FIELD OF THE INVENTION
[0001] The present invention relates to laundry arrangements and in particular to laundry
apparatus having a rotatable drum and to associated control methods for managing out-of-balance
loads thereof. These laundry apparatus encompass washer extractors such as washing
machines that include a water extraction cycle, often referred to as a spinning program.
They also include pure extractors and dehumidifiers such as spin dryer machines and
the like.
BACKGROUND TO THE INVENTION
[0002] Removing liquid from a liquid absorbent article is typically accomplished by rotating
a drum of an extractor or washer extractor at a relatively high speed, so that centrifugal
acceleration forces the load against the interior surface of the drum. As rotation
continues, the liquid absorbed in the load flows out through perforations in the surface
of the drum and is removed out of the machine. Frequently, the load is not evenly
distributed against the interior surface of the drum and the resulting unbalance may
cause vibration while the drum is spinning. If the resulting forces of this vibration
are too big, machine parts can be damaged and it is desirable to limit at least one
of the unbalance or the allowable rotational speed of the drum.
[0003] In some prior art arrangements, it is known to apply a redistribution cycle to more
evenly distribute the load around the interior surface of the drum in order to reduce
unbalance. Often such redistribution is not wholly successful and a residual unbalance
remains as the drum is accelerated towards its final spin speed. This has resulted
in various attempts at detecting such unbalance and in taking precautions against
its potential effects before machine parts are damaged.
[0004] In a free standing machine with a suspension system, it has been known to use mechanical
limit switches to detect eccentric movement of the drum beyond a preset level. In
simple systems, tripping the limit switch results in machine shutdown with possibly
manual redistribution of the load called for. Such tripping may occur in the region
of the natural frequency of the machine,
where eccentric movement of the drum will be at a maximum.
[0005] Sensing out of balance load may be implemented using one or more accelerometers and
using them to measure the vibration by measuring the mechanical forces induced in
a load cell. In similar fashion to the limit switch arrangement, such arrangements
may send a shutdown signal if a fixed vibration is exceeded. This type of detector
may be used on a freestanding machine with a suspension system or on a rigid mounted
machine having no suspension system.
[0006] On rigid mounted machines, it is known to use one or more micro-switches to detect
deformation of certain machine parts. If the deformation exceeds a preset value, the
micro-switch sends a machine shutdown signal.
[0007] Another approach is to monitor load unbalance through variations in the motor load.
One such arrangement is disclosed in US-5543698, in which a motor controller detects
a load unbalance at a relatively low speed and, if the degree of load unbalance is
greater than a predetermined acceptable maximum, produces an alarm signal that indicates
an unbalanced load. Under such circumstances, the proposed arrangement may either
attempt to rebalance the load by redistribution or may stop the motor so that the
unbalance can be manually redistributed. In this fashion, the arrangement of US-5543698
implements a Go/No-Go policy based on a single measure of unbalance detected at a
low speed.
[0008] In EP-1067230 a system is disclosed for measuring load unbalance in a washing machine
and using the value obtained for the load unbalance to calculate a maximum permissible
angular velocity for the drum during the water extraction cycle, i.e. to set an upper
limit for spin speed. In a first step, the drum is accelerated to a relatively low
speed where the value for the unbalanced mass in the drum is determined. Typically
this relatively low speed is the speed needed to produce a centrifugal acceleration
of about 2G on the load and, with a drum diameter of say 750mm, this means a drum
speed of about 69 RPM. Once the mass of the unbalance has been derived, a maximum
safe spinning speed is calculated in dependence on that mass. In this fashion, the
arrangement of EP-1067230 sets a fixed limit to the spin speed based on a single measure
of unbalance detected at a low speed.
[0009] No account is taken in either US-5543698 or in EP-1067230 of the fact that the level
or the effect of unbalance may change as rotational speed of the drum increases or
while spinning at steady state. It is also assumed that the load is homogenous, i.e.
that the water absorption characteristics are the same throughout the load.
[0010] Consideration will first be given to the assumption that the effect of the unbalance
is constant throughout speed and time, assuming for the moment a homogenous load.
If unbalance is measured at a relatively low speed, there is still a lot of liquid
in the liquid absorbing load. This means that an unbalance measured at a relative
low speed may be somewhat bigger than an unbalance from the same starting conditions
will be at high speed, since a lot of liquid will still flow out the load during the
acceleration from the relative low speed to the higher spin speed. If the controller
calculates the maximum safe spinning speed with this unbalance measured at a relatively
low speed, this calculated maximum safe spinning speed may not be optimized and may
err excessively on the safe side, leading consequently to less efficient and longer
extraction cycles and/or longer drying times. This will now be explained by way of
an illustrative example with particular reference for the moment to Figure 1.
[0011] Suppose we have a rigid mounted 16 kg machine loaded with a homogenous 13 kg dry
load and that, when the machine starts the spinning sequence, the load is not equally
divided on the interior surface of the drum. Suppose also that at the relative low
speed (say 90 RPM to produce a centrifugal acceleration of about 2G on the load),
a 6 kg unbalanced mass is measured in the drum. Out of practical experience, an exemplary
homogenous dry load of 13 kg may have a mass of about 34 kg after wash and spin until
90 RPM and a mass of about 23 kg after spinning until 500 RPM is reached. When the
complete load is of the same type with the same absorption coefficient, the unbalanced
mass of 6 kg will also lose the same proportion of liquid. Thus, this unbalanced mass
of 6 kg at 90 RPM will be an unbalanced mass of about 6 x 23/34 = 4,06 kg at 500 RPM.
According to EP-1067230, however, the allowable final spin speed is computed with
the measured unbalanced mass of 6 kg. This computed final spin speed may well be lower
than the true allowable final spin speed, since the unbalanced mass will be only 4,06
kg by the time a spin speed of 500 RPM is reached.
[0012] In summary, the maximum drum speed finally reached may in some cases be lower than
the actual maximum safe spinning speed because no account is taken of the fact that
the unbalanced mass in the drum will reduce (if the load is substantially of the same
type) as drum speed becomes higher and liquid flows out of the load. This has the
disadvantage that the load will have more residual humidity, which causes more drying
time and thus increases the cost to dry the load.
[0013] Turning now to the question of load homogeneity, as mentioned above the simplistic
approach of the prior art presumes that the load is completely homogenous. By this,
the prior proposals assume that the absorption factor throughout the complete load
must be the same. It is quite likely that in many cases the load may comprise laundry
articles of different materials having different liquid absorption coefficients. This
may mean that, under certain circumstances, the actual safe spinning speed for the
machine will be lower than the calculated safe spinning speed and machine damage may
result. This too will now be illustrated by way of example with particular reference
for the moment to Figure 2.
