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page 170

tracks?

(ans. 360°/64steps, 360°/512seps, 360°/4096 steps)

13.4 LINEAR POSITION

13.4.1 Potentiometers

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.

V

Vout=V(a/L)

L

a

13.4.2 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.

page 171

A rod drives the sliding core

AC input

Signal Out

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

-bellows/diaphragm

The signals from theLVDT can be conditioned with the circuit below.

 

 

 

 

 

 

 

 

 

 

 

 

Vac out

 

 

Vdc out

 

 

Vac in

LVDT

page 172

• Near the center of the movement range the voltage is linearly proportional to core movement.

13.4.3 Moire Fringes

By overlapping two fine lines patterns we can measure displacement.

1.The Pattern - two sheets with thin fringes are put at right angles, the optical effect is a darkened strip that runs along the strips. As one of the strips is moved the band will move up or down. When optically magnified the moving strip can be used to determine direction, and distance of motion.

2.The Detector - a collimated light source is shone upon the pattern. The light is then reflected back to a grid of sensors. The sensors then go on/off to indicate the presence of the band.

on off on off

• These are used in high precision applications, and are not available as off the shelf sensors.

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