Abstract:
An apparatus and method for controlling a range of movement of a power piston coupled to an alternator in a linear-motion engine. The apparatus includes a power bridge and a bridge controller. The power bridge is coupled to the alternator and is used to adjust a voltage output from the alternator. The bridge controller is coupled to the power bridge and the alternator and is used to compare the voltage output from the alternator to a reference voltage that corresponds to a desired range of movement of the power piston, and generate a plurality of control signals used to control the operation of the power bridge.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates generally to the field of microcontrollers. More specifically, the invention relates to a microcontroller for controlling a linear-motion engine, such as a Stirling engine.  
         BACKGROUND OF THE INVENTION  
         [0002]    A conventional linear-motion engine, such as a Stirling engine, includes a displacement piston coupled to a power piston. The displacement piston moves as a result of applied thermal energy, which, in turn, causes the power piston to move. Electrical power is generated due to the motion of the power piston. More specifically, the power piston is coupled to an alternator, which converts the power piston&#39;s motion into electrical power.  
           [0003]    The alternator includes two main parts, a stationary part referred to as the stator and a moving part referred to as the armature. Typically, the stator is made from a slotted iron core with copper wire windings wound inside the slots. The armature usually includes a set of permanent magnets mounted on a rigid body, preferably formed from a nonmagnetic material, such as aluminum that is coupled to the power piston.  
           [0004]    The amplitude and polarity of magnetic flux created by the armature&#39;s permanent magnets changes when the power piston moves the armature. A voltage is produced across the stator&#39;s windings as a result of the change in magnetic flux. The voltage produced is referred to as the electromotive force, or emf, and it is defined as follows:  
           
         emf=N×dφ 
         flex 
         /dt  
       
           [0005]    where φ flux =B mag ×W×Xp, and  
           [0006]    emf=electromotive force  
           [0007]    N=the number of turns in the stator&#39;s windings  
           [0008]    d φ flux /dt=the rate of magnetic flux change  
           [0009]    W=the width of the armature&#39;s permanent magnets  
           [0010]    B mag =the magnetic field strength of the armature&#39;s permanent magnets  
           [0011]    Xp=the power piston&#39;s position amplitude  
           [0012]    From the above expression, if φ flux  is a sinusoidal waveform the emf can be expressed as  
           
         emf=B 
         mag 
         ×W×Xp×F  
       
           [0013]    where  
           [0014]    F=the piston frequency  
           [0015]    Thus, a direct relationship exists between the position amplitude of the power piston and the emf of the alternator. For a constant piston frequency, the greater the emf, the greater the range of movement of the power piston. The smaller the emf, the smaller the range of motion of the power piston. Because of this relationship, the power piston&#39;s position can be controlled by controlling the amplitude of the emf. This is particularly desired since Stirling engines are typically optimized to run at a constant piston frequency.  
           [0016]    Under load, the voltage across the terminals of the alternator differs both in magnitude and phase from the emf Primarily, the difference between the voltage across the terminals of the alternator and the emf is due to the impedance, i.e., inductance and resistance, of the stator windings. Typically, the inductive term is the dominant term of the stator winding impedance. In order to control the piston position amplitude, the emf must be directly accessible. Typically, a tuning capacitor is placed in series with the alternator in order to null out the effect of the inductance of the stator windings. Use of the tuning capacitor to facilitate access to the emf, allows for the use of a variety of loads with voltage limiting capability to limit the piston amplitude.  
           [0017]    There are several disadvantages associated with coupling a tuning capacitor between the alternator and the load. First, the capacitance of the tuning capacitor may need to be adjusted due to changes in the impedance of the load over time. Otherwise, the efficiency of the linear-motion engine may be compromised. Second, tuning capacitors are costly since they are low tolerance components. Third, the tuning capacitors may require significant space, depending on the electrical characteristics of the load. Finally, a tuning capacitor does not compensate for the resistance of the stator windings, which contributes to an uncontrolled growth in amplitude, which may result in piston over-stroke.  
           [0018]    Accordingly, there is a need for a low-cost apparatus used to control the movement of a power piston in a linear-motion engine so as to efficiently and effectively drive a load without requiring the use of a tuning capacitor. The present invention satisfies this need.  
