Position – velocity - acceleration

Oscillatory motion

• Position

• Velocity

• Acceleration

)cos(,)( 00 txextx tj

)sin(,)( 000 txexjexdt

dtv tjtj

)cos(,)( 02

02

02

2

txexexdt

dta tjtj

f 2

Consequence 1:

low frequency -> measure position or velocity

high frequency -> measure acceleration

Consequence 2

• Plotting position amplitude emphasizes low frequencies

• Plotting acceleration emphasizes high frequencies

Fig 3: Bearing damage

Fig 2: Misalignment

Vel

ocity

Frequency

Acc

eler

atio

n

Frequency

Velocity is the usual compromise

Acceleration -> Position

• Acceleration is independent of inertial system

• Can be measured without reference

• Can be measured in a closed container

Position can be found by integrating twice:

dtdttatx )()(

BUT

errorsother2

1)(

errors)other)(()()(

2

toffsettx

dtdtoffsettadtdttatx

true

truemeasuredmeasured

=> Measure position if you can

Or, get a grip on your errors

Accelerometer approaches

Fma

Measure F• Compression• Bending

Stress/force based

kxF

Piezoelectric

Piezoresistive

Measure x

Capacitive

(Optical)

(Magnetic)

AC

DC

FP

Thermal

Parallell plateComb

Capacitive spring based

Capacitive finger

Piezoresistive accelerometer

Piezoresistivt akselerometer 2

10 100 1 103 1 10

4 1 105

1 104

1 103

0.01

0.1

1

Frequency (Hz)

Out

put V

/g

Potentiometric level sensor

Tilt sensors

2D tilt sensor

Linear variable differential transformer LVDT

Eddy current sensor

Termisk masse (luft)

http://www.memsic.com/memsic/products/technology.html

Permanant magnet - Hall sensor

Permanent field 1

Permanent field 2

Position sensitive detector

Linear -> Rotary

Position encoders

Ultrasound time of flight

Impedance matching

Magnetic velocity measurement