Dwell control circuit for ignition apparatus

Abstract: A dwell control circuit for developing a digital signal for use in a closed loop dwell control of an ignition apparatus for an electronic internal combustion engine. The circuit has a ramp counter (50) and a down-counter (60). The ramp counter counts-up clock pulses for a period of time beginning when the primary winding (26) of an ignition coil (24) is energized and ending when primary winding current increases to a sensed current limit value. When current limit is reached, the most significant bits of the count in the ramp counter are loaded into the down-counter. The ramp counter is now counted-up and the down-counter is counted-down until the down-counter underflows. The final or ultimate count magnitude in the ramp counter is equal to the count attained by the ramp counter between energization of the primary winding and attainment of current limit added to a fixed percentage of the count attained by the ramp counter.

Description: This invention relates to a dwell control circuit for an ignition apparatus of an internal combustion engine, and more particularly to a dwell control circuit for developing a compensated digital signal that is a function of a time period beginning with energization of the primary winding of an ignition coil and ending when primary winding current increases to a current limit value. US Patent No. 4,711,226 discloses a dwell control circuit wherein the ramp or rise time of the primary winding current of an ignition coil is determined where the ramp time is a time period beginning with energization of the primary winding of an ignition coil and ending when primary winding current increases to a current limit value. This is accomplished by counting clock pulses in a ramp counter where the counting begins when the primary winding is energized and where counting terminates when primary winding current increases to a sensed current limit value. When primary winding current increases to a sensed current limit value, a current limit signal is developed and a Darlington transistor that controls primary winding current is biased into a current limiting mode. The transfer function of the current sensing amplifier of the patent is non-ideal so that it may develop a current limit signal at less primary winding current than a desired or specified current limit value. By way of example, the current limit signal may be developed when primary current increases to 90% of the desired current limit value. In order to compensate for this inherent error mechanism, the closed loop dwell circuitry of US patent no. 4,711,226 uses preset values, which are added to the ramp time, to model the 10% inaccuracy. The preset value is determined from the ignition coil's previous ramp time. This preset value is loaded into the ramp counter before the present start of dwell (SOD) occurs. Once dwell begins, the ramp counter containing the preset, begins counting. When a current limit signal occurs, counting by the ramp counter ceases. Thus, the ramp counter contains all of the ramp time before the current limit signal occurs, plus a fixed number to compensate for the error. The circuit of US patent no. 4,711,226 has a limited number of fixed presets for the full range of coil ramp times and accordingly these presets do not accurately represent the continuous 10% dwell inaccuracy. The more ramp time decodes, and hence fixed presets, the more accurate the model. However, the greater the number of decodes, the larger the programmable logic array (PLA) used in US patent no. 4,711,226 becomes in order to process the decodes and choose the correct preset. This consumes large amounts of silicon area. Further, the PLA will never be completely accurate unless a separate decode and preset are available for every possible ramp time. In US patent no. 4,711,226, the circuit divides the ramp time into only three ranges. Even this small set of ranges requires a large PLA and switching circuitry that could total as high as 700 transistors. A method of developing a signal for a dwell control circuit, and a dwell control circuit, in accordance with the present invention are characterised by the features specified in the characterising portions of claims 1 and 3 respectively. The present invention eliminates the PLA used in US patent no. 4,711,226. Instead of using a PLA, the present invention uses a ramp counter of the type disclosed in US patent no. 4,711,226 that cooperates in a unique manner with a down-counter. The ramp counter is an up counter and it counts constant frequency clock pulses for a period of time beginning when the primary winding of an ignition coil is energized or start of dwell (SOD), and ending when a current sensing amplifier develops a signal indicative of the fact that