Patent Publication Number: US-2016222576-A1

Title: Clothing dryer and method of controlling the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) AND CLAIM OF PRIORITY 
     The present application is related to and claims the benefit of Korean Patent Application No. 10-2015-0014737, filed on Jan. 30, 2015 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a clothing dryer which dries an object to be dried and a method of controlling the same. 
     BACKGROUND 
     Generally, a clothing dryer is an apparatus which rotates a drying drum, in which wet clothing is accommodated, at a low speed and allows high-temperature air to pass through the drying drum and flow in the drying drum in order to dry the clothing in the drying drum. The dryer may be classified as an exhaust type dryer which exhausts high-temperature, humid air that has passed through the drying drum out of the dryer, and a condensation type dryer which removes moisture from high-temperature, humid air that has passed through the drying drum and circulates the air back into the drying drum. Also, the dryer may be classified as an electric drier and a gas type dryer according to a method of heating air, such as a heating means. The electric dryer heats air using electrical resistance heat, and the gas type dryer heats air using heat generated by combustion of gas. However, the gas type dryer is capable of controlling heating power. The dryer may be classified as the electric drier and the gas type dryer according to the method of heating air, such as a heating means. The electric dryer heats air using electrical resistance heat, and the gas type dryer heats air using heat generated by combustion of gas. 
     SUMMARY 
     To address the above-discussed deficiencies, it is a primary object to provide, for use in a dryer capable of efficiently controlling heating power and a method of controlling the same. 
     According to an aspect, a dryer includes a drum for accommodating an object to be dried, a combustion device for combusting gas to heat air, a blowing device for transferring the heated air into the drum, and a valve assembly for controlling a gas discharge amount supplied to the combustion device, wherein the valve assembly may operate in one mode among a high heating power mode which maximizes the gas discharge amount, a low heating power mode which generates 50% or less heating power compared to the high heating power mode, and a standby mode which blocks the gas discharge. Here, the low heating power mode may generate 30% heating power compared to the high heating power mode. 
     In addition, the valve assembly may further include an output control valve for closing a gas flow passage to decrease an open rate. In addition, the dryer may further include a dryness sensor for detecting a dryness level of an object to be dried, and a control unit for comparing the dryness level detected from the dryness sensor to a reference dryness level in order to control the valve assembly such that the operation mode of the valve assembly is changed. Here, the control unit may control the valve assembly to operate alternately between the high heating power mode and the low heating power mode. In addition, the dryer may further include a temperature sensor for measuring a temperature of air which flows into the drum, and a control unit for comparing the temperature measured from the temperature sensor to a reference temperature in order to control the valve assembly such that the operation mode of the valve assembly is changed. Here, the control unit may control the valve assembly to operate alternately between the high heating power mode and the low heating power mode. 
     The dryer may further include a dryness sensor for detecting a dryness level of an object to be dried, and determine a maintenance time of the high heating power mode based on the dryness level change rate detected from the dryness sensor. In addition, the combustion device may further include an igniter for igniting gas, and a control unit for controlling the valve assembly to operate in the high heating power mode when the igniter operates. In addition, the valve assembly may further include a safety valve for determining whether to discharge gas or not. In addition, the control unit may control the safety valve to be opened when the temperature of the igniter reaches an ignition point of gas. According to another aspect, a clothing dryer includes a control unit for controlling an operation, a drum for accommodating an object to be dried, a valve assembly capable controlling heating power by controlling a gas discharge amount, a combustion device for combusting the gas discharged from the valve assembly to generate hot air, and a blowing device for transferring the hot air into the drum. Here, the valve assembly may further include an output control valve for decreasing an open rate of the valve assembly to a predetermined open rate in order to control the gas discharge amount. 
     In addition, the clothing dryer may further include a dryness level measurement unit for measuring a dryness level of the object to be dried, and the output control valve may decrease the open rate of the valve assembly to be low until the dryness level of the object to be dried reaches a preset reference dryness level. In addition, the output control valve may decrease the open rate of the valve assembly for a reference time in which a dryness level of the object to be dried is preset. In addition, the valve assembly may further include a safety valve for determining whether to discharge gas or not. 
     According to another aspect, a method of controlling a clothing dryer which includes a combustion device in which heating power is controlled by a valve assembly which controls a gas discharge amount in accordance with a plurality of predetermined open rates may include controlling the valve assembly to a low open rate among the plurality of open rates to dry an object to be dried in a low heating power mode and, when the dryness level of the object to be dried reaches a preset reference dryness level, controlling the valve assembly to a high open rate among the plurality of open rates to dry an object to be dried in a high heating power mode. 
     Before undertaking the DETAILED DESCRIPTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
         FIG. 1  is a perspective view illustrating an exterior of a clothing dryer according to an embodiment; 
         FIG. 2  is a schematic cross-sectional view of the clothing dryer according to an embodiment; 
         FIG. 3  is a view schematically illustrating a front support plate of the clothing dryer according to an embodiment; 
         FIG. 4  is a view illustrating in detail a guide member of the clothing dryer according to an embodiment; 
         FIG. 5  is a view schematically illustrating a combustion device of the clothing dryer according to an embodiment; 
         FIG. 6  is a view illustrating another embodiment of an igniter of the clothing dryer according to an embodiment; 
         FIG. 7  is a view for describing a gas supply of a valve assembly; 
         FIG. 8  is an exploded perspective view of the valve assembly for describing an embodiment of an output control valve; 
         FIG. 9  is a control block diagram for describing in detail an operation of the clothing dryer according to an embodiment; 
         FIG. 10  is a view illustrating a pulse generation frequency of a dryness level detector in accordance with time according to an embodiment; 
         FIG. 11  is a view for describing a combustion device operation unit of the clothing dryer according to an embodiment; 
         FIG. 12  is a view for describing an operation at the time of ignition of the combustion device operation unit in  FIG. 11 ; 
         FIG. 13  is a flow chart for describing an embodiment of a method of controlling the clothing dryer according to an embodiment; 
         FIG. 14  is a view for describing an air flow in a drying process of the clothing dryer according to an embodiment; 
         FIG. 15  is a flow chart for describing an embodiment of an ignition process in  FIG. 10 ; 
         FIG. 16  is a view illustrating a temperature change in air when exhaust blockage has occurred; 
         FIG. 17  is a view for describing an embodiment of a re-ignition process; 
         FIG. 18  is a flow chart for describing in detail an embodiment of a drying process in  FIG. 13 ; 
         FIG. 19  is a graph illustrating a change in open rates in the drying process of  FIG. 18 ; 
         FIG. 20  is a flow chart for describing in detail another embodiment of the drying process in  FIG. 13 ; 
         FIG. 21  is a flow chart for describing in detail still another embodiment of the drying process in  FIG. 13 ; 
         FIG. 22  is a view for describing another embodiment of a mode change in the drying process; 
         FIG. 23  is a view for describing a control for tracking a temperature; 
         FIG. 24  is a view for describing a drying process which limits an output based on the temperature; 
         FIG. 25  is a flow chart for describing an embodiment of analyzing characteristics of an object to be dried; and 
         FIG. 26  is a view for describing a change in dryness level in accordance with characteristics of an object to be dried. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 through 26 , discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged clothing dryer or other drying device. Hereinafter, a clothing dryer and a method of controlling the same will be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a perspective view illustrating an exterior of a clothing dryer according to an embodiment, and  FIG. 2  is a schematic cross-sectional view of the clothing dryer according to an embodiment. Referring to  FIGS. 1 and 2 , a clothing dryer  1  according to an embodiment may include a housing  10  forming the exterior, a drum  20  rotatably installed within the housing  10 , a hot air supply unit  40  supplying hot air into the drum  20 , a hot air discharge unit  50  through which the air that has dried an object to be dried in the drum  20  is discharged, a combustion device  100  heating the air, and a circulation device circulating the heated air to generate hot air. 
