A New Direct Deformation Measurement System to Enhance Positioning Accuracy of Machine Tools

The positioning accuracy of computer numerical control machines tools and manufacturing systems is affected by structural deformations, especially for large sized systems. Heat induced deformations, long-period deformation of foundations, and the manufacturing process itself, cause time-dependent structural deformations of the machine body, which are difficult to model and predict. In fact, the feasibility of a model-based error prediction is often limited by the complexity of the problem from both the geometrical and the physical point of view. As a consequence, only limited success has been achieved in active error compensation based on the modeling of the relationship between the generalized dynamic loads and the structural deformation field. This paper illustrates a different approach in active error compensation, which exploits a new measurement system, currently in the patenting process, able to measure, in real time, the machine structural displacement field, without any model for the dynamic structural behavior. The first part of the paper illustrates the working principle of the measurement system, which can be described as coupled hardware and software subsystems. The hardware subsystem is basically a triangular mesh of struts, whose nodes are rigidly connected to the underlying structure under measurement. The struts are instrumented with Fibre-optic Bragg Gratings providing their longitudinal strain values. The software part is an algorithm, which evaluates the discrete displacement field by computing the node positions on the basis of the strut longitudinal deformations. The second part of the work focuses on the performance, in terms of accuracy, resolution, and time stability, of a prototype of the above described measurement system. Finally, the third part illustrates two major enhancements on the system design: the design of a monolithic variant of the reticular structure (with higher performances and reduced cost), and a different computation algorithm providing increased accuracy and limited error propagation.