22-08-2012, 03:03 PM
Control Engineering Project - PID Control of a DC Motor
Control Engineering.pdf (Size: 707.32 KB / Downloads: 304)
Introduction
A PID controller comprises three kinds of controller, namely proportional (P), integral (I), and derivative(D). In control system, designing a PID controller is mostly used when the mathematical representation of a plant (system to be controlled) is unknown. Therefore, PID controllers are mostly set and tuned on the field (Ogata 2002, p.681) for practical reason. The mathematical representation (transfer function) of a PID controller itself is given below: ( ) ( )
For the tuning process, Ziegler-Nichols tuning method gives a comfort in providing a place to start. There are two Ziegler-Nichols methods, namely the first and the second method. In the first method, the main focus is the response of the system to a unit step input. On the other hand, the second method is based on the two characteristic variables of the system examined: critical gain (Kcr) and critical period (Pcr). Kcr is based on the first value of Kp that results in a sustained oscillation as its output, while Pcr is defined as the period of the oscillation.
Conclusion
Ziegler-Nichols tuning rules are mostly useful when the plant’s mathematical representation cannot be obtained. It gives the engineers a tuning process starting point with ease. However, these tuning rules are also applicable for those systems with known mathematical models.
As stated before, the Ziegler-Nichols method gives the starting point for the tuning process. Nevertheless, it is critical that an engineer can take it up from there and further tune it (based on experience) so the system can meet the performance specification wanted.