Acclerometer

AccelerometerGonzales, Jamil M.

Tengedan, Billy R.

Accelerometer

An accelerometer is an electromechanical device that will measure acceleration forces.

These forces may be:

1. Static, like the constant force of gravity pulling at your feet.

2. Dynamic - caused by moving or vibrating the accelerometer.

Common Functions

By measuring the amount of static acceleration due to gravity, you can find out the angle the device is tilted at with respect to the earth.

By sensing the amount of dynamic acceleration, you can analyze the way the device is moving.

How do accelerometer work?

Piezoelectric effect - they contain microscopic crystal structures that get stressed by accelerative forces, which causes a voltage to be generated.

Sensing changes in capacitance - If an accelerative force moves one of the structures, then the capacitance will change.

Piezoresistive effect, Hot air bubbles, and Light.

Number of axes - For most projects, two is enough. However, if you want to attempt 3d positioning, you will need a 3 axis accelerometer, or two 2 axis ones mounted at right angles.

Maximum swing - If you only care about measuring tilt using earth's gravity, a ±1.5g accelerometer will be more than enough. If you are going to use the accelerometer to measure the motion of a car, plane or robot, ±2g should give you enough headroom to work with. For a project that experiences very sudden starts or stops, you will need one that can handle ±5g or more.

Sensitivity - Generally speaking, the more sensitivity the better. This means that for a given change in acceleration, there will be a larger change in signal. Since larger signal changes are easier to measure, you will get more accurate readings.

Bandwidth - This means the amount of times per second you can take a reliable acceleration reading. For slow moving tilt sensing applications, a bandwidth of 50Hz will probably suffice. If you intend to do vibration measurement, or control a fast moving machine, you will want a bandwidth of several hundred Hz.

ApplicationsEngineering

Accelerometers can be used to measure vehicle acceleration.

Accelerometers can be used to measure vibration on cars, machines, buildings, process control systems and safety installations.

Accelerometers are increasingly being incorporated into personal electronic devices to detect the orientation of the device, for example, a display screen.

Specification(ADXL335)

From Analog Devices

THEORY OF OPERATION • The ADXL335 is a complete 3-

axis acceleration measurement system. The ADXL335 has a measurement range of ±3 g minimum. It contains a polysilicon surface-micromachined sensor and signal conditioning circuitry to implement an open-loop acceleration measurement architecture. The output signals are analog voltages that are proportional to acceleration. The accelerometer can measure the static acceleration of gravity in tilt-sensing applications as well as dynamic acceleration resulting from motion, shock, or vibration.

• The sensor is a polysilicon surface-micromachined structure built on top of a silicon wafer. Polysilicon springs suspend the structure over the surface of the wafer and provide a resistance against acceleration forces. Deflection of the structure is measured using a differential capacitor that consists of independent fixed plates and plates attached to the moving mass. The fixed plates are driven by 180° out-of-phase square waves. Acceleration deflects the moving mass and unbalances the differential capacitor resulting in a sensor output whose amplitude is proportional to acceleration. Phase-sensitive demodulation techniques are then used to determine the magnitude and direction of the acceleration.

Block Diagram

Pin Configuration

Pin Description

Sources

https://www.sparkfun.com/datasheets/Components/SMD/adxl335.pdf