Automatic Docking Using Second Order Sliding Mode Control

This paper is about the application of second order sliding mode control to solving automatic space docking problem. Two problems are considered herein, first the establishment of a close formation by the steering of the last stage thrust engine, and its proper termination, and then the docking by itself using centre of gravity and attitude thrusters. The design is based on a combination of second order sliding mode control and minimal time (bang-bang) control techniques. Tri-level control is combined with bang-bang control at the last stage of docking to reduce the fuel consumption whereas second order sliding mode control provides required robustness to model uncertainties and considerable simplification of the model used in the design. A continuous (super-twisting) second order sliding mode control is used in controlling of normal and out-of-plane motion, and a pulse width modulation implementation there of is used in the design of the control of longitudinal motion. Computer simulations show that the design achieves excellent performances not withstanding considerable model uncertainties introduced and despite considerable simplification introduced in the model of relative motion used in the control design.