DISPLACEMENT SENSORS – CONCEPT OVERVIEW

Actuators & Sensors

INTRODUCTION

Displacement sensors can measure distance or displacement in various physical ways, such as:
Resistive
Capacitive
Inductive
Piezo effect
Electromagnetic
Optical
This sheet displays these physical working principles and shows the advantages and disadvantages of these principles, along with some typical parameters such as resolution and range. It is very useful if you are to select a displacement sensor or a distance sensor (absolute or incremental).

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Physical principleConceptCLFWSIZVACCRYEURENVLINTDSCOMSURResolutionRangeBandwidth
Physical principleConceptCLFWSIZVACCRYEURENVLINTDSCOMSURResolutionRangeBandwidth
ResistiveLinear/rotary Potentiometer
Resistance proportional to displacement along resistor
10 [µm]1000 [mm]
Strain gauge
Resistance changes when under strain. Secondary position sensor
Depends on primary sensor to convert displacement to strain.
CapacitiveVariable separation, area or dielectric
Change in capacitance due to change in plate area, separation or intermediate dielectric
1 [nm]0.01 – 2 [mm]100 [kHz]
InductiveLVDT
Linear Variable Differential Transformer displacement sensor
0.0001% of Full Scale0.25-250 [mm]50 [kHz]
Eddy Current
Interaction of the magnetic field of two conducting surfaces
[µm][mm]-[cm]100 [kHz]
Variable reluctance sensor
Displacement causes change in total reluctance and hence inductance
10 [mm]Not good for static/low speed
PiezoPiezo electric accelerometer
Accelerometer/force sensor
Depends on primary sensor to convert displacement to force.
Piezo strain gauge
Piezo resistive
Depends on primary sensor to convert displacement to strain.
Electro magneticHall effect
Lorentz force based
100 [µm]10 [mm]100 [kHz]
Variable reluctance sensor (VRS)
Measuring toothed wheel speed
Less good than hall effect sensors.[Hz]- 100 [kHz]
OpticalLaser interferometer
Measuring surface shape and transmitted wave fronts
λ/80000-10[m]Depends on signal conditioning
Encoder
Incremental linear or rotary optical encoders
[nm][nm]-[m][MHz]
Optical proximity detector
Reflection of (usually) IR light
[mm][mm]500 [kHz]
Confocal
Chromatic measurement
10 [nm][mm]100 [kHz]
PSD
Position sensitive detector. Ex triangulation based distance sensor, quad cell
[ µm][mm] to [m][MHz]
Optical fiber
Ex.fibre bragg grating strain or pressure sensor
[ µm][m][Hz]

CL : Contactless
FW: Friction, wear
SIZ : Size
VAC: Vacuum

CRY: Cryo
EUR: Euro (€)
ENV: Environmenal Robustness
LIN: Linearity

TDS : Transverse directional sensitivity
COM: Complexity
SUR: Surface Requirements

Parameter
ContactlessNo contact: long working distanceNo contact: short working distanceContact
Friction, wearNo wear/Significant wear
SizeRelatively small ………….................................. Relatively large
VacuumVacuum compatible version/No vacuum compatible versions
Cryo< 10 [K]>200 [K]Ambient
Relatively low cost ………….................................. Relatively high cost
Environmental robustnessOperatable in every environment/Requires relatively clean environment
LinearityLow linearity error ………….................................. Large linearity error
Transverse directional sensitivityNo influence ………….................................. High influence
From displacement in a transverse direction than measured
ComplexityRelatively simple ………….................................. Relatively complex
System including signal conditioning and processing
Surface requirementsNo measuring surface requirements/Special measuring surface required