TECHNICAL FIELD

Embodiments herein relate to detection of scale on heating elements. In particular, the embodiments herein relate to a method for detecting deposition of scale on a heating element of a household appliance and to a household appliance comprising a heating element. BACKGROUND

Water heating elements in household appliances, as for example dishwashers or washing machines, are used to heat water to some desired temperature. In areas where a heating element contacts water, scale will tend to form on the heating element such that a layer of scale is accumulated over time which slowly reduces the heating ability of the heating element. This is particularly the case in areas where the water supplied is so- called hard water, i.e. water containing some percentage of calcium and magnesium carbonates, bicarbonates, sulphates or chlorides due to long contact with rocky substrates. Scale formed on a heating element makes the element less efficient and may further result in permanent damage of the heating element. This can be avoided by descaling the heating element once it can be detected that scale is present on the heating element. One conventional scale detection method relies on measuring the actual running temperature of a heating element during boiling and comparing the achieved

temperature with a predetermined value.

US20080317091 describes another method for detecting presence of scale on a heating element, where a signal describing the heating element temperature decrease is analysed and presence of scale is indicated if the temperature decrease is greater than a predetermined value. However, the above known methods are not very accurate and reliable and they also require a measurement procedure during boiling of water. SUMMARY

An object of the embodiments herein is to provide an improved method for detecting deposition of scale on a heating element.

According to an aspect, a method is provided for detecting deposition of scale on a heating element of a household appliance. The method comprises: sending an electric pulse to the heating element, determining a behaviour of a parameter of the heating element in response to the electric pulse and evaluating the behaviour in order to detect the deposition of scale on the heating element, wherein the electric pulse comprises a predetermined amount of electric energy.

The behaviour, which may also be called characteristic of the parameter, describes how the parameter of the heating element changes in response to the electric pulse. In other words, the behaviour illustrates how the heating element reacts on the electric pulse. Thus, properties and conditions of the heating element may thereby be described by determining the behaviour of the parameter of the heating element.

The behaviour is obtained by measuring values of the parameter of the heating element during a time period, which measured values of the parameter may be illustrated as a plot or a chart, for example a line chart.

Further, by evaluating of the behaviour, for example by analysing the measured values of the parameter, the deposition of scale on the heating element may be detected.

In this method, the electric pulse comprises a predetermined amount of electric energy, i.e. the amount of electric energy in the electric pulse is determined before the electric pulse is sent to the heating element. Thereby, the amount of electric energy in the pulse is accurately known each time the pulse is sent to the heating element in order to detect deposition of scale on the heating element. By this, an improved method for detecting deposition of scale is achieved because the method is based on a known input i.e. known amount of electric energy in the electric pulse sent to the heating element. The known amount of electric energy in the pulse facilitates evaluation, i.e. analysis of the behaviour achieved by the electric pulse is more accurate.

As a result, the above mentioned object can be achieved.

According to some embodiments, the method may further comprise: obtaining a reference behaviour of the parameter in response to an initial electric pulse comprising the predetermined amount of electric energy when the heating element is considered to be free from deposition of scale, wherein the evaluating of the behaviour is based on the reference behaviour.

The reference behaviour, which may also be called reference characteristic of the parameter, describes how the parameter of the heating element changes in response to the initial electric pulse. In other words, the reference behaviour illustrates how the heating element reacts on the initial electric pulse when the heating element is considered to be free from deposition of scale.

In a similar way to the behaviour described above, the reference behaviour is obtained by measuring values of the parameter of the heating element during a time period, which measured values of the parameter may be illustrated as a plot or a chart, for example a line chart.

Because the initial electric pulse comprises the predetermined amount of electric energy the reference behaviour of the parameter is based on a known amount of electric energy sent to the heating element. The amount of electric energy in the initial electric pulse is the same as the amount of electric energy in the electric pulse. Thereby, the reference behaviour and the behaviour are obtained based on the same and known amount of electric energy in the initial electric pulse and in the electric pulse respectively. Thus, when evaluating the behaviour, the behaviour is compared with the reference behaviour and this comparison is based on the same amount of electric energy in the electric pulse and in the initial electric pulse respectively.

