EP2610401B1

EP2610401B1 - System and method to estimate a laundry-load in a rotatable-drum laundry drying machine

Info

Publication number: EP2610401B1
Application number: EP11195942.5A
Authority: EP
Inventors: Fabio Altinier; Daniele Beninato; Giorgio Coletto; Paolo Driussi
Original assignee: Electrolux Home Products Corp NV
Current assignee: Electrolux Home Products Corp NV
Priority date: 2011-12-28
Filing date: 2011-12-28
Publication date: 2020-04-29
Anticipated expiration: 2031-12-28
Also published as: EP2610401A1

Description

The present invention relates to a system and a method to estimate a laundry-load in a rotatable-drum laundry drying machine.
The present invention relates also to a domestic rotatable-drum laundry drying machine which may correspond to a dryer operable to dry clothes, or to a washer-dryer operable to wash and dry clothes.
As it is known, most of laundry drying appliances currently on the market, generally comprise: a substantially parallelepiped-shaped, boxlike outer casing structured for resting on the floor; a substantially cylindrical rotatable drum which is structured for housing the laundry to be dried and is housed in axially rotating manner inside the casing to rotate about an horizontally-oriented longitudinal reference axis, directly facing a laundry loading/unloading opening formed in the front wall of the casing; a porthole door hinged to the front wall of the casing to rotate to and from a closing position in which the door rests completely against the front wall of the casing to close the laundry loading/unloading opening and airtight seal the revolving drum; an electrically-powered motor assembly which is housed inside the casing and is structured for driving into rotation the rotatable drum about its longitudinal reference axis during drying cycles; a drying circuit which is housed inside the casing and is structured to circulate inside the revolving drum a stream of hot air which has a very low moisture content and flows through the revolving drum and over the laundry inside the drum to dry the laundry; and finally an electronic central control unit which controls both the motor assembly and the drying circuit to perform, on command, one of the user-selectable drying cycles stored in the same electronic central control unit.
The electronic central control unit of the laundry drying machine above disclosed, is usually configured to optimize drying cycle by adjusting automatically the drying cycle time and/or the energy consuming, according to two mainly control parameters, namely the laundry moisture and the laundry load. One of such laundry drying machine is disclosed in EP 2 221 412 .
In-depth research has been carried out by the Applicant to achieve the goal of providing an extremely simple system and method to estimate the laundry load housed in the drum of a laundry drying machine for optimizing the drying cycle.
It is therefore an object of the present invention to provide a solution designed to achieve the above goal. According to the present invention, there is provided a rotatable-drum laundry drying machine as claimed in Claim 1 and preferably, though not necessarily, in any one of the Claims depending directly or indirectly on Claim 1.
A second aspect of the present invention relates to a method to control a rotatable-drum laundry drying machine as claimed in Claim 5 and preferably, though not necessarily, in any one of the Claims depending directly or indirectly on Claim 5.
A third aspect of the present invention relates to an electronic control system as claimed in Claim 9 and preferably, though not necessarily, in any one of the Claims depending directly or indirectly on Claim 9.
A non-limiting embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic view of a rotatable-drum laundry drying machine comprising an electronic control system made according to the present invention;
Figure 2 shows a table and a first graph relating to torque changes during several drying cycle tests performed through the laundry drying machine shown in Figure 1;
Figure 3 is a flow chart of the operating-phases performed through the method to control the rotatable-drum laundry drying machine shown in Figure 1;
Figure 4 is a flow chart of the operating-phases performed through the method to control the rotatable-drum laundry drying machine according to a second embodiment of the present invention; and
Figure 5 shows a table and a second graph relating to torque related signal changes during several drying cycle tests performed through the laundry drying machine according to the second embodiment of the present invention.