[0014] Suppose the liquid absorbent load in the drum includes two different materials that
we will call Mat 1 and Mat 2, Mat 1 having a relatively big liquid absorption coefficient
and Mat 2 having a relatively small liquid absorption coefficient. Suppose also that
we put 2.5kg of Mat 1 and 4kg of Mat 2 in a drum. Once the spin sequence has started,
according to EP-1067230 or US-5543698 the drum is accelerated to a relative low speed
(we will call it speed A)
where the value for the unbalanced mass in the drum is determined. Suppose now that
in the drum Mat 1 is at one side of the drum while Mat 2 is at the other side, as
can be seen in Figure 2. Since the liquid absorption coefficient of Mat 1 is bigger
than the liquid absorption coefficient of Mat 2, the Mat 1 will contain relatively
more liquid than Mat 2. Suppose at this relative low speed A, Mat 1 has absorbed 100%
of its body weight in liquid and that Mat 2 has absorbed 25% of its body weight in
liquid. This means that in the 2.5 kg load of Mat 1, there will be absorbed an additional
2.5 kg liquid and in the 4 kg load of Mat 2 there will be absorbed an additional 1
kg of liquid at this particular relatively low speed A. At speed A the masses in the
drum are thus divided as in Figure 2 and it can be seen that there is therefore no
resultant unbalance in the drum at this speed A.
[0015] According to US-5543698, unbalanced mass is measured at this relatively low speed
A and, based on any such unbalanced mass, a Go/No-Go decision is made as to whether
acceleration to the final spinning speed can start. According to EP-1067230 the unbalanced
mass is measured at this relative low speed A and with this measured unbalanced mass
the maximum safe spinning speed is calculated. In both prior art arrangements, the
controller will decide to accelerate to a certain target speed T of the machine. During
this acceleration water flows out of both materials MAT 1 and 2 of the load. Suppose
that, when the drum reaches the target speed T, 50% of the liquid that was still in
the load at speed A will have flowed out of the load. This means that there will still
be 1.25kg liquid in Mat 1 and 0.5kg liquid in Mat 2 as can be seen in Figure 2. The
total mass of Mat 1 with liquid is 3.75kg and the total mass of Mat 2 with liquid
is 4.5kg such that at this speed T there is now a resulting unbalanced mass of 0.75kg
in the drum. This contradicts the expectation that there would be at worst the same
or more usually even a smaller unbalance than at speed A. This unexpected unbalanced
mass is likely to cause vibration of the machine and damage may result.
[0016] This example shows clearly that measuring the unbalanced mass at a relatively low
speed and making a decision based on this unbalanced mass may not always be the right
decision. In fact, as long as there is still a lot of liquid in the liquid absorbing
load, the unbalanced mass in the drum can still change (and sometimes become bigger)
during the acceleration. It may prove unwise to use a measure of unbalanced mass obtained
at low speed for calculation of the maximum safe spinning speed or for deciding if
the unbalanced mass in the drum is sufficiently low for even starting acceleration
to a fixed final spinning speed. The finally reached maximum drum speed will in some
cases be higher than the actual maximum safe spinning speed because the unbalance
can in some cases increase when the drum speed gets higher, e.g. when the load is
not of the same type. Possible machine damage and/or reduced life time can be consequences.
[0017] Using the arrangement of EP-1067230, the calculation to which maximum safe spinning
speed the drum will be accelerated may not always be the right maximum safe spinning
speed and can possibly damage machine parts. In like manner, the value used for making
a Go/No-Go decision according to US-5543698 may not always lead to the right decision
being made. Using a mechanical limit switch is equivalent to making a measurement
at relative low drum speed (at or below the own frequency of the machine) and making
from its output the decision as to whether acceleration can continue or not. For the
same reasons outlined above, this decision may not always be right.
[0018] Using prior art laundry arrangements may also result in the maximum drum speed reached
being lower than the actual maximum safe spinning speed available because the axial
position of the unbalance in the drum is not determined or taken account of. This
has the disadvantage, in itself or in addition to those discussed above, that the
load may have more residual humidity at the end of a spin cycle. This in turn necessitates
more drying time and thus increases the cost to dry the load.
SUMMARY OF THE INVENTION
[0019] It is an object of the present invention to provide improved laundry arrangements
and in particular to provide improved laundry apparatus having a rotatable drum and
improved control methods associated therewith for managing out-of-balance loads thereof.
These improved laundry apparatus encompass washer extractors such as washing machines
that include a water extraction cycle, often referred to as a spinning program. They
also include pure extractors and dehumidifiers such as spin dryer machines and the
like.
[0020] Accordingly, the present invention provides a laundry apparatus comprising:
a) a drum adapted to receive a load of laundry and to be rotated about an axis;
b) a sensing means adapted to provide an unbalance signal indicative of a substantially
instantaneous unbalance of said load; and
c) a control means which is adapted to monitor said unbalance signal and to control
the rotation of said drum in response thereto;
characterized in that said control means is adapted to compare said instantaneous
unbalance with a predefined relationship between maximum permissible unbalance and
rotational speed of said drum and to alter an acceleration or rotational speed of
said drum if said instantaneous unbalance exceeds a maximum permissible unbalance
defined for a substantially instantaneous rotational speed at which said instantaneous
unbalance was sensed. In this manner, the present invention provides a laundry arrangement
that always operates substantially within the mechanical limits of its components
and avoids the problem of potentially setting too high or low a spin speed. In addition,
there is also a reduced chance of preventing acceleration of the drum all the way
up to a spin speed that the apparatus could in fact usefully handle. The relationship
between unbalance and speed is preferably mapped into a memory of said controller,
but may alternatively be defined using an algorithm or calculation that may be performed
substantially in real time.
[0021] Said relationship may be based on a predetermined design life of one or more components
of said apparatus. Said relationship may be based on a predetermined mechanical limit
of one or more components of said apparatus. Said maximum permissible unbalance may
be derived from a load or durability characteristic of a bearing arrangement adapted
to support said drum.
[0022] Said sensing means may be adapted to sense said instantaneous unbalance during at
least one of acceleration of said drum and at substantially constant rotational speeds
thereof. Said unbalance signal may be compared with said relationship at least one
of substantially continuously or at a plurality of predetermined rotational speeds
of said drum.
[0023] Said apparatus may further comprise a drive motor adapted to rotate said drum under
the control of said control means, wherein said sensing means is adapted to detect
said instantaneous unbalance from a load characteristic of said drive motor.
[0024] Said motor may comprise an alternating current motor and said sensing means may be
adapted to monitor at least one of a motor current, a phase angle between motor voltage
and motor current, a motor power factor, a motor speed, a motor slip characteristic
or a motor torque. Said sensing means may comprise a frequency inverter.
