Abstract:
An automatic variable gain amplifier is provided. The automatic variable gain amplifier automatically adjusts the amplification of a signal, and in one embodiment, an ion signal, based on the amplitude of the peaks of the signal. The automatic variable gain amplifier detects the peaks of the signal, compares them to a threshold value, and, based on this comparison, varies the amount by which the signal is amplified. The automatic variable gain amplifier produces a composite output waveform for an input waveform with an amplitude that may vary over a plurality of orders of magnitude.

Description:
FIELD OF THE INVENTION 
       [0001]    Embodiments of the present invention generally relate to ion current sense circuits for sensing ion current generated during the combustion event in an engine, and, more particularly, to amplifier circuits for use therewith. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the past it was difficult to determine the performance characteristics of an engine due to the fact that it was difficult to determine what was taking place within the combustion chamber of the engine. With the advent of ion sensing came the ability to determine the characteristics of combustion within a combustion chamber, allowing one to determine whether a fuel mixture was too rich or too lean and whether knocking or good combustion was taking place. 
         [0003]    Ion sensing relies on the fact that combustion in an engine creates measurable ionized gas. In such an engine an ion sensor may be installed or, with proper circuitry, the ignition spark plug may be used to sense ion current without installing additional sensors. The ion sensor generates a small current that flows through the ionized gas in the combustion chamber, and amplifier circuitry is used to allow analysis of the combustion ion signal to diagnose engine performance characteristics. 
         [0004]    Testing has identified that the combustion ion signal of a reciprocating engine, for instance, includes a first ion peak and a second ion peak. The first ion peak is due to the chemical ionization of the fuel and air in, or very near, the spark gap (if a spark plug is used) or the ion sensor. The second ion peak, or thermal peak, occurs after most of the fuel is burned and the remaining ion density near the sensor is approximately proportional to overall cylinder pressure. 
         [0005]    Analysis of each of the ion peaks provides different combustion information. The second peak has been studied for some time and correlates with the peak cylinder pressure and indicates the location of the peak temperature, knock, and misfire, for example. 
         [0006]    Difficulty has arisen, however, with regards to analysis of the first peak. Part of this difficulty is because the first peak can be 20 to 100 times the amplitude of the second peak. Prior ion amplifier circuits have been of a fixed, high gain type, optimized for amplifying the amplitude of the very small second peak. However, due to the high dynamic range between the two peaks, these prior ion amplifier circuits were unsuitable for analysis of the first peak. This is because the relatively large amplitude of the first peak compared to the second peak, such amplifiers often clipped this peak or became saturated, resulting in an unusable ion signal output for analysis. 
         [0007]    There is a need in the art, therefore, for an amplifier circuit capable of interfacing with an ion sensor that is capable of producing a related output signal that can be used by a typical Electronic Control Unit (ECU) to analyze both the first ion peak and the second ion peak. 
         [0008]    Embodiments of the present invention provide such a solution. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    In view of the above, embodiments of the present invention provide a new and improved amplifier for use with an ion sensor that overcomes one or more of the problems existing in the art. More particularly, embodiments of the present invention provide a new and improved automatic two gain amplifier for use with an ion sensor. Still more particularly, embodiments of the present invention provide an amplifier capable of producing a useful combustion ion signal that accurately represents both the first ion peak and the second ion peak of an ion current signal generated during a combustion event in an engine. 
         [0010]    Preferably, embodiments of the amplifier are capable of producing a useful ion signal for use with sensors in stoichiometric reciprocating engines, turbine engines, diesel particulate filters, and any other devices that produce an ion signal that has an amplitude that varies over a plurality of orders of magnitude. 
         [0011]    In one embodiment, the automatic two gain amplifier includes a comparator. The comparator is arranged and configured to interface with a peak detector and to compare the output of the peak detector with a threshold value. The comparator is also configured to control a variable amplifier. The variable amplifier is configured to amplify the ion signal produced by an ion sensor, for example, in the combustion chamber of an engine. When the output of the peak detector exceeds the threshold value, the comparator configures the amplifier not to amplify the ion signal or to only amplify the ion signal by a relatively small amount. When the output of the peak detector is less than the threshold value, the comparator configures the amplifier to amplify the ion signal by a relatively large amount. 
