Paul Vanherck

Accurate tracking control of linear synchronous motor machine tool axes

An advanced continuous path tracking controller and its application to accurate control of two different linear synchronous motor machine tool axes are described. The controller consists of state feedback, feedforward, and motor ripple compensation. The feedforward aims at zero tracking errors and consists of inverse model pre-filtering. Ripple compensation is implemented in the position control loop and based on an experimentally identified first order approximation of the motor ripple. Both cogging (magnetic ripple) and force ripple (electro-magnetic ripple) are considered. Experiments carried out on the two machine tool axes show that a simple ripple compensation method is necessary and sufficient in order to achieve acceptable tracking errors.

Contribution of CIRP to the Development of Metrology and Surface Quality Evaluation during the last fifty years

Abstract An overview of the contribution of the members of the C.I.R.P. community to the progress of Metrology and Surface Roughness Quality Evaluation is given. The following items are included in the part on metrology: brief overview of the existing situation before 1950, contribution to the successive definitions of the unit of length and related reference length standards, traceability, preliminary work to standardization, thermal effects, design and construction of precision machine tools and measuring machines, CMM, large scale metrology. In the field of surface quality evaluation, the following items are reviewed: reference profiles and related definitions of parameters, filtering, surface and subsurface integrity, functional meaning of parameters, instrumentation, scanning probe microscopy, 3D surface evaluation. A comprehensive list of references is provided.

Force Measurements for Single Point Incremental Forming: An Experimental Study

In this paper an experimental platform capable of measuring forces in process during an incremental forming procedure is described and the results garnered from it are presented. Some of the earliest measurements of forces in incremental forming and the changes induced on the measured load are reported. Using a table type force dynamometer with incremental forming fixture mounted on top, three components of force were measured throughout the forming process. They were found to vary as the parts were made. The reported experimental test program was focused on the influence of three different process parameters on the forming forces: the vertical step size between consecutive contours, the diameter of the tool and the steepness of the part’s wall. For the tested material, analytical results demonstrating the relationship between the respective process parameters and the induced forces are presented in this paper.

Results of the CIRP-Euromet Intercomparison of Ball Plate-Based Techniques for Determining CMM Parametric Errors

Abstract The general scheme and the results of the intercomparison are presented. The techniques compared are reviewed and the advantages of using ball plates to determine CMM parametric errors are illustrated. The experimental procedures as well as the data reductions are covered by the comparison. The results show the potential of the compared techniques, and that they are essentially equivalent in their capabilities of predicting and compensating for CMM geometrical errors. Improvements in the scheme are recommended for future intercomparisons and extensions to other participants.

Compensation of static and transient thermal errors on CMMs

Abstract Using CMMs under normal workshop conditions necessitates to take into account the influence of environmental temperature on the machine structure. Non-standardised environmental conditions result in temperature dependent measurement errors. The paper presents a parametric approach to describe the relation between transient temperature distributions and resulting deformation of the CMM. The focus lies on broadening the temperature range in which the original accuracy specifications can be guaranteed. Starting from a correction scheme for uniform, invariant temperature situations, an approach for transient environmental loads is developed. Based on a limited number of temperature inputs, the required correction coefficients for the probe position are calculated.

Compensation of thermal deformations in machine tools with neural nets

Abstract The accuracy of a milling machine is limited by thermal deformations. Although the deformations can be minimized by a good design, they cannot be completely avoided, and they depend on operating conditions. We show that a reduction and/or a compensation of these deformations is even more important when exploiting additional degrees of freedom, which is an additional motivation for our research efforts. A neural model was used to compensate thermal deformations of a five-axes milling machine. Neural nets were used because they can approximate complex multivariable non-linear relationships. First, a computer controlled test set-up was built to collect the training and test data. Then, a feedforward neural net was trained to estimate these deformations, given the temperature measurements. A backpropagation algorithm was used, modified with momentum and an adaptive learning rate. The maximum deformation of 150 μm was reduced to 15 u,m by using a single hidden layer with four neurons. Finally, the neural model was tested under operating conditions. Two workpieces were milled, one with and one without compensation. The neural model was able to reduce machining errors from 75 μm to 16μm With the presented approach, we managed to achieve successful results in a relatively short period, after first attempts had failed to reduce the deformations by eliminating the heat source.

A Nanometre-precision, Ultra-stiff Plezostepper Stage for ELID-grinding

Abstract The extremely high machine stiffnesses required for a successful Implementation of ELectrolytic in-process Dressing (ELID) grinding cannot be obtained win conventional technologies. In this paper, an Innovative drive concept is presented, where the actuation and guiding functions in three degrees of freedom are combined in one functional unit, using an original plezo-stepping technology. The stage is actively positioned by a set of six ‘feet’ giving it an infinite static stiffness in all six degrees of freedom. The drive motions in x-, y- and C-directions are obtained by rolling the feet over the base surface in a three-by-three feet gait pattern. The stepping resolution can be as low as 2.5 nm. Hammering is avoided by force-controlled stepping. The x and y stage positions are interferometrically measured. Thermal errors are minimised by a symmetrical design.

Active force feedback in industrial robotic assembly: A case study

This paper describes the application of a robot with active force feedback to an industrial assembly operation involving fragile, medium-sized workpieces supplied with large position and orientation uncertainties. It is shown that active force feedback yields a better and more reliable technical solution than passive force feedback. In addition, active force feedback is also economically justified, since the passive solution requires additional peripheral equipment in order to deal with the large uncertainties. The active force feedback approach presented is general in nature and not application dependent.

A geometrical model of the cut in five-axis milling accounting for the influence of tool orientation

This paper presents a model of the cut geometry in five-axis milling. This allows the establishment of a better model of cutting force to account for the influence of the tool orientation. The formulation of the width and the thickness of the cut were derived and implemented in a computer simulation. The results of simulations were verified experimentally and a good agreement was obtained. The result shows the importance of including the influence of the tool orientation in the cut cross-section calculation.

Accurate modal analysis measurements with programmed sine wave excitation

Abstract Due to the development of several new dynamic analysis methods which use experimental modal analysis results as input data, these results need to be subjected to stringent accuracy demands. One of the techniques to achieve this goal is the use of stepped since excitation. Although the principle of this technique has been known for a long time, a feasible implementation, suited for testing large structures or non-linear systems, has only been possible by making full use of todays techniques in digital signal processing. In this paper, the characteristics of the method are briefly reviewed, for both single input and multiple input testing. An implementation with response adaptive definition of excitation frequency and amplitude is presented and illustrated with some examples. The impact of this method on the estimation of the modal parameters is discussed.