PowerPoint Presentation

ACCELEROMETERS

SUBMITTED BY: NIDHI ARORA(11104047) VAISHALI LALIT(11104088)

AGENDA INTRODUCTION TO ACCELEROMETERS

TYPES OF ACCELEROMETER

COMPARISON BETWEEN TYPES

PRACTICAL APPLICATIONS Anaccelerometeris a device that measuresproper acceleration. Proper acceleration is not the same as coordinate acceleration . For example, an accelerometer at rest on the surface of the Earth will measure an acceleration g straight upwards. By contrast, accelerometers infree fallorbiting and accelerating due to the gravity of Earth will measure zero.Accelerometers are used to measure dynamic forces associated with moving objects. A typical accelerometer consists of a proof mass supported by a spring and a dashpot for damping of the vibrating proof mass.In miniaturizing the accelerometers to the micro-scale, there is no room for the coil spring and the dashpot for damping on the vibrating mass.

TYPES OF ACCELEROMETER

COMPARISON PIEZOELECTRIC ACCELEROMETER UNMATCHED IN UPPER FREQUENCY RANGE LOW PACKGAGED WEIGHT HIGH TEMPERATURE RANGE

PIEZORESISTIVE ACCELEROMETER PREFERRED IN HIGH SHOCK APPLICATIONS

CAPACITIVE ACCELEROMETER SUPERIOR IN LOW FREQUENCY RANGE HIGH STABILITY GOOD LINEARITYPiezoelectric plates are sandwiched between the casing and the seismic mass, which exerts on them a force proportional to acceleration.

Piezoelectric accelerometer

1. seismic mass2. piezoelectric plates (with magnification)3. tension control4. FET preamplifier5. cable attachmentIn MEMS technology a silicon cantilever with deposited piezoelectric film, e.g. BaTiO3 is used.A change in any of these parameters will be measured as a change of capacitance and variation of each of the three variables has been used in MEMS sensing

CAPACITIVE ACCELEROMETERTypical MEMS accelerometer is composed of movable proof mass with plates that is attached through a mechanical suspension system to a reference frame

Movable plates and xed outer plates represent capacitors. The deection of proof mass ismeasured using the capacitance difference The free-space (air) capacitances between themovable plate and two stationary outer plates C1 and C2 are functions of the correspondingdisplacements x1 and x2 :

If the acceleration is zero, the capacitances C1 and C2 are equal because x1 = x2 . The proofmass displacement x results due to acceleration. If x = 0, the capacitance difference is found to beMeasuring C, one nds the displacement x by solving the nonlinear algebraic equation

This equation can be simplied. For small displacements, the term Cx2 is negligible. Thus ,we can say displacement is proportional to change in capacitance.

Vx is square wave with the right amplitude proportional to acceleration. When there is no acceleration(a1 = 0), the proof mass doesnt move, and therefore, the voltage output is zero. If we accelerate the sensor (a1 > 0), the voltage output Vx changes proportional to alternating voltage input V0. To avoid signal attenuation, we read Vx with voltage follower (buffer), therefore signal Vy is actually Vx multiplied by 1. If we inverse the acceleration (a1 < 0), signals Vx and Vy get negative sign. Demodulator then gives us the sign of the acceleration, because it multiplies the input signal Vy with the square waves V0 coming from oscillator. Now we nally have voltage output Vout with the right sign of acceleration and the right amplitude.

APPLICATIONSAIRBAG SYSTEMystemRequired by Government law since 1999 model yearCapacitive Pressure Sensor and Capacitive Accelerometer Sensor are used for passenger protection

Sense a crashCapacitive Accelerometer SensorA change in electrical capacitance with respect to acceleration

Measuring the movement of Wii joystick / orientation and movement of smartphones (COMMERCIAL)