Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR DETERMINING AN IMBALANCE
CONDITION IN AN APPLIANCE
BACKGROUND
Field of the Invention
The present invention generally relates to methods and an apparatus for
detecting imbalance conditions in an appliance.
DESCRIPTION OF THE RELATED ART
Appliances such as automatic washing machines typically contain rotatable
vessels that are designed to hold material or clothing and are known in the
art. The
vessel is contained within a housing and is perforated with apertures. The
apertures
allow water to be pumped into the vessel to wash the material in the vessel
and to
allow soiled water to be forced out of the vessel. A washing machine usually
contains
a main control panel that controls various cycles, typically comprising a wash
cycle,
spin cycle, a rinse cycle, followed by another spin cycle. Water is pumped
into the
vessel during the wash cycle and rinse cycle, while it is extracted via
centrifugal force
during the spin cycle as the vessel rotates or spins. Additionally, a washing
machine
usually contains an agitator that oscillates to facilitate washing where the
vessel
rotates about a vertical axis. In machines that contain vessels that rotate
about a
horizontal axis, an agitator is usually not included as clothes can be tumbled
instead of
agitated in order to facilitate in the washing process.
Appliances that contain rotatable vessels are subject to operating conditions
such as load imbalances. Load imbalances in appliances such as washing
machines
occur when the material contained in the vessels is not evenly distributed
within the
vessel. The material may be unevenly distributed when loaded into the vessel
or may
become unevenly distributed as the vessel rotates. For example, in vertical-
axis
washing machines, when a wash or rinse cycle completes and water is drained
from
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the vessel, the clothes are gathered at the bottom of the vessel without being
evenly
distributed within the vessel. As the motor ramps up the speed for the next
cycle, the
clothes can creep up the sides of the vessel and become imbalanced.
Similarly, in horizontal-axis washing machines, load imbalances can occur
when clothes are not evenly distributed during the machine's distribution
cycle. Load
imbalance conditions can cause various inconveniences such as severe vibration
and
movement of the appliance. Severe vibration occurs when a load is imbalanced,
or
out of balance because the center of mass of the rotating vessel no longer
corresponds
to the geometric axis of the vessel. Severe vibration can cause an appliance
to move
along the surface it rests upon, for example, when a washing machine moves
across
the floor. Additionally, severe vibration can cause the vessel to break free
from its
mountings. Another disadvantage of load imbalance conditions is that the
motor's
power is wasted in the vibrations and movement instead of being fully applied
to
rotating the vessel.
Prior art solutions designed to prevent imbalance conditions were typically
mechanical and include adding masses to the rotatable vessel of the appliance
in order
to counter-balance imbalance conditions.
Other solutions that were designed to detect imbalance conditions are
typically
complex and include comparing the actual power usage of a vess--l to an
expected
power usage and measuring current ripples. One example of such an attempt is
illustrated in U.S. Patent No. 6,640,374, where the amount of current used by
the
motor to rotate the vessel is compared to a threshold value.
Accordingly, there is a need to provide an improved method and apparatus to
detect load imbalance conditions in an appliance to allow for simplified
design and
manufacturing.
BRIEF DESCRIPTION OF THE INVENTION
In one embodiment of the invention, an appliance and method is provided that
can determine load imbalance or out-of-balance conditions. The appliances that
typically apply load imbalance detection are clothes washers. A method of
imbalance
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detection includes identifying parameter values that fluctuate with load
imbalance
over a predetermined sample period and determining a target parameter value
from
the identified parameter values. The method further includes calculating a
parameter
spread of the parameter values by comparing parameter values to the target
parameter
value. The method further includes converting the parameter spread into a
weight
value that reflects an imbalance condition.
In another embodiment of the invention, a computer program embodied on a
computer-readable medium includes identifying parameter values that fluctuate
with
load imbalance over a predetermined sample period and determining a target
parameter value from the identified parameter values. The method further
includes
calculating a parameter spread of the parameter values by comparing parameter
values
to the target parameter value. The method further includes converting the
parameter
spread into a weight value that reflects an imbalance condition.
In another embodiment of the invention, an appliance includes a vessel
mounted for rotation about an axis; a motor for rotating the vessel about an
axis; a
processor configured to determine load imbalance; and a memory for receiving
and
storing parameter data and instructions for determining load imbalance. The
load
imbalance is determined by identifying parameter values that fluctuate with
load
imbalance over a predetermined sample period and determining a target
parameter
value from the identified parameter values. The method further includes
calculating a
parameter spread of the parameter values by comparing parameter values to the
target
parameter value. The method further includes converting the parameter spread
into a
weight value that reflects an imbalance condition.
