This invention relates to a system for balancing the load
in a laundry appliance, particularly but not solely, a system for balancing the
load in a horizontal axis washing machine.
Conventional horizontal axis washing machines involve a
final spin cycle to extract the washed articles of as much of water as possible
to reducing drying time. However, the requirement of a high spin speed is at odds
with quiet operation. At the beginning of a spin the cycle the wash load can be
quite severely unbalanced, such that when the machine tries to accelerate noise
and stressful vibrations result.
The means that washing machine designers have employed
so far to cater for imbalance in the load, is typically to suspend the internal
assembly on springs and dampers in order to isolate its vibration. The difficulty
is these suspension assemblies never isolate the vibration completely, and as the
machine ages they deteriorate and the problem gets worse. Also, these suspension
assemblies require significant internal clearance, and so valuable load capacity
is lost when designing a machine to standard outside dimensions. Further, because
the internal assembly must still withstand the forces due to the imbalance, considerable
extra costs result.
The ideal approach is to eliminate the problem at its source,
for which there are various solutions. The first possibility is to ensure that the
wash load is evenly distributed prior to spinning. This is an effective solution
but it is extremely difficult to achieve in practice. Therefore while steps can
be taken to reduce the degree of imbalance that must be catered for, it is not possible
to eliminate it sufficiently to ignore it there after. Another approach is to determine
the size and nature of the imbalance, and add an imbalance that exactly counteracts
Methods of compensating for imbalance in horizontal axis
washing machines have been disclosed in
US Patent 5,280,660 (Pellerin et al.
European Patent 856604
(Fagor, S.Coop). These disclosures relate to the use of three axially
orientated chambers running the length of the drum, displaced evenly around the
periphery of the drum, which when individually filled with water in the appropriate
amounts can be used to approximately correct imbalances in the axis of rotation.
The disadvantage to these systems is that the imbalance
may not be centered along the axis of rotation, and since no control is available
along the axis of rotation this form of balancing will only ever be partially successful.
This may mean that a suspension system may still be required to isolate the vibrations,
which adds cost and may reduce the useful life of the appliance.
US Patent 2,610,523
British Patent 711531
describe automated balancing systems for hydro-extractors. In each case
two or more receptacles are provided for receiving added balancing mass. The balancing
mass is in the form of water supplied to the receptacles by injectors or conduits
in the drum structure. The receptacles are provided as elongate chambers aligned
with the axis of the drum. An electromechanical system activates valves supplying
water to the receptacles in a trial and error process to improve balance.
When an object of some shape or form is spun about a particular
axis, there are two types of imbalance that it may exhibit Static and Dynamic Static
imbalance is where axis of rotation does not pass through the Centre of Gravity
(CoG) of the object. This means that a force, F, must be applied to the object (acting
through the CoG) to keep accelerating the object towards the axis of rotation. This
force must come from the surrounding structure and of course its direction rotates
with , the object, as illustrated in Figure 1. There are two pieces of information
required to define a static imbalance 3. They are the magnitude of the imbalance
I (the moment of the CoG about the spin axis, which in SI units has dimensions kg
m), and some angle 2 between the direction of the offset of the CoG and some reference
direction within the object 4.
When mounted on a horizontal rotation axis, and under the
influence of gravity, an object with a static imbalance will rotate until its CoG
lies vertically under is axis of rotation. This also has the consequence that a
horizontal axis machine running at speeds slower than its resonance on its suspension
and at constant power input, will exhibit a slight fluctuation in rotation speed
as the CoG goes up one side and down the other. Unfortunately this is not a feasible
technique for determining static imbalance at anything other than very slow speeds.
Dynamic Imbalance is a little more complicated. In Figure
2 the axis of rotation 5 is not parallel with one of the principle axes 6 of the
object. The principal axes of an object are the axes about which the object will
For example, consider a short length of uniform cylinder
7 set to spin about its axis of extrusion, and thus is both statically and dynamically
balanced. Two weights are now attached to the inside of the cylinder, one 8 at one
end and the other 9 at the other end but on the opposite side from the first one.
The CoG 10 of the object has not been moved and so it is still statically balanced,
but now spinning the cylinder will cause vibration; it has a dynamic imbalance.
Static imbalance can be detected statically by seeing which way up the object rolls
over to rest. Dynamic imbalance can only be detected with the cylinder spinning,
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide a balancing
system for a laundry appliance which goes as far as is practical for its purpose
towards overcoming the above mentioned disadvantages.
