GB2361715A

GB2361715A - Out of balance detection

Info

Publication number: GB2361715A
Application number: GB0016266A
Authority: GB
Inventors: Matthew Damien Harrison
Original assignee: Dyson Ltd; Notetry Ltd
Current assignee: Dyson Technology Ltd; Dyson Ltd
Priority date: 2000-04-29
Filing date: 2000-07-04
Publication date: 2001-10-31
Anticipated expiration: 2020-07-04
Also published as: GB0016266D0; GB2361715B; GB0010381D0

Abstract

An appliance, such as a washing machine 10, comprises a drum 50 which is rotatable about an axis 85. Two vibration sensors 100, 110 are spaced axially apart about the drum 50 and sense vibration of the drum 50. The sensed vibrations are indicative of the size and position of an out of balance load within the drum 50. A controller 160 receives and processes vibration signals sent by the sensors 100, 110 and controls the rotational speed of the drum 50 accordingly to avoid an out of balance condition. The sensors may be piezoelectric shock sensors and the controller may be a micro-controller with memory capability. The appliance may comprises two relatively rotatable drum portions.

Description

2361715 Out of balance detection The present invention relates to

monitoring an out of balance condition in an appliance which has a rotatable drum. The invention is particularly suitable for, but not limited to, 5 washing machines and other laundry appliances which rotate a drum at high speed.

Washing machines perform a washing cycle which usually includes a period in which the drum of the machine is rotated at a high spin speed to dry the clothes held within the drum. There is a demand for washing machines which can operate at a high spin speed, since a high spin speed results in drier clothes which require less drying time after they have been removed from the machine. A high spin speed also reduces the length of the spin cycle which is required for a given water extraction from clothing. Washing machines are currently available which can spin at speeds of up to 1800 rpm.

In order that the washing machine can operate at high spin speeds, the drum/tub assembly must be well engineered and the load inside the drum must be well-balanced to prevent rotational and translational vibrations. Such vibrations cause unwanted noise and other undesirable effects such as machine "walking". Under extreme out-of-balance conditions, large drum displacements can result in the drum colliding with the tub within which it is housed, resulting in damage to the machine. The nature and volume of the load within the drum, and the manner in which the user has loaded the drum can significantly affect the balance of the drum at the end of the wash cycle. Washing machines are known which include some form of system for detecting an out-of-balance condition. This allows the machine to detect when the load within the drum is poorly distributed and thus prevent damage to the machine.

One common method for determining whether a drum is adequately balanced is to rotate the drum at a distribute speed and to monitor the resulting speed at which the drum actually rotates. Typically the distribute speed is 83rpm. At this distribute speed the drum speed varies over the course of a cycle; the drum speed decreasing as the motor which drives the drum struggles to lift the out- of-balance mass and then 2 increasing as the drum is accelerated due to gravity aiding the falling out-of-balance mass. The limitation of this method is that it is only effective at low spinning speeds, typically less than 100 rpm. This is not a completely satisfactory solution as it gives a fairly crude estimation of the out-of-balance mass and also because a load which is found to be balanced at distribute speed can become unbalanced at higher speed as different fabrics may allow different degrees of water extraction.

DE 3117106A1 describes a washing machine having a drum rotatable within a tub and a sensor mounted at the front of the tub, nearest the open front of the drum, which monitors vibrational swing of the drum by monitoring proximity of the drum to the tub.

A paper "Direct and Indirect Out-of-Balance Detection for Future Generation Washing Machines" by Christophe Lemaire presented at 1999 Appliance Manufacturer Conference & Expo, September 27-29, Nashville, describes an accelerometer which can be used on a washing machine to sense vibrations when the machine is operating at a high spin speed.

The present invention seeks to provide an improved way of monitoring an out-ofbalance condition of a rotatable drum.

Accordingly, a first aspect of the invention provides an appliance comprising a drum for retaining a load, which drum is rotatable about an axis, and apparatus for monitoring an out of balance condition of the drum, the apparatus comprising: two sensing devices spaced apart in an axial direction for sensing vibration at their respective positions, the sensed vibration being indicative of an out of balance load within the drum, and a controller which is arranged to receive inputs from the sensing devices and to control operation of the appliance according to the received inputs.

