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340 EXPERIMENTING WITH ROBOTIC ARMS

Figure 28- 1 Robotic arm with three joints that
each provide a degree of freedom.

pulleys, and a variety of other mechanical components. Some robot arms provide but 1 DOF;
others provide 3, 4, and even 5 separate DOF.

Degrees of Freedom in a Typical Robotic Arm

Human anatomy offers an inexact comparison with robotic arm systems. Our bone and mus-
cle structure provides movement in a way that is seldom duplicated in robot arms. For exam-
ple, out of simplicity, most robotic arms don’t use a ball joint for the shoulder. In the human
arm, this joint provides multiple degrees of freedom. In the robot version, shoulder motion is
duplicated with two and sometimes three separate joints.
Figure 28- 1 shows a representative robotic arm— it happens to be attached to a mobile
base, which provides yet an additional degree of freedom, though many robot arms are sta-
tionary. The arm provides 3 degrees of freedom; additional freedoms are provided by the
“wrist” and gripper.

• DOF #1 is rotation of the arm at its base. The base may rotate up to 360°, depending on
design, though it’s more common to limit rotation to about 180°. This represents the
mechanical extents of the typical R/C servo, which is often used to move the joints in a
low- cost robotic arm.
• DOF #2 and DOF #3 are essentially the shoulder and elbow joints, respectively. Together
they allow the arm to lift and lower, and to position its gripper at the height and distance
to grasp an object in front of it.

As noted, the wrist mechanism of this arm adds 3 DOF of its own. These include two joints
that form the wrist: it moves the gripper up and down and rotates it to position the gripper
fingers to best grasp the object. The third DOF is the finger mechanism.
All the joints of the arm, including those in the gripper section, work in tandem to locate,
grasp, and move objects. By pitching the shoulder, elbow, and wrist joints forward, the arm
can reach down and pick up something on the ground.

Arm Types

Robot arms are classified by the shape of the area that the end of the arm (where the grip-
per is) can reach. This accessible area is called the work envelope. For simplicity’s sake, the

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