         [0012]    Thus, when the ion signal has a relatively small amplitude, e.g. during the second peak, it can be amplified before being analyzed. When the ion signal has a relatively large amplitude, e.g. during the first peak, it will not be over-amplified, but instead will be amplified only a small amount, or not amplified at all, before being output for analysis. Thus, both a small amplitude ion signal and a large amplitude ion signal can be automatically amplified the proper amount, and the amplifier will output an accurate and usable signal for analysis, even for an ion signal that varies across a diverse range of amplitudes. 
         [0013]    Other aspects, objectives 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 
         [0014]    The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings: 
           [0015]      FIG. 1  is a block diagram illustrating the functionality of an embodiment of an automatic two gain amplifier interfaced with a peak detector and an ion signal source in accordance with the teachings of the present invention; 
           [0016]      FIG. 2  is a graphical illustration of an ion signal and a peak detector output; 
           [0017]      FIG. 3  is a graphical illustration of an output signal of an embodiment of an automatic two gain amplifier in accordance with the teachings of the present invention; and 
           [0018]      FIG. 4  is a schematic illustration of an embodiment of an automatic two gain amplifier in accordance with the teachings of the present invention. 
       
    
    
       [0019]    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 INVENTION 
       [0020]    Turning now to the drawings, there is illustrated in  FIG. 1  an embodiment of an automatic two gain ion amplifier  100  particularly well suited for conditioning an ion current sense signal generated in the combustion chamber of a lean burn reciprocating engine. However, while the following description will utilize such an exemplary environment in describing various features and functionality of embodiments of the present invention, such description should be taken by way of example and not by way of limitation. Indeed, embodiments of the present invention may find use with ion sensor controls for stoichiometric reciprocating engines, turbine engines, and diesel particulate filters, to name just a few. 
         [0021]    As an overview of the system operation and with reference to  FIG. 1 , in one embodiment an ion sensor  102  positioned with a combustion chamber of an engine senses ion current flow and outputs an ion signal  104  to a peak detector circuit  106 . This peak detector circuit  106  tracks the peak amplitude of the ion signal  104 . The comparator  108  compares the output of the peak detector  106  with the threshold value  112  provided by the threshold source  110 . 
         [0022]    When the output of the peak detector  106  is greater than  114  the threshold value  112 , the ion signal  104  will be amplified by a first value  116 . In the exemplary application of a reciprocating combustion engine, the ion signal  104  will typically be larger than the threshold value  112  at or around the time of a first ion peak. As discussed above, this first ion peak will typically be due to the chemical ionization of the fuel and air in or very near the spark gap or ion sensor. Because this first ion peak typically has a relatively large amplitude as compared to the second peak, the first value for amplification will be small if any, e.g. a gain of 1. 
         [0023]    If, however, the output of the peak detector  106  is less than  118  the threshold value  112 , the ion signal  104  will be amplified by a second value  120 . In the exemplary application of a reciprocating combustion engine, the ion signal  104  will typically be smaller than the threshold value  112  at or around the time of a second ion peak. As discussed above, this second ion peak, also referred to as the thermal peak, has a better correlation with the peak cylinder pressure, and occurs after most of the fuel is burned and the remaining ion density near the ion sensor  102  is approximately proportional to the overall cylinder pressure. Because this second ion peak typically has a relatively smaller amplitude than the first peak, possibly by orders of magnitude, the second value for amplification will be much larger than the first value, e.g. 30. 
         [0024]    Following amplification either by the first amount of amplification  116  or by the second amount of amplification  120 , the now amplified ion signal  122  is output for analysis  124  or storage for later analysis. The amplified ion signal  122  may be output, for example, to an Engine Control Unit (ECU) or other controller for combustion parameter detection or ion signal processing in one embodiment. 
