Dryness detection method for clothes dryer based on pulse width

A device and method are provided for detecting a root moisture content of clothing in a clothes dryer. The dryer has two conducting bars situated in the dryer bin. A pulse generator circuit is coupled to the conducting bars. A microcontroller is coupled to an output of the pulse generator circuit. The pulse generator circuit generates a pulse when wet clothing contacts the conducting bars in the dryer bin. The microcontroller receives the pulses and counts the pulses that are longer than a threshold length. The microcontroller issues a termination signal based on the number of counted pulses.

TECHNICAL FIELD

The present disclosure relates to a method and a circuit for detecting the moisture content of articles in an automatic dryer.

DESCRIPTION OF THE RELATED ART

Many clothes dryers allow the user to select a specific amount of time for the clothes dryer to dry a load of laundry. This selection can be made using a dial or a digital interface on the outside of the dryer.

Many dryers alternatively allow the user to select a level of dryness to which the dryer will dry a load of laundry. In this type of dryer there is typically some kind of mechanism for monitoring how dry the laundry is. When the dryer detects that the load of laundry has reached the level of dryness selected by the user, then the drying cycle ends.

In one system the humidity of the air exiting the dryer is monitored. As the dryer dries the clothes, water in the clothes evaporates and is expelled through the dryer vent. At first the air in the dryer is quite humid. But as the clothes become drier, the humidity in the air passing through the vent decreases. In such a system the dryer assumes that the clothes are dry once the humidity of the air passing through the vent has dropped below a threshold value. The dryer then turns off.

A challenge faced by automatic dryers is to ensure that the clothes do not stay in the dryer too long. This is countered by the need to ensure that the clothes are sufficiently dry. Over-drying clothes can damage certain types of delicate clothing and waste electricity. A dryer that frequently continues to operate after the clothes are dry may also shorten its own lifetime.

BRIEF SUMMARY

In one embodiment, two conductors are positioned in the drying bin of a clothes dryer. A pulse generator circuit is coupled to the two conductors to transmit an electric current through the clothes as they dry. An output of the pulse generator circuit is coupled to a microcontroller for determining the dryness of the clothes.

As wet clothing tumbles in the dryer during a drying cycle, the wet clothing periodically comes into contact with the two conductors. When the clothing is in contact with the two conductors, the clothing acts as a conductor having a resistance value that varies with the moisture content of the clothes. It is thus seen by the circuit as a resistor connected between the two conductors. When the resistance between the two conductors is low enough, the pulse generator circuit will charge a capacitor to a threshold value. When the capacitor is charged to a threshold voltage, a transistor is turned on which generates a pulse. The pulses typically indicate that a resistance between the first and second conductors is below a threshold value. The pulses are output to the microcontroller.

In one embodiment the microcontroller compares each pulse to a threshold length of time. If the pulse is longer than the threshold length of time, then the microcontroller counts the pulse. If the pulse is shorter than the threshold length of time, then the microcontroller does not count the pulse. The microcontroller issues a termination signal to end the drying cycle if a rate of counted pulses drops below a threshold rate.

One embodiment is a method for detecting the dryness of clothes. The method comprises drying clothes in a clothes dryer and sensing a resistance between two conductors in a dryer bin; generating a pulse when the resistance between the pulses is lower than a threshold resistance; outputting the pulses to a microcontroller; comparing the length of the pulses to a threshold length; counting the number of pulses longer than the threshold length; and issuing a termination signal when the rate of occurrence of counted pulses drops below a threshold rate.

DETAILED DESCRIPTION

FIG. 1illustrates a dryer10. The dryer10has a dryer bin12in which a user places wet clothing or other articles to be dried. The dryer10has a door14which opens to enable access to the dryer bin12. The dryer10has a panel which has a user input13.

The user can use the user input13to select an automatic drying cycle and a desired level of dryness for the automatic drying cycle. The dryer10is configured to end the automatic drying cycle when clothes placed in the bin12have reached the level of dryness specified by the user.

FIG. 2illustrates a dryness moisture detection circuit20according to one embodiment of the invention. A sensor15is located in the dryer bin12. The sensor15is configured to detect a moisture content of clothing or other articles in the dryer bin12or to enable detection of a moisture content of the clothing or other articles in the dryer bin.

