Linear-rotary direct drive for multi-functional machine tools

Abstract

By combining linear and rotary movement in one single drive, the overall dynamics of machine axes can be enhanced, and the installation space can be reduced. Existing concepts for linear-rotary drives either have low power density or load capacity, and the installation space of the guideways are not sufficient for applications in machine tools. Thus, this paper presents a novel linear-rotary direct drive for machine tools. The electromagnetic coupling between the linear and rotary direction is analyzed, and the control performance is evaluated. The developed drive is characterized by stiffness of up to 205 N/µm, acceleration dynamics of 3,100 mm/s² in linear and 24,100 °/s² in rotary direction, and fine positioning in 0.2 µm and 3.6″ steps.

Introduction

The need to reduce production costs and to machine parts with complex geometry leads to higher demands on the dynamics and accuracy of machine axes [1]. The most commonly used feed drives for machine axes are ball-screw drives and linear direct drives [2]. In the case of ball-screw drives, the mechanical transmission elements limit the attainable dynamics. Whereas, with linear direct drives, the dynamics are limited by the moving mass. Additionally, these axes are usually stacked using a mechanical coupling. This serial kinematic leads to higher moving masses due to the extra mass of the coupling structure and the overlaid axes. Thus, the overall machine axes dynamics is limited by the serial kinematics.

For applications with a linear-rotary motion, at least one linear and one rotary drive are combined. By using 2 degrees of freedom (DOF) linear-rotary direct drives, the complex mechanical coupling structure can be omitted. In this case, the movements in both DOF are accomplished by the same component. The moving part (rotor) is directly actuated in both directions. Compared to a conventional 1-DOF direct drive, the main difference is the orientation of the magnetic field. A 1-DOF drive has either one magnet field in the linear or rotary direction, whereas a 2-DOF drive has both magnet fields [3]. The omission of the serial axes kinematics leads to a lower moving mass and a more integrated and room-saving design [3]. Thus, a higher dynamic [3] and precision are expected.

During the last decades, mainly three different motor types were used for linear-rotary 2-DOF direct drives: switched reluctance motors [4], [5], induction motors (IM) [6], [7], [8], and permanent magnet synchronous motors (PMSM) [9], [10], [11]. Switched reluctance motors achieve high dynamics but also have high torque ripples, which make fine-positioning difficult [3]. Induction motors have the advantage of being simple in design and less expensive than PMSM [3]. However, they have the major disadvantage of a low power density [3]. PMSM allows for high power density while maintaining high efficiency. Moreover, they have good position accuracy [3]. The topology structure of all motor types is largely identical. To generate the force and torque for the linear and rotary motion, either two separated [4,7,9] or one combined motor part is used [[5], [6],8,[10], [11]]. Accordingly, the drive can either be designed to generate two independent magnetic fields [4,7,9] or a single crossing magnetic field [[5], [6],8,[10], [11]]. The drive design with a crossing magnet fields leads to a compact drive design. Moreover, an electromagnetic coupling effect of the two magnetic fields occurs. Due to the electromagnetic coupling, a mutual influence can arise during the positioning along both DOF, which also affects the stability of the controller [3]. The level of influence and the effect are affected by the motor type and its individual design and must therefore be examined.

Currently, 2-DOF linear-rotary direct drives are mainly used in robotic systems, injection machines, and automobile applications [3]. Other approaches of linear-rotary drives use electromagnets to achieve a linear motion. They focus on high precision machining but are limited in their travel distance to less than a millimeter [12,13]. Thus, current linear-rotary 2-DOF direct drives do not meet the requirements of the machine tool industry, regardless of the motor type and drive design [4], [5], [6], [7], [8], [9], [10], [11]. Especially the power density and load capacity of the guideways are not suitable for high process forces in machine tools. Further, the guideways in machine tools are not appropriate for movement in a linear-rotary DOF. Therefore, based on the later application machine, the design specifications of the novel drive were targeted to a feed force of 5200 N and torque of 350 Nm. In addition, process forces of up to 10,000 N must be taken by the guideway. These parameters are in the range of medium-sized multi-functional machine tools.

In this paper, a novel 2-DOF linear-rotary direct drive based on PMSM is presented for use in machine tools. The rotor is actuated by a newly developed cross-winding system and guided by a hydrostatic guideway. The concept is explained in Section 2. As it is unknown whether axis-independent positioning can be carried out by the proposed motor design using cascade control, the electromagnetic interference and control behavior of both DOF are analyzed in Section 3. The characteristics of this motor design are also unknown and are evaluated by experiments in Section 4.