COMPUTER CONTROLLED AIR ISOLATION SYSTEM
DEICON’s Computer Controlled Semi-Active Air Isolation System is the most effective vibration isolation system using air springs originally developed for diesel generators and other operating machinery onboard luxury watercraft (yachts) as well as other mobile platforms. This controlled vibration and shock isolation system has been applied, equally effectively, to stationary as well as mobile machinery and equipment subject to both vibration and shock perturbations, as well as random vibration. This controllable vibration/shock isolation system is capable of meeting the conflicting vibration isolation, shock isolation, and installation integrity requirements of such machines by switching its states, on-demand or automatically, between a) ‘soft and highly underdamped’ for effective vibration isolation (e.g. when the watercraft is at port or anchored and water is calm), b) ‘soft and somewhat damped’ for effective shock isolation, and c) ‘stiff’ for effective installation robustness when the isolated object is subject to excessive perturbations jeopardizing the integrity of its installation (e.g. when the vessel is sailing in choppy waters). The addition of on-demand damping only around the resonant frequency(ies), enhances the shock isolation attributes of the isolated object without deteriorating its vibration and noise isolation attributes.
Vibration and Shock Isolation Conflicting Requirements
An ideal mounting system for machinery and equipment subject to both vibration and shock perturbations, e.g., Diesel generators on-board watercrafts, should:
– Support the weight without excessive static deflection,
– Be soft when the isolated machine is subject to vibration perturbation, only. For example, running Diesel generators on-board a watercrafts while the vessel is not moving and propulsion engines are off and peace, tranquility and quiet are desired. This will prevent the transmission of vibration and noise, through the hull, to the living quarters of the vessel.
– Be stiff when the isolated machine is subject to excessive perturbations endangering the integrity of its installation.
– 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 isolation solution satisfies all the requirements listed above. Even ‘double mounting’ (also known as ‘two stage mounting’), while an effective vibration isolation solution at high frequencies, has less than desirable low-frequency vibration 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 do not help the appeal of the double mounting in retrofit applications, either.
Why Air Isolation
The reasons for choosing air isolation (vibration isolation using air springs) are:
- softness, providing low natural frequency enabling air to provide the highest degree of low-frequency isolation of any type of vibration isolator,
- large load-bearing capacity without excessive static deflection,
- negligible inherent damping, enhancing high-frequency vibration isolation,
- low noise transmission,
- adjustability, and
- lightweight.
Air springs are closed systems that never corrode and are operable in dirty and harsh environments. Because air springs are sealed units, they last a long time and don’t require much service. They contain no moving parts, are friction-free, and give an immediate response in the form of force or compliance.
The low stiffness and low damping which give the air its legendary vibration and structure-borne noise isolation, are also what make shock isolation attributes and installation integrity of air mounting less than desirable.
DEICON’s Computer Controlled Semi-Active Air Isolation System enhances shock isolation capabilities of air mounts while maintaining their highly attractive vibration isolation and structure-borne noise isolation as well as the integrity and robustness of the installation.
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 a) convolution type and b) pneumatic-elastomeric type.
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- (a)
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- (b)
The envelope of convolution type 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.
Pneumatic-elastomeric type 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 excessive bulging of the wall providing a transverse stiffness almost equal to the axial stiffness of the mount. Isolators of this type are more effective than elastomeric isolators but do not provide as much isolation effectiveness as convolution type air mounts do.
Control System
Automatic control of DEICON’s Computer Controlled Semi-Active Air Isolation System is performed by a collection of solenoid and servo-valve subsystems under the constant supervision of a PLC (programmable logic controller), 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 PLC monitors the accelerometers placed on and around the isolated machine and upon detecting accelerations beyond a certain level, switches the states of the isolation system.
It should be noted that DEICON’s computer-controlled air isolation system is quite different from 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 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.
Systems that use controls 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 pressure control. In situations where the configuration of the mounted machine changes resulting in shift 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.
Damping Adjustment
Lack of damping in most isolators/mounts (including air isolators/mounts) enhances their vibration isolation attribute. 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 isolator. It might even, in some applications, endanger the installation integrity of the isolation system.
DEICON’s controlled 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 structure-borne noise abatement. DEICON’s controlled damping technology is based on actively flowing pressurized air in and out of the air mount via a proportional pneumatic valve under feedback control. 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 damping can readily be introduced into such systems with minor modifications to the existing regulating valve(s).
The two time traces in the figure shows the experimentally measured impulse acceleration response 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.
Stiffness Adjustment
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 spectra of acceleration of a 220 lb (100 Kg) machine mounted on a stiffness and damping controlled air isolation system with 3 different controller gains. Evident from the figure, the resonant frequency has changed from 2.9 Hz to 5.9 Hz in steps of 1 Hz, i.e. a factor of 2 change in resonant frequency indicating a factor of 4 (400%) change in stiffness. Moreover, in each stiffness setting, the damping of the air isolation system is changed from low (red traces) to medium (blue traces) to high (black traces). It should be pointed out that stiffness and damping variations occur 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) in conjunction with added damping prevent the isolated machine from undergoing excessive motion in response to abrupt disturbances such as starting up or shutting down the engine.
Vibration Isolation Effectiveness
The effectiveness of rubber (blue trace) and air mounts (red trace) in isolating a 175 KVA diesel generator (weighing around 2000 Kg) 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.
Shock Isolation and Installation 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 installation integrity of DEICON’s air isolation system. The image shows one mounting foot of a diesel generator isolated from the structure by 1 ‘main’ and 2 ‘lateral’ air mounts.
Considering that air mounts do not provide sufficient lateral stiffness needed to secure the isolated object when subject to a shock input (e.g., diesel generator while the boat is in motion), additional lateral support is provided by a set of smaller air 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 between:
– soft and highly underdamped with small lateral stiffness providing excellent vibration and structure-borne noise isolation,
– soft and underdamped providing excellent shock isolation, and
– stiff and damped with high lateral stiffness providing excellent installation integrity.
Comparison of Double Mounting and Controlled Air Mounting
Double mounting, also known as “two-stage mounting” is the isolation scheme used for isolating gensets and other machinery onboard large luxury watercrafts and military vessels. Although very effective in lowering the transmission of vibration and structure-borne noise 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 that exceeds or matches the effectiveness of the double mounting, over the frequency range of interest, without all the above-listed drawbacks, is DEICON’s “Computer Controlled Air Isolation System”. Under the supervision of a computer, 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., less than ideal 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.