Accordingly in a first aspect, the present invention consists
in a laundry appliance having a cabinet, a perforated drum for holding a clothes
load, the drum being supported in the cabinet for rotation about a spin axis with
the spin axis substantially rigidly located in relation to said cabinet, driving
means adapted to rotate said drum about its spin axis thereby dehydrating the load,
and a system for compensating for imbalances caused by the distribution of the load
carried therein during dehydration of the load, said system comprising:
- a pair of sensing means separated along the drum's spin axis for detecting rotational
imbalance in the load, each sensing means providing an output signal representative
of its sensed imbalance,
- a digital processor that receives the output signals from said pair of sensing
means and that is programmed to calculate the size and position of one or more masses
required to be added to the drum to correct the sensed rotational imbalance, and
- correction means adapted to add two or more masses to said drum, wherein in
use at least one of said masses is axially spaced from the remainder of said masses,
there existing a time delay between the correction means initiating the addition
of mass and the effect of that mass addition being registered by one or both sensing
- said system characterised in that,
- said processor, when calculating said size and position of one or more masses
to remove the imbalance on any particular occasion, reduces the calculated value
to account for the anticipated effect of a mass or masses added by the correction
means on a previous occasion or occasions, the full effect of which previously added
mass or masses has not yet been registered by the sensing means.
The invention consists in the foregoing and also envisages
constructions of which the following gives examples.
BRIEF DESCRIPTION OF THE DRAWINGS
One preferred form of the present invention will now be
described with reference to the accompanying drawings in which;
BEST MODE FOR CARRYING OUT THE INVENTION
- Figure 1 is an illustration of the concept of static imbalance,
- Figure 2 is an illustration of the concept of dynamic imbalance,
- Figure 3 is a cutaway perspective view of a washing machine according to the
present invention with the cutaway to show the machine substantially in cross section,
- Figure 4 is an assembly drawing in perspective view of the washing machine of
Figure 3 showing the various major parts that go together to form the machine,
- Figure 5 is an illustration of the drum bearing mount,
- Figure 6 is an illustration of the drum, showing the balancing chambers and
- Figure 7 is a diagrammatic representation of the liquid supply and electrical
systems of the washing machine of Figure 3,
- Figure 8 is a waveform diagram giving example output waveforms from the vibration
- Figure 9 is a graph illustrating the weighting curves,
- Figure 10 is an illustration of the decision making process regarding filling
of the balancing chambers,
- Figure 11 is a flow diagram showing the Imbalance Detection Algorithm ,
- Figure 12 is a flow diagram showing the Balance Correction Algorithm,
- Figure 13 is a flow diagram showing the Spin Algorithm, and
- Figure 14 is a block diagram of the equivalent spring system when the laundry
appliance is supported on a flexible floor.
The present invention provides a novel method of balancing
the load in a laundry appliance, particularly suited to washing machines. Such a
system dispenses with the need for suspension, and this significantly simplifies
the machine design. The following description is with reference to a horizontal
axis machine. However it will be appreciated that the present invention will be
applicable to off horizontal and vertical machines, as well as rotating laundry
appliances in general.
General Appliance Construction
The present invention will be described primarily with
reference to a laundry washing machine although many of the principles are equally
applicable to laundry drying machines. Figures 3 and 4 show a washing machine of
the horizontal axis type, having a perforated drum 11 supported with its axis substantially
horizontal in side-to-side orientation within a cabinet 12. The cabinet 12 includes
surfaces which confine wash or rinse liquid leaving the drum within a water tight
enclosure. Some parts of the cabinet structure 12 may be formed together with the
liquid confining surfaces by for example twin-sheet thermoforming. In particular
the back and side walls of the machine may be formed in this way.
The laundry handling system including the drum and many
other components is preferably contained in a top loading configuration. In Figure
3 the horizontal axis spin drum 11 is contained within a substantially rectangular
cabinet 12 with access being provided via a hinged lid 14 on the top of the machine.
Other horizontal axis configurations may be adopted.
The drum 11 is rotatably supported by bearings 15 at either
end which in turn are each supported by a drum support 16. In the embodiment depicted
the bearings are axially located, externally, on a shaft means 19 protruding from
the hub area 20 of the drum ends 21,22. Other axial configurations are equally possible,
for example internally located in a well in the outer face of the hub area of the
drum to be located on a shaft protruding from the drum support. The drum supports
16 are shown each as a base supported unit and have integrated form, which again
is ideally suited to manufacture by twin sheet thermoforming, blow moulding or the
like. Each drum support preferably includes a strengthening rib area 23 and a drum
accommodating well area 25 as depicted to accommodate the respective drum end 21,
22 of the drum 1. The drum supports 16 engage with sub-structure by interlocking
within complementary surfaces provided in side walls 27,28. Other less preferable
constructions are possible, such as frameworks formed from individual members or
mechanical suspension systems.
The drum supports 16 each include a bearing support well
at the centre of said well area 25. A bearing mount 29 is located within the bearing
support well, and in turn the bearing 15 fits within a boss in the bearing mount
In the preferred embodiment of the invention, as shown
in more detail in Figures 3 and 4, the drum 11 comprises a perforated metal hoop
30, a pair of ends 21, 22 enclosing the ends of the hoop 30 to form a substantially
cylindrical chamber and a pair of vanes 31 extending between the drum ends 21, 22.
