Patent Publication Number: US-11050222-B2

Title: Concurrent method for resonant frequency detection in corona ignition systems

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This U.S. continuation patent application claims the benefit of U.S. utility patent application Ser. No. 14/568,219, which claims the benefit of U.S. provisional patent application No. 61/915,088, filed Dec. 12, 2013; U.S. provisional patent application No. 61/931,131, filed Jan. 24, 2014; U.S. provisional patent application No. 61/950,991, filed Mar. 11, 2014; U.S. provisional patent application No. 62/072,530, filed Oct. 30, 2014; and U.S. provisional patent application No. 62/090,096, filed Dec. 10, 2014, the entire contents of each being incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a corona discharge ignition system, and more particularly to methods for controlling energy supplied to the corona igniter system. 
     2. Related Art 
     Corona discharge ignition systems provide an alternating voltage and current, reversing high and low potential electrodes in rapid succession which enhances the formation of corona discharge and minimizes the opportunity for arc formation. The system includes a corona igniter with a central electrode charged to a high radio frequency voltage potential and creating a strong radio frequency electric field in a combustion chamber. The electric field causes a portion of a mixture of fuel and air in the combustion chamber to ionize and begin dielectric breakdown, facilitating combustion of the fuel-air mixture, which is referred to as an ignition event. The electric field is preferably controlled so that the fuel-air mixture maintains dielectric properties and corona discharge occurs, also referred to as a non-thermal plasma. The ionized portion of the fuel-air mixture forms a flame front which then becomes self-sustaining and combusts the remaining portion of the fuel-air mixture. Preferably, the electric field is controlled so that the fuel-air mixture does not lose all dielectric properties, which would create thermal plasma and an electric arc between the electrode and grounded cylinder walls, piston, metal shell, or other portion of the igniter. An example of a corona discharge ignition system is disclosed in U.S. Pat. No. 6,883,507 to Freen. 
     In addition, the corona discharge ignition system is preferably controlled so that energy is provided to the corona igniter at a drive frequency equal or close to the resonant frequency of the corona igniter. This provides a voltage amplification leading to robust corona discharge in the combustion chamber. Accurately detecting the resonant frequency of the corona igniter is necessary in order to achieve this high level of control. However, accurate detection of the resonant frequency it is difficult to achieve, especially at a wide range of frequencies. Changes in the resonant frequency during operation, for example due to arcing events, also make it difficult to accurately detect the resonant frequency. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention provides an improved method for detecting the resonant frequency of a corona igniter concurrent with operation of the corona igniter in a corona ignition system. The method includes providing a first pulse of energy to a corona igniter at a positive voltage for a first pulse duration causing causes current to flow in the corona igniter; and ceasing the first pulse duration before the current flowing in the corona igniter crosses through zero. A first deadtime duration occurs immediately upon ceasing the first pulse duration, and no energy is provided to the corona igniter during the first deadtime duration. The method further includes detecting when the current flowing in the corona igniter crosses through zero during the first deadtime duration; and providing a second pulse of energy to the corona igniter at a negative voltage for a second pulse duration in response to the zero crossing of the current to cease the first deadtime duration. The method then includes ceasing the second pulse duration before the current crosses through zero. A second deadtime duration occurs immediately upon ceasing the second pulse duration, and no energy is provided to the corona igniter during the second deadtime duration. The method further includes detecting when the current flowing in the corona igniter crosses through zero during the second deadtime duration; and providing a third pulse of energy to the corona igniter at a positive voltage in response to the zero crossing of the current to cease the second deadtime duration. The method then includes obtaining a first resonant frequency value based on a sum of the first pulse duration, the first deadtime duration, the second pulse duration, and the second deadtime duration. 
     Another aspect of the invention provides a method for detecting the resonant frequency of a corona igniter in a corona ignition system, comprising the steps of: providing a first pulse of energy to a corona igniter at a positive voltage causing causes current to flow in the corona igniter; ceasing the first pulse before the current flowing in the corona igniter crosses through zero and providing no energy to the corona igniter for a first deadtime duration immediately upon ceasing the first pulse of energy; and obtaining a first zero crossing duration, wherein the first zero crossing duration begins at the start of the first pulse of energy and ends at the first zero crossing. The method further includes obtaining a first resonant frequency value by doubling the first zero crossing duration. 