         SUMMARY OF THE INVENTION  
         [0019]    An exemplary system that embodies the invention is an apparatus for controlling a range of movement of a power piston coupled to an alternator in a linear-motion engine. The apparatus includes a power bridge and a bridge controller. The power bridge is coupled to the alternator and is used to adjust a voltage output from the alternator. The bridge controller is coupled to the power bridge and the alternator, and is used to compare the voltage output from the alternator to a reference voltage that corresponds to a desired range of movement of the power piston, and to generate a plurality of control signals used to control the operation of the power bridge. In other, more detailed features of the invention, the apparatus includes the linear-motion engine having the alternator coupled to the power piston.  
           [0020]    In other, more detailed features of the invention, the power bridge includes a plurality of switches coupled to the bridge controller and each of the plurality of switches receives one of the plurality of control signals. Also, output power lines are coupled between the power bridge and a load, and at least one capacitor is coupled between the output power lines.  
           [0021]    In other, more detailed features of the invention, the bridge controller includes a root-mean-square (rms) voltage calculator coupled to the alternator that calculates an rms voltage based on the voltage output from the alternator, and an rms current calculator that calculates an rms current based on a current output from the alternator. A current probe is coupled between the rms current calculator and the alternator. Also, in more detailed features of the invention, the bridge controller includes a proportional integral derivative (pid) regulator coupled to the rms voltage calculator. The pid regulator receives the rms voltage and a reference voltage, and generates a dc reference current as a function of a difference between the rms voltage and the reference voltage.  
           [0022]    In other, more detailed features of the invention, the bridge controller further includes a multiplier coupled to the pid regulator and the alternator. The multiplier converts the dc reference current and the voltage output from the alternator into an ac reference current. Also, the bridge controller includes a phase advance coupled to the multiplier and the rms current calculator. The phase advance generates a third reference current that represents a phase-shifted version of the ac reference current. In addition, the bridge controller includes a current regulator coupled to the phase advance and the alternator. The current regulator determines a duty cycle based on a difference between the third reference current and the current output from the alternator. Furthermore, the bridge controller includes a pulse width modulator (pwm) generator coupled between the current regulator and the power bridge. The pwm generator generates the plurality of control signals used to control operation of the power bridge based on the duty cycle. The power bridge also includes a plurality of switches coupled to the pwm generator, and each of the plurality of switches receives one of the plurality of control signals.  
           [0023]    An exemplary method that embodies the invention is a method for controlling a range of movement of a power piston coupled to an alternator in a linear-motion engine. The method includes measuring a voltage output from the alternator, and controlling the range of motion of the power piston by controlling the voltage output from the alternator.  
           [0024]    In other, more detailed features of the invention, the method further includes measuring a current output from the alternator, generating a plurality of control signals based on the voltage and current output from the alternator, and controlling the voltage output from the alternator based on the plurality of control signals.  
           [0025]    In other, more detailed features of the invention, the method further includes calculating an rms voltage from the voltage output from the alternator, generating a dc reference current based on the rms voltage and a reference voltage, measuring a current output from the alternator, and calculating an rms current from the current output from the alternator. Also, the method further includes generating an ac reference current based on the dc reference current and the rms current, generating a third reference current based on the ac reference current and the rms current, and generating a duty cycle based on the third reference current and the current output from the alternator. In addition, the method includes generating a plurality of control signals based on the duty cycle, and controlling the voltage output from the alternator based on the plurality of control signals.  
           [0026]    Other features of the invention should become apparent from the following description of the preferred embodiments taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    [0027]FIG. 1 is a block diagram illustrating a preferred embodiment of the engine controller of the present invention;  
         [0028]    [0028]FIG. 2 is a block diagram illustrating a preferred embodiment of the bridge controller of the engine controller depicted in FIG. 1;  
         [0029]    [0029]FIG. 3 is a flowchart illustrating the steps performed by the bridge controller depicted in FIG. 2; and  
         [0030]    [0030]FIG. 4 is a graph illustrating the signal output from the engine controller depicted in FIGS. 1 and 2, as a function of control signals G 1 -G 4 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]    [0031]FIG. 1 illustrates a preferred embodiment of the engine controller  100  of the present invention. The engine controller is coupled between a linear-motion engine  102 , e.g., a Stirling engine, and a load  104 . The engine controller includes a power bridge  106  coupled between the linear-motion engine and the load, and it further includes a bridge controller  108  coupled between the linear-motion engine and the power bridge. The linear-motion engine includes an alternator  110  that generates ac electrical power in response to the movement of a power piston (not shown). In operation, the engine controller controls the range of motion of the power piston, and thus, controls the range of motion of a displacement piston (not shown) by controlling the alternator&#39;s output voltage, which depends on the value of emf generated by the alternator.  
