A CONTACTLESS AXIS FOR HIGH SPEED CUTTING MACHINES
Machine tools require linear guides with high stiffness and damping at high feed rates. At the same time, a high position accuracy is essential. The linear guides used together with linear direct drives in High Speed Cutting (HSC) Machines have to fulfill these premises along with the necessity to support the high normal forces of a direct drive. Large frictional forces and high wear result from the load on the bearings. Here, a magnetically levitated (maglev) guide supported by linear magnetic bearings could be a solution. A prototype of a maglev guide (cp. figure 1) without linear drive has been developed to examine the potential of this technology. It will be shown that the maglev principle is applicable for a linear machine axis, although further improvements have to be achieved to reach an acceptable stiffness. Due to the high payload necessary for a linear guide in a machine tool, a multivariable control scheme with modal control has to be used to perform the task of FIGURE 1: The maglev guide. levitation. The control is extended to an adaptive control with respect to the changing mass parameters of different workpieces by a COR-LS parameter identification method. This adaptive control is able to identify all mass parameters simultaneously within less than 10% of their real values [1]-[3]. INTRODUCTION For an examination of the suitability of the maglev principle for machine tools, first the driving task has been left out of consideration. It will be dealt with later on. The prototype guide consists of a stiff block. It has a mass of 32kg and is designed to carry an additional payload of up to 100kg. It is levitated and guided by six pairs of electromagnets in differential arrangement. The core of the magnets consists of E-shaped laminated steel. The magnets work against three fixed steel rails. The nominal air gap is 0.35mm, the pole area is 10cm . Switched power amplifiers with high speed current control can source a maximum magnetomotive force of 1000A. Six inductive position sensors measure the displacement of the guide with respect to the rails. Leaving out the feed direction, a modal control is designed for the remaining five degrees of freedom (d.o.f.). The controller is realized on a 200MHz PowerPC VME process computer with 16bit analog I/O using the realtime operating system RTOS-UH from Hannover University. The system is capable to generate flexible sample rates of up to 20kHz while being used as a general purpose computer, allowing even floppy disc access and process graphics. The controller program is written completely in the high-level languague PEARL. This allows a comfortable high-level programming with very effective code output while fulfilling hard real-time demands. MODELING The guide is supposed to be a rigid body which is stabilized by six forces generated by the electromagnets. These constrain the movement in five degrees of freedom, as the feed motion is not yet considered.
Load Adaption of a Linear Maglev Guide for Machine Tools
Machine tools require linear guides for high slide velocities with high stiffness and damping as well as very high position accuracy.
Optimization Methods for Parameter Adaption in Mechatronic Systems
Machine tools require linear guides with high slide velocity and very high position accurancy. The three tasks of a linear guide — supporting, guiding and driving — shall be realized by means of active magnetic bearings (AMB). The resulting linear magnetically levitated (maglev) guide has to accomplish the following characteristics: high stiffness, good damping and low noise as well as low heat production.