Strain and pressure sensors

Microelectromechanical systems (MEMS) combine the electronics of microchips with micromechanical features and microfluidics to create unique devices. The multitude of MEMS applications continues to grow including many types of accelerometers, radio frequency (RF) devices, variable capacitors, strain and pressure sensors, deformable micromirrors for image projection systems, vibrating micro-membranes for acoustic devices, ultrasound probes, micro-optical electromechanical systems (MOEMS) and MEMS gyroscopes, to name a few. 

Moreover, it may also be possible in the next few years to develop electronic skin made entirely out of organic transistors. In particular the possibility of developing strain and pressure sensors that can simultaneously act as switches and as sensors, without the need of any further sensing element, will be an interesting possibility. Furthermore, flexible chemosensitive transistors, biosensors, and temperature sensors could be developed using the same technologies allowing new features for this application. 

Strain and pressure sensors and also biochemosensors for measuring characteristics of the human skin are particularly interesting for this kind of applications because they could measure a wide set of parameters such as posture, breathing activity body fluids composition, etc. in a totally nonintrusive way. This characteristic is in fact very interesting for practical applications. For instance, it would allow doctors to monitor the patient status in real time, 24 h a day additionally, it would afford a better quality of life to patients for whom they would be perceived as noninvasive monitoring systems. 

Knite, M., Teteris, V., Kiploka, A., Kaupuzs, J., 2004. Polyisoprene-carhon hlack nanocomposites as tensile strain and pressure sensor materials. Sensors and Actuators A Physical 110 (1), 142-149. 

Figure 2.6 (a) An unbonded strain gage pressure sensor. The diaphragm is directly coupled by an armature to an unbonded strain-gage system. With increasing pressure, the strain on gage pair B and C is increased, while that on gage pair A and D is decreased. 

Optical fibre-based sensors have drawn most researchers attention, due to their versatility and compatibility with structural composites. They can detect a number of parameters such as temperature, strain and pressure and can be embedded into composites without major changes in the current manufacturing processes. 

Single-walled carbon nanotubes (SWNT) were proposed as nanoscale electromechanical pressure sensors [6]. It was demonstrated by computation that a pressure induced a reversible shape transition in armchair SWNTs, which in turn induced a reversible electrical transition from metal to semiconductor. The potential long lifetime nature of this pressure sensor due to the excellent mechanical durability of the carbon nanotubes was pointed out as a superior aspect. SWNTs can also be used, besides as pressure sensors, as mass, strain, and temperature sensors by sensing the resonant frequency shift of a carbon nanotube resonator when it is subjected to changes in attached mass, external loading, or temperature [7, 8]. The feasibility of such a sensor was illustrated by means of computer... 

Other mechanical sensors rely on the piezoresistive effect, which represents the variation of resistance of a given material when subjected to strain (Fraden, 2010). The change in resistance results from the mechanical deformation of the material and is the physical principle underlying the operation of many force and pressure sensors. 

Pressure. Pressure so defined is sometimes called absolute pressure. The differential pressure is the difference between two absolute pressures. The most common types of pressure-measuring sensors are silicon pressure sensors, mechanical strain gauges, and electromechanical transducers. 

Gurau ef al. [129] presented another apparatus used to measure the in-plane viscous and inertial permeability coefficients. In their method, an annular DL sample was placed between an upper and lower fixture. The gas entered the upper fixture and was then forced fhrough fhe DL info fhe ouflef porfs (open to the atmosphere). A strain sensor was located in the upper fixture in order to determine the thickness of fhe DL (i.e., deformation) because fhe whole assembly was compressed to a determined pressure. In fhis mefhod, the flow rate, temperatures in both fixtures, and pressures were monitored in each test. Once the data were collected, the in-plane permeability was determined from the Forchheimer equation by application of fhe leasf squares fit analysis method. 

It is also interesting to briefly consider online measurements of variables different from temperature [5], Since pressure is defined as the normal force per unit area exerted by a fluid on a surface, the relevant measurements are usually based on the effects deriving from deformation of a proper device. The most common pressure sensors are piezoresistive sensors or strain gages, which exploit the change in electric resistance of a stressed material, and the capacitive sensors, which exploit the deformation of an element of a capacitor. Both these sensors can guarantee an accuracy better than 0.1 percent of the full scale, even if strain gages are temperature sensitive. 

The aim is to eliminate entrance effects as much as possible and any influence on the flow of the pressure tap holes into the channels. This was achieved by integrating on the same silicon chip the microchannel, the pressure taps and the pressure sensors. The fabrication process and the operating mode are described in [28]. The pressure sensors are constituted cf a membrane which is deformed under the fluid pressure and on which is deposited a thin film strain gauge. This strain gauge forms a Wheatstone bridge whose the membrane deformation modifies the electrical resistances. 

Polysilicon and NiCr are currently used for high volume production of high pressure sensors. Because the origin of the change in resistance as a function of the applied strain is different for the two materials, we will analyze the physical contributions to the gauge factor and then discuss these materials in more detail. The Wheatstone bridge, which is commonly used to detect small changes in resistance, is also discussed. 

Three layers have to be patterned to fabricate strain gauges the sensing layer, the contact layer, and the passivation layer. Usually, the highest precision is required when patterning the sensing layer. Minimum line widths are about 30 pm. Examples of layouts for high-pressure sensor elements are shown in Fig. 5.4.11. 

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