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).
| Physical principle | Concept | CL | FW | SIZ | VAC | CRY | EUR | ENV | LIN | TDS | COM | SUR | Resolution | Range | Bandwidth |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Resistive | Linear/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. | ||||||||||||||
| Capacitive | Variable 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] | |||||||||||
| Inductive | LVDT Linear Variable Differential Transformer displacement sensor | 0.0001% of Full Scale | 0.25-250 [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 | |||||||||||||
| Piezo | Piezo 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 magnetic | Hall 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] | |||||||||||||
| Optical | Laser interferometer Measuring surface shape and transmitted wave fronts | λ/8000 | 0-10[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] | ||||||||||||
| Physical principle | Concept | CL | FW | SIZ | VAC | CRY | EUR | ENV | LIN | TDS | COM | SUR | Resolution | Range | Bandwidth |
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 | |||
|---|---|---|---|
| Contactless | No contact: long working distance | No contact: short working distance | Contact |
| Friction, wear | No wear | / | Significant wear |
| Size | Relatively small ………….................................. Relatively large | ||
| Vacuum | Vacuum compatible version | / | No vacuum compatible versions |
| Cryo | < 10 [K] | >200 [K] | Ambient |
| € | Relatively low cost ………….................................. Relatively high cost | ||
| Environmental robustness | Operatable in every environment | / | Requires relatively clean environment |
| Linearity | Low linearity error ………….................................. Large linearity error | ||
| Transverse directional sensitivity | No influence ………….................................. High influence From displacement in a transverse direction than measured |
||
| Complexity | Relatively simple ………….................................. Relatively complex System including signal conditioning and processing |
||
| Surface requirements | No measuring surface requirements | / | Special measuring surface required |