Mike:
We had a member a while back who worked for a company that made these isolation devices. Somewhere on WBF is his post where he went into some length describing differences between active and passive, etc.
Passive Vibration Isolation: Passive vibration isolation systems consist essentially of a mass, spring and damper (dash-pot). The equipment and gears have joints with surrounding objects (the supporting joint - with the support; the unsupporting joint - the pipe duct or cable)
Active Vibration Isolation: Active vibration isolation systems contain, along with the spring, a feedback circuit which consists of a piezoelectric accelerometer, a controller and an electromagnetic transducer. The acceleration (vibration) signal is processed by a control circuit and amplifier. It then feeds the electromagnetic actuator, which amplifies the signal. As a result of such a feedback system, a considerably stronger suppression of vibrations is achieved compared to ordinary damping.
Passive versus Active Isolation: Passive isolation always exhibits amplification within the resonance and the isolation for higher frequencies decreases because of the viscous damping. Another advantage of an active isolation system compared to a passive stage is the short settling time. The passive system needs a long time to come to rest again. In the case of the active system the vibration decays much faster because the active system reacts with its actuators. The actuators are generating forces which counteract the movement of the isolated mass.
Active isolation systems attempt to cancel out vibration by vibrating a platform 180 degrees out of phase with a measured vibration stimulus. A typical system has three main parts; a passive base, a measurement unit, and 6 axis voice coils/ rotational motors. The passive base attempts to limit the transmission of higher frequency vibration (>200 Hz) through passive means such as air bladders, springs or viscoelastic material. The "active" components are usually only active below 200 Hz. The measurement unit accurately, and hopefully quickly measures the actual vibration of the platform on which a component rests. The measuring unit sends this information to the 6 axis voice coils / motors. The voice coils then vibrate 180 degrees out of phase with the measured vibration. That is, if the platform is moving one direction, they move the opposite way to cancel out the motion. The measuring unit continuously senses the motion of the platform and thus forms a feedback loop with the voice coils / rotational motors. Generally, active isolation systems do a good job at isolating a component from vibrations in the structure it is sitting on. They are used extensively in the electron microscope industry and work particularly well with relatively large amplitude vibration sources such as footfall. They have two significant drawbacks however. First, they do less well at dealing with airborne vibration (such as loudspeaker output) and machine generated vibration (such as motors/transport mechanisms. The reason is the sensor that detects the vibration is in the platform, not in the component. Typically the amplitude of the vibration in a component case is reduced by the time it reaches a sensor leading to erroneous inputs (or no input) to the motors. Second, latency causes unwanted voice coil / rotational motor movement. The system feedback is designed to operate quickly, but by definition, the system can't respond instantaneously. Thus the voice coils will still be moving even after the original vibration has stopped. This ring-down effect slows down the settling time of the system.
Pros:
- Attenuates even very low frequency (1-2 Hz) component vibration while providing a stable base.
Cons:
- High cost, large size, extreme complexity (pneumatics, etc).
- Measurements take place in the platform - not the component, therefore ineffective at attenuating vibration that does not reach the sensor.
- Latency and use of servos is source of noise and ring down effect. Voice coils still moving after vibration has stopped.
Wave Kinetics:
A10-U8 Component Control System:
- Rapid settling time - at least as fast as active systems.
- Does not over-damp
- Impedance mismatch prevents flow from structure to component
- Damping at component side dissipates machine generated vibration and air-born vibration transmitted to case.
- Quiescent state is very rigid due to hexagonal close pack of spherical bearing array
2NS Loudspeaker Interface System:
- Reduces signal to floor - reduces unpredictable floor resonances.
- Cleans up the signal going to the floor- materials sequence filters signal from speakers and insures a very sonically flat frequency response.
More information is available on our website:
http://wavekinetics.com