primary current has increased to a sensed current limit value. The count in the ramp counter represents ramp time. When the current limit signal is developed, the most significant bits of the ramp counter are loaded into the down-counter. The ramp counter is now incremented or counted-up and the down-counter is now decremented or counted-down at a constant frequency. This continues until the down-counter underflows whereupon the up-counting of the ramp counter and the down-counting of the down-counter is terminated. The net effect of this is that the ultimate or final count in the ramp counter will be equal to the count attained by the ramp counter between SOD and sensed current limit added to a fixed or constant percentage of the attained count. Since the count in the ramp counter represents elapsed time, the final count in the ramp counter represents ramp time added to a fixed percentage of the ramp time. It will be appreciated that the dwell control circuit of this invention will respond to the entire ramp time range. It accordingly is an object of this invention to provide a new and improved dwell control circuit for developing a compensated digital signal that is related to ramp time wherein a ramp counter is incremented for a period of time beginning when the primary winding of an ignition coil is energized and ending when current limit is reached and wherein the count attained by the ramp counter during this period of time is processed to provide a digital signal that equals the count attained by the ramp counter added to a fixed percentage of the count attained by the ramp counter. Another object of this invention is to provide a dwell control circuit of the type described wherein the processing of the count attained by the ramp count is accomplished by the use of a down-counter and wherein the most significant bits of the count attained in the ramp counter is loaded into the down-counter and wherein the ramp counter is then counted-up and the down-counter counted down until the down-counter underflows. The final or ultimate count in the ramp counter has a count magnitude that is equal to attained count added to a fixed percentage of the attained count. The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 illustrates a current waveform where the primary winding current of an ignition coil is plotted against elapsed time; and Figure 2 illustrates a dwell control circuit made in accordance with this invention. Referring now to the drawings, Figure 1 illustrates a waveform of the primary winding current of an ignition coil plotted against elapsed time. In Figure 1, the primary winding of an ignition coil is energized at the start of dwell (SOD) by biasing a switching transistor conductive. The primary current now increases and ramps up along ramp curve or line 10. When primary winding current reaches a predetermined desired current limit value at point 12, the switching transistor that controls primary winding current is biased into a current limit mode. When this happens, primary winding current is held at a substantially constant value depicted by line 14. The time required for primary winding current to attain the current limit value is the ramp time and it is depicted in Figure 1 for the case where current has attained the desired current limit value. Also depicted in Figure 1 is a current level which is identified as 90% of the current limit value. This occurs at a point identified by reference numeral 13. At the end of dwell point (EOD) the transistor that controls primary winding current is biased nonconductive to cause spark plug firing from the secondary of the ignition coil. The optimum spark event occurs when EOD occurs just after current limit is reached, that is, the transistor that controls primary winding current should be biased nonconductive immediately after point 12 of the waveform of Figure 1. This allows the ignition coil to generate enough energy to cause the spark plugs to fire, without excessive power dissipation which could be caused by operation for too long a time period in the current limit mode along line 14. In describing the dwell control circuit of this invention which is illustrated in Figure 2, references will be made to the circuit disclosed in the above mentioned US patent no. 4,711,226 and the disclosure of that US patent is incorporated herein by reference. Referring now to Figure 2, the reference numerals 16 and 18 designate spark plugs for an internal combustion engine 20. These spark plugs are connected to the secondary winding 22 of an ignition coil 24. The primary winding 26 of the ignition coil 24 is connected between a source 28 of direct voltage and Darlington transistor (transistor switching means) 30. Darlington transistor 30 is connected in series with a current sensing resistor 