• A variable resistor is used to convert a displacement to resistance/voltage.
• These give absolute position readings.
• The basic principle of operation is that a moving wiper (sensor input) moves a contact along a resistor. The ends of the resistor are connected to reference voltages. As the wiper moves the potentiometer acts as a voltage divider and produces a voltage proportional to position.
1.4.2 Linear Variable Differential Transformers (LVDT)
• This is effectively a transformer with a moving inner core. The moving core changes the inductance, and this can be converted to a position.
• In these devices there are three sets of windings. The first set of windings is in the center, and is powered by an AC source. The other two coils are at the opposite sides of the main coils. As the core is moved forward/back the magnetic coupling will change.
• If the core shown is in the center, the output will be 0Vac, as it shifts to the left there will be more coupling between the center and left coil, and the signal magnitude will be larger than the right hand coil. This difference can be converted to a calculated difference.
• This sensor is used for linear position sensing to high accuracies such as,
- load cells
- bourdon tubes
- dimensional measurement for SPC
• The signals from the LVDT can be conditioned with the circuit below.
• Near the center of the movement range the voltage is linearly proportional to core movement.
1.4.3 Moire Fringes
• By overlapping two fine lines patterns we can measure displacement.
• These are used in high precision applications, and are not available as off the shelf sensors.
• Uses the fact that when light is 180 degrees out of phase it cancels out.
• By finding the phase between outgoing and returning light (from lasers typically) the distance can be found.