44 Unit One Principles of Robotics
The rotary motion of most electric actuator drives must be geared down
(reduced) to provide the speed or torque required by the manipulator. However,
manufacturers are beginning to offer robots that use direct-drive motors,
which eliminate some of these problems. These high-torque motors drive the
arm directly, without the need for reducer gears. The prototype of a direct-
drive arm was developed by scientists at Carnegie-Mellon University in 1981.
The basic construction of a direct-drive motor is shown in Figure 2-24.
Coupling the motor with the arm segment to be manipulated eliminates
backlash, reduces friction, and increases the mechanical stiffness of the
drive mechanism. Compare the design of a robot arm using a direct-drive
motor in Figure 2-25 to one with a conventional electric-drive (Figure 2-22).
Using direct-drive motors in robots results in a more streamlined design.
Maintenance requirements are also reduced. Robots that use direct-drive
motors operate at higher speeds, with greater flexibility, and greater accu-
racy than those that use conventional electric-drive motors.
Applications currently being performed by robots with direct-drive
motors are mechanical assembly, electronic assembly, and material
handling. These robots will increasingly meet the demands of advanced,
high-speed, precision applications.
Hydraulic Drive
Many earlier robots were driven by hydraulic actuator drives. A hydraulic
drive system uses fluid and consists of a pump connected to a reservoir tank,
Stator
Rotor
Motor shaft
Housing
Figure 2-24. This drawing shows the basic construction of a direct-drive motor.