[0025] The present invention also provides a method of operating a laundry apparatus, said
apparatus comprising a drum adapted to receive a load of laundry and to be rotated
about an axis under the control of a control means, the method including:
a) sensing an instantaneous unbalance of said load and an instantaneous rotational
speed of said drum;
b) comparing said instantaneous unbalance with a predefined relationship of a mechanical
limit between maximum permissible unbalance and rotational speed of said drum; and
c) altering an acceleration or rotational speed of said drum if said instantaneous
unbalance exceeds a said maximum permissible unbalance defined for a said instantaneous
rotational speed at which said instantaneous unbalance was sensed.
[0026] The method may include stopping an acceleration of said drum, preferably substantially
immediately, in the event that said instantaneous unbalance exceeds said maximum permissible
unbalance for said instantaneous rotational speed. The rotational speed achieved prior
to stopping said acceleration may then be maintained. In this manner, the rotational
speed achieved may be the maximum permissible for the current unbalance and may only
be increased by further acceleration if rotation at that speed removes sufficient
liquid from said load. Such liquid removal may need to be sufficient to reduce the
instantaneous unbalance to a level below the mechanical limit for the rotational speed
at which the excessive unbalance originally caused said acceleration to be stopped.
[0027] The method may include determining, at least in the event of stopping a said acceleration
of said drum, whether or not the rotational speed of said drum is sufficiently high
to achieve a predetermined residual humidity in said load.
[0028] The method may include decelerating said drum to or below a predetermined rotational
speed and implementing a redistribution cycle so as to redistribute said load in said
drum, at least in the event that the rotational speed of said drum prior to halting
an acceleration thereof is insufficient to achieve a predetermined residual humidity
of said load.
[0029] The method may include comparing a said instantaneous unbalance determined during
a distribution cycle or around the start of a drum acceleration with a predetermined
distribute unbalance level for which it is likely that, if unbalance is smaller than
this predetermined level, a preferred minimum rotational speed of said drum is achievable.
If the unbalance in this distribution cycle is higher than this predetermined level
of unbalance, from experience it can be said that during acceleration the mechanical
limit will be reached at a relative low drum speed for which the requested residual
humidity of the load will not be achieved. This predetermined level of unbalance at
distribution speed will be referred to for convenience as a "distribute unbalance
level" and is substantially lower than the mechanical limit of the machine at this
distribution speed.
[0030] The method may include varying said distribute unbalance level after one or more
said redistribution cycles, a variation to said distribute unbalance level depending
on a predetermined cycle requirement.
[0031] The method may include increasing said distribute unbalance level if keeping within
a wash cycle time is said cycle requirement, thereby ensuring that acceleration starts
even if a final drum rotational speed achievable will be lower than a preferred minimum
speed.
[0032] The method may include decreasing said distribute unbalance level if achieving a
predetermined final drum rotational speed is said cycle requirement. A said decreased
distribute unbalance level will on average result in a smaller unbalance at high speed
and will thus on average result in a higher drum speed being achievable.
[0033] The method may include accelerating said drum after a predetermined number of redistribution
cycles even if a said instantaneous unbalance at the start of said acceleration is
equal to or greater than a said unbalance determined prior to the or each said redistribution
cycle.
[0034] The method may include basing said mechanical limit on a predetermined design life
of one or more components of said apparatus, such as for example a load or durability
characteristic of a bearing arrangement adapted to support said drum during rotation.
[0035] The method may include maintaining a rotational speed of said drum for a predetermined
time period, at least in the event that said rotational speed was reached by said
drum prior to halting an acceleration thereof and is sufficient to achieve a predetermined
residual humidity of said load.
[0036] The method may include accelerating said drum in the event that a said unbalance
reduces below a said maximum permissible unbalance for said instantaneous rotational
speed.
[0037] The method may include at least temporarily abandoning an acceleration of said drum
if a said instantaneous unbalance sensed during a said distribution cycle or acceleration
therefrom exceeds said predetermined level.
[0038] The method may include sensing said instantaneous unbalance only up to a predetermined
rotational speed of said drum, at which predetermined rotational speed a said instantaneous
unbalance is detectable within predetermined limits of accuracy, and from that speed
upwards computing a preferred rotational speed of said drum based on one or more sensed
measurements made below said predetermined rotational speed.
[0039] The method may include comparing substantially instantaneous unbalance with said
relationship at least one of substantially continuously or at a plurality of predetermined
rotational speeds of said drum.
[0040] The method may include sensing a said instantaneous unbalance by determining at least
one of an unbalance mass in said drum and from a force exerted on a predetermined
component of said apparatus.
[0041] The method may include sensing a said instantaneous unbalance from a characteristic
of a drive motor rotating said drum, such as from a motor current, a phase angle between
motor voltage and motor current, a motor power factor, a motor speed, a motor slip
characteristic or a motor torque of said drive motor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0042]
Figure 1 is a representation of an unbalanced load in a drum of a laundry apparatus;
Figure 2 is a representation of two opposing unbalanced loads in a drum of a laundry
apparatus;
Figure 3 is a schematic representation of a side view in cross-section of a laundry
apparatus according to an embodiment of the present invention;
Figures 4 and 5 are partial views of a rigid mounted version of the apparatus of Figure
3 showing the effects of position of an unbalance in a drum, front and rear respectively,
on the load felt by a bearing arrangement adapted to support rotation of the drum;
Figure 6 is a graphical representation of a mapped relationship between acceptable
drum speed depending on unbalance mass for the apparatus of Figure 3;
Figure 7 is a graphical representation of a mapped relationship between acceptable
unbalance mass depending on drum speed for the apparatus of Figure 3;
Figure 8 is an exemplary application of the mapped relationship of Figure 7 to an
operational cycle of the laundry apparatus of Figure 3;
Figure 9 is an equivalent view to that of Figure 4 for a free-standing version of
the apparatus of Figure 3, shown with respect to a load and drum speed appropriate
to such a free-standing apparatus;
Figure 10 is a graphical representation of a mapped relationship between acceptable
drum speed depending on unbalance mass for the apparatus of Figure 9;
Figure 11 is a graphical representation of a mapped relationship between acceptable
unbalance mass depending on drum speed for the apparatus of Figure 9;
Figure 12 is a graphical representation of a mapped relationship between acceptable
unbalance mass and tub movement depending on drum speed for the apparatus of Figure
9;
Figure 13 is a representation on one graph of acceptable unbalanced mass for tub movement
and acceptable unbalanced mass for maximum centrifugal force;
Figure 14 is a graphical representation of a resultant mechanical limit derived from
the two limits represented in Figure 13;
Figure 15 is a graphical representation of the application of the mapped relationship
of Figure 14 to the apparatus of Figure 9; and
Figure 16 is a graphical representation of acceptable drum speeds depending on axial
position in a drum of an unbalance.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0043] The present invention will now be described by way of example only, with reference
to certain embodiments and with reference to the above mentioned drawings.
