APPLIANCE, IN PARTICULAR FOR DETECTING THE CONDENSED

WATER LEVEL IN DRYING MACHINES, AND ASSOCIATED METHOD

TECHNICAL FIELD

The present invention relates to an optical level sensor for an electric household appliance, in particular designed to detect the condensed water level in a drying machine or washing/drying machine, as well as to an associated method for detecting the end of the drying cycle in such a machine.

BACKGROUND ART It is known that in drying machines the humidity of the washed laundry is removed by means of a flow of hot air then condensed in the liquid form, collecting the condensed liquid (water) in a specific tank. There are normally two tanks, a bottom tank in which the condensed liquid is collected, and an upper tank in which part of the condensed liquid is pumped to be then manually- eliminated by the user.

As described in DE19701225, the condensed liquid level in the bottom tank is detected by electrodes arranged at the end of a recirculation tube immersed in use into the condensed liquid collected in the bottom tank. According to DE3131543, instead, the achievement of a maximum liquid level in the tank is detected by a sensor consisting of a float or membrane arranged at the end of a tube immersed in use into the condensed liquid and which actuates a switch. Optical level sensors for a liquid in a tank are also known, which however, as in the preceding cases, are able to provide only one signal of the on-off type, i.e. empty/full, because they are based on the change of refraction index caused by the presence of liquid about a transparent tube terminating in a first end provided with oblique surfaces, and provided at a second end with a lid equipped with a photo-emitter diode and a photo-receiver diode, with a corresponding power supply and control electronics . The known sensors are therefore not very useful, because they do not allow to continuously monitor the liquid level in the condensation tank, are not very accurate, large in size and not very reliable.

DISCLOSURE OF INVENTION

It is the object of the present invention to obviate the aforesaid drawbacks by providing a level sensor which is small-sized, low-cost, easy to be assembled and reliable to be operated, and which allows to continuously detect the liquid level between minimum and maximum levels.

The present invention thus relates to a level sensor as defined in claim 1.

In particular, the level sensor according to the invention is an optical sensor and comprises a tubular body provided with a first end and a second end, opposite to the first, the second end being cup-shaped and closed by a lid; at least one photo-emitter diode, at least one photo-receiver diode and electronic power supply and control means for the diodes, carried by an electronic board fixed to an internal face of the lid facing the first end in use; this latter is open and delimited on the inside of the tubular body by an annular edge; the sensor further comprises, in combination, a float accommodated within the tubular body, floating between the first and second ends, the float having at least one first face made to be only partially reflective and arranged facing the lid.

In particular, the first face and an internal side surface of the tubular body are smooth and white and the float consists of a tablet-shaped disc, made of polystyrene, so that the first face always remains over the level of the liquid in which the float is immersed in use.

Thereby, a reflection of at least part of the light emitted by the photo-emitter diode towards the photo- receiver diode caused by the first face of the float is obtained, the distance of which from the diode changes in use as the level of the liquid in which the float floats changes. Therefore, the light scattering which occurs due to the fact that the first face is only partially reflective -in this case defined by a white surface, i.e. a surface having, in addition to a certain reflectance, also a certain degree of absorbance- strongly depends on the distance of the surface from the diodes, thus allowing to pick up a voltage signal which is substantially linear and which continuously changes as the liquid level changes, at the ends of the photo- receiver diode.

Such an effect, surprisingly reproducible, allows to detect the liquid level in a tank in an accurate, highly- reliable and simple manner and in the case of a condensed liquid (water) tank of a drying machine, it allows to implement a method for determining the end of a drying cycle of the machine itself, which may be either a drying machine or a washing/drying machine, according to claim 7.

In particular, such a method comprises the steps of: - continuously monitoring the condensed liquid level in a tank for collecting the condensed liquid of the machine;

- after a predetermined interval of time, checking whether the condensed liquid level has changed; - repeating the preceding step until the condensed liquid level in said predetermined time interval no longer changes; - accordingly, indicating the end of the drying cycle.

BRIEF DESCRIPTION OF THE DRAWINGS Further features and advantages of the present invention will be apparent from the following description of a non-limitative embodiment thereof, with reference to the figures of the accompanying drawings, in which: figure 1 diagrammatically shows a perspective, three-quarter section top view of a level sensor according to the invention; figure 2 diagrammatically shows a perspective, three-quarter section bottom view of the sensor in figure 1, in a different operating condition; and - figure 3 qualitatively shows the electric signal emitted in use by the sensor in figures 1 and 2.

BEST MODE FOR CARRYING OUT THE INVENTION With reference to figures 1 and 2, numeral 1 indicates as a whole an optical-type level sensor for an electric household appliance, adapted to be mounted in a tank 2 of the electric household appliance (not shown) for detecting the level L of a liquid 3 contained therein, e.g. condensed liquid (water) in a bottom tank of a drying machine or washing/drying machine.

Sensor 1 comprises a tubular body 5, in this cylindrical symmetry case shown, delimited by a side wall 6 having an internal side surface 7 and provided with a first open end 8 and a second end 9, opposite to the first and open as well.

End 9, which is larger in cross size than the rest of the body 5, in particular than the external diameter of the side wall 6, is cup-shaped and is closed, on the side opposite to the end 8, by a lid 11; the lid 11 carries at least one photo-emitter diode 12, at least one photo-receiver diode 13 and electronic power supply and control means 14 for the diodes, in this case defined by an electronic board carrying the diodes 12 and 13 onboard and fixed to an internal face 15 of the lid 11, facing the first end 8 in use.

According to an aspect of the invention, the latter is open outwards and delimited on the inside of the tubular body 5 by an annular edge 20, which defines a mouth 21 of the body 5 adapted to let the liquid 3 therein, the level of which is intended to be measured.

