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1.4 Sensors 15

n
0
two consecutive sectors. Accuracy is 360 /2 with commercial devices having
n=12 or so.
Gyros have good accuracy if repeatability problems associated with drift
are compensated for. Directional gyros have accuracies of about 1.5 deg.
Vertical gyros have accuracies of 0.5 deg and are available to measure
multiaxis motion (e.g. pitch and roll). Rate gyros measure velocities directly
with thresholds of 0.05 deg/sec or so.
Various sorts of accelerometers are available based on strain gauges (next
paragraph), gyros, or crystal properties. Commercial devices are available
to measure accelerations along three axes. A popular new technology involves
microelectromechanical systems (MEMS), which are either surface or bulk
micromachined devices. MEMS accelerometers are very small, inexpensive,
robust, and accurate. MEMS sensors have especially been used in the
automotive industry [Eddy 1998].

Force and Torque Sensors. Various torque sensors are available, though they
are often not required; for instance, the internal torques at the joints of a
robot arm can be computed from the motor armature currents. Torque sensors
on a drilling tool, for instance, can indicate when tools are becoming dull.
Linear force can be measured using load cells or strain gauges. A strain gauge
is an elastic sensor whose resistance is a function of applied strain or
deformation. The piezoelectric effect, the generation of a voltage when a
force is applied, may also be used for force sensing. Other force sensing
techniques are based on vacuum diodes, quartz crystals (whose resonant
frequency changes with applied force), etc.
Robot arm force-torque wrist sensors are extremely useful in dexterous
manipulation tasks. Commercially available devices can measure both force
and torque along three perpendicular axes, providing full information about
the Cartesian force vector F. Standard transformations allow computation
of forces and torques in other coordinates. Six-axis force-torque sensors are
quite expensive.
Photoelectric Sensors. A wide variety of photoelectric sensors are available,
some based on fibreoptic principles. These have speeds of response in the
neighborhood of 50 microsec with ranges up to about 45 mm, and are useful
for detecting parts and labeling, scanning optical bar codes, confirming part
passage in sorting tasks, etc.

Other Sensors. Various sensors are available for measuring pressure,
temperature, fluid flow, etc. These are useful in closed-loop servo-control
applications for some processes such as welding, and in job coordination
and/or safety interrupt routines in others.

Copyright © 2004 by Marcel Dekker, Inc.