Virtualizing Physical Measurements Using Wireless Sensor Technology

Electrochem Solutions

Wednesday, October 01 2008

Benefits include the elimination of wired connections and having advanced signal processing capabilities within each sensor.

Virtualizing physical measurements using wireless sensor technology enables a host of new solution choices. Wireless sensors are smart devices that realize measurements without using external data acquisition (DAQ) equipment or external power sources. The hardware needed to directly digitize low-level signals resides within each sensor. The analog-to-digital converter (ADC) found in a highly integrated microcontroller is typically accompanied by a multiplexer (MUX) and programmable gain amplifier (PGA). Further inclusion of other mixed-signal peripherals such as comparators and digital-to-analog converters (DAC) promote the sensor as a complete measurement processing system.

The self-contained sensor can buffer consecutive high-speed measurements within its internal memory in preparation for sophisticated post-processing analytics. A Fourier transform (FFT), for example, can be applied to the captured waveform for conversion to a magnitude and phase component of each frequency. This analysis in the frequency domain reveals which frequencies are present in the waveform and builds a characterization model that describes the waveform’s signature. It is this signature, as opposed to raw measurement data, that can be communicated wirelessly to destination hardware in the sensor network pathway. There is an important benefit to this approach. The power required to periodically transmit high-level descriptive information is magnitudes less than what is required to continuously stream raw measurement data. Since the wireless sensor is battery powered, energy savings is critical.

Ultimately, the waveform signature description is received by another device that can conceivably be another sensor. A sensor that receives high-level description data from another sensor could use this auxiliary information to improve its own accuracy by discriminating against the frequencies of the first sensor. Alternately, the destination device may be a gateway that reconstructs the waveform from the descriptive signature transmitted from one or more sensors in the network. The waveform, in turn, can be fed to the inputs of a legacy DAQ system that would otherwise see the sensor’s direct output.

Before considering the details of data acquisition, post-processing, and RF transmission, one must review the type of physical measurement that is being made and select the appropriate fundamental sensing method.

Pressure measurement can be accomplished using conventional sputtered film technology where a metal diaphragm deflects under pressure and its surface tension and compression regions affect change in the resistive legs of a Wheatstone bridge. A constant current or voltage generated by the microcontroller’s DAC provides excitation to the bridge, whereby a small differential voltage is developed that is proportional to the pressure on the diaphragm. The ADC converts the differential signal with the assistance of the MUX and PGA as previously described.