Error Identification and Error Correction for Spindle Test Setups

Abstract

Geometric accuracy tests are an integral part of machine tool acceptance procedures. With an increasing demand for higher machine tool accuracy at lower costs and the requirement to test under shop floor conditions, the measuring task becomes more and more challenging. The field of spindle metrology is particularly affected. As permissible maximum errors decrease, a corresponding decrease in measurement uncertainty is required. A feasible strategy to cope with this is to increase the accuracy of the measurement instrumentation. However, in shop floor environments disturbances are likely to degrade the measuring performance. Not only sensors and transducers are affected, but also the mechanical setup is prone to disturbing influences. As a consequence, a gap between the measuring performance which is determined under controlled inspection room conditions and the performance under real shop floor application conditions emerges. The objective is therefore to enable the utilization of existing and nominally capable spindle measurement instrumentation in shop floor environments and on machine tools without the abatement of measurement accuracy. The chosen approach to achieve the objective incorporates on-machine and in-process error identifications of the spindle measurement setup. The gained information are used to develop target-oriented countermeasures, improvements of the measurement setup, and correction procedures. Electromagnetic interference as well as environmental temperature variations are identified as main disturbance sources. Due to the characteristics of the related error systems, cap tests are utilized for the experimental error analysis. Electromagnetic interference markedly degrades the performance of capacitive displacement transducers which are commonly utilized for spindle measurements. However, it is shown that an electromagnetic compatible measurement setup is accomplished by the electrical insulation of the measuring circuit against structural components of the machine tool and thus the electrical machine tool system. A testing procedure and a corresponding testing device are proposed which enable the on-machine performance testing of capacitive displacement transducers and the performance verification of the setup recommendation for test setups. In order to identify the thermal distortion of a fixture, an on-machine analysis method is proposed. The achieved measurement uncertainty is almost independent of the fixture dimensions. An experimental thermal analysis of the specific fixture indicates uniformly distributed fixture material temperatures and the admissibility of a lumped capacity assumption, which results in the expectation of thermal expansion leading to dimensional scaling without distortion. This expectation is confirmed by the experimental identification procedure. The experimental data can directly be used for the compensation of thermally induced fixture distortion. The cap test approach is also applied for the analysis of thermally induced errors of axial displacement transducers. Clamping positions of the transducers are considered as boundary conditions in the error identification procedures. In particular, the thermal behaviors of inductive displacement transducers are analyzed. Steady state error coefficients are evaluated which allow a comparison of different transducer models and an appropriate selection of transducers. In addition, a static error correction approach directly makes use of these coefficients. For the correction of dynamic errors an observer-based correction approach is developed. In the tested cases of two inductive displacement transducer models relative error reductions in the range of 85-95% are achieved.