42 Unit One Principles of Robotics
Electric Drive
Three types of motors are commonly used for electric actuator drives:
ac servo motors, dc servo motors, and stepper motors. Both ac and dc servo
motors have built-in methods for controlling exact position. Many newer
robots use servo motors rather than hydraulic or pneumatic ones. Small
and medium-size robots commonly use dc servo motors. Because of their
high torque capabilities, ac servo motors are found in heavy-duty robots,
Figure 2-22. A stepper motor is an incrementally controlled dc motor.
Stepper motors are rarely used in commercial industrial robots, but are
commonly found in educational robots, Figure 2-23.
Conventional, electric-drive motors are quiet, simple, and can be used in
clean-air environments. Robots that use electric actuator drives require less
floor space, and their energy source is readily available. However, the conven-
tionally geared drive causes problems of backlash, friction, compliance, and
wear. These problems cause inaccuracy, poor dynamic response, need for
regular maintenance, poor torque control capability, and limited maximum
speed on longer moves. Loads that are heavy enough to stall (stop) the motor
can cause damage. Conventional electric-drive motors also have poor output
power compared to their weight. This means that a larger, heavier motor must
be mounted on the robot arm when a large amount of torque is needed.
Hydraulic
actuator
Figure 2-21. A large hydraulic actuator provides up-and-down motion to the manipulator arm of this
industrial robot. (FANUC Robotics)