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THE DESIGN OF PHOTOGRAMMETRIC PLOTTERS, HELAVA 127 
However, it will be illustrative to first study some very basic aspects of servo mechanisms. 
A typical servo mechanism that may be used in an analytical plotter is shown in the 
form of a block diagram in Fig. 2. 
The input to the mechanism represents the value to be set by the mechanism. This 
value is compared with the indication of the output measuring device, and in case of dis- 
agreement an error signal is produced. The error signal goes to the controller, where it 
is amplified and then used to actuate the powering device in an appropriate manner to 
cause the error signal to vanish. The powering device must drive some load, and this 
should not be overlooked when considering the performance of servomechanisms. 
The mechanical realization of a servomechanism as shown in Fig. 2 may be based 
on various principles. For example, the input may be in the form of voltage or digital 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
COMPARISON 
OR ERROR POWERING  |OUTPUT 
INPUT — 
ERROR CONTROLLER DEVICE LOAD 
DETECTION 
OUTPUT 
MEASURING 
DEVICE 
Fig. 2. 
number, the powering device may be an electrical motor or a hydraulic counterpart, etc. 
Regardless of the construction principle, the performance limitations are similar and the 
following study is valid for all cases. 
An ideal servo would act instantaneously and reduce the error signal to zero with 
no delay. In practice, however, the ideal is never reached, and the performance of the 
servomechanism must be carefully studied to ensure that the requirements are fulfilled. 
This is the task of the designer of the servomechanism, and he has very sophisticated 
methods and techniques at his disposal to perform this task. Once the requirements are 
set, a relatively straightforward course may be followed in the design. In the case of an 
analytical plotter, much can be done to facilitate the servoing problem by selecting a 
favourable approach in the overall design of the plotter. Therefore, the main purpose of 
our study of the problems involved in the application of servomechanisms is to obtain 
useful information for the design of the plotting instrument. 
9.4.1.1. Simple positioning. 
In general, it is not difficult to design a servo that has a simple task of positioning 
an object very accurately — say a photocarrier — using a lead screw. The only difficulty 
might be in measuring the amount of output with the required degree of accuracy. This 
problem, of course, becomes more difficult as the length of the displacements to be meas- 
ured increases. This we should note as an argument against designs where large displace- 
ments are to be servoed. As will be seen, this argument is supported by many other 
conclusions. 
3.4.1.2. Transient period. 
The simple positioning problem becomes more complicated if the time required to 
perform the positioning is taken into consideration. Obviously, if the servo has to deal 
with small displacements only, the time taken to reduce the error signal to zero is shorter 
than in the case of large displacements. Another factor that has an influence on the