     The housing  10  forms the exterior of the clothing dryer  1 . A control panel  230  for controlling the clothing dryer  1  may be provided at a top side of a front surface of the housing  10 . Also, an inlet  11  for inserting or withdrawing an object to be dried into or from the drum  20  is formed at the front surface of the housing  10 . In addition, a door  15  is coupled to the front surface of the housing  10  by a hinge. The door  15  is provided in a shape corresponding to the inlet  11 . A user may open the door  15  by rotating the door  15  forward, and insert or withdraw the object to be dried into or from the drum  20 . Also, the door  15  may be rotated toward the clothing dryer  1  to close the drum  20 . 
     In addition, a plurality of through-holes  17  may be provided at the housing  10 . Outside air may be introduced into the clothing dryer  1  through the through-holes  17 . As illustrated in  FIG. 1 , the through-holes  17  may be provided at a lower portion of the front surface and a side surface of the housing  10 . Also, as illustrated in  FIG. 2 , the through-holes  17  may be provided at a rear surface of the housing  10 . The drum  20  is rotatably installed within the housing  10 . The drum  20  may include a cylindrical portion  21 , a front support plate  22 , and a rear support plate  23 . The cylindrical portion  21  is formed in a cylindrical shape with open front and rear surfaces, the front support plate  22  is coupled to the front surface of the cylindrical portion  21 , and the rear support plate  23  is coupled to the rear surface thereof. 
     Here, the inlet  11  for inserting or withdrawing the object to be dried is formed at the front support plate  22 , and a plurality of lifts  24  may be provided inside the cylindrical portion  21  along a circumferential direction. The object to be dried inside the drum  20  is lifted and lowered repetitively by the lifts  24 . That is, the lifts  24  lift and lower the object to be dried, thus allowing the object to be dried to be effectively dried. In addition, rollers  30  for supporting the drum  20  are provided at lower ends of an outer circumferential surface of the drum  20 . The rollers  30  may rotatably support the drum  20  by being provided at front and rear lower ends, respectively, of the outer circumferential surface of the drum  20 . Here, the rollers  30  may be fixed by the front support plate  22  and the rear support plate  23 , respectively. 
     The hot air supply unit  40  supplies high-temperature, dry hot air to the drum  20 . The hot air supplied to the drum  20  absorbs moisture of the object to be dried inside the drum  20 . Specifically, the hot air supply unit  40  may include a combustion chamber  41  in which air is heated by the combustion device  100 , a bottom duct  42  for guiding the heated air to a rear duct  43 , the rear duct  43  for guiding the heated air to a hot air outlet  45 , and the hot air outlet  45  through which the hot air is discharged into the drum  20 . Air in the combustion chamber  41  is heated by the combustion device  100  to be described below. The combustion chamber  41  may be provided in a hollow conical shape in which a rear end has a smaller diameter than a front end. 
     Air may be introduced through the front end of the combustion chamber  41 , and the air introduced through the front end of the combustion chamber  41  is heated by the combustion device  100 . A rear end of the combustion chamber  41  is coupled to the bottom duct  42 . The combustion chamber  41  may be coupled to the bottom duct  42  by being inserted therein. For this, the diameter of the bottom duct  42  may be provided to be larger than the diameter of the rear end of the combustion chamber  41 . In addition, due to the difference between the diameter of the rear end of the combustion chamber  41  and the diameter of the bottom duct  42 , air outside the combustion chamber  41  may be introduced into the bottom duct  42 . 
     The air heated in the combustion chamber  41  is guided to the hot air outlet  45  through the bottom duct  42  and the rear duct  43 . The rear duct  43  is provided to be a predetermined distance apart from the rear support plate  23 , thus forming a rear flow passage  44  through which air may move. Meanwhile, a gas sensor  133  for detecting a gas leak may be provided at one side of the combustion chamber  41 . The gas sensor  133  detects whether gas is leaked or not. When a gas leak is detected, the clothing dryer  1  controls a valve assembly ( 120  in  FIG. 6 ) to prevent gas from being discharged to a mixing tube ( 131  in  FIG. 6 ). The gas sensor  133  may be implemented using a catalytic combustion sensor, a semiconductor sensor, a ceramic sensor etc., but is not limited thereto. 
     The hot air outlet  45  may be provided at an upper end of the rear support plate  23 . The heated air that has moved along the bottom duct  42  and the rear duct  43  is introduced into the drum  20  through the hot air outlet  45 , thus absorbing moisture of an object to be dried inside the drum  20 . A first temperature sensor  49  may be provided at the rear duct  43 . The first temperature sensor  49  detects a temperature of air guided into the drum  20  through the rear flow passage  44 . Here, the first temperature sensor  49  may be implemented using a catalytic temperature sensor or a non-catalytic temperature sensor. Specifically, the temperature sensor may be implemented using at least one of a resistance temperature detector (RTD) temperature sensor which uses a change in resistance of a metal in accordance with a change in temperature, a thermistor temperature sensor which uses a change in resistance of a semiconductor in accordance with a change in temperature, a thermocouple temperature sensor which uses an electromotive force generated at both ends of a junction point of two types of metallic lines formed of different materials, and an IC temperature sensor which uses voltages at both ends of a transistor changing in accordance with a temperature or current and voltage characteristics of a P-N junction portion. However, the temperature sensor is not limited thereto and may employ all possible means for detecting a temperature. 
     In addition, although the first temperature sensor  49  is illustrated in  FIG. 2  as being provided at an upper portion of the rear duct  43 , the position of the first temperature sensor  49  is not limited thereto. For example, the first temperature sensor  49  may be disposed at a lower portion of the rear duct  43 , the bottom duct  42 , or the hot air outlet  45 . The hot air discharge unit  50  guides the discharge of air in the drum  20 . The high-temperature, dry air introduced into the drum  20  through the hot air outlet  45  changes to a low-temperature, humid air as the air absorbs moisture of the object to be dried. The low-temperature, humid air may be discharged to the outside through the hot air discharge unit  50 . The hot air discharge unit  50  may be provided at a lower end of the front support plate  22 .  FIG. 3  is a view schematically illustrating a front support plate of the clothing dryer according to an embodiment.  FIG. 4  is a view illustrating in detail a guide member of the clothing dryer according to an embodiment. Referring to  FIGS. 2 to 4 , the hot air discharge unit  50  may include a guide member  51  for guiding an introduction of air in the drum  20  into a front flow passage  53 , and a front duct  54  which forms the front flow passage  53 . The front flow passage  53  may be formed by the front duct  54  provided at a lower portion of a front surface of the clothing dryer  1 . 
     The guide member  51  may be provided at a lower side of the front support plate  22  to guide the introduction of the air in the drum  20  into the front flow passage  53 . A plurality of air inlets  52  may be provided at the guide member  51 , and the air in the drum  20  may be guided to the front flow passage  53  through the plurality of air inlets  52 . A filter member  55  filters foreign substances such as dust or lint included in the air introduced into the front flow passage  53 . A handle  56  which facilitates detachment of the filter member  55  may be provided above the filter member  55 . 
     A blowing device  60  circulates air in the clothing dryer  1 . The blowing device  60  includes a fan casing  61 , a blowing fan  62  provided in the fan casing  61 , and a driving motor  63  which rotates the blowing fan  62 . A front end of the fan casing  61  is connected to the front flow passage  53 , and a rear end of the fan casing is connected to an exhaust duct  67 . The driving motor  63  has a driving shaft extending forward and connected to the blowing fan  62 . Thus, the blowing fan  62  may rotate by the driving motor  63 . Also, the driving shaft of the driving motor  63  may extend backward and be connected to a pulley  64  for driving the drum  20 . Since the pulley  64  and the drum  20  are connected by a belt  65 , the drum  20  may rotate by rotation of the driving motor  63 . That is, the drum  20  and the blowing fan  62  may rotate at the same time by the driving motor  63 . 
