 The key application of capacitive sensors is acceleration measurement. The fundamental principle of operation for a capacitive accelerometer is the property that a repeatable change in capacitance exists when a sensing structure is deflected due to an imposed acceleration. The acceleration creates a force F acting on a suspended flexure of known mass m. The flexure moves predictably and in a controlled manner dictated by its stiffness k. A gas filled gap d exists between surrounding electrodes as shown in figure. The inertial force can be calculated from Newton’s Second Law of Motion as: F = m a Knowing the force, a displacement of the flexure can be estimated using a simple spring calculation: x = F/k However, in practice, Finite Element Analysis (FEA) is employed to model the complicated spring designs. This displacement alters the gaps on either side of the flexure in an equal but opposite proportion. The distance between the flexure and surrounding electrodes (l), is then the nominal (zero g) spacing (d) ± the spring deflection (x) or: I1 = d + x
I2 = d – x Knowing the electrode area (A) and the permittivity constant of the gas (ε), the capacitance formed by the gaps can be determined from: C1 = A ε/I1
C2 = A ε/I2 This capacitance difference causes an imbalance in a bridge network of the internal electronic circuit. Internal signal conditioning incorporates AC excitation and synchronous demodulation. In addition, it provides power for the accelerometer element and outputs an analogue voltage proportional to the acceleration signal. Back to Technology
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