Actuator Saturation Control

The problem of actuator saturation appears in many practical control systems, saturation nonlinearity is unavoidable in most actuators. When an actuator reached such an input limit, it is said to be "saturated", since efforts to further increase the actuator output would not result in any variation in the output. The presence of saturation can debase the performance even lead the closed-loop system to an unstable behavior.With the development of modern technology, the precision of control systems require more and more highly, for example, ultra-precision machining, robot manipulator, assembly of small components and micro devices and hard disk drive system are all needed highly precision. It is very difficult to assure designing accuracy using conventional control system under the influence of saturation. To eliminate the saturation effect, designing and make a more precise mechanical components may lead to a high cost for the over system. However, adopting the advanced saturation compensation scheme makes the use of cheaper mechanical components to meet high-precision need possible. A remarkable progress in mini- and micro computers and power electronics technology has generated more room for control algorithms; designing higher performance and flexibility control strategy becomes a royal road for realizing high-precision control. This paper explores the methods of designing controller for nonlinear systems with input saturation and seeks strategy preferable to compensate the saturation for realizing high-precision tracking control.Some control strategies were reviewed firstly in this paper; in the following, two saturation compensation schemes are presented for different nonlinear systems with input saturation to decrease the influence of saturations and realize tracking control, i.e. the Design of Neural Network Controller. And The Design of Composite Controller.The first design can be applied linear time-variant control systems and the second strategy can be applied linear time-invariant control systems.A neural net-based actuator saturation composition scheme with on-line weights tuning law for nonlinear systems in Brnovsky form is presented in the third method. RBF neural network is adopted to approximate the part exceeding the saturation limit of controller's output. Another most prominent feature of the scheme is which can ensure the system is uniformly ultimately bounded which is proved by Lyapunov theory, and considering the network reconstruction error and the system's external disturbance. The tracking error can be freely adjusted by known form. Considering the plant with unknown function, the method has more extensively practical application. The Design of Composite Controller base on classical control theory, first make the system meeting the expecting control objective by pole placement and then design a nonlinear control law to reduce the overshoot caused by the linear part. This design can guaranteed the closed-loop system with a small damping ratio for a quick response, while the nonlinear control law can increase the damping ratio as the output approaches the target reference.In this paper, every method is given stability provability and simulation example as well results respectively, the results illustrate the effectiveness of the three methods.