Nowadays, feed axes are often equipped with multiple parallel-acting actuators in order to increase the dynamics of the machine tool. Also, additional actuators for active damping are widely used. Normally, the drives or actuators are controlled independently without consideration for the impact on each other. In contrast, by using the modal space control, the system can be decoupled and the modal control loops can be adjusted independently. This control approach is particularly suitable for motion systems, such as machine tools, which have more drives or actuators than degrees of freedom of movement. This paper deals with the pre-investigation of the modal-based vibration control for machine tools with additional actuators. The object of investigation is an elastic system with a movable saddle. The modal-based control is compared with a local control approach. The results obtained experimentally on the test rig are presented. The modal control is superior since, with the modal approach, each control loop corresponds to a specific vibration mode, and the control law for this loop is designed to provide
the desired performance of the control system at the corresponding resonance frequency. The parameterisation of the control loops is simplified by modal control, since the modes can be controlled independently.
Kurzfassung
Mit zunehmender Flexibilisierung der Fertigung und kleineren Serien gewinnt das thermische Verhalten von Werkzeugmaschinen signifikanten Einfluss auf die erreichbare Werkstückqualität. Im SFB/Transregio 96 werden Lösungen zur Beherrschung des thermischen Maschinenverhaltens entwickelt. Das Teilprojekt C06 stellt mit der selektiven Thermografie ein Messverfahren zur Verfügung, mit dem die experimentelle Analyse des thermischen Maschinenverhaltens vereinfacht und verbessert wird.
Gantry stages, which consist of two parallel acting servo drives, are commonly used in machine tools. One drawback of this concept is the crosstalk between both drives due to the structural coupling that can cause stability issues and therefore limits the bandwidth of the position control. This paper deals with the development of compliant joints to solve the coupling between the drives. When compared to solutions containing bearings, the advantages of such flexible elements are low friction and the absence of backlash. To adjust the properties of the joints, packages of spring-steel-sheets are used as compliant links. One design aspect of the flexible joints is a low stiffness relating to the rotation around one specific axis, but a high stiffness relating to the other degrees of freedom. With this method, the dynamic behaviour of the gantry stage is modified and the bandwidth of the controllers can be increased. Additionally, by releasing the mechanical coupling of the drives, the reaction forces the actuators have to provide can be reduced. Both systems with flexible and with rigid connecting elements, are analysed by measured frequency response functions.
This paper presents the modal control applied to motion systems, in particular for machine tools. This control approach is particularly suitable for over-actuated systems that have more actuators than degrees of freedom. By using the modal approach, the parameterisation of the control loops is simplified since each control loop corresponds to a specific eigenmode. A four-variable modal control of a linear motor-driven gantry system equipped with additional active damping devices is presented to achieve active vibration suppression. This approach is experimentally compared with a conventional control method that does not consider the eigenmodes of the system. The influence of the vibration controllers on the closed position loops is investigated.
Typically, the feed dynamics of machine tools are limited to reduce excitations of machine structure oscillations. Consequently, the potential increase in productivity provided by electrical direct drives cannot be exploited. The novel approach of the Kinematically Coupled Force Compensation (KCFC) combines the principles of redundant axes and force compensation to achieve an increase in the machine’s feed dynamics. Because the drive reaction forces are directly applied to the machine frame, they cancel out each other perfectly if the relative motion at the Tool Centre Point (TCP) is split according to the mass ratio of the slides. In this paper, the principle of KCFC is introduced briefly and possible improvements in the design of machine structures and control are presented. The results of experimental investigations obtained by means of a 1D-KCFC Test Bed illustrate the effectiveness of the principle. Moreover, a further increase of the compensation quality can be achieved by decoupling the force flow from the machine frame, by means of elastic elements. Finally, an outlook on future research with reference to the 1D-implementation as well as possible applications of the KCFC in highly productive processes is given.
Gantry stages, consisting of two parallel acting drives, are a well-known example, where a significant interaction between all control systems’ in- and outputs is present due to structural coupling. Stability issues and therefore the limitation of the bandwidth of the position control can be a consequence of this effect. The adjustment of the systems mechanical properties using compliant joints, as well as the implementation of centralised control strategies are two possible approaches to overcome these issues and to improve the dynamic and static behaviour of the stage. Frequency domain methods for multivariable systems are used for controller tuning to consider properties like the controls’ time delay and high frequency mechanical eigenmodes, which are hard to model otherwise.
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