[0044] The present invention provides a laundry apparatus in the form of a washing machine
10 operated using a programmable control means in the form of a cycle controller 12.
The machine 10 comprises a drum 14 which is rotatable inside a tub 16 about a substantially
horizontal axis C/L, the drum 14 being supported for rotation on a horizontal axle
18 supported near the drum 14 by a front bearing 20 and at a driven end distal from
the drum 14 by a rear bearing 22. Drive is provided to the axle 18 through a transmission
24 by an AC motor 26 under the control of the cycle controller 12. A sensing means
is provided which detects the level of unbalance in the drum 14 and provides an unbalance
signal indicative thereof to the cycle controller 12. The sensing means may take a
variety of forms, some embodiments of which will now be discussed.
[0045] In one embodiment, the sensing means comprises for example a frequency inverter 28
or similar associated with or implemented in a motor control portion of the cycle
controller 12.
[0046] The frequency inverter 28 monitors variations in motor load which, if present, are
indications of the level of unbalanced mass in the drum 14. An advantage of this approach
is that often commercially available cycle controllers 12 already include at least
the hardware necessary to implement such a sensing means so that extra costs are minimized.
All that is needed is a calculation to determine the size of the motor load variation,
which can generally be achieved merely by activating a software function in the cycle
controller 12. The load value or motor characteristic which the sensing means monitors
may include the motor current, the phase angle between motor voltage and motor current,
the power factor, the motor speed, the motor slip in case of an asynchronous motor
or the motor torque.
[0047] Besides the above described disadvantage of measuring the unbalanced mass when its
position in the drum 14 cannot be determined, often there is still the disadvantage
that the unbalanced mass may not be accurately measurable until a relatively high
drum speed has been reached. This can be explained as follows:
[0048] The variation in motor load is caused by the fact the unbalanced mass will be influenced
by gravity acceleration. When unbalance in the drum 14 is going down, the gravity
acceleration will help to pull the unbalanced mass down which will reduce the motor
load. When the drum speed is low, the variation in speed and thus the variation in
motor load will be big, because of the gravity acceleration. This can be understood
more easily when considering the definition of acceleration:
![](https://data.epo.org/publication-server/image?imagePath=2003/37/DOC/EPNWA1/EP02447033NWA1/imgb0001)
where "δv" is the variation in speed under influence of the acceleration "a" for
the time "δt".
[0049] Since in this case the acceleration is the acceleration "g" due to gravity and is
thus substantially constant at g = 9.81m/s
2, when δt and δy are big so too will be the variation in motor load. At low drum speed,
the time between the highest and the lowest points of the unbalanced mass will be
high, thus leading to a high δt which in turn may cause δv to be high. Thus at low
speed the variation in motor load will be high. At high drum speed now, the time between
the unbalanced mass being at the topside of the drum 14 and at the downside of the
drum 14 will be low. This means that δt is low and thus so too is δv. Thus, at high
drum speed the variation in motor load will be low.
[0050] To minimize this disadvantage, the cycle controller 12 of the present invention computes
a preferred angular velocity of the drum 14 on the basis of a measurement of the unbalanced
mass for the highest possible drum speed where the unbalance can still be accurately
measured in order to have only a small influence of the amount of water that is still
in the washing load.
[0051] The arrangement that is disclosed in US-5677606 may prove suitable as the basis of
a sensing means associated with a cycle controller 12 of the present invention. The
Mitsubishi invention is directed to a method and a device in which the instantaneous
load of a motor is sensed and a current average value of that motor load over time
is established. This current average value is compared with a mapped value for average
motor load and the number of times the current average value exceeds the mapped average
value is counted. If the current average value exceeds the mapped average value more
than a predetermined number of times in a predetermined period, then motion of the
load is stopped.
[0052] An alternative to motor load monitoring is to use measuring techniques which detect
forces in the material they are attached to and do not measure unbalanced masses in
the drum 14. In this manner, forces induced by unbalanced loads can be measured and
the position of the unbalanced mass in the drum 14 is automatically taken into account.
These sensing means may be attached to or between machine parts in which the mechanical
force is proportional to the force in one or both bearings 20, 22. Sensing means may
be used that substantially continuously provide to the cycle controller 12 a signal
that is an indication for the value of the measured force exhibited by the unbalance.
The sensing means may be implemented using one or more accelerometers and using them
to measure the vibration by measuring the mechanical forces induced in a load cell.
Other examples would be to use piezo-electric elements or strain gauges adapted to
sense distortion of bearing housings 20a, 22a.
[0053] In similar fashion to the embodiment which employs motor load monitoring, the mechanical
force felt by these and equivalent sensing means during acceleration is again substantially
continuously compared with the maximum allowable mechanical force mapped in the cycle
controller 12. If that maximum allowable mechanical force is exceeded, acceleration
is stopped instantly and the achieved rotational speed of the drum 14 substantially
maintained. If the machine 10 is operating at a steady state and the sensing means
of any embodiment indicates that the mechanical limit curve has been exceeded, the
rotational speed of the drum 14 is altered to bring the force below that limit curve,
e.g. by deceleration.
[0054] Using the force measuring techniques discussed above may mean that it is difficult
to execute accurate measurements at low drum speeds when unbalanced forces are small.
Therefore it may become difficult to estimate with high certainty at low speed if
the speed that can be achieved during an extraction cycle will be sufficiently high
in order to achieve the expected and desired low residual humidity. Therefore time
will be lost by initially accelerating the drum and then having to decelerate it to
re-divide the load if the achieved spin speed is too low. The motor load monitoring
approach discussed above may therefore prove preferable, although it may be used in
addition to, as well as instead of, the force sensing in particular at low drum speeds.
[0055] Sensing means may be used in the form of a switch, such as a piezo-electric switch
adapted to change state if a predetermined force is reached. Such a sensing means
must then be adjusted to switch on or before the designed limit of vibration which
stays within the lifecycle limits of the machine. If the allowable mechanical force
is reached and the sensing means switches, acceleration may be stopped substantially
instantly in similar fashion to other embodiments. In that way, the maximum spinning
speed reached will always be within the mechanical limit that the machine can handle
for its proposed lifetime.
[0056] Within today's measuring techniques, there are many types of sensor and techniques
available that can measure the unbalanced mass in the drum over a wide drum speed
range with the required accuracy and/or measure forces in a material or between two
machine parts. The final choice of the sensing technique or combination of sensing
techniques may depend on quality and cost requirements or on other detail design considerations.
