Framework for Overcoming Structural Dynamic Limitations of Machine Tools using Additional Tool Center Point Measurements
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Date
2021
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Doctoral Thesis
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Abstract
In addition to geometrical and thermal, mainly dynamic errors occur on mechatronic systems such as machine tools. The dynamic errors are of transient type and caused by forces, which are acting on the structural components of the mechatronic systems during motion, interacting with a motion controller reacting to them.
Besides static and transient internal and external disturbances, mainly inertial forces lead to deformations of the compliant structural components, causing deviations at the tool center point (TCP). Especially for high dynamic systems with relatively soft and therefore compliant structural components, which are moved at high accelerations, the dynamic errors become dominant. Mainly by restrictions related to the applied measurement principle, these errors are not fully detectable by the axes measurement systems. Often linear encoders are used for measuring the locations of the moving structural components, which are forced to be placed far from the TCP due to limitations in design. Accordingly all dynamic deviations in-between the measuring locations of the encoder and TCP are invisible.
Within this work the available sensor principles and their usability for measuring the dynamic deviations between encoder and TCP are investigated. It is shown that a sensor fusion approach, combining the linear encoder with additional accelerometer measurements at the workpiece (WP) and the TCP leads to a significant reduction of the dynamic measuring error between encoder and TCP. For the sensor fusion task a linear filter based method is presented, which combines the encoder with the accelerometer measurements with a quasi-static deformation model. Further a parametrisation strategy for the filtering task is derived based on a simplified model of the dynamic error between WP and TCP. Using simulations and measurements on a validated multi-axes mechatronic system it is shown that the dynamic error can be measured with an accuracy of at least 10 um for highly dynamic systems. Further an estimation error calculation is derived. As nowadays reasonably accurate accelerometers are available at low costs on the market, the method might be broadly applied on machine tools, in order to enhance the measurement accuracy.
In addition to the measurement of the dynamic error, the reduction of it is the main objective, which might be achieved by either online or offline dynamic compensation methods.
Offline dynamic compensation methods often rely on accurate measurements of the actual deviations at the TCP . These algorithms might directly benefit from the increased measurement accuracy of the method described above.
As a first approach for online dynamic compensation, within this work a new parametrisation strategy for the standard cascaded loop motion controller is presented, which is minimising the dynamic error at the TCP. It considers the dynamics between encoder and TCP in order to compensate for quasi-static deviations and maximising the structural damping actively at the same time. The parametrisation strategy is validated using measurements and simulations. As it is applicable to almost every industrial numerical control (NC), it might directly be used for the dynamic performance optimisation of industrial machine tools.
The second presented approach involves an additional velocity feedback loop, added to the standard cascaded loop motion controller. The known principle in literature is applied to machine tool direct drive structures including the derivation of a hands-on parametrisation strategy. Using the additional velocity feedback allows to increase the controller bandwidth significantly without loosing structural damping. By this the disturbance rejection and tracking accuracy is increased without induction of unwanted vibrations. The method is applied to simulations, showing promising results.
For the practical applicability of the approach online measurements of the velocity at the TCP are needed, where the accelerometer based measuring method is suggested to be extended from offline to online applicability.
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ETH Zurich
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Subject
MACHINE TOOLS AND MACHINERY (MANUFACTURING TECHNOLOGY); Mechatronic Systems; Structural Deformation Monitoring; CONTROL (MECHANICAL ENGINEERING)
Organisational unit
03641 - Wegener, Konrad (emeritus) / Wegener, Konrad (emeritus)