In the preferred form of the invention the drum is driven
only from one end 21 and consequently one purpose of the vanes 31 is to transmit
rotational torque to the nondriven drum end 22. The vanes also provide longitudinal
rigidity to the drum assembly 11. To these ends the vanes 31 are wide and shallow,
although they have sufficient depth and internal reinforcing to achieve any required
resistance to buckling due to unbalanced dynamic loads. Preferably the vanes 31
have a distinct form, including a leading and trailing edge to assist in tumbling
the washing load. In the preferred embodiment the vanes 31 are oriented oppositely
in a rotational direction, so that under rotation in either direction one vane is
going forwards and the other backwards. This vane configuration provides further
benefits in providing a user friendly opening into the washing chamber as is described
In the preferred embodiment of the washing machine incorporating
the invention the drum 11 is supported between a pair of drum supports 16 one at
either end thereof. Access to the interior of the drum 11 is provided through a
slide away hatch section 33 in the cylindrical wall 30 of the drum. The hatch section
is connected through a latching mechanism 34, 35, 36, 37, 38 such that it is connected
in a continuous loop during operation. Accordingly the cabinet 12 of the washing
machine is formed to provide access to the drum 11 in a substantially top loading
fashion, rather than the traditional front loading fashion more common to horizontal
The washing machine includes an electric motor (rotor 39
and stator 40 visible in Figure 4) to effect rotation of the drum during all phases
of operation (wash, rinse and spin dry). In the preferred form of the washing machine
incorporating the present invention the motor is a direct drive inside-out electronically
commutated brushless dc motor having a permanent magnet rotor 39 coupled to one
end 21 of the drum 11 and stator 40 coupled to the drum support 16. A suitable form
of motor is described in EF0361775.
A user interface 24 is provided, allowing user control
over the functions and operation of the Machine. The control electronics are integrally
contained within the interface module, and provide electronic control over the operation
of the machine.
In the present invention the forces caused by an out-of-balance
load during high speed rotation of drum 11 to effect spin drying are minimised by
a dynamically controlled balancing system This balancing system uses electrical
signals generated by the deformation of load cells in the bearing mounts 29 at each
end of the shaft 19 to assess the required weight distribution correction that is
required to dynamically rebalance the drum 11. Each bearing mount 29 is formed with
a pair of bending bridges 140,41 and mounted on each bending bridge is a load cell
42 as shown in Figure 5. The outputs of the load cells 42 are fed to control processor
of the laundry machine to effect the balancing task, which is achieved by the addition
of water to one or more of the six balancing chambers 43,46,47,80,81,82 located
in the drum, as shown in Figure 6. There are three such chambers at each end spaced
120° apart and positioned on the extremity of the drum end 21,22.
In more detail the balancing system is illustrated in Figure
7. The output from the load cells 42 is first passed through filtering 50 before
connection To the inputs of a microprocessor 51, which may be task specific or the
main control processor for the laundry machine. The various algorithms (detailed
later) programmed into the microprocessor 51, will dictate spin commands (eg: speed
up/slow down) to the motor controller 52 and balancing corrections (eg: open/close
valve 54) to the valve driver 53. The motor controller 52 in turn, will vary its
energisation of the motor windings to achieve the spin command. The valve driver
53 will open or close the appropriate balancing valve 54, which allows water to
flow through the injector 44 into the relevant slot 45 whereupon it is channelled
to the appropriate chamber. The valve driver 53 also allows switching between coarse
and fine control modes by switching the water flow through the high 55 and low 56
flow rate valves respectively.
To correct an imbalance, it is necessary to artificially
add equal and opposite static and dynamic imbalances. To add a static imbalance
only requires to add a certain amount of mass at some radius and rotation angle
(or 'phase' angle), at the same location along the spin axis as the CoG. However,
to add a dynamic imbalance requires to add two equal and opposite imbalances at
two locations along the spin axis that are evenly spaced either side of the CoG.
The end result is that both static and dynamic imbalances can be corrected by adding,
at two separate locations along the spin axis, two independent masses (both may
be at the same radius) at two independent phase angles. There are four variables
to be defined, and so four useful pieces of information about the nature of the
imbalance must be obtained.
These pieces of information are typically obtained by measuring
either acceleration, velocity, force, or displacement at two independent locations
on the vibrating system. The reason that only two sensor locations are required
and not four is that because the relevant signals are sinusoidal in time and therefore
contain two pieces of information. One is the magnitude of the signal, and the other
is the "phase" angle with respect to some reference point on the spinning system.
Once the signal magnitude and phase angle at two independent
locations are acquired, a method is required to calculate the two masses and their
phase angles with which to correct the imbalance. This is done by representing the
signal data and mass data as vectors of two complex numbers, and the relationship
between them as a square matrix of four complex numbers. This matrix, when for mapping
the mass vector to the signal vector, is called a response matrix, and it is its
inverse that is used to map the signal vector back to the mass vector representing
The technique for acquiring data on the imbalance is difficult
to implement in practice. This is because some types of signal are more difficult
to measure than others, and even if good signals are obtained, the response matrix
can become a unpredictable and difficult thing to know (or learn) depending where
the signals are measured. In the preferred embodiment of the present invention the
imbalance is characterised using force or stress measurement. Of the available alternatives
force is easy to measure and the signal level is quite adequate at low speeds.
