Active Vibration Control
Active vibration control systems, also called active vibration isolation or active vibration cancellation, are isolation systems that dynamically react to incoming vibrations. That is, they sense incoming vibrations and react to them, rather than passively reducing their effect by virtue of their mechanical structure.
There are two general types of active vibration cancellation systems: feedforward and feedback systems. Feed-forward systems are specifically programmed to compensate for regular periodic vibrations. Feedback systems continually sense and react to incoming vibrations. Typical feedback systems have a sensing mechanism which senses incoming vibrations and an actuator which reacts to these vibrations, either by tuning an isolator to reduce the incoming vibrations or creating a signal which cancels them out.
Herzan is the exclusive distributor for Table Stable active vibration isolation systems in the Americas, Australia, and New Zealand. The systems are designed and produced in Table Stable’s workshop in Zwillikon, Switzerland just outside Zurich.
The Table Stable active vibration control systems were developed twenty-five years ago by Dr. John Sandercock to support his interferometer, the Sandercock Interferometer. Interferometers are sensitive to incoming vibrations, so Dr. Sandercock developed the active vibration isolation technology to improve the performance of his interferometers. The active isolation systems became popular in their own right, so Dr. Sandercock founded Table Stable to manufacture and sell the systems.
Strictly speaking, the Table Stable isolation systems are hybrid systems which incorporate passive and active isolation elements. The passive component consists of stiff metal springs which support the load weight and provide isolation over a broad spectrum. These springs provide a basic level of isolation in the lower frequencies and excellent isolation in the higher frequencies (above 200 Hz). They also support the load while allowing for travel of the actuators in the active component.
The performance of the springs is augmented and corrected by an active isolation component. The active isolation component consists of vibration sensors, control electronics, and actuators. The vibration sensors are piezo accelerometers. There are at least eight sensors in each isolation system; they are positioned in different orientations to sense in all six degrees of freedom. The piezo accelerometers convert kinetic vibration energy into electrical signals which are transmitted to the control electronics. The electronics reconcile and process the signals from the various sensors using a proprietary algorithm. The electronics then send a cancellation signal to the actuators.
The actuators are piezo actuators which are coupled to the sensors so they appear in the same number, location, and orientation as the sensors. The actuators generate vibrations that are equal to the incoming vibrations but out of phase in relation to the incoming vibrations (see Illustration). This results in cancellation of the incoming noise, leaving the load on top of the system undisturbed. This process occurs within 5 – 20 milliseconds of a vibration entering the system.
The sensors, electronics, and actuators create a feedback loop in the system. In addition to providing an extremely high level of attenuation for vibrations within the active range, the feedback loop provides other benefits. Foremost, the feedback loop effectively removes resonances that would otherwise be present in the system. It removes the low frequency resonance which plagues all passive vibration isolation systems. The feedback loop also performs ‘housekeeping’, removing incidental resonances which are inherent to simple mechanical structures. The active systems’ performance curve exhibits a smooth roll-off moving into the higher frequencies rather than a spiky curve which would indicate degraded performance at certain frequencies.
Another benefit of the feedback loop is that the system can utilize stiff metal springs for the passive component. Soft springs, like air or bungee springs, are utilized in passive isolation systems to push the low frequency resonance of the system as low as possible in order to increase the effective range of the isolator. This practice results in a very soft mount for the instrument that is loaded on top. The softer a mount is, the longer its settling time will be. Some systems require as long as a minute to settle and begin isolating again after disturbance from shock or bumping. Because they rely on stiff metal springs to support the load, Table Stable active isolation systems settle out and begin isolating within a second of a shock or disturbance.
Table Stable active vibration isolation systems offer the following advantages:
Download the original white paper describing the technology behind Table Stable active vibration control by clicking here.