     Another aspect of the invention provides a method for detecting the resonant frequency of a corona igniter in a corona ignition system, comprising the steps of: providing a first pulse of energy to a corona igniter at a positive voltage causing causes current to flow in the corona igniter; ceasing the first pulse before the current flowing in the corona igniter crosses through zero and providing no energy to the corona igniter for a first deadtime duration immediately upon ceasing the first pulse of energy; and obtaining a first zero crossing duration, wherein the first zero crossing duration begins at the start of the first pulse of energy and ends when the current flowing in the corona igniter crosses through zero during the first deadtime duration. The method next includes providing a second pulse of energy to the corona igniter at a negative voltage in response to the first zero crossing of the current to cease the first deadtime duration; ceasing the second pulse of energy before the current crosses through zero and providing no energy to the corona igniter for a second deadtime duration immediately upon ceasing the second pulse of energy; and obtaining a second zero crossing duration, wherein the second zero crossing duration begins at the first zero crossing and ends when the current flowing in the corona igniter crosses through zero during the second deadtime duration. The method then includes obtaining a first resonant frequency value based on a sum of the first zero crossing duration and the second zero crossing duration. 
     Another aspect of the invention provides a system for detecting the resonant frequency of a corona igniter. The system includes a first switch providing a first pulse of energy from an energy supply to a corona igniter at a positive voltage for a first pulse duration causing current to flow in the corona igniter. The first switch ceases the first pulse duration before the current in the corona igniter crosses through zero. No energy is provided to the corona igniter for a first deadtime duration which occurs immediately upon ceasing the first pulse duration. A frequency detector detects when the current flowing in the corona igniter crosses through zero during the first deadtime duration and initiates a drive signal to provide a second pulse of energy to the corona igniter in response to the zero crossing of the current. A second switch receives the drive signal and provides the second pulse of energy from the energy supply to the corona igniter at a negative voltage for a second pulse duration to cease the first deadtime duration. The second switch ceases the second pulse duration before the current flowing in the corona igniter crosses through zero. No energy is provided to the corona igniter for a second deadtime duration which occurs immediately upon ceasing the second pulse duration. The frequency detector detects when the current flowing in the corona igniter crosses through zero during the second deadtime duration and initiates a drive signal to provide a third pulse of energy to the corona igniter in response to the zero crossing of the current. The first switch receives the drive signal and provides the third pulse of energy from the energy supply to the corona igniter at a positive voltage to cease the second deadtime duration. The frequency detector then obtains a first resonant frequency value based on a sum of the first pulse duration, the first deadtime duration, the second pulse duration, and the second deadtime duration. 
     Yet another aspect of the invention provides a system for detecting the resonant frequency of a corona igniter in a corona ignition system, comprising a first switch and a frequency detector. The first switch provides a first pulse of energy from an energy supply to a corona igniter at a positive voltage causing current to flow in the corona igniter The first switch then ceasing the first pulse of energy before the current in the corona igniter crosses through zero and provides no energy to the corona igniter for a first deadtime duration immediately upon ceasing the first pulse of energy. The frequency detector then obtains a resonant frequency value by doubling a first zero crossing duration, wherein the first zero crossing duration begins at the start of the first pulse of energy and ends at the first zero crossing. 
     Another aspect of the invention provides a system for detecting the resonant frequency of a corona igniter in a corona ignition system, comprising a first switch, a second switch, and a frequency detector. The first switch provides a first pulse of energy from an energy supply to a corona igniter at a positive voltage causing current to flow in the corona igniter. The first switch ceases the first pulse duration before the current in the corona igniter crosses through zero and provides no energy to the corona igniter for a first deadtime duration immediately upon ceasing the first pulse of energy. The frequency detector detects when the current flowing in the corona igniter crosses through zero during the first deadtime duration and initiates a drive signal to provide a second pulse of energy to the corona igniter in response to the first zero crossing of the current. The frequency detector obtains a first zero crossing duration, wherein the first zero crossing duration begins at the start of the first pulse of energy and ends at the first zero crossing. The second switch receives the drive signal and provides the second pulse of energy from the energy supply to the corona igniter at a negative voltage to cease the first deadtime duration. The second switch ceases the second pulse duration before the current flowing in the corona igniter crosses through zero and provides no energy to the corona igniter for a second deadtime duration immediately upon ceasing the second pulse of energy. The frequency detector detects when the current flowing in the corona igniter crosses through zero during the second deadtime duration. The frequency detector obtains a second zero crossing duration, wherein the second zero crossing duration begins at the first zero crossing and ends at the second zero crossing. The frequency detector obtains a first resonant frequency value based on a sum of the first zero crossing duration and the second zero crossing duration. 