BRIEF DESCRIPTION OF THE DRAWINGS
The drawings are described herein:
Fig. I a is a view of a horizontal-axis washing machine.
Fig. lb is a diagram showing a horizontal-axis washing machine.
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Fig. 2 is a block diagram showing a system for detecting a load imbalance in
an embodiment of the present invention.
Fig. 3 is a flow diagram showing a method for detecting a load imbalance
condition.
Fig. 4 is a table containing load imbalance data.
Fig. 5 is a graph of the load imbalance data of Fig. 4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
One embodiment of the present invention concerns a method and a circuit for
detecting a load imbalance in an appliance that is simple to implement.
Referring now to Fig. 1 a, a horizontal-axis washing machine in accordance
with one embodiment of the present invention is illustrated generally at 10. A
horizontal-axis washing machine includes a vessel that rotates about a
horizontal axis
within the cabinet. Another embodiment of the invention includes a vertical-
axis
machine where the vessel rotates about a vertical axis within the cabinet. One
of
ordinary skill in the art can perform the exemplary embodiments of the
invention
described herein using either configuration. Washer 10 includes a motor 12 and
a
motor control unit 14 (see Fig. lb) that can also be integrated within one
unit. Washer
includes an outer housing or cabinet 20 supporting a fixed tub 22, a vessel or
moving tub ("tub") 25, motor 12, and motor control unit 14, within which there
is a
processor 102, in a known manner. Vessel drive shaft 30 is also illustrated.
Tub 25 is
configured to hold articles (not shown) such as clothes to be washed.
In the horizontal washer configuration, a direct belt drive is configured to
transmit rotary motion imparted on a motor shaft 36 by motor 12 to tub 25 via
drive
belt 29. Fig. 1 b illustrates a side view of the exemplary washer of the
present invention.
During a spin cycle, liquid within the articles is removed by the centrifugal
force imparted by the spinning vessel and is allowed to exit the basket
through
apertures (not shown). During the spin cycle, articles or clothing becomes
plastered
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to the wall of tub 25 at a first speed or plaster speed. Plaster refers to the
centrifugal
force of the spin cycle pushing the clothing against the wall or structure of
the basket.
The clothes remain positioned by centrifugal force during a time period the
first speed
or plaster speed to a second speed or maximum speed of the spinning basket.
The
plastered speed and maximum speed can be determined by one of ordinary skill
in the
art. Load imbalance conditions can occur when the clothes are unevenly
plastered
throughout the vessel.
Fig. 2 depicts a block diagram showing an appliance for detecting a load
imbalance in an embodiment of the present invention. The appliance, which
could be
an automatic washing machine 10, can include a cabinet 20, a vessel 25, a
motor 12,
and a motor control unit 14 including a memory 101, a processor 102, and a
sensor 103.
The cabinet 20 contains the vesse125 that can be loaded with material, for
example
clothes. The motor 12 drives the vessel and can be directly attached to the
vessel by a
belt, clutch, or a direct coupling, for example. The motor can be any type,
including
an induction motor. The sensor 103 can detect the rotation speed of the motor
shaft
along with other parameters that fluctuate due to load imbalance, such as
voltage
amplitude, torque and motor current, for example. Any type of sensor can be
used,
including a hall sensor to detect the rotation speed of the motor shaft. The
memory 101
stores the executable instructions for controlling the functions of an
appliance. The
processor 102 executes the instructions stored in the memory 101. The memory
101
can either be external or internal to the processor. The processor may
comprise any
type of processor including microcontroller or a microprocessor. The processor
102
executes the instructions to determine when an imbalance condition exists
according
to the method shown in Fig. 3 and takes corrective action if a sufficiently
high
imbalance or out of-balance (OOB) condition is detected. The actions taken
when a
sufficiently high imbalance condition is detected may include stopping the
motor,
attempts to re-balance the clothes in the tub, or a reduction of the allowed
top speed.
