reading for utility companies and also helps users plan their monthly expenses.

All of the above charging methods are based on the basic functions of the utility meter. It seems that the development time of the meter will increase because the development includes two parts: the basic meter function and the additional functions such as anti-tampering, AMR, security and billing method. Next, this article will introduce how to simplify the basic meter function design to a simple pulse counter and focus on the design of the user interface. Most MCUs can count the external clock input to the I/O pin through an internal timer. Some MCUs have timers that can count in low-power mode and wake up the device when the timer overflows. This function is very flexible because gas meters, water meters and heat meters may not have a local power supply and are powered by batteries.

Gas and water meters

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Gas and water meters are the simplest meters to design. Both meters use a mechanical device to measure gas or water flow, and their output is usually a rotating shaft (in gas meters) or a rotating magnet (in water meters). Figure 2 shows the block diagram of a gas meter. The output shaft of a gas meter has a slotted disk and a reflector that outputs a stream of pulses. Each pulse represents a certain amount of airflow. A water meter usually uses a rotating magnet and a Hall effect sensor that generates an output pulse each time the magnet passes. The pulse streams from gas and water meters can be connected to the clock input of the counter inside the MCU. A major challenge in the design of gas and water meters is that they are generally not near an AC power source. This means that they must be powered by batteries or solar energy. Solar cells are expensive and add additional mechanical costs to the installation of the meter. The design in this article uses a low-power MCU that can count the pulses, periodically save the data to non-volatile memory, and upload the billing information once a month. The example shown in Figure 2 uses a PIC16F9xx series MCU from Microchip. This series of MCUs has 4-8KB Flash program memory, up to 336 bytes of RAM, 256 bytes of data EEPROM, built-in 8MHz crystal oscillator, 10-bit A/D, I2C, SPI, USART interface, and can drive 168 pixels. These functions, coupled with low power consumption (typical current 0.5uA in sleep mode, typical current 190uA at 1MHz), make this MCU very suitable for battery-powered gas and water meters.

Figure 4 Electricity meter structure diagram

Hot Table

The heating method may be different in different regions and countries where users live. It is common to use hot water flowing through radiators to provide heating. The structure of heat meters is slightly more complicated than gas meters or water meters because the way thermodynamics calculates heat involves temperature and flow. Heat meters measure the temperature of the radiator inlet and outlet at the same time, and also measure the flow rate of water flowing through the radiator. Based on these measurements, the MCU calculates the heat energy usage according to thermodynamic formulas. Figure 3 shows an example of a heat meter. To reduce the cost of heat meters, we can use MCU to calibrate and adjust the temperature sensor. Temperature sensors are usually RTD (resistance temperature detector) or similar devices that can work when immersed in liquid. A calibration table can be stored in the MCU to convert the analog output of the sensor into a linear temperature value. The flow meter used in the heat meter is similar to that in the water meter and also generates output pulses. There is another challenge in the design of heat meters that gas and water meters do not have. Heat meters are installed in the user's home, unlike gas and water meters that can be installed outdoors. Without AMR function, the user must cooperate at home when the meter reader records the heat energy usage. MCU-based heat meters can easily implement RF functions, so that users can read meters even when they are not at home. The example in Figure 3 also uses the PIC16F9XX series MCU, which has low power consumption and an integrated LCD module.

Electricity Meter

The most interesting focus of electronic meters is probably the electricity meter. The problem of electricity theft has always been the main reason for the development of electronic meters in developing countries. Not only can the meter be tampered to reduce the amount of electricity it shows, but the meter reader can also easily tamper with the meter reading data by accepting bribes from the user. Therefore, electronic meters with automatic meter reading capabilities can greatly reduce the revenue lost by utility companies. The biggest challenge in the design of electricity meters is the need to accurately record the amount of electricity used. As mentioned earlier, some manufacturers require accuracy as high as 0.2%. The meter must also be able to handle large inductive loads, such as appliances such as refrigerators, HVAC (heating, ventilation and air conditioning), washers and dryers. Therefore, it is best for designers to use MCUs or discrete components. Fortunately, some manufacturers provide both types of meters. To simplify the design, the discrete design provides the load and power interface, uses a measurement engine to measure the current and voltage and calculate the power consumption, and uses a simple pulse output method. The example shown in Figure 4 uses a PIC16F9XX device as the MCU and a Microchip MCP3905 to measure the power consumption. The MCP3905 has a typical accuracy of 0.1% and features a power reversal indication function, using a shunt resistor to measure current. The power output drives a mechanical two-phase stepper motor, but can also drive a counter input of an MCU.

Conclusion

Compared with mechanical meters, electronic meters are small, reliable, and highly accurate, and can use tamper-proof circuits and methods to increase the revenue of utility companies and reduce user expenses. The electronic meter solution using pulse counting can greatly reduce the complexity of meter design. This allows designers to focus on the design of more convenient data collection and charging functions.