Computer Controlled Air Isolation System

 Computer Controlled Air Isolation System

DEICON’s Computer Controlled Air Isolation System is the most effective mounting scheme developed for diesel generators and other operating machinery on-board luxury watercrafts (yachts) as well as other mobile platforms.The controllable vibration isolation system is capable of meeting the conflicting mounting requirements of such machines. This pneumatic system switches its states, on-demand or automatically, between ‘soft’ for effective vibration isolation (when the craft/vehicle is not moving, e.g. watercraft is at port or anchored and water is calm) and ‘stiff’ for effective shock isolation and structural integrity when the craft/vehicle is in motion (e.g. vessel is sailing). The addition of active damping only around the resonant frequency, further enhances the shock isolation attributes of the mounted system without deteriorating its vibration and noise isolation attributes.

Independent reviews of DEICON’s vibration isolation system, published in The SuperYacht Report and the Superyacht Business magazines are listed below:

Conflicting Mounting Requirements

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An ideal mounting system for diesel generators and other running machinery on-board watercrafts should:

1. support the weight without excessive static deflection.

2. be soft while the vessel is not moving, i.e., the propulsion engines are off and peace, tranquility and quiet are needed. This will prevent the transmission of vibration and noise, through the hull, to the living quarters.

3. be stiff when the vessel is moving or in rough waters and the isolated machine need to be securely attached to the structure.

4. be damped around the resonant frequency (to mitigate the resonance problem) and highly underdamped at high frequencies to avoid transmitting noise and high frequency vibration to the hull.

No one passive solution quite satisfies all the requirements listed above. Even the popular double mounting, while effective at high frequencies, has less than desirable low-frequency isolation effectiveness. Shock isolation of double mounting is also inferior to that of single mounting. In addition, double mounting imposes unfavorable weight penalty and space penalty. The complexity and difficulties of converting an existing mounting system to double mounting does not help the appeal of double mounting in retrofit applications.

When sailing, noise and vibration produced by diesel generators and other machinery are normally masked by the noise and vibration created by the propulsion engines, propellers induced turbulence, etc. Thus, noise and vibration isolation attributes of diesel generator mounting system have a lower priority than its shock isolation attributes requiring mounts with large stiffness and damping. On the other hand, while the vessel is docked (and not on shore power) or anchored, diesel generators are the main source of noise and vibration, which if not isolated properly, will transmit their noise and vibration through the hull to the living quarters, disturbing the occupants.

Why Air Isolation

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The reasons for choosing air isolation (using air springs) are: 1) softness, providing low natural frequency enabling air to provide the highest degree of low-frequency isolation of any type vibration isolator, 2) large load-bearing without, despite its low stiffness, excessive static deflection, 3) negligible overall damping enhancing high-frequency isolation, 4) adjustability, and 5) light weight. Air springs are closed systems that never corrode and are operable in dirty and harsh environments—such as underneath trucks (as suspension elements). Because air springs are sealed units, they last a long time and don’t require a great deal of service. They contain no moving parts, are friction-free and give immediate response in the form of force or compliance.

The low stiffness and low damping which give air its legendary vibration and structure-borne noise isolation, are also what make shock isolation attributes of air less than desirable. DEICON’s Computer Controlled Air Isolation System enhances shock isolation capabilities of air mounts while maintaining their highly attractive vibration isolation, structure-borne noise isolation, and light weight attributes.

Air springs are rubber and fabric bellows that contain compressed air as the load bearing medium. They are constructed out of fabric-reinforced rubber enclosures. While the material itself does not provide force or support (it is the compressed air operating inside that does that) it is exceptionally enduring—a fact that has been proven both in the field and through formal, rigorous testing.

Types of Air Mounts/Isolators

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An air mount (air spring) is simply an enclosed compressed air. They have been used in vibration isolation for the last 70 years. Commercially available air mounts are of two basic types known as convolution (bellow) type and pneumatic-elastomeric type.

Figure (a) depicts a single convolution air mount. The envelope is made of two or three relatively thin plies of fabric reinforced rubber, sealed to hold pressure typically up to 100 or 150 Psi (7 to 10 Bars) depending the number of plies. The metal plates on the top and bottom are for locating and loading the air spring. The shape and relatively thin wall of convolution type air springs are mainly meant to hold the air and do not provide much lateral stiffness.