Because the machine has no suspension the cabinet is effectively
rigidly connected to the spin axis of the drum. This means that the response matrix
that relates imbalance to force at the bearing assemblies is reasonably diagonal
and does not vary in a complex and/or unpredictable manner with speed where the
appliance is supported on a rigid floor. Thus a radial component of force (vertical
for instance) at the bearing assemblies at each end of the drum, is the most useful
signal to measure for the purpose of balancing, with a rigid floor. Where the floor
supporting the appliance is flexible a different relationship applies, which is
To perform a complete static and dynamic balance requires
four useful pieces of information to be known about the nature of the imbalance.
It has also been shown that the desirable signals for the purpose of balancing are
a radial component of force at each bearing assembly supporting the drum, and thus
two load cells of some sort are required. In the preferred embodiment a pair of
sensors 42 are located at either end of the shaft 19 as shown in Figure 4.
A strain sensor suited to this application is the piezo
disc. This type of sensor produces a large signal output and so is not significantly
affected by RFL However a piezo strain sensor can only measure fluctuations in load
due to charge leakage across the disc.
The piezo disc will have a particular response in relation
to applied force. Since force is proportional to frequency squared and the response
magnitude is proportional to force frequency, the relationship between sensor output
and rpm of the drum is cubic.
In more detail the bearing mount looks like two concentric
cylindrical rings 146, 147 as illustrated in Figure 5. The load bridges 140, 41
described previously are connected at the top and bottom of the inner ring 147,
respectively, and to opposite parts of the upper periphery of the outer ring 146.
A piezo disc 42 is adhered to the loading bridge onto the side facing the outer
ring. The load from the drum is taken through a bearing 15 mounted in the internal
ring 147, through the load bridges 48 and load cell 42 into the outer ring 146,
and out into the external structure. It will be appreciated that in this fashion
the load bridges will flex according to any vertical forces from the spinning of
the drum, thus deforming the piezo disc and providing a signal representative of
the imbalance force.
In the preferred embodiment of the invention a dynamic
control method is used. This is not in any way to be confused with static and dynamic
imbalance as explained earlier, it simply refers to the nature of the control methodology.
The alternative control methodology is 'static'. A static control method does not
make use of or retain data on the time dependent behaviour of its target system.
As a result the method is executed as a 'single shot' attempt to restore equilibrium,
and sufficient time must be allowed to lapse after each execution so that the system
has returned to a steady state condition prior to the next execution. Whereas a
dynamic control method can anticipate the time dependent behaviour of the system
and by storing recent past actions it is able to continuously correct the system,
even while the system is in transient response.
The main advantage of the preferred dynamic control is
that the control loop is able to adjust for discrepancies as and when they appear
rather than having to wait for the next execution time to come round. For systems
with slow time response this is a considerable advantage. To work effectively the
controller must be programmed with an estimate of the time dependent response of
the target system. However, provided it has no significant quirks, this only needs
to be roughly approximated and the approach will still work well. Also, because
the dynamic controller runs on a fast decision loop, any noise on the input parameters
will result in many small corrections being made that are completely unnecessary.
For this reason a minimum threshold correction level must be established where there
is any cost or difficulty associated with effecting a correction.
Listing the main sources of time dependent behaviour:
If in the spin cycle the machine is to ramp from 100 to 1000 rpm in about 3 min
then the machine will almost certainly be in a state of transient response for the
duration of this period. Consequently the controller must be able to respond to
changes in the balance state of the machine without the machine ever being in a
steady state condition.
- Given an instantaneous change in balance state of the machine, it will take
a few revolutions to reach a steady state of vibration.
- The forgetting factor averaging on the load cell data acquisition means that
the averaged data also takes a number of revolutions to respond to a new vibration
- Change in balance state of the machine is never instantaneous; water addition
requires anything from 0.1 to 60 seconds.
- Water extraction from the load means the balance state of the machine may change
quite rapidly as its spin speed ramps up.
As previously stated for dynamic control to be implemented
the present controller must be programmed with an approximation of the time dependent
behaviour of the machine. More precisely it must know how much to weight its past
actions (as a function of how long ago they were made) when deciding on what corrections,
if any, are to be implemented. In this application, for each water chamber the sum
of the appropriately weighted past history of water addition can be considered to
be 'Effect in Waiting'; i.e. the controller is still anticipating that the effect
of a certain quantity is still to come through on the signals, and thus must subtract
this 'Effect in Waiting' from the presently calculated water requirements when deciding
which valves should be on and which should be off at present.
