Abstract:
An apparatus for and method for protecting the electronic circuitry within a gas appliance. The electronic circuitry is arranged on a circuit board. A temperature sensor, located on the circuit board is periodically read to determine the current temperature. If the current temperature exceeds a first predetermined value, a microprocessor commands a stepper motor to reduce the input fuel pressure to the main burner. If the current temperature is within an acceptable range, the microprocessor permits the user to increase main valve outlet pressure.

Description:
CROSS REFERENCE TO CO-PENDING APPLICATIONS 
     U.S. patent application Ser. No. 09/447,611, filed Nov. 23, 1999, and entitled, “LOW INPUT VOLTAGE, LOW COST, MICRO-POWER DC-DC CONVERTER”; U.S. patent application Ser. No. 09/447,999, filed Nov. 23, 1999, and entitled, “STEPPER MOTOR DRIVING A LINEAR ACTUATOR OPERATING A PRESSURE CONTROL REGULATOR”; U.S. patent application Ser. No. 09/447,612, filed Nov. 23, 1999, and entitled, “LOW INPUT VOLTAGE, HIGH EFFICIENCY, DUAL OUTPUT DC TO DC CONVERTER”; U.S. patent application Ser. No. 09/450,077, filed Nov. 29, 1999, and entitled, “ELECTRONIC FUEL CONVERTIBILITY SELECTION”; and U.S. patent application Ser. No. 09/448,000, filed Nov. 23, 1999, and entitled, “ELECTRONIC DETECTING OF FLAME LOSS BY SENSING POWER OUTPUT FROM THERMOPILE” are commonly assigned co-pending applications incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to systems for control of a gas appliance incorporating a flame and more particularly relates to fuel control valve systems. 
     2. Description of the Prior Art 
     It is known in the art to employ various appliances for household and industrial applications which utilize a fuel such as natural gas (i.e., methane), propane, or similar gaseous hydrocarbons. Typically, such appliances have the primary heat supplied by a main burner with a substantial pressurized gas input regulated via a main valve. Ordinarily, the main burner consumes so much fuel and generates so much heat that the main burner is ignited only as necessary. At other times (e.g., the appliance is not used, etc.), the main valve is closed extinguishing the main burner flame. 
     A customary approach to reigniting the main burner whenever needed is through the use of a pilot light. The pilot light is a second, much smaller burner, having a small pressurized gas input regulated via a pilot valve. In most installations, the pilot light is intended to burn perpetually. Thus, turning the main valve on provides fuel to the main burner which is quickly ignited by the pilot light flame. Turning the main valve off, extinguishes the main burner, which can readily be reignited by the presence of the pilot light. 
     These fuels, being toxic and highly flammable, are particularly dangerous in a gaseous state if released into the ambient. Therefore, it is customary to provide certain safety features for ensuring that the pilot valve and main valve are never open when a flame is not present preventing release of the fuel into the atmosphere. A standard approach uses a thermogenerative electrical device (e.g., thermocouple, thermopile, etc.) in close proximity to the properly operating flame. Whenever the corresponding flame is present, the thermocouple generates a current. A solenoid operated portion of the pilot valve and the main valve require the presence of a current from the thermocouple to maintain the corresponding valve in the open position. Therefore, if no flame is present and the thermocouple(s) is cold and not generating current, neither the pilot valve nor the main valve will release any fuel. 
     In practice, the pilot light is ignited infrequently such as at installation, loss of fuel supply, etc. Ignition is accomplished by manually overriding the safety feature and holding the pilot valve open while the pilot light is lit using a match or piezo igniter. The manual override is held until the heat from the pilot flame is sufficient to cause the thermocouple to generate enough current to hold the safety solenoid. The pilot valve remains open as long as the thermocouple continues to generate sufficient current to actuate the pilot valve solenoid. 