Figure (b) depicts a pneumatic-elastomeric mount. These air springs have a thick-walled cylindrical body with their top shaped as a diaphragm coupling the body to the top plate (where the load sits). This thick wall in conjunction with 3 steel rings built into the make-up of the mount prevents the bulging of the wall providing a transverse stiffness almost equal to the axial stiffness of the mount. Isolators of this type, although more effective than a elastomeric isolators but do not provide as much isolation effectiveness as convolution type air mount.



Control System

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Automatic control of DEICON’s Computer Controlled Air Isolation System is performed by a collection of solenoid and servo-valve subsystems under the constant supervision of a computer, all housed in a control cabinet. In addition to feedback control of either pressure in individual mounts or mounting height of the machine, the control system switches (on-demand) between different pre-programmed isolation states.

Various degrees of automation can be incorporated into the control system to initiate the change in the state of the isolation system. At one end of this automation spectrum, a selector switch can be used for commanding the computer to switch the states. At the other end of the spectrum, the control computer monitors the accelerations and on detecting accelerations beyond a certain level, switching of the states gets enabled.

It should be noted that DEICON’s computer controlled air isolation system is quite different form an ‘active isolation system’ discussed in the literature and available in the marketplace. Active vibration isolation systems use full authority actuators (mostly electromagnetic or hydraulic) in parallel to, in place of, or in conjunction with (if proof-mass actuators are used) the passive mounts (springs) in a traditional isolation system. The actuators put out most or all of the vibration control force. In DEICON’s technology, no full authority actuator is used. We still use the air springs as the mounts in our isolation system, and only take advantage their adjustability to actively or semi-actively adjust their parameters (stiffness and damping) at the right time for the circumstances in hand. The comparison of one such active isolation system with DEICON’s ‘Computer Controlled Air Isolation System’, is presented here.

Systems that use controls (active or semi-active) for adjusting their parameters, such as DEICON’s Computer Controlled Air Isolation System, are normally categorized as semi-active systems. Since semi-active control systems only use energy to slightly modify their parameters, occasionally, their needed energy is by far less than their fully active counterparts. In addition, semi-active systems are by far less complex, less costly, and more reliable than fully active systems.

Active Pressure or Height Control

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In its standard configuration DEICON’s Computer Controlled Air Isolation System is under tight pressure control. In situations where the configuration of the mounted machine changes resulting in shifts in its center of gravity, height control in place of pressure control is recommended. Active height control, requiring displacement sensors on 3 of the air springs, maintains the isolation effectiveness of DEICON’s air isolation system even when the mounted machine undergoes drastic changes in its configuration. With this feature, the control system reacts quickly to changes in the supported load and center of gravity shifts by automatically re-leveling the isolated machine.

Active Damping

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Lack of damping in most mounts (including air mounts) enhances high-frequency vibration isolation. The side-effect of this desirable attribute is the creation of a highly underdamped, low-frequency resonance which could deteriorate the shock isolation capability of the mount.

DEICON’s active damping technology for air isolation systems addresses the shortcoming of underdamped resonance leading to undesirable shock isolation, while maintaining the desirable attributes of vibration isolation and noise abatement. DEICON’s active damping technology is based on actively flowing pressurized air in and out of the air mount via a pneumatic servo-valve under feedback control according to a proprietary damping scheme/algorithm. Considering that air mounts in most isolation applications (including DEICON’s Computer Controlled Air Isolation System) are either under pressure or height regulation, the active system is readily realizable with minor modifications to the existing regulating valve(s).

The two time traces in the figure shows the experimentally measured acceleration of a 500 lb (240 Kg) machine mounted on an uncontrolled and controlled air mount. Clear from this figure, active damping can introduce an appreciable amount of energy dissipation into the system. Comparison of the two traces in this figure indicates the increase in damping form 2.5% to 21%, i.e. in excess of 8 fold increase, more than adequate for taming the machine bounce at resonance contributing to shock isolation.

Active Damping of Air Mounted Systems

Active Stiffness Control

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With optional active stiffness control the stiffness of the mount can be lowered (softened) or increased (stiffened) without physically changing the mounting arrangement or connecting the mount to a bulky and heavy auxiliary air reservoir.

