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Chapter 28

Experimenting

with Robotic Arms

obots without arms can’t “reach out and touch someone.” Arms extend the reach of
Rrobots and make them more like humans. For all the extra capabilities arms provide a
robot, it’s interesting that they aren’t difficult to build. Your arm designs can be used for
factory- style, stationary “pick- and- place” robots, or they can be attached to a mobile robot as
an appendage.
This chapter deals with the concept and design theory of robotic arms. Incidentally, when
we speak of arms, we will usually mean just the arm mechanism minus the hand (also called
the gripper). Chapter 29, “Experimenting with Robotic Grippers,” talks about how to con-
struct robotic hands and how you can add them to arms to make a complete, functioning
appendage.

The Human Arm

Take a close look at your own arms for a moment. You’ll quickly notice several important
points. First, your arms are amazingly adept mechanisms, no doubt about it. Each arm has
two major joints: the shoulder and the elbow (the wrist, as far as robotics is concerned, is usu-
ally considered part of the gripper mechanism). Your shoulder can move in two planes, both
up and down and back and forth. The elbow joint is capable of moving in two planes as well:
back and forth and up and down.
The joints in your arm, and your ability to move them, are called degrees of freedom
(DOF). Your shoulder provides 2 DOF in itself: shoulder rotation and shoulder flexion/exten-
sion (shoulder flexion is motion upward to the front; shoulder extension is motion downward
to the rear). The elbow joint adds a third and fourth degree of freedom: elbow flexion/exten-
sion and elbow rotation.
Robotic arms also have degrees of freedom. But instead of muscles, tendons, ball- and- socket
joints, and bones, robot arms are made from metal, plastic, wood, motors, solenoids, gears,
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