Two spaced-apart sensing devices provide sensor information from two different positions in the axial direction. This sensor information is indicative of the position of 3 the load imbalance in the axial direction. The use of two spaced-apart sensing devices can allow the drum to be safely rotated at a higher speed than would normally be allowed with the information provided by a single sensor. This is particularly useful with a drum which has a significant depth along its rotational axis. The appliance is controlled according to the measurements, such as by restricting the speed at which the drum is rotated.

One type of appliance which has a significant depth along its rotational axis is one where the drum comprises two rotatable portions which are positioned adjacent one another along the rotational axis. An appliance of this type is described more fully in International Patent Application W099/58753. During a wash cycle the portions are rotated relative to one another to agitate the load within the drum.

The sensing devices are spaced apart in an axial direction. In a washing machine or similar appliance the drum is rotatable within a tub, the tub forniing a watertight enclosure. The sensors can be conveniently mounted on the tub, the two sensors being spaced apart in an axial direction along the tub. Alternatively, the sensors can be mounted on the supports for the tub, the sensors being spaced apart in an axial direction.

In an appliance where the drum is supported at both ends of its rotational axis and access to the drum is via a hatch in the outer surface of the drum, the sensors can be placed on the rotational axis itself.

It is preferable to use the inputs from the sensing devices to determine the position of the out-of-balance mass and to control operation of the machine according to the position and size of the out-of-balance mass. While it is possible to determine the precise position of the out-of-balance along the rotational axis, it is sufficient to determine the position to within one of a plurality of zones along the rotational axis. These can be a central zone and the two end zones adjacent the central zone along the axis. Each zone has a threshold or some other criteria for an allowable out-of-balance mass. It is preferred that the threshold for an allowable out-of-balance mass varies with the speed at which the drum is rotated as the effects of an out-of-balance mass vary with 4 speed. It is also possible to directly use the inputs from the sensing devices, without necessarily determining the position of the out-of- balance mass.

Preferably, the controller is arranged to cause the appliance to operate sequentially at increasing rotational speeds and, before progressing to a higher rotational speed, to monitor outputs from the sensors to determine if the appliance should progress to the higher rotational speed. Thus, the appliance determines whether it can safely operate at a higher speed before progressing to that speed. This avoids wasting energy in raising the drum to a higher speed only to find that the drum cannot safely operate at the new 10 speed.

Preferably, the sensors are piezoelectric sensors which are particularly effective at responding to vibration at the frequencies experienced during a washing machine spin cycle.

The invention is particularly suitable for a washing machine or other laundry appliance but is not restricted to use in such machines.

Other aspects of the invention provide a control apparatus for controlling operation of 20 an appliance and a method of operating an appliance in this way.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 is an overall cross-sectional view of a washing machine; Figures 2A and 2B show the effect of an out-of-balance mass on the tub of the machine of Figure 1; Figure 3 is a chart showing how a permitted out-of-balance mass varies with distance along the rotational axis of the drum of the machine; Figure 4 shows apparatus for monitoring and controlling an out-of-balance condition in the machine of Figure 1; Figure 5 is a side view of one type of sensor that can be used in the arrangement of Figure 4; Figure 6 shows the output of one of the sensors; Figures 7A 7C show how the relative values of the sensor measurements vary according to the position of the out-of-balance mass; Figure 8 is a chart showing the limits for an out-of-balance mass in the different positions along the rotational axis of the machine and at different rotational speeds.

Figure 9 shows a washing cycle which the machine of Figure 1 may perform; Figure 10 is a flow diagram of the steps taken by the machine of Figure 1 to perform the cycle shown in Figure 9; and, Figure 11 shows the drum of the machine and the parameters that are used in a method of calculating an allowable spin speed for the machine.

