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Solving the challenge of smart meter design

using smart meters gives enterprises and engineers more opportunities to design metering solutions that meet evolving global standards. These solutions can meet future needs and will become part of mass solutions, that is, low-cost solutions. However, many difficulties need to be overcome in order to design a successful measurement solution for vehicle forward design and green supply chain construction

many times, the designers who develop measurement chips do not even realize the challenges and needs faced by measurement solutions. In this case, designers are prone to design problems, so that the product cannot be used for the final solution because of small design defects

this paper will introduce some main problems in the design of measurement SOC, and put forward some solutions that can achieve the expected goals. At the same time, this paper also enables SOC designers to understand the challenges in advance, so that they can calmly cope with and design effective solutions

challenge 1: accuracy

accuracy is the key to the success of measurement applications, because service providers will never use instruments that cannot be measured accurately. Accuracy is particularly important for meter applications because meters rely more on analog on-chip components than gas/water meter models. Usually, the meter uses on-chip ADC to measure the level of current and voltage (because off chip ADC will increase the price of the final solution). On the other hand, the gas flowmeter uses an off chip sensor to sense the speed of the gas flow

these sensors can provide digital outputs in the form of a series of pulses, which are proportional to the flow rate. Because these sensors generally use digital interfaces, the overall accuracy depends less on SOC and more on external sensors

on the other hand, for electric energy measurement, the accuracy depends on two aspects: how the transmission line is connected to the instrument (using transformer, sensor, Rogowski coil, etc.) and the measurement accuracy of on-chip AFE (analog front end) for voltage and current

therefore, for gas/water flow meters, the accuracy largely depends on the accuracy of the connected sensor. For electric meters, accuracy depends on two factors: AFE of SOC and off chip analog interface of SOC. Next we will discuss one by one

analog front end (AFE) from the perspective of customers, the accuracy of AFE is the most important factor. Usually, the result of ADC determines the scalability of SOC

the accuracy of the simulation system mainly depends on the selection of ADC. Σ-Δ ADC and successive approximation (SAR) ADC are the most commonly used in metrology applications. These two kinds of ADC have their own advantages and disadvantages. SAR ADC uses successive approximation algorithm, Σ-Δ ADC uses oversampling technology to sample the input and perform conversion. SAR ADC is very suitable for power sensitive applications

however, they may not be suitable for use in very noisy environments. Therefore, according to the performance of ADC and use case environment, low-pass filter can be used at the input of ADC to filter noise. At the same time, with Σ-Δ Compared with ADC, they also have lower stabilization time - the time required to stabilize ADC to give accurate conversion value

therefore, SAR ADC is more suitable for applications that require fast switching of input channels, which will lead to rapid changes in input levels. Σ-Δ ADC requires a high frequency clock to shorten the stabilization 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 multiplication and addition operations between the current and voltage values. It is easy to determine the input load voltage; However, it is indeed difficult to determine the current consumption

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

the scale factor of current and voltage measurement is constant, so it can be calculated appropriately. One limitation of this "current measurement" process is the need for a low-cost ADC that can directly measure current

another option is to use the known load resistance to convert the current into the corresponding voltage, and then measure the voltage through ADC, which corresponds to the actual current consumption. This provides a more feasible low-cost solution for current measurement, and there are various technologies available for current measurement. Some of the most widely used technologies include shunt resistors, Rogowski coils, and current transformers

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

the method of current transformer (CT) is the same as that of ordinary transformer. The magnetic flux of load current (consumed current) generates a small amount of current in the secondary CT coil, and then converts the current into corresponding voltage through the load resistor, and then feeds it to the AFE of MCU