To do this accurately requires a complete record of the
controllers past actions for as many points back as it needs to remember, and a
table of weighting values for as many points, which in this application will be
at least ten. If we call this number of points N, then to store the history of six
control output channels with N points each requires 6N data points. Also, to then
calculate the effect of this history will require 6N multiplications. One simplification
would be to approximate the exact weighting curve 60 with a 'table top' curve 61
as shown in Figure 9. This then eliminates the need for a stored table of weighting
values, and reduces the 6N multiplications to 6N additions, but even this is still
to complicated. A very crude approximation of the exact weighting curve is the negative
exponential 62 also shown in Figure 9. While this sounds complicated it is in fact
extremely easy to achieve, it is simply a forgetting factor type average. All that
needs to be done is this: for each water control channel, create an effect in waiting
variable and each time the control loop executes multiply it by a certain factor
(between zero and one) and add to it some increment value if the water control valve
for this channel was on during the last loop. Computationally all that is required
is six multiplications and six additions with each control loop execution; a vast
saving. To avoid the need to have different forgetting factors dependent on speed;
the control loop must be executed on a per revolution basis. This is simply achieved
by executing the balance control code with the once per rotation sensor, directly
after the data acquisition conversion code. Of course all quantities of water must
now be calculated in terms of revolutions at the present speed rather than time,
but this is a simple matter in that the magnitude calibration factor will now vary
like rpm rather than rpm squared.
Another point to consider is that, considering one end
at a time, if the out of balance load is directly opposite one of the chambers (say
chamber number 43) then the data acquisition routine will identify this chamber
as the primary one needing water, however, due to noise on the signals, it will
almost certainly also say that one of the other chambers needs a small amount of
water as well. This second water requirement will be much smaller than the other
one and will sometimes be chamber 46 and sometimes chamber 47 depending on just
what the noise was in the last few revolutions. If the balance control routine addresses
these secondary small water requirements then over the relatively long period of
addressing chamber 43 it will also gradually fill chambers 46 and 47, thus negating
some of the water going into chamber 43, and leaving less headroom for further balancing
corrections later on. Clearly the balance controller must not address two chambers
at once at one end unless it is clear that neither of them could be due to noise,
i.e. both of them require a similar amount of water. Similarly because the ends
of the machine are not truly independent systems but are weakly coupled (as will
be discussed later) then large out of balance forces at one end cause 'ghost images'
at the other, thus the balance controller must not address two ends at the same
time unless it is clear that neither of them could be ghost images, i.e. both ends
require a similar amount of water. The easiest way to address both of these problems
is identify the maximum water requirement out of the six chambers and to then set
a dynamic 'noise' threshold equal to half of this value of water (as shown in Figure
10). A water valve (e.g. 5) is then only turned on if the result 72 of its present
requirements 70, minus its present effect in waiting 71, minus the noise value,
is greater than the increment value mentioned above. It is here that we perform
our magnitude calibration by adjusting this increment value.
Finally, a small amount of hysteresis is necessary to prevent
repetitive short valve actuations. This is simply achieved by using the above criterion
for deciding when to turn a valve on, but using a different criterion when deciding
when to turn it off again. The off criterion is more simple: a water valve is only
turned off once its present requirements is less than its present effect in waiting.
In other words once the valve is on it is not turned off until its chamber requirements
The task of spinning while balancing actively can be subdivided
into three sub-tasks or algorithms:
Imbalance Detection Algorithm
Balance Correction Algorithm
The Imbalance Detection Algorithm (IDA) (shown in Figure
11) is concerned solely with the acquisition of imbalance related data, and is embedded
in the motor control routine. It is active whenever the motor is turning, and makes
its results available for the Balance Correction Algorithm (BCA) to see.
The Spin Algorithm (SA) (shown in Figure 13) is concerned
solely with executing the spin profile asked of it. It ramps the speed of the machine
according to the profile requested and the vibration level determined by IDA.
BCA (shown in Figure 12) is concerned solely with correcting
whatever imbalance IDA has determined is there. It is an advanced control algorithm
that takes into account the time dependent behaviour of both the machine and IDA.
BCA is active whenever the rotation speed of the machine is greater than approximately
Signal Analysis - IDA Processing
To determine the imbalance in the load requires the magnitude
and phase angle of the once per rotation sinusoidal component in each of the signals.
Unfortunately the signal does not look like a clean sinusoid, but is messy due to
structural non-linearities in the machine as well as Radio Frequency Interference
(RFI). The once per rotation component or 'fundamental component' must be somehow
obtained out of such a signal.
This is done by digitally sampling the signal and using
the discrete Fourier Transform technique. It is not necessary to compute an entire
transform, which would give us half as many frequency components as we have signal
samples inside of one revolution (and would also take some time in an 8-bit microprocessor),
but just the fundamental component. The way this is done is to multiply each of
the signal data points obtained by the value of a once per rotation cosine wave
at the equivalent phase angle lag after the rotational reference mark, and sum each
of these results over a whole revolution, and then divide by the number of results.
