118 SAMPLED-DATA CONTROL SYSTEMS 
Within the restriction of design of the single-loop system shown in Fig. 
6.1 for T; = T. = T, there are two fundamentally different approaches, 
based upon the restriction placed on the characteristics of the controller 
available to the designer. In the application of conventional feedback- 
system-design techniques usually implied by the frequency methods 
associated with the names Nyquist, Bode, or Nichols, the designer 
typically selects a specific network, such as lead, lag, or lag-lead, and tries 
the resulting design against his specifications. A series of such cut-and- 
try steps leads, hopefully, to satisfactory performance. The present 
chapter will outline the extension of these conventional techniques to 
sampled-data-system design. 





Continuous Plant 
controller 

Digital 
controller 
Fic. 6.1. Single-loop error-sampled control system. 
A technique for the realization of arbitrary pulse transfer functions for 
purposes of controller design by the use of essentially conventional RC 
networks is also presented. This technique was originally suggested by 
Barker! and developed by Sklansky.*® In the following chapter a method 
of direct time-domain synthesis will be presented which can be applied, 
provided that the necessary complexity of controller is available. 
6.1 Limitations to the Application of Conventional Techniques to 
Sampled-data-system Design 
A direct attempt to apply conventional frequency techniques to the 
design of shaping networks for the error-sampled system leads to the 
block diagram shown in Fig. 6.2. In this case the form of the transfer 





Hold 
circuit 
Shaping 
network 


Fra. 6.2. A block diagram for the application of conventional techniques to design. 
function of the shaping network N(s) is chosen at the outset as, for 
example, (1 + a7s)/(1 + 7s), and one wishes to choose a and 7 in such a 
fashion that the system gives satisfactory performance. The reasons for 
the rather widespread use of frequency techniques are fairly easy to 
understand. The methods guarantee at the outset that the compensat- 
