Addressing the challenges of smart utility meter design

Adoption of Smart Utility Meters has thrown open a plethora of opportunities for companies and engineers to come up with metering solutions which could comply with the evolving global norms, have the potential to serve and adapt to future demands and could be part of a mass-appeal solution i.e. low costs solution. However, it has also thrown open a Pandora-box of challenges which lie in the path of achieving a successful metering solution.

Many a time, a designer working on a metering chip might not be even aware of the challenges and the demands of the metering solutions. In such a case, he/she is very prone to faltering in the design, making the product unsuitable for the end-solution due to a minor fault in the design.

This article aims to highlight some of the major issues of metering SoC (System on Chip) design and also propose some possible solutions to achieve the intended goals. It also aims to make the SoC designer aware of the challenges beforehand so that he/she could attack them head-on and an effective solution can be rolled out.

Challenge #1:  Accuracy

Accuracy is the key to success in metering applications as no services provider would go for a meter which is not able to give accurate reading of the consumption. This gains more importance for energy meter applications as they are heavily dependent on the analog on-chip components than their gas/water/flow counterparts. Generally, energy meters make use of on-chip ADCs (Analog to Digital Converters) to measure the current and voltages levels (as off-chip ADCs would only increase the price of the end-solution). On the other hand, gas/flow meters use off-chip sensors to sense the rate of flow of fluid/gas.

These sensors can give digital output in the form of a train of pulses, directly proportional to the rate of flow. As these sensors generally have more of a digital interface, the overall accuracy depends very less on the SoC (System on Chip) and more on the external sensor.

On the other hand, in case of energy metering, the accuracy depends on two things -- how the power-lines are interfaced with the meter (using transformers, sensors, Rogowski coil, etc) and how accurately voltages and currents are measured by on-chip AFE (Analog Front End).

Therefore, for gas/water/flow meter, accuracy is largely a function of the accuracy of the sensor interfaced. And for an energy meter, accuracy is dependent upon two factors – AFE of the SoC and the off-chip analog interface of the SoC. Now let’s consider each of them one by one.

Analog Front End (AFE). From a customer perspective, accuracy of the AFE is the most important factor. And very often, it is the results of the ADC that decide the salability of the SoC.

The accuracy of the analog system depends majorly on the choice of the ADC. Sigma-delta (SD) and Successive Approximation (SAR) are two most commonly used ADCs in metering applications. Both the ADCs have their own advantages and disadvantages.

SAR ADCs make use of the successive approximation algorithm and Sigma-Delta ADCs use over-sampling technique to sample the input and do the conversion. SAR ADCs are very well suited for applications which are power-sensitive.

However, their performance might not sustain in very noisy environment. Therefore, depending upon the performance of the ADC and the use-case-environment, one may use Low Pass Filters at the input of the ADCs to filter out the noise. Also, they have low settling time – time which an ADC require to stabilize to give accurate conversion value – as compared to SD ADCs.

Therefore, SAR ADCs are more preferable for applications that require faster switching of the input channels which also results in faster changes of the input level. SD ADCs require very high frequency clocks to reduce this settling time. Therefore, this results in the overall increase of the final cost of the solution and the power consumption.

Load-Line Interface. Energy Consumption calculation involves various multiplications and additions of currents and voltage quantities. Determination of the input load voltage is not a major issue; however, determination of the current consumption does pose challenges.

Whole of the current being consumed by the house/industry/building cannot be fed to the chip. Instead, a proportional quantity (current or voltage) is determined which is fed to the AFE and then measured using the ADC.

The scaling factor of the current and voltage measurements is maintained so that appropriate calculations could be done. One limitation of this ‘current measurement’ process is the availability of the low-cost ADCs which can measure currents directly.

Instead, this current is converted into a corresponding voltage, using a known load resistance, and then this voltage is measured by the ADC which corresponds to the actual current consumption. This provides a more viable low cost solution for the current measurement process.There are various techniques available for the current measurement. Some of the most widely used techniques are – Shunt resistor, Rogowski Coils, Current transformers.

The Shunt Resistor technique uses a small (shunt) resister placed in the path of the load-current. When load current flows through this resistance, a small voltage drop is developed across it. This voltage drop is fed as an input to the AFE which measures the corresponding current consumption.

The Current transformer (CT) approach works the same way a normal transformer works where the magnetic flux of the load current (current consumed) generates a small amount of current in the secondary coil of CT. Then this current is passed through a load resistance to convert it into a corresponding voltage which in turn is fed to the AFE of the MCU.

The Rogowski Coil is another method (Figure 1, below) used for the measurement of the current. This class of coils gives very good results even for highly varying currents. However, they give output in the time differentiated form. That is why one needs to have an integrator to get the corresponding current value.

Figure 1. Rogowski coil arrangement (Source www.allaboutcircuit.com)

Comparing all the above mentioned three modes, Shunt Resistor technique is the cheapest of all; however, it has severe limitations in the high current measurements and also suffers from DC offset issues. Current Transformers (CT) can measure more currents than their shunt resistor counterparts, however, they have their own issues associated with them -- they cost more, suffer from Saturation, Hysteresis and DC/High current saturation problems, etc.

On the other hand, Rogowski coils, third alternative, are not as expansive as CTs and offer very good linearity over a large range of currents, do not have Saturation, Hysteresis or DC/High current saturation problems associated with them.

However, they cost little more than the shunt resistors. Given the types of current variations and consumption, Shunt Resistor technique is primarily employed in consumer/residential applications and Rogowski coils are more popular with the industrial applications.