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

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.