Precision Matters: Designing for Nanometers · 2013. 10. 30. · 1 of 24 2007 MIT LMP Manufacturing Summit Precision Matters: Designing for Nanometers Stephen Ludwick, Ph.D. [email protected]

1 of 24 2007 MIT LMP Manufacturing Summit

Precision Matters: Designing for Nanometers

Stephen Ludwick, [email protected]



Aerotech: Big parts, small features

• Sub-micron motion control needed over centimeters of travel.

• Payloads measured in kg

• “Off the Shelf” design

• Industries– Semiconductor– Data-storage– Medical devices– Optics


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Core Technologies and Capabilities –Submicron Precision Mechanical Systems

Air Mechanical Vacuum



Software Electronics Mechanics

Core Technologies and Capabilities


Typical High-Precision Stage System

• Aluminum or Steel Construction• Air, crossed-roller, or recirculating ball bearings• Linear motor drive• Encoder Feedback (0.2 - 50 nm resolution)• Micron-level error motions• Sub-micron repeatability (or better)• Nanometer-level incremental motions


Enabling Technologies

• High-quality bearings

• Direct-drive motors

• High-resolution, long travel position encoders

• “Open-architecture” PC-based controllers.


Bearings

• Rolling-element bearings dominate market– Economical, adapt to many environments– Friction difficult to quantify

• Air-bearings common– Almost frictionless (except for cables)– Difficult to miniaturize

• Flexures used in short-travel applications– Travel limited to hundreds of microns


Crossed-roller, linear bearing stage; interferometer metrology

Repeatability can be a few nanometers at short time scales.


Effect of Friction on System Response

• Ball bearings look like springs at small motions; effect must be considered at nanometer scales.

Small Signal Inputs

Large Signal Inputs

Frequency

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Key Dates for Linear Motors

• 1890’s – Linear motor concept described

• 1966 – Samarium Cobalt magnets• 1983 - Neodymium-Iron-Boron magnets

• 1992 – Linear Motors in Aerotech catalog• 1994 – Linear Motor Stages added as

Aerotech standard product offerings.


Lorentz Force on a Coil

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Figure by A. Hazelton


Typical Direct-Drive Motor Designs

• Force applied directly to payload; no transmission• Higher-speeds, higher-bandwidth system• Enabled by high-strength magnets designs


Feedback Element: Linear Encoders

• Tape scale released in early 1990’s.– 20 um fundamental period– Resolution < 5 nm

• Some scales available with 200-400 nm period.

• Sub-nanometer resolution after interpolation common.


Alternate Feedback Devices

• Laser interferometers– Standard in highest-performance systems– Atmospheric effects troublesome

• Capacitance probes– Excellent over short travel (hundreds of microns)– Limited by noise in digital conversion.

• MEMS inertial sensors– Lots of opportunity


Position Stability

• Rolling-element bearings, 350 mm travel.• Linear motor drive, encoder feedback


PC-Based Controllers

• First popular open-architecture, PC-based controllers available in 1980’s.

• Still very common


Typical Controller Capabilities

• Windows-based interface• Conversational programming language• Multi-dimensional mapping capabilities• Digital servo loop closure (PID) at 5-20 kHz• Integrated encoder multipliers (up to 65536x)• Extensive setup and tuning wizards


Loop Transmission Tuning Utilities

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PID Algorithm Amp, Motor, and Stage

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Advanced Control Capabilities

• Learning control for repeated motions

• Cross-axis compensation (multivariable)

• Model-based feedforward algorithms

• Friction compensation


Present Controller Paradigm

• Distributed digital control– Multiple networked drives – scalable– Amplifiers and controllers merged– Rely on software for all interfacing


Future Precision Systems

• All components integrated into one “system” tuned for highest performance.

• Similar to consumer electronics (e.g. printers)


Take Away

• “Commercial Grade” motion control possible at nanometers.

• Advances made possible by improvements to all components.

• Mechanics, electronics, and controls in future systems will be tightly coupled.


www.aerotech.com

Aerotech… Your Worldwide Partner in Advanced Motion Control and Automation Solutions

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Precision Matters: Designing for Nanometers · 2013. 10. 30. · 1 of 24 2007 MIT LMP Manufacturing Summit Precision Matters: Designing for Nanometers Stephen Ludwick, Ph.D. [email protected]