Achieving the lowest power in wireless sensor networks

As physical sensing becomes more pervasive, the need for accurate and efficient wireless sensors and communication nodes become more important. A wireless sensor node is a platform that has basic components including a sensor, conditioning circuit, and data communication. A set of such nodes spatially distributed in order to co-operatively sense and pass the data forms a wireless sensor network (WSN).

Today, wireless sensor nodes are embedded in many devices. For instance, many of our electronic gadgets come with different sensors whose data can be harnessed for a variety of needs. The accelerometers in our mobile phones can track our activity, and the pressure sensors in the wheels of automobiles keep the pressure at optimal levels.

Wireless sensor nodes are a class of emerging applications for which energy consumption is a key metric. The successful deployment of wireless sensor nodes on a large scale depends on advances in many areas such as distributed computing, networking, wireless communication, and, most importantly, low power circuit design. Some of the applications of a wireless sensor network include industrial, home automation, medical monitoring, habitat monitoring, and agriculture resource management.


Fig1. Representation of a Generic Wireless Sensor Network


Generic Wireless Sensor Node


Fig2. Block diagram of a simple generic wireless sensor node.

A generic wireless sensor node includes a processor, which is the core of the system. This controls the various sensing activities of the node as well as schedules data transmission. Features like cooperative sensing and energy management control can be incorporated in the algorithms programmed into the processor. Sensors form the acquisition system followed by processing and control by the processor. The entire system can be powered by battery or stored harvested energy. Since different voltage levels are needed, boost conversion is needed for low voltage input, making a regulator part of the system as well. The data collected must be periodically transmitted wirelessly to a data center which is facilitated by low range RF or GSM to upload the data onto the internet.

General Requirements of a Wireless Sensor Node
Low Energy Consumption: Since the deployment of a wireless sensor network tends to be large, replacement of batteries would be a difficult, if not impossible, task. Therefore, any processing the node performs has to be low power and energy efficient. Energy harvesting methods need to be used to make nodes self-sustained.

Self-healing Structures: A fault in one nodes should not overtly impact the operation of the network. There should be easy access to debugging and rectifying the fault through support from the other networked nodes. Communication failures should be minimal and in the event of such occurrences, back up control should take over in order to avoid losing valuable data.

Robust: Since these nodes are deployed under different harsh physical conditions, they need to be able to operate accurately for long periods of time without any problems.

Challenges in the Future:

• The size of the typical node has gone from a few cm3 to sub cm3 regime. Size is determined by energy storage and harvesters.
• Power distribution to the different part of systems needs to be efficient.
• Scaling also affects the storage element as well as harvesting sensor size which puts a lot of importance on the efficiency of conversion. Battery size also tends to reduce by the same scale.
• Standby operating mode is dominant and therefore leakage/quiescent consumption becomes a crucial design parameter.
• Nodes are subjected to different non-ideal physical conditions under which these nodes should operate. The successful deployment of wireless nodes depends on the thorough characterization and investigation of operating in practical settings.