Figure 1 shows a washing machine 10 which includes an outer casing 12 in which a stationary tub 40 is located. A drum 50 is mounted inside the tub 40 so as to be rotatable about an axis 85. The tub 40 is watertight except for an inlet 21 and outlet 22. The washing machine 10 includes a soap tray 20 capable of receiving detergent in a known manner. At least one water inlet 23 communicates with the soap tray 20 and is 30 provided with suitable means for connection to a water supply within the environment in which the washing machine 10 is to be used. A conduit 21 is provided between the 6 soap tray 20 and the tub 40 so as to allow water introduced via the inlet 23 to enter the tub 40. The tub 40 has a sump 26 located beneath the drum 50. A drainage pipe 28 conununicates with the sump 26 and leads to a water outlet 30 via which water can be discharged from the washing machine 10. A pump 42 is provided to allow water to be pumped from the sump 26 to the water outlet 30 at appropriate stages of the washing cycle carried out by the washing machine 10. Tub 40 is supported within the easing 12 by dampers 90, 92 and springs 93, 94.

The drum 50 is rotatably mounted about the axis 85 by way of a shaft 80. The shaft 80 is mounted in a known manner, allowing the tub 40 to remain stationary whilst the drum is rotatable with the shaft 80. The shaft 80 is rotatably driven by a motor (not shown) mounted within the outer casing 12 of the washing machine 10. A door 66 is located in the front panel 12a of the outer casing 12 to allow access to the interior of the drum 50. It is via the door 66 that a wash load can be deposited within the drum 50 before a wash cycle commences and removed from the drum 50 at the end of the wash cycle.

Drum 50 can be a conventional single drum or, as shown in Figure 1, can comprise two portions 60, 70 which are mounted such that they can be rotated with respect to one another. A drum of this type is described more fully in International Patent Application W099/58753. Typically the drum portions 60, 70 are rotated in opposite directions to one another, i.e. one portion clockwise, one counter-clockwise, but they can also be rotated together in the same direction. The drum 50 is mounted in a cantilever fashion on the wall of the tub 40 remote from the door 66. The first outer rotatable portion 60, is supported on a hollow cylindrical shaft 81. An angular contact bearing 82 is located between the rear wall of the tub 40 and the hollow cylindrical shaft 81. The outer rotatable portion 60 is dimensioned so as to substantially fill the interior of the tub 40. More specifically, the outer rotatable portion 60 has a generally circular rear wall 63 extending from the hollow cylindrical shaft 81 towards the cylindrical wall of the tub 40, a generally cylindrical wall 61 extending generally parallel to the cylindrical walls of the tub 40 from the rear wall 63 towards the front wall of the tub 40, and a generally 7 annular front face 64 extending from the cylindrical wall 61 towards the door 66. Sufficient clearance is allowed between the walls 61, 63, 64 of the outer rotatable portion 60 and the tub 40 to prevent the outer rotatable portion 60 from coming into contact with the tub 40 when the drum 50 is made to spin.

An inner cylindrical wall 62 is also provided on the interior of the cylindrical wall 61 of the outer rotatable portion 60. The inner cylindrical wall 62 extends from a point which is substantially midway between the rear wall 63 and the front face 64, to the front face 64. The space between the interior cylindrical wall 62 and the cylindrical wall 61 is hollow but, if desired, could be filled with a strengthening material. In this event, the strengthening material must be lightweight. The provision of parallel cylindrical walls 61, 62 in the portion of the outer rotatable portion 60 closest to the front face 64 provides strength to the whole of the outer rotatable portion 60 whilst reducing the internal diameter of the outer rotatable portion 60 in this region.

The inner rotatable portion 70 is supported on a central shaft 80, which in turn, is supported by deep groove bearings 83 located between the central shaft 80 and the hollow cylindrical shaft 81. The inner rotatable portion 70 essentially comprises a generally circular rear wall 71 extending from the central shaft 80 towards the cylindrical wall of the tub 40, and a cylindrical wall 74 extending from the periphery of the rear wall 71 towards the front wall of the tub 40. The diameter of the cylindrical wall 74 of the inner rotatable portion 70 is substantially the same as the diameter of the inner cylindrical wall 62 of the outer rotatable portion 60. The cylindrical wall 74 of the inner rotatable portion 70 is dimensioned so that its distal end approaches the end of the cylindrical wall 62 closest to it. It is advantageous to keep the gap between these two cylindrical walls 62, 74 as small as possible. An annular sealing ring 76 is located on the cylindrical wall 61 of the outer cylindrical portion 60 immediately adjacent to the end of the inner cylindrical wall 62 closest to the inner cylindrical portion 70 so as to provide support for the distal end of the cylindrical wall 76 thereof.

