Papastathis, Thomas
(2011)
Modelling and design methodology for fully-active fixtures.
PhD thesis, University of Nottingham.
Abstract
Fixtures are devices designed to repeatedly and accurately locate the processed workpiece in a desired position and orientation, and securely hold it in the location throughout the manufacturing process. Fixtures are also charged with the task of supporting the workpiece to minimise deflection under the loads arising from the manufacturing process. As a result, fixtures have a large impact on the outcome of a manufacturing process, especially when the workpiece presents low rigidity. Traditionally, in manufacturing environments, where thin-walled components are produced, the utilised fixtures are dedicated solutions, designed for a specific workpiece geometry. However, in the recent decades, when the manufacturing philosophy has shifted towards mass customisation, there is a constant technological pull towards manufacturing equipment that exhibits high production rates and increased flexibility/reconfigurability, without any compromise in the quality of the end result. Therefore, fixtures have been the focal point of a plethora of research work, targeting mainly towards either more reconfigurable, or more intelligent/adaptive solutions. However, there have been no attempts so far to merge these two concepts to generate a new fixturing approach. Such an approach, referred to in this work as fully-active fixrturing, would have the added ability to reposition its elements and adapt the forces it exerts on-line, maximising the local support to the workpiece, and thus reducing vibration amplitude and elastic deformation. This results in a tighter adherence to the nominal dimensions of the machined profile and an improved surface-finish quality.
This research work sets out to study the impact of such fixturing solutions, through developing suitable models which reflect the fixture-workpiece system behaviour, and a design methodology that can support and plan the operation of fully-active fixtures. The developed model is based on a finite elements representation of the workpiece, capturing the dynamic response of a thin-walled workpiece that is being subjected to distributed moving harmonic loads. At the same time, the workpiece is in contact with an active element that operates in closed-loop control. An electromechanical actuator is charged with the role of the active elements, and it is modelled via first-principle based equations. Two control strategies are examined experimentally to identify the best performing approach. The direct force/torque control strategy with a Proportional-Integral action compensator is found to lead to a system that responds faster. This control architecture is included in the model of the active elements of the fixture. The behaviour of the contact between the fixture and the workpiece is approximated via a combination of a spring and a damper. The overall model is assembled using the impedance coupling technique and has been verified by comparing its response with the time-domain response of an experimental set-up.
The developed model serves as the backbone of the fully-active fixture design methodology. The latter is capable of establishing important fixturing parameters, such as the pattern of motion of the movable fixture element, the points on the surface of the workpiece that formulate the motion path of the fixture element, the time instant at which the element needs to change position, and the clamping forces the fixture needs to apply and maintain. The methodology is applied on a thin plate test case. Such a plate has been also used in a series of machining experiments, for which the fixturing parameters used are those that resulted from the test case. A very good quantitative agreement between both experiments and theory was observed, revealing the capabilities of the methodology itself and of the fully-active fixturing approach in general.
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