With reference to Figure 1, referral number 1 indicates as a whole a laundry drying machine which may be embodied as a rotatable-drum laundry dryer, or a rotatable-drum laundry washer-dryer.
According to the embodiment shown in Figure 1, rotatable-drum laundry drying machine 1 comprises an outer casing 2 that preferably rests on the floor on a number of feet. Casing 2 supports a rotatable laundry drum 3, which defines a chamber 4 for receiving laundry 5 and rotates about a preferably, though not necessarily, horizontal axis of rotation 6. In an alternative embodiment not shown, axis of rotation 6 may be vertical or inclined. Chamber 4 has a front access opening 7 closable by a door 8 preferably hinged to casing 2.
Rotatable-drum 3 is operable to be rotated about the axis of rotation 6 by an electric motor 9 (shown schematically in Figure 1) which is mechanically connected to the rotatable drum 3 through a drive member 10 for transmitting the motion for driving the rotatable-drum 3 in rotation about its axis 6.
Referring to Figure 1, rotatable-drum laundry drying machine 1 also comprises a hot-air generator 11 housed inside the casing 2, and designed to circulate through rotatable-drum 3 a stream of hot air having a low moisture level, and which flows over and dries the laundry 5 inside chamber 4.
During a drying cycle, hot-air generator 11 provides for gradually drawing air from rotatable-drum 3; extracting moisture from the air drawn from rotatable-drum 3; heating the dehumidified air to a predetermined temperature, normally higher than the temperature of the air from rotatable-drum 3; and supplying the heated, dehumidified air cyclically back into rotatable-drum 3, where it flows over, to dry, the laundry 5 inside the rotatable-drum 3.
Referring to Figure 1, hot-air generator 11 provides for dehumidifying and heating the air circulating inside rotatable-drum 3 to dry the laundry 5 inside the drum 3/chamber 4, and substantially comprises:

an air recirculating conduit 12 presenting two opposite ends connected to the rotatable laundry drum 3 on opposite opened sides of the latter;
a fan 13 located along the recirculating conduit 12 to produce inside the latter an airflow which flows into rotatable-drum 3 and over the laundry 5 inside rotatable-drum 3;
preferably, a moisture condensing device 14, which is operable to remove moisture from the airflow coming out from rotatable drum 3/chamber 4 ; and
heating device 15, which is able to heat the airflow returning back into rotatable drum 3, so that the airflow entering into revolving drum 3 is heated to a temperature higher than or equal to that of the same air flowing out of the drum 3.

