EP2597186A1 - Method for estimating the load of laundry in a washing machine - Google Patents

Abstract

A method for estimating the load of laundry in a washing machine having a drum rotating in a tub with a substantially horizontal axis (B) comprises rotating the drum at a tumbling speed and measuring a motor torque or an electrical parameter related to such torque for driving the drum at said speed, particularly during water filling of the tub.

Description

• The present invention relates to a method for estimating the load of laundry in a washing machine comprising a drum rotating in a tub and having a substantially horizontal axis. The present invention relates also to a washing machine adapted to carry out the above method.
• Methods for measuring the load of laundry in a washing machine are well known in the art. EP 143685 discloses a method for determining the charge of dry or dewatered laundry comprising the determination of the moment of inertia of the mass of laundry for a measured value of acceleration of the drum and from a measured value of the motor torque for driving the drum to said acceleration. The motor torque is easily detected by measuring the current used by the electric motor driving the drum. For this kind of load measure the clothes have to be somehow plastered by centrifugal force on the circular wall of the drum.
• A similar method, in which the load assessment is carried out at a constant speed, is disclosed by US 6842928 where the drum is driven by a sensorless brushless DC motor (BLDC); in this case the laundry weight is determined based on a speed of the motor after a predetermined time has elapsed when the washer operates in a torque control mode. In this solution a quite expensive kind of motor is needed and, also in this case, the speed has to be sufficiently higher in order to have the clothes maintained by the centrifugal force in contact with the circular wall of the drum.
• EP 1447469 discloses a method for determining the load in a drum of a washing machine by establishing the moment of mass inertia of the drum from the electrical power consumption of the drive motor for the drum which rotates at different rotation speeds above the resting rotation speed.
• US 7484258 discloses a method for detecting a laundry weight in a washing machine where the machine is driven at a first speed and then to a second speed (both speed being above the resting rotation speed), the laundry load being detected on the basis of a value of a first current for driving the motor at the first speed and a value of a second current for driving the motor at the second speed.
• All the above known solutions are focused on detecting a quite precise value of the load in order to adjust the washing program accordingly, including the amount of detergent. The carry out such detection at the beginning of the washing cycle when the clothes are preferably still dry. This can cause either some damages to delicate clothes due to the dry friction of clothes for instance between the drum, gasket and door, and moreover it adds time to the whole cycle of the washing program.
• Another problem with the known method relates to the speed at which such load assessment is made, which has to be higher that the tumbling speed, i.e. the speed at which the speed of rotation of the drum is not sufficient for keeping the laundry "plastered" against the wall of the drum. This means that in a condition where no unbalance assessment has been already made, there is the possibility of a high vibration of the whole washer, particularly when the laundry is partially or totally wet.
• A further problem of the known methods relates to the quite complex algorithm needed for assessing the mass of laundry from electrical parameter when a certain precision of the mass signal is needed; this can increase the cost of the electronic control and therefore of the overall washing machine.
• It is an object of the invention to provide a method of the type specified at the beginning of the description which does not present the above problems.
• It is another objects of the present invention to provide a method for estimating the load size at the very beginning of the washing cycle, in order to fill the correct amount of water.
• The above objects are reached thanks to the features listed in the appended claims.
• According to a preferred feature of the invention, the drum is rotated at a tumbling speed, that could be constant or not, and a motor torque or an electrical parameter related to such torque for driving the drum is measured at said speed.
• According to a preferred embodiment of the invention, the method for estimating the load of clothes in household washing unit uses fire angle signals of the motor during water filling.
• The method according to the invention can discriminate quickly the load size in the drum. There is no added time to the traditional washing program timing, and the risk of damaging delicate clothes in a dry sate by rotating the drum at a speed typical of intermediate spinning speed is greatly reduced.
• According to the present invention, a high torque, measured at tumbling speed, is indicative of a low charge of laundry, and vice versa.
• Further advantages and features according to the present invention will be clear from the detailed description, with reference to the attached drawings in which:
• Figure 1 is a schematic view of a washing machine according to the invention;
• Figure 2 is a schematic view of a washing machine in which the drum is in a full load configuration;
• Figure 3 is a view similar to figure 2 in which the drum is in a partially loaded configuration;
• Figure 4 is a diagram which links the fire angle of the motor to the torque exerted by the motor;
• Figures 5 and 6 show how the fire angle changes with time and with respect to the amount of water fed into the tub, in two different condition of load, i.e. high load and medium load respectively;
• Figure 7 shows a diagram which links the fire angle at the end of the water filling and the mass of the laundry; and
• Figure 8 is a diagram similar to figure 7 in which the mass of laundry is linked to the difference of firing angle at two different instants of the water filling.
• With reference to Figure 1, a washer machine 10 is composed by the following components: a rotating drum 12 actuated by an electric motor 14, an electronic board driving the system (not shown), several electromechanical devices to fill or drain out the water in the system and a main voltage sensor and a speed sensor.
• One of the main features of a method according to the invention is to use the motor torque variation during the water filling, in order to evaluate quickly the load size inside the washing unit. During the water filling the drum is rotating at slow speed (tumbling speed) and the clothes inside the washing unit starts to absorb water.
• Simplifying the system model, we can assume that the motor torque is due mainly to:
• the static friction (F),
• the dynamic friction (β·ω)
• inertial torque (J·ω̇)
• the gravitational component due to fabric mass and water mass $$\left(m_{\mathit{clothes}}+m_{\mathit{water}}\right)\cdot g\cdot d\cdot \sin \left(\vartheta \right)$$
  
