Phase and Gain Margins with Third Order Sliding Mode Control: An Integrated Guidance Application

Abstract

Department of Defense Regulation requires designers to evaluate the robustness of their designs using phase and gain margin criteria. Given that such criteria are applicable to linear designs, their inapplicability in their current definition to Higher Order Sliding Mode (HOSM) control designs; non-linear designs techniques in the time domain appeared to constitute a major hindrance to the application of HOSM techniques to aircraft and missiles. This paper illustrates the usage of new phase and gain margin techniques applicable to HOSM designs for providing measures of robustness such designs. The technique consists in replacing the -1 singularity point used with linear designs with describing function based representations of the HOSM controller. It is applied here to general problem of guidance of aerospace vehicles steered with aerodynamic lift that is, to a control problem with relative degree = 3. In the application case we assume the unavailability of attitude measurements. Results show that whether the guidance enforces the collision condition or a proportional integral linear manifold has little effect on the robustness. The concept of practical Relative Degree is introduced and applied instead of the strict definition thereof.