Figures 2A and 2B show just the tub 40 of Figure 1 and illustrate, by dashed lines, how the tub 40 can vibrate during use. If a load within the drum 50 is unbalanced, i.e. the mass of the load within the drum 50 is unevenly distributed around the rotational axis 85, the drum will vibrate as it is rotated. There is said to be an out-of-balance mass at some angular position about the rotational axis 85. The greater the size of the out-ofbalance mass, the greater the amplitude of the drum displacement, and hence the tub 40 displacement, about the normal position in which they lie.

The out-of-balance mass will also be located at some distance along the rotational axis 85 of the drum. The position of the out-of-balance mass along the rotational axis, and the size of that mass, determines the vibration profile of the tub 40. Clearly, the machine 10 must restrict the drum displacement to within a limit that will not cause damage to the machine. Figures 2A and 2B show extents of displacement of the tub 40 about the normal axis 85 in which the tub lies. Figure 2A shows, in an exaggerated form, the extents of displacement of tub 40 when an out-of-balance mass is positioned towards the front of the drum. Similarly, Figure 2B shows the extents of displacement of tub 40 when an out-of-balance mass is positioned towards the rear of the drum. Figure 3 is a chart which shows the allowable range of values for the out-of-balance mass against distance from the back of the tub. The size of the permitted OOB mass generally decreases with distance from the rear of the tub, as an out-of- balance mass causes a greater vibrational effect on the drum when it is located towards the front of the drum. This chart is derived by experiments made by placing an out-of-balance mass within the drum and monitoring the effects of that out-of-balance mass.

Figure 4 shows the same arrangement as previously shown in Figures 1 and 2 with the control apparatus for monitoring an out-of-balance condition. Two sensors 100, 110 are mounted on the outside surface of tub 40. These sensors 100, 110 are responsive to vibrations of the tub and provide a respective output signal 101, 111 indicative of the sensed vibration. An out-of-balance mass generates a force on the drum 50 which is transrrdtted to the tub 40 via the shafts 80, 81 and bearings 82, 83. Vibration of the tub 9 is linearly proportional to the force exerted by the out-of-balance mass on the drum 50.

A first sensor 100 is positioned at the rear of the tub 40 and a second sensor 110 is mounted mid-way along the longitudinal extent of the tub 40. Sensor 115 indicates an alternative position for the second sensor at the front of the tub 40. Other combinations of positions are possible. It is important to provide a good spacing of the positions at which the two sensors are mounted. The front position 115 offers a better spacing than mid-way position 110 but in many machines the front upper surface of the tub is covered by a counterweight which serves to dampen vibration of the tub. Both sensors are shown mounted on the upper face of the tub, but it is possible to mount the sensors at other angular positions around the tub. It is not essential that the two sensors are mounted in a line parallel to the axis of rotation; e.g. one sensor could be mounted on the upper face and one on the lower face or some intermediate position on the tub between the upper and lower face. However, mounting both sensors along a line parallel to the axis of rotation minimises the distance between the sensors and the distance by which the output signals, which are relatively weak in electrical strength (few tens of mV), need to be carried. Sensors 100, 110 can take various forms. It is preferred to use piezoelectric shock sensors which can readily respond to vibrations in the frequency range of interest. By using two sensors 100, 110 which are spaced apart in the direction of the rotational axis of the drum, it is possible to determine the size of the out-of-balance mass and the position of the out-of-balance mass along the rotational axis of the drum.

