Page 385 - The Mechatronics Handbook

Resistive

One of the simplest and least expensive ways to measure rotational or linear motion is using a variable
resistor called a potentiometer or rheostat. We focus on rotary potentiometers, or “pots” for short, but
the principle of operation is the same in the linear case.
A pot consists of three terminals (Fig. 19.11(a,b)). Two end terminals, call them terminals 1 and 3,
connect to either end of a length of resistive material, such as partially conductive plastic, ceramic, or a
long thin wire. (For compactness, the long wire is wound around in loops to make a coil, leading to the
name wirewound potentiometer.)
The other terminal, terminal 2, is connected to a wiper, which slides over the material as the pot shaft
rotates. The total resistance of the pot R 13 is equal to the sum of the resistance R 12 between terminal 1
and the wiper, and the resistance R 23 between the wiper and terminal 3. Typically the wiper can rotate
from one end of the resistive material (R 13 = R 12 ) to the other (R 13 = R 23 ). If the full motion of the wiper
is caused by one revolution of the shaft or less, the pot is called a single-turn pot. If the full motion is
caused by multiple revolutions, it is called a multi-turn pot.
Typically a pot is used by connecting terminal 1 to a voltage V, terminal 3 to ground, and using the
voltage at the wiper as a measure of the rotation. The voltage observed at the wiper is V(R 23 /R 13 ) and is
a linear function of the rotation of the shaft.
A remarkably simple absolute sensor for a wide range of distances is the string pot or draw-wire sensor
(Fig. 19.12). It consists of a string wrapped on a spool, with a potentiometer to monitor rotations of the
spool. A return spring keeps the string taut. Lengths up to many meters may be measured, using sensors
incorporating multi-turn pots. The same technique is similarly useful for short distances (a few centi-
meters) using compact single-turn pots and a small spool. Both tolerate misalignment or arc-like motion
well. String pots are susceptible to damage to the string in exposed applications, but the sensor element
is small and unobtrusive. Manufacturers include RDP Electronics, SpaceAge Control, and UniMeasure.

(a) 1

wiper
2

1
2
3
3

FIGURE 19.11 (a) As the shaft of the potentiometer rotates, the wiper moves from one end of the resistive material
to the other. (b) The inside of a typical potentiometer, showing the wiper contacting a resistive strip.

FIGURE 19.12 A string pot, courtesy of Space Age Control, Inc.

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