This gives the real (or x) component of the complex number result. The imaginary
(or y) component is derived using the same technique but using a sin wave instead
of a cosine wave. The resulting complex number may then be converted in polar form,
giving magnitude and phase angle of the fundamental component in the signal. Also
to prevent aliasing the input signal is passed through an analogue filter first
to remove frequency components higher than half of the sampling frequency.
The discrete Fourier analysis may be made considerably
more simple if the sampling is performed using a fixed number of samples per revolution
rather than a fixed frequency. This of course requires a rotary encoder, which in
this application is already provided in the form of a DC Brush-less motor. It is
therefore necessary to use a number of points per revolution that divides exactly
into the number of commutations per revolution executed by the motor. This also
enables the sine values that will be required to be pre-programmed as a table (termed
the 'sine table'), from which the cosine values may be obtained by offsetting forwards
by a quarter of the number of samples per period. It is necessary to have a reasonable
number of sampling points per revolution so that the order of harmonics that are
aliased onto the fundamental component is well beyond the cut-off frequency of the
low pass filter. This means that the number of sampling points must be at least
12 to obtain reliable sampling at speeds upwards of 200 rpm. An even number of points
per revolution for sampling should be used so that the sine table is perfectly symmetrical,
i.e. the positive sequence and the negative sequence are identical apart from their
sign. This ensures that the DC offset on the input signal does not influence the
fundamental component. Figure 8 illustrates the signal after filtering 57 and the
extracted fundamental component 58.
Alternatively, if a more powerful microprocessor is employed
then by maximising its data acquisition capabilities the noise problem will be further
reduced. This would mean instead of fixed sampling on a per revolution basis, it
would be on a fixed frequency basis - at a higher rate. Further, the sine and cosine
valves could be either calculated or interpolated from a table, which simplifies
much of the calculations.
Once the fundamental component of the source signals is
obtained it will inevitably contain some noise component (i.e. consecutive measurements
will have some variance). The best way to get rid of this is to ensure that the
signal source is accurate, clean, and has linear response. Once the source end has
been addressed then averaging techniques may be used to address the remainder of
One such technique is to implement a 'Forgetting Factor'.
This is where every time a new measurement is acquired the new average is equal
to for example 70% of the old averaged value plus in this case 30% (=100%-70%) of
the new measurement. Here the forgetting factor used was 0.3 since 0.3 of the old
average is forgotten and replaced it with 0.3 of the new measurement. This form
of averaging suits microprocessor based application since it is inexpensive with
respect to both memory space and processor time.
The main disadvantage with averaging the measurements is
that the response time of the imbalance detection goes down. This is simply a result
of the fact that the averaged result must incorporate several measurements in order
to reduce the noise, which of course can only be obtained from past measurements,
not future ones. The lower the forgetting factor, the more the averaged value remembers
from past measurements, and thus the slower it responds to a change in the machine's
Because the balancing can only be executed over many iterations
(due to water extraction from the load) it is not necessary to be able to obtain
a perfect balance in one 'hit'. From this point of view it is then acceptable to
make a few 'approximations', the biggest of which is to treat the machine as two
independent single degree of freedom (SDOF) systems associated with each signal
source. The main advantage of doing this is that the micro does not have to calculate
and invert the 2x2 response matrix, it only has to estimate the two SDOF responses
for each end.
Since the measurement data are complex numbers in Cartesian
format (x & y), whereas the responses are in polar format (magnitude & phase), a
format conversion and complex division is required at each end to obtain the water
correction vector. While this is not impossible to execute conventionally, there
is a more simple approach: take the phases of the response and incorporate them
directly into the discrete Fourier technique as offsets each of an integer number
of points when referencing the table of sine values. These offsets may then adjusted
as the machine changes speed for phase angle calibration. Alternatively phase calibration
may be performed using a rotation matrix acting on the vectors as calculated without
any applied offset to the sine table. Magnitude calibration however, is performed
later in the dynamic control routine.
Once having obtained the x and y components of the imbalance
at each end of the drum, it is then required to calculate how much water each chamber
at each end needs since the chambers are 120 degrees apart. If the chambers were
90 degrees apart, (i.e. orthogonal like the x and y axes) then the problem would
be trivial, but this would require four chambers for each end and thus two more
water control valves and associated drivers than necessary. A more simple approach
is to calculate the projection of the signal vector onto axes that are 120 degrees
apart, the same as the chambers.
The way to implement this is very simple. The Fourier technique
uses sine and cosine wave forms to extract the orthogonal x and y projections. This
follows quite naturally from the fact that a cosine wave is a sine wave that is
has been shifted to the left by 90 degrees. Therefore to split the signal vectors
into projections that are 120 degrees apart simply requires to replace the cosine
wave form with a sine wave form that has been shifted to the left by 120 degrees,
i.e. one third of a rotation.
The phase calibrated signals now represent the projection
of the imbalance onto the first two chambers. To obtain the projection of the imbalance
onto the third chamber to we may use the vector identity that the sum of three vectors
of equal magnitude and all spaced 120 degrees apart must be equal to zero. Hence
the sum of all three projections must be zero, i.e. the projection onto the third
chamber is the negative of the sum of the projections onto the first two chambers.