With regard to heating device 15, it may advantageously comprise a number of electric heating components, such as electric resistors (not shown) located inside the air recirculating conduit 12 to dissipate electric power by Joule effect so as to heat the air supplied to rotatable-drum 3.
Regarding the moisture condensing device 14, it may comprise a heat-exchanger operable to condense moisture in the airflow through the air recirculating conduit 12. It should be pointed out that condensing device 14 applies, purely by way of example, to one possible embodiment of the present invention, and may be omitted in the case of a vented-type rotatable-drum laundry drier 1 (i.e. in which the hot and moisture-laden drying air from the rotatable-drum 3 is exhausted directly out of rotatable-drum laundry drier 1).
According to one possible embodiment of the present invention, heating device 15 and moisture condensing device 14 may be embodied as two heat exchangers, i.e. a refrigerant fluid condenser and a refrigerant fluid evaporator respectively, comprised within a heat pump assembly (not shown).
In actual use, fan 13 blows a stream of drying air, heated by heating device 15 into rotatable-drum 3. After contacting laundry 5 inside drum 3, the moisture-laden drying air flows out of rotatable-drum 3 and it is preferably directed to the moisture condensing device 14, which removes moisture from the drying air to condense the moisture inside it.
Rotatable-drum laundry drying machine 1 also comprises an electronic control system 16 configured to control rotatable-drum laundry drier 1 preferably on the basis of a drying cycle selected by a user through a user control interface 18, according to the control method which will be hereinafter disclosed in detail.
According to the present invention, the electronic control system 16 comprises a sensing device 19 which outputs a sensed signal S1 which is indicative of the torque value provided to the rotatable drum 3 by the electric motor 9, i.e. a signal which is indicative of the effort made by the drum motor 9 (force exerted by the motor 9 on the drum 3) to rotate the drum 3 itself during a laundry drying process.
The sensed signal S1 may be advantageously a motor torque signal or any electrical parameter which is directly or indirectly correlated with the motor torque. For example, the signal S1 output by the sensing device 19 may be the current S1=I supplied to/through the electric motor, and/or the voltage S1=V supplied to the motor, and/or the magnetic flux S1=Φ in the electric motor and the like.
The electronic control system 16 further comprises an electronic control unit 20 which is configured to: receive the sensed signal S1 which is indicative of a torque that electric motor 9 provides to the rotatable laundry drum 3; determine a value of a parameter that is indicative of the variation of the sensed signal S1 in time; and estimate the laundry-load contained within said drum 3 based on the determined value of the parameter.
According to a preferred embodiment of the present solution electronic control unit 20 is configured to: receive the sensed signal S1; determine the value of a parameter that is indicative of the variation of the sensed signal S1 in time (i.e. gradient of the sensed signal in time), based on a signal variation period/interval TIME corresponding to the elapsed time of the sensed signal S1 to change from a first prefixed signal value STH1 to a second prefixed signal value STH2; and estimate the laundry-load based on the determined variation period/interval TIME.
To this end, it should be pointed out that in-depth research carried out by the Applicant has demonstrated that, during the drying cycle, there is a strong correlation between the laundry-load and the time that passes between two prefixed sensed signals values indicative of the torque provided to drum 3 by the electric motor 9 during a drying cycle.
As a matter of the fact, during the drying cycle, the laundry moisture decreases gradually so reducing step by step the total weight of the wet clothes, and the loss in weight affects directly the torque that electric motor 9 has to provide to the drum 3 to maintain a prefixed drum speed. Also any parameter indicative of the motor supplied torque, such as the current I through the electric motor, the voltage V of the motor, the magnetic flux Φ in the electric motor and the like, are affected by the laundry weight loss in a similar manner as the torque supplied by the electric motor 9.
In order to clarify the correlation between the laundry-load and the torque value, Figure 2 shows, as an example, some experimental data measured by Applicant during experimental tests, wherein it is shown how torque provided by the electric motor 9 to the drum 3 decreases during a drying cycle for different laundry loads, i.e. a first laundry load associated to a low load weight (indicated "L1" in Figure 2), a second laundry load associated to a medium load weight, i.e. about half of the maximum load accepted by the drum chamber 4 (indicated "L2" in Figure 2), and a third laundry load associated to high load weight, i.e. about the maximum load accepted by the drum chamber 4 (indicated "L3" in Figure 2).
Through analogous experimental tests, similar graphs may be found representing the variation during a drying process time of a sensed signal S1 indicative of the torque developed by the electric motor 9 to rotate the drum 3, such as, for example, the current I through the motor 9, the voltage V of the motor, the magnetic flux Φ in the motor, and the like, instead of representing the torque itself as shown in Figure 2.
