Power efficiency trade-offs key to wireless sensing success

As applications multiply for autonomous wireless sensor nodes, the trade-offs resulting from limited available power are coming sharply into focus. Until a breakthrough is made in battery design or energy scavenging, designers have some clear choices regarding sample rate, signal resolution and filtering, data storage and transmission, and even what’s to be sensed in order to hit their power and lifetime budgets. For wireless sensor nodes, the key is either to find more power or to be smarter about how to use it.

Some applications, such as the structural health monitoring of bridges, can tolerate a larger sensor node, with a battery sized for a five- to 10-year lifetime. Other applications require smaller form factors. Smart power usage, designed for the sensing application, is a key enabler of these smaller, cheaper nodes.

To maximize power efficiency, focus on when and what to sense. Identify the specific data that must be measured and configure your power parameters around those criteria. Strategically targeting the data capture will allow the sensor node to record only the data required. Try to avoid recording and then transmitting a lot of high-precision zeros.

In terms of when to sense, several schemes can be used depending on the application. A local sentry sensor—a low-power, low-performance sensor in each node—can watch for signals over a defined threshold. The sentry can trigger on the high-performance, high-resolution sensors, recording an analysis to capture the desired event, and then put the system back to sleep once its task is completed. The local sentry sensor then returns to the signal watch mode until the next event occurs, allowing the wireless sensor node system to maintain its measurement and analysis coverage while conserving power.

Low-power schemes

Many vendors have released tiny, low-power sensors suitable for sentry duty. Sensor fusion brings together many sensor variables into a wireless node that allows for such redundancies as accelerometers with different power/performance levels measuring in the same axis.

Another low-power scheme is to use a high-performance sensor in a simple threshold mode. A capacitance-based accelerometer can be used with a single op-amp charge detector. Motion of the sensor node will build up a charge to a certain threshold and can automatically trigger the high-performance electronics to boot up and capture at full resolution. Delay electronics can return the node to sleep mode once the event is complete.

Finally, a remote sentry scheme can enable power savings without significant performance trade-offs. In this “team” approach, the sensor nodes alternate running at full power. While the lead node is running, the other nodes are in sleep mode but check for alerts at specific intervals. When an alert occurs, all the nodes wake up, capture at full resolution, and then drop back into sleep mode to conserve power.

In all of these schemes, fast system wakeup response times are an integral part of electronics design, particularly for applications requiring high performance, low power and long life cycles. The stability of calibrated sensor offsets and scale factors from turn-on to turn-on (ToT) becomes hugely important to the accuracy of data over long time scales. ToT stability requirements demand careful attention to sensor, packaging and system design.

Some sensor node apps, such as structural integrity monitoring, can tolerate a large battery form factor for extended deployment lifetime. Source: HP Labs.