Feedback Control for Computer Systems

Chapter 19. Case Study Wrap-Up

In wrapping up this collection of case studies, I’d like to point out a few recurrent themes.

Simple Controllers, Simple Loops

The elements of a feedback system are not complicated: the basic loop structure and a simple controller are all that is required. In fact, all the case studies were completed with nothing more complicated than the generic feedback loop as implemented in the closed_loop() convenience function!

Most controllers, also, were of the generic PID type, although particular situations sometimes called for specially designed controllers. But even those controllers were very simple and did nothing more than calculate an output based on the input while maintaining only minimal internal state.

The natural temptation to build more complicated controllers should probably be resisted in most cases. The feedback principle is not about clever (and complex) algorithms; rather it works with simple components put together in a straightforward fashion. What makes feedback work is that corrective actions are calculated and applied constantly. Because of the iterative nature implied by the feedback scheme, the components and calculations themselves can (and should) be simple.

Measuring and Tuning

Given all the details and specific methods to “measure the transfer function” presented in Chapter 8, it is easy to forget that we are really after only a few pieces of basic information:

• What is the directionality of the input/output relationship? Does increasing ...