DSP technology keeps stage aimed straight

July 11, 2001
Triathlon X-Y stages couple the airbearing stage directly to the lapped granite base plate to provide superflat travel.

Triathlon X-Y stages couple the airbearing stage directly to the lapped granite base plate to provide superflat travel.


Adigital signal processor implements dual servoloop control on the axes of an air-bearing stage. The resulting motion is extremely straight (to ±0.5° rotation) and smooth.

Triathlon Series of air-bearing X-Y stages from Dover Instrument Corp., Westboro, Mass., dynamically correct the yaw motion between the dc linear motors driving the X and Y axes. The angle correction values compensate for guideway mechanical errors. The on-board DSP can also make fine angular adjustments in the stage to handle part or fixture error or misalignment.

The motion controller employs a PMAC control card from Delta Tau in Chatsworth, Calif. The Motorola DSP on the card implements servoloops that can be tuned individually because of the decoupled dynamics of the stage's linear and rotational degrees of freedom. The resulting servosystem has a high bandwidth and displays no cross coupling between the parallel motors.

An advanced, magnetically preloaded air bearing rides along a precision-ground, nickel-plated steel rail to provide straightness in the Y axis. For super-high levels of straightness, error compensation via look-up table provides a compensated straight path for the servosystem to follow.

Both axes have low inertia, leading to fast step and settle times. Special software algorithms develop a motion profile that takes into consideration the natural resonance frequencies of the structure, then modifies rates of acceleration and deceleration so as to avoid exciting the system at these frequencies.

The stages use a method of direct interpolation that produces 5-nm resolution at speeds reaching 2,000 mm/sec. A licensed algorithm for yaw control makes use of dual encoder feedback in the control loop of the Y axis to compensate yaw and straightness in the X-axis direction of travel. The technique provides small rotary motions of the X axis to actively control yaw of the X-axis rail. The same technique can make fine angular adjustments in the stage to accommodate for substrate or fixture error or misalignment.

Positional repeatability on the clean-room-compatible units is <±0.25 m over standard travel envelopes that can range as much as 900 900 mm.

Thermoplastics conduct heat but not electricity
The first-ever line of thermoplastics designed to conduct heat while remaining electrically insulative recently emerged from LNP Engineering Plastics. Called Konduit, they are a composite of plastic resins and thermally conductive additives that overcome the base resins' natural insulation properties. They provide 10 to 50 times more thermal conductivity than typical unfilled and reinforced thermoplastic materials.

Expectations are that heat sinks injection molded from the new compound can do double duty as structural parts. Use of the material to encapsulate transformers as found in neon signs, fluorescent lights, and other applications may reduce the number of windings and steel laminations required in these components, thereby reducing their size.

The Konduit line will sport two levels of thermal conductivity. Low-cost resins will provide 2 W/m°k through use of a ceramic additive. A high-performance version provides up to 10 W/m°k and employs a specialty carbon fiber additive.

LNP Engineering Plastics Inc., 475 Creamery Way, Exton, PA 19341-2537, (610) 363-4500, www.lnp.com

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