         [0032]    The alternator  110  supplies electrical power on a first input power line  112  to the power bridge  106 . The electrical power supplied by the alternator has an instantaneous voltage V A (t) and an instantaneous current I A (t) at time t. Also, the alternator supplies V A (t) on the first input power line to the bridge controller  108 . Electrical power is returned from the power bridge on the second input power line  116  to the alternator. As illustrated in FIG. 1, a current probe  118  for measuring I A (t) flowing between the power bridge and the alternator is located adjacent to the second input power line. The current probe supplies the value of I A (t) on a current probe line  120  to the bridge controller.  
         [0033]    The power bridge  106  supplies electrical power on a first output power line  122  to the load  104 . The electrical power supplied by the power bridge has an instantaneous voltage V PN (t) and an instantaneous current I PN (t) at time t, which is returned from the load on a second output power line  124  to the power bridge. In preferred embodiments, a filter capacitor  126 , which reduces variations in the electrical power output from the power bridge to the load, is coupled between the first and second output power lines.  
         [0034]    The power bridge  106  includes four semiconductor switches Q 1 -Q 4 . The four semiconductor switches may be bipolar junction transistors (not shown) or MOSFET devices with their corresponding anti-parallel diodes (not shown). Electrical power generated by the alternator  110  is supplied on the first input power line  112  to both the first and third switches Q 1  and Q 3 , respectively. Electrical power is returned from the second and fourth switches Q 2  and Q 4 , respectively, on the second input power line  116  to the alternator. The first and second switches provide electrical power on the first output line  122  to the load  104 . The load returns electrical power on the second output line  124  to the third and fourth switches.  
         [0035]    The bridge controller  108  generates four control signals G 1 , G 2 , G 3 , and G 4  that are supplied on four switch interface lines  128 ,  130 ,  132 , and  134  to the four semiconductor switches Q 1 , Q 2 , Q 3 , and Q 4 , respectively. Each of the semiconductor switches is ON when its respective control signal is high or “1,” and correspondingly, each of the semiconductor switch is OFF when its respective control signal is low or “0.” 
         [0036]    [0036]FIG. 2 is a block diagram of the alternator  110 , power bridge  106 , and components that make up the bridge controller  108 . As illustrated in FIG. 2, the bridge controller includes an rms voltage calculator  136 , a pid regulator  138 , a multiplier  140 , a phase advance  142 , an rms current calculator  144 , a current regulator  146 , and a pwm generator  148 . The alternator provides V A (t) on the first input power line  112  to the rms voltage calculator and the multiplier. The rms voltage calculator calculates the rms voltage, V rms , from measurements of V A (t) between times t 0  and t n , where t 0 -t n  equals one half of the time period.  
         [0037]    The rms voltage calculator  136  provides V rms  on a first pid regulator interface line  150  to the pid regulator  138 . The pid regulator also receives a reverence voltage V ref , which is set to the voltage desired to be output from alternator  110 , on a second pid regulator interface line  152 . The pid regulator compares V ref  with V rms  and supplies a dc reference current I ref1 , which is proportional in value to the difference between V rms  and V ref , on a multiplier interface line  154  to the multiplier  140 . The multiplier multiplies I ref1  by V A (t), and in doing so converts I ref1  into ac reference current I ref2 , which has the amplitude necessary to maintain the desired emf given the phase of V A (t). The multiplier supplies I ref2  on a first phase advance interface line  156  to the phase advance  142 .  
         [0038]    The current probe  118  supplies I A (t) on the current probe line  120  to both the rms current calculator  144  and the current regulator  146 . The rms current calculator calculates I rms  from measurements of I A (t) between times t 0  and t n . The rms current calculator supplies I rms  on a second phase advance interface line  158  to the phase advance  142 . The phase advance generates a third reference current I ref3 , which represents a phase-shifted version of I ref2  based on I rms , and includes the phase information necessary for proper control of the emf. The phase advance provides I ref3  on a current regulator interface line  160  to the current regulator.  