Figure 5 shows a piezoelectric sensor 650 comprising a piezoelectric crystal disc 651 bonded to a thin disc of metal 652. Leads 653, 654 from a coaxial cable 655 are electrically bonded to the discs 651, 652. The sensor 650 is housed within a rugged plastics shell and the cable 655 can be secured in a convoluted channel, such as an 'S' shaped channel, within the shell which serves to prevent the cable being pulled and damaging the delicate connections to the discs 651, 652. As an altemative to mounting the sensors on the tub, they can be mounted as part of supports (see sensors 95, 96 in Fig. 1) which support the tub 40 within the casing 12. In this alternative arrangement, the sensors should be mounted in a position where they are responsive to vibrations of the supports 93, 94 and the supports should be spaced apart in the direction of the rotational axis 85 of the drum 50.

Outputs 101, 111 from vibration sensors 100, 110 are fed to a multiplexor 120 which, under the control of controller 160, switches between the outputs and feeds one of the outputs through a processing chain which comprises an amplifier 130, band- pass filter and analogue-to-digital converter 150. The multiplexor simply avoids the need for two such processing chains and can be omitted by providing two separate processing chains, one for each sensor output. The band-pass filter 140 filters the amplified analogue signal, passing only signals in the frequency band which is expected at the rotational speeds of the drum at which measurements are being made. This is the range 800 - 1400rpm. Typically the filter has a band-pass region of 5 - 40Hz. The output of the A-D converter 150 is fed to controller 160. The controller 160 operates upon the two received signals, as will be more fully described below, and provides control signals 161, 162 for controlling the drives 170, 180 to rotate the drum portions 60, 70 at a required spin speed. Two drives 170, 180 are shown here, each drive separately driving a portion 60, 70 of the drum 50. As an alternative, a single drive and gearbox can be used to drive the portions 60, 70 of the drum 50. The controller is preferably a microcontroller which includes software, stored on a non-volatile memory, and working memory for implementing the signal processing described below. However, a skilled person will readily appreciate that this could alternatively be implemented entirely in hardware.

The inputs to the controller 160 are processed as follows. Each input to the controller comprises a series of digitised values of the vibrational signal sensed by sensor 100 or 110. The vibration has a frequency which is equal to the rotational frequency of the drum 50. The maximum amplitude of each sensor signal is determined, which represents the maximum extent of the vibration. It has been found that each sensor returns a waveform that includes a useful component representing vibration of the tub as well as a noise component. This means that a simple inspection of the maximum value of the waveform supplied by each sensor would not give a reliable indication of the amplitude of the vibration. The noise results from electro-magnetically induced noise from power cables and harmonic components due to non-linearities in the sensors 100, 110. A form of Discrete Fourier Transform is used to extract the maximum value over a number of cycles of the sensor waveform. This has the effect of comparing the sampled waveform with a pure sinusoidal signal at a given frequency. Typically four cycles of the sensor waveform are used, with eight samples being taken each cycle.

Figure 6 shows one of the sensor waveforms and the position of the first eight sampling points (yO - y8). Two sums: sumi and sum2 are calculated as follows for each waveform:

sumI = 0.75y2 + y3 + 0.75y4 - 0.75y6 - y7 - 0.75y8.......

sum2 = yO + 0.75yl - 0.75y3 - y4 - 0.75y5 + 0.75y7 The overall value of amplitude, called 'Result' is:

Result [(SUM 1)2 + (sum2)2] The above steps are performed for both sensors 100, 110 to produce two values. Resultl is the amplitude given by the middle sensor 110 and Result2 is the amplitude given by the rear sensor 100.

The sensor measurements are used to determine whether the out-of-balance mass within the drum is allowable for a given spin speed. Several different methods for using the sensor measurements will now be described.

Position-based method The relative size of the two sensor measurements (Result 1, Result 2) indicates the position of the out-of-balance mass within the drum. The difference between the two 12 values (Result 1, Result 2) is taken, i.e. (Resultl - Result2). The sign (positive or negative) of the value that results from this calculation indicates the position of the out of-balance mass along the rotational axis and therefore the bending moment exerted on the tub 40. Figures 7A - 7C show the values of the displacement measurements made by sensors 100, 110. A positive difference value indicates that the out- of-balance mass is located towards the front of the tub (Figure 7A). A negative difference value indicates that the out-of-balance mass is located towards the back of the tub (Figure 7Q. If the difference value is zero, or in the region of zero, this indicates that the out of-balance mass is located centrally (Figure 713). Determining the position of the out of-balance mass to one of three zones has been found to be sufficient. However, a larger number of zones can be used if desired. Figure 8 shows the three zones: rear zone, central zone, front zone on a chart.

