Page 39 - Introduction to AI Robotics

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1 From Teleoperation To Autonomy

Figure 1.3 An RT3300 industrial manipulator. (Photograph courtesy of Seiko Instru-
ments.)

bow, and wrist), two of which are complex (shoulder and wrist), yielding six
degrees of freedom.
Control theory is extremely important in industrial manipulators. Rapidly
moving around a large tool like a welding gun introduces interesting prob-
lems, like when to start decelerating so the gun will stop in the correct loca-
tion without overshooting and colliding with the part to be welded. Also,
oscillatory motion, in general, is undesirable. Another interesting problem is
the joint configuration. If a robot arm has a wrist, elbow and shoulder joints
like a human, there are redundant degrees of freedom. Redundant degrees
of freedom means there are multiple ways of moving the joints that will ac-
complish the same motion. Which one is better, more efficient, less stressful
on the mechanisms?
It is interesting to note that most manipulator control was assumed to be
BALLISTIC CONTROL ballistic control,or open loop control. In ballistic control, the position trajectory
OPEN LOOP CONTROL and velocity profile is computed once, then the arm carries it out. There
are no “in-flight” corrections, just like a ballistic missile doesn’t make any
course corrections. In order to accomplish a precise task with ballistic control,
everything about the device and how it works has to be modeled and figured
CLOSED-LOOP into the computation. The opposite of ballistic control is closed-loop control,
CONTROL where the error between the goal and current position is noted by a sensor(s),