     The blowing fan  62  rotates inside the fan casing  61  to generate an air flow inside the clothing dryer  1 . The air in the front flow passage  53  is discharged out of the clothing dryer  1  through the exhaust duct  67  by the rotation of the blowing fan  62 . When the air in the front flow passage  53  is discharged through the exhaust duct  67 , the pressure in the front flow passage  53  decreases, and air in the drum  20  moves into the front flow passage  53 . Also, the introduction of air heated by the combustion device  100  increases when the pressure of the air in the drum  20  decreases. That is, a supply of heated air from the hot air supply unit  40  is facilitated when the blowing fan  62  rotates. 
     Meanwhile, a sensor mounting unit  58  on which a dryness level detection unit  210  is mounted may be provided at the guide member  51 . Here, the dryness level detection unit  210  is an element that generates an electrical signal in accordance with the amount of moisture contained in an object to be dried. The dryness level detection unit  210  will be described in detail herein. 
     A second temperature sensor  59  may be provided at the front duct  54 . The second temperature sensor  59  detects a temperature of air guided to the front duct  54 . Here, the second temperature sensor  59  may be implemented using a catalytic temperature sensor or a non-catalytic temperature sensor. In addition, although the second temperature sensor  59  is illustrated in  FIG. 2  as being provided at the front duct  54 , the position of the second temperature sensor  59  is not limited thereto. For example, the second temperature sensor  59  may be provided at one surface of an inside of the guide member  51 . 
     The combustion device  100  heats air to generate hot air. Here, a heating power of the combustion device  100  may be controlled to control the temperature of the hot air. The heating power increases when a great amount of gas is combusted, and the temperature of the hot air increases when the heating power increases. The heating power decreases when a small amount of gas is combusted, and the temperature of the hot air decreases when the heating power decreases. The heating power of the combustion device  100  may be controlled in multiple stages. For example, the combustion device  100  may be controlled to be one operation mode among a high heating power mode, a low heating power mode, and a standby mode. The high heating power mode refers to a state in which the gas discharge amount is the maximum, and is a mode having the highest heating power. The low heating power mode refers to a state in which a smaller amount of gas is discharged compared to the gas discharge amount in the high heating power mode, and is a mode having a heating power lower than that in the high heating power mode, such as 50% or less heating power compared to that in the high heating power mode. The standby mode refers to a state in which the gas discharge is blocked and gas is not combusted in the combustion device  100 . 
     Since the mode of the combustion device  100  changes in accordance with the amount of gas discharged to the combustion device  100 , the mode of the combustion device  100  may be determined by the valve assembly  120  which controls the amount of gas discharged to the combustion device  100 . Hereinafter, each configuration of the combustion device  100  will be described in detail. 
       FIG. 5  is a view schematically illustrating a combustion device of the clothing dryer according to an embodiment.  FIG. 6  is a view illustrating another embodiment of an igniter of a combustion device  100   a  of the clothing dryer according to an embodiment.  FIG. 7  is a view for describing a gas supply of a valve assembly.  FIG. 8  is an exploded perspective view of the valve assembly  120  for describing an embodiment of an output control valve. Referring to  FIGS. 5 to 8 , the combustion device  100  may include a valve support  110 , the valve assembly  120  for discharging gas, and a combustion unit  130  for combusting gas. The valve support  110  is coupled to the valve assembly  120  to support the valve assembly  120 . The valve support  110  includes a first support  111  extending from one end of the valve support  110  to support the valve assembly  120 , and a second support  115  extending from the one end of the valve support  110  to support the other end of the valve assembly  120 . 
     The first support  111  and the second support  115  may respectively include inclined surfaces  112  and  116  extending from one end of the valve support  110  and vertically bent to be parallel to a gas injection direction. The second support  115  extends in a shorter length compared to the first support  111 , and the inclined surface  116  of the second support  115  is positioned at a lower side than the inclined surface  112  of the first support  111 . The valve support  110  and the valve assembly  120  may be coupled by screws. For the screw coupling, screw fastening holes  113  and  117  may be formed at the inclined surface  112  of the first support  111  and the inclined surface  116  of the second support  115 , and screw fastening holes may also be formed at the valve assembly  120  at positions corresponding to the screw fastening holes  113  and  117  of the valve support  110 . 
     The valve assembly  120  may control the discharge amount of gas to control the heating power of the combustion device  100 . The valve assembly  120  may control the gas discharge amount in multiple stages in accordance with the operation modes. Specifically, the valve assembly  120  may maximize the gas discharge amount in the high heating power mode, decrease the gas discharge amount such that the heating power is 50% or less compared to the high heating power mode in the low heating power mode, and block the gas discharge in the standby mode. The gas discharge amount of the valve assembly  120  in accordance with the operation modes is controlled in accordance with the open rate of the valve assembly  120 . The open rate is an index which shows an extent to which the valve is open. For example, the open rate is 100% when the valve is completely open, and the open rate is 0% when the valve is closed. 
     That is, the gas discharge amount increases and the heating power increases as the open rate of the valve assembly  120  is higher, and the gas discharge amount decreases and the heating power decreases as the open rate of the valve assembly  120  is lower. The open rate of the valve assembly  120  may be set as one of a plurality of predetermined open rates. For example, the open rate of the valve assembly  120  may be set as one of 100%, 30%, and 0%. When the valve assembly  120  has the above open rates, the open rate may be 100% in the high heating power mode, 30% in the low heating power mode, and 0% in the standby mode. Specifically, the valve assembly  120  may include a decompressor  122 , a plurality of safety valves  123  and  124 , and an output control valve  125 . 
     The valve assembly  120  receives gas through a gas inlet  121 . The gas inlet  121  is connected to a gas tube  140  to which gas is guided from a gas supply source outside the housing  10 . The decompressor  122  controls the pressure of the gas introduced through the gas inlet  121 . Since the pressure of the gas introduced through the gas inlet  121  is high, the pressure of the gas needs to be lowered to a pressure suitable for combustion. The decompressor  122  decreases the pressure of the gas supplied through the gas inlet  121 . The gas, whose pressure is lowered by the decompressor  122 , is applied to the output control valve  125 . The plurality of safety valves  123  and  124  may control whether the gas is discharged or not. That is, the gas is discharged only when both of a first safety valve  123  and a second safety valve  124  are open. The combustion device  100  with the plurality of safety valves  123  and  124  is provided, such that an accident due to a gas leak may be prevented. 
     The output control valve  125  may be provided between the first safety valve  123  and the decompressor  122 . The output control valve  125  may control the open rate of the valve assembly  120 . That is, the output control valve  125  may control the heating power of the combustion device  100 . Since the gas discharge amount of the valve assembly  120  is controlled in accordance with the open rate, and the heating power is controlled in accordance with the gas discharge amount as mentioned above, the output control device may control the open rate of the valve to control the heating power of the combustion device  100 . The output control valve  125  may be implemented using a solenoid valve. Hereinafter, an embodiment of the output control valve  125  will be described in detail with reference to  FIG. 8 . The output control valve  125  is formed of an orifice  125   b  and a valve body  125   a.  A coil is provided in the valve body  125   a.  A magnetic field is formed when current flows through the coil in the valve body  125   a,  and the orifice  125   b  may move by the magnetic field. 