     The safety thermocouple(s) can be replaced with a thermopile(s) for generation of additional electrical current. This additional current may be desired for operating various indicators or for powering interfaces to equipment external to the appliance. Normally, this requires conversion of the electrical energy produced by the thermopile to a voltage useful to these additional loads. Though not suitable for this application, U.S. Pat. No. 5,822,200, issued to Stasz; U.S. Pat. No. 5,804,950, issued to Hwang et al.; U.S. Pat. No. 5,381,298, issued to Shaw et al.; U.S. Pat. No. 4,014,165, issued to Barton; and U.S. Pat. No. 3,992,585, issued to Turner et al. all discuss some form of voltage conversion. 
     Upon loss of flame (e.g., from loss of fuel pressure), the thermocouple(s) ceases generating electrical current and the pilot valve and main valve are closed, of course, in keeping with normal safety requirements. Yet this function involves only a binary result (i.e., valve completely on or valve completely off). Though it is common within vehicles, such as automobiles, to provide variable fuel valve control as discussed in U.S. Pat. No. 5,546,908, issued to Stokes, and U.S. Pat. No. 5,311,849, issued to Lambert et al., it is normal to provide static gas appliances with a simple on or off, linearly actuated valve having the desired safety features. 
     Yet, there are occasions when it is desirable to adjust the outlet pressure regulation point of the main burner supply valve of a standard gas appliance. These include changes in mode (i.e., changes in the desired intensity of the flame) and changes in the fuel type (e.g., a change from propane to methane). U.S. Pat. No. 5,234,196, issued to Harris; U.S. Pat. No. 4,816,987, issued to Brooks et al.; U.S. Pat. No. 5,873,351, issued to Vars et al.; and U.S. Pat. No. 5,150,685, issued to Porter et al., suggest approaches to variable valve positioning of a gas appliance. However, the introduction of an entirely new valve design is likely to introduce severe regulatory difficulties. The present safety valve approach has been used for such a long time with satisfactory results. Proof of safe operation of a new approach to valve design would require substantial costly end user testing. 
     One of the problems associated with the use of a gas appliance, such as a gas fireplace, is temperature regulation. It is customary to provide a main burner, main valve, and fuel pressure regulator combination which can produce a desired combustion chamber temperature and output heat transfer. However, with fixed fuel supply systems, certain details of usage can have a major impact on combustion chamber temperature. For example, a user may choose to close glass combustion chamber doors, even when advised against such action, producing highly elevated combustion chamber temperatures. Similarly, variable output circulation fans may cause similarly large temperature variations. 
     As electronic components are added to the gas appliance, such temperature variations can cause damage. Though undesirable, the gas appliance could be sized such that the combustion chamber would never exceed a maximum temperature. At least equally undesirable especially from a cost perspective, special electronic components could be developed to operate at abnormally high ambient temperatures. Also costly, particularly at installation, the electronic components could be mounted remotely from the combustion chamber. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the disadvantages of the prior art by providing a main burner valve for a gas appliance having variable outlet pressure. The main burner valve of the present invention utilizes a standard, linearly actuated valve design having proven safety features, but which also offers precisely controllable fuel input. A temperature sensor on the electronic circuit board enables control of the main valve to prevent overheating of the electronic circuitry. 
     In accordance with the preferred mode of the present invention, a thermopile is thermally coupled to the pilot flame. As current is generated by the thermopile, it is converted via a DC-to-DC converter to a regulated output and an unregulated output. The regulated output powers a microprocessor and other electronic circuitry which control operation of the main fuel valve in response to sensed temperature conditions. The unregulated output powers various mechanical components including a stepper motor. 
     The stepper motor is mechanically coupled to a linear actuator which precisely positions the main fuel valve. Because the main fuel valve is linearly actuated, it operates in known fashion with respect to the industry proven flame out safety features. Yet, the stepper motor, under direct control of the microprocessor, positions the linear actuator for precise valve positioning and therefore, fuel input modulation to the main burner. 
     The use of a stepper motor means that any selected valve position is held statically by the internal rachet action of the stepper motor without quiescent consumption of any electrical energy. That rhakes the electrical duty cycle of the stepper motor/valve positioning system extremely low. This is a very important feature which permits the system to operate under the power of the thermopile without any necessary external electrical power source. In fact, the stepper motor duty cycle is sufficiently low, that the power supply can charge a capacitor slowly over time such that when needed, that capacitor can power the stepper motor to change the position of the linear actuator and hence the outlet pressure regulation point of the main fuel valve. 