The three traces in the figure show the experimentally measured power spectrum of acceleration of a 500 lb (240 Kg) machine mounted on a stiffness-controlled air mount for 3 different controller gains. Evident from the figure, the resonant frequency has changed from 2.25 Hz to 4.5 Hz, i.e. a factor of 2 change in resonant frequency indicating a factor of 4 (400%) change in stiffness. It should be pointed out that stiffness variation occurs in a matter of milli-seconds.

Lowering the natural frequency (softening the mount) further enhances the isolation performance of the air mounted system beyond what the uncontrolled system provides.

Increasing the natural frequency (stiffening the mount) prevents the genset from undergoing excessive motion in response to abrupt disturbances such as starting up or shutting down the engine.

Active Stiffness Control of Air Mounted Systems

Vibration Isolation Effectiveness

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The effectiveness of rubber (blue trace) and air mounts (red trace) in isolating a 175 KVA diesel generator on-board a luxury vessel is compared in the figure depicting power spectra of acceleration at a location on the engine room floor. Clear from the figure, the vibration isolation effectiveness of air mounts by far exceeds that of rubber mounts.

More performance data.

Shock Isolation and Structural Integrity

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In addition to or in place of active damping control and active stiffness control in the vertical/heave direction , stiffening in lateral directions are used to enhance the shock isolation attributes of DEICON’s air isolation system.

Considering that air mounts do not provide sufficient lateral stiffness needed to secure the diesel generator while the boat is in motion, additional lateral support is provided by a set of smaller mounts which will be engaged, when needed. These mounts not only provide lateral stiffness, they also increase the heave stiffness of the mounting system. By selecting any of the pre-programmed states, via a selector switch or automatically, the attributes of the mounting system under computer control changes from soft, highly underdamped with small lateral stiffness (providing excellent vibration and structure-borne noise isolation), to stiff, underdamped with high lateral stiffness (providing excellent shock isolation). The figure shows one mounting foot of a diesel generator isolated from the structure by 1 ‘main’ and 2 ‘lateral’ air mounts.

Comparison of Double Mounting and Controlled Air Mounting

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Double mounting, also known as “two-stage mounting” is the scheme commonly used for isolating gensets and other machinery onboard superyachts and megayachts. Although very effective in lowering the transmission of vibration at high frequencies, double mounting, as with any other passive isolation technique, has its own drawbacks including the design complexity, weight penalty, large space requirement, and excessive cost associated with the added mass (also known as auxiliary mass) which depending on the design, could weigh up to 100% of the weight of the isolated machine. Note that a double mounted isolated system has twice as many resonant frequencies (at least 12) as those of a single mounted system; this is assuming the auxiliary mass is designed and fabricated properly so that it is rigid enough and does not introduce its own flexible-body resonant frequencies into the mix. Keeping all these resonant frequencies from matching any of the harmonics of engine vibration is a major challenge contributing to the design complexity of two stage mounting systems.

An alternative isolation strategy, without all the above-listed drawbacks, that exceeds or matches the effectiveness of the double mounting, over the frequency range of interest, is DEICON’s “Computer Controlled Air Isolation System”. Under the supervision of a computer, semi-active and active control strategies are used to keep the desirable attributes of air mounting, i.e., unsurpassed isolation specially at low frequencies, and address the undesirable attributes, i.e., ineffective shock isolation , low lateral stiffness, etc.

The figure depicts the transmissibilities (a) and motion (b) of a single degree of freedom isolation system, using 3 different arrangements of 1) single elastomeric mounting (black/dotted line), 2) double elastomeric mounting (blue/dashed line) with M_aux/M_machine=0.25, and 3) air mounting under the control of a computer (red/solid line).

Comparison of single and double elastomeric mounting (black/dotted line and blue/dashed line) clearly shows the advantage of double mounting at higher frequencies.

On the other hand, the vibration isolation effectiveness (judged by the transmissibility traces of Figure (a) ) of air mounting system is almost as good as double mounting at high frequencies and is by far superior to double mounting at low frequencies. The lack of damping in air mounts can be addressed by active targeted damping incorporated into the system; note that this damping scheme adds damping to the resonant frequency only without deteriorating the high frequency vibration isolation effectiveness. The higher motion of the machine at low frequencies can also be addressed by the active and semi-active stiffness control.