         [0025]      FIG. 2  graphically illustrates the problem of adequately analyzing the two peaks of an ion signal  104 . Specifically, the ion signal  104  in this exemplary embodiment typically has three main phases, to with, a sparking period  126 , a first peak region  128 , and a second peak region  130 .  FIG. 2  also shows some coil ringing that occurs after the sparking period  126  and the first peak region  128 . Because of the relative magnitudes of the first peak and the second peak, this second peak cannot be adequately viewed in this  FIG. 2 . As is clear, however, this second peak, or more particularly the output  132  of the peak detection circuitry, is below the threshold  112  for use in determining which gain to use in the amplifier circuitry to be discussed below. 
         [0026]    In this exemplary application, the ignition coil of the engine is sparking during the sparking period  126  causing the high amplitude peaks and high frequency fluctuation of the ion signal  104  illustrated in  FIG. 2 . Shortly after the ignition coil of the engine stops sparking, the first peak period  128  begins, during which the ion sensor  102  will output the first peak due to changes in conditions inside the combustion chamber of the engine. The first peak output of the ion sensor  102  will have a high amplitude, in the exemplary embodiment the first peak is approximately 4.0 volts and 144 microamperes. The output  132  of the peak detector tracks this high amplitude ion signal  104  value during the first peak period  128 . The output  132  of the peak detector will have an amplitude slightly less than the ion signal  104  due to the fact that in this embodiment an ideal diode is not used. Instead, a silicon diode, which has a forward voltage drop of, for example, 0.7V is used, creating an output of the peak detector  132  which is slightly lower than the ion signal  104 . 
         [0027]    After the first peak period  128 , due to changing conditions in the engine&#39;s combustion chamber, the ion current signal  104  generates a second peak during the second peak period  130 . The amplitude of the second peak is much smaller than the amplitude of the first peak, on the order of less than half a volt and approximately 2 microamperes in the illustrated embodiment. As may be seen, the output  132  of the peak detector is below the threshold  112  during the entire second peak period  130 . During this second peak period  130 , the second peak of the ion signal  104  cannot effectively be analyzed because its amplitude is too small, particularly as compared with the first peak. 
         [0028]    However, the variable amplification employed by embodiments of the present invention allows for both peaks to be effectively analyzed. The result of this variable amplification can be seen through an analysis of  FIG. 3 , which represents the amplified output  134  of the ion signal  104  of  FIG. 2 . 
         [0029]    During the sparking period  126 , the automatic two gain amplifier  100  remains in a low-gain mode, amplifying the ion signal  104  by only a small amount (in this embodiment a gain of only 1), because the amplitude of the output of the peak detector  132  (see  FIG. 2 ) is greater than the threshold value  112 . During the first peak period  128 , the output of the peak detector  132  is again (still) greater than the threshold value  112 . As such, the automatic two gain amplifier  100  remains in its low gain mode, using a gain of only 1, to generate the output signal  134  shown in  FIG. 3 . 
         [0030]    After the first peak period  128  has ended, the output of the peak detector  132  drops below the threshold value  112  as seen in  FIG. 2 . This causes the automatic two gain amplifier  100  to switch from its low gain mode to a high gain mode. During this high gain mode, the automatic two gain amplifier  100  utilized a much larger gain, e.g. a gain of 30 in the illustrated embodiment, to amplify the ion signal  104 . As may be seen from  FIG. 3 , during the second peak period  130 , the automatic two gain amplifier  100 , now in high gain mode, greatly amplifies the ion signal  104  so that the second peak is now clearly visible in the output  134 , and can effectively be analyzed by the same analysis circuitry used for analysis of the first peak. 
         [0031]      FIG. 4  illustrates one embodiment of a hardware implementation of the automatic two gain amplifier  100  constructed in accordance with the teachings of the present invention. An illustrated, an ion sensor  102  is provided to sense the ion current in the combustion chamber of the engine. As discussed above, this ion sensor  102  may be a dedicated ion sensor or may be an ignition spark plug. In any event, the ion sensor  102  detects the ionization within the combustion chamber and outputs the ion current signal  104 ′. The ion current signal  104 ′ flows through a sense resistor  140  to develop the ion signal  104  (voltage) illustrated in  FIG. 2 . This ion signal  104  is input to the peak detector circuit  106  and to the amplification circuit  136  as will be discussed in detail below. 