Having determined where the out-of-balance mass is located, the size of the out-of balance mass is determined. The values of Result 1 and Result2 are indicative of the size of the out-of-balance mass. The next part of the method sums Resultl and Result2 and compares this with a threshold value.

The sum (Resultl + Result2) is compared with a table which specifies a safe threshold value of this sum for each position of the out-of-balance mass for each spin speed at which the machine can operate. Figure 8 shows the effect of using a table in this way.

Line 300 shows the allowable out-of-balance mass against position along the rotational axis at a spin speed of 11 0Orpm. The threshold within each of the front, centre and back zones along the rotational axis 85 are shown as horizontal lines 301, 302, 303. Within each zone, the threshold is taken as the most restrictive value of out-of- balance mass according to the 1 10Orpm limit 300.

Threshold value Target Spin Speed/rpm Front Centre Back 1100 XX xx xX 1400 XX xX xX 13 If the result of the calculation described above is less than the safe threshold then the machine can progress to that spin speed. If not, the machine should remain at the current spin speed. Each spin speed has a set of threshold values associated with it.

Each threshold value is an acceptable value for the vibration measurement at that position for that spin speed, which represents an acceptable value for the out-of-balance mass. The machine is allowed to operate at a high spin speed if the position and size of the out-of-balance mass is within the required limits. Higher spin speeds have a stricter requirement than lower spin speeds as a given out-of-balance mass has a greater vibrational effect at higher spin speeds.

Each of the thresholds represents an allowable limit for the vibrational measurements at the spin speed at which the measurements will be taken (e. g. 80Orpm) in order that the machine can operate at the target spin speed (e.g. 1 10Orpm). The actual values of these thresholds is dependent on the type and position of the sensors, the calibration of the sensors, and the degree of tub displacement that it is desired to tolerate.

Figures 9 and 10 show a typical wash programme which washing machine 10 can perform. Figure 9 shows the stages on a time line and Figure 10 shows the corresponding steps that the control system performs to operate in this manner. After one or more wash and rinsing cycles 200 the machine enters the spin cycle 202. Firstly, at step 205, the drum is rotated at a distribute speed which attempts to distribute the load evenly around the inner wall of the drum 50. Typically the distribute speed is 83rpm. At this distribute speed the drum speed varies over the course of a cycle; the drum speed decreasing as the motor which drives the drum struggles to lift the out- of-balance mass and then increasing as the drum is accelerated due to gravity aiding the falling out-ofbalance mass. During the distribute operation controller 160 monitors the resulting speed at which the drum actually rotates and determines, from the speed, the size of the out-of- balance mass (step 210). Providing, at step 210, the load is determined to be sufficiently balanced, the machine enters a period when it operates at progressively 14 higher spin speeds. If the load is deemed to be insufficiently balanced the controller, at step 208, controls the machine to redistribute the contents of the drum 50.

Firstly, the machine progresses to the next highest spin speed of 80Orpm (step 220).

The controller uses the vibration sensors 100, 110 and the corresponding control apparatus to measure the out-of-balance mass. A check is made whether the out-of balance mass calculated using the vibration sensors corresponds to the out-of-balance mass calculated during the distribute operation. If the two measurements do not correspond, allowing for a margin of error, this indicates that there is some fault in the control system and the controller progresses to step 218 and limits the spin speed to 80Orpm. or it stops and informs the user, via the control panel, that servicing is required.

If the measurements correspond, then at step 215 the controller uses the estimation of size and position of the out-of-balance mass provided by the vibration sensors to decide whether the load is sufficiently balanced to operate at the next highest speed of 11 0Orpm. If the measurementsindicate that the size and position of the out-of-balance mass is appropriate for the higher spin speed, then the controller maintains the machine progresses to the next highest spin speed of 1 10Orpm. If the measurements indicate that the size and position of the out-of-balance mass is inappropriate for the higher spin speed, then the controller maintains the machine at 80Orpm. The controller can measure the out-of-balance mass at regular intervals to determine if it has changed to an appropriate size and position to permit the machine to operate at a higher spin speed.