With reference to Figure 2, experimental data prove that the time needed for drying the clothes such that the laundry moisture reaches a prefixed moisture threshold depends on the load-laundry, whereas the water quantity per minute removable from the clothes, during a drying cycle, is substantially limited by the maximum heating power provided to the clothes by the hot-air generator 11. This behavior influences the total drying time and consequently the gradient in time of the torque, or a torque-related parameter, such as, for example, the current I through the motor 9, the voltage V of the motor, the magnetic flux Φ in the motor.
Therefore, Applicant found-out that it is possible to estimate the laundry load by measuring the elapsed time between two prefixed torque, or a torque-related signal values. As a matter of the fact, as showed in Figure 2, that refers to torque as motor parameter sensed by the sensing device 19, heavier is the laundry-load and longer is the time needed for intersecting the two predetermined torque limits STH1 and STH2.
In detail, Figure 2 shows that the elapsed time TIME1 associated with first laundry load L1 (low load weight) is lower than the elapsed time TIME2 (TIME1<TIME2) associated with second laundry load L2 (medium load weight), and that the latest elapsed time TIME2 is lower than elapsed time TIME3 (TIME2<TIME3) associated with third laundry load L3 (high load weight).
As mentioned above, even if in Figure 2 it has been represented the drum motor torque on drying process time, other parameters indicative of the torque supplied by the drum motor, such as the current I, the voltage V, the magnetic flux Φ, and the like, could be detected to obtain a similar graph. It is therefore possible to identify two threshold values STH1 and STH2 of the sensed signal S1 indicative of the motor supplied torque, wherein the time elapsed for the signal S1 to pass from threshold value STH1 to threshold valueSTH2 can be put in relation to a laundry load weight range, thereby providing an estimation of the laundry mass contained in the chamber 4.
Evidently it is possible to identify the most appropriate threshold values STH1, STH2 of a sensed signal S1 indicative of the drum motor torque for each of the drying programs that the machine is able to carry out on a laundry load placed within the drum chamber 4. A set of threshold values corresponding to the user selectable drying programs is therefore stored in a memory support accessible to the electronic control unit 20.
Figure 3 shows a flow chart of the operating-phases performed by the electronic control system 16 when it carries out the control method according to the present invention, and, more specifically estimates the laundry-load.
At the beginning of the method, electronic control unit 20 starts a drying cycle selected by a user through the user control interface 18 (Block 100). In such operating phase, the electronic control unit 20 sets both a time control parameter to zero (T=0), and a sampling time dt to a predetermined time value ΔT (dt=ΔT), for example one second.
The electronic control unit 20 controls the electric motor 9 to cause the drum 3 to rotate at pre-set rotation speeds about the axis of rotation 6 according to the drying cycle which user has selected. At the same time, the electronic control unit 20 switches-on the heating device 15 and controls the fan 13 to supply drying air into the rotatable-drum 3 (Block 110).
During the drying cycle, electronic control unit 20 acquires from the sensing device 19 the signal S1 indicative of the torque provided to the rotatable-drum 3 by the electric motor 9 (Block 120).
Electronic control unit 20 checks, at every sampling time dt=ΔT, for example, every second, whether the sensed signal S1 value is equal to, or lower than, the first comparison signal threshold value STH1, which is retrieved by a memory support accessible to the electronic control unit 20 based on the drying cycle selected by the user (Block 130), and, if not (NO output from Block 130), it repeats operating-phases disclosed in Blocks 120 and 130 namely, receives from the sensing device 19 a new sensed signal S1 which is indicative of the torque provided to the rotatable drum 3 by the electric motor 9 in a next measuring instant, and performs comparisons between the last sensed signal S1 and the first comparison signal threshold value STH1.
If the sensed signal S1 is equal to, or lower than, the first comparison signal threshold value STH1 (YES output from Block 130), then electronic control unit 20 starts to measure/count the signal variation interval - TIME - (Block 140).
Electronic control unit 20 continues to acquire/receive sensed signal S1 from the sensing device 19 (Block 150) and update the time control parameter T=T+dT (Block 160).
Electronic control unit 20 checks at every sampling time dt= ΔT, whether the sensed signal S1 value is equal to, or lower than, the second comparison signal threshold value STH2 (Block 170) and, if not (NO output from Block 170), it repeats operating phases disclosed in Blocks 150 and 160 namely, receives a new sensed signal S1 from the sensing device 19 in a next measuring instant T+dT, and performs comparisons between the last determined sensed signal S1 value and the second comparison signal threshold value STH2.