‘Computer Controlled Air Isolation System’ with its unsurpassed vibration isolation effectiveness (particularly at low frequencies), and without the drawbacks associated with plain air mounting, is a highly attractive isolation alternative to the more traditional elastomeric double mounting. This is especially true when a) the added weight, space requirement, and cost associated with double mounting are concerns (which they always are) and b) when addressing low-frequency vibration and structure-borne noise is of utmost importance.

American Bureau of Shipping (ABS) Approval

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After thorough evaluation of DEICON’s ‘Computer Controlled Air Isolation System’, American Bureau of Shipping (ABS) has determined conformance with specifications and awarded DEICON its Product Design Assessment(PDA) certificate. This PDA pre-approves DEICON’s ‘Computer Controlled Air Isolation System’ for use on a variety of ABS class ships, reducing the turn around time for approval on a specific ship. When a specific vessel is chosen, ABS would then verify that the product, as already approved, is suitable for the intended use. This can be done with a simple review of the PDA and not require submital of further documentation from the manufacturer.

Frequently Asked Questions

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Q. Is DEICON’s air isolation system suitable for main/propulsion engines or is it just for Diesel generators?

A. DEICON’s ‘Computer Controlled Air Isolation’ technology is originally developed for Diesel generators and other machinery such as air conditioning compressors which work irrespective of whether the yacht is sailing or not. Note that except when the vessel is on shore power, Diesel generators always work even when the yacht is not sailing (at shore or anchored) and the occupants expect a high level of peace and quiet; noise and vibration specifications are very stringent.

On the other hand noise and vibration specifications are less stringent when sailing (propulsion engines are working); people expect (and thus do not mind) some noise and vibration. Thus the more traditional mounting of propulsion engines might be adequate. Having said this, with the gearbox being elastomerically mounted and flexibly coupled to the engine,‘Computer Controlled Air Isolation System’ can be used to isolate Diesel mechanic propulsion engines.

We recommend our isolation technology for all the gensets in Diesel-electric propulsion vessels.

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Q. Is the control system commercially available (requiring fine tuning) or is it a complete custom install?

A. The control scheme used in DEICON’s Computer Controlled Air Isolation System is made up of pneumatic components (mainly high speed valves), sensors, and computers. Although all the components making up the control system are off-the-shelf, commercially available items, the way they are integrated and the software running them are proprietary. Depending on the desire of our clients, the control strategy can be as simple as just providing lateral stiffness by engaging and disengaging lateral mounts all the way to active damping control, active stiffness control, and active level/height control. How elaborate the control scheme is, dictates what and how many pneumatic components, sensors, and how involved of a control software are needed.

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Q. What is typically done with regards to attached systems such as exhaust lines?

A. The attached items, e.g., exhaust, should be connected to the engine in a flexible manner. In addition, the way these items/subsystems are hung on the walls/ceilings influence the transmission of vibration to the surrounding. Isolating the engine effectively but not extending such isolation effectiveness to subsystems such as the exhaust hangers would not be a sound practice.

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Q. Are there problems with any of the system’s components failing and if so, would it be possible to replace critical components at sea (e.g. air mounts & air lines)?

A. The components used in DEICON’s ‘Computer Controlled Air Isolation System’ are of highest quality, but still like any other high-quality component unexpected failure is a remote possibility. The failures that are based on the life expectancy of the components are addressed in regular maintenance schedule. The unexpected ones, which rarely happen, are addressed by building redundancy into the design of the system. For example, mounts are inflated to no more than 80% of their allowable pressure thus up to 20% of the mounts can be out, for whatever reason, and the remaining mounts can carry the load. The mounts can be replaced, at sea, easily (note that the failed mount is deflated and can easily be replaced with a spare one which is deflated while being installed and will be inflated when it is installed). Lastly, our suppliers have a strong international presence, thus the components used in DEICON’s isolation system are readily available, globally.

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Q. What life expectancy do the air mounts have when subjected to the hot (and possibly oily) environment expected in engine rooms?

A. Majority of the air mounts used in DEICON’s air isolation system are made of a composite of synthetic rubber and fabric. The standard material provides operating temperatures of up to 135 deg F. Most of the mounts we use are also available in a ‘high temperature compound’ which also have very good oil resistance; such mounts can operate at temperatures of up to 225 deg F.

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Q. What is the difference between height control and stiffness control?

