Non-Servo Robots

Degrees of Freedom

Degrees of freedom (DOF) refers to how many " axes" of movement your robot has.

The number of degrees of freedom defines the robot’s configuration. For example, many simple applications 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 traverseis movement on a vertical axis. This is the side-to-side swivelof the robot’s arm on its base. The radialtraverseis the extension and retraction of the arm, creating in-and-out motion relative to the base. The vertical traverseprovides 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 yawis the side-to-side movement, and the roll, or swivel, involves rotation.

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.

Classifying Robots

Robots can be classified in various ways, depending on their components, configuration, and use. Three common methods of classifying robots are by the types of control system used, the type of actuator drive used, and the shape of the work envelope.

Type of Control System

Robots may use one of two control systems—non-servo and servo. The earliest type of robot was non-servo, which is considered a non-intelligent robot.

The second type is the servo robot. These robots are classified as either intelligent or highly intelligent. The primary difference between an intelligent and highly intelligent robot is the level of awareness of its environment.

Non-Servo Robots

Non-servo robotsare the simplest robots and are often referred to as “limited sequence, ” “pick-and-place, ” or “fixed-stop robots.” The non-servo robot is an open-loop system. In an open-loop system, no feedback mechanism is used to compare programmed positions to actual positions.

Non-servo robots are also limited in their movement and these limitations are usually in the form of a mechanical stop (limit switch) which signals the controller when it is activated. This form of robot is excellent in repetitive tasks, such as material transfer.

Characteristics of non-servo robots:

• Relatively inexpensive compared to servo robots.

• Simple to understand and operate.

• Precise and reliable.

• Simple to maintain.

• Capable of fairly high speeds of operation.

• Small in size.

• Limited to relatively simple programs.