If the sensed signal S1 value is equal to, or lower than, the second comparison signal threshold value STH2 (YES output from Block 170), the electronic control unit 20 measures/determines the signal variation interval TIME=T (Block 180), which is the elapsed time of the sensed signal S1, in which the sensed signal S1 value changes from the first prefixed signal threshold value STH1 to the second prefixed signal threshold value STH2. In other words, the signal variation interval TIME=T (Block 180) is the elapsed time from the time instant in which signal S1 crosses the first prefixed signal threshold value STH1 to the time instant in which signal S1 crosses the second prefixed signal threshold value STH2.
Electronic control unit 20 estimates the laundry-load based on calculated signal variation interval TIME. It should be pointed out that the conditions to be checked in Block 130 (S1 ≤ STH1) and in Block 170 (S1 ≤ STH2) take into account that sensed signal S1 changes, in particular decreases, during progression of a user selected drying cycle. Therefore, since it may happen that signal S1 crosses the threshold values STH1, STH2 in a moment between a signal S1 acquisition instant and the next one, i.e. when the sampling time dt is not yet elapsed, the condition that signal S1 has crossed the threshold values STH1, STH2 is considered verified in Blocks 130 and 170 whether such signal S1 is sensed equal to, or lower than, STH1 and STH2, respectively.
Preferably, the laundry-load quantity may be estimated by using an appropriate mathematical function of time wherein "TIME" is the time needed so that sensed signal S1 intersects, i.e. crosses, the two prefixed signal threshold values STH1, STH2:
Laundry_Load = f(TIME)
According to a preferred embodiment, mathematical function may correspond to a table (stored-data in a memory support accessible to the electronic control unit 20) wherein a number of signal variation intervals - TIME - are associated with respective estimated laundry load.
In detail, Figure 3 shows a simplifying example of table containing three signal variation intervals - TIME - which are associated with three estimated load, respectively. For example, if the measured signal variation interval TIME is 30 minutes, i.e. comprised within the second TIME (25<T<60) then, according to the function/table, the estimated laundry load is comprised between 2 kg and 4 kg.
Preferably, electronic control unit 20 optimizes the drying cycle based on the estimated load by performing known optimization procedure (Block 190), such as regulating drying airflow rate, the heating power supplied by the drying air heating devices, the drum revolution speed and so on.
According to a second embodiment shown at Figure 4, electronic control unit 20 may be configured to estimate a laundry-load contained within the drum 3 based on a signal variation ΔS1, which is indicative of the torque variation determined at the end of a prefixed time period ΔTS measured as from an instant T1 in which the sensed signal S1 has crossed the first prefixed value STH1.
In detail, Figure 4 shows a flow chart of the operating-phases performed by the electronic control system 16 when it carries out the control method, to estimate the laundry-load, according to the second embodiment of the present invention.
At the beginning of the method, electronic control unit 20 starts a drying cycle selected by a user through the user control interface 18 (Block 200).
The electronic control unit 20 controls the electric motor 9 to cause the drum 3 to rotate at pre-set rotation speeds about the axis of rotation 6 according to the drying cycle which user has selected. At the same time, the electronic control unit 20 switches-on the heating device 15 and controls the fan 13 to supply drying air into the rotatable-drum 3 (Block 210).
During the drying cycle, electronic control unit 20 receives from the sensing device 19 the sensed signal S1 indicative of the torque provided to the rotatable-drum 3 by the electric motor 9 (Block 220).
Electronic control unit 20 checks whether the sensed signal S1 value is equal to, or lower than, a first comparison signal threshold value STH1, which is retrieved by a memory support accessible to the electronic control unit 20 based on the drying cycle selected by the user (Block 230) and, if not (NO output from Block 230), it repeats operating-phases disclosed in Blocks 220 and 230. It should be pointed out that considerations made with reference to Block 130 about the condition that signal S1 has crossed the threshold values STH1, are valid also for Block 230 here disclosed.
If the sensed signal S1 value is equal to, or lower than, the first comparison signal threshold value STH1 (YES output from Block 230), then electronic control unit 20 starts to measure the elapsed time, indicated hereinafter as "T" (Block 240) and, instant by instant, update the time values T (Block 250) and verifies whether measured time T is equal to a prefixed time period ΔTS (T= ΔTS) which is retrieved by a memory support accessible to the electronic control unit 20 based on the drying cycle selected by the user (Block 260).
If electronic control unit 20 determinates that measured time T is not equal to the prefixed time period ΔTS (NO output from Block 260), it performs operating-phases disclosed in Blocks 250 (updating time T), while if measured time T is equal to the prefixed period ΔTS (YES output from Block 260), the electronic control unit 20 acquires a signal S1(ΔTS) as sensed by the sensing device 19 at the instant of time T= ΔTS (Block 270), and assigns the value of the acquired signal S1(ΔTS) at the instant of time T= ΔTS, to a second signal value STH2, i.e. STH2=S1(ΔTS) (Block 280).