The sensor15is coupled to a pulse generator circuit18. When wet clothes contact the sensor15, the pulse generator circuit18outputs a pulse to a processor24. The processor24is coupled to a clock26, a memory28, a counter30, a timer31, and a filter33. The memory28stores and retrieves data. The data includes information regarding pulses received from the pulse generator, software to enable execution of programs by the processor24, or any other data which may be used by the processor24or other components

The counter30counts a number of pulses received by the processor24from the pulse generator circuit18. The timer31may be used to measure a time duration of pulses sent from the pulse generator circuit18. The filter33filters pulses which are shorter than a threshold length. In one embodiment pulses that are shorter than a threshold length will not be counted by the counter30.

In one embodiment, the processor24monitors the counter30to determine if the number of counted pulses in a selected time period is smaller than a threshold number. If the number of counted pulses is smaller than a threshold number then the processor24issues a termination signal to end the drying cycle.

Other embodiments may have fewer or more components than those shown inFIG. 2. Also, the components may be connected differently to each other without departing from the scope of the present disclosure.

FIG. 3illustrates an alternative embodiment of the invention. The sensor15is coupled to a voltage source Vsource. The output of the sensor is coupled to sense node Ns, a capacitor C1, a resistor R, and a switch35. When articles or clothing in the dryer bin12contact the sensor15the capacitor C1begins to charge. The capacitor C1will charge towards a voltage dependent on a moisture content of the clothing. If the moisture content is high enough, then the capacitor C1will charge quickly beyond a threshold voltage of the switch35and activate the switch35. The switch35causes a pulse to be output to a microcontroller22when the voltage on the capacitor C1charges beyond the threshold voltage of the switch35. The value of the resistor R is selected to permit the capacitor to charge to the threshold value when wet clothes are present under normal operating conditions. The value of R is usually a high resistance, such as in the mega ohm range; after the clothes are no longer in contact with the sensor, the capacitor will discharge through R to be ready for the next sensing event.

If the resistance of the clothes is low, as will be the case for moist clothes, then current through the resistor R will be low compared to the charging current through the dry clothes, which will permit the capacitor to charge to the threshold voltage. If the resistance of the clothes is high, when the clothes are dry enough, then the voltage dropped across the clothes will prevent the capacitor from charging to the threshold voltage and the switch will not be activated. In other words, if the resistance of the clothes is high the current flow to charge the capacitor will be low. Further, the current will bleed off via resistor R at a rate that prevents the capacitor from charging to the threshold voltage. If the current through resistor R is higher than the current through the clothes, the capacitor C1will never charge.

In one embodiment the microcontroller22may include the processor24, the clock26, the memory28, the counter30, the timer31, and the filter33. The microcontroller22receives pulses from the switch35. Counter30counts the pulses. The filter33filters pulses that are shorter than a threshold length of time and cause the counter to count only those pulses which are longer than the threshold length of time. Counter30counts the pulses. The processor24monitors the counter30to determine if the number of counted pulses in a selected time period is smaller than a threshold number. If the number of counted pulses is less than a threshold number, then the processor24issues a termination signal to end the drying cycle.

FIG. 4illustrates a view of the inside of the dryer bin12, looking at the door14, from the inside of the dryer bin12. In one embodiment, the sensor15is two conducting bars16and17positioned below the door14. In one embodiment, the conducting bars16and17are between eight and ten inches in length, and are spaced apart by about an inch. In other embodiments, the bars16and17are 2-3 inches long and spaced apart by ⅛ of an inch. The conducting bars16and17are electrically insulated from each other when the dryer bin12is empty. The conductors16and17may of course be other shapes than bars and may be other sizes and spaced differently than described above.

Prior to the beginning of a drying cycle, wet clothes or other articles are loaded into the bin12of the dryer10. The user then selects an automatic drying cycle at the user input13and begins the drying cycle. During the drying cycle the dryer10tumbles the clothes. The clothes are thus moved about throughout the bin12. As the clothes tumble, individual items of clothing randomly and momentarily come into contact with both conducting bars16and17below the door14. If an item of clothing contacts both conducting bars16and17simultaneously, then the clothing momentarily acts as a conductor having a resistance value connected between the two conducting bars16and17. Of course, two items of clothing that are in contact with each other, while each is in contact with respective conductive bars, will also act as a resistive electrical conductor between the conducting bars16and17.

Wet clothing generally has a lower resistance than dry clothing. When wet clothing contacts the conductive bars16and17there is a lower resistance between the conducting bars16and17than if dry clothing contacts the conductive bars16and17. This configuration can be utilized to sense a relative moisture content (RMC) of the clothing. When the RMC of the clothing drops below a threshold level, according to the automatic drying cycle selected, the dryer10automatically shuts off.