     The orifice  125   b  may move back and forth along an axis parallel to a gas flow passage  129 . Specifically, the orifice  125   b  moves forward toward the gas flow passage  129  when the magnetic field is formed in the valve body  125   a,  and the orifice  125   b  moves backward toward the valve body  125   a  when the magnetic field is not formed in the valve body  125   a.  When the orifice  125   b  is positioned in the valve body  125   a,  the gas flow passage  129  is completely open and has a high first open rate (such as 100%). However, when the orifice  125   b  moves forward toward the gas flow passage  129 , the gas flow passage  129  is closed by the orifice  125   b.  In addition, gas moves along an inner flow passage  125   c  in the orifice  125   b.  That is, the gas flow passage  129  is closed by the orifice  125   b,  causing the open rate of the gas flow passage  129  to be lowered to a second open rate (such as 30%). When the open rate is lowered from the first open rate to the second open rate by the orifice  125   b,  the amount of gas discharged to the combustion unit  130  reduces, such that the heating power of the combustion device  100  decreases. 
     In other words, the output control valve  125  decreases the open rate of the gas flow passage  129  to a predetermined second open rate. Here, the predetermined second open rate may be determined according to the size and structure of the orifice  125   b.  For example, the second open rate may be determined according to the size of the inner flow passage  125   c  formed in the orifice  125   b.  The clothing dryer  1  may control the output control valve  125  to control the heating power mode. Specifically, the clothing dryer  1  may control back and forth motions of the orifice  125   b  of the output control valve  125  to control the operation mode. In other words, since the gas flow passage  129  is completely open when the orifice  125   b  moves forward, the gas discharge amount of the combustion device  100  becomes the maximum, and the combustion device  100  operates in the high heating power mode. 
     Since the gas flow passage  129  is partially closed when the orifice  125   b  moves backward, the gas discharge amount of the combustion device  100  decreases, and the combustion device  100  operates in the low heating power mode. As described herein, since the heating power may be controlled by controlling the back and forth motions of the orifice  125   b,  the combustion device  100  may control the heating power without repeating extinction and ignition. Meanwhile, when all of the safety valves are open, the gas may be discharged through a gas outlet  126 . Here, a front end portion of the mixing tube  131  may be positioned in the combustion chamber  41 . The gas discharged through the gas outlet  126  is mixed with air in the mixing tube  131 . An igniter  132  may be provided at the front end portion of the mixing tube  131 . The igniter  132  ignites the gas mixed with air. The ignited gas heats surrounding air while being continuously combusted with air. 
     The igniter  132  applies a temperature higher than the ignition point of gas to ignite the gas mixed with air. The igniter  132  may be a heated type igniter  132  which is heated up to a temperature greater than the ignition point of the gas as illustrated in  FIG. 5 , but is not limited thereto. For example, the igniter  132  may be implemented using an ignition plug  132   a  which generates and ignites electric flames as illustrated in  FIG. 6 . The operation of the clothing dryer  1  is described in detail herein. 
       FIG. 9  is a control block diagram for describing in detail an operation of the clothing dryer according to an embodiment. Referring to  FIG. 9 , the clothing dryer  1  may include the dryness level detection unit  210  for detecting the dryness level of an object to be dried, a state detection unit  220  for detecting a state of the clothing dryer  1 , the control panel  230  for receiving a control command from a user and providing information to the user, a storage unit  240  for storing data to operate the clothing dryer  1 , and a control unit  270  for controlling the overall operation of the clothing dryer  1 . The dryness level detection unit  210  may detect the dryness level of an object to be dried. The dryness level detection unit  210  may be installed in the drum  20  and come in contact with the object to be dried which rotates in the drum  20  in order to detect the dryness level of the object to be dried. Here, the dryness level is an index which shows an extent to which the moisture included in the object to be dried is dried. A higher dryness level signifies that a greater amount of moisture is included in the object to be dried, and a lower dryness level signifies that a lesser amount of moisture is included in the object to be dried. Hereinafter, an embodiment of the dryness level detection unit  210  will be described with reference to  FIGS. 3 and 4 . 
     Referring to  FIGS. 3 and 4 , the dryness level detection unit  210  may include a first electrode  211  and a second electrode  212 . The first electrode  211  and the second electrode  212  may be provided at the guide member  51 . The first electrode  211  and the second electrode  212  are mounted on the sensor mounting unit  58  provided at the guide member  51 . The first electrode  211  and the second electrode  212  may be provided to be curved in accordance with the shape of the sensor mounting unit  58 . The first electrode  211  and the second electrode  212  may be mounted on the sensor mounting unit  58  formed at the guide member  51  while being spaced apart from each other. Since the first electrode  211  and the second electrode  212  are provided to be a predetermined distance apart from each other, the first electrode  211  and the second electrode  212  remain electrically open to each other. 
     When an object to be dried which has moisture comes in contact with the first electrode  211  and the second electrode  212  at the same time, the first electrode  211  and the second electrode  212  are shorted by the moisture included in the object to be dried, causing current to flow between the first electrode  211  and the second electrode  212 . That is, a current pulse is generated at the first electrode  211  and the second electrode  212  by the object to be dried which has moisture. Thus, the dryness level detection unit  210  may detect the dryness level of the object to be dried based on the current pulse generated by the moisture of the object to be dried.  FIG. 10  is a view illustrating a pulse generation frequency in accordance with time of a dryness level detector according to an embodiment. 
     When the clothing dryer  1  operates, the frequency of the current pulse generated at the first electrode  211  and the second electrode  212  may change as illustrated in  FIG. 10 . The object to be dried has high dryness level at an early stage of drying. Thus, the current pulse is frequently generated by the object to be dried when the drum  20  begins rotating, and the frequency of the current pulse generation is also high as illustrated in  FIG. 10 . Meanwhile, humidity of the object to be dried gradually decreases as the object to be dried is dried by the hot air. Thus, as the object to be dried is dried, the frequency of the current pulse generation gradually decreases as illustrated in  FIG. 10 . 
     In other words, the dryness level detection unit  210  may detect a change in the dryness level of the object to be dried based on the frequency of the current pulse generation. Since the moisture content in the object to be dried is high when the dryness level is low, a current flow frequently occurs between the first electrode  211  and the second electrode  212 , such that the frequency of the current pulse generation is high. Since the frequency of the current flow generation between the first electrode  211  and the second electrode  212  decreases when the object to be dried is dried and the dryness level thereof is lowered, the frequency of the current pulse generation decreases. Although the first electrode  211  and the second electrode  212  of the dryness level detection unit  210  have been described as being provided at the guide member  51 , the position of the dryness level detection unit  210  is not limited thereto. 
     In addition, although the first electrode  211  and the second electrode  212  are illustrated as having curved shapes in  FIGS. 3 and 4 , the shapes of the first electrode  211  and the second electrode  212  are not limited thereto. For example, the first electrode  211  and the second electrode  212  may be formed in the shape of a rod. The state detection unit  220  may detect the state of the clothing dryer  1 . Here, the state of the clothing dryer  1  refers to various types of information such as a temperature of air in the clothing dryer  1  and a gas ignition state to operate the clothing dryer  1 . The state detection unit  220  may include a temperature sensor for detecting the temperature of air, the gas sensor  133  for detecting a gas leak, etc. Specifically, the state detection unit  220  may detect the temperature of air introduced into the drum  20  using the first temperature sensor  49  and detect the temperature of air discharged from the drum  20  using the second temperature sensor  59 . 