Electronic control unit 20 then calculates the signal variation ΔS1=STH1-STH2 of the sensed signal S1 at the end of the prefixed period ΔTS, by making the difference between the first comparison signal threshold value STH1 and the second signal value STH2 (Block 290), and estimate the laundry-load based on the determined signal variation ΔS1 (Block 300).
Preferably, the laundry-load quantity may be estimated by using an appropriate mathematical function based on the signal variation ΔS1 of the signal S1 during a prefixed time period ΔTS whose values depend on the drying cycle selected by the user and are stored in a memory support accessible to the electronic control unit 20.
Laundry_Load = f(ΔS1)
According to a preferred embodiment, mathematical function may correspond to a table (stored-data in a memory support accessible to the electronic control unit 20) wherein a number of signal variation ΔS1 of the signal S1 are associated with respective estimated laundry load.
It should be pointed out that it is possible to identify the most appropriate prefixed time period ΔTS for each of the drying programs that the machine is able to carry out on a laundry load placed within the drum chamber 4. A set of prefixed time periods ΔTS corresponding to the user selectable drying programs is therefore stored in a memory support accessible to the electronic control unit 20.
Figure 5 shows an example of table containing three signal variations AS1 which are associated with three estimated loads, respectively. For example, if the measured signal variation ΔS1 is comprised within Val1 and Val2 then, according to the function/table, the estimated laundry load is comprised between L1 and L2.
In addition Figure 5 shows experimental data that prove that, during a same prefixed period ΔTS, signal variation ΔS1 is inversely proportional to the load quantity to be dried. As a matter of the fact, as showed in Figure 5 that refers to the signal S1 sensed by the sensing device 19, heavier is the laundry-load and lower is the signal variation ΔS1 of the sensed signal S1 (L3). More in detail, graph of Figure 5 shows that the signal variation ΔS1 associated with first laundry load L1 (low load weight) is higher than the signal variation ΔS1 associated with second laundry load L2 (medium load weight), and that signal variation ΔS1 associated with second laundry load L2 is higher than signal variation ΔS1 associated with third laundry load L3 (high load weight).
Therefore, Applicant found that it is possible to estimate the laundry load by determining the signal variation ΔS1 during a prefixed time, i.e. by determining the gradient of the sensed signal S1 in a prefixed amount of time.
The sensed signal S1 shown in Figure 5 may be advantageously a motor torque signal or any electrical parameter which is directly or indirectly correlated with the motor torque. For example, the signal S1 output by the sensing device 19 may be the current S1=1 supplied to/through the electric motor, and/or the voltage S1=V supplied to the motor, and/or the magnetic flux S1=Φ in the electric motor and the like.
As it has been disclosed in the description above, the electronic control system 16 can determine the value of a parameter, that is ΔS1 or TIME, such parameter being correlated to variation of the sensed signal S1 in time. In other words, the electronic control system 16 can determine a sensed signal variation ΔS1 during a prefixed time period ΔTS, or it can determine a period TIME in which the sensed signal S1 changes from a first prefixed signal value STH1 to a second prefixed signal value STH2. Based on said sensed signal variation ΔS1, or on said determined period TIME, the electronic control system 16 estimates the laundry-load contained within said chamber (4).
According to the method of the invention, the electronic control system 16 is configured to provide a value of a first or a second parameter ΔS1, TIME, that are both correlated with variation of the sensed signal S1 in time, by maintaining a prefixed value of said second ΔTS or respectively first parameter ΔS1=(STH1-STH2). In this way the electronic control system 16 may estimate the weight of the laundry load placed into the drum chamber 4 by determining the slope of the curve representing the signal S1 variation during a user selected drying process time, samples of such curves being represented in Figures 2 and 5.
Preferably, electronic control unit 20 optimizes the drying cycle based on the estimated load by performing known optimization procedure (Block 310), such as regulating drying airflow rate, the heating power supplied by the drying air heating devices, the drum revolution speed and so on.
The operating-phases of the methods above disclosed may be contained in a computer program which is loadable in the electronic control system so that, when running, the electronic control system causes the rotatable-drum laundry drying machine 1 to operate according to what above disclosed.
Rotatable-drum laundry drying machine and method to estimate the laundry load of such machine according to the present invention are extremely simple to be performed.
Moreover, the above-described dryer and method are also advantageous as, thanks to the estimation of load, it is possible to adequately optimize drying cycle by adjusting automatically the cycle time and/or the energy consumption, based on estimated load. Clearly, changes may be made to the rotatable-drum laundry drier or to the method as described and illustrated herein without, however, departing from the scope of the present invention, as defined by the appended claims.