     The system and method provide numerous advantages. First, the resonant frequency values obtained become more accurate over time, and are equal to, or very close to, the actual resonant frequency of the corona igniter. In addition, the resonant frequency values are obtained while energy is being supplied to the corona igniter, and typically while the corona igniter provides corona discharge. Thus, additional power phase or measurement periods are not required. Furthermore, the drive frequency of the energy provided to the corona igniter can be immediately adjusted concurrent with operation of the corona igniter, or prior to a future corona event, to match the detected resonant frequency value, for example in response to changes caused by arcing events or other conditions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other 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 wherein: 
         FIG. 1  is a block diagram of a corona discharge ignition system according to a first exemplary embodiment of the invention; 
         FIG. 2  is a block diagram of a corona discharge ignition system according to a second exemplary embodiment of the invention; 
         FIG. 3  is a block diagram of a corona discharge ignition system according to a third exemplary embodiment of the invention; 
         FIG. 4  is a graph illustrating current flowing in the corona igniter and voltage provided to the corona igniter at the beginning of a corona event; and 
         FIG. 5  shows the current flowing in the corona igniter and the voltage provided to the corona igniter after  20  cycles in the corona event of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     The present invention provides an improved system  20  and method for detecting the resonant frequency of a corona igniter  22  concurrent with operation of the corona igniter  22 . The resonant frequency values obtained using the method are equal to, or very close to, the actual resonant frequency of the corona igniter  22 . The drive frequency of the energy provided to the corona igniter  22  can be adjusted to match the detected resonant frequency value while the energy is being supplied to the corona igniter  22  and while the corona igniter  22  provides corona discharge  26 . In addition, immediate adjustments to the drive frequency can be made during operation of the corona igniter  22  based on the detected resonant frequency value, for example in response to changes caused by arcing events or other conditions. 
     Exemplary embodiments of the system  20  capable of implementing the improved resonant frequency detection are shown in  FIGS. 1-3 . These systems  20  are also described in related U.S. patent application Ser. Nos. 14/568,219, 14/568,330, and 14/568,438, which are incorporated herein by reference. In each embodiment, the system  20  includes the corona igniter  22  coupled to an induction coil L, which are together referred to as the load operating at a resonant frequency. This resonant frequency is referred to herein as “the resonant frequency of the corona igniter  22 .” The corona igniter  22  receives energy at a drive frequency and voltage level causing current to flow in the corona igniter  22 . This current and voltage can be measured at an output  24  of the corona igniter  22 . During operation in an internal combustion engine, the corona igniter  22  preferably forms a high radio frequency electric field at a firing end, referred to as corona discharge  26 , to ignite a mixture of fuel and air in a combustion chamber of the engine. 
     The system  20  also includes the controller  28  and a pair of switches  30 A,  30 B that control the drive frequency provided to the corona igniter  22 , and the capacitance/inductance circuit of the system  20 , so that the drive frequency is maintained at or close to the resonant frequency of the corona igniter  22 . Operating the system  20  such that the drive frequency is equal to the resonant frequency provides voltage amplification leading to robust corona discharge  26  in the combustion chamber. 
     The controller  28  of the exemplary embodiments activates one of the switches  30 A or  30 B at predetermined times to achieve the desired drive frequency. When one of the switches  30 A or  30 B is active, energy can flow from the power supply V 3  through the active switch  30 A or  30 B to the corona igniter  22 . When the switches  30 A,  30 B are not active, energy cannot flow through to the corona igniter  22 . Switch  30 A is referred to as a first switch, and switch  30 B is referred to as a second switch, but the switch  30 B could alternatively be referred to as the first switch, and the switch  30 A could be referred to as the second switch. In each case, only one of the switches  30 A or  30 B is active and providing energy to the corona igniter  22  at any given time during operation of the corona ignition system  20 . Thus, the controller  28  deactivates the first switch  30 A before activating the second switch  30 B, and vice versa, so that the two switches  30 A,  30 B are not active at the same time. Preferably, activation of the switches  30 A,  30 B is synchronized with the resonant frequency of the corona igniter  22 . For example, in one embodiment, the first switch  30 A is active and thus provides energy to the corona igniter  22  whenever the current at the output  24  is positive, and the second switch  30 B is active and thus provides energy to the corona igniter  22  whenever the current at the output  24  is negative. The system  20  also includes a frequency detector for detecting the resonant frequency of the corona igniter  22 . The frequency detector is typically provided by a combination of components working together, for example the controller  28  working in combination with a current sensor  36 , or other components of the system  20 . 
