Victory
over vibrations
A
good example of unwanted vibratory motion is a washing machine in its
spin cycle trying to walk out of the room. Magnetorheological damping
can correct this
As
motion control systems become more refined, vibration characteristics
become more important to a system’s overall design and functionality.
Engineers, however, have tended to look at motion control and vibration
as separate issues. Motion control, it might be said, presents fairly
familiar design engineering problems while vibration suggests more subtle
problems. Few design engineers have either the hands-on experience or
the training to address both sets of problems in a single design solution.
Engineers
faced with this challenge often arrive at solutions that offer, at best,
a compromise in terms of optimum vibration control, overall system performance,
energy efficiency, and cost. Many system designers have therefore come
to accept such compromises as givens. To arrive at significant design
improvements, engineers might begin by first questioning some of these
“givens”.
The
walking washing machine
The
common household washing machine represents a standard compromise between
controlling vibration associated with the spin cycle and achieving optimum
system performance and efficiency. The tub in a conventional machine
is suspended by a number of coil springs that provide mechanical support
as well as vibration isolation at high frequency. To prevent potentially
damaging vibratory excursions when the drum velocity passes through
resonance as it accelerates during the ramp-up to the spin cycle, static
vibration dampers are added to the suspension.
A
variable damping system based on magnetorheological (MR) fluid sponges
can help control the vibratory motion of a household washing machine
during its spin cycle. Damping is switched on as the drum passes through
resonance and off again at the highest speeds for optimum vibration
isolation. The system permits the drum to rotate at speeds high enough
to function as a centrifuge, but without the violent shaking familiar
to every user.
This
arrangement represents a compromise; while conventional dampers easily
control the tub’s motion at resonance, they can significantly degrade
high-speed vibration isolation. This tendency limits the size of the
tub and to some extent dictates the dimensions of the housing that must
accommodate the overall motion of the tub.
In
terms of the machine’s overall performance, static damping draws energy
from the motor that might otherwise go toward maximising spin speeds
for optimum water removal from clothes and shortened drying time. So
can vibration control, performance, and energy efficiency be improved
without raising the price of the appliance to more than the average
consumer is willing to pay?
The
European response
European
manufacturers of horizontal-axis, front-loading washing machines are
pushing for increased performance and efficiency. Smaller in load capacity
and typically installed in apartment kitchens, these machines are being
designed for maximum energy efficiency and quiet operation (in small
spaces and high-occupancy buildings, neighbours are never far away).
Because
many households have only a washing machine and not a dryer, tub speeds
are reaching 2,000 revolutions per minute effectively becoming centrifuges
that remove almost all the water from the washload. In fact, manufacturers
have had to reduce the size of the drain holes in the tub to prevent
extrusion of small items of clothing during the spin cycle.
To
achieve this level of performance, manufacturers have incorporated a
controllable damping system designed around magnetorheological (MR)
fluid.
These
can simply be turned off at high spin speeds for an increased degree
of vibration isolation.
Advantages
of MR damping
The
MR fluid sponge damper requires neither seals nor bearings, and uses
the same inexpensive components found in existing passive dampers, but
with a few important modifications. The damper consists of a layer of
open-celled, polyurethane foam, or other suitable absorbent matrix materials,
saturated with approximately 3 ml of MR fluid surrounding a steel bobbin
and coil.
Together
these elements form a piston on the end of the shaft that is free to
move axially inside a steel housing that provides the magnetic flux
return path. Damping force is proportional to the sponge’s active area.
The
application of a magnetic field causes the MR fluid in the matrix to
develop yield strength and resist shear motion. The amount of force
produced is proportional to the area of active MR sponge that is exposed
to the magnetic field. This arrangement can be applied in both linear
and rotary configurations wherever a direct shear mode of operation
would be used.
During
passage through resonance, these controllable dampers may be energised
to provide a high level of damping that totally controls the excursions
of the tub.
At
high speed, the MR sponge dampers are turned off to enable a high level
of vibration isolation. With enhanced vibration control, the drum may
be....
....CONTD