Claims

A laundry drying machine (1) comprising a rotatable laundry drum (3) designed to rotate about an axis of rotation (6), said drum having a chamber (4) adapted to receive laundry to be dried, an electric motor (9), which is mechanically connected with the rotatable laundry drum (3) for rotating said rotatable laundry drum (3) about its axis of rotation (6); hot-air generator means (11) configured to supply a heated airflow to the rotatable laundry drum (3) during a drying cycle;
the drying machine (1) comprising an electronic control system (16) which comprises an electronic control unit (20) configured to:
- retrieve prefixed threshold values (STH1, STH2) or prefixed time period (ΔTS), which are stored in a memory support accessible to the electronic control unit (20), according to the selected drying program;

- provide a sensed signal (S1) which is indicative of a torque that said electric motor (9) provides to the rotatable laundry drum (3);

- determine the value of a parameter (ΔS1)(TIME) that is correlated to variation of the sensed signal (S1) in time, based on said retrieved prefixed values (ΔTS) (STH1, STH2), and

- estimate the laundry-load contained within said chamber (4) based on said determined value of said parameter (ΔS1, TIME).
A laundry drying machine according to claim 1, wherein said value of said parameter (ΔS1, TIME) is a signal variation (ΔS1) of the sensed signal (S1) calculated at the end of a prefixed time period (ΔTS) counted as from an instant in which sensed signal (S1) has crossed a first prefixed value (STH1);
said electronic control system (16) being further configured to:
- estimate the laundry-load contained within said chamber (4) based on said determined signal variation (ΔS1).
A laundry drying machine according to claim 1, wherein said value of said parameter (ΔS1, TIME) is a signal variation period (TIME), which is indicative of the elapsed time of the sensed signal (S1) to change from a first prefixed signal value (STH1) to a second prefixed signal value (STH2);
said electronic control system (16) being further configured to:
- estimate the laundry-load contained within said chamber (4) based on said determined signal variation period (TIME).
A laundry drying machine according to any preceding claim, wherein said electronic control system (16) comprises a sensing device (19) to provide said sensed signal (S1); said sensed signal (S1) being the current (I) through the electric motor (9) or the voltage (V) of the electric motor (9), or the magnetic flux (Φ) in the motor (9), or the torque (TE) developed by the electric motor (9).
Method to estimate a laundry-load in a laundry drying machine (1) comprising a rotatable laundry drum (3) designed to rotate about an axis of rotation (6), said drum having a chamber (4) adapted to receive laundry to be dried, an electric motor (9), which is mechanically connected with the rotatable laundry drum (3) for rotating said rotatable laundry drum (3) about its axis of rotation (6); hot-air generator means (11) configured to supply a heated airflow to the rotatable laundry drum (3) during a drying cycle; the drying machine (1) comprising an electronic control system (16) which comprises an electronic control unit (20),
the method comprising:
- retrieving prefixed thresholds values (STH1, STH2) or prefixed time period (ΔTS), which are stored in a memory support accessible to the electronic control unit (20), according to the selected drying program;

