Sensing Technologies used for Accelerometers
PCB Piezotronics utilizes a number of transduction techniques when designing accelerometers including piezoelectric, piezoresistive, and capacitive.
The Piezoelectric Effect is an inherent, or induced property of certain crystals.
As the crystal undergoes stress due to applied force, negative and positive ions will accumulate onto the opposed surfaces of the crystal. The amount of accumulated charge is directly proportional to the applied force. Piezoelectric sensors will bleed this charge off through an amplifier in order to measure the amplitude of the charge. The geometry of the sensor determines the physical parameters (typically force, pressure, or acceleration) to which the crystal is sensitive.
Piezoelectric accelerometers employ either natural quartz crystals or man-made polycrystalline ceramics as their sensing elements. A proof mass is mated with the crystal and output is generated when a force is imposed upon the crystal during acceleration. This force causes stress in the crystal, which then generates an electrical charge that is relative to the applied force - the piezoelectric effect. The amount of force is proportional to applied acceleration as governed by Newton's law of motion F=ma. Piezoelectric accelerometers cannot measure constant acceleration because they are inherently AC coupled, however, they are typically the most versatile and economic choice for measuring fast transient and periodic acceleration.
Piezoresistive accelerometers may be fabricated from metal strain gauges, piezoresistive silicon, or as a MEMS device. In such designs resistive material is typically bonded to a cantilever beam that undergoes bending under the influence of acceleration. This bending causes deformation of the resistor, leading to a change in its resistance. The resistors are normally configured into a Wheatstone bridge circuit, which provides a change in output voltage that is proportional to acceleration. Piezoresistive accelerometers are capable of measuring constant, transient, and periodic acceleration.
Capacitive accelerometers utilize the properties of an opposed plate capacitor for which the distance between the plates varies proportionally to applied acceleration thus altering capacitance. This variable is used in a circuit to ultimately deliver a voltage signal that is proportional to acceleration. Capacitive accelerometers are capable of measuring constant as well as slow transient and periodic acceleration.