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The initial development of sensors involves modeling knowledge drawn from many different disciplineFlitterFBD1s. Inertial sensors design requires lumped parameter dynamic modeling supported by stress analysis of beams, thermal analysis of plates, fluid flow analysis and squeeze film damping. As their size and shapes are altered so does their behavior. The adjoining figure shows the free body diagram of the inertial element used in Waddan’s square gyro.


The closed loop devices employ a pickoff (implemented as a set of parallel plates capacitors either with a fixed area or with a fixed gap), a PID controller, and an electrostatic forcer. The level of pick-off signal and forcing action depends upon the nominal gaps and the areas of the capacitor plates. The response of the servo loops, their controlling behavior, stability and bandwidth are analyzed prior to their electrical implementation. The figures below show the fixed area and the fixed gap configurations used as capacitive pickoffs. The displacement sensitivity of these pickoffs is dependent upon the area involved which can be determined from the following limitations:



The tunnel diode pickoff uses the current jumping across a gap between a flat surface and a sharp tip. Obviously, these are suited to sensors where one of the electrodes can be built as a tip with a very small area in close proximity of another flat electrode.

The electrostatic forcing action can be also generated by parallel plate capacitors. Again it is done either using fixed area or fixed gap plates as shown in the above figures.

The residual stresses left after wafer processing and their affect on sensor response is also investigated. Waddan Systems employs a dual approach investigation where experimentation and analysis work in parallel. If a simple processing experiment can shed some light on a difficult to model behavior, then that is performed to simplify modeling. The simplified model becomes a part of the 'What if' integrated development package.
The diaphragm like structures used in pressure sensors and hydrophones are modeled as thin plates flexing under dynamic pressure waves. Embedded impedance bridges are fabricated with piezo -electric and/or piezo-resistive elements to measure the deflection of the diaphragm. The size, shape and layout of the embedded elements are also determined by analysis.
The coriolis sensing devices and the gyros are analyzed with detailed math modeling. This sort of modeling works well with bulk micromachined or micromolded sensors. Bulk micromachining devices allow bi-polar approach to implement simple devices like transistors, diodes, piezo-electric and piezo-resistive elements, diffused capacitors, and tunnel diodes.
With surface micromachined devices the lumped parameter modeling does not work well because the inertial elements cannot be made rigid, because the thickness of structural polysilicon cannot be increased significantly (the stiffness of the plate increases with increasing thickness). The flexing and warping of thinner structures introduces high levels of unmodelable errors in sensor outputs.


The optical devices involve propagation of light through fibers or waveguides with embedded sensitive segments, where they get reflected, polarized, split, modulated, or interfered with due to phase difference. The propagation delays involved are measured either by measuring phase differences or by phase nulling.



For biological sensing, the balanced Wheatstone bridge approach is also used in SiC based nanowires sensing mechanism as shown in figure. If the fluid passing in the channel is contaminated, the contaminants will bind to the nanowires and change their conductivity across them, thus, imbalancing the bridge, which can be easily measured.
Optical interrogation technique employs a long period grating on an inverted waveguide and coated with capture antibodies of different pathogenic microorganisms. This structure is placed across the micro-fluidic channel. When the fluid passing in the channel has the target pathogens, the specific antibodies will bind to them, and can be selectively identified by the fluorescent dye molecules.


The change in temperature affects the behavior of all sensors. Either sensors are calibrated for response at different temperatures or a differential approach is implemented for common mode rejection.

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