- providing a sensed signal (S1) which is indicative of a torque that said electric motor (9) provides to the rotatable laundry drum (3);

- determining the value of a parameter (ΔS1, TIME) that is correlated to the variation of the sensed signal (S1) in time based on said retrieved prefixed values (ΔTS) (STH1, STH2); and

- estimating the laundry-load contained within said chamber (4) based on said determined value of said parameter (ΔS1, TIME);
Method according to claim 5, comprising the steps of:
- determining said value of said parameter (ΔS1, TIME) on the basis of a signal variation (ΔS1) of the sensed signal (S1) calculated at the end of a prefixed time period (ΔTS) measured as from an instant in which sensed signal (S1) has crossed a first prefixed value (STH1);

- estimating the laundry-load contained within said chamber (4) based on said determined signal variation (ΔS1).
Method according to claim 5, comprising the steps of:
- determining said value of said parameter (ΔS1, TIME) on the basis of a signal variation period (TIME), which is indicative of the elapsed time of the sensed signal (S1) to change from a first prefixed signal value (STH1) to a second prefixed signal value (STH2);

- estimating the laundry-load contained within said chamber (4) based on said determined signal variation period (TIME).
Method according to any claim 5 to 7, wherein said sensed signal (S1) being the current (I) through the electric motor (9) or the voltage (V) of the electric motor (9), or the magnetic flux (Φ) in the motor (9), or the torque (TE) developed by the electric motor (9).
Electronic control system (16) configured to estimate laundry-load in a laundry drying machine (1) comprising a rotatable laundry drum (3) designed to rotate about an axis of rotation (6), said drum having a chamber (4) adapted to receive laundry to be dried, an electric motor (9), which is mechanically connected with the rotatable laundry drum (3) for rotating said rotatable laundry drum (3) about its axis of rotation (6); hot-air generator means (11) configured to supply a heated airflow to the rotatable laundry drum (3) during a drying cycle;
the electronic control system (16) comprising:
- an electronic control unit (20) configured to retrieve prefixed threshold values (STH1, STH2) or prefixed time period (ΔTS), which are stored in a memory support accessible to the electronic control unit (20), according to a selected drying program;

- a sensing device (19) to provide a sensed signal (S1) which is indicative of a torque that said electric motor (9) provides to the rotatable laundry drum (3);
said control unit (20) being further configured to:
- receiving said sensed signal (S1);

- determining the value of a parameter (ΔS1, TIME) that is correlated to the variation of the sensed signal (S1) in time based on said retrieved prefixed values (ΔTS) (STH1, STH2);

- estimating the laundry-load contained within said chamber (4) based on said determined value of said parameter (ΔS1, TIME).
Electronic control system (16) according to claim 9, wherein said value of said parameter (ΔS1, TIME) is determined on the basis of a signal variation (ΔS1) of the sensed signal (S1) calculated at a prefixed time period (ΔTS) measured as from an instant in which sensed signal (S1) has crossed a first prefixed value (STH1);
said control unit (20) being further configured to:
- estimate the laundry-load contained within said chamber (4) based on said determined signal variation (ΔS1).
Electronic control system (16) according to claim 9, wherein said value of said parameter (ΔS1, TIME) is determined on the basis of a signal variation period (TIME), which is indicative of the elapsed time, during which the sensed signal (S1) changes from a first prefixed signal value (STH1) to a second prefixed signal value (STH2); said control unit (20) being further configured to:
- estimate the laundry-load contained within said chamber (4) based on said determined signal variation period (TIME).
Electronic control system (16) according to any claim 9 to 11, wherein said sensed signal (S1) being the current (I) through the electric motor (9) or the voltage (V) of the electric motor (9), or the magnetic flux (Φ) in the motor (9), or the torque (TE) developed by the electric motor (9).