At the next highest spin speed, 1 10Orpm, the above steps are repeated. Controller 160 uses the estimation of size and position of the out-of-balance mass provided by the vibration sensors to decide whether the load is sufficiently balanced to operate at 140Orpm. If the measurements indicate that the size and position of the out-of-balance mass is appropriate for the higher spin speed, then the controller maintains the machine progresses to the next highest spin speed of 140Orpm. If the measurements indicate that the size and position of the out-of-balance mass is inappropriate for the higher spin speed, then the controller maintains the machine at 1 10Orpm. After a period of spinning at 140Orpm, (step 240) the controller 160 stops spinning the drum 50.

It will be appreciated that this method can be repeated before progressing to other spin 5 speeds, and that any of the spin speeds can take values different from those used above.

Maximum speed calculation This method also uses the two sensor measurements Result 1, Result2 which are here called s,, s, The measurements are used in a formula which determines a maximum safe spin speed. The derivation of this formula will now be described with reference to Figure 11. Figure 11 shows the cylindrical drum 40 of the washing machine having a radius r, the middle sensing position producing a sensor measurement s, and the rear sensing position producing a sensor measurementS2. An out-of-balance mass of mass m is positioned at a distance p from the back of the drum. The drum rotates at a speed v.

The size of the out-of-balance mass can be expressed as:

S S (1) m=A-' +B - v 2 V2 where A, B are constants.

The position of the out-of-balance mass can be expressed as:

Cs' +Ds2 (2) p =( v v m where C, D are constants.

The force exerted by the out-of-balance mass is:

F = mrOJ2 16 The bending moment BM exerted on the drum bearings by the out-of-balance mass is:

(3) BM =Fxp=mroj 2 p By substituting (1) and (2) into (3):

BM = ra)2 C s12 +D si.

2 2 v v In order to limit stress on the bearings, a maximum limit is set for the value of the 10 bending moment, called BM.,,,. This limit can be found by calculations or by experiments on a rig.

(4) BMLImIT = r (Cs, + DS2) v where (o is the angular speed at which the bending limit occurs.

Rearranging, this becomes:

2 v 2 1 (5) BM umT - r (Cs, + Ds.) to v r(BM 2gf = 2z RPM Cs, + r(C + 1 S2) 60 i.e. RPM = 60 v-,rB-M -L,.IT - 2z D IrrC ''I +RS2 (6) RPM - kv V -s1 _+ES2 where k = 604B-MIjmT 2z--rC 17 and RPM is the speed in revolutions per minute at which the limiting bending moment is reached.

Thus, from the sensor measurements s,, s, and knowing the various constants and the value of BM., equation (6) gives the maximum speed at which the drum can rotate to restrict the force on the bearings to within the bending moment limit. The maximum permitted speed given by equation (6) is dependent on the position of the of the out-of balance mass within the drum, even though position does not directly appear as one of the variables in equation (6).

The equation can be applied in the same manner as previously described with reference to Figures 9 and 10, with the controller causing the machine to operate at successively higher spin speeds and using the sensor measurements s, s, in equation (6) above to test whether the machine can proceed to the next highest spin speed. The machine operates at the next highest spin speed or the speed given by equation (6), whichever is the lower. For example, when testing at 800rprn as to whether the machine can progress to 11 OOrpm, but equation (6) only permits a maximum speed of 1 OOOrpm, the machine will spin at 10OOrpm. If equation (6) had permitted a maximum speed of 1200rpm, the machine would operate at the next highest required speed of 1 100rpm.

It is not necessary for the machine to use the maximum speed result in this way. For example, the controller can use the results of a test at a fairly low speed (e.g. 500rpm) to progress directly to the maximum speed, without any inten-nediate steps.

The position-based method may be simpler to implement than the maximum speed method as it requires the control processor to perform fewer calculations.

Alternatives and variants to the described embodiments will be apparent to a skilled person and are intended to fall within the scope of this invention.