The specific techniques used herein are not essential or limiting. What does matter
is that the technique used enables the cycle controller 12 to make a realistic comparison
between a substantially instantaneous unbalance and a mapped relationship between
maximum permissible unbalance and substantially instantaneous rotational speed of
the drum 14. Such a relationship is preferably developed through durability testing
to represent a mechanical limit curve for one or more components of the machine 10
in question across its life, as will now be discussed.
[0057] The present invention thus provides a laundry arrangement adapted to ensure that
the spinning speed of the machine 10 concerned does not exceed a level at which abnormal
wear or damage will affect machine durability or reliability. In order to achieve
this, it may be useful to consider how limits to such criteria may be set.
[0058] Generally, when a new model of washing machine 10 is designed it will be constructed
to a predetermined minimum durability. Durability will be analyzed and tested to ensure
that, amongst other things, the machine 10 has the ability to withstand for a predetermined
time a predetermined amount of unbalanced mass at a predetermined speed. The durability
requirements depend upon many influences, including for example market aspirations
for expected speed and lifetime, arrangements for counteracting unbalanced mass in
the drum 14 and not least whether it is to be rigid mounted or a free standing machine
type.
[0059] By way of example of durability requirements, reference is made for the moment in
particular to Figure 3 in which the effects of axial position in the drum 14 of an
unbalanced load are considered.
[0060] A machine with a capacity of 10kg is designed for a lifetime of 20000 cycles, with
a spinning speed of 450 RPM having an unbalanced mass of 4 kg at the front of the
drum, i.e. at the side of the door for a front-loading washing machine. The centrifugal
force because of the unbalanced mass in the drum 14 that the machine 10 can handle
can be written as:
![](https://data.epo.org/publication-server/image?imagePath=2003/37/DOC/EPNWA1/EP02447033NWA1/imgb0002)
With
Fc = Centrifugal force
M = Unbalanced mass in the drum = 4kg
W = Angular velocity = 2 x Pi x f = 2 x Pi x n/60 = 47.1225 rad/s
n = drum speed = 450 RPM
Pi = 3,1415
f = frequency (of the drum) = n/60 = 7.5 Hz
R = Radius from the center of rotation until the center of gravity of the unbalanced
mass = 0.265
[0061] From the above, it can be seen that the machine 10 can survive a predefined level
of unbalance in the form of a centrifugal force Fc = 2353.76 N at the front of the
drum 14 for its expected lifetime. If the unbalanced mass were to be positioned in
the center or at the back of the drum 14, the centrifugal force Fc that the machine
10 could withstand for its lifetime would be bigger. This will be explained by the
following example. The determining factor to reach the expected lifetime of the machine
10 may be that it should not exceed the maximum allowable force that the bearings
20, 22 can cope with. The mechanical construction of the machine 10 has to be strong
enough to handle the forces that the drum axle 18 brings on the bearings 20, 22. These
forces are carried further from the bearings 20, 22 to the bearing housings 20a, 22a
and led further from the bearing housings 20a, 22a to the machine frame (not shown
separately from the tub 16). Suppose now for the moment that the distances between
point of application of the centrifugal force of the unbalance and the front bearing
20 and rear bearing 22 are as indicated on Figure 4, in which:
Fc = Centrifugal force of the unbalanced mass on the drum = 2353,8 N
R1 = Reaction force of the first bearing
R2 = Reaction force of the second bearing.
Because of the principle of moments, the above can be written as:
![](https://data.epo.org/publication-server/image?imagePath=2003/37/DOC/EPNWA1/EP02447033NWA1/imgb0003)
This means
![](https://data.epo.org/publication-server/image?imagePath=2003/37/DOC/EPNWA1/EP02447033NWA1/imgb0004)
The sum of forces must be zero, so this can also be written as:
![](https://data.epo.org/publication-server/image?imagePath=2003/37/DOC/EPNWA1/EP02447033NWA1/imgb0005)
Thus:
![](https://data.epo.org/publication-server/image?imagePath=2003/37/DOC/EPNWA1/EP02447033NWA1/imgb0006)
[0062] So, if the machine can withstand Fc = 2353,8 at the front of the drum 14 for its
proposed lifetime, this means in fact that the machine 10 can withstand for its proposed
lifetime a force of 7061,4 N in the front bearing 20 and a force of 4707,6 N in its
rear bearing 22.
[0063] Suppose now that the unbalanced mass is not positioned at the front of the drum 14,
but at the back of the drum 14 as shown in Figure 5. In this situation, the allowable
centrifugal force can be recalculated in order to have the same force on the front
bearing 20 as calculated above:
[0064] So if R1 is taken as the same (R1 = 7061,4 N), then Fc and R2 can be calculated.
Again, because of principle of moments, this can be written as:
![](https://data.epo.org/publication-server/image?imagePath=2003/37/DOC/EPNWA1/EP02447033NWA1/imgb0007)
This means that
![](https://data.epo.org/publication-server/image?imagePath=2003/37/DOC/EPNWA1/EP02447033NWA1/imgb0008)
Also, because the sum of forces must be zero:
![](https://data.epo.org/publication-server/image?imagePath=2003/37/DOC/EPNWA1/EP02447033NWA1/imgb0009)
Thus:
![](https://data.epo.org/publication-server/image?imagePath=2003/37/DOC/EPNWA1/EP02447033NWA1/imgb0010)
Thus:
![](https://data.epo.org/publication-server/image?imagePath=2003/37/DOC/EPNWA1/EP02447033NWA1/imgb0011)
And
![](https://data.epo.org/publication-server/image?imagePath=2003/37/DOC/EPNWA1/EP02447033NWA1/imgb0012)
[0065] Out of this calculation it is clear that a centrifugal force Fc of 2353,8 N at the
front of the drum 14 causes the same force on the front bearing 20 as the centrifugal
force Fc of 4707,6 N at the back of the drum 14. With the centrifugal force of 4707,6
N at the back of the drum 14, the force on the second bearing 22 is smaller than with
the centrifugal force of 2353,8 N at the front of the drum 14. This means also that
the lifetime of the machine 10 will be equal or bigger with the centrifugal force
of 4707,6 N at the back of the drum. So when an unbalance is positioned at the back
of the drum 14 bigger centrifugal forces can be tolerated, which means that for the
same unbalance a higher drum speed can be allowed or, for the same drum speed, there
can be allowed a higher unbalance at the back of the drum.
[0066] In general, it can be seen that measuring unbalanced masses in the drum 14 without
knowing the position of the unbalance in the drum 14 is not sufficient to always reach
the maximum drum speed that the machine 10 can handle for its proposed lifetime.
