Degrees of Freedom

Although robots have a certain amount of dexterity, it does not compare to human dexterity. The movements of the human hand are controlled by 35 muscles. Fifteen of these muscles are located in the forearm. The arrangement of muscles in the hand provides great strength to the fingers and thumb for grasping objects. Each finger can act alone or together with the thumb.

This enables the hand to do many intricate and delicate tasks. In addition, the human hand has 27 bones. This bone, joint, and muscle arrangement gives the hand its dexterity.

Degrees of freedom (DOF) is a term used to describe a robot’s freedom of motion in three dimension al space—specifically, the ability to move forward and backward, up and down, and to the left and to the right. For each degree of freedom, a joint is required. A robot requires six degrees of freedom to be completely versatile. Its movements are clumsier than those of a human hand, which has 22 degrees of freedom.

The number of degrees of freedom defines the robot’s configuration. For example, many simple application s require movement along three axes: X, Y, and Z. The three degrees of freedom in the robot arm are the rotational traverse, the radial traverse, and the vertical traverse. The rotational traverse is movement on a vertical axis. This is the side-to-side swivel of the robot’s arm on its base. The radial traverse is the extension and retraction of the arm, creating in-and-out motion relative to the base.

The vertical traverse provides up-and-down motion.

For applications that require more freedom, additional degrees can be obtained from the wrist, which gives the end effector its flexibility. The three degrees of freedom in the wrist have aeronautical names: pitch, yaw, and roll. The pitch, or bend, is the up-and-down movement of the wrist. The yaw is the side-to-side movement, and the roll, or swivel, involves rotation.

The arrangement of bones and joints found in the human hand provides dexterity. Each joint represents a degree of freedom; there are 22 joints, and thus, 22 degrees of freedom in the human hand.

A robot requires a total of six degrees of freedom to locate and orient its hand at any point in its work envelope. Although six degrees of freedom are required for maximum flexibility, most applications require only three to five. When more degrees of freedom are required, the robot’s motions and controller design become more complex. Some industrial robots have seven or eight degrees of freedom. These additional degrees are achieved by mounting the robot on a track or moving base. This addition also increases the robot’s reach.

Although the robot’s freedom of motion is limited in comparison with that of a human, the range of movement in each of its joints is considerably greater. For example, the human hand has a bending range of only about 165 degrees.