     The method of detecting the resonant frequency is conducted concurrent with operation of the corona igniter  22  in an internal combustion engine. This this case, the method is conducted while energy is provided to the corona igniter  22  and typically while the corona igniter  22  provides corona discharge  26 . However, the method can also be conducted at a reduced duty cycle, wherein the energy provided to the corona igniter  22  is at lower level so that the corona discharge  26  is not created. The method could also being at the reduced duty cycle, and the duty cycle can be increased over time. 
       FIGS. 4 and 5  provide an example of the voltage provided to the corona igniter  22  and the current flowing in the corona igniter  22  while implementing the method of the present invention.  FIG. 4  shows the current and voltage at the beginning of a corona event, and  FIG. 5  shows the current and voltage after  20  cycles of the same corona event. The “corona event” is a period of time during energy is provided to the corona igniter  22  and the corona igniter  22  provides a corona discharge  26 . These current and voltage levels are used to detect the resonant frequency of the corona igniter  22 , as will be discussed further below. 
     In general, the method includes employing the first switch  30 A to provide a first pulse of energy from an energy supply, for example V 3 , to the corona igniter  22 . The first pulse of energy is provided at a positive voltage for a first pulse duration  101  causing a positive current to flow in the corona igniter  22 . The first switch  30 A then ceases the first pulse duration  101  before the current in the corona igniter  22  crosses through zero, as shown in  FIG. 4 . The controller  28  typically sets the length of the first pulse duration  101  based on any delays of components of the system  20 , so that the first pulse duration  101  ends before the current flowing in the corona igniter  22  crosses through zero. 
     A first deadtime duration  201  then occurs immediately upon ceasing the first pulse duration  101 . No energy is provided from the energy supply V 3 , or from any other type of energy source, to the corona igniter  22  during the first deadtime duration  201 , and the voltage level is at zero during the first deadtime duration  201 , as shown in  FIG. 4 . Since the energy is powered off during the first deadtime duration  201 , there are no problems associated with noise in the corona circuit due to switching. 
     During the first deadtime duration  201 , the frequency detector, such as a combination of the controller  28  and current sensor  36 , detects when the current flowing in the corona igniter  22  crosses through zero. The current crosses zero only once during the first deadtime duration  201 . In one embodiment, the current sensor  36  obtains the current flowing in the corona igniter  22  from the output  24  and determines the zero crossings of the current. This zero crossing measurement is typically conveyed from the current sensor  36  to the controller  28  in an output signal  54 . 
     The controller  28  receives the output signal  54 , and in response to the zero crossing of the current, the controller  28  initiates a drive signal  50  to provide a second pulse of energy to the corona igniter  22 . The second switch  30 B receives the drive signal  50  and provides the second pulse of energy from the energy supply V 3  to the corona igniter  22  at a negative voltage for a second pulse duration  102  to cease the first deadtime duration  201 . The second switch  30 B ceases the second pulse duration  102  before the negative current flowing in the corona igniter  22  crosses through zero, as shown in  FIG. 4 . The controller  28  can also set the length of the second pulse duration  102  based on any delays of components of the system  20 , so that the second pulse duration  102  ends before the current flowing in the corona igniter  22  crosses through zero. 
     A second deadtime duration  202  then occurs immediately upon ceasing the second pulse duration  102 . No energy is provided from the energy supply V 3 , or from any other energy source, to the corona igniter  22  during the second deadtime duration  202 , and the voltage level during the second deadtime duration  202  is at zero, as shown in  FIG. 4 . The second pulse duration  102  is greater than the first pulse duration  101 , and the second deadtime duration  202  is less than the first deadtime duration  201 . 
     During the second deadtime duration  202 , the frequency detector again detects when the current flowing in the corona igniter  22  crosses through zero, in the same manner as during the first deadtime duration  201 . The current crosses zero only once during the second deadtime duration  202 . In response to the zero crossing of the current, the frequency detector, typically the controller  28 , initiates another drive signal  50  to provide a third pulse of energy to the corona igniter  22 . The first switch  30 A receives the drive signal  50  and provides the third pulse of energy from the energy supply V 3  to the corona igniter  22  at a positive voltage to cease the second deadtime duration  202 . 
