38 Unit One Principles of Robotics
Non-servo robots are also limited in their movement and these limita-
tions 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.
The diagram in Figure 2-17 represents a pneumatic (air-controlled),
non-servo robot.
1. At the beginning of the cycle, the controller 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.
Controller
Signal
Air supply
End stop
Control
valve
Exhaust
Air cylinder
Manipulator
Limit
switch
Figure 2-17. In a non-servo system, movement is regulated by devices such as a limit switch, which
signals the controller when it is activated.