Servo Robots

The servo robot is a closed-loop system because it allows for feedback. In a closed-loop system, the feedback signal sent to the servo amplifier affects the output of the system. A servo amplifier translates signals from the controller into motor voltage and current signals. Servo amplifiers are used in motion control systems where precise control of position or velocity is necessary. In a sense, a servomechanism is a type of control system that detects and corrects for errors

The principle of servo control can be compared to many tasks performed by human beings. One example is cutting a circle from a piece of stock on a power bandsaw ленточно-пильный станок. The machine operator’s eye studies the position of the stock to be cut in relation to the cutting edge of the blade.

The eye transmits a signal to the brain. The brain compares the actual position to the desired position. The brain then sends a signal to the arms to move the stock beneath the cutting edge of the blade. The eyes are used as a feedback sensing device, while the brain compares desired locations with actual locations. The brain sends signals to the arms to make necessary adjustment s. This process is repeated as the operator follows the scribed line during the sawing operation.

The diagram in Figure 2-20 details one of the axes used in a hydraulic robot and helps to explain its operation.

1. When the cycle begins, the controller searches the robot’s programming for the desired locations along each axis.

2. Using the feedback signals, the controller determines the actual locations on the various axes of the manipulator.

3. The desired locations and actual locations are compared.

4. When these locations do not match, an error signal is generated and fed back to the servo amplifier. The greater the error, the higher the

intensity of the signal.

5. These error signals are increased by the servo amplifier and applied to the control valve on the appropriate axis.

6. The valve opens in proportion to the intensity of the signal received.

The opened valve admits fluid to the proper actuator to move the various segments of the manipulator.

7. New signals are generated as the manipulator moves.

8. The servo control valves close when there are no more error signals, shutting off the flow of fluid.

9. The manipulator comes to rest at the desired position.

10. The controller then addresses the next instruction in the program, which may be to move to another location or operate some peripheral equipment.

The process is repeated until all steps of the program are completed.

Characteristics of servo robots:

• Relatively expensive to purchase, operate, and maintain.

• Use a sophisticated, closed-loop controller.

• Wide range of capabilities.

• Can transfer objects from one point to another, as well as along a controlled, continuous path.

• Respond to very sophisticated programming.

• Use a manipulator arm that can be programmed to avoid obstructions within the work envelope.

Type of Actuator Drive

One common method of classifying robots is the type of drive required by the actuators.

• Electrical actuators use electric power.

• Pneumatic actuators use pneumatic (air) power.

• Hydraulic actuators use hydraulic (fluid) power.