Sohail Iqbal

Robust feedback linearization using higher order sliding mode observer

A novel robust feedback linearization scheme is proposed in this paper based on a modified robust exact differentiator. The states and drift terms in the system are estimated simultaneously by the observer using back injection of the control effort. The estimated drift term is used in the feedback loop to compensate the disturbances and observed states are used for feedback linearization. Finite time convergence of the complete closed-loop system is proved and thus a form of separation principle is satisfied, i.e., the controller and observer can be separately designed. The design is verified through simulations and by experiments on a DC motor rig.

Dynamic Analysis and Robust Control Design for Stewart Platform with Moving Payloads

The novelty of the paper lies in an extended nonlinear model for the Stewart platform with moving payload. The models found in the existing literature are most of the time only valid for static loads with symmetric configurations, as reflected by the presumed assumption on Moments of Inertia (MOI) being static and symmetric. Such assumption precludes a wide range of systems having asymmetric or moving payloads e.g. stabilized platforms used to stabilize satellite antenna trackers or surgical tables etc. In this paper the actual MOI of the Stabilizing Stewart platform with moving payload are computed and used to parameterize the extended nonlinear model. This model is subsequently used for designing sliding mode controller. The control performance of the proposed algorithm is verified with computer simulations. These simulations demonstrate better stability properties and performance of the proposed dynamic model with much lower uncertainty bounds, as compared to that of the controller designed on the prevalent nonlinear model.

A smooth second-order sliding mode controller for relative degree two systems

A novel smooth second order sliding mode controller based on a modified “twisting” algorithm for relative degree two systems is discussed to provide robustness as well as accuracy. A robust exact observer is introduced in the closed loop system to reject the drift signals. A Lyapunov method combined with homogeneity concept is used to prove finite time stability of the overall system. The method is verified through simulations on a benchmark example of a DC motor.

A novel reaching law for smooth sliding mode control using inverse hyperbolic function

This paper pertains to an important improvement in sliding mode control. The Sliding mode control is robust nonlinear control design technique well known for its robustness against certain class of disturbances and model uncertainties. The mentioned nonlinear design technique suffers from a phenomenon called chattering which consists of high frequency discontinuities introduced by the sliding mode controller. Various techniques have been suggested to remove the chattering however these techniques either result in decrease in the rigor or increase in the complexity of the resulting system. The purpose of this work is to formulate a new controller that intends to reduce the chattering effect and the Reaching Phase of the sliding mode control. The proposed Reaching law has been formulated by using inverse hyperbolic function in its gain term. The gain at the start of reaching phase is high which results in faster convergence towards the surface. Once the Sliding Mode starts the gain is reduced which reduces the chattering. Simulation results show significant reduction of chattering in the control input as well as in the output of the system.

Robust Feedback Linearization for Minimum Phase Systems

Abstract In this paper, synthesis of robust feedback linearization procedure for nonlinear systems with stable zero dynamics is explored employing robust exact observer for estimation of the states as well as drift terms simultaneously on the basis of just available output of the system. The detail knowledge of the mathematical model of the system is no more crucial. Finite time convergence of the complete closed loop system is proved and thus a form of separation principle is satisfied i.e., the controller and observer can be designed separately. The simulation results verify robustness as well as performance of the proposed technique.

Parameter estimation of uncertain nonlinear MIMO three tank systems using higher order sliding modes

This paper presents a technique for parameter estimation of uncertain nonlinear systems based on the computation of an accurate and robust derivative using higher order sliding modes. The novelty of the method is to facilitate the determination of system parameters via calculating the derivatives of measured system input-outputs. To validate the technique, simulations have been performed for a laboratory benchmark uncertain nonlinear MIMO ‘Three Tank System’. The simulation results demonstrate that the convergence of estimates to their nominal values faster than existing techniques even in the presence of measurement noise. The presented parameter estimation scheme is relatively simple and can be employed for the controller design and fault diagnosis of nonlinear systems.

Determination of realistic uncertainty bounds for the Stewart Platform with payload dynamics

Robust Control of Stewart Platform has been successfully demonstrated by various authors [8-12]. The work done so far is based on the assumption that the bounds on uncertainty are known and the chosen reachability gains are greater than these bounds. This assumption can only be justified if those bounds could be quantified, which is not the case in the existing approaches. The problem gets severe when the controller has to compete against the variations in payload, especially when the payload is asymmetric. This paper addresses such uncertainties. The novelty of the paper lies in the extension of existing nonlinear model to include asymmetric payloads and quantification of uncertainties arising from the use of a variety of asymmetric payloads. The actual Moments of Inertia (MOI) of the Stewart Platform with asymmetric payload are computed and used to find worst case uncertainty bounds. The control performance of the proposed algorithm is verified by computer simulations. These simulations show that the system follows the desired trajectory and errors converge to equilibrium points efficiently.

Smooth second-order sliding mode control design of PEM fuel cell system

A novel smooth second-order sliding mode (SSOSM) control is applied to tackle the problem of PEM fuel cell system output voltage stabilization along with controlling oxygen excess ratio. The oxygen excess ratio is controlled in the inner control loop using PID while the regulation of output voltage is being treated in the outer loop through smooth second-order sliding mode control. The control is applied on the validated nonlinear model of the PEM fuel cell system. The simulation results show that the scheme with SSOSM control provides smoothness (chattering free) with robust performance and the fuel consumption minimization.