Figure 1 shows a demonstrator, fitted to a laser cutting machine using a conventional planar motor with reluctance actuators formed using hybrid electromagnets (fig. 2).
This motor uses a Philips Applied Technologies-designed hybrid variable reluctance actuator consisting of an iron core, a permanent magnet and a current conducting coil. Due to the magnetic field from the permanent magnet, the iron core is attracted to a steel counterpart; when current is applied to the coil, either positive or negative, the flux from the permanent magnet is enhanced or reduced respectively, and as a result, a force actuator is created. The magnitude of the indirect force generated between the reluctance actuator and the steel counterpart is related non-linearly to the flux carried by the coil.

A second conventional linear planar motor uses Lorentz actuators. The Lorentz effect generates a direct force as a result of the interaction of a magnet and current carrying coil. The magnet generates a field inside the coil that acts on the charge carriers producing the localized direct force.

More recently, Philips Applied Technologies has unveiled its inverted planar technology, called Planar Maglev (see figure 3). This is a unique development in maglev technology using Lorentz actuators to provide the motive force. The free-floating platform houses the permanent magnets (in a chess-board pattern where the “black” squares are “north”, and the “white” squares are “south”) while the fixed bed contains the coils. The force generated by the Lorentz actuators is related linearly to the flux carried by the coils.

 

 

According to Peter Frissen, mechatronics engineer with Philips Applied Technologies and project leader for Planar Maglev, by using Lorentz actuators, the load distribution over the coils depends on the location of the floating stage. In contrast, the conventional planar motors (with moving coils and either Lorentz or variable reluctance actuators) carry an equal load all over the workspace no matter where the moving platform is located.

Advantages of Planar Maglev technology
Both the inverted Planar Maglev’s and the conventional planar motors’ actuators dissipate energy and generate heat when current is applied to the coils. Forced cooling is necessary to prevent the system from overheating. Because the coils for conventional planar motors are mounted on the floating platform itself, the hoses carrying the coolant (together with the cables carrying the coils’ power supply) have to be attached to the plate itself. This effectively “tethers” the plate, restricting its movement to a stroke determined by the length of the cooling hoses.

In comparison, the inverted Planar Maglev’s coils are embedded on the fixed bed, leaving the floating plate unencumbered. Consequently the plate’s movements are completely unrestricted, with the stroke limited only by the dimensions of the fixed bed. If data or power is needed on the floating platform (for example, to operate a gripper) it can be transmitted wirelessly.

The elimination of the cooling hoses also removes a source of contamination when the platform is working in a high-vacuum environment. Moreover, it allows the supported component to be transported through a number of process steps without being transferred to a different platform. The limited stroke of a moving coil configuration makes this impossible. This is a key advantage in semiconductor wafer fabrication because it limits the number of times the wafer has to be handled. Wafer handling compromises accuracy, extends processing time, and increases the risk of wafer damage.

The Planar Maglev is mechanically less complex than a Lorentz actuator conventional planar motor, making it less expensive to manufacture. For customers this means it is easier to design-in, has a lower cost of ownership, and reduced maintenance costs. “Much of the complexity of the system has been moved from the mechanical elements to the system’s electronics and software,” says Frissen. “In effect, we have ‘digitized the mechatronics’.”
Finally, the Planar Maglev is easier to control than the reluctance actuator conventional planar motor because the motive forces vary linearly with coil flux. In contrast, indirect forces that vary non-linearly with coil flux move the reluctance actuator conventional planar. This means the inverted planar motor, with further development, has the potential to be the more accurate maglev technology. The inaccuracies of the magnets and coils can be calibrated out by ‘self learning calibrations’ (to less than 0.1 %).

(Note: it is possible to manufacture an inverted motor using variable reluctance actuators. However, due to the actuators’ design, the system would require split axes with two linear long-stroke stages for “unlimited” planar motion. By using Lorentz actuators, Philips Applied Technologies’ inverted planar motor is a single stage design boasting “unlimited” motion in X and Y directions.)

The third Philips Applied Technologies’ design, the Lorentz actuator conventional linear planar motor, boasts the simplicity and ease of control of its inverted counterpart, but does not have the “unlimited” stroke because of its moving coil design.

However, according to Prof. van Eijk the reluctance actuator conventional planar motor does have some key advantages over the inverted planar motor that will ensure its adoption in many general industrial applications. “Chief among these is its much lower power consumption and less complex electronics,” he notes. “In addition, because the magnetic fields generated by the actuators can be shielded, the platform can be used in proximity to charged particle beam applications such as e-beam lithography. That’s not possible with the Planar Maglev technology.”

Planar Maglev positional control
Controlling the movements and positioning of a free-floating maglev platform is a complex engineering challenge. The key problem is to determine the position of the platform at any moment using an appropriate metrology system and then to control the actuators so the platform moves to the desired new position. For a system with six degrees of freedom this demands considerable computing power and sophisticated software.

