DEICON’s patent-pending 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 isolation system, published in March 2006 issue of The Yacht Report magazine and in December 2009 issue of the Superyacht Business magazine are presented below: The Yacht Report Review SuperYacht Business Review.

Conflicting Mounting Requirements

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.

Why Air Isolation

The reasons for choosing air isolation 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. 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.

Types of Air Mounts/Isolators

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 springs have a thick-walled cylindrical body and the 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 about 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.

(a) (b)

Control System

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 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 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 patent-pending 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 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

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

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 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 patent-pending “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.