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Active Vibration Control vs. Passive Vibration Control - Part 1

In part one of a three-part series, Wes Wigglesworth, TMC’s Product Manager for Active Systems explains the difference between active and passive vibration control. Some passive systems, such as TMC’s PEPS system can have features of active control systems, but they are not active vibration control systems. Wes explains the details. 

The key elements required to call something “Active vibration control” are an inertial sensor (that measures the vibration), a controller with a feedback (or feedforward) loop, and an actuator. The term “active vibration control” is sometimes used  to describe passive vibration control, and the terms are often confused. 

Active Vibration Control

In the industry, many things can be considered a passive isolator. This would include a simple spring, a rubber damper, or an air bladder between two plates. This type of isolator supports a payload and attenuates vibration at high frequencies (like a low pass filter, low frequency passes through). The performance of the system is dependent on the resonance of the springs. They are easy to set up, but there is no height control, and they are low performance.

Active Vibration Control 2

Typical applications for simple passive rubber mounts could include:
Optical microscopes that have low power and low magnification
Reducing the vibration impact of certain machinery in a factory, like pumps or compressors
Other things that are not extremely sensitive to floor vibration 

The next step up from a simple passive isolator, is a pneumatic isolator, like the TMC Micro-g. While these systems are a little more difficult to set up, a pneumatic isolation system provides better vibration isolation than a rubber mount or a steel spring because it is a softer isolator with a resonant frequency of around 2 Hz. These isolation systems usually include a mechanical self-leveling system that links to the payload and re-levels the payload in response to a deflection or motion. 
Active Vibration Control3

Typical applications for pneumatic isolators could include:
Confocal microscopy
Patch Clamping
Atomic Force Microscopy
Laser spectroscopy

 A pneumatic isolator with self-leveling control is sometimes called an active vibration control system, but it is not. While it does attenuate floor vibration and continuously level the payload, it has no inertial sensor and there is no active feedback or feedforward loop. The mechanical height control valve can be upgraded to non-contacting electro-pneumatic height control for highly precise position repeatability. While this is a control system with electronic proximity sensors and precision electronic valves for actuation of the payload, it is not an active vibration control system as there is no inertial sensor, i.e., nothing is measuring vibration.

For improved performance at or near the natural frequency of the isolator, inertial sensors can be added to the payload which send a signal back to a controller. The controller then processes this signal and very quickly opens and closes the electronic valve to cancel the motion. This is considered to be active damping as it reduces the amplification from the resonance of the air isolator. This is an active control system, but it is not improving the overall vibration isolation outside of its bandwidth which is very narrow, around the resonance of the air isolator. Since the sensor is on the payload it isn’t actively measuring and cancelling floor vibration over a wide frequency range. 

Stay tuned for our next episode, where Wes will explain the difference between parallel and serial active vibration isolation.

Click here to see part two.

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