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Sensors/MEMS??

Making low power possible in wireless sensor networks

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

Keywords:wireless sensor network? node? energy harvesting?

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.

Future challenges
???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.

Sensors
Sensors form the first contact to the physical environment. MEMS technology has made advances in both reduction of size as well as increasing accuracy of the sensed quantity. MEMS-based sensors that monitor temperature, voltage, humidity, photo, vibration, and gas have been part of the onboard sensor for wireless sensor nodes. These sensors result in an analog signal which further has to be amplified in order to be able to process the signal and differentiate it from noise. The external sensors can be configured or controlled by serial interfaces like SPI,I2C.

Energy requirements, sources
Usually the major part of energy consumption goes into the analog sensors. For instance, active sensors like metal oxide-based gas sensors consume a large amount power for heating. Next in line comes the communication link which is usually characterized by bursts of packets at regular intervals. Finally, there is the digital processor which does the sensing/conversion of the signal and low-level processing before ordering the data to be transmitted.

A typical node consists of a sensor to capture the external phenomenon, an ADC with signal processing circuitry to convert sensor information into a reliable digital form, and a radio for communication of this data. Typically, energy consumption will be dominated by the radio. Depending on the duty cycle of sensing activity, sometimes the sensor will dominate the energy consumption. Studies quote values between 1uW to 20uW for the amount of energy required for operation of a node. Some techniques like collaborative processing of data jointly by a set of nodes is done to complete computationally intense tasks.

Ambient energy
There are numerous sources of energy in the environment which can serve as a continuous source of energy for wireless sensor nodes over their lifetime. We will concentrate on RF energy, mechanical energy and solar energy harvesting which have been shown in literature as the viable options for powering wireless nodes.

RF energy refers to the energy available through the public telecommunications system. The energy levels provided by GSM at a distance of 30-100m from the base station are between 0.1 to 1mW/m2. This is not sufficient for any viable harvesting mechanism. WLAN, on the other hand, is a couple of orders lower compared to GSM. One possibility to harness this energy from GSM is to use a large area antenna or to have a dedicated source of RF energy. It should be noted here that these levels transmitted should be within the guidelines for the maximum amount permissible.

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