Stefan Rothenbücher

Analyzing Energy Consumption of Machine Tool Spindle Units and Identification of Potential for Improvements of Efficiency

This paper describes the results of the analyses of a representative machine tool spindle unit and the thereof identified potentials for increasing the energy efficiency. This is done within the project “EnergieMSP”, whose aim it is to develop an energy optimized spindle unit with an adapted electric drive train. The holistic analysis of the system is done in a machine tool to determine the consumption of pressurized air, the energy demand for the tool change and hydraulic stand-by losses. The frictional losses in the bearings and rotary union and the electrical drive train where separately investigated at test benches.

Cartesian compliance model for industrial robots using virtual joints

Industrial robots represent a promising, cost-saving and flexible alternative for machining applications. Due to the kinematics of a vertical articulated robot the system behavior is quite different compared to a conventional machine tool. The robot’s stiffness is not only much smaller but also position dependent in a non-linear way. This article describes the modeling of the robot structure and the identification of its parameters with focus on the analysis of the system’s stiffness. Therefore a method for the calculation of the Cartesian stiffness based on the polar stiffness and the use of the Jacobian matrix is introduced. Furthermore, so called virtual joints are used. With this method it is possible to model each joint of the robot with three degrees of freedom. Beside the gear stiffness the method allows the consideration of the tilting rigidity of the bearing and the link deformations to improve the model accuracy. Based on the results of the parameter identification and the calculation of the Cartesian stiffness the experimental model validation is done.

System identification during milling via active magnetic bearing

One of the main focuses of recent machine tool development is to increase the machining performance. But the enhancement of the metal removal rate (MRR) is often limited by the stability of the milling process. To prevent unintentional chatter vibrations and to observe changes in the system’s behavior, there is a need to observe the state of the milling process during machining. Beside others, one approach is the development of an adaptronic motor spindle using active magnetic bearings (AMB) combined with conventional roller bearings in a so called “hybrid approach”. The AMB acts as a contact less sensor and actuator, in order to estimate the frequency response function (FRF) of the spindle-tool system during the milling process. The two major dependencies on the FRF are the spindle speed and the cross-sectional area of the chip. Also the closed loop damping can be observed to estimate the stability margin. This paper presents the identification of the varying eigenvalues during the machining.

Digitally controlled output filters for improvement of temperature behavior of machine tool main spindle drives

Variable speed main spindle drives of machine tools are usually fed by three-phase two-level inverters. Because of the switching operating mode of these inverters, the motor voltages and currents contain harmonics which do not contribute to torque formation but solely to an undesirable warming of motor spindle elements. This warming results in thermal elongation of the spindle shaft and therefore to a deviation in dimensional accuracy. The increased wear of the spindle bearings through high temperature differences between the inner and outer rings of the bearings is another drawback. This paper proposes a novel high-performance control system of an interior permanent magnet synchronous motor (IPMSM) using an inverter with an LC output filter. The combination of inverter and filter reduces the motor losses by compensating the switching ripples and therefore leads to an evident improvement of the temperature behavior of the main spindle drive.