31. Voltage divider resistors 32 and 34 having a node or junction 36 are connected across current sensing resistor 31. When Darlington transistor 30 is biased conductive primary winding current flows through primary winding 26, through Darlington transistor 30 and then through current sensing resistor 31 to ground. The voltage that is developed at junction 36 is a function of primary winding current magnitude and this voltage follows the waveform shown in Figure 1. The voltage at junction 36 is applied to a control circuit 38 via line 40. The control circuit 38 is further connected to the base of Darlington transistor 30 by line 42 and to a line 44. A current limit signal CLI is developed on line 44 whenever primary winding current attains a current limit value. The control circuit 38 applies a square wave signal to line 42 which causes Darlington transistor 30 to be biased either conductive or nonconductive. The control circuit 38 takes the form shown in Figure 3 of the above-referenced US patent no. 4,711,226, and defines biasing means for the Darlington transistor (30), current sensing means, and developing means for developing current limit signal CLI. When an SOD signal transition is applied to line 42, Darlington transistor 30 is biased to a conductive saturated condition. Primary current now increases along ramp line 10. When primary winding current reaches the current limit value, the voltage developed at junction 36 causes Darlington transistor 30 to be brought out of saturation and to come biased into a current limiting mode (line 14 of Figure 1). When it is desired to fire spark plugs 16 and 18, a signal transition occurs on line 42 which biases Darlington transistor 30 nonconductive. When Darlington transistor 30 goes nonconductive, a voltage is developed in secondary winding 22 to cause spark plugs 16 and 18 to be fired. The circuit of Figure 2 has two clock pulse sources designated respectively as clocks 46 and 48. The clock 46 develops square wave clock pulses at a constant frequency of about 10 kHz where internal combustion engine 20 is a four cylinder engine. If internal combustion engine 20 were a six cylinder engine, the frequency of clock 46 would be about 16 kHz. The clock 48 also develops square wave clock pulses at a constant frequency that is higher than the frequency of clock 46. Thus, the frequency of clock 48 may be about 125 kHz. The clock 46 is connected to the clock input of a ramp counter 50 via line 52, gate 54, line 55 and line 56. The ramp counter 50 is an up-counter. As will be more fully described hereinafter gate 54 (which defines applying means), is actuated to a closed condition wherein it connects clock 46 to the clock input of ramp counter 50 at SOD or in other words at the time Darlington transistor 30 is biased conductive. Gate 54 is actuated to an open condition by a signal developed on line 44 to terminate the application of clock pulses to ramp counter 50 when primary winding current increases to a current limit value to thereby cause Darlington transistor 30 to be biased into a current limit mode. The clock 48 is connected to the clock input of ramp counter 50 via line 57, latched gate 58, line 59 and line 56. The clock 48 is also connected to the clock input of a down counter 60 (processing means) via line 57, line 61, latched gate 62 and line 64. As will be more fully described hereinafter, the latched gates 58 and 62 are at times actuated to a closed condition to connect clock 48 to ramp counter 50 and down counter 60. The ramp counter 50 is a nine-bit up-counter and the down counter 60 is a six-bit down-counter. As will be more fully described hereinafter, the six most significant bits of ramp counter 50 are periodically loaded into down-counter 60 via the six bit lines 67 that are connected to bit output terminals Q4-Q9 of ramp counter 50. The ramp counter 50 and down counter 60 are so-called ripple counters and are comprised of a plurality of flip-flops. The digital count value in ramp counter 50 can be applied to a dwell and advance control circuit 70 via line 72. The dwell and advance control circuit 70 has an anti-dwell counter and various other elements as disclosed in the above-referenced US patent no. 4,711,226. The dwell and advance control circuit 70 may include latches in a manner described in US patent no. 4,711,226 for receiving and storing the count attained by ramp counter 50. The crankshaft of internal combustion engine 20 is connected to apparatus designated as 74 for developing crankshaft position pulses. These crankshaft position pulses are applied to dwell and advance control circuit 70 and to an electronic control module (ECM) 76 that supplies spark timing information to dwell and advance control circuit 70. The ECM 76 is connected to sense various engine