Technical Paper: Courage for a highly dynamic drive for precise positioning

20 years success story of direct drives for high-precision positioning systems.

Linear and rotary direct drives offer significant advantages in the operation of demanding positioning systems. They deliver high speeds and acceleration values. In addition, they are low-maintenance, provide very long service lives and have the best characteristics for synchronisation and positioning accuracy. Nevertheless, there are still reservations in mechanical engineering. A possible extra effort in the control system commissioning of the axes frightens some designers. However, Föhrenbach’s more than 20 years of experience in the use of direct drives in high-precision positioning systems is clear: The deployment is worthwhile in a sustainable manner, and commissioning is easier than expected thanks to good cooperation between the machine manufacturer and the positioning system experts.

The technical properties of linear motors and the design features of directly driven positioning axes lead to numerous positive properties that predestine the corresponding positioning systems for particularly demanding applications. What are these properties?

Low-maintenance machine design for maximum productivity

Direct drives eliminate many mechanical components such as gears, pinions, racks, spindles, timing belts and chains. The slim design of the mechanism significantly reduces wear, resulting in longer system life. Machine operators benefit from reduced maintenance and increased machine availability. And thanks to their complete integration into the machine, linear motor axes offer particularly high reliability even under harsh operating conditions. At high cycle speeds, energy savings of up to 70% compared to spindle drives are possible.

Economically, these advantages are particularly relevant in applications where above-average productivity is required and a machine downtime causes high costs. The high acceleration and traversing speed also ensure fast processing times. Optimized high-volume production is therefore reliably and economically supported by a machine solution with direct-drive positioning axes.

Of course, a direct drive is not a miracle component of the machine design. It can only develop its robust technical characteristics if the entire positioning system is first of all designed coherently, secondly it has been dimensioned to suit the application, and thirdly, the highest precision in the mechanical machining of the axes systems is guaranteed.

Quality due to excellent positioning accuracy and synchronous properties

The elimination of the above-mentioned mechanical elements also has a positive effect on the precision of the positioning. It eliminates the typical inaccuracies due to gear play, toothed belt strain, wave torsion, resonance or hysteresis. Given the careful mechanical processing of the linear axes, it is ultimately the measuring system for directly driven linear axes that determines the accuracy of the overall application.

Therefore, in conjunction with a high-resolution measuring system, there are very good synchronization properties, even at low travel speeds. Also, positioning and repeat accuracy are improved over traditional mechanical transmission systems. And thanks to the optimum ratio of torque or force to intrinsic inertia or power to weight, direct-drive positioning systems are characterized by particularly high dynamics. The high rigidity of the mechanical system allows a higher gain of the control circuits.

The quality of the axles sets standards

Especially in terms of accuracy, however, the overall system is only as good as its weakest component. In conjunction with the potentially very high precision of the direct-drive and a finely resolved measuring system, the quality of the mechanical machining of the axes ultimately determines the performance of the application.

Föhrenbach therefore develops its positioning systems on the principle of “everything from one source”. The motors used are in-house developments with very high power density. Above all, however, when it comes to fine milling and grinding of the axles, the company sets above-average standards. This is achieved with very high performance and vertical integration by the use of a specialised machinery including the measurement of produced axes by means of a laser interferometer. In this combination of experienced design and the most demanding production, Föhrenbach succeeds in implementing positioning systems with a repeatability and positioning accuracy in the range of ≤ 1 μm.

Föhrenbach has all the necessary measuring equipment for detecting position spread, running accuracy ≤ 1“(tilting, yawing and rolling), as well as angular and tumbling errors in rotary axes.

The precision in the machining of the axes is also the guarantee for the robustness of Föhrenbach's directly driven positioning systems described at the beginning. Only in conjunction with a particularly precise design of the remaining mechanical components, the long service life and the minimum maintenance of the systems can be effectively realised in the long term.

No fear of direct-drives - Smooth commissioning included

Direct drives allow high acceleration values compared to mechanical transmission systems. This positive system characteristic places increased demands on the regulation and control of the axes or on the setup and commissioning of drive controllers.

From this aspect as well, Föhrenbach’s integrated approach of supplying all the required components from a single source offers a proven solution. Föhrenbach has developed its own portfolio of controllers and drive controllers, which are delivered completely to customers in the own control cabinet. For customers who choose this integral solution for the positioning technology of their machines, Föhrenbach takes over the setting of the controllers and the commissioning of the axis and multi-axis systems with applied loads. The systems are handed over to the customer ready for operation in an optimized state.

However, even if another drive and control technology is to be used in consideration of equipment regulations or according to customer requirements, the Föhrenbach service technicians offer a commissioning service. According to the respective device specifications, commissioning is prepared at the factory and accompanied on site by the end customer. Especially in view of the low maintenance and the long service life of the direct-drive positioning systems worth this initial effort.

Years of successful use in demanding industries

For more than 40 years, Föhrenbach has been developing and manufacturing high-precision components, units and systems for linear and rotary motion in automation technology and precision engineering. For around 20 years, our own developments have been using integrated linear motors that are adapted to the customer's requirements. Thus, a portfolio of systems of different sizes and different strong linear motors have emerged, which, depending on the tightness requirements, have adequate solutions for covering the slide guide. For requirements that cannot be realised with these high-quality standard components, Föhrenbach develops tailor-made systems.

These high-precision positioning solutions are used in customer applications with particularly high demands on the quality and/or availability of processing machines. Examples are laser processing, laboratory applications or the printed circuit board industry. In view of many years of customer relations, the developers and service of Föhrenbach can fully confirm the outstanding features and operating advantages of linear and rotary direct drives in positioning tasks. In view of the partly decisive competitive advantages, Föhrenbach is convinced that these solutions will find their way into machine automation concepts to an even greater extent in the future.