9 
Digital Servos 
Figure 5 shows a digitai servo system which is conceptually advanced 
in that the servo loop is closed inside a computer. The blocks shown in 
side the computer block are actually software functions. This design 
approach is of historical interest because it is not unlike the way the 
early analytical plotters handled the problem. The main difference is 
that the closing was implemented in terms of increments and with con 
siderable hardware involvement rather than a whole number fashion and 
in software as shown in the figure. In retrospect, the system is con 
ceptually advanced; at the time of those early designs that was the only 
way the job could be done. The servo closing ended up inside the com 
puter, because the computer and the interface were practically inseparable. 
Separation of the computer and the interface has been desirable for 
many years. Perhaps the strongest reason for this has been the phenomenal 
development rate of computers. A separate interface would allow, at least 
theoretically, replacement of one type of computer with a newer and better 
one. One system approach to achieve separation is shown in Figure 6. 
The computer still keeps track of the totals as far as the desired posi 
tion and the present position are concerned. The servo works in an incre 
mental environment, without any information within itself about the abso 
lute distances traveled, or still to be traveled. The computer "meters 
out" suitable command increments and the servo loop is closed on the 
basis of similar incremental feedback quantities originating from servo 
shaft encoders. 
Another approach for closing the servo loop in the interface, or 
within a special digital servo module, is shown in Figure 7. This is a 
fundamentally different approach, because here the computer has relin 
quished a significant part of its control. It simply sends to the servo 
the desired position and then trusts that the servo brings about the 
necessary motion to make the present position coincide with the desired 
one. Obviously, the servo logic must be reliable. In practice this is 
not an unreasonable objective; the servo logic need not meet reliability 
specifications any different from those set for a feedback data trans 
mission back to the computer. However, there is a less obvious require 
ment: in continuous real-time operation the servo response must be 
sufficiently good to avoid system problems. Such a response is not 
difficult to achieve when the system is designed for a human operator. 
His time constant is much longer than that of a well designed servo. In 
automatic systems, however, the servo is frequently a limiting factor. 
An extension of the design concept discussed above is shown in Fig 
ure 8. Here the logic of a digital servo is implemented by a micropro 
cessor. With increasing speed and memory capacity of microprocessor, 
this approach can be extended to include additional useful functions, 
such as acceleration, deceleration, and rate limiting as well as delayed 
action apd adaptive characteristics.