Page 92 - Concise Encyclopedia of Robotics

Displacement Transducer
As an example, suppose a mobile robot is programmed to proceed at
a speed of 1.500 meters per second (m/s) at an azimuth bearing of 90.00°
(due east) on a level surface. After 10 s, this robot can be expected to be
15.00 m due east of its starting position. If the robot encounters an up-
ward incline, the displacement might be less than 15.00 m; if the robot
encounters a downslope, the displacement might be more. If the surface
banks to the left or the right, the direction of motion can be expected to
change, causing the robot to end up to the north or south of its position
had it traveled on a level surface. In the ideal scenario, terrain irregularity
would not affect the speed or direction of the machine; the displacement
error would therefore be zero.
Displacement errors can result from the accumulation of kinematic
error over time. Compare KINEMATIC ERROR.
DISPLACEMENT TRANSDUCER
A displacement transducer is a device that measures a distance or angle
traversed, or the distance or angle separating two points. Some displace-
ment transducers convert an electrical current or signal into movement
over a certain distance or angle. A transducer that measures distance in a
straight line is a linear displacement transducer. If it measures an angle, it
is an angular displacement transducer.
Suppose you want a robot arm to rotate 28° in the horizontal plane—
no more and no less. You give a command to the robot controller such as
“BR = 28”(base rotation = 28°). The controller sends a signal to the robot
arm,so that it rotates clockwise.An angular displacement transducer keeps
track of the angle of rotation, sending a signal back to the computer. This
signal increases in linear proportion to the angle that the arm has turned.
By issuing the command “BR = 28,” you tell the controller two things:
1. Start the base of the arm rotating.
2. Stop the rotation when the arm has turned through 28°.
The second component of the command sets a threshold level for the return
signal.As the signal from the displacement transducer increases, it reaches
this threshold at 28° of rotation. The controller is programmed to stop
the arm at this time.
There are other ways to get a robot arm to move, besides using displace-
ment transducers. The above is just one example of how such a transducer
might be used in a robotic system.
See also ROBOT ARM and TEACH BOX.

DISTANCE MEASUREMENT
Distance measurement, also called ranging, is a scheme that an autonomous
robot can use to navigate in its work environment. It also allows a central

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