Post on 18-Jan-2018
description
Gonzales, Jamil M. Tengedan, Billy R.
Accelerometer Gonzales, Jamil M. Tengedan, Billy R. Accelerometer
An accelerometer is an electromechanical devicethat will measure
acceleration forces. These forces may be: Static, like the constant
force of gravity pulling atyour feet. Dynamic - caused by moving or
vibrating theaccelerometer. Common Functions By measuring the
amount of staticacceleration due to gravity, you canfind out the
angle the device is tilted atwith respect to the earth. By sensing
the amount of dynamicacceleration, you can analyze the waythe
device is moving. How do accelerometer work?
Piezoelectric effect - they contain microscopiccrystal structures
that get stressed by accelerativeforces, which causes a voltage to
be generated. Sensing changes in capacitance - If anaccelerative
force moves one of the structures,then the capacitance will change.
Piezoresistive effect, Hot air bubbles, and Light. Piezoelectric
Effect is the ability of certain materials to generate an electric
charge in response to applied mechanical stress Number of axes -
For most projects, two is enough
Number of axes - For most projects, two isenough. However, if you
want to attempt 3dpositioning, you will need a 3 axisaccelerometer,
or two 2 axis ones mounted atright angles. Maximum swing - If you
only care aboutmeasuring tilt using earth's gravity, a
1.5gaccelerometer will be more than enough. Ifyou are going to use
the accelerometer tomeasure the motion of a car, plane orrobot, 2g
should give you enoughheadroom to work with. For a project
thatexperiences very sudden starts or stops, youwill need one that
can handle 5g or more. Although the symbol is sometimes used for
standard gravity, (without a suffix) can also mean the local
acceleration due to local gravity and centrifugal acceleration,
which varies depending on one's position on Earth (see Earth's
gravity). Sensitivity - Generally speaking, the more sensitivity
the better
Sensitivity - Generally speaking, the more sensitivity the
better.This means that for a given change in acceleration, there
willbe a larger change in signal. Since larger signal changes
areeasier to measure, you will get more accurate readings.
Bandwidth - This means the amount of times per second youcan take a
reliable acceleration reading. For slow moving tiltsensing
applications, a bandwidth of 50Hz will probablysuffice. If you
intend to do vibration measurement, or control afast moving
machine, you will want a bandwidth of severalhundred Hz.
Applications Engineering
Accelerometers can be used to measure vehicle acceleration.
Accelerometers can be used to measure vibration oncars, machines,
buildings, process control systems andsafety installations.
Accelerometers are increasingly being incorporatedinto personal
electronic devices to detect theorientation of the device, for
example, a display screen. Specification (ADXL335) From Analog
Devices THEORY OF OPERATION 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. 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. Block
Diagram Pin Configuration Pin Description Sources
https://www.sparkfun.com/datasheets/Components/S MD/adxl335.pdf