Precision Matters:
Designing for Nanometers
Stephen Ludwick, Ph.D.
[email protected]
1 of 24
2007 MIT LMP Manufacturing Summit
2 of 24
2007 MIT LMP Manufacturing Summit
Aerotech: Big parts, small features
•
Sub-micron motion control
needed over centimeters
of travel.
•
Payloads measured in kg
•
“Off the Shelf” design
•
Industries
–
–
–
–
3 of 24
Semiconductor
Data-storage
Medical devices
Optics
2007 MIT LMP Manufacturing Summit
www.AEROTECH.com
Dedicated to the Science of Motion Since 1970
ISO 9001 Registered as of 1995
4 of 24
2007 MIT LMP Manufacturing Summit
Core Technologies and Capabilities –
Submicron Precision Mechanical Systems
Air
5 of 24
Mechanical
2007 MIT LMP Manufacturing Summit
Vacuum
6 of 24
2007 MIT LMP Manufacturing Summit
Core Technologies and Capabilities
Software
7 of 24
Electronics
2007 MIT LMP Manufacturing Summit
Mechanics
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
8 of 24
2007 MIT LMP Manufacturing Summit
Enabling Technologies
• High-quality bearings
• Direct-drive motors
• High-resolution, long travel position encoders
• “Open-architecture” PC-based controllers.
9 of 24
2007 MIT LMP Manufacturing Summit
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
10 of 24
2007 MIT LMP Manufacturing Summit
Crossed-roller, linear bearing stage; interferometer metrology
Repeatability can be a few nanometers at short time scales.
11 of 24
2007 MIT LMP Manufacturing Summit
Effect of Friction on System Response
Magnitude
Large Signal Inputs
Small Signal Inputs
Frequency
• Ball bearings look like springs at small motions;
effect must be considered at nanometer scales.
12 of 24
2007 MIT LMP Manufacturing Summit
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.
13 of 24
2007 MIT LMP Manufacturing Summit
Lorentz Force on a Coil
B
B
i
i
F
Fcoil = li × B = nturn liB
Figure by A. Hazelton
14 of 24
2007 MIT LMP Manufacturing Summit
nturn = number of wires in coil
Typical Direct-Drive Motor Designs
• Force applied directly to payload; no transmission
• Higher-speeds, higher-bandwidth system
• Enabled by high-strength magnets designs
15 of 24
2007 MIT LMP Manufacturing Summit
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.
16 of 24
2007 MIT LMP Manufacturing Summit
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
17 of 24
2007 MIT LMP Manufacturing Summit
Position Stability
•
•
Rolling-element bearings, 350 mm travel.
Linear motor drive, encoder feedback
18 of 24
2007 MIT LMP Manufacturing Summit
PC-Based Controllers
• First popular open-architecture, PC-based controllers
available in 1980’s.
• Still very common
19 of 24
2007 MIT LMP Manufacturing Summit
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
20 of 24
2007 MIT LMP Manufacturing Summit
Loop Transmission Tuning Utilities
w
e
r
-
21 of 24
C
PID Algorithm
u2
u1
P
Amp, Motor, and Stage
2007 MIT LMP Manufacturing Summit
y
Advanced Control Capabilities
• Learning control for repeated motions
• Cross-axis compensation (multivariable)
• Model-based feedforward algorithms
• Friction compensation
22 of 24
2007 MIT LMP Manufacturing Summit
Present Controller Paradigm
• Distributed digital control
– Multiple networked drives – scalable
– Amplifiers and controllers merged
– Rely on software for all interfacing
23 of 24
2007 MIT LMP Manufacturing Summit
Future Precision Systems
• All components integrated into one “system” tuned for
highest performance.
• Similar to consumer electronics (e.g. printers)
24 of 24
2007 MIT LMP Manufacturing Summit
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.
25 of 24
2007 MIT LMP Manufacturing Summit
Aerotech… Your Worldwide Partner in Advanced
Motion Control and Automation Solutions
Questions ?
www.aerotech.com
26 of 24
2007 MIT LMP Manufacturing Summit