Page 695 - Mechanical Engineers' Handbook (Volume 2)

686 General-Purpose Control Devices

Figure 8 Sequential function chart example.

received within a time interval specified by the variable CMD, the system enters an error
step and issues the error message TIMEOUT ERROR. This error condition is cleared when
acknowledged by the signal ACK, for example, from an operator’s console, combined with
the feedback (DONE) that the motion has been accomplished. The system then reenters the
initial step and waits for another command.

1.5 Path Control
General-purpose control devices must often perform path control, that is, the coordinated
control of several variables at once along a continuous path through time. Typical kinds of
path control include:

• The path of a metal-cutting tool or a robot manipulator
• The trajectory of a set of continuous process variables such as temperature, pressure,
and composition
• The startup of a set of velocity and tension variables in a paper or steel processing
line

The numerical control of metal-cutting tools and robot manipulators is discussed in more
detail in Sections 3 and 4, respectively.
A typical application to path control of the general-purpose architecture shown in Fig.
3 has the control processor planning the motions to be accomplished and issuing commands
to the closed-loop controllers to perform the required motions. Coordination between the
closed-loop controllers may be performed by the control processor, or by direct interaction
among the closed-loop controllers, using the I/O bus or special interconnections.
In addition to performing path planning, the control processor also performs sequencing
of individual motions and coordination of the motions with other control actions, typically
using programming mechanisms such as the SFCs discussed in Section 1.4.