Dipl.-Ing. Thorsten Rabenschlag, Siemens Linear Motor Systems GmbH & Co. KG München
1,138 words
1 April 2004
Control Engineering Europe
Volume 5, Issue 2
English
© 2004, Control Engineering Europe, Reed Business Information, a division of Reed Elsevier, Inc. All Rights Reserved

About ten years ago, the machine tool industry was amazed to see the first product from Ex-Cell-O equipped with linear motors. Since then, manufacturers such as Deckel Maho Gildemeister and COMAU Renault Automation have surprised the industry with a wide portfolio of machine tools with linear motors. According to these manufacturers, their tools are surpassing all expectations when it comes to sales figures.

The next step for linear motors is to move into general series production machines. This appears to be reasonably close.

New application areas are crystallising. The linear motor has already established itself in machining centres, HSC (high speed cutting) production and laser cutting machines, as well as applications in the areas of turning and grinding. At one time not so long ago, use of linear motors in these areas would have been considered 'taboo.'

For several years, the linear direct drive has been used in so-called pick-up lathes. The long traversing distances and the fast and precise positioning required mean that in this case, it is the optimum drive. Gildemeister uses the Siemens linear motor in the X slides of its CTX 320/420 linear universal lathes. Gildemeister's Rüdiger Kapitza , Head of the Board, explains the reason quite simply: 'The two machines with direct linear drive guarantee an increase of productivity of at least 10 per cent.'

For applications in grinding machines, the long term accuracy always did support the argument for using direct drives. Until now, the only problem was the grinding dust that becomes a sludge during machining. The motor's secondary section magnetically attracts the dust and there was a considerable danger that it would be deposited in the air gap between the secondary and primary sections.

Of course, this grinding dust is still present today. However because the linear motor can be integrated into the machine in many ways, dust is kept away from critical locations. Machine builders, together with the specialists from Siemens Linear Motor Systems, utilise this feature by optimising the mounting position. This means that almost none of this dangerous material comes close to the motors. Further, the linear motors are completely encapsulated and are additionally shielded by combined covers—for example, using bellows and telescopic covers. And even if the dust penetrates the secondary section of the drive, a 'curtain' of sealing air or overpressure prevents it from penetrating and blocking the air gap.

The long-term precision of the linear motor speaks for itself. For applications in grinding machines, it is possible to significantly reduce the main machining times—up to a factor of five. It is therefore quite easy to estimate the potential that the linear direct drive has in this application area.

The possibility of combined machining using different technologies, such as hard turning and grinding, is interesting. In this case, the dynamic performance of the motor can be fully utilised when turning, and it provides the necessary precision when grinding. Schaudt Mikrosa BWF's STRATOS M machine is equipped this way.

Impulse neutralisation

When it comes to high-tech, linear motor specialists are expecting the next quantum leap in the near future: the so-called impulse neutralisation. With this concept, the secondary section is designed so that it can move in a longitudinal direction to absorb the jerk that normally has a significant impact on the machine bed as a result of the high dynamic performance. Such a solution has already been implemented in an elliptical machining lathe. In this particular case, acceleration levels of 30g and more with a jerk of 100,000 m/s2 could be achieved.

However, the associated control technology and mechanical design for impulse neutralisation are still extremely costly. For this reason, these solutions are only interesting for highly dynamic applications.

Siemens engineers are hoping a new material will significantly simplify the situation. This is a material that is presently in the development phase as part of the EffeNDi research project. In the future, the secondary section could be de-coupled from the machine bed using this material.

According to Joachim Berkemer, the Project Manager, basic concepts are already available: 'However, it will be about a year before we can present the first solutions,' he says. 'It should be possible to implement a broad range of applications after an additional year in the test phase.'

This technology will then be available for general machine tool construction. In many cases, it may be possible to change over from conventional spindle to linear drives without re-designing the machine, as the stability of the mechanical design would then no longer be a problem.

Thermal problems

In addition to the many advantages that a linear direct drive has, there are some well known problem areas. For example, high power consumption and the corresponding temperature increase are often negative features cited by linear drive sceptics.

But linear motor manufacturers have a handle on the thermal issues.

For instance, Siemens Linear Motor Systems has developed a so-called thermo-sandwich design. It is made up of a power cooler that dissipates most of the thermal energy, and a precision cooler that dissipates the rest. Using this technique, the 100 degree temperature developed by the linear motor in continuous operation in its 'hot' core can be significantly cooled down within a few millimetres to the surface of the drive. The temperature difference between the motor and the interfaces to the machine are, depending on the motor type, not more that 2 to 4 degrees above the cooler intake temperature.

The fact that a linear motor generates a lot of heat is an indication of unused energy. In discussions it is often considered to be an energy waster.

It is true the linear motor has relatively high power losses. But this should not be considered in isolation, and the rated power losses alone should not be used to make a comparison between a linear motor and a ballscrew drive.

The energy consumption of the complete machining process is of interest and when this is taken into consideration, the situation is quickly clarified. Further, for this evaluation, an apple for apple comparison should be made.

There is a good reason why many suppliers of ballscrews in the market offer water-cooled spindles. When these conventional drives move at 120 m/min and accelerate with almost 2g, there is no way around water cooling.

And, by the way: At these rates of acceleration and speed, the linear motor with its various advantages starts to come into its own.

thorsten.rabenschlag@siemens.com