     After the second deadtime duration  202 , the frequency detector obtains a first resonant frequency value based on a sum of the first pulse duration  101 , the first deadtime duration  201 , the second pulse duration  102 , and the second deadtime duration  202 . Typically, the controller  28  receives information about the current and voltage from other components of the system, and then uses that information to determine the pulse durations  101 ,  102  and deadtime durations  201 ,  202 . The controller  28  then uses the sum of those durations  101 ,  102 ,  201 ,  202  to determine the first resonant frequency value. Various different methods can be used to determine the first resonant frequency value based on the sum, such as algorithms performed by software of the controller  28 . For example, the step of obtaining the first resonant frequency value can include dividing the sum in half to determine the duration of one half cycle of the resonant frequency. As indicated above, the gathering of information and evaluation conducted by the controller  28  to obtain the first resonant frequency value can be conducted during the corona event, while providing the energy to the corona igniter  22 . 
     In another embodiment, the frequency detector, such as the current sensor  36  and the controller  28 , determines the first resonant frequency value by obtaining the time between adjacent zero crossings, which can include the time between the start of the first pulse of energy and the first zero crossing, or the time between two consecutive zero crossings. For example, the controller  28  can obtain the first resonant frequency value by doubling a first zero crossing duration X 1 . As shown in  FIG. 4 , the first zero crossing duration X 1  begins at the start of a pulse of energy and ends at a first zero crossing. The first zero crossing could be at startup, as shown in  FIG. 4 , but could alternatively be the time between two later zero crossings. 
     The controller  28  could alternatively obtain the first resonant frequency value based on the time between three consecutive zero crossings, which could be the first three consecutive zero crossings, as shown in  FIG. 4 , or three later zero crossings. In this embodiment, the controller  28  obtains the first zero crossing duration X 1  and a second zero crossing duration X 2 , wherein the second zero crossing duration X 2  begins at the first zero crossing and ends at the second zero crossing. The controller  28  obtains a first resonant frequency value based on a sum of the first zero crossing duration X 1  and the second zero crossing duration X 2 . 
     After the first resonant frequency value is obtained, the controller  28  can adjust the drive frequency of the energy provided to the corona igniter  22  to equal the obtained first resonant frequency value, concurrently with operation of the corona igniter  22 . The controller  28  typically instructs the switches  30 A,  30 B to provide the energy from the energy supply V 3  to the corona igniter  22  at the first resonant frequency value while the corona igniter  22  continues to provide the corona discharge  26 . Determining the first resonant frequency value and adjusting the drive frequency to match the obtained first resonant frequency value can all occur in the same corona event. 
     The above steps are typically repeated over several cycles or time periods, as shown in  FIGS. 4 and 5 , to obtain additional resonant frequency values, wherein each consecutive resonant frequency value obtained is closer to the actual resonant frequency of the corona igniter  22  than the previous value obtained. Typically, at the start of each corona event, as shown in  FIG. 4 , a reduced duty cycle is used and the pulse durations are selected to allow a predefined range of frequencies to be accurately detected. The pulse durations are increased throughout the process of detecting the resonant frequency values while the deadtime durations decrease, as shown in  FIGS. 4 and 5 , until a maximum pulse duration is achieved. The actual resonant frequency value of the corona igniter  22  is fully identified when the maximum pulse duration is achieved. 
     For example, the method can include using the first switch  30 A to provide the third pulse of energy from the energy supply V 3  to the corona igniter  22  for a third pulse duration  103 , which is longer than the second pulse duration  102 ; and ceasing the third pulse duration  103  before the positive current flowing in the corona igniter  22  crosses through zero. No energy is provided to the corona igniter  22  for a third deadtime duration  203  immediately upon ceasing the third pulse duration  103 . The third deadtime duration  203  is shorter than the second deadtime duration  202 . 
     The frequency detector then detects when the current flowing in the corona igniter  22  crosses through zero during the third deadtime duration  203 . The second switch  30 B provides a fourth pulse of energy to the corona igniter  22  at a negative voltage for a fourth pulse duration  104  in response to the zero crossing of the current to cease the third deadtime duration  203 . The fourth pulse duration  104  is ceased before the current flowing in the corona igniter  22  crosses through zero. No energy is provided to the corona igniter  22  for a fourth deadtime duration  204  immediately upon ceasing the fourth pulse duration  104 . 
