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. 09/448,102, filed Nov. 23, 1999, and entitled, “LOW INPUT VOLTAGE, HIGH EFFICIENCY, DUAL OUTPUT DC TO DC CONVERTER”; 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 valve 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. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the disadvantages of the prior art by providing a main burner valve for a gas appliance which offers the user the opportunity to quickly and easily change the main valve outlet pressure regulation point to accommodate changes in fuel type. 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 differing outlet pressure. Linear actuation is important, because it offers the normal safety features associated with the industry standard of full off upon flame out. However, because the valve of the present invention may be positioned along the entire length of its travel from full open to full closed, the valve is totally adjustable permitting changes in mode, fuel input, and other outlet pressure related features. 
     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 conditions, operator inputs, and certain stored data. 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 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 makes 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 main fuel valve outlet pressure regulation point. 
     In accordance with the present invention, the gas appliance is calibrated during the manufacturing process. The stepper motor values and hence the valve positioning data corresponding to the desired valve settings are determined empirically for the various fuel types. This information is stored within non-volatile memory of the microprocessor. Thus, a table of stepper motor commands are available to the microprocessor for rapid changes of fuel type. 
    
    
     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 calibration of the valve assembly. 
    
    
     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) 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 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 higer 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 the second device 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. 
     The two primary inputs to microprocessor  60  are the thermopile output voltage received via input  62  and the manual mode change information received via input  64 . The thermopile output voltage is input once per second. The mode change information, on the other hand, is received a periodically in response to manual action by the user. 
     Output  66  controls operation of the stepper motor. As is explained in more detail below, this affects management of the main fuel valve orifice size. 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 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 regulation point 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  60  to direct the stepper motor to set the main burner valve outlet pressure to the proper value. The method for determining the valve positioning table entries is described in detail below. 
     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. 
     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  15  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 entries are empirically determined for the valve positioning table. Entry is via element  160 . The propane valve positioning values are determined first. The stepper motor opens the valve to its maximum position at element  164 . 
     At element  166 , the stepper motor decrements the outlet pressure of the valve. The outlet pressure is determined at element  168 . If the pressure is not as desired, control is returned to element  166  for a further decrement of the outlet pressure. When the valve pressure has been decremented to the desired point, control is given by element  168  to element  170 . The stepper motor positioning command is stored in the valve positioning table by element  170 . Element  72  determines whether there are other propane entries to be determined. If yes, control is given to element  166  to continue the process. 
     After element  172  finds that all of the propane entries have been made in the valve positioning table, control is given to element  174  to initialize for determine the methane (or natural gas) values. The process is essentially repeated for methane. Element  176  opens the valve to the maximum outlet pressure. Decrementation of the valve outlet pressure is accomplished by element  178 . Element  180  determines if the desired value has been reached. If no, the process continues at element  178 . If yes, element  182  records the stepper motor value. Element  184  ascertains whether all of the methane values have been determined. If not, control is given to element  178 . If yes, element  186  completes the valve positioning table, and exit is made via element  188 . 
     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.
An apparatus for and method for providing easy and rapid conversion from a first fuel to a second fuel in a gas appliance. The gas appliance has a variable fuel valve controlled by a microprocessor. A table stored in non-volatile memory has an entry for each of the fuels to be burned in the gas appliance. The table entries are empirically determined at the time of manufacture.