EP1342826A1 - System for managing out-of-balance of loads in a laundry apparatus - Google Patents

Abstract

A laundry arrangement is disclosed including a laundry apparatus 10 comprising:

• a) a drum 14 adapted to receive a load of laundry and to be rotated about an axis C/L;
• b) a sensing means 28 adapted to provide an unbalance signal indicative of a substantially instantaneous unbalance of said load; and
• c) a control means 12 which is adapted to monitor said unbalance signal and to control the rotation of said drum 14 in response thereto;

   characterized in that said control means 12 is adapted to compare said instantaneous unbalance with a predefined relationship between maximum permissible unbalance and rotational speed of said drum 14 and to alter an acceleration or rotational speed of said drum 14 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, a laundry apparatus according to the invention is adapted to operate always within the mechanical limits imposed by its design life.

Description

FIELD OF THE INVENTION
• 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
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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 andMat 2 having a relatively small liquid absorption coefficient. Suppose also that we put 2.5kg ofMat 1 and 4kg ofMat 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 ofMat 1 is bigger than the liquid absorption coefficient ofMat 2, theMat 1 will contain relatively more liquid thanMat 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 ofMat 1, there will be absorbed an additional 2.5 kg liquid and in the 4 kg load ofMat 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.
• 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 bothmaterials 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 inMat 1 and 0.5kg liquid inMat 2 as can be seen in Figure 2. The total mass ofMat 1 with liquid is 3.75kg and the total mass ofMat 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.
• 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.
• 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.
• 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
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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
• 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
• 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.
• The present invention provides a laundry apparatus in the form of awashing machine 10 operated using a programmable control means in the form of acycle controller 12. Themachine 10 comprises adrum 14 which is rotatable inside atub 16 about a substantially horizontal axis C/L, thedrum 14 being supported for rotation on ahorizontal axle 18 supported near thedrum 14 by afront bearing 20 and at a driven end distal from thedrum 14 by arear bearing 22. Drive is provided to theaxle 18 through atransmission 24 by anAC motor 26 under the control of thecycle controller 12. A sensing means is provided which detects the level of unbalance in thedrum 14 and provides an unbalance signal indicative thereof to thecycle controller 12. The sensing means may take a variety of forms, some embodiments of which will now be discussed.
• In one embodiment, the sensing means comprises for example afrequency inverter 28 or similar associated with or implemented in a motor control portion of thecycle controller 12.
• Thefrequency inverter 28 monitors variations in motor load which, if present, are indications of the level of unbalanced mass in thedrum 14. An advantage of this approach is that often commerciallyavailable 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 thecycle 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.
• Besides the above described disadvantage of measuring the unbalanced mass when its position in thedrum 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:
• The variation in motor load is caused by the fact the unbalanced mass will be influenced by gravity acceleration. When unbalance in thedrum 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:$$\text{a =δv / δt}$$

  

     where "δv" is the variation in speed under influence of the acceleration "a" for the time "δt".
• Since in this case the acceleration is the acceleration "g" due to gravity and is thus substantially constant at g = 9.81m/s², 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 thedrum 14 and at the downside of thedrum 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.
• To minimize this disadvantage, thecycle controller 12 of the present invention computes a preferred angular velocity of thedrum 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.
• The arrangement that is disclosed in US-5677606 may prove suitable as the basis of a sensing means associated with acycle 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.
• 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 thedrum 14. In this manner, forces induced by unbalanced loads can be measured and the position of the unbalanced mass in thedrum 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 bothbearings 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 bearinghousings 20a, 22a.
• 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 thecycle controller 12. If that maximum allowable mechanical force is exceeded, acceleration is stopped instantly and the achieved rotational speed of thedrum 14 substantially maintained. If themachine 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 thedrum 14 is altered to bring the force below that limit curve, e.g. by deceleration.
• 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.
• 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.
• 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 thecycle 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 thedrum 14. Such a relationship is preferably developed through durability testing to represent a mechanical limit curve for one or more components of themachine 10 in question across its life, as will now be discussed.
• The present invention thus provides a laundry arrangement adapted to ensure that the spinning speed of themachine 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.
• Generally, when a new model ofwashing 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, themachine 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 thedrum 14 and not least whether it is to be rigid mounted or a free standing machine type.
• 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 thedrum 14 of an unbalanced load are considered.
• 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 thedrum 14 that themachine 10 can handle can be written as:$${\text{Fc = MxRxW}}^{\text{2}}{\text{= MxRx(2xPixf)}}^{\text{2}}{\text{= 4x0,265x(47,1225)}}^{\text{2}}\text{= 2353,76 N}$$

  

  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
• From the above, it can be seen that themachine 10 can survive a predefined level of unbalance in the form of a centrifugal force Fc = 2353.76 N at the front of thedrum 14 for its expected lifetime. If the unbalanced mass were to be positioned in the center or at the back of thedrum 14, the centrifugal force Fc that themachine 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 themachine 10 may be that it should not exceed the maximum allowable force that thebearings 20, 22 can cope with. The mechanical construction of themachine 10 has to be strong enough to handle the forces that thedrum axle 18 brings on thebearings 20, 22. These forces are carried further from thebearings 20, 22 to thebearing housings 20a, 22a and led further from the bearinghousings 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 thefront bearing 20 andrear 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:$$\text{Fc x 0,4 =<figure-callout label="R2 x" filenames="imgf0002.png,imgf0006.png" state="{{state}}"><figure-callout label="R2 x" filenames="imgf0002.png,imgf0007.png" state="{{state}}">R2 x</figure-callout></figure-callout> 0,2}$$

