Networking & Communications

Power Management: Critical Choices for Wireless

January 1, 2007 By: Tom Kevan Sensors

In the past, the devices making up sensor networks received power through the hard-wired infrastructure; managing this essential operational resource meant simply guaranteeing clean, constant electrical energy. If the electrical system was in good working order, the network’s needs were met. Wireless installations, however, are challenging traditional solutions. Although an estimated 80% of current wireless systems do not require off-the-grid power sources, the industry is increasingly moving to truly untethered sensors, which rely on batteries as the energy supply. As a result, power management is a critical issue and the methods and technologies used in the process have become much more complex. In the broad sense, power management means optimizing performance to minimize energy consumption. Ultimately, this approach will extend the battery life of a sensor system to be as long as the end user requires, enabling autonomous sensing systems that don’t rely on main power distribution.

A Holistic Approach

To attain the best system performance, therefore, you must optimize the performance of every component—the sensors, microcontrollers, and radios—to consume as little power as possible while still meeting the requirements of the application, in terms of data throughput, latency, and reliability

“Power management touches the process almost everywhere,” says Rob Conant, co-founder and vice president of marketing and business development for Dust Networks. “It is incredibly important for wireless sensor networks because in many of these applications the battery-operated devices are supposed to last for years without user intervention. Traditionally, that has been a huge challenge. There aren’t many devices that are battery operated and function for years at a time. So power management techniques are incredibly important, and they have an impact on almost every aspect of the design.”

Each component manufacturer contributes to power management by providing the most efficient component possible. But optimal power management is implemented by taking a holistic view of the wireless sensor network, and the person responsible is often the system integrator, who has knowledge of individual components, as well as the applications and working environment. “You have to consider all these factors to put together a complete power management system,” says John Suh, senior application engineer for Crossbow Technology. “The sum is greater than its parts.”

The Sensors

Power management in sensors is achieved through hardware design and software functionality.

Hardware specifications define the current required to operate the sensor and the efficiency with which it performs certain processes. For example, when you turn on a sensor, there is a transient state during which time it has to stabilize before you get an accurate reading. The quicker a device stabilizes, the faster it can take a measurement and return to sleep mode, consequently using less power (Figure 1).

Figure 1. A timeline of events that shows one of Crossbow Technology’s motes, programmed with a sensor firmware using the company’s XMesh low-power network routing protocol
Figure 1. A timeline of events that shows one of Crossbow Technology’s motes, programmed with a sensor firmware using the company’s XMesh low-power network routing protocol
Different sensors have different characteristics, and some manufacturers do a better job of making devices that stabilize faster than others. For the most part, though, this is an area that requires improvement. “So far, I haven’t seen the sensor manufacturers of the world coming up with very power-efficient sensors,” says Crossbow’s Suh. “There are some; I am not saying there are none. But in terms of effort and bringing products to market, sensor manufacturers could further the cause of power management by creating sensors that have fast stabilization times.” But you cannot focus solely on stabilization. “You have to balance the overall power that the sensor consumes,” says Suh. “If you have a sensor that takes 10 times longer to warm up, but uses 20 times less current, you have to balance the total power used by the system.”

On the other hand, software enables power management in the sensor by controlling duty cycling: turning on the sensor, taking a measurement, and putting the sensor back into a low-power sleep mode. Controlling the duty cycle is the most common method of implementing power management in a sensor. Setup and configuration of the process are usually done through a graphical user interface (GUI).

Choosing the most appropriate sensor can also play a big role in managing power consumption. Are you using low-power sensors? Can you sense the property of interest in a different way, using a type of sensor that consumes the least amount of power? For instance, you could use either an accelerometer or a tilt switch to monitor tilt. But the switch is a simpler device that uses less power than the accelerometer.

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