Angular measuring system for direct drive precision rotary axes

No contradictions: High load carrying capacity and highest level of precision

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AXA Entwicklungs- und Maschinenbau GmbH relies on Schaeffler Group Industrial’s bearing series with integrated angular measuring system for achieving high speeds and high load carrying capacity as well as the highest level of precision, rigidity and dynamics in the new turning and milling center VHC50-D.

Author: Dipl.-Ing. (FH) Guenter Schmid, Schaeffler KG, Herzogenaurach

High speed capacity and high load carrying capacity as well as the highest level of precision, rigidity and dynamics were the challenging requirements AXA Maschinenbau GmbH in Schoeppingen defined for the development of a new multi-axis machine tool (Figure 1). An important task in this regard was the new development of a rotary table (Figure 2) that can be used for drilling and 5-axis milling as well as for turning. This way, vertical turning and face turning as well as milling and drilling operations can be performed in one single clamping operation. AXA Maschinenbau did not make any compromises in the selection of the drive components for this rotary table. In addition to a torque motor, the company opted for the high speed axial/radial cylindrical roller bearing YRTSM460 of Schaeffer Group Industrial (INA/FAG). The new bearing series with integrated angular measuring system was specially developed for highly dynamic direct drive precision rotary axes. The advantages are highest precision in machining and extremely economical manufacture of complex components.
Rotary table bearing with integrated magneto-resistive angular measuring system
The bearing is equipped with a magneto-resistive angular measuring system integrated in the bearing design envelope and is specially matched to the requirements of direct drive rotary tables for machine tools (Figure 3). In this system, the magnetic angular scale is physically applied to the rotating shaft washer of the bearing without resulting joints. It is scanned by two sensor heads that are offset by 180° and directly flange located to the base component of the rotary table. The twin head scanning system ensures that any center displacement caused by machining forces is compensated within the system. Absolute measuring accuracies of less than +/-3 angular seconds are thus achieved. In addition, this type of scanning doubles the angular resolution and transmits analog signals of a total of 7008 increments per revolution to the controller. The signals from the two heads are combined in the electronic evaluation system that is connected to the NC controller.
Electronic evaluation system with shortest possible computing time
During the development of the electronic evaluation system, emphasis was placed on achieving the shortest possible computing times so as to ensure maximum control quality. This extremely fast electronic evaluation system and the special bearing design allow a limiting speed of 560 revolutions per minute for the selected bearing size. Both sensor heads are flange located and screw mounted with high mechanical rigidity to the base component of the rotary table. Due to the rigid mounting of the angular scale on the rotating rotary table and the robust, vibration-resistant mounting of the sensor heads on the base component (Figure 4), highest amplification factors in the control loop are achieved and therefore the best possible control quality, control stability and dynamics. Furthermore, the system is compactly integrated in the bearing design envelope and the center of the rotary table is available for cables or pipes. This gives more compact machine subassemblies and cost savings in design, procurement and mounting.
Insensitive to cooling lubricants and rolling bearing greases and oils
The sensor heads and the electronic evaluation system are protected against chemical, mechanical and electrical influences from the environment, which has been demonstrated in comprehensive environmental simulations. In particular, the magneto-resistive measurement principle has proved insensitive to cooling lubricants and rolling bearing greases and oils. The operational suitability of the angular measuring system in machine tools under production conditions has been confirmed. Machining tests showed contour deviations of less than two angular seconds.
For extremely rigid and dynamic controller settings for the torque motor drive
With the new bearing series YRTSM, Schaeffler Group Industrial has developed an angular measuring system that is optimally matched to the requirements of direct drive precision rotary axes for machine tools. It is available for bearing inside diameters of 200 mm to 460 mm and allows limiting speeds between 560 and 1160 revolutions per minute depending on the bearing size. The measuring system combines very high measuring speed and limiting speed with high measurement accuracy and also allows extremely rigid and dynamic controller settings for the torque motor drive. Schaeffler Group Industrial thus fully meets the trend towards high-speed precision rotary tables for economical complete machining through milling and turning.
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