By adding half a rotation to the response phase angles the three values obtained
are made to represent the projection of the restoring water balance required onto
each balancing chamber.
Finally, at least one of these three projections will be
negative, representing water to be removed from that chamber. This cannot be done
and so we simply add a constant to all three numbers so that the most negative number
becomes zero and the other two are guaranteed positive.
Overall Control Strategy - SA
The overall control over the spin process is assigned to
the spin algorithm SA. It begins with the bowl speed at zero, and disables the BCA.
Its first task is to better distribute the wash load to allow spinning to begin.
If at a very low spin speed the vibration is below the initial threshold, it is
allowed to spin to the minimum BCA speed at which point BCA is enabled. If the vibration
is not below the threshold, redistribution is retried a number of times before stopping
and displaying an error message. Once BCA has attained the target level of spin
speed the spin is allowed to continue for the desired period after which the bowl
is stopped, valves are closed and BCA is disabled.
Dynamic Balancing - BCA
In more detail the balance correction algorithm shown in
Figure 12 begins with calibration of the phase information from the IDA. The step
of vector rotation is optional depending on the method used (alternative is to apply
in offset to the sine table). Following this the vectors are normalised and the
level of vibration is calculated. If the enable flag is true and the level of vibration
is below a predefined critical limit the decision making process begins. Firstly
the vibration level is compared to a number of threshold values to assess whether
to enable increase of the bowl speed. Then depending on the level of vibration fine
or coarse (low or high flow rate to valves) correction is enabled. The effect in
waiting of past actions is then updated, and together with the current vector information
and the status of each valve a decision is made whether to open or close each valve.
Then if the hold bowl speed flat is not enabled i.e. acceleration is allowed, and
the speed is not currently at the desired target level, the bowl speed is allowed
to increase to the target level. At this point it loops to the start and begins
another iteration, effectively continuously correcting and accelerating until it
reaches the target speed.
It will be appreciated in the preceding embodiments that
the washing machine is assumed to be supported on a rigid surface such as a concrete
floor. Where this is not the case, for example, wooden floors, and the entire washing
machine is permitted substantial displacement during the spin cycle, then those
techniques previously described will not be entirely successful. Therefore, in a
further improvement the present invention also provides a method and apparatus for
correcting for spin imbalances when the washing machine is supported on a non-rigid
The equivalent spring system which represents the spin
drum 100, the machine frame 102 and the reference surface is shown in Figure 14.
The first spring 106 between the spring drum 100 and the machine frame 102 effectively
represents the elasticity of the load bridge which connects the bearing mount to
the drum support or frame of the washing machine. This bridge also forms the basis
of the load cell which measures the forces between the drum and the frame of the
washing machine. The second spring component 108 in this case represents the elasticity
of the support surface, for example, flexible wooden floorboards. The second spring
108 is complex and includes a damping component 110. In order to measure the acceleration
or displacement of the drum 100 relative to the reference surface 104, i.e. a stationary
reference point, a accelerometer 112 is connected either to a non-rotating part
of the bearing itself or on an adjacent section of the load cell bridge.
Now, consider that the machine is spinning at a particular
speed and is in a perfectly balanced state. Suppose we now add a small "Out Of Balance"
(FO/B) load at one end (by injecting some water into one of the balance
chambers). If the ends of the machine behaved entirely as independent mechanical
systems then we would expect that we would now measure a force vector at the end
to which we added water, and that nothing would change at the other end: the other
end would remain perfectly balanced. However, the ends of the machine are not independent
systems, and in reality we find that we now measure a force vector at both ends
of the machine. The two ends are said to be 'coupled' together. As a result of this
coupling, the observed force vector at one end of the machine is related not only
to the "out of balance" FO/B vector at the same end, but it is also related
to the FO/B vector at the other end of the machine. Thus:
Where F1 is the force vector measured at one
end 1 of the machine, FO/B1 and FO/B2 are the FO/B
vectors at ends 1 and 2 respectively and R11 and R12 are the
individual response factors that FO/B1 and FO/B2 have at end
(Note that R11 and R12 are also vectors; each consisting of
magnitude, and phase lag of the response)
Similarly at end 2 we may write
Where F2 is now the force vector as measured
at end 2 and R21 and R22 are the individual response factors
that FO/B1 and FO/B2 have at end 2.
These two equations may be mathematically combined as a
Where F is the column vector (of vectors)
FO/B is the column vector (of vectors)
R is the response matrix (of vectors)
Now, if the machine held the bowl absolutely rigid while
spinning then we would expect the force transducers to measure precisely the force
vectors required for the centripetal acceleration of the FO/B load vectors.
