Page 187 - Robot Builders Source Book - Gordon McComb

5

Feedback Sensors

Referring to Figure 1.5 we see that, beginning from level 8, feedbacks are introduced
into the design of an automatic machine or robot. These serve to control the machine
or process, assuring automatic correction response of the system when conditions
change. Sensors are the principal elements of a feedback system. This chapter pre-
sents a brief review of the most important feedback domains and sensors appropriate
to them. The sensors can be divided into two main groups: analog and digital. To the
first group belong those sensors that respond to changes in the measured value by
changing some other physical value in their output, say, voltage, resistance, pressure,
etc. In contrast, digital sensors transform the measured value into a sequence of elec-
trical pulses. Information is carried encoded as the amount of pulses (say, the higher
the number of pulses, the larger the measured dimension), as the frequency of pulses,
or as some other pulse-duration parameter. The amplitude of the pulses usually has
no importance in information transmission.

5.1 Linear and Angular Displacement Sensors

The most common task of a feedback is to gather information about the real loca-
tions of robot or machine links using, for example, sensors that respond to displace-
ment or changes in location. There are several kinds of these sensors, some of which
will be considered here.

Electrical sensors

The simplest displacement sensor is a potentiometer: a variable electrical resistor
in which the slide arm is mechanically connected to the moving link. Thus, the resis-
tance changes in accordance with the displacement. The electrical displacement or
location sensors are usually a part of an electrical bridge, the layout of which is shown
TEAM LRN 175