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Making low power possible in wireless sensor networks

Posted: 16 Nov 2011 ?? ?Print Version ?Bookmark and Share

Keywords:wireless sensor network? node? energy harvesting?

A rechargeable battery can be used too for supplying and storing harvested energy. The voltage level can be monitored and system operation can be based on the available power. For example, the entire system can go into sleep/hibernate when the battery signals that it is low. Before dropping into sleep while waiting for the battery to recharge, the system sends a signal to the nearby node/network gateway to update its status.

The boost converter is configured by adjusting the values of the capacitors and inductors as shown in figure 4. The input and output voltage can be specified depending on the requirements of the harvester and the end application. The switching frequency can be controlled based on the allowed size of inductor (inversely proportional to switching frequency) and switching losses (proportional to the frequency).

Figure 4: Configuration window of boost converter for the application.

Low power design
Severely energy constrained systems need to have a long battery lifetime due to the inherent difficulty in physically accessing them to replace batteries as well as the sheer scale of deployment. With shrinking sizes of the nodes, this becomes a challenge both in terms of storage and harvesting area available to the node. Low power design is very important in energy constrained mobile applications, and therefore SoCs must be designed to ensure the lowest quiescent energy is consumed during idling states where the node spends most of its time.

Many processors offer multiple operating and low power modes to improve power efficiency. For example, there are four modes available in PSoC3. Active mode is the primary mode where the CPU controls specific modules to be operational. Alternate active powers only specific sub systems while the CPU is halted. Sleep mode is where all modules and CPU are disabled except some supervisory systems like the Watch Dog Timer. Finally, the lowest energy consumption is hibernate mode where all modules including supervisory modules are turned off and consumption is less than 200nA. The device only wakes upon a hardware pin interrupt. These different operating modes are very useful, depending on the state in which the wireless sensor node is operating. This could be dependent on external environment, time of day, or the activity levels/frequency to be sensed. This kind of software-based state machine control is useful for the end user to choose the right mode and easily change power consumption based on the application's current requirements.

Some other techniques can be used in order to avoid unnecessary power consumption like leaving I/O unused which could be driving a load. Instead, have a default high impedance on such I/O pins at the top level. Also, using a higher frequency than needed for a certain application is sub-optimal. Similarly, use of a PLL with source clock at a low frequency is more optimal than running the main clock frequency at higher cycles.

Wireless sensor nodes continue to become more sophisticated as well as more computationally intensive. Features like low power as well as integration of modules in a single chip are vital for a rugged and efficient node development.

References
1. Vullers et al., "Micropower Energy Harvesting", Solid State Electronics 2009.
2.Kantesh Kudapali, "Using sleep and hibernate modes in PSoC?3" AN66083 : http://www.cypress.com/?docID=26820
3.Course Report,U.R Kamath,"Energy Conversion Circuits for Wireless Autonomous Systems", Delft University of Technology.
4. Arun Mahodaya et al., "Urban air quality monitoring through large scale wireless sensor network", CISTUP 2010.

About the author
Umanath Kamath is a Technical Consultant for Cypress Semiconductors. He works on designing and supporting systems around Cypress' PSoC 1/3 and Power PSoC range of processors. He holds a Bachelors degree in Electronics and Communication from M.S.Ramaiah Institute of Technology, Bangalore. He enjoys designing analog circuits and systems and sensor interfaces.

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