The Rogowski coil is another method of measuring current (see Figure 1). This kind of coil also has a good measurement effect for the current with large changes. However, they provide output in time differential form. This is why an integrator is needed to obtain the corresponding current value

diagram of various instrument connection options of the solution

compared with the above three methods, shunt resistor technology is the cheapest; However, this technology is difficult to meet the requirements of high current measurement, and there is a problem of DC offset. Current transformers (CTS) can measure more current than shunt resistor technology. However, they also have problems: their cost is higher, and there are problems such as saturation, hysteresis and dc/high current saturation

the measurement range of the third Rogowski coil method is smaller than that of CT, showing better linear characteristics for large current range, and there are no problems of saturation, hysteresis or dc/high current saturation

however, its cost is only slightly higher than that of shunt resistor. Considering the current variation and consumption type, shunt resistor technology is mainly used in consumer/residential applications, and Rogowski coil is more widely used in industrial applications

Challenge 2: current consumption

soc current consumption is the main factor affecting the battery life of applications/solutions. Therefore, applications that operate in battery powered mode require SOC to have very low current consumption. The gas meter/flowmeter is not directly connected to the power supply

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

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

in addition, the consumption mode of gas/water/heat is different from that of electric 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 believe that the current range of low-power mode is 1.1 μ A-2 μ A (standby current in sleep mode)

another area of concern is SOC start-up time and related current consumption. Since the application requires that the instrument must wake up regularly, the starting time and starting current will be very critical. Therefore, the kernel used in such SOC is more important than other factors such as processing speed

challenge 3: security, protection and detection

security, tamper protection and detection performance mainly depend on the complexity of the final application. Meeting this requirement can be very simple. It only needs to be able to detect whether someone is trying to open the instrument cover, or whether someone illegally accesses the SOC and changes the billing software

however, it may also be very complex. It is a part of the gprs/cdma/zigbee network solution to enable the instrument connected to Ethernet to prevent hackers from attacking or protect the user data in the instrument. These requirements vary widely because measurement can or should be able to support different types of solutions

for independent solutions, instruments are not part of network-based measurement solutions. Meter reading and billing are carried out manually, and the requirements for security, protection and detection will be very low, because attacking a single instrument will not affect other instruments. Therefore, the service provider may choose the relatively simple detection scheme mentioned above

form a current path between the instrument window and the instrument cover to detect whether the instrument cover is opened. As long as someone tries to open the meter, the current will be interrupted, as will the operation of tampering with the meter

password protection of SOC internal registers can prevent unauthorized reprogramming of SOC. Unless there is a correct password, it cannot be reprogrammed, and any such failed attempt will be shown as a tampering attempt

for network-based solutions, security problems cannot be solved only through detection or simple password protection. More strict protection is needed, because the instrument is a part of the network. If a node (instrument) is attacked by hackers, the whole network will be exposed to hackers

in these cases, security is divided into software and hardware layers, which are further divided into multiple layers. In order to solve these problems, the industry has formulated many agreements such as en13757, Homeplug, isa100.11a, ansi/eia/cea-709.1-b-2000 and en14908

the rise of the measurement revolution largely depends on the development of the communication mode supported by smart meters. This kind of Communication puts forward high requirements for security. Because among all communication modes, this kind of communication mode will make the instrument/instrument network most vulnerable to hacker attacks

take prepaid measurement based on smart card as an example. This solution uses SPI (serial peripheral interface) to transmit data between smart card and instrument MCU. The smart card stores the amount in its internal memory. After inserting it into the meter, the meter will continuously deduct the amount according to the consumption

a simple attack may be to reprogram or copy the smart card. In this case, one way to prevent such tampering is to encrypt the data (such as authenticity data and amount) stored in the smart card. The instrument decrypts these data first, and then processes them

the same encryption process will be followed when writing back data on the smart card. In this way, as long as the encryption algorithm and encryption key are not exposed, the instrument will be protected. In fact, no matter which communication mode is adopted, almost all measurement solutions use encryption function to ensure that security will not be damaged

the type and complexity of encryption mainly depend on the type of communication protocol used. Gps/gprs/cdma, Ethernet and other communication protocols require intermediates and polymers that will dominate and require more complex encryption. Therefore, special hardware is also used to reduce software dependency, and the chip performance is enhanced by reducing kernel overhead

challenge 4: instant software update

due to the high cost involved in replacing the instrument, the service provider hopes that the instrument can be used for more than ten years, or even as long as 15 years. Therefore,

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