  
  where d is the distance between black dot B and gray dot G of figure 2 and figure 3, i.e. the distance between the axis of the drum and the center of gravity of laundry and water; and ϑ is the angle between the vertical axis crossing the drum axis and the line connecting black dot B (drum axis) and gray dot G (center of gravity of laundry plus water) $$\tau =F+\beta \cdot \omega +J\cdot \dot{\omega }+\left(m_{\mathit{clothes}}+m_{\mathit{water}}\right)\cdot g\cdot d\cdot \sin \left(\vartheta \right)$$
  
• The inertial torque (J·ω̇) is negligible if the speed is almost constant.
• If the clothes volume is high (i.e. close to drum volume) (Fig 2) the laundry will rotate in almost fixed position inside the drum and the water will be absorbed uniformly through outside surface. This means that the system center of gravity (gray dot in Fig. 2) will be close to drum axis (black dot in Fig. 2) and the gravitational component will be small or close to zero.
• If the clothes volume is lower than drum volume (Fig 3) the laundry can move inside the drum and after being lifted up by the drum movement, will fall down continuously at the tumbling speed due the gravitational force. As soon as the tumble starts (in example counter clock wise) the clothes move in the bottom right side of the drum.
• In this situation the center of gravity of the system (gray dot G in fig 3) will change position during the rotation of the drum so the torque component will have a gravitational component bigger than in previous case (Fig 2).
• In general, if m_water doesn't change too much, the amount of gravitational component will increase when the load mass inside the drum is decreasing.
• In the following examples, there will be disclosed two possible ways to use the gravitational torque component and in particular two applications for the universal motor based on the triac actuation. In general, these applications can be used for any kind of motor.
Example 1 (load mass estimator using torque measure at water filling end)
• In some low cost applications (for example a washing machine equipped with a universal motor) is not possible to measure the torque directly. In this case the torque can be estimated by using the fire angle target used to drive the universal motor (figure 4)
• Figure 5 and 6 show two tests (one with 4 kg of laundry and one with 8 kg of laundry) carried out with a commercial washing machine AWOE 91200 produced and sold by the applicant. These charts show how the fire angle increases during the water filling: according to such experimental tests, the fire angle of the 4kg load (figure 6) increases more than the fire angle of the 8kg load (figure 6), therefore in full agreement with the theory set forth above.
• The fire angle value (or any other electrical measure linked to torque) at the end of water filling can be used to evaluate the load size of clothes inside the washing machine (figure 7)
Example 2 (load mass estimator using torque measure during water filling)
• Starting from the above equation 1 it is possible to evaluate the torque difference between starting time (close to opening valve time) and end time (close to closing valve time) $${\tau }_{\mathit{start}}=F+\beta \cdot \omega +\left(m_{\mathit{clothes}}\right)\cdot g\cdot d_{\mathit{start}}\cdot \sin \left({\vartheta }_{\mathit{start}}\right)$$

  

  $${\tau }_{\mathit{end}}=F+\beta \cdot \omega +\left(m_{\mathit{clothes}}+m_{\mathit{water\_end}}\right)\cdot g\cdot d_{\mathit{end}}\cdot \sin \left({\vartheta }_{\mathit{end}}\right)$$

  

  $$\mathrm{\Delta }\tau ={\tau }_{\mathit{end}}-{\tau }_{\mathit{start}}=\left(m_{\mathit{clothes}}+m_{\mathit{water\_end}}\right)\cdot g\cdot d_{\mathit{end}}\cdot \sin \left({\vartheta }_{\mathit{end}}\right)\cdot g\cdot d_{\mathit{start}}\cdot \sin \left({\vartheta }_{\mathit{start}}\right)$$

  
• Assuming that:
• d_start ≅ d_end
• ϑ _start ≅ ϑ _end
• The equation 4 can be simplified as follows: $$\mathrm{\Delta }\tau ={\tau }_{\mathit{end}}-{\tau }_{\mathit{start}}\cong m_{\mathit{water\_end}}\cdot g\cdot d$$

  
• The amount of water can be evaluated starting from opening valve time and water flow: $$m_{\mathit{water\_end}}=\mathit{water\_flow}\cdot \left({\mathit{time}}_{\mathit{end}}\mathit{-}{\mathit{time}}_{\mathit{start}}\right)$$

