Ph. Mullhaupt

A simple output feedback PD controller for nonlinear cranes

A simple output feedback PD controller is proposed that stabilizes a nonlinear crane. Global asymptotic stability is achieved at any equilibrium point specified by the controller. The control scheme relies solely on the winches position and velocity and hence no cable angle measurement, or no direct measurement of the load position, is needed. The controller can be extended to many different kinds of existing cranes.

Predictive control of fast unstable and nonminimum-phase nonlinear systems

The problem of controlling fast, unstable, and nonminimum-phase systems is considered. With standard predictive control, the time required for optimization is typically larger than the sampling interval that is needed for stabilization of the fast dynamics. On the other hand, due to the nonminimum-phase behavior, control based on input-output feedback linearisation leads to unstable internal dynamics. In the paper, a cascade structure is proposed, with control based on input-output feedback linearisation forming the inner loop and predictive control the outer loop. Assuming high-gain feedback for the inner loop, a stability analysis of the global scheme is provided based on singular perturbation theory. The approach is illustrated via the simulation of an inverted pendulum system.

Condition for bifurcation of the region of attraction in linear planar systems with saturated linear feedback

Bifurcation of the region of attraction for planar systems with one stable and one unstable pole under saturated linear state feedback is considered. The boundary of the region of attraction can either possess an unbounded hyperbolic shape or be a bounded limit cycle. The main contribution of this paper is to provide an analytical condition under which bifurcation occurs. This condition is based on characteristics and position of the stable and unstable manifolds. Furthermore, the exact shape of the region of attraction is provided.

On the nonminimum-phase characteristics of two-link underactuated mechanical systems

Underactuated mechanical systems are those which possess fewer actuators than the number of degrees of freedom. It is shown that, except for some pathological cases, exclusively kinetic, two-link underactuated mechanisms are of nonminimum phase when the natural outputs are considered.

Dedicated controller design for a dual-stage opto-mechatronic system

To improve the resolution of the very large telescope interferometer (VLTI) a two-stage mechanical system, a so called differential delay line (DDL), is developed jointly by the EPFL and the Observatory of Geneva. The system is designed to reach nanometer accuracy at high bandwidth over large displacements. The coarse stage features a permanent magnet (PM) stepper motor driving a lead screw connected to a double-parallelogram flexure with notch-hinges (blade) guiding system, and the fine stage features a stacked piezoelectric device, combine to one single measurable output. This paper compares different control approaches for the DDL with their respective advantages and disadvantages. The developed control methods are based on modern linear and nonlinear control theory. The performance of the control schemes is illustrated via simulation and measurement on the available prototype. The new developed methods are compared to the currently implemented decoupled SISO design which features a direct-coil controller for the coarse stage and a simple PID-controller for the fine one.

A two-time-scale controller for a differentially cross-coupled system

Control of underactuated mechanical systems often leads to unstable internal dynamics, which can be handled by resorting to prediction when the system bandwidth is small. The paper considers systems with a high bandwidth and proposes a two-time-scale controller for decoupling the system while ensuring internal stability. A toy helicopter in which the speed of the propellers is manipulated to vary the aerodynamic force is taken up as a case study.