Active vibration control
Active vibration control is the active application of force in an equal and opposite fashion to the forces imposed by external vibration. With this application, a precision industrial process can be maintained on a platform essentially vibration-free.
Many precision industrial processes cannot take place if the machinery is being affected by vibration. For example, the production of semiconductor wafers requires that the machines used for the photolithography steps be used in an essentially vibration-free environment or the sub-micrometre features will be blurred.
In the past, passive techniques were used. These include traditional vibration dampers, shock absorbers, and the like, all of which attempt to use friction to convert vibration into heat. But these are not sufficient for the increasing requirements for a vibration-free environment.
The typical system uses several components:
- A massive platform suspended by several active drivers (that may use voice coils, hydraulics, or other techniques)
- Three accelerometers that measure acceleration in the three degrees of freedom
- An electronic amplifier system that amplifies and inverts the signals from the accelerometers. A PID controller can be used to get better performance than a simple inverting amplifier.
- For very large systems, hydraulic components that provide the high drive power required.
If the vibration is periodic, the control system may adapt to the ongoing vibration, thereby providing better cancellation than would have been provided simply by reacting to each new acceleration without referring to past accelerations.