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Claims

1. An appliance comprising a drum for retaining a load, which drum is rotatable about an axis, and apparatus for monitoring an out of balance condition of the drum, the apparatus comprising: two sensing devices spaced apart in an axial direction for sensing vibration at their respective positions, the sensed vibration being indicative of an out of balance load within the drum, and a controller which is arranged to receive inputs from the sensing devices and to control operation of the appliance according to the received inputs.

2. An appliance according to claim 1 wherein the controller is arranged to determine the position of the out of balance along the axis from the sensor inputs.

3. An appliance according to claim 1 or 2 wherein the axis is divided into a plurality of zones and the controller is arranged to determine the position of the out of balance to within one of the zones.

4. An appliance according to claim 3 wherein the controller has a threshold value associated with each of the zones which represents an allowable out-of- balance for that zone and the controller is arranged to compare actual sensor measurements with the threshold values to control the appliance.

5. An appliance according to claim 4 wherein there is a plurality of sets of thresholds for the zones, each set being associated with a rotational speed for the drum.

6. An appliance according to any one of claims 3 to 5 wherein there are three zones representing the central and the end regions of the axis passing through the drum.

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7. An appliance according to any one of claims 2 to 6 wherein the controller is arranged to determine the position of the out of balance by determining a difference between the sensor inputs.

8. An appliance according to claim 1 wherein the controller is arranged to use the sensor inputs to calculate a maximum allowable rotational speed for the drum.

9. An appliance according to claim 8 wherein a maximum limit is set for the allowable bending moment on the bearings of the drum and the controller is arranged to 10 also use this maximum limit to calculate the allowable rotational speed for the drum.

10. An appliance according to any one of the preceding claims wherein the controller controls the speed at which the drum can rotate.

11. An appliance according to claim 10 wherein the controller is arranged to cause the appliance to operate sequentially at increasing rotational speeds and, before progressing to a higher rotational speed, to monitor outputs from the sensors to determine if the appliance should progress to the higher rotational speed. 20

12. An appliance according to any one of the preceding claims wherein the sensors are offset from the axis of rotation of the drum.

13. An appliance according to claim 12 wherein the sensors are mounted along an axial line which is parallel to the axis of rotation. 25

14. An appliance according to claim 12 or 13 wherein the drum is housed within a tub and the sensors are mounted on the tub.

15. An appliance according to any one of the preceding claims wherein the sensors 30 are mounted near each end of the drum.

16. An appliance according to any one of claims 1 to 14 wherein a first of the sensors is mounted near one end of the drum and the second of the sensors is mounted in the central region between the ends of the drum.

17. An appliance according to any one of the preceding claims wherein the drum has an open end for allowing the drum to be loaded and a closed end, a rotatable shaft being mounted to the closed end of the drum, and wherein one of the sensors is mounted near the closed end of the drum.

18. An appliance according to any one of the preceding claims comprising supports for the drum and wherein the sensors are mounted on the supports.

19. An appliance according to any one of the preceding claims further comprising processing means for processing the output of a sensor and switching means for 15 selectively routing the output of one of the sensors through the processing means.

20. An appliance according to any one of the preceding claims wherein the sensors are piezoelectric sensors.

21. An appliance according to any one of the preceding claims wherein the drum comprises two rotatable portions and a drive for causing relative rotation of the portions of the drum.

22. An appliance according to any one of the preceding claims in the form of a 25 washing machine.

23. A control apparatus for controlling operation of an appliance which comprises a drum for retaining a load, which drum is rotatable about an axis, the control apparatus comprising:

21 an input for receiving inputs from two sensing devices spaced apart in an axial direction, the sensing devices sensing vibration at their respective positions which is indicative of an out of balance load within the drum, and a processing means which is arranged to control operation of the appliance 5 according to the received inputs.

24. A method of operating an appliance which comprises a drum for retaining a load which is rotatable about an axis, the method comprising receiving inputs from two sensing devices which are spaced apart in an axial direction, the devices sensing vibration at their respective positions which is indicative of an out of balance load within the drum, and - controlling operation of the appliance according to the received inputs.

25. An appliance, a control apparatus for controlling operation of an appliance or a method of operating an appliance substantially as described herein with reference to the accompanying drawings.