Towards the Development of MEMS g-Switches
Accelerometers are sensors that measure and record the acceleration of an object in motion. They typically contain spring and mass elements that are set into vibration upon any impressed acceleration. The characteristics of their continuous vibration are used to extract the acceleration information. MEMS g-switch accelerometers are a special class of such sensors that sense a threshold acceleration and act as a switch for some other action that must ensue only if the threshold acceleration is achieved. Such accelerometers are used for impact detection and very high-speed propulsion, e.g., rockets. There are two types of g-switches — Latch type and Non-latch type. Non-latch type accelerometers have a proof-mass suspended by springs (beams) and two stationary electrodes, on experiencing an acceleration, this proof-mass touches the two electrodes closing an external electrical circuit and allowing an electric current to flow through the circuit as long as there is contact. This current flow indicates that a particular level of acceleration is reached. Latch type accelerometers, on the other hand, have compliant electrodes that latch on to the proof-mass on experiencing the threshold acceleration, maintaining the switch in ON state permanently from the instant the threshold acceleration is reached. We have designed, developed and studied both types of g-switches in this work. Time of contact (in case of non-latch type switches), contact resistance, and response time are key parameters that affect the performance of these devices. Energy-based approach is used in the design of these switches with appropriate displacement and velocity constraints that enforce the threshold condition. Dynamic characterisation is done on the fabricated devices to extract their modal parameters. An impulse acceleration is generated by means of an impact using a drop test equipment and high-speed video imaging is used to extract the device response. The experimental response is compared with FEM simulation results. The high-speed video imaging is also used to study the dynamics of the contact at the micro-second time scale. Electrical switching experiments are done for varying magnitude of the shock profile to find the threshold g-level of the switches. For the latch type accelerometers, to reset the switch and make them reusable after latching, a Chevron beam (bent beam) type thermal actuator is designed to accomplish the unlatching. In case of large vertical off-set between the proof-mass and the electrode, a horizontally actuated bent beam mechanism is used to accomplish the unlatching. Thus the study undertaken reports a complete development of the target g-switches.