     Next, the frequency detector detects when the current flowing in the corona igniter  22  crosses through zero during the fourth deadtime duration  204 , and provides a fifth pulse of energy to the corona igniter  22  at a positive voltage and for a fifth pulse duration  105  in response to the zero crossing of the current to cease the fourth deadtime duration  204 . The fourth pulse duration  104  is greater than the third pulse duration  103 , and the fourth deadtime duration  204  is less than the third deadtime duration  203 . 
     The frequency detector then obtains a second resonant frequency value based on a sum of the third pulse duration  103 , the third deadtime duration  203 , the fourth pulse duration  104 , and the fourth deadtime duration  204 , in the same manner as the first resonant frequency value was obtained. The detected resonant frequency values become more accurate over time, so the second resonant frequency value obtained is typically closer to the actual resonant frequency of the corona igniter  22  than the first resonant frequency value. 
     As shown in  FIGS. 4 and 5 , the method typically continues in the same manner, preferably until the actual resonant frequency of the corona igniter  22  is detected, or very close to being detected. A plurality of additional pulses of energy can be provided to the corona igniter  22  after the fifth pulse of energy, wherein each additional pulse of energy is spaced from the next pulse by a deadtime duration during which no energy is provided to the corona igniter  22 . The pulse durations continuously increase over time and the deadtime durations continuously decrease over time. 
     The zero crossings and pulse durations are detected and evaluated to obtain additional resonant frequency values, in the same manner as the first and second resonant frequency values were obtained. Obtaining the additional resonant frequency values is also conducted concurrently with operation of the corona igniter  22 , while energy is provided to the corona igniter  22 . The controller  28  can also continue to adjust the drive frequency to match the obtained resonant frequency values concurrent with operation of the corona igniter  22  to continuously improve the performance of the system  20 . Alternatively, the last resonant frequency value obtained at the end of the resonant frequency detection process, specifically the value based on the last two pulse durations and last two deadtime durations, can be used as the starting drive frequency in a future corona event, or as the drive frequency during a future corona event. 
     In addition to accurately detect the resonant frequency of the corona igniter  22 , the system is also able to make immediate adjustments to the drive frequency, for example in response to resonant frequency changes, in order to maintain the drive frequency equal to, or very close to, the actual resonant frequency of the corona igniter  22 . The system  20  is also able to efficiently track and respond to resonant frequency changes caused by arcing events. Quick acquisition of the resonant frequency and rapid real-time adjustment of the drive frequency is possible to maintain the best possible performance. It is also noted that other methods of resonant frequency control which can be employed in the system described herein are disclosed in related U.S. patent application Ser. Nos. 14/568,219, 14/568,330 and 14/568,438, which are incorporated herein by reference. Each application lists the same inventor and was filed on the same day as the present application. 
       FIG. 1  is a block diagram of the corona discharge ignition system  20  according to a first exemplary embodiment which is capable of implementing the concurrent method for resonant frequency detection of the present invention, and capable of rapidly responding to resonant frequency changes and arc formation concurrent with operation of the corona igniter  22 , in order to maintain the drive frequency equal to or approximately equal to the resonant frequency. In addition to the controller  28 , switches  30 A,  30 B, corona igniter  22 , induction coil L, and the current sensor  36  described above, the system  20  also includes a pair of drivers  32 A,  32 B, referred to as a first driver  32 A and a second driver  32 B. The system  20  of  FIG. 1  further includes a transformer  34 , a first low-pass filter  38 , and a first signal conditioner  40 . The voltage provided to the corona igniter  22  and current flowing in the corona igniter  22  is detected at the output  24 . 
     The system  20  is controlled by the controller  28 , which is preferably a programmable digital or mixed-signal controller, such as a digital signal processor (DSP), complex programmable logic device (CPLD), field-programmable gate array (FPGA), microcontroller, or microprocessor. The controller  28  receives a trigger input signal  42  which commands the controller  28  to initiate the production of corona discharge  26  in the combustion chamber. The controller  28  also provides an arc detect output signal  44  to inform any external control system (not shown) that an arc has been detected, and a feedback output signal  46  to provide additional data about the health and operation of the circuit to any external control system. The trigger input signal  42 , arc detect output signal  44 , and feedback output signal  46  conveyed to and from the controller  28  are filtered by electromagnetic capability filters, referred to as EMC filters  48 , and other input filters  49 . In response to the trigger input signal  42 , the controller  28  provides the drive signals  50  to the drivers  32 A,  32 B which control the switches  30 A,  30 B. When one of the switches  30 A or  30 B is active, the energy supply V 3 , which is a DC voltage, is applied to a primary winding  52  of the transformer  34 . The transformer  34  then provides energy through the output  24  and to the corona igniter  22  at the drive frequency. In the exemplary embodiment, the transformer  34  has a configuration known in the art as a “push-pull” configuration. 