Sensor 1 further comprises a float 25 accommodated within the tubular body 5 floating between the first end 8 and the second end 9, e.g. sliding with a clearance within the same, along the internal side surface 7, acting as a sliding guide thereof; the float 25 has at least one first face 27 made, according to the invention, so as to be only partially reflective, i.e. not made as a mirror-like surface (which would be totally reflective) , and arranged towards the lid 11, i.e. intended to be the face of float 25 facing upwards in use.

According to the invention, face 27 and internal side face 7 of the tubular body 5 are smooth (i.e. without embossing, ridges or other irregularities which could alter the reflection of light rays incident thereon) and white surfaces, which white is the color of the light spectrum provided with the lowest absorbance.

Therefore, they are reflective, but not totally reflective, because they however have a given absorbance and further display the feature that the amount of reflected light depends on the incidence angle of the light thereon.

In this case, face 27 is flat and float 25 consists of a round, tablet-shaped disc made of polystyrene (or other material with similar optical features) so that the first face 27 always remains over the level L of the liquid 3 in which the float 25 is immersed in use.

According to a further feature of the invention, sensor 1 not necessarily but preferably comprises a screen 30 (figure 1) arranged facing the first end 8, at a given distance D from the same, so as to delimit in use an annular side access opening 31 into the tubular body 5 for the liquid 3, the level L of which is intended to be detected. For example, screen 30 consists of a disc made in one piece with the body 5, to which it is connected by means of fins 33, by molding of a synthetic plastic material . The annular edge 20 of the first end 8 is shaped to act as a lower abutting element for the float 25, in order to prevent its escape from the tubular body 5 through the first end 8, if the level L of liquid 3 is so low that it does not reach the float 25.

Finally, to allow its easy, fluid-tight assembly in use, the tubular body 5 is provided, outside and immediately close to the second end 9, with an annular collar 40 made of an elastomeric material and snappingly mounted between a bottom wall 41 of the end 9, facing part of lid 11 in use, and an annular ridge 42 of the tubular body 5.

In use, sensor 1 is easily assembled by inserting the float 25 through the end 9, which is transversally wider than the end 8 and which is not provided with the internal annular stop edge 20, and then by mounting the lid 11, once the electronic board 14 with diodes 12 and

13 has been mounted thereto. Finally, the collar 40 is mounted by fitting it onto the side of the end 8, until it couples with the ridge 42.

The so assembled sensor 1 is then mounted in the tank 2, through an appropriate hole (not shown); the collar 40 fluid-tightly cooperates with the edge of the hole, thus placing the end 8 at a relatively small, predetermined distance P from a bottom wall 200 of the tank 2.

In use, the liquid 3 which is gradually collected in the tank 2 enters into the body 5 through the mouth 21, passing through the annular opening 31 if the screen 30 is present, thus reaching the float 25, which is thus gradually taken closer to the diodes 12 and 13 as the level L increases.

The radiation emitted by the diode 12 (e.g. infrared or in the light field) is partially absorbed and partially reflected by the only partially reflective surface consisting of the exposed face 27 of the float 25, thus reaching the diode 13, which generates a voltage signal S, shown in the diagram in figure 3. Such a signal S will depend on the amount of radiation which is reflected towards the diode 13, but either absorbed by the surface of face 27 or reflected by the latter towards the internal side wall 7 of body 5, where the radiation is partially reflected and partially absorbed again.

Therefore, a signal S which changes in a substantially linear manner according to the entity of the level L of the liquid 3 present in the tank 2 is obtained.

Experimental tests conduced by the Applicant demonstrated how the mathematical relationship between signal S and distance of the float 25 from the end 9 is constant and reproducible. During the drying cycle in a drying machine or washing/drying machine, the level L changes over time and raises until all the humidity present in the laundry is removed. Once this condition has been reached, the level L of condensed liquid 3 will tend to stabilize, having reached its maximum.

By virtue of the linear relationship between the level L of liquid 3 and the signal S, the relationship obtained due to the particular described structure of sensor 1, a method for determining the end of the drying cycle in a drying machine or washing/drying machine may be easily implemented. According to the invention, such a method comprises the steps of: continuously monitoring the level L of the condensed liquid 3 in tank 2 for collecting the condensed liquid 3 of the machine; - after a predetermined interval of time T, checking whether the level L of the condensed liquid 3 has changed (i.e. whether the signal S has changed in intensity);

- repeating the preceding step until the level L of the condensed liquid 3 in the predetermined time interval T no longer changes (segment C of the curve shown in figure 3) ;

- accordingly, indicating the end of the drying cycle, because such a condition corresponds to the absence of further humidity to be removed from the laundry.

Obviously, such a method may be efficiently implemented only by carrying out the step of continuously monitoring the level L of condensed liquid 3 in the tank 2, by arranging an optical level sensor 1 in the collection tank 2, as described above.

Finally, it should be noted that the presence of the optional screen 30 allows to preserve reliability of sensor 1 over time.

Indeed, due to the removal of humidity from the laundry by means of a hot air flow, microscopic fabric filaments may be detached from the laundry and then remain suspended in the condensed liquid 3 in the form of lint which, in the known optical sensors, sticks to the outer wall of the sensor, thus strongly altering its reading. In the case of sensor 1, the lint could penetrate into the body 5 thus interfering with the free movement of the float 25.

According to the invention, this does not happen because most of the lint which manages to introduce itself into the annular opening 31 accumulates, instead, against an external face 300 (i.e. facing the opposite side of the end 8) of the disc 30, thus preserving the float 25.