In order to define more precisely control systems, one can separate
them into two different categories:
- The open-loop Control Systems (nonfeedback systems)
- The closed-loop Control Systems (feedback is used in part of the
input given to the control system (or controller))
In fact, feedback can be seen as a link between the output and the
input of the system; it is used to obtain a more accurate control by
being compared to the input, in order to correct an error.
However, beside this purpose, feedback has numerous other effects,
among which stability, bandwith, overall gain, impedance, and
sensitivity can be noticed, effects which have their importance in any
control application.
Dealing with the properties of control systems, a reader familiar with
Digital Signal Processing business will easily recognize numerous
properties among which linearity and time-invariance of systems are
used to describe the behavior of control systems.`
We have been, so far, describing control systems regardless of them
being continuous- or discrete-data systems. The fundamental difference
between those two is that a discrete-data system has, at one or more
points in it, a signal which is either a pulse train or a digital
code.
Digital control systems are commonly separated into two classes:
- Sampled-data
- Digital Control Systems
The signal in a sampled-data control system is in the form of pulse
data. On the other hand, when a digital computer or controller is used
in a control system, it is referred to as a Digital Control System.
Generally, a sampled-data system receives data or information only at
specific instants of time, intermittently, since the signal we are
dealing with in this case is digital.
Incorporating sampling into a control system presents numerous
advantages among which 2 main can be noticed:
- The sampling operation enables equipement to be time-shared among
several control channels
- Usually pulse data are less susceptible to noise