FIG. 5illustrates a moisture detection device20according to one embodiment of the present invention. A pulse generator circuit18is coupled to the conductive bars16and17. The pulse generator circuit18typically is not located in the dryer bin, but may be located in any suitable portion of the dryer that protects the circuit from being damaged.

A resistor R1, for example 4 kΩ, is connected between a high positive voltage supply Vph, for example 17V, and the first conductive bar. The second conductive bar is not electrically connected to the first conductive bar in the situation illustrated inFIG. 4. When clothes touch both bar16and bar17at the same time, a conductor having the resistance value Rccouples the two bars together. The value of Rcwill vary from less than 4 kΩ when the clothes are wet to greater than 5 MΩ when the clothes are dry. The value of Rcis a sufficiently reliable measure of the amount of moisture in the clothing for use in this circuit to determine when to shut off the dryer. A resistor R2, for example 4 kΩ, is coupled between the second conductive bar and node N1. A capacitor C1, for example 3.3 nF, is coupled between node N1and ground. A resistor R3, for example 5 MΩ, is coupled between N1and ground. The base of transistor T1is coupled to N1. Resistor R4, for example 750 kΩ, is coupled between the high positive voltage supply and the collector of T1. The emitter of T1is coupled to node N2. A resistor R5, for example 68 kΩ, is coupled between N2and ground. The base of transistor T2is also coupled to N2. The emitter of T2is coupled to ground. The collector of T2is coupled to an input In1of microcontroller22. Resistor R6, for example 100 kΩ, is coupled between a low positive voltage supply Vp1, for example, 5V and In1. The specific values and configuration of circuit components are given merely by way of example and are not limiting. The circuit components may be arranged in many other configurations and have many other values according to other embodiments of the invention. In particular, transistors T1and T2may be implemented as MOS transistors or any other suitable transistor according to other embodiments of the pulse generator circuit18. Transistors T1ad T2may also be replaced by a comparator circuit with a threshold set by a resistor divider network, or other acceptable detection circuit, or some other acceptable transition circuit.

Operation of the circuit ofFIG. 5will now be described. When clothes placed in the bin12undergo a drying cycle, they periodically come into contact with the conductive bars16and17. An item of clothing in contact with both bars16and17acts as a conductor connected between the bars16and17. This conduction allows an electric current I1to flow between the two bars16and17at a value related to the resistance of the clothes, Rc. I1flows from the high positive voltage source Vph through R1, through Rc(the clothes), and R2. I1causes the capacitor C1to start to charge. If transistors T1, T3, and T4are off, then I1will reach the following steady state current:

I1=VphR1+Rc+R2+R3
where Rcis the resistance of the clothing between the bars16and17.

The current I1will charge the capacitor to a voltage Vcdependent on the resistance of the clothes Rcaccording to the following relationship:

If the voltage Vcat node N1on the capacitor C1is greater than the base-emitter turn on voltage Vbe1of transistor T1, then T1will turn on. If the voltage Vcon the capacitor C1is greater than Vbe1plus the base-emitter turn on voltage Vbe2of transistor T2, then T2will turn on as well and the voltage at the base of T1will be clamped to the sum of Vbe1plus Vbe2. When T2is turned on, current I2flows from the low positive voltage source through resistor R6. This causes the voltage to drop at In1. This drop in voltage acts as a pulse at In1. The microcontroller22receives the pulses at In1.

In order for a pulse to be sent to the microcontroller22, the voltage Vcon the capacitor C1must be equal to or greater than a double threshold voltage Vt:
Vt=Vbe1+Vbe2.

The voltage to which the capacitor C1will charge depends in part on the resistance Rcof the clothing in contact with the bars16and17. Thus, the resistance Rcof clothing which has contacted the bars16and17must be below a threshold resistance if the voltage Vcon N1is to exceed Vt.

The duration of a pulse corresponds to the length of time that the wet clothing contacts the bars16and17and to the wetness of the clothing. Once a pulse has been generated on the output Out, the pulse will continue as long as the wet clothing remains in contact with the bars. When the clothing is no longer in contact with the bars16and17, the capacitor C1discharges through the resistor R3to ground. The discharge of the capacitor C1causes the voltage Vcof the node N1to drop. Once the voltage Vchas dropped below the threshold voltage Vt, the transistor T2turns off and current I2no longer flows. The voltage at In1increases to the level of the power supply Vp1. The return of the voltage at In1to Vp1, is the trailing edge of the pulse, which is the end of the pulse.

