Page 469 - Sensors and Control Systems in Manufacturing

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Ei g h t
Cha p te r

• Degrees of freedom. Usually the same as the number of axes.
• Working envelope. The region of space a robot can reach.
• Kinematics. The actual arrangement of rigid members and joints
in the robot, which determines the robot’s possible motions.
Classes of robot kinematics include articulated, Cartesian,
parallel, and SCARA.
• Carrying capacity or payload. How much weight a robot can lift.
• Speed. How fast the robot can position the end of its arm.
This may be defined in terms of the angular or linear speed of
each axis or as a compound speed (i.e., the speed of the end
of the arm when all axes are moving).
• Acceleration. How quickly an axis can accelerate. Since this
is a limiting factor, a robot may not be able to reach it’s speci-
fied maximum speed for movements over a short distance or
a complex path requiring frequent changes of direction.
• Accuracy. How closely a robot can reach a commanded posi-
tion. Accuracy can vary with speed and position within the
working envelope and with payload (see Compliance). It can
be improved by robot calibration.
• Repeatability. How well the robot will return to a pro-
grammed position. This is not the same as accuracy. It may be
that when told to go to a certain X-Y-Z position, it gets to
within only 1 mm of that position. This would be its accuracy,
which may be improved by calibration. But if that position
is taught into controller memory and each time it is sent there
it returns to within 0.1 mm of the taught position, then the
repeatability will be within 0.1 mm.
• Motion control. For some applications, such as simple pick-
and-place assembly, the robot need merely return repeatably
to a limited number of pre-taught positions. For more sophis-
ticated applications, such as welding and finishing (spray
painting), motion must be continuously controlled to follow
a path in space, with controlled orientation and velocity.
• Power source. Some robots use electric motors, others use
hydraulic actuators. The former are faster, the latter are stron-
ger and advantageous in applications such as spray painting
where a spark could set off an explosion; however, low inter-
nal air-pressurization of the arm can prevent ingress of flam-
mable vapors as well as other contaminants.
• Drive. Some robots connect electric motors to the joints via
gears; others connect the motor to the joint directly (direct drive).
Using gears results in measurable “backlash,” which is free
movement in an axis. In smaller robot arms with DC electric
motors, because DC motors are high-speed low-torque motors,