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 dimensional 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 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 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.

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 robots are 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.

A good example of an open-loop system is the operating cycle of a washing machine. At the beginning of the operation, the dirty clothes and the detergent are placed in the machine’s tub. The cycle selector is set for the proper cleaning cycle and the machine is activated by the start button. The machine fills with water and begins to go through the various washing, rinsing, and spinning cycles. The machine finally stops after the set sequence is completed. The washing machine is considered an open-loop system for two reasons:

• The clothes are never examined by sensors during the washing cycle to see if they are clean.

• The length of the cycle is not automatically adjusted to compensate for the amount of dirt remaining in the clothes. The cycle and its time span are determined by the fixed sequence of the cycle selector. Non-servo robots are also limited in their movement and these limitations are usually in the form of a mechanical stop. This form of robot is excellent in repetitive tasks, such as material transfer. One may question if the non-servo robots qualify as a robot based on the definition provided by the Robot Institute of America. However, if these robots are equipped with a programmable logic controller (PLC) they easily meet the requirement of a reprogrammable device, thus allowing them to be classified as a robot.

1. At the beginning of the cycle, the controller of a pneumatic (air-controlled), non-servo robot sends a signal to the control valve of the manipulator.

2. As the valve opens, air passes into the air cylinder, causing the rod in the cylinder to move. As long as the valve remains open, this rod continues to move until it is restrained by the end stop.

3. After the rod reaches the limit of its travel, a limit switch tells the controller to close the control valve.

4. The controller sends the control valve a signal to close.

5. The controller then moves to the next step in the program and initiates the necessary signals. If the signals go to the robot’s end effector, for example, they might cause the gripper to close in order to grasp an object.

The process is repeated until all the steps in the program have been completed.

Characteristics of non-servo robots:

• Relatively inexpensive compared to servo robots.

• Simple to understand and operate.

• Precise and reliable.

In a non-servo system, movement is regulated by devices such as a limit switch, which signals the controller when it is activated.

• Simple to maintain.

• Capable of fairly high speeds of operation.

• Small in size.

• Limited to relatively simple programs.