Because the reference behaviour relates to conditions when the heating element is consider being free from deposition of scale, the comparison between the behaviour and the reference behaviour may give quick and reliable answer if scale is present on the heating element.

By this, an improved method for detecting deposition of scale on the heating element can be achieved. The evaluating of the behaviour may comprise integrating the behaviour during an integration time period and evaluating the integrated behaviour based on integrated reference behaviour during the integration time period.

As described above, the behaviour and the reference behaviour are determined by measuring values of the parameter during a time period. Further the measured values of parameter may be illustrated as a plot in a chart, for example a line chart.

Thus, the behaviour and the reference behaviour may be described by a mathematical function and by a reference mathematical function respectively. Then, integrating of the behaviour may comprise calculation of an integral of the mathematical function and integrating of the reference behaviour may comprise calculation of a reference integral of the reference mathematical function. Further, the integral may be evaluated, i.e. compared with the reference integral.

As an alternative, integrating of the behaviour may comprise summarizing the measured values of the parameter that form the behaviour during the time period to obtain a summarized value of the parameter. In a similar way, integrating of the reference behaviour may comprise summarizing the measured values of the parameter that form the reference behaviour during the time period to obtain a summarized reference value of the parameter. The summarized value of the parameter is then evaluated, i.e. is compared with the summarized reference value of the parameter.

By this a simple and reliable way of evaluating the behaviour may be achieved.

The evaluating of the behaviour may comprise identifying a maximum absolute value of the parameter and evaluating the maximum absolute value based on a reference maximum absolute value as the reference behaviour. The maximum absolute value may be determined i.e. identified by analysing the behaviour and the reference maximum absolute value may be determined i.e. identified by analysing the reference behaviour. The maximum absolute value may refer to a maximum value of the parameter or to a minimum value of the parameter. In a similar way the reference maximum absolute value may refer to a reference maximum value of the parameter or to a reference minimum value of the parameter. The maximum value or the minimum value used for the maximum absolute value depends on the parameter of the heating element. When the maximum absolute value has been identified, the maximum absolute value is evaluated i.e. is compared with the reference maximum absolute value. By comparing the maximum absolute value with the reference maximum absolute value of the parameter a simple way of evaluating of the behaviour may be achieved.

The method may comprise measuring an initial value of the parameter before sending the electric pulse. By this, an improved method for detecting deposition of scale on the heating element can be achieved because value of the parameter before sending the electric pulse is known. Thus, evaluating of the behaviour may be facilitated.

Optionally, the evaluating of the behaviour may comprise determining a stabilization time it takes for the parameter to reach a stabilization value associated to the initial value of the parameter and evaluating the stabilization time based on a reference stabilization time as the reference behaviour.

The stabilization time refers to a time period it takes for the parameter to reach the stabilization value and the stabilization value refers to a value of the parameter that is stable, i.e. the value is substantially constant and does not change or there are very small changes in the parameter values.

The stabilization value of the parameter may refer to an average value of at least two values of the parameter. The at least two values of the parameter are measured after said electric pulse has been sent. The average value is calculated if the difference between the at least two values is not greater than a predefined value of the difference. With other words, when a rate of change of the parameter values is not greater than a predefined rate the stabilization value may be calculated. The at least two values may be equal. The stabilization value may also be determined by analysing a derivative of the behaviour when said behaviour is described as a mathematical function as named above. Thus, the stabilization value is reached when the derivative maintains a predefined value.

The stabilization value is associated to the initial value of the parameter i.e. the stabilization value is compared with the initial value. The stabilization value may be equal to the initial value or it may be a predetermined offset between the stabilization value and the initial value.

Further, evaluating of the behaviour may comprise evaluating the stabilization time based on a reference stabilization time as the reference behaviour. This means that, the stabilization time is compared with the reference stabilization time in order to detect deposition of scale on the heating element. The reference stabilization time is obtained by examining the reference behaviour in a similar way as the stabilization time is obtained by analysing the behaviour as described above.

By comparing the stabilization time with the reference stabilization time a simple way of evaluating the behaviour may be achieved.