     In addition, the state detection unit  220  may detect whether gas is leaked or not based on the gas sensor  133 . For example, the gas sensor  133  may be provided in the combustion chamber  41  as a catalytic combustion sensor as illustrated in  FIG. 2  to detect the temperature in the combustion chamber  41 . When the temperature in the combustion chamber  41  detected by the catalytic combustion sensor is lower than a preset temperature even though gas is being discharged to the combustion chamber  41 , the state detection unit  220  may detect that gas is leaking. The control panel  230  may receive a control command from a user or provide information related to the operation of the clothing dryer  1  to the user. The control panel  230  may be provided at an upper side of the front surface of the clothing dryer  1  as illustrated in  FIG. 1  to be easily manipulated by the user. 
     Specifically, the control panel  230  may include an input unit  231  which receives a control command from the user. The user may select one drying course among a plurality of preset drying courses through the input unit  231 . The drying courses may be classified in accordance with the type, weight, etc., of the object to be dried. Also, the drying courses may be classified in accordance with energy efficiency or a target dryness level. Here, the target dryness level refers to the final dryness level of the object to be dried after drying is ended. The moisture content included in the object to be dried that has reached the target dryness level is low. 
     The input unit  231  may be implemented using devices such as a touch sensor, a push button, a membrane button, a dial, and a slider switch. Here, the touch sensor is a device which detects a touch input of a user, and an electrostatic capacitive technology, a resistance type technology, an infrared ray technology, and a surface acoustic wave technology may be used for the touch sensor, but the technologies are not limited thereto. In addition, the control panel  230  may include a display unit  232  for displaying information to the user. The display unit  232  may display a state of the clothing dryer  1  or a time remaining until drying is finished. The display unit  232  may be implemented using display means such as a plasma display panel, a liquid crystal display panel, a light-emitting diode panel, an organic light-emitting diode panel, or an active organic light-emitting diode panel, but is not limited thereto. 
     The storage unit  240  stores various types of data for the operation of the clothing dryer  1 . For example, the storage unit  240  may store firmware or various types of applications for the operation of the clothing dryer  1 . In addition, a drying algorithm may be stored in the storage unit  240 . The drying algorithm is related to a procedure for drying an object to be dried. A proper drying procedure differs in accordance with characteristics of the object to be dried such as a material of the object to be dried or the amount of object to be dried. Drying algorithms may be different for each of the drying courses mentioned above. The storage unit  240  may include a high-speed random access memory (RAM), a magnetic disk, a static RAM (S-RAM), a dynamic RAM (D-RAM), a read-only memory (ROM), etc., but is not limited thereto. 
     A blowing device operation unit  250  may operate the blowing device  60  in accordance with a control signal of the control unit  270 . Specifically, the blowing device operation unit  250  may rotate the driving motor  63  in accordance with the control signal of the control unit  270  to rotate the blowing fan  62  and the drum  20 . Here, a rotation speed and a rotation direction of the driving motor  63  may be controlled by the blowing device operation unit  250 . When the driving motor  63  rotates by the blowing device operation unit  250 , the blowing fan  62  rotates such that humid air in the drum  20  is discharged through an air inlet, and dry, hot air is introduced into the drum  20  through the hot air outlet  45  due to the pressure difference. Also, when the drum  20  rotates in accordance with the rotation of the driving motor  63 , the object to be dried in the drum  20  is dried by the dry, hot air while being lifted and lowered repetitively. 
     A combustion device operation unit  260  may operate the combustion device  100  in accordance with the control signal of the control unit  270 . Hereinafter, an embodiment of the combustion device operation unit  260  will be described in detail.  FIG. 11  is a view for describing a combustion device operation unit of the clothing dryer according to an embodiment.  FIG. 12  is a view for describing an operation at the time of ignition of the combustion device operation unit in  FIG. 11 . Referring to  FIGS. 11 and 12 , the combustion device operation unit  260  may include a plurality of switches  271  and  276 , a plurality of coils  272 ,  273 , and  274 , and a variable resistor  275 . A first switch  271  may be in an off-state and a second switch  276  provided at the igniter  132  may remain in an on-state at an initial state. Here, the first switch  271  may be turned on or off in accordance with the control signal of the control unit  270 , and the second switch  276  may be turned on or off in accordance with the temperature of the igniter  132 . 
     When the first switch  271  is converted to an on-state in accordance with an ignition control signal of the control unit  270 , voltage is applied to a first valve coil  272 , a booster coil  273 , and the variable resistor  275 . When voltage is applied to the first valve coil  272 , the first safety valve  123  is opened by a magnetic field generated at the first valve coil  272 . Also, when voltage is applied to the variable resistor  275 , the igniter  132  is heated by resistive heat. Here, the resistance value of the variable resistor  275  may be controlled. When the igniter  132  is heated by the variable resistor  275  and the igniter  132  reaches a preset ignition temperature, the second switch  276  is converted to an off-state as illustrated in  FIG. 12 . Here, the ignition temperature is set as a temperature higher than the ignition point of gas. 
     When the igniter  132  reaches the ignition temperature and the second switch  276  is converted to the off-state, voltage is applied to a second valve coil  274 . When voltage is applied to the second valve coil  274 , a magnetic field is generated at the second valve coil  274 . The second safety valve  124  is opened by the magnetic field formed at the second valve coil  274 . Since the first safety valve  123  and the second safety valve  124  are both opened when the second switch  276  is converted to the off-state, gas is discharged through the gas outlet  126 . The discharged gas is mixed with air in the mixing tube  131 , and the gas mixed with air is ignited by the igniter  132  having a temperature higher than the ignition point of the gas. Here, the output control valve  125  operates in the high heating power mode. Specifically, the output control valve  125  may maintain the first open rate and discharge gas with the maximum output, thus facilitating gas ignition. 
     The control unit  270  controls the overall operation of the clothing dryer  1 . The control unit  270  may be one or more processors. Here, the one or more processors may be implemented by a plurality of arrays of logic gates or by a combination of a universal microprocessor and a memory in which a program capable of being executed in the microprocessor is stored. The control unit  270  may control operation units to dry the object to be dried. The control unit  270  may operate each configuration in accordance with a drying algorithm stored in the storage unit  240 . Specifically, the control unit  270  may operate each configuration in accordance with a drying algorithm corresponding to a drying course input through the control panel  230 . 
     In addition, the control unit  270  may control each configuration based on a state of the clothing dryer  1  detected in the state detection unit  220 . Specifically, the control unit  270  may control the operation mode of the combustion device  100  based on values detected in the first temperature sensor  49  and the second temperature sensor  59 . In addition, the control unit  270  may determine the amount of objects to be dried based on the dryness level detected in the dryness level detection unit  210 , and perform a drying algorithm in accordance with the amount of the objects to be dried. Also, the control unit  270  may analyze drying characteristics of the object to be dried, and perform a drying algorithm in accordance with the analyzed drying characteristics of the object to be dried. For example, the control unit  270  may determine a temperature of hot air or determine a time at which the hot air will be supplied in accordance With the drying characteristics of the object to be dried. Hereinafter, a method of controlling a clothing dryer will be described in detail with reference to  FIG. 13 . 
       FIG. 13  is a flow chart for describing an embodiment of a method of controlling the clothing dryer according to an embodiment.  FIG. 14  is a view for describing an air flow in a drying process of the clothing dryer according to an embodiment. Referring to  FIGS. 2, 9, and 13 , the user may insert an object to be dried into the drum  20  and use the control panel  230  to set a drying course at step  510 . Here, the drying course may be classified in accordance with the type of the object to be dried, but is not limited thereto. For example, the drying course may also be classified in accordance with the target dryness level, characteristics of the object to be dried, etc. 
     The clothing dryer  1  determines whether an operation command has been input from the user at step  520 . When a drying command is input (YES at step  520 ), the clothing dryer  1  measures the amount of the objects to be dried at step  530 . Although there are no limitations to a method of measuring the amount of the object to be dried, the amount of the objects to be dried may be measured based on the dryness level detected in the dryness level detection unit  210 . For example, the amount of the objects to be dried may be determined to be greater as the frequency of the current pulse generation is higher, and the amount of the objects to be dried may be determined to be smaller as the frequency of the current pulse generation is lower. However, measuring the amount of the objects to be dried may be omitted when the user has input the amount of the objects to be dried. 
     The clothing dryer  1  begins an ignition process at step  540 . Specifically, the control unit  270  may control the valve assembly  120  to discharge gas, and apply a temperature higher than the ignition point of the gas to the gas being discharged to ignite the gas. Here, the valve assembly  120  may discharge the gas with the maximum output. The clothing dryer  1  begins a drying process at step  550 . When the drying process begins, the control unit  270  controls the driving motor  63  to rotate a circulation fan and the drum  20 , and controls the combustion device  100  to heat air. The air circulates in the drying process as illustrated in  FIG. 14 . 
     Specifically, the gas discharged from the valve assembly  120  is combusted in the combustion chamber  41  after passing through the mixing tube  131 . The air around the combustion chamber  41  is heated by the combustion of gas. The heated air is introduced into the drum  20  along the rear duct  43 . The air introduced into the drum as above absorbs the moisture of the object to be dried which is lifted and lowered repetitively. The air that has absorbed the moisture is suctioned by the blowing device  60  and discharged through an exhaust tube. The pressure in the drum  20  decreases as the humid air in the drum  20  is discharged to the outside as above, thus further accelerating the introduction of the air heated in the combustion chamber  41 . 
     The clothing dryer  1  begins a cooling process at step  560 . Since the object to be dried is dried by the hot air generated in the combustion chamber  41 , the temperature of the object to be dried is higher when drying is finished. Thus, the temperature in the drum  20  should be lowered through the cooling process. The control unit  270  may close the safety valves of the valve assembly  120  to stop the combustion of gas, and drive the driving motor  63  to emit the hot air in the drum  20  to the outside. Meanwhile, although the step  520  is illustrated in  FIG. 13  as being performed before the ignition process, embodiments are not limited thereto. For example, the step  520  may be performed during the ignition process or the drying process. 
     Hereinafter, an embodiment of an ignition process will be described in detail with reference to  FIG. 15 .  FIG. 15  is a flow chart for describing an embodiment of an ignition process in  FIG. 10 . 
     Referring to  FIGS. 2, 9, and 15 , the clothing dryer  1  opens the first safety valve  123  at step  511 , and heats the igniter  132  at step  512 . As illustrated in  FIG. 11 , when the first switch  271  is converted to the on-state by the control command of the control unit  270 , voltage is applied to the first valve coil  272  and a magnetic field is generated. The first safety valve  123  is opened by the magnetic field generated at the first valve coil  272 . Also, when the first switch  271  is converted to the on-state, voltage is applied to the variable resistor  275 , and the igniter  132  is heated. 
     The clothing dryer  1  determines whether the temperature of the igniter  132  is greater than the ignition temperature at step  513 . When the temperature of the igniter  132  reaches the ignition temperature (YES at step  513 ), the second safety valve  124  is opened at step  514 . As illustrated in  FIG. 12 , the igniter  132  reaches the ignition temperature, and the second switch  276  is converted to the off-state by the igniter  132 . When the second switch  276  is converted to the off-state, voltage is applied to the second valve coil  274 , and the second safety valve  124  is opened by the magnetic field generated at the second valve coil  274 . 
     That is, gas is discharged only when both of the first safety valve  123  and the second safety valve  124  are opened. The gas discharged to the mixing tube  131  is mixed with air in the mixing tube  131 . Here, since the igniter  132  has the ignition temperature higher than the ignition point of the gas, the gas which is mixed with air and discharged begins to be combusted by the igniter  132 . Meanwhile, the clothing dryer  1  may operate in the high heating power mode at the time of ignition. Specifically, the output control valve  125  remains opened to maximize the gas discharge amount. The clothing dryer  1  determines whether the gas ignition has succeeded at step  515 . There are no limitations to a method of determining whether the gas ignition has succeeded. For example, the control unit  270  may determine that the gas ignition has succeeded when the temperature of air detected in the first temperature sensor  49  is a preset temperature, or determine that the ignition has succeeded as long as a gas leak is not detected by the gas sensor  133 . 
     When the gas ignition is determined to be successful, at step  516  the clothing dryer  1  closes the output control valve  125 . When the output control valve  125  is closed, the open rate of the valve assembly  120  decreases, and the gas discharge amount decreases due to the decrease in the open rate. That is, when the ignition of the clothing dryer  1  is finished, the operation mode is changed from the high heating power mode to the low heating power mode. Meanwhile, when the gas ignition is determined to have failed, the clothing dryer  1  initializes the safety valves at step  517 , and returns to the step  511  and begins the ignition process again. Specifically, the control unit  270  opens the first switch  271  and closes both of the first safety valve  123  and the second safety valve  124 . In addition, since it is preferable that the gas discharge amount be set high at the time of ignition, the control unit  270  opens the output control valve  125  and changes the operation mode to the high heating power mode. 
       FIG. 16  is a view illustrating a temperature change in air when exhaust blockage has occurred.  FIG. 17  is a view for describing an embodiment of a re-ignition process. Meanwhile, the clothing dryer  1  may perform the re-ignition process in the drying process. The re-ignition process may be used in the drying process for various reasons. For example, the temperature of the hot air supplied to the drum  20  may be excessively high and the combustion may be stopped to prevent the object to be dried from being damaged. The re-ignition process can be used when the combustion is stopped as the above. In addition, the combustion may be stopped unintentionally due to exhaust blockage, etc. The control unit  270  may detect an unintentional stop of the combustion due to the exhaust blockage, etc. and perform re-ignition. 
     For example, when a temperature value detected in the first temperature sensor  49  drops below a combustion determination temperature F as illustrated in  FIG. 16 , it may be determined that the combustion has stopped and the re-ignition process may begin. Here, the combustion determination temperature F may be preset. Hereinafter, the re-ignition process will be described while focusing on differences with the ignition process. 
     Referring to  FIG. 17 , the clothing dryer  1  initializes valve states at step  611 . Since the valve states when the combustion is finished are unclear, the control unit  270  controls the valve states to be initialized. Specifically, the control unit  270  closes the first safety valve  123  and the second safety valve  124  and opens the output control valve  125  to control the clothing dryer  1  to operate in the high heating power mode. The clothing dryer  1  opens the first safety valve  123  at step  612 , and heats the igniter  132  at step  613 . When the temperature of the igniter  132  becomes greater than the ignition temperature (YES at step  614 ), the second safety valve  124  is opened at step  615 . When the second safety valve  124  is opened, gas is discharged to the mixing tube, and the discharged gas is ignited by the igniter  132 . 
     When it is determined that the gas ignition has succeeded (YES at step  616 ), the clothing dryer  1  closes the output control valve  125  at step  617 . That is, the operation mode may be changed from the high heating power mode to the low heating power mode when the ignition is finished. However, the high heating power mode may be continuously maintained as needed. Meanwhile, when it is determined that the gas ignition has failed, the clothing dryer  1  initializes the valve states again at step  611 . The clothing dryer  1  may control the valve assembly  120  to control combustion modes. Hereinafter, the drying process will be described in detail.  FIG. 18  is a flow chart for describing in detail an embodiment of the drying process in  FIG. 13 .  FIG. 19  is a graph illustrating a change in open rates in the drying process of  FIG. 18 . 
     Referring to  FIG. 18 , the clothing dryer  1  dries the object to be dried in the low heating power mode at step  621 . At an initial stage of drying, the object to be dried contains a great amount of moisture. When the object to be dried contains a great amount of moisture, the moisture of the object to be dried may be efficiently removed even when low-temperature hot air is used. Thus, the clothing dryer  1  may operate in the low heating power mode at the initial stage of drying, thus increasing the gas efficiency of the clothing dryer  1 . 
     The valve assembly  120  maintains a first open rate  01  in the low heating power mode as illustrated in  FIG. 19  and discharges a smaller amount of gas compared to the high heating power mode. Here, the first open rate O 1  may be 50% or less of a second open rate O 2 . For example, the first open rate O 1  may be 30%. Specifically, the output control valve  125  maintains the on-state in the low heating power mode. When the output control valve  125  is turned on, the orifice  125   b  moves forward into the gas flow passage  129 . When the orifice  125   b  moves forward into the gas flow passage  129 , the gas flow passage  129  is closed by the orifice  125   b,  and the gas moves along the inner flow passage  125   c  provided in the orifice  125   b.  When the open rate of the gas flow passage  129  becomes the first open rate as above, the amount of gas discharged to the mixing tube  131  also decreases, such that the heating power of the combustion device  100  decreases and the hot air of low temperature is generated. 
     The clothing dryer  1  detects the dryness level of the object to be dried at step  622 . The dryness level detection unit  210  may detect the dryness level every predetermined period. For example, the dryness level detection unit  210  may count a number of operation pulses generated during a predetermined time (such as one minute) and calculate the dryness level of the object to be dried based on the number of operation pulses generated. The clothing dryer  1  determines whether the detected dryness level is below the reference dryness level at step  623 . When the amount of moisture contained in the object to be dried drops below a predetermined level by the low heating power mode, the object to be dried is not dried well with the hot air of low temperature. As above, the dryness level at which the object to be dried is not dried well with the hot air of low temperature is referred to as the reference dryness level. The reference dryness level may be preset, and may be set differently in accordance with the amount of the objects to be dried and the characteristics of the object to be dried. 
     When the detected dryness level is below the reference dryness level (YES at step  623 ), the clothing dryer  1  is converted to the high heating power mode at step  624 . The control unit  270  opens the output control valve  125 . As illustrated in  FIG. 8 , the orifice  125   b  that was blocking the flow passage moves backward toward the valve body  125   a  when the output control valve  125  is opened. When the orifice  125   b  moves backward, the open rate of the gas flow passage  129  increases. When the open rate of the gas flow passage  129  increases, the amount of gas discharged to the mixing tube  131  increases, such that the heating power of the combustion device  100  increases and hot air of high temperature is generated. 
     The clothing dryer  1  detects the dryness level of the object to be dried at step  625 , and determines whether the detected dryness level is below the target dryness level at step  626 . When the detected dryness level is below the target dryness level (YES at step  626 ), the clothing dryer  1  closes the safety valves at step  627 . When the safety valves are closed, the gas discharge stops. Also, the clothing dryer  1  performs the cooling process of cooling the object to be dried. That is, the clothing dryer  1  is converted to the standby mode. Here, the target dryness level refers to the dryness level at which drying is finished, and may be preset. Same as the reference dryness level, the target dryness level may also be set differently in accordance with the amount of the objects to be dried or the characteristics of the object to be dried. In addition, the target dryness level may also be set differently for each drying course. For example, the target dryness level may be set higher than that of a normal drying course when an anti-wrinkle function is selected in order to prevent the object to be dried from being wrinkled. 
     Meanwhile, although it has been described in  FIG. 18  that the drying process is performed based on a change in the dryness level, the drying process may also be performed based on time. Hereinafter, a drying process performed based on time will be described with reference to  FIG. 20 .  FIG. 20  is a flow chart for describing in detail another embodiment of the drying process in  FIG. 13 . Hereinafter, another embodiment of the drying process will be described while focusing on differences from that in  FIG. 18 . The clothing dryer  1  dries the object to be dried in the low heating power mode at step  631 . As illustrated in  FIG. 19 , the output control valve  125  is turned on and the open rate of the valve assembly  120  is set as the first open rate in the low heating power mode. The clothing dryer  1  determines whether a low-temperature drying time has elapsed at step  632 . 
     When the low-temperature drying time has elapsed (YES at step  632 ), the clothing dryer  1  dries the object to be dried in the high heating power mode at step  633 . Here, the low-temperature drying time may be preset. In addition, the low-temperature drying time may be set differently in accordance with the amount of the objects to be dried or the dryness level detected at the initial stage of drying. For example, the low-temperature drying time may be set longer as the amount of the objects to be dried is greater, or set shorter as the dryness level detected at the initial stage of drying is higher. 
     The clothing dryer  1  determines whether a high-temperature drying time has elapsed at step  634 . As illustrated in  FIG. 19 , the output control valve  125  is turned off and the open rate of the valve assembly  120  increases from the first open rate to the second open rate in the high heating power mode. Here, the second open rate may be 100% at maximum. When the high-temperature drying time has elapsed (YES at step  634 ), the clothing dryer  1  closes the safety valves at step  635 . That is, the clothing dryer  1  is converted to the standby mode. Here, the high-temperature drying time may be preset. In addition, the high-temperature drying time may be set differently in accordance with the amount of the objects to be dried or the dryness level detected at the initial stage of drying. For example, the high-temperature drying time may be set longer as the amount of the objects to be dried is greater, or set shorter as the dryness level detected at the initial stage of drying is higher. In addition, the high-temperature drying time may be set differently in accordance with the change in the dryness level. For example, when the change in the dryness level is great, the control unit  270  may determine that the object to be dried may be easily dried and set the high-temperature drying time to be short. 
     Meanwhile, although it has been described in  FIG. 20  that the drying process is performed in accordance with a preset time, the drying process may also be performed using a combination of the dryness level and the time. For example, the clothing dryer  1  may be converted from the low heating power mode to the high heating power mode when one of the preset reference dryness level condition and the low-temperature drying time condition is satisfied, or converted from the low heating power mode to the high heating power mode when both of the reference dryness level condition and the low-temperature drying time condition are satisfied. In addition, the high heating power mode may end when one of the preset target dryness level condition and the high-temperature drying time condition is satisfied, or the high heating power mode may end when both of the target dryness level condition and the high-temperature drying time condition are satisfied. 
     Hereinafter, an embodiment of a drying process which uses a combination of the dryness level and time conditions will be described with reference to  FIG. 21 .  FIG. 21  is a flow chart for describing in detail still another embodiment of the drying process in  FIG. 13 . Referring to  FIG. 21 , the clothing dryer  1  dries object to be dried in the low heating power mode at step  641 . The clothing dryer  1  detects the dryness level of the object to be dried at step  642 , and determines whether the detected dryness level is below the reference dryness level at step  643 . When the detected dryness level is below the reference dryness level (YES at step  643 ), the clothing dryer  1  dries the object to be dried in the high heating power mode at step  644 . The clothing dryer  1  determines whether the high-temperature drying time has elapsed at step  645 , and when the high-temperature time has elapsed (YES at step  645 ), the clothing dryer  1  closes the safety valves at step  646 . Here, the high-temperature drying time may be determined by the change in the dryness level in the low heating power mode. 
       FIG. 22  is a view for describing another embodiment of mode change in the drying process. Although it has been described though  FIGS. 19 to 21  that the drying process is classified as the low heating power mode which generates the low-temperature hot air in accordance with the first open rate and the high heating power mode which generates the high-temperature hot air in accordance with the second open rate, the modes of the drying process are not limited thereto. That is, as illustrated in  FIG. 22 , the drying process may be configured of more operation modes. Specifically, the drying process may include a first heating power mode which discharges gas at the open rate of 30%, a second heating power mode which discharges gas at the open rate of 60%, and a third heating power mode which discharges gas at the open rate of 100%. 
     Here, the valve assembly  120  may include a plurality of output control valves  125 . For example, the valve assembly  120  may include a first output control valve which lowers the open rate to 30% and a second output control valve which lowers the open rate to 60%. Meanwhile, although it has been described through  FIGS. 19 to 21  that the valve assembly  120  maintains the first open rate in the low heating power mode and maintains the second open rate in the high heating power mode, embodiments are not limited thereto. 
     In one embodiment, when a temperature detected in the first temperature sensor  49  is greater than a first preset critical temperature, the valve assembly  120  may be controlled such that the heating power decreases. The first critical temperature refers to a temperature at which the object to be dried may be damaged. The first critical temperature may be set differently in accordance with the type of the object to be dried. Specifically, the clothing dryer  1  may be converted from the high heating power mode to the low heating power mode when the temperature of air introduced into the drum  20  becomes greater than the first critical temperature, or converted from the low heating power mode to the standby mode to prevent damage to the object to be dried. 
     In another embodiment, when the temperature detected in the first temperature sensor  49  drops below a preset second critical temperature, the clothing dryer  1  may increase the heating power. The second critical temperature refers to a temperature at which the drying efficiency of the object to be dried decreases, and may be set differently in accordance with the type of the object to be dried. Specifically, the clothing dryer  1  may be converted from the standby mode to the low heating power mode or the high heating power mode when the temperature of air introduced into the drum  20  becomes lower than the second critical temperature, or converted from the low heating power mode to the high heating power mode to increase the drying efficiency. 
       FIG. 23  is a view for describing a control for tracking a temperature.  FIG. 24  is a view for describing the drying process which limits an output based on the temperature. In still another embodiment, the clothing dryer  1  may perform the drying process by tracking a preset temperature. Since drying is well-performed even if hot air of a relatively low temperature is supplied at the initial stage of drying, the clothing dryer  1  may operate in a low temperature mode which maintains a low temperature at the initial stage of drying and operate in a high temperature mode which maintains a high temperature after drying is performed to some extent as illustrated in  FIG. 23 . The clothing dryer  1  may control a combustion mode such that hot air of a preset low temperature (such as 35° C.) is generated in the low temperature mode, and control the combustion mode such that hot air of a preset high temperature (such as 55° C.) is generated in the high temperature mode. Specifically, the control unit  270  may change the operation mode to increase the heating power when the actual temperature of hot air is lower than a preset temperature of hot air and change the operation mode to decrease the heating power when the actual temperature of hot air is higher than the preset temperature of hot air, thus tracking the preset hot air temperature. Hereinafter, a method of tracking the hot air temperature by controlling the operation mode will be described in detail with reference to  FIGS. 24 and 25 . 
     Referring to  FIGS. 23 and 24 , the clothing dryer  1  detects a temperature of hot air at step  711 . Since the hot air generated by the combustion device  100  is supplied to the drum  20  along the rear flow passage  44 , the clothing dryer  1  may detect the temperature of hot air using the first temperature sensor  49 , but the method of detecting the hot air temperature is not limited thereto. The clothing dryer  1  determines whether the hot air temperature is lower than a first critical temperature t 1  at step  712 . The first critical temperature tl refers to a minimum maintenance temperature and may be set differently in the low temperature mode and the high temperature mode. For example, the first critical temperature tl may be set as al in the low temperature mode and a 3  in the high temperature mode. When the hot air temperature is below the first critical temperature t 1  (YES at step  712 ), the clothing dryer  1  turns off the output control valve  125  at step  713 . That is, the clothing dryer  1  converts the output control valve  125  to the off-state and increases the open rate of the valve assembly  120  to change from the low heating power mode to the high heating power mode. Since the gas discharge amount increases when the operation mode is changed from the low heating power mode to the high heating power mode, the heating power of the combustion device  100  increases and the hot air temperature also increases. 
     When the hot air temperature is higher than the first critical temperature tl (NO at step  712 ), the clothing dryer  1  determines whether the hot air temperature exceeds a second critical temperature t 2  at step  714 . The second critical temperature t 2  refers to the maximum maintenance temperature and may be set differently in the low temperature mode and the high temperature mode. For example, the second critical temperature may be set as a 2  in the low temperature mode and a 4  in the high temperature mode. Meanwhile, when the hot air temperature is lower than the second critical temperature t 2  (NO at step  714 ), the clothing dryer  1  detects the hot air temperature again at step  711 . That is, the clothing dryer  1  may determine that the hot air temperature is within a reference range and maintain the heating power. When the hot air temperature exceeds the second critical temperature t 2  (YES at step  714 ), the clothing dryer  1  determines whether the output control valve  125  is open at step  715 . When the output control valve  125  is determined to be opened (YES at step  715 ), the clothing dryer  1  closes the output control valve  125  at step  716  and detects the hot air temperature again at step  711 . That is, the clothing dryer  1  may control the output control valve  125  to be turned on and decrease the open rate. Since the gas discharge amount decreases when the open rate decreases, the heating power of the combustion device  100  decreases and the hot air temperature also drops. That is, the clothing dryer  1  is converted from the high heating power mode to the low heating power mode. 
     Meanwhile, when the output control valve  125  is determined to be closed (NO at step  715 ), the clothing dryer  1  closes the safety valves at step  717 , and re-ignites after a predetermined amount of time at step  718 . The object to be dried may be protected by extinguishing the combustion device  100  as above. That is, the clothing dryer  1  is converted from the low heating power mode to the standby mode. Meanwhile, each parameter of the drying process may be adjusted in accordance with a change in a dried amount. Hereinafter, this will be described in detail.  FIG. 25  is a flow chart for describing an embodiment of analyzing characteristics of an object to be dried.  FIG. 26  is a view for describing a change in a dryness level in accordance with the characteristics of an object to be dried. 
     Referring to  FIGS. 25 and 26 , the clothing dryer  1  measures a first dryness level at step  801 . As illustrated in  FIG. 26 , the first dryness level may be measured at a first preset time Q 1 . The clothing dryer  1  measures a second dryness level at step  802 . As illustrated in  FIG. 26 , the second dryness level may be measured at a second preset time Q 2 . The clothing dryer  1  analyzes characteristics of an object to be dried based on the first dryness level and the second dryness level at step  803 . Specifically, the control unit  270  detects a change in the dryness level based on the first dryness level and the second dryness level, and analyzes the characteristics of the object to be dried. That is, a dryness characteristic of the object to be dried may be analyzed. The clothing dryer  1  adjusts parameters based on the characteristics of the object to be dried at step  804 . Here, the parameters refer to various types of variables such as the above-mentioned reference dryness level, target dryness level, low-temperature drying time, high-temperature drying time, critical temperature, and the like which can be used for the drying process. 
     For example, the clothing dryer  1  may analyze the object to be dried having a rapid change in dryness level such as D 1  illustrated in  FIG. 26  as a synthetic fiber, and revise the low-temperature drying time and the high-temperature drying time to be shorter. In addition, since the synthetic fiber is vulnerable to heat, the critical temperature of hot air may be set lower to prevent the synthetic fiber from being damaged by the hot air. In addition, the clothing dryer  1  may determine that the object to be dried having a slow change in dryness level such as D 2  illustrated in  FIG. 26  uses additional drying, and increase the high-temperature drying time or set the reference dryness level or the target dryness level to be lower. 
     As described above, since heating power is controlled in accordance with predetermined open rates, gas efficiency may be increased. In addition, a valve assembly is controlled by only the predetermined open rates, thus facilitating the heating power control. 
     Although the present disclosure has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present disclosure encompass such changes and modifications as fall within the scope of the appended claims.