Page 23 - Electric Drives and Electromechanical Systems

Chapter 1 Electromechanical systems 15

performance, for example if the wrist has a significant mass, it will reduce the overall
capabilities of the robot. The arm and the wrist give the robot the required six degrees of
freedom which permit the tool to be positioned and orientated without restrictions in
three-dimension space as required by the task.
The selection of a robot can be a significant problem for a design engineer, and the
choice depends on a rage of factors, including the task to be performed. One of the
earliest applications of robotics was within a foundry; such environments are hazardous
to human operators due to noise, heat, and fumes from the process. This is a classic
application of a robot being used to replace workers because of environmental hazards.
Other tasks which suggest the use of robots include repetitive work cycles, the moving of
difficult or hazardous materials, and requirements for multishift operation. Robots that
have been installed in manufacturing industry are normally employed in one of four
application groups: materials handling, process operations, assembly, or inspection. The
control of a robot in the performance of a task necessitates that all the joints can be
accurately controlled. A basic robot controller is configured as a hierarchical structure,
similar to that of a CNC machine tool; each joint actuator has a local motion controller,
with a main supervisory controller which coordinates the motion of each joint to achieve
the end effector trajectory that is required by the task. As robot control theory has
developed so the sophistication of the controller and the algorithms has increased.
Controllers can be broadly classified into one of four groups:

Limited sequence control. This was used on low-cost robots which are typically
designed for pick-and-place operation. Control was usually achieved by the use of
mechanical stops on the robot’s joint which control the end positions of each
movement. A step-by-step sequential controller is used to sequence the joints and
hence to produce the correct cycle.
Stored program with point-to-point control. Instead of the mechanical stops of the
limited-sequence robot, the locations are stored in memory and played back as
required. However, the end effector’s path is not controlled; only the joint end
points are verified before the program moves to the next step.
Stored program with continuous-path control. The path control is similar to a CNC
contouring controller. During the robot’s motion the joint’s position and speed are
continually measured and are controlled against the values stored in the program.
Intelligent-robot control. Using sensors, the robot is capable of interacting with its
environment for example, by following a welding seam or undertaking a detailed
component inspection. As the intelligence increases so the complexity of the con-
trol hardware and its software also increase.
In order for the robot to completes at task, it is required to move the end effector from
its initial position to the final position. To achieve this, the robot’s control system must
plan and execute a motion trajectory; this trajectory is a sequence of individual joint
positions, velocities, and accelerations that will ensure that the robot’s end effector
moves through the correct sequence of positions. It should be recognised that even