A method and a device for detecting failure of the drum drive system of a tumble clothes dryer

Abstract

 A method for monitoring failure of a drum rotation system of a clothes dryer comprises acquiring a conductivity signal from the conductivity sensor of the dryer, and processing said signal in the frequency domain in order to retain a frequency spectra of the clothes tumbling process while rejecting the frequency spectra of noises and/or the like.

Description

[0001] The present invention relates to a method for detecting the breakage of the transmission system providing rotating force to the drum of a tumble dryer. More particularly, the present invention relates to a method for monitoring failure of said transmission system in a dryer having a conductivity sensor for assessing electrical resistance across electrodes being touched by tumbling clothes. The present invention relates also to a clothes dryer that performs the above method. Usually, the drum rotating system of a clothes dryer comprises a motor and a belt. The breakage of the belt is a dangerous event, potentially leading to a fire risk if such condition is not detected. The root cause of the fire risk associated with the failure of the drum motion is the persistence of the same piece of clothes against the hot air inlet of the dryer. The present invention can be used in those dryers equipped with moisture sensors based on analogue resistance measuring systems, as those disclosed in EP-A-940494.

[0002] The methods currently used to detect the breakage of the drum belt are currently relying on electro-mechanical switches kept in close position when the belt is operating correctly and conversely open in case of belt rupture or loosening. These methods (and the dryers associated thereto) are disclosed by W02005064065 and US2005161310.

[0003] Those known electromechanical devices are expensive and sometime unreliable as they imply the use of additional moving part, holding systems and harness. Such electromechanical switches are usually electrically connected in series with the heating elements of the dryer for triggering, in case of belt rupture, the immediate interruption of heat generation inside the machine. Those methods are able to detect just the breakage of the transmission belt, not being able to reliably detect also a belt loosening or failures of the motor.

[0004] Other methods, disclosed in US5166592 and US5006778, rely on saturating ferrite sensors to detect the decrease in motor torque upon breakage of the transmission belt. These known methods are able to detect also motor failures but require additional electrical sensors with the associated cost.

[0005] Methods like those disclosed in the US2005/0016013 are applicable on those machines equipped with moisture sensors able to output a digital signal whenever a wet cloth touches a couple of electrode strips. Unfortunately this control method fails in providing a reliable information when the machine is so heavily loaded that the clothes are always exceeding the detection threshold or the items are so dried to be unable to exceed that threshold, those two conditions being actually the most dangerous as far as the transmission failure is concerned.

[0006] Other methods, substantially relying on the detection of the abrupt speed changes in case of transmission rupture, in addition of requiring additional hardware to read such speed, are again ineffective in detecting the failure at the very beginning of the cycle, when no baseline for normal speed does exists.

[0007] An object of the present invention is to provide a detection method and system which does not cause an increase of the overall cost of the dryer and is able to identify and give a signal indicative of the absence of drum motion, caused by a failure in the motor and/or in the transmission system. The invention is applicable in the dryers equipped with conductivity strips circuits (originally used for moisture detection) of the type providing a continuos (i.e. analogue) signal, proportional to the instantaneous electrical impedance offered by the clothes touching the strips. This well-known type of sensor relies usually on two electrodes, placed inside the machine cavity in such position to be subjected to the mechanical contact with the tumbling clothes. The electrodes may either made of a couple of metal strips located in proximity of the drum rim but not rotating with the drum itself (i.e. integral with the machine chassis) or directly placed on the drum and then connected to the electronic circuitry trough sliding contacts. These sensors are offered by most of the dryer manufacturer in the high-end segment of the market and allow a better monitoring of the drying state by actually reading the residual water content in the fabric that is known to be proportional to the conductivity with a known relationship. An advantage of those continuous conductivity sensors vs. those just providing voltage pulses upon touching of a cloth sufficiently wet lies in the possibility to have increased moisture detection accuracy. This is due to the extended range of resistance measurement typically spanning from 50kΩ to 50MΩ as opposed to ability of "wet hit" sensor to just discriminate if the instantaneous resistance is above or below a given threshold (typically fixed around 1 MΩ).

[0008] In order to provide electrical insulation with respect to the control board, said analogue conductivity signal is typically converted into a frequency or pulse width signal and the optically isolated before being fed into a digital counter of the control unit. This well-known technique, known as pulse width modulation (PWM) is substantially equivalent to a low cost analogue to digital conversion.

[0009] Once the continuous clothes impedance signal is converted into a digital sequence, this is usually processed in known manner by the control unit in order to determine the dryness state of the clothes.

[0010] The present invention aims at using the same continuous conductivity signal in order to assess the absence of drum motion, with high reliability over the whole drying cycle. Since the same conductivity signal is used also for another purpose, the overall cost of the system according to the invention is very low, being included in an updated software embedded in the microprocessor of the control process unit.

[0011] Another object of the present invention is to use said signal of absence of drum motion to trigger a safety program in the machine control unit aimed at placing the appliance in safe conditions.