A. Pressure inside an air mount, at any instant in time, is the combined static pressure, mainly supporting the weight, and dynamic pressure, responding to the vibration of the isolated machine; note that when the isolated machine is not working the dynamic pressure is zero.

Height control is normally a slow (low-bandwidth) control scheme suitable for applications such as leveling a coordinate measuring machine (CMM) or raising (for off-road driving)/lowering(for highway driving) the ride height of a sport utility vehicle (SUV); this control is done by adjusting the static pressure. For example, when the measuring probe (which along with its carrier is somewhat heavy) moves on a CMM going from one end of the machine to the other, the center of gravity of the machine changes and as such the machine loses its level. Height controller gently/slowly inflates the mount(s) that experience more deflection and level the machine again. So, height controllers by themselves are not meant to be fast trying to react to the dynamics of the system in milliseconds. Moreover, height control schemes do not address the air mounts lateral stiffness or lack there of. Stiffness control on the other hand is a fast (high-bandwidth) control scheme meant to modify the dynamics of a mounted system; this control is done by high-speed adjustment of the dynamic pressure.

Since DEICON’s stiffness control scheme has all the ingredients needed for height control we do offer height control, as an option, as well. This option does not replace stiffness control but complements it, with the purpose of providing slow disturbance rejection, e.g. slow loss of pressure due to a leaky fitting.

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Q. If soft mounting enhances vibration isolation effectiveness why is it that DEICON promotes stiffness control (to stiffen up the system)?

A. Well, softness provides very effective vibration isolation but at the expense of excessive motion in response to abruptive stimuli to the machine (such as starting and stopping a Diesel engine). We all have seen how an engine moves (mostly rolls) when it goes thru shut down and start up even when it is mounted on elastomeric mounts. Soft mounting would exacerbate this impulse response problem even further. When stiff mounts are used to avoid this excessive motion problem (in response to impulsive/shock inputs), too much vibration will be transmitted to the base and when soft mounts are used then vibration transmission is addressed but not the impulse response. Traditionally, practitioners have dealt with this problem by optimizing the mount to be not too stiff and not too soft; although a pragmatic solution but definitely not an ideal solution. We avoid this compromise by using softer air mounts and then stiffening it up, dynamically (very quickly), when needed and then going back to the soft state when the need for stiffening is gone. This has become possible with the availability of high-speed servo-valves, computers, and sensors all at reasonable prices.

How the lateral stiffness is provided depends on what type of air mounts is being used. With convoluted mounts we will use two additional mounts, laterally installed, to provide on-demand lateral stiffness (they get engaged and disengaged in milliseconds). With pneumatic mounts, the meaty/stiff wall of the mounts provide adequate lateral stiffness so there might not be a need for additional lateral mounts.

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Q. How is DEICON’s ‘Computer Controlled Air Isolation System’ compared to MTU-Paulstra’s Active Isolation System?

A. MTU-Paulstra active isolation system is a respectable piece of isolation technology. It is a combination of an active scheme comprised of an extensive combination of hardware and software and a traditional passive isolation (mainly a single stage rubber mounting) system. The active component lowers the extent of the engine vibration at a set of predetermined vibration harmonics by partially canceling them, at the source (the machine). The passive component of the isolation system lowers the transmission of vibration, to the base, of all the harmonics including the actively controlled ones which are already small in magnitude. The shock isolation attributes of the MTU-Paulstra scheme is what the passive component of that isolation scheme delivers.

DEICON’s system, as discribed in detail in this page, is just an adjustable passive mounting scheme taking advantage of the adjustability of air as the mounting medium. Via the use of controls, it exploits the adjustability of air, to do on-demand adjustment of the parameters (mainly damping and stiffness) of the isolation system. DEICON’s system uses no additional actuators to exercise the control. It is by far less elaborate and less expensive than the MTU-Paulstra active isolation system while delivering nearly as much vibration isolation effectiveness as the MTU-Paulstra system delivers at its actively controlled harmonics, and more at the uncontrolled harmonics. Besides, DEICON’s system also enhances the shock attributes of the isolation performance whereas the active component of the MTU-Paulstra scheme has no effect on shock isolation which is solely determined by the passive component of that scheme.

A more detailed comparison of MTU-Paulstra’s Active Isolation System and DEICON’s ‘Computer Controlled Air Isolation System’, is presented here.