Bakker, Otto Jan
(2010)
Control methodology and modelling of active fixtures.
PhD thesis, University of Nottingham.
Abstract
Fixtures are used to fixate, position and support workpieces, and are critical elements in manufacturing processes. Machining is one of these manufacturing processes, and this is often done by computer numerical control (CNC) machines. A major trend observed in production industry is that manufacturing is increasingly done in small batches in combination with a quick changeover from one product to another, in combination with a surge in automation. Several novel fixture concepts have been developed that allow for a reconfiguration of the fixture layout, such that different types of workpieces can be fixtured using the same fixture components. However, the initial novel fixturing concepts lacked accuracy, and, in addition, required long set-up times. Recently, a new fixturing concept has been developed, the so-called intelligent fixturing system. Sensors and actuators are integrated in an intelligent fixturing system, which allows for an automatic and precise reconfiguration of the fixturing elements. Additionally, the actuated fixture elements can be used to exert optimal clamping forces to minimise the workpiece deflection during the machining process, this is called active fixturing.
A literature survey has been carried out, in which it has been established that the main process variables to control in active fixturing, are the reaction forces at the contacts where the workpiece is fixated and supported by the fixture (the locating points), and/or the part or fixture displacements. Furthermore, four knowledge gaps were identified: (1) a lack of computationally efficient models of workpiece response during machining; (2) a lack of methodic structural analysis approach of part-fixture interaction; (3) a lack of model-based control design, which can potentially speed up the fixture design process; and (4) a lack of control design methodology for active fixturing systems.
An active fixturing system can be divided into the following subsystems: the part, the part-fixture contact interface, passive fixture elements, the actuated clamp, sensors and the controller(s). In the thesis, a methodical research approach has been applied to address the knowledge gaps by analysing the active fixturing subsystems. In addition, a model-based control design methodology has been proposed. The research has aimed to establish mathematical models, or the necessary tools and methodology to build the subsystem models, and methods to connect the subsystem models into an overall model of the active fixturing system. On basis of the subsystem analyses, two simple, yet complete, active fixturing systems have been modelled. Parameter studies have been held to assess the performance of the control design. In addition, an industrial case study has been analysed, using the developed control design methodology.
The study of the subsystems resulted in the comprehensive structural dynamic analysis of workpieces: a finite element model of the workpiece is built. Typically, finite element models contain too many degrees of freedom for real-time control applications. It was found that model reduction techniques can be used to reduce significantly the number of degrees of freedom. Methodologies for the selection of the degrees of freedom and for ensuring that the model reduction is accurate enough for practical use have been established. Mathematical models for hydraulically and electromechanically actuated clamps have been established. Compensators for closed-loop servo-control of the clamps have been investigated and control strategies to maintain workholding stability are found. Finally, a methodology to establish the overall model of an active fixturing system has been implemented. The control design methodology, and the mathematical tools established in the thesis have been verified against case studies of simple active fixturing systems. Furthermore, from the industrial case study it is concluded that the control design methodology can be successfully applied on complex fixturing systems. Additionally, a mathematical model for a piezoelectrically actuated clamp was derived, which also demonstrates the general applicability of the control design methodology derived here, as a new established actuator model is integrated in the control design. The overall conclusion, is hence that a good methodology for the model-based control design of active part-fixturing systems has been developed, which enables the engineer to speed up the design process of active fixturing systems.
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