Acceleration Sensors

Basic Description

Linear acceleration sensors, also called G-force sensors, are devices that measure acceleration caused by movement, vibration, collision, etc. All acceleration sensors operate based on a simple principle in which Newton's second law of motion is applied to a spring-mass system. A mass is connected to the base of the acceleration sensor through an equivalent spring (Fig-1). Since the force between the mass and base is proportional to the acceleration of the mass and the relative distance between them has a linear relationship with the force due to the spring, the acceleration can be calculated from a measurement of the relative position of the mass or force on the spring as it varies with time. Generally, the most common types of acceleration sensors include: piezoelectric, piezoresistive, variable capacitance and variable reluctance.

Piezoelectric

A piezoelectric acceleration sensor utilizes the piezoelectric effect to measure the relative distance between the mass and sensor's base, and then represents the acceleration in terms of an output voltage. Piezoelectric acceleration sensors are widely used due to their compact size and light weight, but they cannot be used to measure steady-state accelerations.

Piezoresistive

In a piezoresistive acceleration sensor, a piezoresistive material is positioned so that it is deformed by the position o the mass changing its resistance. This type of acceleration sensor has a small size, large signal amplitude and good linearity. It can be used to measure both steady-state and dynamic accelerations.

Variable Capacitance

A variable capacitance acceleration sensor uses changes in capacitance caused by a displacement in the mass to detect its position.

Variable Reluctance

A variable reluctance acceleration sensor uses changes in the inductance of a coil caused by a displacement in a mass made of magnetic material to detect the position of the mass.

In most acceleration sensors, the spring is not a coiled wire spring, but it is something that tends to restore the mass to its initial position. Some sensors employ pendulums or diaphragms in place of the spring.

Servo force balance sensors operate in a closed-loop manner. These sensors monitor the balance of forces between the mass and spring and keep them in an equilibrium state. This mechanism minimizes the errors caused by nonlinearity of the spring but increases the cost.

MEMS Acceleration Sensors: Acceleration sensors based on MEMS (MicroElectromechanical Systems) technology are becoming increasingly popular in automotive systems. MEMS devices are relatively small and rugged compared to other technologies. They are made by etching tiny mechanical structure in silicon wafers where they are readily integrated with system electronics.

In MEMS acceleration sensors, the sensitive element is a comblike structure of differential capacitors arranged in parallel on a beam (forming the seismic mass) supported by springs etched from the silicon substrate. The differential capacitor forms a capacitive half-bridge driven by a high frequency square wave generator. When acceleration is applied perpendicular to the seismic mass, the differential capacitor is mismatched and a non-zero voltage appears on the central plate. This signal is preamplified, demodulated, amplified and output as a voltage proportional to the applied acceleration.

Manufacturers

ÅAC Microtec, Bosch, Continental, Denso, Freescale, Sensata, STMicroelectronics, TRW

For More Information

[1] Accelerometer, Wikipedia

[2] The Principles of Acceleration, Shock, and Vibration Sensors, Craig Aszkler, Sensors, May 1, 2005.

[3] STMicroelectronics MEMS Accelerometers, YouTube, June 8, 2009.

[4] MEMS Accelerometers, Matej Andrejasic, 2008 [pdf].

[5] Some Recent Developments in Accelertion Sensors,George Juraj Stein, Measurement Science Review, 2001 [pdf].