Optionally, the method may further comprise determining a voltage of the electric pulse and/or a time duration of the electric pulse to achieve the predetermined amount of electric energy. Thus, the amount of electric energy in the electric pulse may be described as a function of voltage and/or of the time duration. Thereby, determined voltage and/or the determined time duration is to be used to calculate the predetermined amount of energy when sending the electric pulse to the heating element. For example, if voltage has been determined, i.e. obtained by measuring, the time duration is then adjusted to achieve the predetermined amount of electric energy. If time duration of the electric pulse has been determined, i.e. decided when sending the electric pulse, the voltage is then adjusted to achieve the predetermined amount of electric energy.

Thereby, the predetermined amount of electric energy in the electric pulse may easily be determined by determining the voltage for the electric pulse and/or the time duration of the electric pulse. The parameter may comprise at least one of temperature of the heating element and resistance of the heating element.

According to another aspect a household appliance comprising a heating element is provided. The household appliance being configured to: send an electric pulse to the heating element, determine a behaviour of a parameter of the heating element in response to the pulse, and evaluate the behaviour in order to detect deposition of scale on the heating element, wherein the electric pulse comprises a predetermined amount of electric energy.

Because the household appliance is configured to send the electric pulse comprising the predetermined amount of electric energy, i. e. the amount of electric energy is known when the electric pulse is sent to the heating element, detection of scale on the heating element is facilitated. Thereby an improved household appliance is achieved. As a result, the above mentioned object can be achieved.

Optionally, the household appliance may be configured to determine a voltage of the electric pulse and/or a time duration of said electric pulse to achieve the predetermined amount of electric energy. Thereby, the predetermined amount of electric energy in the electric pulse may easily be determined by determining the voltage for the electric pulse and/or the time duration of the electric pulse.

Further features and advantages of the embodiments herein will become apparent when studying the appended claims and the following detailed description. Those skilled in the art will realize that the different features described may be combined to create embodiments other than those described in the following, without departing from the scope defined by the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

The various aspects mentioned above, including their particular features and advantages, will be readily understood from the following detailed description and the accompanying drawings, in which: Fig. 1 is a flow chart illustrating a method for detecting deposition of scale on a heating element,

Fig. 2a is a diagram illustrating a reference behaviour of a parameter of a clean heating element,

Fig. 2b is a diagram illustrating behaviour of the parameter in Fig. 2a of a scaled heating element, and

Fig. 3 is a plane view of a household appliance comprising a heating element.

DETAILED DESCRIPTION

The embodiments herein will now be described in more detail with reference to the accompanying drawings, in which example embodiments are shown. Disclosed features of example embodiments may be combined. Like numbers refer to like elements throughout. Well-known functions or constructions will not necessarily be described in detail for brevity and/or clarity. Fig. 1 illustrates an example of actions in a procedure for implementing a method 100 that can be performed for detecting deposition of scale on a heating element of a household appliance. The method 100 may for example be carried by a control unit connected to or arranged at the household appliance. The method 100 comprises sending 103 an electric pulse to the heating element, determining 105 a behaviour of the parameter in response to the electric pulse, and evaluating 107 the behaviour, wherein the electric pulse comprises a predetermined amount of electric energy. The amount of electric energy to use in the electric pulse is determined before the electric pulse is sent to the heating element. The electric energy W in the electric pulse may for example be calculated as:

W = (td-U ² )/R (1 ) where:

td is time duration of the electric pulse in seconds [s],

U is voltage in volts [V] and

R is resistance of the heating element in ohms [Ω].

The voltage U is measured in a connection to an electrical socket the heating element is connected to. Usually the voltage U of an electrical socket is approximately 230 V in Northern Europe, however the voltage U may vary for example in a range of 220 -250 V.

According to some embodiments illustrated in Fig. 1 the method 100 may comprise measuring 101 an initial value of the parameter before sending 103 the electric pulse. Further, the method 100 may comprise obtaining 109 a reference behaviour of the parameter which represents a response to an initial electric pulse comprising the predetermined amount of electric energy when the heating element is consider to be free from deposition of scale, wherein the evaluating 107 of the behaviour may comprise evaluating the behaviour based on the reference behaviour. The reference behaviour is described in details in conjunction with Fig. 2a. Further the procedure 100 may comprise determining 1 1 1 a voltage of the electric pulse and/or a time duration of the electric pulse to achieve the predetermined amount of electric energy.

