Date of final exam: 22/02/2000

E-mail: coccoli@dist.unige.it

Tutor: Prof.  G. Bartolini, Università di Genova

  ___________________________________________________________________________________________________________

Sliding mode control of uncertain non-linear systems with mono-directional control actions. Case study: DIST-AMADEUS gripper   ___________________________________________________________________________________________________________

Advisor:

Prof.  G. Bartolini, Università di Genova

Summary of the thesis

Many complex mechanism can be actuated by mono-directional control actions. The so called tendon arms, the jet actuated vehicles and the system described in this thesis are significant examples of systems of this kind. Often the corresponding actuators are characterized by an on/off behaviour and/or imprecise relationships between the electrical input signals and the mechanical outputs. This situation constitutes a further source of uncertainties with respect to the standard ones usually present in the mathematical description of the systems' dynamics.

A very promising approach for this kind of system is represented by simplex of control vectors sliding mode methodology. This method consists in identifying a collocation of the actuation system in the structure so that the control vector generated by the action of a single actuator (thruster) at a time form a simplex in a suitable vector space. The control objective, in accordance with sliding mode control theory, is represented by the zeroing of a suitable output vector. The problem consists in identifying a switching logic causing the finite reaching of the control objective in which a single vector of the control simplex is univocally associated to a single region among the ones in which the output space is partitioned.

With the aim to demonstrate tha effectiveness of this approach, a practical implementation involving a prototype of a three-fingered gripper is considered which has been designed in order to cope the theoretical assumption underlying the proposed methodology. Indeed the number of actuators is exactly the minimum required for controlling the relevant degree-of-freedom with mono-directional actions. Due to its constructive features and to the high bandwidth of the chosen actuators, this prototype allows the implementation of almost-ideal sliding regimes, provided that the sampling frequency allowed by the digital implementation of the control algorithm is of the same order of the motors' bandwidth. In order to fulfil the requirements of the proposed simplex methodology, a set of digital second order sliding mode observer/differentiator has been included in the control scheme. Extensive experimentations and simulation activity have confirmed the validity of the proposed approach.
 

  _______________________________________