     In the system  20  of  FIG. 1 , the current flowing in the corona igniter  22  (the output current) is measured at the output  24  by the first current sensor  36 . The first current sensor  36  can also collect information about the voltage provided to the corona igniter  22  at the output  24 , such as the length of the pulse durations  101 ,  102 ,  103 ,  104  and the deadtime durations  201 ,  202 ,  203 ,  204 . The first current sensor  36  can be a shunt resistor, hall-effect sensor, or current transformer, for example. The current sensor  36  identifies the zero crossing of the current during each deadtime duration  201 ,  202 ,  203 ,  204 , and sends the output signal  54  including this information toward the controller  28 . The first current sensor  36  can use various different techniques to identify the zero crossing. The current sensor can also determine the length of the pulse durations  101 ,  102 ,  103 ,  104  and the length of the deadtime durations  201 ,  202 ,  203 ,  204 , and can send this information in the output signal  54 . 
     Preferably, the output signal  54  is lightly filtered by the first low-pass filter  38  before being conveyed to the controller  28 . The first low-pass filter  38  creates a phase shift in the output signal  54  which is smaller than the period of oscillation of the current. In one embodiment, the phase shift is 180 degrees, but preferably the phase shift is less than 180 degrees, and more preferably the phase shift is less than 90 degrees, which is less than one half cycle. The first low-pass filter  38  also removes unwanted high frequency noise generated by switching high current and voltages. The filtered output signal  54  is then transferred to the first signal conditioner  40 , which makes the output signal  54  safe for transferring to the controller  28 . Thus, the output signal  54  is at a level that can be safely handled by the controller  28 . The output signal  54  is typically provided to the controller  28  after each zero crossing. 
     The controller  28  receives the output signal  54  with the current and voltage information obtained by the first current sensor  36 , and uses the information to initiate correct timing of the switches  30 A,  30 B. The length of the first pulse duration  101  is predetermined by the controller  28  before the corona event, but the first deadtime duration  201  is not predetermined. Thus, the controller  28  monitors the current flowing in the corona igniter  22  for the zero crossing via the output signal  54 . The zero crossing detection is preferably corrected to account for any delay in measuring the current, delay in the first low-pass filter  38 , and the delay in other analogue or digital circuit elements. Immediately upon identifying the zero crossing of the current, the controller  28  terminates the first deadtime duration  201  by activating one of the switches  30 A,  30 B. 
     The controller  28  also uses the information contained in the output signal  54  to identify the resonant frequency of the corona igniter  22  concurrent with operation of the corona igniter  22 . As discussed above, the controller  28  can use various different techniques to identify the resonant frequency value based on the sum of the pulse durations  101 ,  102 ,  103 ,  104  and the deadtime durations  201 ,  202 ,  203 ,  204 . Once the resonant frequency value is obtained, the controller  28  activates the switches  30 A,  30 B at the correct times so that the drive frequency is equal to that resonant frequency value. 
     In the exemplary embodiment, once the controller  28  determines the timing of the first switch  30 A or second switch  30 B required, the controller  28  instructs the first driver  32 A to activate the first switch  30 A or instructs the second driver  32 B to activate the second switch  30 B at the required time. The drivers  32 A,  32 B are instructed to activate the switches  30 A,  30 B at the predetermined times, so that the drive frequency of the energy conveyed through the switches  30 A,  30 B to the transformer  34  and ultimately to the corona igniter  22  is equal to the resonant frequency value of the corona igniter  22  obtained by the controller  28 . The controller  28  can also adjust the timing of the switches  30 A,  30 B whenever needed, for example in response to changes of conditions in the system  20 , concurrent with operation of the corona igniter  22 . 