The microcontroller22comprises a processor24, a clock26, a system memory28, a counter30, a timer31, and a filter33, as shown inFIG. 2. The clock26may be a crystal oscillator, a resonant circuit, an Rccircuit, or any other means suitable for generating a clock signal. The system memory28is coupled to processor24and is configured to store and retrieve data. The memory28may store program data for the operation of the microcontroller22, data regarding pulse counts and pulse lengths, or any other data. The memory28may include one or more arrays of ROM, EPROM, EEPROM, Flash memory, SRAM, DRAM, or any other suitable memory. The counter31is either a register in the processor24or is coupled to the processor24and serves to count pulses received from the pulse generator circuit18at input In1. In practice, the microcontroller22may have many more or different components and the components may be connected differently than is shown inFIG. 5.

When the pulse generator circuit18generates a pulse at the input In1, the processor24detects the pulse and causes the counter30to increment. The counter30thus counts the number of pulses generated by the pulse generator circuit18.

In one embodiment, the processor24monitors the number of pulses generated during each of a plurality of defined counting periods. At the end of each counting period, the processor24monitors the counter30to determine the number of pulses received during the counting period. The number of pulses received during the counting period defines a rate at which pulses are being received. At the end of the counting period, a new counting period begins and the rate of pulses is monitored again for the new counting period. In one embodiment, each counting period is about two seconds.

The rate at which pulses are being received corresponds to the RMC of the clothing in the dryer bin12. If the clothes are wetter, then the pulses will be generated more frequently. If the rate at which pulses are received drops below a threshold pulse rate for a number of counting periods, then the processor24determines that the clothes are dry and issues a shutdown signal which terminates a drying cycle of the clothes dryer10. In one embodiment, the processor24issues the shutdown signal if the rate of pulses drops below the threshold rate for two consecutive counting periods. In other embodiments, the processor24may issue the shutdown signal after more or fewer counting periods than two.

Under some circumstances, the rate of pulses may falsely indicate that the clothing is wet when the clothing is in fact dry. These errors may arise due to static discharge of the clothing in the dryer bin12. As the clothing becomes drier, certain types of fabric tend to frequently build up a static charge. When an item of clothing that has a build up of static charge contacts the second conductive bar, the static charge discharges through the second conductive bar. This static discharge quickly charges the capacitor C1beyond the threshold Vtand a pulse is generated as previously described. Thus, as the clothes become drier, static electricity may cause many pulses to be sent to the microcontroller22. If not filtered for length, these pulses would increment the counter30and the microcontroller22might interpret the rate of pulses to mean that the clothing is wet. The pulses due to static discharge may cause the dryer10to continue drying after the clothes are already dry. The prolonged drying cycle needlessly wastes energy. The clothing may also be damaged if it remains in the dryer10longer than necessary.

The pulses generated due to static discharge are generally very short compared to the pulses generated due to contact of wet clothing with the conductive bars16and17. The reason for this is that a static charge discharges very rapidly as a very small current. A static discharge will quickly charge the capacitor C1and then cease delivering current. When current is no longer supplied, capacitor C1discharges through the resistor R3. Pulses generated due to static discharge are thus much shorter than those due to wet clothing.

To overcome this problem, the microcontroller22is configured to compare each pulse to a threshold pulse length. The microcontroller22will count the pulses that are longer than a threshold time and disregard the pulses that are shorter than the threshold time. The threshold time is selected to be longer than a typical pulse due to static discharge and shorter than a typical pulse due to wet clothing.

In one embodiment the microcontroller22is configured to trigger an interrupt at the processor24when the leading edge of a pulse is received from the pulse generator circuit18. The interrupt will last a predetermined number of clock cycles that is considered longer than a pulse due to static discharge. If the pulse is still present after the interrupt is over, the processor24causes the counter30to increment. If the pulse is not present upon return from the interrupt then the processor24does not cause the counter30to increment. This is one way to carry out the function of filter33. Thus the microcontroller22does not count pulses which are shorter than a threshold time or pulse length. In this way pulses due to static discharge are not counted. Only pulses longer than a threshold time are counted and the rate of pulses during a counting period more accurately reflects the RMC of the clothing. In one embodiment, the interrupt and counting as described above may be implemented by running software installed on the memory28of the microcontroller22.

In one embodiment, the microcontroller22is configured to start a timer31when the leading edge of a pulse is received. The timer31counts either down from or up to the threshold time. If the timer31counts to the threshold time before the trailing edge of the pulse is received, then the pulse is counted. If the trailing edge of the pulse is received before the timer31counts to the threshold time then the pulse is not counted.

Various embodiments for the function of filter33to filter out pulses that are shorter than the threshold time and cause the counter30to increment only if the pulse is longer than the threshold time have been described.

