Page 380 - The Mechatronics Handbook

19.1 Linear and Rotational Sensors

Kevin M. Lynch and Michael A. Peshkin

By far the most common motions in mechanical systems are linear translation along a fixed axis and
angular rotation about a fixed axis. More complex motions are usually accomplished by composing these
simpler motions. In this chapter we provide a summary of some of the many technologies available for
sensing linear and rotational motion along a single axis. We have arranged the sensing modalities
according to the physical effect exploited to provide the measurement.

Contact
The simplest kind of displacement sensor is a mechanical switch which returns one bit of information:
touching or not touching. A typical microswitch consists of a lever which, when depressed, creates a
mechanical contact within the switch, which closes an electrical connection (Fig. 19.1). Microswitches
may be used as bump sensors for mobile robots, often by attaching a compliant material to the lever
(such as a whisker) to protect the robot body from impact with a rigid obstacle. Another popular
application of the microswitch in robotics is as a limit switch, indicating that a joint has reached the limit
of its allowable travel.
Figure 19.2 shows a typical configuration for a microswitch. The pull-up resistor keeps the signal at
+V until the switch closes, sending the signal to ground. As the switch closes, however, a series of micro-
impacts may lead to “bounce” in the signal. A “debouncing” circuit may be necessary to clean up the
output signal.
Switches may be designated NO or NC for normally open or normally closed, where “normally”
indicates the unactivated or unpressed state of the switch. A switch may also have multiple poles (P) and
one or two throws (T) for each pole. A pole moves as the switch is activated, and the throws are the
possible contact points for the pole. Thus an SPDT (single pole double throw) switch switches a single
pole from contact with one throw to the other, and a DPST (double pole single throw) switch switches
two poles from open to closed circuit (Fig. 19.3).

FIGURE 19.1 A typical microswitch.

+V

FIGURE 19.2 Signal bounce at a closing switch.

©2002 CRC Press LLC