[0067] In the following explanations, certain values for this time, speed and unbalanced
mass will be used to calculate exemplary durability requirements expressed as predefined
relationships in the form of exemplary mapped relationships between maximum permitted
unbalance against instantaneous rotational speed. It will be appreciated that, while
in each embodiment the relationship between permissible unbalance and rotational speed
is predefined by mapping it into a memory associated with the controller 12, the relationship
may be defined in other ways such as by using an algorithm or calculation which may
be performed substantially in real time. The values used herein are only meant for
explanation and in practice may well differ from the values used. The following explanation
will be divided into two parts. The first part will be for rigid mounted machines,
while the second part will be for free standing machines having a suspension system.
Rigid mounted machines (without suspension systems)
[0068] Referring for the moment back to the example of Figure 3, a case is given in which
a machine 10 with a capacity of 10kg is designed for a lifetime of 20000 cycles, with
a spinning speed of 450 RPM and an unbalanced mass of 4 kg at the front of the drum,
i.e. at the side of the door 16a for a front-loading washing machine 10.
[0069] There it was calculated that this machine 10 could handle the centrifugal force Fc
= 2353.76 N at the front of the drum 14 for its expected lifetime. If this value is
used in equation (1) for Fc, then the maximum safe spinning speed can be calculated
for each possible unbalance. Or by way of a corollary, there can be calculated for
each possible drum speed the maximum allowable unbalance at the front of the drum
14. By doing this, the mapped relationships shown in Figures 6 and 7 can be generated.
The area below the curves represents a working area that the machine 10 can handle
for its proposed lifetime. So from the point of view of mechanical durability, it
is sufficient to stay below these curves to reach the expected lifetime of the machine
10. For convenience, these curves will be considered to represent the mechanical limit
curve of the machine 10, seen from two complementary approaches, which in common with
other embodiments may be based on for example the design life of one or more components
of the machine such as a load or durability characteristic of a bearing arrangement
adapted to support rotation of the drum. If at some point during a drum acceleration
or while operating at a steady state speed the unbalanced mass goes above the mechanical
limit curve, the acceleration is stopped instantly or the rotational speed reduced
as the case may be. This will occur in each embodiment of the present invention if
a substantially instantaneous unbalance exceeds the predefined value for maximum permissible
unbalance defined or calculated with respect to the substantially instantaneous speed
for which that particular instantaneous unbalance was sensed.
[0070] If this mechanical limit curve is used only for determining the maximum safe spinning
speed then, in some cases when the unbalanced mass is high, the final spinning speed
may be low. This may leave the load with a higher than desirable residual humidity
at the end of a spin cycle. The energy needed to completely dry this load in a drying
cycle would therefore increase, as would the cost associated with drying this load.
It is therefore desirable to have an adequately fast spinning speed that can be guaranteed
as achievable in almost all cases.
[0071] Suppose for the moment that a spin speed of 400 RPM meets the extraction requirements
in most cases. Referring for the moment in particular to Figure 7, the drum speed
of 400 RPM corresponds with a mapped maximum permissible unbalanced mass in the drum
14 of 5 kg. To be almost sure that the spin speed of 400 RPM will be reached, the
cycle controller 12 is programmed as follows: if the acceleration is stopped before
the spin speed of 400 RPM is reached, the controller 12 will give command to decelerate
the drum 14 again until it reaches a low speed (see Figure 8 "Try 1) where a redistribution
cycle is implemented, i.e. a sequence of drum movements to try to divide the load
more equally on the interior surface of the drum 14. Then acceleration of the drum
14 starts again with probably a more equally divided load in the drum 14 so that there
will be less unbalanced mass in the drum 14. This should mean that acceleration will
be stopped at a higher drum speed, to which end the reader is referred to Figure 8
"Try 2". Suppose acceleration is again stopped before 400 RPM, in which case the controller
12 will give the same commands again as explained above. This may be repeated for
a predetermined number of times, for instance a maximum 10 times. If after trying
this 10 times the machine 10 still does not succeed in reaching the 400 RPM spin speed,
the machine 10 will decelerate and finish its cycle. However, it is anticipated that
in practice in almost all cases the speed of 400 RPM will be reached in the 10 possible
tries. So there will always an attempt to reach a final spinning speed above "zone
1" in Figure 8, because this zone would leave a too big a residual humidity in the
load.
[0072] Optionally the drum may be accelerated up to whatever limit it can reach, for example
after a predetermined number of redistribution events, such that the load can be spun-dry
at least to the extent possible within the mechanical limit, i.e. at the speed achievable
before acceleration is stopped. This may be performed even if the instantaneous unbalance
detected at the start of this acceleration is equal to or possibly even greater than
the instantaneous unbalance determined prior to any one or more of those redistribution
cycles.
[0073] Depending on the available motor power, the machine 10 will also have a maximum spin
speed that can be supplied by the motor 26. Suppose the motor power is such that the
maximum possible drum speed is 550 RPM. This means that "zone 2" in Figure 8 can never
be reached because of the limited motor power. Thus in the example shown, the final
spin speed will be between 400 and 550 RPM in almost all cases.
[0074] Another parameter that is important for the washing or extraction process is the
cycle time. The cycle time should be as short as possible to save time and money.
In order to reduce the cycle time, another function can be added into the controller
12. When acceleration to spin speed is started, very soon (thus at relatively low
speed) it can be seen with high certainty from the value for the unbalanced mass whether
or not the machine 10 is likely to reach the desired spinning frequency of 400 RPM.
Out of practical tests it can be determined that for instance an unbalanced mass in
the drum of 8 kg or higher at 100 RPM will in 90 percent of the cases lead to a final
spin speed lower than 400 RPM. Therefore, if the controller 12 sees that the unbalanced
mass at very low speed is above a certain limit, the controller 12 will decide to
reduce speed again and do a sequence of movements in order to divide the load more
equally on the interior surface of the drum 14 and then start the spinning sequence
again. In this way the time needed for increasing the speed until the "mechanical
limit" curve is reached and the time for decreasing the speed again are potentially
reduced, along with the average cycle time.
[0075] It will be noted that all the mapped values of speeds, unbalanced mass and retrying
times of this fictive example may be adjusted in accordance with projected market
needs and machine type.
Free standing machines (with suspension systems)
[0076] Basically, the same method can be followed as for rigid mounted machines, but a free
standing machine has another "mechanical limit" curve. A rigid mounted machine can
handle very big unbalances at low speeds, since the centrifugal force that the unbalance
causes at low speeds is not big yet. However in a free standing machine there is the
suspension system that allows the tub assembly to move. At low speeds and when unbalanced
mass in the drum is very high, this movement of the tub can become too big so that
the tub assembly can touch other machine parts and can damage them.
[0077] Suppose a free standing machine with a drum capacity of 10kg is designed for a lifetime
of 20000 cycles, with a spinning speed of 1000 RPM with an unbalanced mass of 2,5
kg at the front of the drum.