Many other embodiments implementing hardware and/or software to filter pulses due to static discharge are possible. In some embodiments a filter to filter pulses due to static discharge may be implemented as hardware or software in the microcontroller22. In one embodiment, the pulse generator circuit18may be configured to not generate a pulse at all due to static discharge. Many other embodiments of the pulse generator circuit18and the microcontroller22are apparent in light of the present disclosure and fall within the scope thereof. Specific embodiments are illustrated only by way of non-limiting example.

FIGS. 6A and 6Bare sample graphs of the voltage on the capacitor C1and the voltage on the input In1, respectively, during a portion of a drying cycle.FIG. 6Acharts the voltage on the capacitor C1during a 500 millisecond sample of an end portion of a drying cycle.FIG. 6Billustrates the voltage at the microcontroller input In1for the same time period as shown inFIG. 6A.

InFIG. 6A, the capacitor reaches the threshold voltage of about 1.3V at the point labeled34. At this time, the voltage at In1(illustrated inFIG. 6B) drops from 5 volts to about 0 volts. This drop from 5 volts to 0 volts constitutes the leading edge or first edge of pulse35. InFIG. 6Aat36, the voltage on the capacitor drops below the threshold voltage. At this time the voltage at In1ofFIG. 6Breturns to 5V. This constitutes the trailing edge or end of the pulse35. This pulse35lasts about 50 milliseconds.

InFIG. 6B, pulse37begins when the voltage on the capacitor inFIG. 6Areaches the threshold voltage at38. Two very brief pulses,39and41, occur when the voltage on the capacitor briefly reaches the threshold at40and42, respectively. These last two very short pulses40and41are so short that they are considered to be due to static discharge from the clothing or local noise in the system. A dryer circuit is in an electrically noisy environment and noise may be generated in the sensing circuit from a number of locations, such as from the 60 Hz power line, spiking in the power supplies, the switching control signals, the power for driving the motor that is rotating the drum, the electrical control panel, or even from such sources as the filter mesh, a person banging the lid, or other unexpected locations. The dryness detection circuit20as described above is configured to not count pulses generated from sources other than the wetness of the clothing, whether the source is static electricity or some other source of noise. In one embodiment, a threshold time of 10 milliseconds is appropriate to filter out the pulses due to static discharge and noise. In other embodiments, a 20 millisecond threshold time is used to mask noise, while a 5 millisecond time is sufficient to mask noise in some environments. Of course, in some dryers, the numbers might be different and be given in microseconds or seconds based on the dimensions of the bars and how far apart they are from each other.

In the example illustrated inFIGS. 6A and 6B, pulses39and41are comparatively brief and can be identified as spurious pulses due to static electricity or other noise. The filter33of the dryness detection circuit can identify these short pulses and cause them to be filtered so that the counter30does not increment. If, for example, the threshold time is 10 ms, then inFIG. 6B, the counter30would increment at the trailing edge of pulses35and37because these pulses are longer than the threshold time. The filter33prevents the counter30from incrementing for pulses39and41because the pulses39and41are shorter than the threshold time. In this way, the dryness detection circuit20ignores pulses that are due to static discharge and more accurately determines the dryness of the clothes. Of course, the threshold time may be larger or smaller depending on the dryness detection system and components thereof.

FIG. 7shows a flow diagram100which illustrates a method for monitoring and modifying the RMC of clothes in a clothes dryer10according to one embodiment. At102a drying cycle is begun. This includes putting wet clothing in the dryer bin12and selecting a drying cycle at the user input13of the dryer10. Upon beginning the drying cycle, the dryer10tumbles the clothes in the bin12.

At104wet clothing comes into contact with conductive bars16and17located in the dryer bin12. If the clothing is wet enough, then the resistance between the two bars16and17will drop below a threshold resistance and a capacitor C1will charge to a voltage higher than a threshold voltage and turn on transistor T2. When transistor T2turns on, a pulse is sent to the microcontroller22.

At106the microcontroller22compares the pulse duration to a threshold time.

At108if the length of the pulse is shorter than the threshold time, then the pulse is disregarded and the counter30is not incremented, as shown at110. If the pulse is longer than the threshold time, then the counter30is incremented at112.

At the end of a counting period at114, the processor24monitors the number of pulses that have been counted. The number of pulses received during the counting period corresponds to a rate of pulses received. If the rate of pulses is lower than a threshold rate, then a termination signal is issued at118. In one embodiment, the termination signal is issued only if the rate of pulses is lower than the threshold rate in two or more consecutive counting periods.