196 SAMPLED-DATA CONTROL SYSTEMS 
mentation or by mixed implementation of a fully digital implementation, 
is practical. 
One of the advantages of digital controllers is that they are particularly 
applicable to systems having large time constants, on the order of minutes 
or even hours. If such systems were to be compensated by conventional 
continuous methods, inordinately large time constants would be required 

i : iE iB 
Fic. 7.24. Laboratory implementation of digital controller. (a) Early analogue 
model. (b) Later model using digital number storage and analogue arithmetic 
operations. 
in the compensating networks. For instance, there are some chemical 
processes whose time constants are measured in hours or fractions thereof. 
Since the digital controller can store data for very long periods, provided 
that drift-free methods are used, it is relatively simple to implement a 
digital controller for such a plant. The sampling interval would be 
chosen consistent with the plant characteristics and, regardless of its 
magnitude, a digital controller could be designed. In those systems 