But this is not the case. The external structure of the machine is not infinitely
stiff, and neither is the floor, the house, or even the ground under the house for
that matter. As a result the force transducers also measure a component due to the
mechanical response of the machine which is a function of all of the above (machine
structure, floor, house ...), and also of bowl rotation speed. Note that it is this
extra component of machine response that makes the coupling terms in the matrix
(R12 and R21) significant and the whole matrix in general
impossible to pre-calibrate.
It is here that two possible techniques emerge:
- 1) By measuring acceleration vectors at each end we may determine the machine's
mechanical response, and then by appropriately combining force vector and acceleration
vector at each end we can make a new vector quantity for which the response matrix
is uncoupled (i.e. R11 and R22 are the only significant terms).
Further the matrix is not a function of unknown parameters and thus can be factory
- 2) Or by making small, but known, changes to the FO/B vectors and
measuring the resultant change in force vectors, it is possible to learn this response
matrix 'R' during the spin cycle.
The first technique is very robust, but requires the addition
of acceleration sensors to measure absolute vertical acceleration of the drum.
The second technique is very clever, but has several difficulties
associated with it which are outlined further on.
First Method - acceleration measurement
From the system described above it will be apparent that
the force measured by the load cell will not be an accurate measure of the imbalance.
In order to determine the imbalance to correct the controller must take account
of the effect of the complex system external to that of the washing machine. It
will be appreciated therefore that the absolute force Fa acting on the
spin bowl can be expressed as
where m1 is the mass of the spin drum and aa is the absolute
acceleration of the drum, as measured by the accelerometer. This force in turn is
then composed of:
where Fo/b is the out of balance force and F1 is the force
measured by the load cell. By rearrangement the out of balance force Fo/b
may be expressed in terms of known variables
and calculated by the controller. Whereas F1 would be available from
IDA as previously described, the output of the accelerometer would need to be put
through a similar filtering process to the IDA, in order to provide a useful signal.
The drum mass m1 is estimated based on the known weight of the drum,
the amount of water added to the load and known characteristics of the load based
on the "type" of load. The "type" of load may be determined using any one of a number
of well known fabric sensing techniques such as that disclosed in our
US patent 4857814
The above makes the assumption that each end of the drum
may be treated separately. We have found that by using this method this is a satisfactory
assumption. However in some cases this may not be adequate and therefore a more
accurate system may be required. In this case it is necessary to take into account
the coupling between each end of the drum. To this end a coupling matrix y may be
determined by successive tests on the system, where &xgr; is the ratio of the
position of the centre of gravity to the length of the drum, and &agr; is the
Second Method - determining the system response
from this we may calculate the out of balance force:
where the acceleration vector A may be represented
and the force measure of the load bridge F1 as
If the response of the machine is relatively linear
Where dF and dFO/B are still 2*1 column vectors,
and R is the 2*2 response matrix. dF represents the change in the force vectors
as a result of adding FO/B vectors dFO/B. However, in the
real world we will want to find out the FO/B vectors needed to remove
the F vectors measured. To do this we need to rearrange by multiplying each side
by the inverse of R:
Since any matrix times it's inverse gives the identity
matrix. Let us also call the inverse of R 'A' since it is really the 'action' matrix
that tells us what to do given what we measure. Thus:
The problem is we want to find out A. The way to do this
is to add a small, but known, additional imbalance to one end and nothing to the
other. Let us denote the addition as dFO/Ba, and the corresponding changes
in the force vectors as dFa. Remember dFO/Ba and dfa
are both column vectors (of vectors). Now repeat the exercise but this time adding
another small addition to the other end. This time let us denote the addition as
dFO/Bb, and similarly the corresponding change in force vectors dFb.
Now we can combine the two experiments together to write:
Where DFO/B and DF are now the 2*2 matricies
formed by joining two 2*1 column vectors side by side. Multiplying each side of
the equation by the inverse of DF:
And thus the action matrix is now known, and may be used
to calculate the correction required eliminating the measured F vectors. To illustrate
all this here is a worked example. Suppose the machine in presently spinning at
some constant speed, and the force vectors we measure at each end are:
Now suppose we add one unit of water at 90° at end
1, and nothing at end 2, and the new force vectors become:
Now for the second run suppose we add 0.5 units of water
at 0° at end 2, and nothing at end 1, and the new force vectors become:
With A now calculated and knowing F as measured by the
load bridge, the required correction to counteract the imbalance can be calculated.
Intially the action matrix is completely unknown thus we must make random guesses
for the inital FO/B vectors. After we have some knowledge of the matrix
we may make better guesses for the initial FO/B vectors.
Overall System Advantages
The advantages for the Washing Machine of employing and
active balancing system are:
- Forces due to imbalance are eliminated prior to bearing assemblies. Thus structural
requirements are reduced, enabling less and/or cheaper material to be employed.
- Suspension which wears out and deteriorates is eliminated.
- Wash cylinder clearances reduced enabling ample load capacity in a machine of
- Complexity of door opening mechanism also reduced because it no longer needs
to cope with height changes on a suspension.
- Quiet smooth spinning at all times.
- Able to cope with variable external conditions.