Thus, to achieve the electric energy that is set to a constant value, the voltage U may be measured and depending on the measured voltage U the time duration td is adjusted to achieve the predetermined value of the electric energy W. From the equation (1 ) above the time duration td for the electric pulse is calculated according to: td = (W-R)/U ² (2)

For example, if the electric energy W is set to 8 kJ, the resistance R of the heating element is 26 Ω and the voltage U is measured to 223.4 V the time duration td for the electric pulse will be 4.17 s.

Fig. 2a shows a reference behaviour ChO of temperature of a clean heating element. With clean heating element is meant that the heating element is free from scale or the scale level is very low. The reference behaviour ChO represents a response to an initial electric pulse comprising the predetermined amount of electric energy W when the heating element is considered to be free from deposition of scale. The reference behaviour ChO in Fig. 2a is illustrated as a line chart and represents measured values of the temperature of the heating element in response to the initial electric pulse sent to the heating element at time tip in the figure. Depending on voltage U the time duration tdi for the initial pulse is controlled to achieve the predetermined value of the electric energy in the initial pulse. The time duration tdi for the initial pulse may for example be equal to 4.17 s according to the example above. This time duration is used when the measured voltage U is 223.4 V and the predetermined value of the electric energy W is set to 8 kJ. As can be seen in Fig. 2a, there is a time delay for approximately 3 s from when the initial pulse has been sent at the time tip to the heating element to when a rise in temperature is measured. The time delay may vary for example within a range of 2-3 s. The temperature from time 0 to the time tip when the initial pulse has been sent represent room temperature of the heating element, i.e. the temperature of the heating element is then equal to ambient temperature. The reference behaviour ChO may be created by a manufacturer of the heating element before the heating element is mounted in a household appliance or when the heating element is mounted in the household appliance but before the household appliance is used. In other words, the reference behaviour ChO is created when the heating element is new and free from deposition of scale. The reference behaviour may alternatively be created by a user of the household appliance comprising the heating element before the household appliance has been used for its intended purpose. For example, if the heating element is present in a dishwasher, the reference behaviour may be created before a first wash of dishes is executed in the dishwasher. The reference behaviour ChO may be created on wet heating element. With other words, when the heating element is surrounded by a liquid, as for example water, i.e. when the heating element is arranged in a cavity filled with the liquid. Alternatively the reference behaviour may be created on a dry heating element, i.e. when the heating element has only contact with a gas, as for example air. Fig. 2b shows a behaviour Ch of temperature of a scaled heating element i.e. heating element having a deposition of scale. The behaviour Ch represents a response to an electric pulse comprising the predetermined amount of electric energy W. The behaviour Ch in Fig. 2b is illustrated as a line chart of measured values of the temperature of the heating element in response to the electric pulse sent to the heating element at time tp in the figure.

The electric pulse is sent at time tp to the heating element. The time duration td of the electric pulse depends on the measured voltage when the electric pulse is intended to be sent to the heating element. Thus, if the voltage has been measured to for example 230 V and the predetermined value of the electric energy W is the same as for the reference behaviour, i.e. 8 kJ, the time duration td for the electric pulse is 3,93 s.

The x-axis in Fig. 2a and Fig. 2b represents time in seconds [s] and the y-axis in Fig. 2a and Fig. 2b represents temperature in degrees Celsius. If the reference behaviour ChO is created on wet heating element, as described above, the reference behaviour is also created on wet heating element. Further, if the reference behaviour is created on dry heating element, as described above, the reference behaviour is also created on dry heating element. With other words, the behaviour Ch and the reference behaviour ChO are preferably determined when the heating element is in substantially the same environment.

A correction factor may be used if conditions when determining the behaviour Ch deviate i.e. differ from the conditions when the reference behaviour has been determined. For example, if there are differences regarding an initial temperature of the heater, temperature of liquid or gas the heating element is in.

