Page 46 - Instrumentation and Measurement

3.6 Digital Control Systems

With analogue control systems all the signals are analogues, i.e. a scaled version, of the quantities
they represent. Digital signals, however, are sequences of pulses, i.e. on-off signals, with the value
of the quantity being represented by the sequence of on-off signals. Most of the signals being
controlled are analogue and thus it is necessary with a digital control system to convert analogue
inputs into digital signals for the controller and then the digital outputs from the controller to analogue
for the process being controlled. Thus analogue-to-digital converters (ADCs) and digital-to-analogue
converters (DACs) are used. Figure 3.19 shows the basic form of a closed-loop digital control system.

FIGURE 3.19 The basic elements of a digital closed-loop control system.

Digital controllers can be microcontrollers or computers. A microcontroller is an integration of a
microprocessor with memory, input/output interfaces and other peripherals such as timers on a single
chip. The microcontroller effectively incorporates the comparison element, the controller, the DAC
and the ADC. At some particular instant the microcontroller samples its input signals. There is then
an input of the required analogue value to the microcontroller and the measured analogue value of
the output. It then carries out its program and gives an analogue output to the correction element. The
microcontroller then repeats its program for the next sample of signals. The program followed by the
microcontroller is thus:

Take a sample of the signals at its inputs, i.e. read the required value input and read the actual
measured value of the output, i.e. the feedback signal, at its ADC input port
Calculate the error signal
Calculate the required controller output
Send the controller output to its DAC output port
Wait for the next sampling interval
Repeat the process.

Digital control has advantages over analogue control in that digital operations can be easily controlled
by a program, i.e. a piece of software, information storage is easier, accuracy can be greater and
digital circuits are less affected by noise.

As an illustration of a digital control system, Figure 3.20 shows the system for the control of the
speed of rotation of a motor shaft which was represented by an analogue system in Section 4.4.1.

Page | 46