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Design smart gas and water meters with superior energy efficiency

Posted: 13 Jan 2012 ?? ?Print Version ?Bookmark and Share

Keywords:Electronic water? gas meters? TX link?

Electronic water and gas meters come with some of the most difficult low-power design challenges for embedded control systems requiring RF connectivity. The nature of these applications requires them to be battery powered as electricity is rarely provided at the point of service for gas or water utilities.

The expected battery life for these systems is often greater than 20 years. This requirement is dictated by the utility provider since a single service call from a technician can often exceed the entire cost of the smart meter. Because of this long-life design requirement, nearly all water and gas meters use a battery chemistry of lithium thionyl chloride (LiSOCl2). This battery chemistry is chosen because of its very low self-discharge behavior and resulting ability to last for up to 20 years in these applications. However, these batteries are very expensive (as much as $1.5/A-hr) resulting in battery bill of material (BOM) costs of up to $10 to $15 per water or gas meter.

Many smart meter providers have determined that they can further differentiate their products by extending their communication range. In their system network topology, a fixed number of meters would communicate usage and billing information to a single repeater mounted on a utility pole through a sub-GHz proprietary network. The repeater would aggregate and transmit the collected information back to the utility provider over a cellular network modem or other backhaul channel.

A single repeater could support approximately 1000m nodes. However, the cost of the repeater can be anywhere from 10 to 100 times greater than a single meter node. Metering suppliers often face pressure from their customers to reduce the number of repeaters in a given network. This can be most readily achieved by improving the robustness of the transmitter (TX) link.

Figure 1: Comparison of power budgets for smart meter applications.

There are a number of ways to improve the TX link budget. The most obvious solution is to increase the output power of the transmitter using a power amplifier (PA). This is also the most costly approach in terms of battery life. Another strategy is to enhance the protocol to minimize the number of dropped messages and subsequent retransmissions. Although a much lower power approach than simply adding a larger PA, this technique can still increase the new TX power budget by as much as 40 percent over the current power budgets.

Let's consider three design requirements for one particular smart meter redesign:
???Allocate 40 percent more power budget to TX functions to increase range
???Maintain existing LiSOCl2 battery size (A) and capacity (3650 mA-hr)
???Maintain existing battery service life of 20 years

The strategy is clear: Increase the power within the TX budget while not increasing its total power budget. The reductions would have to be found in other functional areas, namely the RX, active and sleep mode budgets. Figure 1 shows the original power budget and the target budget after redesign.

Higher efficiency voltage conversion
To increase performance and reduce the power requirements of CMOS circuits, chip designers use the smallest practical device geometry to build their integrated circuits. It is common to find embedded processors and RF transceivers designed in 0.18?m, 0.13?m and even 90 nm geometries. One of the keys to reducing the power consumed by the device is reducing the internal operating voltage, thus reducing the CVf switching losses.

iswitching loss = Cgate Vgate frequency

Even though the battery supplying the device may have a terminal voltage of 3.6V, the device will operate at a much lower voltage internally.

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