Page 473 - Sensors and Control Systems in Manufacturing

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Cha p te r
Ei g h t

Transformations, which may need to be performed iteratively or
recursively for a multiple axis robot. The mathematics of the relation-
ship between joint angles and actual spatial coordinates is called
kinematics.
Positioning by Cartesian coordinates may be done by entering
the coordinates into the system or by using a teach pendant, which
moves the robot in X-Y-Z directions. It is much easier for a human
operator to visualize motions up/down, left/right, and so forth than
to move each joint one at a time. When the desired position is reached,
it is then defined in some way particular to the robot software in use
(e.g., P1–P5 cited earlier).
The American National Standard for Industrial Robots and Robot
Systems—Safety Requirements (ANSI/RIA R15.06-1999) defines a
singularity as “a condition caused by the collinear alignment of two
or more robot axes resulting in unpredictable robot motion and veloc-
ities.” It is most common in robot arms that utilize a “triple-roll wrist.”
This is a wrist about which the three axes of the wrist, controlling
yaw, pitch, and roll, all pass through a common point. An example of
a wrist singularity is when the path through which the robot is travel-
ing causes the first and third axes of the robot’s wrist to line up. The
second wrist axis then attempts to spin 360° in zero time to maintain
the orientation of the end effector. Another common term for this sin-
gularity is a “wrist flip.” The result of a singularity can be quite dra-
matic and can have adverse effects on the robot arm, the end effector,
and the process. Some industrial robot manufacturers have attempted
to sidestep the situation by slightly altering the robot’s path to pre-
vent this condition. Another method is to slow the robot’s travel
speed, thus reducing the speed required for the wrist to make the
transition. The ANSI/RIA has mandated that robot manufacturers
shall make the user aware of singularities if they occur while the sys-
tem is being manually manipulated.

8.15 Robot Programming
The robot is programmed in the teach-in mode with a standard pen-
dant. Different subprograms, which are called from the control com-
puter, are thus realized. The robot has 100 registers in which positions
and orientations can be stored by a control computer. In this way,
position and orientation determined by the vision module can be
transferred to the robot controller. The search and contour tracing
options of the robot controller are used for adaptive control.
The products arrive in a varying order and in an undefined posi-
tion. Because the parts (except for bolts) are supplied on a flat surface
in their most stable position (i.e., resting on their flat side), they have
three degrees of freedom, namely the x and y coordinates and the
orientation angle. The different parts are recognized by the vision