         [0039]    The current regulator  146  generates a duty cycle based on the phase difference between I A (t) and I ref3 . Thus, the duty cycle output from the current regulator is determined in accordance with the desired voltage, V ref , output from the alternator  110 , which is directly related to both the desired emf and desired range of motion of the displacement piston (not shown). The greater the phase difference between I A (t) and I ref , the larger the duty cycle. In contrast, the smaller the phase difference between I A (t) and I ref , the smaller the duty cycle. Finally, the current regulator supplies the duty cycle on a pwm generator interface line  162  to the pwm generator  148 . The pwm generator generates the four control signals G 1 , G 2 , G 3 , and G 4 , which control the operation of the semiconductor switches Q 1 , Q 2 , Q 3 , and Q 4 , respectively, in the power bridge  106 .  
         [0040]    [0040]FIG. 3 is a flowchart that illustrates the operation of the bridge controller  108 . First, the bridge controller receives V A (t) and I A (t) measurements from the alternator  110  (Step  200 ). Next, the rms voltage calculator  136  and rms current calculator  144  calculate V rms  and I rms  based on the values of V A (t) and I A (t), respectively (Step  202 ). Then, the pid regulator  138  compares V rms  with V ref , and generates I ref1  (Step  204 ). The multiplier  140  then converts I ref1  into I ref2  based on V A (t) (Step  206 ). Next, the phase advance  142  shifts the phase of I ref1  based on I rms , and generates I ref3  (Step  208 ). The current regulator  146  then compares I A (t) and I ref3  and generates the duty cycle based on the phase and magnitude difference between I A (t) and I ref3  (Step  210 ). Finally, using the duty cycle, the pwm generator  148  generates control signals G 1 -G 4  (Step  212 ).  
         [0041]    [0041]FIG. 4 illustrates an example of control signals G 1 -G 4 , shown in dotted lines, and the resulting V A (t) output from the alternator  110 , shown in solid lines. For simplicity, control signals G 1  and G 4  are represented by a single pulse, and control signals G 2  and G 3  are represented by a single pulse. However, it is understood that identical pulses are simultaneously applied to each of the respective switches Q 1 -Q 4 .  
         [0042]    In a preferred embodiment, switches Q 1  and Q 4  are ON at the same time and switches Q 2  and Q 3  are ON at the same time. As depicted in FIG. 4, the amplitude of V A (t), and thus, the amplitude of V PN (t) and I PN (t), increases as the duty cycle of the control signals G 1  and G 4 , and G 2  and G 3  increases. In contrast, as the duty cycle of control signals G 1  and G 4 , and G 2  and G 3  decreases, the amplitude of V A (t), and thus, the amplitude of V PN (t) and I PN (t), decreases.  
         [0043]    Accordingly, by controlling the duty cycle of the control signals G 1 -G 4 , the power bridge  106  controls V PN (t) and I PN (t) output from the power bridge as well as V A (t) and I A (t) output from the alternator  110 . By controlling V A (t) and I A (t) output from the alternator, the power bridge also controls the emf of the alternator, the range of motion of the power piston (not shown), and the range of motion of the displacement piston (not shown).  
         [0044]    Therefore, the engine controller  100  offers one mode of operation, the constant-amplitude mode, in which the engine controller maintains the range of motion of the power piston (not shown) by maintaining a constant alternator emf. The engine controller can also be used in a second mode of operation, known as a variable-amplitude mode, where the engine controller adjusts the range of motion of the power piston by increasing or decreasing the alternator&#39;s emf.  
         [0045]    Unlike, prior art linear-motion engines, the linear-motion engine  102  of the present invention is not coupled directly to the load  104 , rather, the linear-motion engine is coupled to the load via the power bridge  106 . This configuration is advantageous because it eliminates the need to couple a tuning capacitor between the linear-motion engine and the load. With this approach, the tuning capacitor is eliminated resulting in increased reliability and reduced cost.  
         [0046]    The foregoing detailed description of the present invention is provided for purposes of illustration, and it is not intended to be exhaustive or to limit the invention to the particular embodiments disclosed. The embodiments may provide different capabilities and benefits, depending on the configuration used to implement the key features of the invention. Accordingly, the scope of the invention is defined only by the following claims.