Philips Applied Technologies’ engineers have spent almost an entire decade developing “Soft Motion” software. Soft Motion was first developed to control the variable reluctance conventional linear planar motor, and is now used as the basis for controlling the inverted planar motor. The key to accurate control is Philips Applied Technologies’ in-depth understanding of the dynamics of six-axis maglev systems.

The software works by constructing a model of the maglev system using data from the metrology system’s sensors. It then computes the forces required to move the platform to a new position using the model. Finally, these forces are translated into the commands required to generate the real forces via the electronics and actuators. The system operates as a tightly controlled closed loop.

“The software is the key enabler that makes the relatively simple mechanics of the Planar Maglev maneuver in six axes to nanometer precision,” says Frissen. “It is something that requires efficient algorithms developed over years of studying the dynamics of maglev systems and is not something that can be easily duplicated by others.”

Potential applications
Philips Applied Technologies’ variable reluctance actuator conventional linear planar motor uses operational principles that are also used as the basis for magnetic bearings for rotary applications. This technology is ready for commercial adoption and is widely accepted as a practical solution by the engineering community. Because it features lower power consumption than the inverted planar motor, and can be shielded to mitigate the effects of stray magnetic fields, the variable reluctance actuator conventional planar motor is applicable to many industrial processes requiring a precision positioning platform. Examples include laser cutting, laser drilling, e-beam lithography and e-beam inspection.

Philips Applied Technologies has targeted next generation semiconductor nano-imprint lithography and optical wafer inspection as the first practical application for the variable reluctance actuator conventional planar motor. “There are many different ways to build the next generation of lithography tools but the one with the most potential is the floating platform architecture based on magnetic levitation. It’s an essential development and will be the dominant factor in achieving the next level of accuracy,” says Prof. van Eijk.

The Lorentz actuator conventional planar motor has a number of advantages that make it suitable for particular applications such as the moving of lithography stages in vacuum chambers. The Planar Maglev suits niche applications where its inherent advantages of “infinite” stroke, compact and clean design, and resistance to aggressive (chemical) conditions override its additional cost. “The inverted Planar Maglev technology is attractive for all applications where you want to move a flat substrate with high accuracy relative to a process, ” explains Prof. van Eijk “Tools work better if you are able to bring the point of interest more accurately under the microscope or probe into processes working at the nanometer scale.”

Future developments
The ultimate accuracy of maglev technologies depends highly on the metrology systems used to feedback the platform’s position to the supervisory computer, the disturbance-level to the platform and the servo-control bandwidth that is used to reject these disturbances. The metrology system used for the inverted planar motor is based on Hall sensors, providing a positional accuracy of ± 0.1 mm. For practical applications this needs to improve to at least ± 10 nm and on to ± 1 nm which requires a laser interferometer-based metrology system and further software development. It should be noted that current two-stage platforms already approach this accuracy, but Philips Applied Technologies is aiming to achieve this precision with a single, mechanically simple, long stroke stage.

One technical limitation with laser interferometry is that it registers the moving platform itself and not the work piece. Precision is compromised due to inaccuracies generated by the system’s finite stiffness introducing uncertainties in the relative positions of the platform and work piece. To resolve this problem, Philips Applied Technologies is investigating metrology systems that reference from the point of action on the work piece instead.

Philips Applied Technologies will pursue commercial opportunities for its maglev technologies via partnerships with leading precision positioning platform manufacturers. “Maglev is ready for industrial use and is supported by a body of knowledge from Philips Applied Technologies that is being added to continuously,” says Prof. van Eijk. “There is a lot of general research on maglev using both reluctance and Lorentz actuators. Philips Applied Technologies continues to research and develop both types and our conventional maglev platform using reluctance actuators is close to commercialization while the Lorentz actuated inverted planar motor demonstrates one of the more exotic things we can do with magnetic platforms.

“While reluctance-actuated maglev platforms have been used for rotary applications they feature only very short strokes; Philips Applied Technologies planar motors boast X and Y strokes of about half a meter. Both our planar motor platforms are unique developments with no parallels anywhere else in the world.”

 

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Press inquiries please contact:

US West Coast J. David Hammett Philips Applied Technologies North America 1101 McKay Drive M/S 16 San Jose, CA 95131 USA Phone: +1 408 474-9001 (phone) Fax: +1 408 474-9055 (fax) E-mail: j.david.hammett@philips.com	Europe Joost Maltha Philips Applied Technologies High Tech Campus 5 5656 AE Eindhoven The Netherlands Phone: +31 40 27 48882 E-mail:joost.maltha@philips.com
US East Coast Ton Peijnenburg Philips Applied Technologies North America 201 South Johnson Road Building 1, Suite 102 Houston, PA 15342 Phone +1 724) 743 5372 Fax +1 724) 743 5371 Cellular +1 408) 458 0325 E-mail: a.t.a.peijnenburg@philips.com

 

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