Short History of SGS
The theoretical foundation for Strain Gauge Sensors was already laid in 1856, when Lord Kelvin discovered the phenomena of the piezoresistence effect. In 1954 Smith established then the physical relationship between resistance and stress with a silicon and germanium structure (source: http://cdn.intechopen.com/pdfs-wm/6124.pdf).
In the eighties this technology found its broad entrance into pressure sensors. Among many other applications, piezoresistive materials are also used in accelerometers. It is worthwhile to mention that carbon filled polymers are also working like piezoresistive materials. Due to straining, the distance between carbon particles increase and thus the conductivity decrease. Typically piezoresistive materials have breakages much below 10% of strain. In the late 90ties elastomer based piezoresistive materials were developed for textile applications.
Piezoresistive vs. Piezoelectric:
There is always a large confusion between the terms piezoelectric and piezoresistive. This goes as far as that our patent applications were compared to applications using the opposite technology. Piezo or the Greek form “piezin” means “squeeze” or “apply some pressure”. But beyond the similarities of name and that both technologies are used for similar purposes; there are also fundamental differences with regard to the technology and the signal characteristics. In an article published in the online magazine electronicdesign.com the following summary can be found.
“Piezoelectric components convert mechanical energy to electrical energy and vice versa, while piezoresistive devices convert mechanical energy to resistance values and that’s it. They do not work in reverse like their piezoelectric counterparts.”
With regard to the sensor labelling Empa generated the name Soft Condensed Matter Sensors (SCMS) to better separate between polymeric and elastomer based hybrid sensor materials.
Primary Signal vs. Noise Signal:
Captured signals can be basically come in the form of prime signals or noise signals. Capturing the dynamics around the wrist includes the wave form of the arteries, muscle and tendon movements or nerve reflexes. Tests have shown that many of those dynamics can be clearly distinguished using just one strain gauge sensor. As a result, strain gauge sensor signals can be used to filter distorted prime signals such as the arterial pulse wave signal captured with another sensor-type. Here, the noise signal function is key point and initial issue for all developments of STBL in the last years. Distinguishable and reproducible signals can be also used for many purposes including gesture control systems. Strain gauge sensor signals do have in such circumstances a prime signal function.
Deletion vs. Elimination:
It should be noted, that filtering can basically come in two forms, deletion and elimination. When dynamics cannot be properly characterised, distorted measuring periods can only be deleted or excluded from averaging periods. However, if dynamics can be quantitatively characterized, there remains the possibility to eliminate noise and calculate a filtered result.