Page 318 - Concise Encyclopedia of Robotics

Stereoscopic Vision
second (rps). A stepper motor, however, usually runs less than 180 rpm,
or 3 rps. Often the speed is much slower than that. There is no lower
limit; a robot arm might be programmed to move just 1° per day, if a
speed that slow is necessary.
In a conventional motor, the torque, or turning force, increases as the
motor runs faster. With a stepper motor, however, the torque decreases as
the motor runs faster. Because of this, a stepper motor has the most turning
power when it is running at slow speed. In general, stepper motors are
less powerful than conventional motors.
Two-phase and four-phase
The most common stepper motors are of two types: two-phase and
four-phase. A two-phase stepper motor has two coils, called phases, con-
trolled by four wires.A four-phase stepper motor has four phases and eight
wires.The motors are stepped by applying current sequentially to the phases.
The illustration shows schematic diagrams of two-phase and four-
phase stepper motors.
When a pulsed current is supplied to a stepper motor, with the current
rotating through the phases, the motor rotates in steps, one step for each
pulse.In this way,a precise speed can be maintained.Because of the braking
effect, this speed is constant for a wide range of mechanical turning
resistances. Most stepper motors can work with pulse rates up to about
200 per second.
Control
Stepper motors can be controlled using microcomputers. Several stepper
motors, all under the control of a single microcomputer, are typical in
robot arms of all geometries. Stepper motors are especially well suited for
point-to-point motion. Complicated, intricate tasks can be done by robot
arms using stepper motors controlled by software. The task can be
changed by changing the software. This can be as simple as launching a
new program with a spoken or keyed-in command. Compare SELSYN,
SERVOMECHANISM, and SYNCHRO.
See also MOTOR, POINT-TO-POINT MOTION, and ROBOT ARM.
STEREOSCOPIC VISION
See BINOCULAR MACHINE VISION.
SUBMARINE ROBOT
Human SCUBA divers cannot normally descend to levels deeper than
about 300 m (1000 ft). Rarely do they descend below 100 m (330 ft). Even
at this depth, a tedious period of decompression is necessary to prevent

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