Page 456 - Mechanical Engineers' Handbook (Volume 2)

1 Introduction 447

Figure 4 Multilevel hierarchical control structure.

puter processes only numbers. All control algorithms, however, need not be implemented
digitally. As an example, for most feedback control loops, the PID algorithm implemented
with analog circuitry is superior to the PID algorithm implemented in digital software. The
derivative mode, for instance, requires very high resolution to approach the quality of analog
performance. Thus it is sometimes advantageous to use hybrid systems for controller imple-
mentation.

1.6 Real Systems with Digital Control

Real systems are typically mixtures of analog and digital signals. Typical plant inputs and
outputs are analog signals; that is, actuators and sensors are analog systems. If a digital

Figure 5 DDC configuration.