This means$$\text{R2 = Fc x (0,4/0,2) = 2353,8 x 2 = 4707,6 N}$$

The sum of forces must be zero, so this can also be written as:$$\text{Fc + R1 + R2 = 0}$$

Thus:$$\text{R1 = -Fc - R2 = -2353,8 - 4707,6 = -7061,4 N}$$

• So, if the machine can withstand Fc = 2353,8 at the front of thedrum 14 for its proposed lifetime, this means in fact that themachine 10 can withstand for its proposed lifetime a force of 7061,4 N in thefront bearing 20 and a force of 4707,6 N in itsrear bearing 22.
• Suppose now that the unbalanced mass is not positioned at the front of thedrum 14, but at the back of thedrum 14 as shown in Figure 5. In this situation, the allowable centrifugal force can be recalculated in order to have the same force on thefront bearing 20 as calculated above:
• 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:$$\text{Fcx0,1 =<figure-callout label="R2 x" filenames="imgf0002.png,imgf0006.png" state="{{state}}"><figure-callout label="R2 x" filenames="imgf0002.png,imgf0007.png" state="{{state}}">R2 x</figure-callout></figure-callout> 0,2}$$

  

  This means that$$\text{Fc = R2 x (0,2/0,1) = R2 x 2}$$

  

  Also, because the sum of forces must be zero:$$\text{Fc + R1 + R2 = 0}$$

  

  Thus:$$\text{(R2 x 2) + R1 + R2 = 0}$$

  

  Thus:$$\text{R2 = -R1/3 = -7061,4 N / 3 = 2353,8 N}$$

  

  And$$\text{Fc = R2 x 2 = 2353,8 x 2 = 4707,6 N}$$

  
• Out of this calculation it is clear that a centrifugal force Fc of 2353,8 N at the front of thedrum 14 causes the same force on thefront bearing 20 as the centrifugal force Fc of 4707,6 N at the back of thedrum 14. With the centrifugal force of 4707,6 N at the back of thedrum 14, the force on thesecond bearing 22 is smaller than with the centrifugal force of 2353,8 N at the front of thedrum 14. This means also that the lifetime of themachine 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 thedrum 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.
• In general, it can be seen that measuring unbalanced masses in thedrum 14 without knowing the position of the unbalance in thedrum 14 is not sufficient to always reach the maximum drum speed that themachine 10 can handle for its proposed lifetime.
• 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 thecontroller 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)
• Referring for the moment back to the example of Figure 3, a case is given in which amachine 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 thedoor 16a for a front-loading washing machine 10.
• There it was calculated that thismachine 10 could handle the centrifugal force Fc = 2353.76 N at the front of thedrum 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 thedrum 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 themachine 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 themachine 10. For convenience, these curves will be considered to represent the mechanical limit curve of themachine 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.
• 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.
• 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 thedrum 14 of 5 kg. To be almost sure that the spin speed of 400 RPM will be reached, thecycle controller 12 is programmed as follows: if the acceleration is stopped before the spin speed of 400 RPM is reached, thecontroller 12 will give command to decelerate thedrum 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 thedrum 14. Then acceleration of thedrum 14 starts again with probably a more equally divided load in thedrum 14 so that there will be less unbalanced mass in thedrum 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 thecontroller 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 themachine 10 still does not succeed in reaching the 400 RPM spin speed, themachine 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.
• 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.
• Depending on the available motor power, themachine 10 will also have a maximum spin speed that can be supplied by themotor 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.
• 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 thecontroller 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 themachine 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 thecontroller 12 sees that the unbalanced mass at very low speed is above a certain limit, thecontroller 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 thedrum 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.
• 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)
• 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.
• 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.
• The centrifugal force because of the unbalanced mass in the drum that the machine can handle can be written as:$${\text{Equation (1) Fc = MxRxW}}^{\text{2}}{\text{= MxRx(2xPixf)}}^{\text{2}}{\text{= 2,5x0,265x(104,72)}}^{\text{2}}\text{= 7264,7 N}$$

  

  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
• 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.
• Graphs have now been developed of the two mechanical limit curves for thefree 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.
• 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 .
• 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.
• In the examples of both rigid mounted and free standing machines, the principle of handling unbalanced loads in thedrum 14 is based on measuring unbalanced masses without knowing the position of the unbalance in the drum. As mentioned above, measuring unbalanced masses in thedrum 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 thedrum 14 causes the same force on thefront bearing 20 as the centrifugal force Fc of 4707,6 N at the back of the drum.
• In Figure 16, the mechanical limit curve of the unbalanced mass at the front of thedrum 14 and at the back of thedrum 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 thedrum 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.
• 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 thedrum 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.
• 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.
• 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.
• 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.
• 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.
• 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.

Claims (26)

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.

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