parameters via line 78, such as engine temperature and engine manifold pressure and other factors well known to those skilled in the art. The dwell and advance control circuit 70 develops an SOD signal that is applied to line 80 whenever Darlington transistor 30 is biased conductive or in other words at start of dwell. The manner in which this signal is developed is described in US patent no. 4,711,226. The line 80 is connected to gate 54. When an SOD signal is applied to line 80, it causes gate 54 to be actuated to a closed conductive state so that clock pulses from clock 46 are now applied to the clock input of ramp counter 50 to cause the ramp counter 50 to count-up. As previously mentioned, a current limit signal CLI is developed on line 44 whenever primary winding current attains a current limit value. The line 44 is connected as an input to dwell and advance control circuit 70. The dwell control circuit of Figure 2 has a clock pulse counter 82 which is connected to clock 48 via line 84, a clock supply control 86 and line 88. The clock pulse counter 82 is connected to four output or bit lines 90, 92, 94 and 96. As clock pulse counter 82 is counted up by clock pulse, signals are sequentially developed on bit lines 90-96 in accordance with the count attained by the clock pulse counter. The bit line 90 is connected to the load terminal of down-counter 60. The bit line 92 is connected to the latched gates 58 and 62 and to clock supply control 86 via line 93. The bit line 94 is connected to dwell and advance control circuit 70 and the bit line 96 is connected to the reset terminal of ramp counter 50. The clock supply control 86 enables or disables the supply of clock pulses to clock pulse counter 82 from clock 48. Clock supply control 86 is connected to dwell and advance control circuit 70 by a line 98. An end of dwell or EOD signal is developed on line 98 when dwell and advance control circuit 70 develops a signal to cause Darlington transistor 30 to be biased nonconductive to in turn cause the spark plugs 16,18 to be fired. The clock supply control 86 is also connected to a control line 100. The control line 100 is connected to the Q output of a flip-flop 102. This output of flip-flop 102 is also connected to latched gates 58 and 62 via line 104. The CB input of flip-flop 102 is connected to down-counter 60 by line 106. The operation of the dwell control circuit shown in Figure 2 will now be described. When an SOD signal is developed on line 80, gate 54 is actuated to a condition wherein clock pulses are supplied to ramp counter 50 and it counts up. Current is now supplied to primary winding 26 and current increases along ramp line 10 of Figure 1. When primary winding current increases to a current limit value, a current limit signal CLI signal is developed on line 44. The signal on line 44 actuates gate 54 to an open condition so that clock 46 is disconnected from ramp counter 50 and accordingly the supply of clock pulses to ramp counter 50 is terminated. The CLI signal is also applied to dwell and advance control circuit 70 to signify that the circuit is ready to fire a plug. When dwell and advance control circuit 70 issues an EOD signal to line 98, the signal on line 98 causes clock supply control 86 to supply clock pulses to clock pulse counter 82. At a first attained count of clock pulse counter 82, a signal is developed on the bit line 90 that is connected to the load terminal of down-counter 60. This causes the six most significant bits of the count in ramp counter 50 to be loaded into down counter 60 via bit lines 67. As clock pulse counter 82 continues to count-up, it will reach another higher count magnitude which causes a signal to be developed on bit line 92. The signal on bit line 92 causes latched gates 58 and 62 to be both actuated to a closed condition so that the clock pulses from clock 48 are now applied to ramp counter 50 and down counter 60. When a signal is developed on bit line 92, the supply of clock pulses to clock pulse counter 82 is temporarily disabled via line 93 that is connected to clock supply control 86 to disable clock supply control 86. The ramp counter 50 now counts up from its previously attained count and the down counter 60 counts down from the count it received when the six most significant bits of ramp counter 50 were loaded into down counter 60. The down counter 60 continues to count down or decrement until it reaches a count of all zeros. At the next clock pulse, the down counter 60 will underflow to all ones. This underflow sets the flip-flop 102 via line 106 to a one that is applied to control line 100 and line 104. The line 104 is connected to latched gates 58 and 62 and when the down-counter 60 underflows to produce a signal on line 104, the latched gates 58 and 62 are actuated to