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Active Vibration Control vs. Passive Vibration Control - Part 2

In part one of our video Wes explained the difference between Active and Passive isolation systems. In this section he continues to discuss pneumatic isolators and Parallel-Type active vibration control.

To recap, typical applications for passive pneumatic isolation are:
• High magnification optical microscopes
• Atomic Force Microscopes (AFM)
• Confocal microscopy
• Electrophysiology applications
• Other life science research techniques
Pneumatic Isolator

An example of a platform using pneumatic isolators is TMC’s CleanBench with Micro-g isolators. The CleanBench is a laboratory table supported by pneumatic isolators with self-leveling height control. While this provides excellent vibration isolation above roughly 4Hz, it is a soft isolation system and will deflect in response to a payload force or impact to the tabletop (for instance a person leaning against it, or a moving stage). With good damping and automatic leveling, the payload will settle within a few seconds which is acceptable for many applications. For faster settling time, or just more stability, many customers choose to use our MaxDamp isolator legs.

If even faster settling time is required, particularly for applications that include an automated rotation or translation stage, a specific vibration control system known as Parallel-Type active vibration control is used. A Parallel-Type vibration control system typically includes a pneumatic isolator with non-contacting electro-pneumatic height control. Active damping of the payload is added using linear motors as an actuator in parallel with the spring. When this is combined with a feedback system, it is exceptionally good at canceling deflection due to stage motion. The Parallel-Type system is a high-performing, complex system that is more involved to set up and tune but provides superior performance compared to a simple pneumatic isolator, particularly to control and cancel payload-generated vibration and motion.

Parallel-Type Active

Parallel-Type systems, such as TMC’s Electro-Damp, are typically designed in to machines, supporting payloads that have a moving stage. When the stage moves, a pneumatic spring would normally deflect easily, but by adding the linear motor in parallel with the pneumatic spring as part of a feedback system, it sends a reaction force to the payload to cancel the deflection of that payload. Another benefit is it can be connected to the stage of the tool and information from the stage can be used in a feed forward manner to cancel that stage motion more aggressively. Semiconductor metrology, requiring very fast settling times after moving the wafer, demanding high throughput, is a classic example of an application requiring this type of active control inside the tool.

Parallel-Type Diagram

This is clearly an active vibration control system as it contains a sensor, a feedback loop and a force actuator. Isolation from floor vibration is defined by the spring used, in this case the pneumatic isolator. Parallel active vibration control systems are ideal for minimizing deflection from moving stages, but do not provide the highest attenuation of floor vibration, particularly at low frequencies. To learn how to get the best vibration isolation at low frequency, tune in next time when Wes discusses active serial vibration isolation systems, such as TMC’s STACIS system.

Click here to see part three.

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