Accelerometer Approaches

Outline Material from - Accelerometers • Fraden chap. 7 - Velocity • Fraden chap. 8 - Position + Misc!! • Wikipedia • Web – Analog Devices – MEMSIC – Chipworks • SINTEF

Bruk av akselerometere

• Crash sensor (biler, harddisker etc) • Sekundærsystem for navigasjon • Inklinometere (vater) • Vibrasjonsmålinger • Spill o.l. • Billedstabilisering • Seismiske undersøkelser

Position – velocity - acceleration

Oscillatory motion   2f jt • Position x(t)  x0e , x0 cos(t) d v(t)  x e jt  jx e jt ,  x  sin(t) • Velocity dt 0 0 0 d 2 • Acceleration a(t)  x e jt   2 x e jt ,  2 x cos(t) dt 2 0 0 0

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

Acceleration Velocity Fig 3: Bearing damage Frequency

Velocity is the usual compromise Fig 2: Misalignment Frequency 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: x(t)   a(t)dtdt BUT

x (t)  a (t)dtdt  (a (t)  offset  othererrors) dtdt  measured  measured  true 1 x (t)  offset t 2  othererrors true 2

=> Measure position if you can

Or, get a grip on your errors Accelerometer approaches

Piezoelectric

Measure F Piezoresistive • Compression • Bending

Parallell plate ma  F Comb AC Capacitive F  kx DC

dx FP v  (Optical) dt Measure x Cantilever Fiber (Magnetic) Measure v

Inductive Thermal Capacitive spring based Capacitive displacement sensing 3

Q i dt i V     C C jC Capacitive finger

Analog Devices ST-Accelerometer Iphone 4s

www.chipworks.com Piezoresistive accelerometer Piezoresistivt akselerometer 2

0.1

0.01 Output V/g Output

 3 110

 4 110 3 4 5 10 100 110 110 110

Frequency (Hz) Termisk masse (luft)

http://www.memsic.com/memsic/products/technology.html Tilt sensors 2D tilt sensor Magnetic velocity measurement Doppler Potentiometric level sensor Linear variable differential transformer LVDT Eddy current sensor

Permanant magnet - Hall sensor Permanent field 1 Permanent field 2 Position sensitive detector

Linear -> Rotary Position encoders Ultrasound time of flight

7.42 Triangulering - GPS

Wikipedia Innendørs triangulering - ultralyd Optisk mus

+ billedanalyse