Accelerometers

MEMS Technology

The basis of ifm’s vibration products is a Micro-Electro-Mechanical System (MEMS) accelerometer integrated onto an IC chip that powers the sensor. MEMS technology uses nano-sized (10^-9) mechanical components with nearly no mass or movement. The image below shows the relative gap distance between components. As these gaps change with vibration, the capacitance of the MEMS chip changes in direct proportion to the vibration.

Advantages of MEMS technology:

Integrated mechanical system and measurement electronics for rugged and reliable operation
Active self-test
Flat temperature response (< 2%) between -30…125 °C
No aging, so periodic calibration verification is not required
No saturation of the signal due to shock, cable bending or compression
Connection using standard M12 shielded cables

Disadvantages of MEMS technology

Fewer variants
Increased noise floor
Noise dependent on bandwidth

Piezoelectric accelerometers

Piezoelectric elements accumulate electrical charges in response to mechanical stress, such as vibration. The amount of accumulated charges is proportional to the level of vibration.

Piezoelectric accelerometers consist of a seismic mass attached to a piezo material (like ceramic). When the accelerometer is subjected to vibration, a force is generated and a small millivolt change is measured. This voltage is proportional to the acceleration of the mass.

Advantages of piezo technology:

Many variants and special versions available
Wide measuring range (0.005…50 g)
High accuracy over the total frequency range (Class 1:+/- 5%)
Wide operating temperature range (-55…125 °C)
Low noise floor independent of bandwidth

Disadvantages of piezo technology

No self-test ability
Cable bends, fast moving equipment can cause signal saturation

Very sensitive to temperature changes (up to 30%) between -30…125 C
Typically special armored cables are used because of cable compression and bending