[0012] Another object of the present invention is to minimise the occurrence of false alarms (i.e. undue generation of the signal of absence of drum motion) and simultaneously minimise the probability of missed alarms (e.g. missed generation of the signal of absence of drum motion in case of a real failure), even in presence of heavy electrical noises superimposed to the moisture sensor signal and/or in case of external shaking of the dryer caused - for example - by a washing machine stacked below the dryer.

[0013] Another object of the present invention is the ability to perform the detection of the drum motion system even in conditions usually not reliably identifiable by existing prior art based on digital clothes touch system, like heavily loaded machine with very wet items or when the load is getting so dry to be unable to trigger the so called "wet hit" sensor.

[0014] In order to achieve these objects and other advantages and in accordance with the purpose of the invention, a method for processing said analogue resistance signal output by a known continuos conductivity sensor based on electrodes is disclosed, in order to detect the state of motion of the drum, by analysing the frequency content of the signal and outputting a "motor still" or "drum still" signal if such signal doesn't contain sufficient energy at those frequency typically excited by the process clothes tumbling.

[0015] The invention will now be described in more details, with reference to the attached drawings, in which:

• figure 1 is a schematic view of a clothes dryer according to the present invention and of the control unit thereof;
• figure 2 is a conductivity signal spectra of the dryer of figure 1; and
• figure 3 is an experimental diagram showing clothes damages in clothes dryer as a function of temperature and time.

[0016] With reference to figure 1, a clothes dryer 10 comprises a drum 12, a motor 14 with a pulley 16 and a transmission belt 18 around the pulley 16 and the drum 12. The invention is based on the observation that the frequency spectra of the continuous conductivity signal provided by the electrodes contacted by the tumbling clothes in the drum 12 may be substantially described by four frequency components graphically shown in figure 2:

s1. a pink noise, (also called 1/f noise or flicker noise) being generated by the pseudo-chaotic process of the clothes tubing inside the rotating drum;

s2. a frequency component at a frequency f₀ substantially related to the fundament periodicity of the rotation of the clothes bulk against the electrodes;

s3. a number of harmonics (i.e. multiple integers) of the frequency f₀;

s4. a D.C. component (substantially equivalent to the average value of the conductivity signal).

[0017] In addition to those physical process "signatures", the signal is always affected by a number of disturbing component or noises, typically:

n1.the mains frequency noise pickup, having frequency substantially equivalent to the mains service (i.e. 50 Hz or 60Hz)

n2.the machine external vibration noise, being caused by external mechanical forces as, for example, the shaking induced when the dryer is stacked on top of a washing machine operating in the spinning phase.

[0018] These observations explain why a simple algorithms based of the detection on absence of change of the conductivity signal over time would not be reliable enough, not being able to distinguish between signal variations caused by the tumbling of clothes and variations caused by said noise sources.

[0019] The invention is also based on the fact that the conditions for fabric damage or fabric ignition are function of a combination of the factors temperature, time and humidity content. The applicant has discovered by empirical methods the set of time/temperature/humidity that defines the boundary of fabric damage and fabric ignition. An example of such physical relationship is shown in figure 3.

[0020] The present invention achieves the object of robust and reliable detection of absence of drum motion by processing the signal in the frequency domain in order to retain the frequency spectra of the clothes tumbling process while rejecting the frequency spectra of the noise sources. Said signal processing in the frequency domain is therefore carried out in order to extract the energy associated only with the frequency bands being excited by the clothes tumbling process. It is not necessary to disclose in details how the above signal processing in the frequency domain is carried out, since this is well known to the man skilled in the art of appliance control. The extraction of the frequency band stimulated by the clothes tumbling is done for instance by calculating the discrete Fourier transform of the time sampled analogue conductivity at the frequencies of interest (typically 0.5Hz to 6 Hz) and then averaging the magnitude of these spectral lines.

[0021] According to another embodiment, the extraction of the frequency band stimulated by the clothes tumbling is done by fast Fourier transforming the analogue signal and then grouping and averaging the spectral lines corresponding to the above frequencies of interest. According to another embodiment of the invention, the extraction of the frequency band stimulated by the tumbling clothes is done by implementing a band-pass filter - either in analogue or discrete-time form - whose pass-band is substantially overlapped to the above frequencies of interest.

[0022] At the end of this process a signal is produced, substantially carrying an information related to of amount of clothes motion against the conductivity measuring electrodes.

[0023] The above signal can be already used by a control process unit 20 of the dryer 10 by comparing it to a threshold value in order to detect a failure of the drum transmission system. Such threshold value can be set as a fraction of the average value of the above motion signal in normal operating conditions. Such motion threshold can be dynamically and continuously set as a fraction of the motion signal continuously averaged during the drying process.

[0024] The control process unit 20 comprises a resistance to voltage converter 20a, a frequency domain processor 20b, a safety state machine processor 20c, a humidity estimator 20d and a risk supervisor processor 20e. The signal coming from the resistance to voltage converter 20a is fed to the humidity estimator 20d and to the frequency domain processor 20b. The safety state machine processor 20c is fed with signals coming from the frequency domain processor 20b and by the risk supervisor 20e. This latter processes not only data on humidity coming from the block 20d, but also signal h, k and z coming from other components of the dryer and indicative of the heat power delivered by the heaters, of the fabric type and of the process air temperature. Therefore the main signal coming from the frequency domain processor 20b, signal which reflects the actual motion state of the drum, is somehow "adjusted" in the block 20c in order to take into account other signals useful to update the actual degree of risk.