     In accordance with the present invention, the output of the temperature sensor on the electronic circuit board is determined periodically. If the sensed temperature is greater than a desired level (e.g., 185 degrees f.), the microprocessor instructs the stepper motor to reduce the main valve pressure to reduce operating temperature. If the sensed temperature is within acceptable limits of the control, the microprocessor instructs the stepper motor to increase the main valve pressure, if requested by the user. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects of the present invention and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, in which like reference numerals designate like parts throughout the figures thereof and wherein: 
     FIG. 1 is a simplified electrical schematic diagram of the present invention; 
     FIG. 2 is a simplified block diagram of the microprocessor of the present invention; 
     FIG. 3 is a detailed electrical block diagram; 
     FIG. 4 is a plan view of the valve assembly; and 
     FIG. 5 is a flow diagram showing the temperature regulation algorithm. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a very basic electrical diagram  22  of the power circuitry of the present invention. Thermopile  24  is structured in accordance with the prior art. Resistor  26  represents the internal resistance of thermopile  24 . 
     Pilot valve  28  has a solenoid (not separately shown) which holds the pilot valve closed whenever sufficient current flows through the circuit. Similarly, the internal solenoid (also not separately shown) of main valve  32  holds the main valve closed whenever sufficient current flows through the associated circuit. 
     DC-to-DC conversion facility  36  converts the relatively low voltage output of thermopile  24  to a sufficiently large voltage to power the electronic control circuitry, including the microprocessor. In accordance with the preferred mode of the present invention, DC-to-DC conversion facility  36  consists of two DC-to-DC converters. The first converter operates at the extremely low thermopile output voltages experienced during combustion chamber warm up to generate a higher voltage to start the higher efficiency, second DC-to-DC converter. The other DC-to-DC converter, once started, can keep converting at much lower input voltage and generate much more power from the limited thermopile output for the system during normal operation. A more detailed description of these devices is available in the above identified and incorporated, commonly assigned, co-pending U.S. Patent Applications. 
     FIG. 2 is a simplified diagram showing the basic inputs and outputs of microprocessor  60 . In the preferred mode, microprocessor  60  is an 8-bit AVR model AT90LS8535 microprocessor available from ATMEL. It is a high performance, low power, restricted instruction set (i.e., RISC) microprocessor. In the preferred mode, microprocessor  60  is clocked at one megahertz to save power, even though the selected device may be clocked at up to four megahertz. 
     To practice the present invention, the two primary inputs to microprocessor  60  are the thermopile output voltage, received via input  62 , and the temperature sensor output, received via input  64 . The thermopile output voltage and temperature sensor output are received once per second. 
     Output  66  controls operation of the stepper motor. As is explained in more detail below, this affects management of the main fuel valve pressure. Output  68  is the on/off control for the external circulation fan. Output  70  controls the radio frequency receiver through which an operator can communicate via a remote control device. 
     FIG. 3 is a detailed block diagram of the inputs and outputs of microprocessor  60 . One megahertz crystal  84  clocks microprocessor  60 . The output of crystal  84  is also divided down to provide an interrupt to microprocessor  60  once per second. This interval is utilized for sampling of the thermopile output voltage and temperature sensor output. Indicator  112  permits early notification of flame on to the user. 
     Manual mode switch  86  permits an operator to select local mode or remote mode. Similarly, manual switch  88  is used to select the input fuel type, so that the main valve outlet pressure can be switched between propane and methane. Each of these alternative switch positions cause microprocessor  60  to consult a particular corresponding entry within the valve positioning table stored in the non-volatile memory of microprocessor  60 . These entries provide the necessary information for microprocessor to direct the stepper motor to set the main burner valve outlet pressure to the proper level. The method for determining the valve positioning table entries is described in detail in the above referenced co-pending patent application. 
