Smart meter design challenges many successful metering solutions

The use of smart meters will give companies and engineers more opportunities to design metering solutions that meet evolving global standards that will meet future needs and will become part of the mass solution, low-cost solutions. However, to design a successful metering solution, many challenges need to be overcome.

In many cases, designers who develop metrology chips are not even aware of the challenges and needs of metrology solutions. In this case, the designer is prone to design problems and the product cannot be used for the final solution because of a small design defect.

This article will introduce some of the major issues in metering SoC design, and propose some solutions that can achieve the desired goals. At the same time, this article also enables SoC designers to understand the challenges in advance so that they can easily deal with and design effective solutions.

Challenge 1: Accuracy

Accuracy is the key to successful metering applications because service providers will never use instruments that cannot measure accurately. Accuracy is especially important for meter applications because the meter is more dependent on analog on-chip components than the natural gas/water flow meter model. Typically, the meter uses on-chip ADCs to measure current and voltage levels (because off-chip ADCs increase the price of the final solution). Gas flow meters, on the other hand, use off-chip sensors to sense the velocity of gas flow.

These sensors can provide digital output in the form of a series of pulses that are proportional to the flow rate. Because these sensors generally use digital interfaces, overall accuracy is less dependent on SoCs and more dependent on external sensors.

On the other hand, for energy metering, the accuracy depends on two aspects: how the power line is connected to the instrument (using transformers, sensors, Rogowski coils, etc.) and on-chip AFE (Analog Front End) voltage and current measurement accuracy.

Therefore, for gas/water flow meters, the accuracy depends to a large extent on the accuracy of the connected sensors. For the meter, the accuracy depends on two factors: the AFE of the SoC and the off-chip analog interface of the SoC. We will discuss one by one below.

Analog Front End (AFE) From the customer's point of view, the accuracy of the AFE is the most important factor. In general, the result of the ADC determines the scalability of the SoC.

The accuracy of the analog system depends mainly on the choice of ADC. The sigma-delta ADC and successive approximation (SAR) ADCs are the most commonly used in metering applications. Both ADCs have their own advantages and disadvantages. The SAR ADC uses a successive approximation algorithm, and the sigma-delta ADC uses an oversampling technique to sample the input and perform the conversion. SAR ADCs are ideal for power-sensitive applications.

However, they may not be suitable for use in very noisy environments. Therefore, depending on the performance of the ADC and the use case environment, a low-pass filter can be used to filter noise at the ADC input. At the same time, they also have lower settling time compared to sigma-delta ADCs - the time required to stabilize the ADC to give an accurate conversion value.

Therefore, SAR ADCs are more suitable for applications that require fast switching of input channels. Fast switching of channels can result in rapid changes in input levels. The Σ-Δ ADC requires a high-frequency clock, which reduces the settling time. Therefore, this will increase the final cost of the solution and increase power consumption.

The energy consumption calculation of the load line interface requires multiple multiplications and additions between the current and voltage values. Determining the input load voltage is easy; however, determining the current consumption is indeed difficult.

The total current consumed by home/industrial/buildings cannot be fed to the chip. However, a ratio value (current or voltage) can be determined and fed to the AFE and then measured using the ADC.

The scale factor for current and voltage measurements is constant, so proper calculations can be made. One limitation of this "current measurement" process is the need for a low-cost ADC that can measure current directly.

Another option is to use a known load resistor to convert this current to a corresponding voltage, which is then measured by the ADC, which corresponds to the actual current consumption. This provides a more feasible, low-cost solution for current measurement, and there are various techniques that can be used for current measurement. Some of the most widely used technologies include - shunt resistors, Rogowski coils, current transformers

The shunt resistor technology uses small (shunt) resistors placed on the load current line. When the load current passes through this resistor, a small voltage drop is formed. This voltage drop is fed as input to the AFE, which can measure the corresponding current consumption.

The current transformer (CT) method works in the same way as an ordinary transformer. The load current (consumed current) magnetic flux generates a small amount of current in the secondary CT coil, and then passes the current through the load resistor and converts it to the corresponding voltage. Then feed the MCU's AFE.

The Rogowski coil is another way to measure the current. This kind of coil also has a good measurement effect for a large change of current. However, they provide output in the form of time difference. This is why an integrator is required to obtain the corresponding current value.

Compared with the above three methods, the shunt resistor technology is the cheapest; however, this technology is difficult to meet the high current measurement requirements, and there is a problem of DC offset. Current transformers (CTs) can measure more current than current-resistor technology. However, they are inherently problematic: they are more costly, have problems with saturation, hysteresis, and DC/high current saturation.

The third Rogowski coil method has a smaller measurement range than CT, exhibits good linearity over a large current range, and does not suffer from saturation, hysteresis, or DC/high current saturation issues.

However, its cost is only slightly higher than the flow resistors. Considering the current variation and the type of consumption, the shunt resistor technology is mainly used in consumer/domestic applications, and Rogowski coils are more widely used in industrial applications.

Challenge 2: Current Consumption

The current consumption of the SoC is a major factor affecting the battery life of the application/solution. Therefore, applications that operate in battery-powered mode require a very low current consumption of the SoC. The gas meter/flow meter is not directly connected to the power supply.

Therefore, they can only be powered by batteries. Therefore, these applications are more sensitive to current than electric meters. This feature is very important because the average life of the meter is about 15 years, and customers certainly do not want to replace the battery every few years.

Therefore, gas/flow meter applications are more sensitive to these limitations than meters. In a typical gas/flow meter solution, the meter remains in a low energy state most of the time. It will periodically wake up to calculate energy consumption, store values, and possibly reset pulse counters.

In addition, gas/water/heat consumption patterns are different from electrical energy because they are not used all the time like electricity. Therefore, the kernel does not always have to be powered on. "Low-power mode current" will play an important role. Many companies consider the low-power mode current range 1.1μA-2μA (sleep mode standby current).

Another area of ​​concern is the SoC startup time and the associated current consumption. Because the application requires that the meter must wake up periodically, the start-up time and start-up current will be critical. Therefore, the kernel used in this type of SoC is more important than other factors such as processing speed.

Challenge 3: Safety, Protection, and Detection

Security, tamper protection, and detection performance depend primarily on the complexity of the end application. Satisfying this requirement can be as simple as detecting whether someone is trying to open the meter cover or illegally accessing the SoC and changing billing software.

However, it can also be very complicated to allow Ethernet-connected instruments to prevent hackers from attacking or protecting user data in the instrument. This is part of the GPRS/CDMA/ZigBee network solution. These requirements are very different because metering can or should be able to support different types of solutions.

For stand-alone solutions, the meter is not part of a web-based metering solution. Meter reading and billing are performed manually. The requirements for security, protection, and detection are low because attacking a single meter will not affect other meters. . Therefore, the service provider may choose the aforementioned relatively simple detection scheme.