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14 Commercial Robot Manipulators

Distance sensors include time-of-flight rangefinder devices such as sonar
and lasers. The commercially available Polaroid sonar offers accuracy of
about 1 in. up to 5 feet, with angular sector accuracy of about 15 deg. For
360 deg. coverage in navigation applications for mobile robots, both scanning
sonars and ring-mounted multiple sonars are available. Sonar is typically
noisy with spurious readings, and requires low-pass filtering and other data
processing aimed at reducing the false alarm rate. The more expensive laser
rangefinders are extremely accurate in distance and have very high angular
resolution.

Position, Velocity, and Acceleration Sensors. Linear position-measuring
devices include linear potentiometers and the sonar and laser rangefinders
just discussed. Linear velocity sensors may be laser- or sonar-based Doppler-
effect devices.

Figure 1.4.1: Optical Encoders, (a) Incremental optical encoder, (b) Absolute optical
encoder with n=4 using Grey code. (Snyder, W.E., 1985. Industrial Robots, Prentice-
Hall, NJ, with permission.)

Joint-angle position and velocity proprioceptors are an important part of
the robot arm servocontrol drive axis. Angular position sensors include
potentiometers, which use dc voltage, and resolvers, which use ac voltage
and have accuracies of 15 min. Optical encoders can provide extreme accuracy
using digital techniques. Incremental optical encoders use three optical sensors
and a single ring of alternating opaque/clear areas, Figure 1.4.1(a), to provide
angular position relative to a reference point and angular velocity information;
commercial devices may have 1200 slots per turn. More expensive absolute
optical encoders, Figure 1.4.1(b), have n concentric rings of alternating
opaque/clear areas and require n optical sensors. They offer increased
accuracy and minimize errors associated with data reading and transmission,
particularly if they employ the Grey code, where only one bit changes between

Copyright © 2004 by Marcel Dekker, Inc.