Fig. 3 is a flow diagram showing a method for detecting a load imbalance
condition. The method includes selecting a parameter that fluctuates due to
load
imbalance, in this example, voltage amplitude which is required to maintain a
constant speed. The method further includes identifying parameter values in a
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predetermined sample period 201 and determining a target parameter value 202,
which is the average of the parameter values in this example. In one exemplary
embodiment, the parameter measured is voltage amplitude, required to maintain
a
constant speed, which is measured at a fixed interval of time, (i.e. every
50ms) of a
pre-determined sample period. The sample period represents the predetermined
sample period, the time during which the parameter values are read. The
parameter
values are stored in a data buffer of predetermined length in memory. If the
buffer is
full, return to the beginning of the data buffer so the parameter data will
overwrite the
oldest parameter data. The effect of this method is that of a moving data
window or
moving sample period.
In one embodiment, the amplitude required to maintain the current speed is
calculated in the microprocessor software. The software increases or decreases
voltage amplitude according to input from a speed sensor 103 to maintain a
constant
speed. The speed sensor 103 may be employed to detect speed fluctuations of
the
motor shaft 36.
The method further provides calculating a parameter spread of the parameter
values described above 203. The parameter spread is calculated whenever the
buffer
window is full of parameter values, for example or in another embodiment,
whenever
a total number of parameter values is reached over a given sample period.
Also,
several sample periods may be taken over time, which constitutes a moving
sample
period. In one embodiment of the invention, the parameter spread comprises an
average deviation based on the difference between the average of the parameter
values in the sample period and a particular parameter value, although other
methods
of determining parameter spread may be used.
In accordance with the present embodiment, the processor 102 compares
individual parameter values to a the target parameter value to get an average
deviation
of the parameter spread as shown below:
'; 1I X; -X l
N
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The parameter spread shown by the equation above requires calculating the
average (X) of the total number of parameter values from the sample period,
summing
the absolute value of each parameter value (X;), which can be a real-time
reading of
the fluctuating parameter, minus the average (X), and dividing the sum by a
predetermined number of values. The processor can calculate the average
parameter
error by retrieving the parameter value data from memory at predetermined
intervals.
Furthermore, the parameter spread may be converted to an actual weight value
(Lbs or Kg) 204 that can be used to determine the existence of an imbalance
condition
205 by the following equation:
OOB Lbs = Parameter Spread / Load Constant
Wherein the load constant is calculated by applying a predetermined linear
equation to the current load size in the washing machine tub. The current load
size
can be determined in various ways as determined by one of ordinary skill in
the art,
and stored in memory as a weight value (lbs or kg). The slope and offset
comprise
predetermined values that are constants calibrated using known or
predetermined
imbalance loads.
Load Constant = (Load Size) (Slope) + Offset
An example of calibration includes calculating the average deviation for a
chosen parameter for each known imbalance load, which is a known actual
imbalance
that has a weight value (lbs or kg). When voltage is the measured parameter,
the
voltage deviation is measured as an A/D value, where 1 VDC = 2.0277 A/D units.
In practical embodiments the load constant may be determined through
empirical data that may stored in tabular format in the memory 101. To
accomplish
this, the load constant may be generated through the use of empirical data
such as that
provided in Figure 4 utilizing the following equation wherein the parameter
spread is
the average deviation as provided in Figure 4.
Load Constant = Parameter Spread / Actual OOB
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When voltage amplitude is the measured parameter, the equation for the load
constant can be modified as shown below:
Load Constant = Parameter Spread * Speed / Actual OOB
The modification to the equation includes multiplying the parameter spread by
the speed of the motor in order to normalize the voltage amplitude spread. The
modification to the equation above is not required, although desirable due to
the drop
in amplitude spread as speed increases. If the amplitude spread is multiplied
by the
speed, the resulting load constant curve is flatter and provides an improved
imbalance
calculation. In the current implementation, the OOB calculation is optimized
between
90 basket RPM (or plaster speed) and about 150 basket RPM. This range may vary
slightly based on machine dynamics.
Referring now to Fig. 5, a linear graph of the data from the table of Fig. 4
showing slope 300 gives the equation:
Load Constant =-7.2623 * Load Size + 152.46
Wherein the load size is the actual weight of the clothes in the vessel.
The imbalance weight value (OOB Lbs) can be determined by the using the
equation:
OOB = Parameter Spread / Load Constant
An imbalance condition will be detected when the OOB value is above a
predetermined value.
The particular embodiments of the invention described above are merely
illustrative as the invention may be practiced in different but equivalent
manners
apparent to those skilled in the art. Similarly, the protection sought is to
be found in
the claims and is not to be limited by the descriptions of the embodiments
above.
Therefore, the particular embodiments disclosed above may be altered or
modified
and all such variations are considered within the scope of the invention.
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