Microelectromechanical Systems (MEMS)

Microelectromechanical Systems (MEMS)
for Sensing Heading and Acceleration

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Defined:
- Microscopic electro-mechanical devices with moving parts
Typical Components:
- A microprocessor
- One or more microsensors
Size:
- Components are typically between 1 and 100 micrometers (i.e., 0.001 and 0.1 mm)
- Devices are typically between 0.02 and 1.0 mm

Measuring the Heading

Objective:
- Measure the magnetic field (e.g., to determine a heading)
Historical Technology:
- A magnetic compass consists of a magnetized needle that can rotate (e.g., on a pivot, floating in water). It points north (after settling down) because the Earth's magnetic fild exerts torque on the needle pulling the "north" end toward the Earth's magnetic North pole
Other Modern Technology:
- A gyrocompass consists of a spinning wheel that is acted on by friction caused by the rotation of the Earth and, hence, points toward true North

MEMS Magnetometers

A Common General Approach:
- Measure the mechanical motion of a component that results from the Lorentz force acting on a current-carrying conductor (in the magnetic field)
Specifics:
- Measure the movement of the component using an optical sensor (e.g., a laser or LED)
- Measure the strain using piezo-resistors (which change their electrical resistivity when mechanical strain is applied)

Measuring Acceleration

Objective:
- Measure the proper acceleration (i.e., the acceleration relative to freefall)
Historical Technology:
- When a damped mass on a spring is accelerated the mass is moved to the point that the spring can push the mass at the same speed, hence, the compression of the spring measures the acceleration

MEMS Accelerometers

A Common General Approach:
- The mass is on a cantilever beam
- The entire structure is in a gas filled chamber (with the gas causing the damping)
Specifics:
- Measure the motion using a piezoelectric material (that generate an electric charge in response to applied mechanical stress)
- Measure the strain using piezo-resistors (which change their electrical resistivity when mechanical strain is applied)