  
• The distance between drum axis B and center of gravity G is proportional to the amount of laundry inside the washing machine and can be evaluated in the following way: $$\mathit{load\_amount}\propto d\cong \frac{\mathrm{\Delta }\tau }{\mathit{water\_flow}\cdot \left({\mathit{time}}_{\mathit{end}}-{\mathit{time}}_{\mathit{start}}\right)}$$

  
• In some low cost applications (for example a washing machine equipped with a universal motor and without the flow meter) is not possible to measure the torque and the water flow. In this case the torque can be estimated using the fire angle target used to drive the universal motor, while the water flow can be considered nominal: $$\mathit{load\_amount}\propto \frac{\mathrm{\Delta }\mathit{Firing\_angle}}{\mathit{waterflow\_nom}\cdot \left({\mathit{time}}_{\mathit{end}}-{\mathit{time}}_{\mathit{start}}\right)}$$

  
• If the opening valve time is constant Equation 8 is equivalent to Equation 9: $$\mathit{load\_amount}\propto \mathrm{\Delta }\mathit{Firing\_angle}$$

  
• Fig. 8 reports the relationship between the fire angle increase and the load mass inside the washing machine.
• The motor torque measurement, or any signal related to it (measured current, target current, fire angle etc.), during the water filling, can be used to estimate the load mass inside the washing machine.
• This estimation can be used together with other estimators (e.g. inertia estimators, wetting dynamic estimator etc) at following stages of the washing program in order to optimize the water filling process and the selection of washing cycle parameters.
• The motor torque measurement, or any signal related to it (measured current, target current, fire angle etc.), can be also used to understand the mechanical stress applied to fabric. This information can be used in order to adapt the tumbling speed to increase or reduce the mechanical action according to the fabric type and/or the selected cycle.
• In the present example only two inputs are needed for carrying out a quick estimation of load during the initial phase of washing program, i.e. a motor torque measurement (this information can be obtained using other system variables such as the motor current, the required current, the triac fire angle etc.) and a valve status on/off (this information allows to know when the water starts/ends to enter the washing unit.
• In order to adapt the motor speed to the desired mechanical stress applied to the clothes and for improving torque estimation a motor speed sensor, a voltage sensor and a water flow sensor can be used, even if they are not strictly needed to evaluate the laundry load mass.
• The present invention can be applied in any kind of washing machine having a drum with an horizontal or tilted axis; the motor can drive the drum through a traditional pulleys and belt system (as in figure 1) or through a direct drive system.

Claims (10)

1. Method for estimating the load of laundry in a washing machine (10) comprising a drum (12) rotating in a tub and having a substantially horizontal axis (B), characterised in that the drum (12) is rotated at a tumbling speed and a motor torque or an electrical parameter related to such torque for driving the drum at said speed is measured.
2. Method according to claim 1, wherein the estimation of load is carried out during water filling of the tub.
3. Method according to claim 1 or 2, wherein the electrical parameter is motor current.
4. Method according to claim 1 or 2, wherein the electrical parameter is triac firing angle of the motor.
5. Method according to claim 4, wherein the measure of firing angle is carried out at the end of water filling of the tub.
6. Method according to claim 4, wherein a difference between firing angle at the beginning and at the end of the water filling is measured.
7. Method according to claim 2, wherein the torque is measured on the basis of the following formula: $$\tau =F+\beta \cdot \omega +J\cdot \dot{\omega }+\left(m_{\mathit{clothes}}+m_{\mathit{water}}\right)\cdot g\cdot d\cdot \sin \left(\vartheta \right)$$

   

   
   Where
   τ is the torque;
   F is the static friction;
   ω is the rotational speed of the drum;
   (β · ω) is the dynamic friction;
   (J · ω̇) is the inertial torque (negligible if ω is almost constant)
   (m_clothes + m_water ) · g · d · sin(ϑ) is the gravitational component due to fabric mass and water mass;
   (m_clothes ) is the mass of laundry;
   (m_water ) is the mass of water loaded in the drum;
   g is the gravity acceleration;
   d is the distance between the axis (B) of the drum (12) and the center of gravity (G) of laundry and water; and
   ϑ is the angle between the vertical axis crossing the drum axis (B) and the line connecting the drum axis (B) and the center of gravity (G) of laundry plus water.
8. Method according to claim 6, wherein the load is assessed on the basis of the following formula: $$\mathit{load\_amount}\propto \mathrm{\Delta }\mathit{Firing\_angle}$$

   

   
   where
   Δ Firing_angle is the difference of triac firing angle at the beginning and at the end of water filling.
9. Washing machine comprising a drum (12) having a substantially horizontal axis, driven by a motor and rotatably mounted in a tub, characterized in that it comprises a control unit adapted to drive the drum at a tumbling speed and to measure motor torque or an electrical parameter related to such torque in order to estimate the load of laundry.
10. Washing machine according to claim 9, wherein the control unit is adapted to carry out said assessment during water filling of the tub (12).

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