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Poetry in motion
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28/01/2008
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A linear motor that allows movement in two axes could pave the way for lighter, more controllable precision motion systems. Jonathan Ward reports
High precision motion control is one of the unsung heroes of the semiconductor industry. As chip makers push the limits of the physics to pack more on to the surface of a silicon wafer, so the machines used to manipulate and inspect the chips must become ever more precise. The drive to create high precision motion control systems has tested the ingenuity of machine designers. At the same time, challenging operating conditions expose problems with conventional technologies.
Rolling element bearings can be built to extraordinary precision for extremely smooth motion. Yet at a microscopic scale, slight imperfections can produce damaging noise and vibration. Air bearings suggest themselves as a promising alternative, but many critical silicon-processing operations take place in a vacuum, making their use unfeasible. These challenges have led designers to explore the use of magnetic levitation systems for motion control. They deliver smooth, precise motion control in demanding conditions. They can allow the same machine elements to deliver both support and motion functions – and, critically, they allow active software control of the position of a component.
Philips Applied Technologies in the Netherlands is one organisation that has extensive experience of magnetic motion control technology. The company has developed a number of technologies that use different types of actuators to control the position and of free-floating platforms magnetically. Along the way, it has produced high-precision platforms, with six degree of freedom, for inspection and chip fabrication applications. Until now, however, most of these magnetic levitation designs have relied on complex, highly bespoke components, limiting their market to very high end uses.
Now motion control specialists at the company have taken a different approach to the problem, producing an innovation that allows extremely accurate three-dimensional motion control, using conventional linear motor components. Philips claims that the advantages of its new technology, which it calls ‘Nforcer’, will be felt beyond only high-end applications such as chip manufacture. The low cost of the new approach will also enable it to be used economically in a wide range of more general motion control applications.
At the heart of NForcer is a modification to conventional linear motor design. In a standard ironless, multi-phase linear motor, a set of conducting coils sits between two rows of magnets. By applying a current to the coils in sequence, a force is generated that pushes the coils along the track. In a standard motor, just the vertical sides of the coils sit in the magnetic field. As a result, the motor only generates lateral motion. To achieve two-dimensional motion from one motor, the technologists at Philips have shifted the position of the coils, with respect to the magnet track. Now, the lower horizontal section of the coils also sits in the magnetic field, where it generates force and consequent motion in the vertical direction.
As the horizontal and vertical forces are perpendicular to one another, they do not interact. This makes control of the new motor design straightforward, according to Dr Georgo Angelis, senior scientist at the company.
“To control the second axis, we just add a second commutation algorithm to the software controller, which sends appropriate signals to the linear motor amplifier and appropriate currents to the coils,” he explains.
The NForcer technology can be used to build full floating, bearingless platforms that will operate in vacuum conditions. But while the new design provides a solution to several of the key challenges in high-end motion control for chip production, it offers some other advantages too.
The ability to deliver two axes of motion from a single set of linear motor hardware and control electronics means the control hardware takes up less space and has less mass. This gives machine designers the option of building smaller, faster-moving machines – which are therefore more productive.
Cutting down on hardware and making use of conventional, off-the-shelf components has significant cost benefits as well, and it is this aspect of the NForcer technology that will open up a host of potential new markets.
“We think that pick-and-place assembly equipment might be an ideal application,” notes Angelis.
One of the company’s first demonstrations of the NForcer concept is a single-track roller bearing pick-and-place mechanism, with 20-cm horizontal and 3-cm vertical movements, achieved with a single forcer.
Combinations of NForcer actuators can also deliver complex multi-axis movement with high accuracy. Using only six forcers in four magnet tracks, the Philips team has produced platforms that move in six axes – a long x axis, shorter y and z axes and a few milli-radians of tilt and twist – to “nanometre accuracy”. Such devices, says Angelis, will be highly desirable in silicon inspection applications.
“Chip makers need their inspection equipment to follow tiny surface irregularities,” he says. “This technology can do that in a compact, mechanically simple design.”
Philips says that NForcer is available for commercial exploitation today. It expects initial applications to be in chip production equipment, which it is developing with partners. At the same time, it has not ruled out licensing the technology – which is protected by a number of patents – to manufacturers of conventional linear motors.
“Around half the linear motors on the market today could adopt NForcer technology very easily,” states Angelis. “The others have mechanical designs that make it less straightforward, but there are so many benefits to this technology that we are expecting a lot of interest.”
Pointers
* Modification of coil design has led to a linear motor that can move in two axes
Initial target is the semiconductor industry, but it could also be used in less demanding applications such as pick-and-place machines
* Two axes of motion are delivered from a single set of linear motor hardware and control electronics, leading to smaller, faster-moving machines
The 2008 Linear Motion Masterclass is now confirmed. The event takes place at the Kaetsu Centre in Cambridge on Tuesday 18 March.
Sponsors are Aerotech (linear motors), Igus (bearings), Renishaw (encoders) and Steinmeyer (ballscrews).
The event is the latest in Eureka’s series of Design Days, which combine technical presentations with hands-on workshops to help design engineers understand aspects of new technology
For more information go to www.designdays.co.uk/
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Author
Tom Shelley
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