[0078] The centrifugal force because of the unbalanced mass in the drum that the machine
can handle can be written as:
![](https://data.epo.org/publication-server/image?imagePath=2003/37/DOC/EPNWA1/EP02447033NWA1/imgb0013)
With
Fc = Centrifugal force
M = Unbalanced mass in the drum = 2,5kg
W = Angular velocity = 2 x Pi x f = 2 x Pi x n/60 = 104,72 rad/s
n = drum speed = 1000 RPM
Pi = 3,1415
f = frequency (of the drum) = n/60 = 16,67 Hz
R = Radius from the center of rotation until the center of gravity of the unbalanced
mass = 0,265
[0079] It is known that the machine can handle a centrifugal force Fc = 7264,7 N at the
front of the drum for its expected lifetime (see Figure 9). If this value is used
in equation (1) for Fc, then for each possible unbalance there can again be calculated
the maximum safe spinning speed. By doing this, curves as shown in Figures 10 and
11 may be derived and the relationship between maximum permissible unbalance and instantaneous
rotational speed mapped. As stated above, there is an additional mechanical limit
for free standing machines. Care has to be taken that movement of the tub assembly
does not ever damage machine parts. Therefore the maximum unbalance can be derived
from practical tests used to derive the maximum drum movement that can be tolerated
which is small enough to be sure that no machine parts can be damaged. Suppose the
result of this test is as in Figure 12.
[0080] Graphs have now been developed of the two mechanical limit curves for the free standing
machine 10. These curves can be put together on a single graph, and such a resultant
graph is shown in Figure 13. Since it is always necessary to stay below both curves,
the lowest points of both curves for every drum speed comprise in effect the real
"mechanical limit" curve. This can be seen in Figure 14, where only the lowest of
the two curves for each drum speed is drawn.
[0081] The area below the resultant limit curve of Figure 14 represents a working area that
the machine can handle for its proposed lifetime. So, from a mechanical point of view,
it is sufficient to stay below that resultant curve to reach the expected lifetime
of the machine. This resultant curve will be referred to for convenience and consistency
as the "the mechanical limit" curve of the machine .
[0082] In similar fashion to the operation of the rigid mounted machine, if at some point
during the acceleration the unbalanced mass comes above the mechanical limit curve,
acceleration is stopped instantly or the speed of rotation altered as the case may
be. Here again a zone "1" is defined where the residual humidity will be too big.
Again, if the finally reached drum speed is not above this zone (925 RPM), the controller
will act in similar fashion to that discussed above in relation to the rigid machine.
Thus the drum will be decelerated, the load re-divided and acceleration started again.
Also, a zone "2" that cannot be reached because of the limited motor power can be
indicated again. Such a situation can be seen by way of example with particular reference
to Figure 15.
[0083] In the examples of both rigid mounted and free standing machines, the principle of
handling unbalanced loads in the drum 14 is based on measuring unbalanced masses without
knowing the position of the unbalance in the drum. As mentioned above, measuring unbalanced
masses in the drum 14 without knowing the position of the unbalance in the drum may
not always be sufficient to reach the maximum drum speed that the machine can handle
for its proposed lifetime. Out of the example in relation to Figure 3, it has been
shown that a centrifugal force Fc of 2353,8 N at the front of the drum 14 causes the
same force on the front bearing 20 as the centrifugal force Fc of 4707,6 N at the
back of the drum.
[0084] In Figure 16, the mechanical limit curve of the unbalanced mass at the front of the
drum 14 and at the back of the drum 14 are drawn based on these allowable centrifugal
forces. In Figure 16, it is clear that the maximum allowable drum speed for the same
instantaneous unbalance is higher at the back of the drum 14, or in opposite fashion
it can be said that for the same drum speed a bigger unbalance at the back of the
drum can be allowed.
[0085] It can therefore be seen that measuring instantaneous unbalanced masses in the drum
during acceleration and comparing this with the "mechanical limit curve" of the machine
is a safe method. Because the position in the drum 14 cannot easily be detected, however,
the "mechanical limit curve" can be followed by way of compromise as if the unbalanced
mass were to be positioned at the front of the drum (worst case). Therefore, when
the unbalanced mass is not at the front of the drum, the drum speed will be limited
at a drum speed that is lower than the drum speed that the machine can handle for
its proposed lifetime.
[0086] It can thus be seen that acceleration will only be cut out once the highest speed
has been reached that falls within the mechanical limits of the laundry apparatus,
the mechanical limit depending for example on the predetermined design life or load
carrying/durability of certain machine components such as drum bearings. In this manner,
the present invention avoids the problem of potentially fixing too low or high a spin
speed for the mechanical limit. In addition, there is a reduced chance of preventing
acceleration in the first place to a useful spin speed that it could in fact handle
within its mechanical limits.
[0087] If the drum speed reached before acceleration is stopped is such that a predetermined
residual humidity of the load can be achieved in an acceptable predetermined period,
drum speed remains constant at the speed reached before acceleration was halted. If
it is determined that this speed will not achieve a predetermined and preferred level
of liquid extraction or residual humidity in the load, the drum may be decelerated
to a predetermined rotational speed so as to perform one or more redistribution events
and then re-accelerated in order to try again to reach a preferred minimum spinning
speed, this operation being repeatable. Comparison of the substantially instantaneous
unbalance sensed by the sensing means may be performed substantially continuously
or at a plurality of predetermined rotational speed of the drum.
[0088] Cycle requirements can be prioritized such that a level of unbalance referred to
as a "distribute unbalance level" is varied by either being pushed up or down so as
to allow a washing cycle or spinning cycle to stand a better chance of being completed.
This distribute unbalance level is a limit set for instantaneous unbalance detected
during a distribution cycle or around the start of an acceleration therefrom. The
level is such that, if an instantaneous unbalance detected during a distribution cycle
exceeds this level, it is unlikely that a preferred minimum spinning speed will be
reached. This is because, from experience and/or development testing, during acceleration
the mechanical limit will be reached at too low a drum speed to achieve the desired
humidity in the load. It will be noted that the "distribute unbalance" is at a level
that may be significantly lower than the mechanical limit of the machine concerned
at the speeds this comparison is made. Increasing the distribute unbalance level allows,
for example, temporary modification of cycle control to implement acceleration of
the drum after a predetermined number of redistribution events regardless of whether
the drum speed achievable is likely to meet the preferred minimum, so that liquid
extraction can take place at least to the minimum level possible under the circumstances
without damaging the machine. Temporarily decreasing the distribute unbalance level
allows on average smaller unbalances to be achieved and thus on average a higher final
spin speed before the mechanical limit of the machine is reached.
[0089] In some embodiments of the present invention, difficulty may be experienced in sensing
unbalance up to a preferred target minimum rotational speed. Under such circumstances,
it may be found advantageous to actively sense instantaneous unbalance only up to
a rotational speed which falls within a predetermined level of accuracy and, from
that speed upwards, to compute a preferred rotational speed based on measurements
made below the speed limit of accurate measurement.
