Sliding Mode Control Applied to Offshore Dynamic Positioning Systems

The authors introduce a new controller, aimed at industrial domains, that improves the performance and accuracy of positioning systems based on Stewart platforms. More specifically, this paper presents, and validates experimentally, a sliding mode control for precisely positioning a Stewart platform used as a mobile platform in non-destructive inspection (NDI) applications. The NDI application involves exploring the specimen surface of aeronautical coupons at different heights. In order to avoid defocusing and blurred images, the platform must be positioned accurately to keep a uniform distance between the camera and the surface of the specimen. This operation requires the coordinated control of the six electro mechanic actuators (EMAs). The platform trajectory and the EMA lengths can be calculated by means of the forward and inverse kinematics of the Stewart platform. Typically, a proportional integral (PI) control approach is used for this purpose but unfortunately this control scheme is unable to position the platform accurately enough. For this reason, a sliding mode control (SMC) strategy is proposed. The SMC requires: (1) a priori knowledge of the bounds on system uncertainties, and (2) the analysis of the system stability in order to ensure that the strategy executes adequately. The results of this work show a higher performance of the SMC when compared with the PI control strategy: the average absolute error is reduced from 3.45 mm in PI to 0.78 mm in the SMC. Additionally, the duty cycle analysis shows that although PI control demands a smoother actuator response, the power consumption is similar.

Download Full-text

Experimental Analysis of Higher-Order Sliding Mode Control Applied to Dynamic Positioning Systems

Volume 1: Offshore Technology ◽

10.1115/omae2013-10496 ◽

2013 ◽

Cited By ~ 2

Author(s):

Tadeu F. de Sousa ◽

Eduardo A. Tannuri

Keyword(s):

Robust Control ◽

Sliding Mode Control ◽

Real Time ◽

Sliding Mode ◽

Higher Order ◽

Scale Model ◽

Dynamic Positioning ◽

Control Techniques ◽

Mode Control ◽

Position Signal

The control algorithm normally used in Dynamic Positioning (DP) Systems is based on linear control theory (proportional-derivative or linear quadratic MIMO controller), coupled to an Extended Kalman Filter (EKF) to estimate the environmental forces and wave filtering. Such controllers and estimators have problems of performance and stability related to large variations of loading (for tankers for example) or environmental conditions. The adjustment of controller gains and parameters of EKF is a complex process. Therefore, other techniques are being applied. An investigation into the area of control of mechanical systems was made, carrying out theoretical and experimental studies involving nonlinear robust control techniques applied to dynamic positioning of floating vessels. Two robust control techniques were applied and compared: first order sliding mode control (SMC) and higher order sliding mode control (HOSM). It is known that the main drawback of SMC is the presence of high-frequency oscillations called chattering. This undesirable effect can be eliminated by using HOSM. In order to ascertain the performance of the controller under the DP system, time-domain simulations were done. Furthermore, the technique of sliding mode requires higher order derivatives of the vessel’s position signal. Therefore was developed an exact real-time differentiator, a mathematical technique used to obtain the signal derived from the position signal in real time. To validate the simulated controller, experimental tests were performed considering a small-scale model of a DP tanker. The results confirmed the robustness of the HOSM controller, the good performance of the differentiator and the elimination of the chattering problem.

Download Full-text