     DC-to-DC converter  36  can receiver inputs from up to two thermopiles. Inputs  94  and  96  provide the positive and negative inputs from the first thermopile, whereas inputs  90  and  92  provide the positive and negative inputs from the second thermopile, respectively. Output  102  is the unregulated output of DC-to-DC converter  36 . This output has a voltage varying between about 6 volts and 10 volts. The unregulated output powers the mechanical components, including the stepper motor. Line  104  is a 3 volt regulated output. It powers microprocessor  60  and the most critical electronic components. Line  106  permits microprocessor to power DC-to-DC converter  36  up and down. This is consistent with the voltage sampling and analysis by microprocessor  60  which predicts flame out conditions. 
     Line  72  enables and disables pilot valve driver  72  coupled to the pilot valve via line  98 . Similarly, line  110  controls main valve driver  74  coupled to the main valve via line  100 . This is important because microprocessor  60  can predict flame out conditions and shut down the pilot and main valves long before the output of the thermopile is insufficient to hold the valves open. A more detailed description of this significant feature may be found in the above referenced, co-pending, commonly assigned, and incorporated U.S. Patent Applications. 
     Temperature sensor  103  is a standard device known in the art. It is physically mounted on the electronic circuit board, as the temperature control is primarily to ensure that the electronic components are not overheated. The output of temperature sensor  103  is provided on line  105 . 
     Stepper motor drivers  76  are semiconductor switches which permit the output of discrete signals from microprocessor  60  to control the relatively heavy current required to drive the stepper motor. In that way, line  66  controls the stepper motor positioning in accordance with the direction of the microprocessor firmware. Line  114  permits sensing of the stepper motor status. Lines  122 ,  124 ,  126 , and  130  provide the actual stepper motor current. 
     In the preferred mode of practicing the present invention, the gas appliance is a fireplace. The thermopile output is not sufficient to power the desired fan. However, the system can control operation of the fan. Therefore, line  132  provides the external power which is controlled by fan driver  80 . Lines  128  and  129  couple to optical isolation device  78  for coupling via lines  68 ,  116 , and  118  to microprocessor  60 . Line  134  actually powers the fan. 
     The fireplace of the preferred mode also has radio frequency remote control. A battery operated transmitter communicates with rf receiver  82  via antenna  136 . Lines  70  and  120  provide the interface to microprocessor  60 . Rf receiver  82  is powered by the 3 volt regulated output of DC-to-DC converter  36  found on line  104 . 
     FIG. 4 is a plan view of the valve assembly  140  of the preferred mode of the present invention. Fuel inlet  150  has standard fittings. Similarly, gas outlet  148  includes a standard coupling. Regulator cap  142  fits within housing cap  144  as shown (a better view is found in the section of FIG.  5 ). Motor housing  146  contains the linear actuator and stepper motor (neither shown in this view). 
     FIG. 5 is a flow diagram showing the manner in which the temperature control functions are shown. This logic is intended to operate in a continuous loop. In the preferred mode, the microprocessor is awakened once per second. At that time, the thermopile output voltage is sensed, in addition to the temperature. A detailed description of the output voltage sensing is available in the cross-referenced applications. 
     The electronic circuit board temperature is sensed at element  160 . As explained above, this is accomplished at one second intervals. The value is available from temperature sensor  103  (see also FIG.  3 ). Element  164  looks for an over temperature condition. The critical temperature is 185 degrees Fahrenheit, as that is the maximum operating temperature for typical electronic circuitry components. It is some times preferable to sense at a slightly lower temperature to provide a safety margin. 
     If element  164  determines that the temperature is too high, control is given to element  166  to command the stepper motor to incrementally reduce the main valve outlet pressure. Similarly, element  168  determines whether the temperature in an acceptable range. This temperature is much less critical in nature and is chosen at a convenient point, such as 150 degrees Fahrenheit. If the current temperature is acceptable, control is given to element  170  to command the stepper motor to increase the main valve outlet pressure to the desired level commanded by the user. 
     Element  172  represents the one second delay until the process is repeated at element  160 . 
     Having thus described the preferred embodiments of the present invention, those of skill in the art will be readily able to adapt the teachings found herein to yet other embodiments within the scope of the claims hereto attached.