[0090] While the present invention has been particularly shown and described with respect
to a preferred embodiment, it will be understood by those skilled in the art that
changes in form and detail may be made without departing from the scope and spirit
of the invention.
1. A laundry apparatus comprising:
a) a drum adapted to receive a load of laundry and to be rotated about an axis;
b) a sensing means adapted to provide an unbalance signal indicative of a substantially
instantaneous unbalance of said load; and
c) a control means which is adapted to monitor said unbalance signal and to control
the rotation of said drum in response thereto;
characterized in that said control means is adapted to compare said instantaneous unbalance with a predefined
relationship between maximum permissible unbalance and rotational speed of said drum
and to alter an acceleration or rotational speed of said drum if said instantaneous
unbalance exceeds a maximum permissible unbalance defined for a substantially instantaneous
rotational speed at which said instantaneous unbalance was sensed.
2. An apparatus according to claim 1, wherein said relationship is based on a predetermined
design life of one or more components of said apparatus.
3. An apparatus according to claim 1 or claim 2, wherein said relationship is based on
a predetermined mechanical limit of one or more components of said apparatus.
4. An apparatus according to any preceding claim, wherein said maximum permissible unbalance
is derived from a load or durability characteristic of a bearing arrangement adapted
to support said drum.
5. An apparatus according to any preceding claim, wherein said sensing means is adapted
to sense said instantaneous unbalance during at least one of acceleration of said
drum and at substantially constant rotational speeds thereof.
6. An apparatus according to any preceding claim, wherein said unbalance signal is compared
with said relationship at least one of substantially continuously or at a plurality
of predetermined rotational speeds of said drum.
7. An apparatus according to any preceding claim, said apparatus further comprising a
drive motor adapted to rotate said drum under the control of said control means, wherein
said sensing means is adapted to detect said instantaneous unbalance from a load characteristic
of said drive motor.
8. An apparatus according to claim 7, wherein said motor comprises an alternating current
motor and said sensing means is adapted to monitor at least one of a motor current,
a phase angle between motor voltage and motor current, a motor power factor, a motor
speed, a motor slip characteristic or a motor torque.
9. A method of operating a laundry apparatus, said apparatus comprising a drum adapted
to receive a load of laundry and to be rotated about an axis under the control of
a control means, the method including:
a) sensing an instantaneous unbalance of said load and an instantaneous rotational
speed of said drum;
b) comparing said instantaneous unbalance with a predefined relationship of a mechanical
limit between maximum permissible unbalance and rotational speed of said drum; and
c) altering an acceleration or rotational speed of said drum if said instantaneous
unbalance exceeds a said maximum permissible unbalance defined for a said instantaneous
rotational speed at which said instantaneous unbalance was sensed.
10. A method according to claim 9, including stopping an acceleration of said drum, preferably
substantially immediately, in the event that said instantaneous unbalance exceeds
said maximum permissible unbalance for said instantaneous rotational speed.
11. .A method according to claim 9 or claim 10, including determining, at least in the
event of stopping a said acceleration of said drum, whether or not the rotational
speed of said drum is sufficiently high to achieve a predetermined residual humidity
in said load.
12. A method according to any one of claims 9 to 11, including decelerating said drum
to or below a predetermined rotational speed and implementing a redistribution cycle
so as to redistribute said load in said drum, at least in the event that the rotational
speed of said drum prior to halting an acceleration thereof is insufficient to achieve
a predetermined residual humidity of said load.
13. A method according to claim 12, including comparing a said instantaneous unbalance
determined during a distribution cycle or around the start of a drum acceleration
with a predetermined distribute unbalance level for which it is likely that, if said
instantaneous unbalance is smaller than said predetermined distribute unbalance level,
a preferred minimum rotational speed of said drum is achievable.
14. A method according to claim 13, including varying said distribute unbalance level
after one or more said redistribution cycles, a variation to said distribute unbalance
level depending on a predetermined cycle requirement.
15. A method according to claim 14, including increasing said distribute unbalance level
if keeping within a predetermined wash cycle time is said cycle requirement, thereby
ensuring that acceleration starts even if a final drum rotational speed achievable
will be lower than a preferred minimum speed.
16. A method according to claim 14, including decreasing said distribute unbalance level
if achieving a predetermined final drum rotational speed is said cycle requirement.
17. A method according to any one of claims 12 to 16, including accelerating said drum
after a predetermined number of redistribution cycles even if a said instantaneous
unbalance at the start of said acceleration is equal to or greater than a said instantaneous
unbalance determined prior to the or each said redistribution cycle.
18. A method according to any one of claims 9 to 17, including basing said mechanical
limit on a predetermined design life of one or more components of said apparatus,
such as for example a load or durability characteristic of a bearing arrangement adapted
to support said drum during rotation.
19. A method according to any one of claims 9 to 18, including maintaining a rotational
speed of said drum for a predetermined time period, at least in the event that said
rotational speed was reached by said drum prior to halting an acceleration thereof
and is sufficient to achieve a predetermined residual humidity of said load.
20. A method according to any one of claims 9 to 19, including accelerating said drum
in the event that a said instantaneous unbalance reduces below a said maximum permissible
unbalance for said instantaneous rotational speed.
21. A method according to any one of claims 9 to 20, including at least temporarily abandoning
an acceleration of said drum if a said unbalance sensed during a said distribution
cycle or acceleration therefrom exceeds said predetermined level.
22. A method according to any one of claims 9 to 21, including sensing said instantaneous
unbalance only up to a predetermined rotational speed of said drum, at which predetermined
rotational speed a said instantaneous unbalance is detectable within predetermined
limits of accuracy, and from that speed upwards computing a preferred rotational speed
of said drum based on one or more sensed measurements made below said predetermined
rotational speed.
23. A method according to any one of claims 9 to 22, including comparing substantially
instantaneous unbalance with said relationship at least one of substantially continuously
or at a plurality of predetermined rotational speeds of said drum.
24. A method according to any one of claims 9 to 23, including sensing said instantaneous
unbalance by determining at least one of an unbalance mass in said drum and from a
force exerted on a predetermined component of said apparatus.
25. A method according to any one of claims 9 to 24, including sensing a said instantaneous
unbalance from a characteristic of a drive motor rotating said drum, such as from
a motor current, a phase angle between motor voltage and motor current, a motor power
factor, a motor speed, a motor slip characteristic or a motor torque of said drive
motor.
26. An apparatus according to any one of claims 1 to 8 or a method according to any one
of claims 9 to 25, wherein said relationship is a mapped relationship, preferably
stored in association with said control means.