Conflicting Mounting Requirements
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
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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.
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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.
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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.
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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.
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(a)
(b)
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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.
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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.
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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
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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.
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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.
Evidient 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.
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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
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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 spectrums 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.
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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.
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The figure depicts the transmissibilites (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.
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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
unsurrpassed 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.
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