= Microelectromechanical system
= MEMS
{c}
{synonym}
A <microelectromechanical system> combines small mechanical structures with electrical sensing or actuation. Fabrication methods related to <semiconductor device fabrication> can create suspended beams, membranes and movable masses together with their <electrodes>. <Sensors> and <actuators> can consequently be integrated into compact devices.
A <capacitive accelerometer> illustrates the mechanism. A suspended <proof mass> moves relative to its support under <acceleration>. In a simple <spring> model, a slowly varying support <acceleration> $a$ produces a relative displacement $x\approx-ma/k$, where $m$ is the <mass> and $k$ is the <spring constant>. Measuring a change of <capacitance> then measures that displacement. The ADXL335 uses suspended <polysilicon> structures and a differential <capacitor> to turn motion into an electrical signal.<ADXL335 Data Sheet by Analog Devices>[Revision B, page 10, Theory of Operation and Mechanical Sensor]{ref}
For an idealized pair of parallel-plate <capacitors> with equal area $A$ and gaps $g\mp x$, the <capacitance> difference is
$$
C_+-C_-=\frac{\varepsilon A}{g-x}-\frac{\varepsilon A}{g+x}\approx\frac{2\varepsilon A x}{g^2}\quad(|x|\ll g).
$$
This model explains why a small mechanical displacement can produce a signed electrical measurement; it is not a detailed model of every commercial <MEMS> geometry.
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