[0025] A subsequent step of the control method is preferably used to further process said instantaneous amount of motion signal over a given time observation window in order to understand if the lack of signal is just temporarily due to extreme unfavourable conditions - as loads close to almost complete dryness, or the amount of load is such to have low probability to fall on the strips causing absence of signal- or is actually due to a motor or transmission failure.

[0026] A further preferred step of the method is to trigger a first safety state in the machine control unit 20 upon the condition that the amount of motion signal stayed below the given motion threshold for a time longer than a given observation time-window, being said first safety state a reversible state, reverting to normal drying state upon detection that the amount of motion signal has remained above a given threshold for a given amount of time. When in the first safety state, the control unit may be conveniently programmed to turn off the heating elements or to reduce the associated power to a safe value.

[0027] The reliability of the method is further increased by making the time observation window for said further processing depending on the output of a fire risk assessment algorithm (processed in the risk supervisor 20e) that takes into account the instantaneous state of the machine (i.e. power delivered to the heating elements, dryness state of the load, process air temperature etc.), allowing longer reaction times (i.e. longer observation time-windows) in case of low risk and forcing shorter reaction time in case of critical conditions. Such observation time-window can be changed during the drying process in a way proportional to residual humidity of clothes and inversely proportional to the average power delivered to heaters.

[0028] Another preferred step of the present invention is to check the motion detection signal when the machine is in the first reversible safety status and if this signal exceeds the motion thresholds again the dryer will be turned back in the nominal operations unless a certain time limit has passed since the first entrance in the safety state. If this said time limit has passed the first safety state becomes irreversible until the end of the cycle and the machine cannot reach the nominal operations anymore.

Claims

1. A method for monitoring failure of a drum rotation system of a clothes dryer (10), such dryer comprising a conductivity sensor for assessing electrical resistance across electrodes being touched by tumbling clothes, characterised in that it comprises acquiring a conductivity signal from the conductivity sensor, and processing said signal in the frequency domain in order to retain a frequency spectra of the clothes tumbling process while rejecting the frequency spectra of noises and/or the like.

2. A method according to claim 1, characterised in that the conductivity signal is processed in order to extract a clothes motion signal associated with frequency bands excited by tumbling clothes, such clothes motion signal being compared with a predetermined threshold value for issuing a state signal indicative of the safe or unsafe state of the dryer.

3. A method according to claim 2, characterised in that said clothes motion signal is monitored in predetermined time intervals, and in that said state signal is issued when the clothes motion signal remains above or below the predetermined threshold value for a period equal or longer than the time interval.

4. A method according to claim 2 characterised in that while in the said state signal the said motion signal is compared to the amount of motion signal and if it exceeds the amount of motion threshold the machine is reversed to normal operations unless a predetermined time threshold has passed, if this said time threshold has passed the dryer is in a irreversible safety state until the end of the cycle.

5. A method according to claim 3, characterised in that the threshold value is the clothes motion signal detected at the previous time interval.

6. A method according to claim 3, characterised in that said threshold value is set as a predetermined fraction of an average value of the clothes motion signal in normal operating conditions.

7. A method according to claim 3, characterised in that the length of said time intervals is periodically changed during the drying process in a way proportional to residual humidity of clothes and inversely proportional to the average power delivered to heaters.

8. A method according to claim 2 where the extraction of the frequency band stimulated by the clothes tumbling is done by calculating the discrete Fourier transform of the time sampled analogue conductivity at the frequencies of interest and then averaging the magnitude of these spectral lines.

9. A method according to claim 2 where the extraction of the frequency band stimulated by the clothes tumbling is done by fast Fourier transforming the analogue signal and then grouping and averaging the spectral lines corresponding to the frequencies of interest.

10. A method according to claim 3 where the extraction of the frequency band stimulated by the tumbling clothes is done by implementing a band-pass filter - either in analogue or discrete-time form - whose pass-band is substantially overlapped to the frequencies of interest.

11. A clothes dryer, of the type comprising a drum (12), a drum rotation system (14, 16, 18), a conductivity sensor for assessing electrical resistance across electrodes being touched by tumbling clothes, a control process unit (20) adapted to receive a conductivity signal indicative of said electrical resistance in order to control the drying process, characterised in that the control process unit (20) is adapted to process said conductivity signal in the frequency domain in order to retain a frequency spectra of the tumbling clothes while rejecting the frequency spectra of noises and the like, and in that the central process unit (20) is adapted to issue a failure signal when the frequency spectra of the tumbling clothes or a value extracted therefrom differs from a predetermined threshold value for more than a predetermined observation time window.

12. A clothes dryer according to claim 10, characterised in that the control process unit (20) comprises a frequency domain processor (20b) and a risk supervisor processor (20e) which are adapted to input signals to a safety state processor (20c) in order to take into account other operative conditions of the dryer, as the heating power, the fabric type, the process air temperature and the dryness state of the load.

Drawing