       FIG. 2  is a block diagram of a corona discharge  26  ignition system  20  according to a second exemplary embodiment of the invention, which operates like the system  20  of  FIG. 1 , but includes several additional features. One additional feature is that that the various functional sections of the system  20  include a control system ground  56 , a power system ground  58 , and load ground  60  which are separated from one another. This technique is used to improve EMI and/or electromagnetic capability (EMC). The control system ground  56  is isolated from a power system ground  58  by galvanic isolation  62 . The transformer  34  isolates the power system ground  58  from the load ground  60 , and this isolation must be maintained between the first current sensor  36  and the controller  28 . The isolation between the power system ground  58  and the load ground  60  may be achieved by adding galvanic isolation  62  at the first low-pass filter  38  or the first signal conditioner  40 . Alternatively, the isolation between the power system ground  58  and the load ground  60  can be achieved by operating the first low-pass filter  38  or the first signal conditioner  40  in a differential mode where only a negligible current can flow through the device. In the system  20  of  FIG. 2 , only the first signal conditioner  40  operates in differential mode to isolate the power system ground  58  from the load ground  60 . One or more of these methods may be employed. 
     Another additional feature of the system  20  of  FIG. 2  is a second current sensor  64  to measure the amplitude of the current in the second switch  30 B on the primary side of the transformer  34 . The second current sensor  64  specifically measures the current at the output of the second switch  30 B. Alternatively, there could be a second current sensor  64  at each of the switches  30 A,  30 B. The second current sensor  64  provides an additional feedback signal  55  to the controller  28 , giving valuable diagnostic information which is not possible through the phase measurement of only the first current sensor  36 . For example, it is possible to detect an open or short circuit in the load circuit by measuring the current at the output of the switches  30 A,  30 B. In addition, the system  20  of  FIG. 2  includes a second low-pass filter  66  located between the current sensor and the controller  28  to lightly filter the current output signal  54  before providing the feedback signal  55  to the controller  28 . 
       FIG. 3  is a block diagram of a corona discharge  26  ignition system  20  according to a third exemplary embodiment of the invention. The system  20  of  FIG. 3  also includes the galvanic isolation  62 , but in this embodiment, the galvanic isolation  62  is located on both the energy input and energy output sides of the controller  28 , and completely separates the three grounds  56 ,  58 ,  60 . One or both of the barriers provided by the galvanic isolation  62  can be omitted if the circuit is designed to operate using fewer grounds. 
     The system  20  of  FIG. 3  further includes another winding, referred to as a voltage feedback winding  68 . The voltage provided by the voltage feedback winding  68  reflects the voltage at the output  24  of the corona igniter  22 . A voltage sensor  78  is preferably located at the output of the voltage feedback winding  68  to measure this voltage. An output signal  80  including the output voltage is then transferred through the second low-pass filter  66  to the controller  28 . The second low-pass filter  66  lightly filters the voltage output signal  80  before providing the output signal  80  to the controller  28 . The controller  28  can determine the length of the pulse durations  101 ,  102 ,  103 ,  104  and the length of the deadtime durations  201 ,  202 ,  203 ,  204  from the information contained in this output signal  80 . Also, unlike the systems  20  of  FIGS. 1 and 2 , a control signal  72  is provided to the controller  28  of  FIG. 3 . The control signal  72  can include any information related to operation of the corona igniter  22 , such as whether arcing occurred or the desired voltage. 
     The features of the exemplary systems  20  shown in  FIGS. 1-3 , as well as those shown in the related applications, may be used in various combinations, other than those specifically described herein. However, the system  20  should have the ability to drive the corona igniter  22  with an AC signal at or near its resonant frequency; enable and disable this AC drive signal; adjust the duty cycle of the voltage supplied to the corona igniter  22  independent of the frequency of the voltage supplied to the corona igniter  22 . 
     The system  20  and method of the present invention provides multiple advantages over comparative systems. As discussed above, the system and method can detect a resonant frequency value that is equal to, or very close to, the actual resonant frequency of the corona igniter  22  concurrent with operation of the corona igniter  22 . A complete measurement of the resonant frequency after each cycle can be made, and the measurement can be evaluated and used on a per-cycle basis. Measurement over multiple cycles can also be done to improve the accuracy of the resonant frequency value detected. Immediate adjustments of the drive frequency can be made to maintain the drive frequency at or close to the actual resonant frequency of the corona igniter  22  and thus maintain a robust corona discharge  26 . 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings and may be practiced otherwise than as specifically described while within the scope of the claims.