11.1: Introduction to Control Fundamentals
- Page ID
- 47286
11.1.1: Plants, Inputs, and Outputs
Controller design is about creating dynamic systems that behave in useful ways. Many target systems are physical; we employ controllers to steer ships, fly jets, position electric motors and hydraulic actuators, and distill alcohol. Controllers are also applied in macroeconomics and many other important, non-physical systems.
It is the fundamental concept of controller design that a set of input variables acts through a given “plant” to create an output. Feedback control then uses sensed plant outputs to apply corrective plant inputs:
| Plant | Inputs | Outputs | Sensors |
|---|---|---|---|
| Jet aircraft | elevator, rudder, etc. | altitude, heading | altimeter, GPS |
| Marine vessel | rudder angle | heading | gyrocompass |
| Hydraulic robot | valve position | tip position | joint angle |
| U.S. economy | federal interest rate, etc. | prosperity, inflation | inflation, M1 |
| Nuclear reactor | cooling, neutron flux | heat, power level | temperature, pressure |
11.1.2: The Need for Modeling
Effective control system design usually benefits from an accurate model of the plant, although it must be noted that many industrial controllers can be tuned up satisfactorily with no knowledge of the plant. Ziegler and Nichols, for example, developed a general heuristic recipe which we detail later. In any event, plant models simply do not match real-world systems exactly; we can only hope to capture the basic components in the form of differential or other equations.
Beyond prediction of plant behavior based on physics, system identification generates a plant model from actual data. The process is often problematic, however, since the measured response could be corrupted by sensor noise or physical disturbances in the system which cause it to behave in unpredictable ways. At some frequency high enough, most systems exhibit effects that are difficult to model or reproduce, and this is a limit to controller performance.
11.1.3: Nonlinear Control
The bulk of this subject is taught using the tools of linear systems analysis. The main reason for this restriction is that nonlinear systems are difficult to model, difficult to design controllers for, and difficult overall! Within the paradigm of linear systems, there are many sets of powerful tools available. The reader interested in nonlinear control is referred to the book Applied Nonlinear Control by Slotine and Li (1991).