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1946 |
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Geometrical Modification of the Tricept Machine to Maximize Stiffness |
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Equipment, Machines & Instruments: Analysis & Modeling |
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Tricept machine is one type of parallel link manipulators. In parallel link manipulators there is a mobile platform connected to a stationary base via a number of links, hence, an end-effector load gets distributed between the different links leading to smaller deformation per link and therefore, higher stiffness and better accuracy. However, due to this closed chain structure, the work volume to machine volume ratio for parallel link manipulators is smaller than its counterpart in serial manipulators. The most popular type of the parallel link manipulator machines is the Stewart platform which has 6 degrees of freedom and many machine tools are based on this configuration. The Tricept machine is another configuration, however, it is a hybrid serial-parallel link manipulator machine that has 5 degrees of freedom (3UPR in addition to 2 rotary axis fixed at the end of the parallel manipulator; these rotary axes are essential for the Tricept concept because in its parallel structure there is coupling between translation and rotation). This Tricept machine finds best applications in the machine tool field where high speed, good accuracy, and high stiffness are desirable.
The potential issues with the Tricept machine are the low stiffness regimes due to the hybrid structure. Current parallel structure geometry contributes also to the low stiffness of the machine. The other issue is the poor work volume to machine volume ratio which is a general problem for parallel kinematics machines, and in the Tricept machine this may prove to be a smaller issue because of the hybrid structure leading to believe that the Tricept machine has better positioning in terms of this ratio relative to the other types of parallel structures.
The purpose of this study is to alter the Tricept geometry while keeping the Tricept topology so as to increase the machine stiffness and accuracy. The work volume to machine tool volume ratio may suffer due to this geometry change, hence a quantitative value for the relationship between stiffness gains versus work volume is shown which can be used as a design tool. A numerical example would be provided to quantitatively demonstrate the mathematical derivation.
The results of this paper can be used as a design tool to improve the machine performance and/or can be imbedded in the decision making and control systems of the machine by which the usable workspace can be tailored to meet certain stiffness/accuracy constraints and hence improving the machining performance results, i.e. it can be used as a post processing tool for already existing machines.
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