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Passive vs. Active Vibration Isolation for AOI and Metrology Machines
Euchang Tech Engineering Team · Technical note · October 2026
Summary. Passive isolators (air springs, elastomers) are mechanical low-pass filters: they amplify vibration near their natural frequency fn and only start to isolate above √2·fn, so a ~3 Hz air-spring system begins isolating at about 4.2 Hz. Active systems add sensors, a controller and actuators to damp the resonance, extend isolation to lower frequencies and, with feedforward from the stage, cancel the machine's own motion reactions. A peer-reviewed analysis found that properly selected, well-damped passive isolation meets the requirements of most precision equipment; active isolation is justified mainly where the floor is poor at low frequency or stage reaction forces dominate.
1. Two kinds of disturbance
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Floor (base) excitation: building vibration from HVAC, pumps, traffic and nearby tools, transmitted up through the isolators.
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Payload (direct) forces: reaction forces from the machine's own stages and robots, which act on the isolated mass and make soft mounts rock.
The two pull the design in opposite directions: soft mounts isolate the floor better but let the machine tilt more when its stage accelerates [1].
2. Facility criteria: the VC curves
Vibration-sensitive facilities are commonly specified with the generic vibration criterion (VC) curves, expressed as RMS velocity in one-third-octave bands; the curves span 4–100 Hz (constant-velocity limits apply from 8 Hz), and Gordon notes that air-spring isolators resonate at 1–3 Hz, below the original range [2]:
| Curve | RMS velocity | Typical use (per Gordon 1999) |
|---|---|---|
| VC-A | 50 µm/s | Optical microscopes to 400×, microbalances, proximity/projection aligners |
| VC-B | 25 µm/s | Optical microscopes to 1000×; inspection and lithography to 3 µm line width |
| VC-C | 12.5 µm/s | Most lithography and inspection equipment to 1 µm detail size |
| VC-D | 6 µm/s | Most demanding equipment incl. TEM/SEM and e-beam systems |
| VC-E | 3 µm/s | Long-path laser-based and other extraordinarily sensitive systems |
Your tool's own vibration specification, measured on its actual floor, should drive the isolation choice.
3. Passive isolation: the physics
For a single-degree-of-freedom isolator with damping ratio ζ and frequency ratio r = f/fn, transmissibility is
T = √[(1 + (2ζr)²) / ((1 − r²)² + (2ζr)²)]
| Region | Behaviour |
|---|---|
| r ≪ 1 | T ≈ 1 — the system follows the floor |
| r ≈ 1 | Resonance; peak T ≈ 1/(2ζ) for light damping |
| r = √2 | T = 1 for any damping — isolation begins here |
| r ≫ 1 | T falls toward −40 dB/decade (undamped); viscous damping reduces this toward −20 dB/decade |
Example: an air-spring system with fn ≈ 3 Hz amplifies around 3 Hz, starts isolating at ≈4.2 Hz, and at 30 Hz (r = 10) an undamped model gives T ≈ 1/99 ≈ 1 %. More damping lowers the resonance peak but costs some high-frequency isolation — the classic trade-off.
Strengths: no electronics, no power, very reliable, low cost; effective above a few hertz. Limits: the resonance itself; soft mounts allow rocking and long settling after stage moves; isolation below ~√2·fn is impossible.
4. Active isolation
An active system adds inertial sensors (geophones or accelerometers), a real-time controller and actuators (voice-coil or piezo), usually in parallel with passive air springs.
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Feedback ("skyhook" damping) suppresses the resonance peak and extends isolation to lower frequencies.
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Feedforward from the stage's trajectory generator applies counter-forces as the stage accelerates, reducing tilt and settling time.
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Costs: power, cabling, commissioning and tuning, interaction with structural modes, higher price.
5. Comparison
| Aspect | Passive (air spring / elastomer) | Active (sensors + actuators) |
|---|---|---|
| Isolation starts | ≈ √2·fn (≈ 4 Hz for a 3 Hz air spring) | Lower, depending on sensors/controller |
| Resonance | Amplification peak 1/(2ζ) | Actively damped |
| Stage-induced rocking | Larger; longer settling | Reduced with feedforward |
| Utilities | Clean compressed air | Electrical power (+ air if hybrid) |
| Complexity | Leveling and air regulation | Controller set-up and tuning |
6. Choosing in practice
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Measure the floor in one-third-octave bands and compare with the tool specification.
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If the problem is above ~5 Hz, a well-damped passive system on a stiff, heavy base is usually enough [1].
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If the floor is poor below ~4 Hz, or step-and-settle throughput is limited by stage reactions, evaluate active or hybrid isolation.
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In all cases the structure above the isolators must be stiff, so that its internal resonances lie well above the stage servo and isolator control bandwidths. A heavy granite base also lowers the ratio of moving mass to isolated mass, which directly reduces rocking.
Where Euchang fits
Euchang machines precision granite bases and structures (single piece up to 4,000 × 2,500 × 300 mm and 10 t, flatness to 0.015 mm depending on size and grade) and ceramic structural parts that sit on top of whichever isolation system you select. We can design the base around your isolator mounting points and stage layout.
FAQ
Why does isolation only begin at √2 times the natural frequency? Below that ratio the transmissibility of a spring–mass system is greater than or equal to one, so floor motion is passed through or amplified; above it, transmissibility drops below one.
What natural frequency do pneumatic isolators have? Gordon (1999) cites 1–3 Hz as typical air-spring resonances; a 3 Hz system starts isolating at about 4.2 Hz (√2 × 3 Hz).
Can passive isolators be upgraded to active later? Sometimes, if there is space and load margin for actuators and sensors, but the control loop interacts with the structure's modes, so the assembly must be re-evaluated.
What is hybrid isolation? Passive air springs carry the static load; active actuators in parallel damp resonance and counter stage forces.
Does a heavier base help? Yes for payload disturbances: more isolated mass means less tilt for the same stage force. It does not change floor isolation, which depends on fn and damping.
References
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E. I. Rivin, "Vibration isolation of precision equipment," Precision Engineering 17(1) (1995) 41–56. https://doi.org/10.1016/0141-6359(94)00006-L
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C. G. Gordon, "Generic vibration criteria for vibration-sensitive equipment," Proc. SPIE 3786 (1999). https://doi.org/10.1117/12.363802 (full text: https://www.octava.info/files/%D0%A1%D1%82%D0%B0%D1%82%D1%8C%D0%B8/Gordon-SPIE99.pdf)
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C. G. Gordon, "Generic criteria for vibration-sensitive equipment," Proc. SPIE 1619 (1992) 71–85. https://doi.org/10.1117/12.56826
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E. I. Rivin, "Vibration Isolation of Precision Objects," Sound and Vibration, July 2006. http://www.sandv.com/downloads/0607rivi.pdf
Author: Euchang Tech Engineering Team, EUCHANG TECH CO., LTD., Taichung, Taiwan.
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