PE2-5.5 MEMS-Based Sensors & Actuators

What this is and why it exists

The field divides into devices that measure and devices that move. The two halves are not equally hard.

Sensors dominate the market. Actuators are the harder half, because producing useful force at this scale is genuinely difficult. That difficulty shapes every actuator here.

The vocabulary

• Transduction mechanism — the physical effect converting between the measured quantity and a signal.
• Piezoresistive readout — sensing strain as a resistance change.
• Capacitive readout — sensing movement as a capacitance change.
• Electrostatic actuation — moving a structure by the attraction between charged surfaces.
• Gyroscope — a device sensing rotation.
• Coriolis effect — the sideways force felt by something moving within a rotating frame.
• Microgripper — a device that grips at microscopic scale.
• Micropump — a device moving fluid in microscopic channels.
• Lab on a chip — an entire laboratory process carried out on one small device.

The mental model

For every device below, ask one question first. What is the transduction mechanism? Identify that and the device's strengths and limits usually follow.

Pressure sensing was the field's first commercial success and it is the device from the previous topic: a diaphragm with resistors that change when strained. Temperature sensing at this scale is straightforward and is often integrated beside another sensor to correct its drift.

Humidity sensing uses a material whose electrical properties change with the water it absorbs. Its practical limits come from the same mechanism. Absorption takes time, so the response is slow, and the material ages and gets contaminated, so the calibration drifts.

The accelerometer and gyroscope pair is worth studying together. They sit in the same package in every phone, and both act on a proof mass.

The accelerometer is the seismic principle at small scale. Accelerate the device and the proof mass lags on its springs. The relative movement is read as a capacitance.

The gyroscope senses rotation, which is harder. There is no rotational equivalent of a mass lagging behind. The usual answer is to drive the proof mass into a steady vibration. If the whole device then rotates, the vibrating mass experiences a sideways force at right angles to both its motion and the rotation axis. That is the Coriolis effect, and the sideways movement it causes is measured, again capacitively. So one structure vibrates continuously to make a second, much smaller motion detectable.

Biomedical and chemical sensors face the problems already named in the sensor module. Selectivity is harder than sensitivity. Packaging matters more here than anywhere else in the field, because the device must meet a hostile environment and remain safe in it.

Now the actuators, and the difficulty is worth stating plainly. Forces available at this scale are small. Electrostatic actuation is the usual answer. The attraction between charged surfaces grows sharply as the gap narrows, and gaps at this scale are already tiny. It needs no special material, and it consumes almost no current, because nothing conducts across the gap.

Its limits follow from the same equation. The force falls quickly as the gap widens, so the travel is short. And it needs high voltages relative to what the electronics run on, which is why many of these devices carry a charge pump.

A microgripper demonstrates how different actuation is at this size. Ordinary mechanisms with pivots and bearings cannot exist, because friction and stiction dominate and nothing can be assembled at that scale anyway. So a gripper is one piece of material with flexures where a hinge would be, deformed by an actuator. There is nothing to wear and nothing to assemble.

Micropumps close the course and point at where the field is growing. Moving fluid without conventional moving parts is what a laboratory on a chip needs. It is achieved by deforming a chamber, by heating a bubble, or by acting on the fluid electrically. The goal is a device that takes a drop of sample and performs a whole analysis on it. Much of the field's current growth is there.

What you should now be able to explain or do

Identify the transduction mechanism in each device before anything else. Say why sensors dominate the market and actuators are harder. Explain a micro accelerometer and a micro gyroscope, including why the gyroscope must vibrate. Say why electrostatic actuation is chosen and what its two limits are. Explain why a microgripper uses flexures rather than hinges.

Check yourself