Page 246 - Electric Drives and Electromechanical Systems

Chapter 10 Controllers for automation 243

FIG. 10.2 Two approach to controlling the linear position of the load, in (A) any non-linearity in the leadscrew or
thermal expansion will not be compensated, in approach (B) any non-linearities in or thermal expansion of the
leadscrew will be compensated. (A) Position of the load is determined by the encoder position and the lead of
the ballscrew. (B) The position of the load is directly measured by a linear encoder.

can produce the required torque, and hence the acceleration that is necessary to follow
the required motion profile within the allowable error. If the drive is not capable of
matching this basic requirement, there is no way that the overall system can ever meet its
specification.
While the control problems which are typically encountered in robotics and machine
tools can be simply stated, their full solution is anything but simple because of the
additional complications of variable inertial forces, for example the dynamic coupling
between axes, in particular within robots, and in some cases gravitational loads.
The general route to the development of a control system is first to develop a full
dynamic model of the application, and then solve it to obtain the control laws or
strategies for the desired performance. In the analysis which is required, both small and
large movements which are associated with the interaction between the workpiece and
the mechanical system must be considered. This task has now been made considerably
easier with the introduction of a range of computer-based tools, either generic control-
simulation packages (of which the best known is perhaps MATLAB and its associated
tool-boxes) or packages designed to support a manufacturer’s motion-control system.
The increasing use of digital controllers has permitted the integration of simulation and
parameter optimisation in one computer package, which can directly interface with the
drive via a communication link and hence permit real-time configuration of a drive’s
stability terms.
It is clear from a review of the commercial literature that the majority of modern
position-control systems are based on digital processing; however, as an introduction, it