146 SAMPLED-DATA CONTROL SYSTEMS 
In contrast to this approach, an alternative method is shown in Fig. 7.2. 
In this case, the sampled error Ef is applied to a unit identified here as 
the digital controller. This unit computes or generates a sequence of 
output numbers Hy which are linearly related to the input number 
sequence Hi. The sequence EH} is then reconstructed into a continuous 

Fic. 7.1. Sampled-data feedback control system using continuous network 
compensation. 
command function M, which is applied to the plant. The difference is 
apparent when it is noted that continuous compensation first recon- 
structs and then modifies the error signal, while the digital controller 
first modifies the error sequence and then reconstructs the modified error 
function. The compensating element which carries out the number- 
sequence reconstruction is called the digital controller. 
D(z) G(z) — a 
ret 
pa SRomeg 
| 
Ej Digital E3 | 
controller 
Fic. 7.2. Sampled-data feedback control system using digital controller. 

Cc 


The digital controller may contain a pulsed network which is merely 
a passive network to which is applied a sequence of samples. The output 
sequence of numbers or pulses is related to the input sequence by the 
pulse transfer function of the network N(z). On the other hand, the 
digital controller may contain an active element such as a numerical 
processing unit or a program on a digital computer. In this case, the 
restrictions which are imposed on the pulse transfer function of networks 
can be overcome and more complete freedom in the choice of stabilizing 
functions can be achieved. One of the more interesting aspects of 
sampled-data systems is the possibility of using a digital computer to 
compensate the system. It should be mentioned here that a digital con- 
troller need not have any resemblance to a large-scale digital computer 
since it is a special-purpose device and can be constructed much more 
simply than the general-purpose computer. In addition, the digital con- 
troller can be implemented entirely in analogue form if it is considered 
