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Introduction

Introduction

The term "digital control" appears when the controller function is implemented by a digital processing unit. In digital control, all continuous data need to be converted into digital signals.

Some benefits of digital control are:
  • cheaper
  • reliable
  • easily reprogrammed.
  • ...
To undertand how a digital controller works it is important to review some basic notions of signals.

Types of signals

Continuous signal:
  • Signal defined over a continuous range of time.
  • Its amplitude may take any value in a given range (analog signal): 

Ejemplo se señal analógica 

 or may take only a finite number of values (continuous-time quantized signal):

Sample of a Continous-time quantized signal

Discrete-time signal:
  • Signal defined only at discrete instants of time.
  • If the signal takes any value in a continuous range: sampled-data signal.
  • If the signal can only takes values from a finite set: digital signal (normally represented in binary form).

Digital vs Discrete-event control systems

The terms discrete-time control systems, sampled-data control systems or digital control systems normally refers to similar types of control systems (although strictly speaking there are differences among them). A digital control system generally involves both continuous (from the process under control, sensors, etc.) and discrete-time signals (controller, communication devices, etc). In general, a fixed-sample time is considered in digital control.

There are processes without dynamics in which the changes from one state to another occurs instantly. The changes does not occur at fixed time-instants. Instead, changes are triggered by the occurrence of an event or by the completion of a time period. The control of this kind of discrete systems are called sequential control systems, control of discrete-event systems or logic control. Methodologies for digital control and for sequential control are completely different.  An example of a discrete-event process is an assembly line: