Abstract:
A water temperature control circuit for a washing machine includes window detector and dual water valve drivers to modulate the hot and cold water valves to maintain the water temperature at a desired level. A temperature selector switch provides hot, regulated hot, warm, regulated warm, and cold temperature settings for the washing machine. During regulated temperature operation the window detector modulates one of the water valves to maintain the water temperature within a narrow temperature band. The window comparator includes two comparators that receive a common temperature input from a temperature sensing element. As such, no shunt resistor is required for one or the other comparators, thus improving accuracy. The control circuit operates open loop with respect to the hot and cold water valves that it controls, utilizing instead only the temperature input for controlling the proper water valve when enabled.

Description:
FIELD OF THE INVENTION 
     This invention is directed to fluid temperature controllers, and more particularly to fluid temperature controllers suited for regulating the temperature of the water used in a clothes washing machine. 
     BACKGROUND OF THE INVENTION 
     Continuing advances and changes in the types of fabrics, materials, and colors used in the apparel industry are putting increasing pressure on the garment and clothes cleaning industry. Brighter colors and more delicate fabrics require specialized care and tighter tolerances on every aspect of the cleaning cycle, including better regulation of water temperature. While businesses may be able to afford more sophisticated controls for their industrial washing machines to allow them to be better regulated, modern pressures on time and personal budgets diminish the desirability of new fashions that require professional cleaning only. Unfortunately, the current state of the art for water temperature control in consumer washing machines may not maintain the water temperature within tight enough tolerances for these new fabrics, materials, or colors. 
     In current washing machines the water temperature control regulates temperature of the water during the fill of the tub. This is done by having either the cold or the hot water valve on, and then modulating the other valve to maintain the aggregate temperature of the water in the tub within a certain range. The particular valve that is commanded to be continuously on (cold or hot) is selected via a user actuated water temperature control switch. If the user selects HOT on the temperature selector switch, for example, the controller may turn on the hot water valve continuously and modulate the cold water valve on and off to maintain the temperature at a pre-selected setpoint. If, on the other hand, a user selects WARM, the controller may turn on the cold water valve continuously and modulate the hot water valve to maintain the water temperature in the tub at a different pre-selected setpoint. 
     One problem with many conventional water temperature controllers which introduces an unacceptable tolerance error in the control set points may be better understood with reference to FIG.  5 . As may be seen from this conventional temperature controller, a single reference voltage Vcc is used for both temperature set points as monitored by comparators  13  and  15 . The actual temperature measurement is made by a thermistor  17  placed in the water flow. A pull-up resistor  19  biases the thermistor  17  to establish a voltage at node  11  used by comparator  13  to turn on a water solenoid driver circuit  21 . A shunt resistor  23  coupled across the thermistor  17  is used to develop the second temperature reading for the comparator  15  to turn on a water solenoid driver circuit  21 . 
     The particular water valve that is controlled by the solenoid driver circuitry  21  is determined by the position of selector switch  25 . The particular comparator  13 ,  15  that is enabled for temperature sensing control is determined by comparators  27 ,  29  based on the position of the hot water valve (via input  31 ). When the hot water valve is turned on full time, comparator  15  modulates the cold water valve, and when the cold water valve is turned on full time, comparator  13  modulates the hot water valve. Unfortunately, this sensing of the hot water valve at input  31  and selection via comparators  27  and  29  increases the cost and complexity, and reduces the reliability of this circuit. Further, the requirement for the valve driver selector switch  25  and its associated driver circuitry  33  also adds to the cost and detracts from the reliability of the water temperature controller. In the highly competitive and cost conscience consumer appliance industry, the requirement for four comparators  13 ,  15 ,  27 , and  29 , a hot water valve sense input  31 , and a selector switch  25  (and its associated circuitry  33 ) in the water temperature control circuit detrimentally impacts the marketability of appliances that incorporate such high cost circuitry. 
     There, therefore, exists a need in the art for a new and improved water temperature controller that has tighter tolerance on temperature variation, that is more cost effective, and that is less complex with a higher reliability than current designs. 
     SUMMARY OF THE INVENTION 
     In view of the above, it is an object of the instant invention to provide a new and improved fluid temperature control circuit that overcomes the above described and other problems existing in the art. Specifically, it is an object of the instant invention to provide a new and improved temperature control circuit that provides temperature regulation within narrow tolerances. Further, it is an object of the invention to provide a temperature controller that is less complex and more cost effective with improved reliability over prior designs. Additionally, it is an object of the invention to provide a fluid temperature controller that is suitable for use in a washing machine environment to control the water temperature. 
     In view of these objects, it is a feature of the instant invention to provide a fluid temperature controller that modulates the hot and cold water valves to regulate the water temperature of the fill water in a washing machine. It is a further feature that the proper modulation of the hot and cold valves is accomplished without input sensing of either these valves, and without an output driver selection switch. It is also a feature of the instant invention that the water temperature for both the hot and cold valve modulation is accomplished with a single thermistor without the necessity of a shunt resistor. Another feature of the instant invention is the use of a window comparator having two thresholds, with hysteresis, to maintain the temperature at the desired level. 
     In view of these objects and features of the invention, a preferred embodiment of the instant invention provides a fluid temperature control circuit for a washing machine having a hot water valve and a cold water valve controllable to provide a flow of hot and cold water, respectively, into a washing chamber. A temperature selector switch having an output to energize the hot water valve, an output to energize the cold water valve, and an output to enable operation of the fluid temperature control circuit. The fluid temperature control circuit comprises a temperature sensing element positioned to sense a temperature of water flowing into the washing chamber and a window detector circuit in sensory communication with the temperature sensing element. The window detector circuit includes a first and a second output, the first output being active when the temperature of the water is below a first threshold, and the second output being active when the temperature of the water is above a second threshold. The control circuit also includes a pair of water valve driver circuits responsive to the first and the second outputs of the window detector circuit to energize the hot water valve and the cold water valve. 
     Preferably, the temperature sensing element is a thermistor. Further, the first output of the window detector circuit is inactive when the temperature of the water is above a third threshold. The first and the third thresholds define a temperature regulation band in which the water temperature is maintained when the second and third outputs of the temperature selector switch are energized. Additionally, the second output of the window detector circuit is inactive when the temperature of the water is below a fourth threshold. In this embodiment, the second and the fourth thresholds define a temperature regulation band in which the water temperature is maintained when the first and third outputs of the temperature selector switch are energized. The window detector circuit preferably comprises a first and a second comparator, each having an inverting and a non-inverting input and an output. The inverting input of the first comparator and the non-inverting input of the second comparator are coupled to a common node that is in sensory communication with the temperature sensing element. 
     In an alternate preferred embodiment of the invention, a system for controlling the temperature of an aggregate fluid having hot and cold fluid constituents is provided. A hot fluid valve and a cold fluid valve separately control each of the hot and the cold fluid constituents. The system comprises a temperature selection switch having a hot enable output coupled to the hot fluid valve, a cold enable output coupled to the cold fluid valve, and an automatic temperature controller (ATC) enable output. A temperature sensing element is positioned to sense a temperature of the aggregate fluid. An automatic temperature controller (ATC) circuit having an input coupled to the ATC enable output, a first output coupled to the hot fluid valve, and a second output coupled to the cold fluid valve is also included. Preferably, the temperature selection switch provides at least one temperature regulated selection, a hot selection, and a cold selection. This temperature selection switch also provides a regulated hot selection and a regulated warm selection. The regulated hot selection provides the hot enable output and the ATC enable output, and the regulated warm selection provides the cold enable output and the ATC enable output. 
     In one embodiment, the ATC circuit comprises a window detector circuit having a first and a second comparator. These two comparators have a common temperature input in sensory communication with the temperature sensing element. Preferably, the common temperature input is coupled to an inverting terminal of the first comparator and to a non-inverting terminal of the second comparator. Also included is fluid valve driver circuitry having a first and a second pair of fluid valve drivers. The first fluid valve driver has an output coupled to the hot fluid valve, and the second fluid valve driver has an output coupled to the cold fluid valve. The comparators preferably establish a first and a second fluid valve control threshold. 
     The first comparator provides an output to the first fluid valve driver to open the hot fluid valve when the temperature of the aggregate fluid is below the first threshold, and the second comparator provides an output to the second fluid valve driver to open the cold fluid valve when the temperature of the aggregate fluid is above the second threshold. The first comparator further establishes a third threshold, and removes the output from the first fluid valve driver to close the hot fluid valve when the temperature of the aggregate fluid is above the third threshold. The second comparator further establishes a fourth threshold, and removes the output from the second fluid valve driver to close the cold fluid valve when the temperature of the aggregate fluid is below the fourth threshold. The ATC circuit preferably operates open loop with respect to the hot and the cold fluid valves. 
     In an alternate preferred embodiment of the invention, a water temperature controller for a washing machine having a temperature sensing element positioned therein to sense water temperature, a hot water control valve, and a cold water control valve, comprises a temperature sense conditioning circuit coupled to the temperature sensing element. This temperature sense conditioning circuit provides a temperature output signal representative of the water temperature. The controller also includes a first comparator having an input coupled to the temperature output signal that generates a first output enable signal when the water temperature is below a first threshold. A second comparator is also included having an input coupled to the temperature output signal that generates a second output enable signal when the water temperature is above a second threshold. 
     The controller also includes a hot water valve driver circuit coupled to the first comparator. The hot water valve driver circuit energizes the hot water valve to supply hot water to the washing machine upon receipt of the first output enable signal. A cold water valve driver circuit is coupled to the second comparator, and energizes the cold water valve to supply cold water to the washing machine upon receipt of the second output enable signal. The first comparator includes hysteresis such that once the first output enable signal is generated, the first output enable signal is removed when the water temperature is above a third threshold (the third threshold being above the first threshold). The second comparator also includes hysteresis such that once the second output enable signal is generated, the second output enable signal is remove when the water temperature is below a fourth threshold (the fourth threshold being below the second threshold). Preferably, the temperature output signal is coupled to an inverting input of the first comparator and to a non-inverting input of the second comparator. 
     Other objects and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified block diagram of a temperature control system for a washing machine in accordance with an embodiment of the instant invention; 
     FIG. 2 is a logic state table illustrating operating states of the temperature control system of FIG. 1; 
     FIG. 3 is a graphical illustration of the dual threshold window control for the hot and cold water valves in accordance with an embodiment of the instant invention; 
     FIG. 4 is a single line electrical schematic of an embodiment of a temperature control circuit constructed in accordance with the teachings of the instant invention; and 
     FIG. 5 is a single line electrical schematic of a conventional water temperature control circuit for use in washing machines. 
    
    
     While the invention will be described in connection with certain preferred embodiments, there is no intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As described above, there is a great need in the appliance industry, and in particular with respect to washing machines, for a temperature controller that regulates the temperature of water using hot and cold water valves. While the instant invention is particularly well suited for such application, it should be understood that the teachings of the instant invention may well be suited to other applications for fluid temperature control. Therefore, the following discussion is presented by way of example and not by way of limitation. 
     Turning now to the drawings, and in particular to FIG. 1, an embodiment of a temperature control system  10  is illustrated for use in a washing machine. The system  10  includes a conventional pressure switch  12  coupled between a source of power (V) and a program timer fill switch  14 . This timer fill switch provides proper coordination of the filling of the washing tub with the washing cycle. Also included in this system  10  is a user-actuated temperature selector switch  16 . This switch  16  allows the user to select the desired temperature for the particular type of clothes that are to be cleaned by the washing machine. Alternatively or additionally, an automated wash cycle control (not shown) may also provide the proper temperature selection for different types of wash cycles, depending on the washer&#39;s features design. 
     In a preferred embodiment, the switch  16  includes a selection for Hot, Regulated Hot, Warm, Regulated Warm, and Cold, although other (additional, fewer, or different) settings are possible. In this embodiment, the switch  16  includes outputs  18 ,  20 , and  22 . Output  18  (Hot Enable) provides power to the hot water valve  24 , output  22  (Cold Enable) provides power to the cold water valve  26 , and output  20  (ATC Enable) provides power to the automatic temperature control (ATC) module  28 . The ATC  28  also includes inputs for the sensing of a thermistor  30  that is positioned within the washing machine to sense the temperature of the water. The ATC  28  of the instant invention also includes two outputs  32 ,  34  to control the hot valve  24  and the cold valve  26  respectively. 
     Turning now to the logic state diagram of FIG. 2, the operation of the temperature selector switch  16 , in terms of its outputs, is illustrated. As may be seen from this state diagram, when the temperature selected by the user is Hot, the Hot Enable output  18  is energized to open the hot water valve. The other outputs  20 ,  22  are not energized or are off. If the user (or automatic control) selects the Regulated Hot setting, both the Hot Enable output  18  and the ATC Enable output  20  are energized, while the Cold Enable output  22  remains off. If the Warm temperature setting is selected, both the Hot Enable output  18  and the Cold Enable output  22  are on while the ATC Enable output  20  is off. For a selection of Regulated Warm, the Cold Enable output  22  and the ATC Enable output  20  are on while the Hot Enable output  18  is off. Finally, if the temperature selection is Cold, the Cold Enable output  22  is on while both the Hot Enable output  18  and the ATC Enable output  20  are off. 
     While it is clear from the foregoing description and an analysis of FIGS. 1 and 2 that the hot valve is open whenever the Hot Enable output  18  is energized and that the cold valve is open whenever the Cold Enable output  22  is energized, the operation of the ATC requires further discussion. Specifically, the ATC  28  operates to control the two water valves based on the temperature of water sensed by the thermistor  30  (see FIG.  1 ). While various types of control are contemplated, modulation control of the hot and cold valves will be described herein. 
     The ATC  28  operates on a window detector principle as illustrated in FIG.  3 . As will be recognized, this window detection control operates to turn on the hot valve by energizing output  32  when the sensed temperature of the water falls into region  36  of the graph of FIG.  3 . Similarly, the ATC  28  operates to turn on the cold valve by energizing output  34  when the sensed water temperature is in region  38  of this temperature graph. Within region  40  the ATC  28  does not energize either output  32 ,  34 , relying on the Hot Enable output  18  or Cold Enable output  22  to bring the water temperature into one of the temperature control zones  36 ,  38 , depending on whether the Regulated Hot or Regulated Warm temperature settings have been selected, respectively. 
     A hot regulation band  42 , defined by a cold turn on limit  42   a  and a cold turn off limit  42   b,  is utilized by the ATC  28  to maintain the water temperature within a narrow tolerance for the Regulated Hot setting. Likewise, a cold regulation band  44 , defined by a hot turn on limit  44   a  and a hot turn off limit  44   b,  is utilized by the ATC  28  to maintain the water temperature within a narrow tolerance for the Regulated Warm setting. While the various limits may be set at any level based on system requirements, preferably the bands  42 ,  44  are each approximately 2° F. As will be described in greater detail below, each of these bands may be individually set to different tolerances as desired. Likewise, the positioning of the bands (the setting of the average temperature for the regulated settings may also be set to different user or system defined levels. 
     Upon initiation of a wash fill cycle when a user or automatic control has designated the Regulated Hot temperature setting on the Temperature Selector Switch  16 , the Hot Enable output  18  energizes the hot valve  24  to begin the fill process. At the same time the ATC  28  is also enabled (see FIG. 2) via the ATC Enable output  20 . The window detector control of the ATC  28  will sense a temperature below the hot turn on level  44   a  (see FIG.  3 ), and energize output  32  to open the hot valve. Of course, the hot valve  24  has already been energized by the Hot Enable output  18  and so is already open. As the temperature of the water rises beyond the hot turn off level  44   b  of the ATC as sensed by the thermistor  30 , the output  32  is turned off. However, since the Hot Enable line  18  is still energized, the hot valve stays open, filling the tub with hot water. As the temperature of the water exceeds the cold turn on setpoint  42   a,  the cold valve  26  is energized by the ATC  28  via output  34 . With the cold water valve open, the temperature of the water will begin to fall. Once the falling water temperature has reached the cold turn off limit  42   b,  the ATC turns off output  34  to close the cold water valve. With the cold water off, the temperature will again begin to rise since the Hot Enable line  18  is still energized. The ATC will maintain the water temperature within the band  42  by continuing this cycle as the water temperature rises and falls between the two set points  42   a  and  42   b.    
     Upon initiation of a wash fill cycle when a user or automatic control has designated the Regulated Warm temperature setting on the Temperature Selector Switch  16 , the Cold Enable output  22  energizes the cold valve  26  to begin the fill process. At the same time the ATC  28  is also enabled (see FIG. 2) via the ATC Enable output  20 . The window detector control of the ATC  28  will sense a temperature below the hot turn on level  44   a  (see FIG.  3 ), and energize output  32  to open the hot valve. As the temperature of the water rises beyond the hot turn off level  44   b  of the ATC as sensed by the thermistor  30 , the output  32  is turned off. Since the Cold Enable line  22  is still energized, the cold valve stays open, filling the tub with cold water. As the temperature of the water drops below the hot turn on setpoint  44   a,  the hot valve  24  is energized by the ATC  28  via output  32 . With the hot water valve open, the temperature of the water will begin to rise. Once the rising water temperature has reached the hot turn off limit  44   b,  the ATC turns off output  32  to close the hot water valve. With the hot water off, the temperature will again begin to fall since the Cold Enable line  22  is still energized. The ATC will maintain the water temperature within the band  44  by continuing this cycle as the water temperature rises and falls between the two set points  44   a  and  44   b.    
     With an understanding of the operation of the system and method of the instant invention, attention is now directed to FIG. 4, which illustrates an exemplary embodiment of a circuit constructed in accordance with the teachings of the invention. As may be seen from a brief overview of this schematic, only two comparators  46 ,  48  are required to perform the functions described above. This represents a significant saving in both complexity and cost over prior designs. Further, it may be noted that this circuit contains no input sense of either the hot or cold valve, which also reduces the complexity of this circuit. It may also be noted that no output switch is required to select one of the hot or cold water valves for energization as with the prior designs, presenting yet a further improvement. 
     Specifically, this embodiment of the invention includes an input sense of thermistor  30  that has an inverse temperature/resistance coefficient. That is to say, as the temperature of the water increases, the resistance of the thermistor  30  decreases. The voltage developed across the resistor network  50  is used by the window detector circuitry  52  to indicate which water valve should be energized via driver circuitry  54 ,  56 . To accomplish the windowing performance illustrated in FIG. 3, the temperature sense voltage is input to the negative input of comparator  46 , and the positive input of comparator  48  in this exemplary embodiment utilizing a negative power supply (e.g. −24 Vdc)  58  and ground  20  (supplied in this embodiment by the Temperature Selector Switch  16  via ATC Enable  20  line). While a negative supply is used in this exemplary embodiment, one skilled in the art will recognize that this circuitry  28  could also be constructed with a positive power supply and appropriate polarity changes. 
     Upon initially enabling circuitry  28  (by supplying ground  20  via the temperature selector switch  16  to the resistor network  50 ), the window detector  52  is active. If the water temperature sensed by thermistor  30  is low, the voltage input to the negative input of comparator  46  is high, resulting in its output is low. This energizes the driver circuitry  54  and results in the opening of the hot water valve to supply hot water in accordance with FIG.  3 . As the temperature sensed by thermistor  30  increases, the voltage input to the comparators  46 ,  48  decreases to a point where the output of comparator  46  goes high (see setpoint  44   b  in FIG.  3 ). This turns off the hot water valve. If the temperature sensed by thermistor  30  continues to increase, the voltage input to the comparators  46 ,  48  decreases to a point where the output of comparator  48  goes low (see setpoint  42   a  in FIG.  3 ). This energizes the driver circuitry  56  and turns on the cold water valve. 
     If the temperature of the water sensed by thermistor  30  begins to drop, the voltage input to the comparators  46 ,  48  increases to a point where the output of comparator  48  goes high (see setpoint  42   b  in FIG.  3 ). This turns off the cold water valve. Hysteresis resistor  60  establishes this voltage point ( 42   b ) at a voltage level different than the turn on level ( 42   a ). If the temperature continues to decrease, the input voltage to the comparators  46 ,  48  increases to a point ( 44   a ) where the output of comparator  46  goes low, thus turning on the hot valve. Hysteresis resistor  62  establishes this voltage point ( 44   a ) at a voltage level different than the turn off level ( 44   b ). 
     Numerous modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only and is for the purposes of teaching those skilled in the art the best mode for carrying out the invention. The details of the structure may be varied substantially without departing from the spirit of the invention, and exclusive use of all modifications that come within the scope of the appended claims is reserved.