As an alternative, the evaluating of the behaviour Ch may comprise integrating the behaviour Ch during a integration time period, for example from the time tp the pulse has been sent to 120 s after that and evaluating the integrated behaviour Chlnt based on integrated reference behaviour ChOInt during the same time period, i.e. for example from the time tip the initial pulse has been sent to 120 s after that. An Integrated behaviour Chlnt greater than the integrated reference behaviour ChOInt indicates that scale may be present on the heating element. A threshold value equal, for example, to 20% between the integrated behaviour Chlnt and the integrated reference behaviour ChOInt may indicate that scale is present on the heating element and an action should be taken, for example the heating element should be cleaned.

Further, the evaluating of the behaviour Ch may comprise identifying a maximum absolute value of the temperature in the behaviour Ch and evaluating the maximum absolute value with a reference maximum absolute value of the temperature in the reference behaviour ChO. According to Fig. 2a and Fig. 2b the maximum absolute value corresponds to a maximum value Tmax of the temperature and the reference maximum absolute value corresponds to a reference maximum value TOmax of the temperature. If the maximum absolute value is greater that the reference maximum absolute value, it can be concluded that scale may be present on the heating element. The maximum absolute value greater than the reference maximum absolute value with for example 15 degrees may indicate that scale is present on the heating element and an action should be taken, for example the heating element should be descaled. As further alternative, an initial value TO of the temperature of the heating element may be measured before sending the electric pulse to the heating element. In this case, the evaluating of the behaviour Ch may comprise measuring a stabilization time ts it takes for the temperature to reach a stabilization value Ts associated to the initial value TO of the temperature and evaluating the stabilization time ts with a reference stabilization time tOs as the reference behaviour. The stabilization time ts and the reference stabilization time tOs may be measured from time tp and tip respectively, i.e. when the electric pulse and the reference electric pulse respectively has been sent. As can be seen in Fig. 2a and Fig. 2b there is delay in the response to the reference electric pulse and electric pulse respectively.

The stabilization value Ts of the temperature may for example be determined as an average value of at least two values (T1 , T2) of the temperature. In this case, the at least two values (T1 , T2) of the temperature may be measured after said electric pulse has been sent and a difference between the at least two values (T1 , T2) is not greater than a predefined value of the difference. For example the difference may be 2 °C. Wherein the stabilization time ts is a time when the stabilization temperature Ts is reached.

In a similar way a reference stabilization temperature TOs and a reference stabilization time tOs may be determined.

The stabilization time ts greater than the reference stabilization time tOs, indicates that scale may be present on the heating element. Difference between the stabilization time ts and the reference stabilization time tOs, for example about 40 seconds, may indicate that scale is present on the heating element and an action should be taken, for example the heating element should be cleaned from scale. A method for removing scale from a heating element may sometimes be called descaling.

As an alternative, resistance of the heating element may be used for creating a reference behaviour and a behaviour of the resistance of the heating element.

Fig. 3 illustrates an example of a dishwasher 3 comprising a heating element 1. The heating element 1 is arranged in a cavity 4 where water is supplied through an inlet 6 comprising a filter. Water is supplied to the dishwasher through a second inlet 14. In the cavity 4, water is heated by the heating element 1 and is then pumped to a wash arm arrangement 2 of the dishwasher 3 by a pump 8. The heating element 1 is connected to a power grid through a control unit 10. The heating of the heating element 1 is electrical and is achieved in a regular manner, which is therefore not necessary to described in detail. The control unit 10 comprises a voltage measuring device (not shown), as for example an ADC, and a time controlling device, as for example a crystal oscillator (not shown), arranged to control the electrical pulse and the reference electric pulse to the heating element 1. The control unit 10 may also comprise a voltage adapting means (not shown) for adjusting voltage to the heater.

A sensor 12 for, for example a temperature sensor, is arranged on the heating element in order to detect changes in the parameter and thereby to create the reference behaviour and the behaviour described in Fig. 2a and Fig. 2b. The sensor 12 is connected to the control unit 10. The control unit 10 is arranged to enable processing of data measured by the sensor 12. The sensor 12 may be any type of sensor as for example a NTC resistor.

Even though embodiments of the various aspects have been described, many different alterations, modifications and the like thereof will become apparent for those skilled in the art. The described embodiments are therefore not intended to limit the scope of the present disclosure.