Page 443 - Mechanical Engineers' Handbook (Volume 2)

434 Basic Control Systems Design

12.1 Digital Control Hardware
Commercially available controllers have different capabilities, such as different speeds and
operator interfaces, depending on their targeted application.

Programmable Logic Controllers (PLCs)
These are controllers that are programmed with relay ladder logic, which is based on Boolean
algebra. Now designed around microprocessors, they are the successors to the large relay
panels, mechanical counters, and drum programmers used up to the 1960s for sequencing
control and control applications requiring only a finite set of output values (for example,
opening and closing of valves). Some models now have the ability to perform advanced
mathematical calculations required for PID control, thus allowing them to be used for mod-
ulated control as well as finite-state control. There are numerous manufacturers of PLCs.

Digital Signal Processors (DSPs)
A modern development is the digital signal processor (DSP), which has proved useful for
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feedback control as well as signal processing. This special type of processor chip has sep-
arate buses for moving data and instructions and is constructed to perform rapidly the kind
of mathematical operations required for digital filtering and signal processing. The separate
buses allow the data and the instructions to move in parallel rather than sequentially. Because
the PID control algorithm can be written in the form of a digital filter, DSPs can also be
used as controllers.
The DSP architecture was developed to handle the types of calculations required for
digital filters and discrete Fourier transforms, which form the basis of most signal-processing
operations. DSPs usually lack the extensive memory management capabilities of general-
purpose computers because they need not store large programs or large amounts of data.
Some DSPs contain A/D and D/A converters, serial ports, timers, and other features. They
are programmed with specialized software that runs on popular personal computers. Low-
cost DSPs are now widely used in consumer electronics and automotive applications, with
Texas Instruments being a major supplier.

Motion Controllers
Motion controllers are specialized control systems that provide feedback control for one or
more motors. They also provide a convenient operator interface for generating the com-
manded trajectories. Motion controllers are particularly well suited for applications requiring
coordinated motion of two or more axes and for applications where the commanded trajectory
is complicated. A higher level host computer might transmit required distance, speed, and
acceleration rates to the motion controller, which then constructs and implements the con-
tinuous position profile required for each motor. For example, the host computer would
supply the required total displacement, the acceleration and deceleration times, and the de-
sired slew speed (the speed during the zero acceleration phase). The motion controller would
generate the commanded position versus time for each motor. The motion controller also
has the task of providing feedback control for each motor to ensure that the system follows
the required position profile.
Figure 50 shows the functional elements of a typical motion controller, such as those
built by Galil Motion Control, Inc. Provision for both analog and digital input signals allows
these controllers to perform other control tasks besides motion control. Compared to DSPs,
such controllers generally have greater capabilities for motion control and have operator
interfaces that are better suited for such applications. Motion controllers are available as
plug-in cards for most computer bus types. Some are available as stand-alone units.