Comparison of Passive and Active Vibration Isolation Systems

In some applications, active isolation systems like the Table Stable systems available from Herzan can deliver the required performance, while passive isolation systems are adequate for applications which are not so sensitive.

Low Frequency Resonance

Low frequency resonance problems are common in all passive vibration isolation systems. Hence, a passive isolation system will often increase the low frequency noise instead of damping it, creating problem for instruments such as SEMs and AFMs that are susceptible to low frequency vibration.

Conversely, the Table Stable active isolation systems eliminate the low frequency resonance by means of an internal feedback loop which allows them to cancel vibrations at sub-hertz frequencies.

Moreover, the feedback loop ensures effective isolation at all frequencies, thanks to its ability to do ‘housekeeping’ in higher frequencies. Figures 1 and 2 illustrate a resonant system and a non-resonant system.

Resonant System

Figure 1. Resonant System

Non-Resonant System

Figure 2. Non-Resonant System

Performance Comparison

The performance of passive isolation systems increases rapidly once they start damping, substantially reducing vibration within a few hertz. Additionally, the level of damping is further decreased by other mechanical resonances present in a passive system.

Since the resonances in a Table Stable active isolation system are removed by its feedback loop, it has a smooth roll-off moving into the higher frequencies. This means that active systems maintain high level of attenuation in higher frequencies once they reach their peak performance at 10Hz.

Nevertheless, both active and passive systems provide 98% to 99% reduction at frequencies above 30Hz.

Sensitivity to the Instrument Size

Since passive isolation systems are simple mechanical structures, their performance relies on the weight, size, and center of gravity of the equipment loaded on top of the isolation system. The equipment affects the performance of the isolation system by changing the mechanical structure and resonances of the system.

Also, the mounting platform size has to be in proportion to the location of the center of gravity of the equipment.

Mechanical variations are offset by the feedback loop on the Table Stable systems, enabling the systems to deliver superior performance as long as the equipment is within their footprint and load capacity.

Multiple Axis Isolation

Most passive isolation systems cancel out only Z axis vibrations and are incapable of isolating horizontal vibrations. Different isolation mechanisms need to be used to allow a passive system to isolate horizontal vibrations.

However, the system cannot handle vibrations which are not purely X, Y, or Z. Table Stable active systems isolate vibrations in six degrees of freedom using the same sensing and damping mechanism used in passive systems. There is communication between the sensors, thus allowing the Table Stable system to provide the isolation at the same level across all axes.

Stiffness

Passive systems are made very soft, in order to push their low frequency resonance to the lowest level possible.

When soft systems are exposed to a major disturbance, they take more time to settle, typically on the order of seconds. Some systems take minutes before they are isolating again, increasing measurement times and demanding the restart of long scans.

Moreover, soft systems are more sensitive to load changes, air currents, user bumping compared to stiffer systems.

Since low frequency resonance is removed in the Table Stable systems, they can use very stiff springs. Hence, they are less prone to be unsettled and their settling time is on the order of milliseconds (Figure 3).

Settling Time - TS vs. Passive

Figure 3. Settling Time - TS vs. Passive

Cost

Passive systems are significantly less expensive than active systems. Rubber mounts and pads can offer adequate level of isolation for applications that are not so sensitive at a cost of few dollars.

Very high levels of vibration isolation can be achieved using bungee systems with appropriate geometry and tuning at a cost of less than hundred dollars. Air-based isolation systems can provide adequate vibration cancellation for a variety of research instruments at a cost of few thousand dollars.

The cost of active systems is typically more than five thousand dollars. Hence, they are not an economical option for less expensive instruments.

Isolation at the Instrument Level

Passive systems can only isolate ground-borne vibrations, whereas Table Stable systems sense and isolate vibrations at the instrument level.

Hence, they can damp certain noises originating from the equipment, including parasitic noise from cabling, air movements, acoustic noise coupled into the casing, and even certain noises generated by the equipment itself.

Site and Maintenance Requirements

An air compressor or clean air supply in the installation area is often required for passive systems. Air diaphragms of air-based isolation systems need to be replaced after several years of service.

Air is not required for Table Stable systems, but 110 – 240V AC power is required. Table Stable systems run maintenance free for years and require ventilation if in a confined space where heat dissipation is a problem.

Table Stable active vibration control systems are designed to run continuously for years with no maintenance. These systems do not require air; their only requirement is 110 – 240V AC power. If the system is being used in a confined space and heat dissipation is a concern, then ventilation should be used.

Comparison Table

The following table compares passive and active vibration control systems:

PASSIVE VS. ACTIVE - Comparison Table
FEATURE PASSIVE ACTIVE
Isolation Range >5Hz >.7Hz
Low Frequency Resonance 2 - 5 Hz None
Amplification at Resonance 4-9 times N/A
Load Sensitive Yes No
High Center of Gravity Decreases Efficiency Yes No
Stiffness Soft 500x Stiffer than Passive
Isolation in All Six Degrees of Freedom No Yes
Air Supply Requirements Hand Pump or Air Supply None
AC Power None 110-240 VAC
Settling Time 2 - 10 seconds 10-20 milliseconds
Automatic Load Adjustment -M – No
-A - Yes
AVI – No
TS - Yes
Noise Isolated Ground-borne vibrations Ground, instrument, and acoustic noise
Price Low to Moderate Moderate to High

About Herzan

Herzan provides high performance environmental solutions for precision research instruments. They include acoustic enclosures, vibration isolation systems, Faraday cages, and site survey tools. Herzan specializes in supporting nanotechnology research, but also offers solutions for product testing, in-vitro fertilization, and many other applications.

Herzan understands that every application and environment is different, so it collaborates with customers to create comprehensive integrated solutions that satisfy their unique demands.

Herzan was founded in 1992 by Ann Scanlan in Orange County, California. Originally, Herzan was established as an American subsidiary of Herz Company Ltd., a Japanese company specializing in vibration control. The name Herzan comes from the amalgamation of 'Herz' and 'Ann'.

This information has been sourced, reviewed and adapted from materials provided by Herzan LLC.

For more information on this source, please visit Herzan LLC.

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