         [0032]    In the illustrated embodiment the peak detector  106  includes an operational amplifier  142  configured as a voltage follower, with an output resistor  144  and blocking diode  107 . The peak detector  106  also includes the resistor  146 ,  148  capacitor  109  combination that determines the tracking response time of the peak detector  106  output to the comparator  108 . As will be apparent to those skilled in the art, the output of the operational amplifier  142  charges the capacitor  109  through resistor  146  to or approximately to the peak of the ion signal  104 . As the ion signal  104  falls below the voltage stored on the capacitor  109 , the diode  107  becomes reverse biased, and the capacitor  109  then discharges through resistor  148 . The values of resistors  146 ,  148  and capacitor  109  may be varied to vary the tracking response time of the peak detector circuit  106 . 
         [0033]    As shown in  FIG. 4 , this peak tracking voltage on capacitor  109  is input to the comparator  108 . The threshold value for comparison with the voltage on capacitor  109  is set by the resistor divider of resistors  150  and  152  coupled to the supply source  156  (supply source  158  providing the negative supply for the operational amplifiers  142 ,  160  in the illustrated embodiment). The comparator  108  compares the instantaneous output of the peak detector  132  with this threshold value. 
         [0034]    When the peak value is greater than the threshold, the output of the comparator  108  is low which pulls down the voltage on capacitor  162 , the operation of which will be described more fully below with regard to the amplification circuit  136 . When the peak value is less than the threshold, the output of the comparator goes high. This allows capacitor  162  to charge through resistor  154 , turning on transistor  138  and changing the gain of the amplification circuit  136  as will be discussed below. When the peak of the ion signal  104  again rises above the threshold, the output of comparator  108  goes low, discharging capacitor  162  and turning off transistor  138 , to return the gain of the amplification circuit  136  to its previous value. Although not shown for simplicity, it is expected that one skilled in the art may employ hysteresis within the comparator  108  circuits as appropriate to prevent oscillations or false triggering due to noise. 
         [0035]    Turning now specifically to the amplification circuit  136 , the operational amplifier  160  receives the ion signal  104  on its non-inverting input and outputs the amplified output  134  for use by, in an exemplary environment, an ECU or other engine controller. The gain of operational amplifier  160  is set by resistors  164 ,  166 , depending on the operational state of transistor  138 . That is to say, during the period when transistor  138  is off (corresponding to a low output from comparator  108  generated because the peak value of the ion signal  104  is above the threshold as discussed above), the gain of the operational amplifier  160  is unity determined by the un-attenuated feedback of resistor  166 . In this embodiment, this gain is set to one to allow the amplified output  134  to follow the actual first peak of the ion signal  104  without being clipped or saturating as was the case with prior circuits that only had a single gain set to allow analysis of the second peak. Further, during the period when transistor  138  is on (corresponding to a high output from comparator  108  generated because the peak value of the ion signal  104  is below the threshold as discussed above), the gain of the operational amplifier  160  is determined by resistors  164 , and  166  forming a feedback voltage divider. In one embodiment, this gain is set to 30 to allow substantial amplification of the ion signal  104  so that the second peak of the ion signal  104  can be analyzed. Resistor  168  is simply an output load resistor. 
         [0036]    The foregoing has been described with respect to an exemplary lean burn reciprocating engine. However, the present application need not be limited to such an exemplary application. Instead, embodiments of the automatic two gain amplifier of the present application may be used in conjunction with a stoichiometric reciprocating engine, a turbine engine, a diesel particulate filter, or any other application in which two ion signal peak amplitudes are of interest but differ by orders of magnitude. 
         [0037]    All references, including publications, patent applications, and patents cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein. 
         [0038]    The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
         [0039]    Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.