an open condition to terminate the application of clock pulses to ramp counter 50 and down counter 60. When a signal is developed on control line 100, clock supply control 86 is re-enabled so that clock pulse counter 82 once more counts up. When clock pulse counter 82 counts up to a count that causes a signal to be developed on bit line 94, dwell and advance control circuit 70 is actuated to cause dwell and advance control circuit 70 to be loaded from ramp counter 50. As clock pulse counter 82 counts up further, a signal is developed on bit line 96 that is connected to the reset terminal of ramp counter 50. This resets ramp counter 50 to zero count. In the operation of the dwell control circuit shown in Figure 2, the amplifier in control circuit 38, which is also shown in Figure 3 of US patent no. 4,711,226 that senses primary winding current has a non-ideal transfer function. Thus, if it is assumed that the actual current limit value should be 9 A (point 12 of Figure 1), the transfer function of the amplifier may be such that the current limit signal CLI will occur at 90% to 100% of the actual or desired current limit value of 9 A. Thus, it is possible that the current limit signal will be developed at 90% of the desired current limit value or at point 13 on the waveform of Figure 1. This creates a possible 10% error in the amount of time that a particular ignition coil should be allowed to be turned on during its next ignition cycle. The dwell control circuit of Figure 2 compensates for the above mentioned possible 10% error by increasing the sensed ramp time by a fixed percentage of the sensed ramp time. Thus, the ultimate ramp time signal that is developed for use in a closed loop dwell control will be equal to the sensed ramp time added to a fixed percentage of the sensed ramp time. The manner in which this is accomplished will now be described. It will be appreciated that since ramp counter 50 and down counter 60 are supplied with constant frequency clock pulses, the digital count values attained by these counters represents or is a function of elapsed time. Let it be assumed that the desired current limit value is 9 A (point 12 of Figure 1) but that the transfer function of the current limit amplifier is such that the current limit signal CLI is developed at 90% (point 13 of Figure 1) of the desired current limit value. It can be seen in Figure 1 that the sensed ramp time has been reduced from the desired ramp time and the dwell control circuit of this invention compensates for this. When Darlington transistor 30 is biased conductive at SOD the ramp counter 50 begins to count-up and it counts the clock pulses from clock 46 until the current limit signal CLI is developed which has been assumed to be at 90% of the desired current limit value. The ramp counter 50 now contains a count value that corresponds to the sensed ramp time and subsequently the six most significant bits of ramp counter 50 are loaded into the down-counter 60. This essentially performs a logical divide by eight in regard to the count in the ramp counter 50 or in other words the count in down-counter 60 will be 1/8 of the count previously attained by ramp counter 50. Therefore, since ramp counter 50 contained 90% of the desired coil ramp time, the down counter 60 contains 11.25% of the total desired ramp time (0.9 x 1/8 = 0.1125) . The ramp counter 50 and down counter 60 now begin counting at the frequency of clock 48 with ramp counter 50 counting-up and down counter 60 counting-down. This continues until down counter 60 underflows in a manner previously described. The ramp counter 50 now contains the sensed ramp time (SOD to CLI), plus 11.25% of that sensed ramp time. The count so attained by ramp counter 50 can then be loaded into an anti-dwell counter in dwell and advance control circuit 70 of the type disclosed in US patent no. 4,711,226 in order to provide closed loop dwell control. In summary, the ultimate count that is attained by ramp counter 50 will be a count value related to sensed ramp time added to a fixed or constant percentage (11.25%) of the sensed ramp time. In the description of this invention only one ignition coil 24 has been illustrated. The dwell control circuit will include additional ignition coils as disclosed in US patent no. 4,711,226 and as previously described can use latches arranged such that data collected for a given ignition coil is used to subsequently control the dwell time of this same ignition coil. In the description of this invention, it has been pointed out that gates control the periodic application of clock pulses to ramp counter 50 and down counter 60. This same function could be accomplished by selectively enabling and disabling the clocks. The reason that clock 48 has a higher frequency than clock 46 is to speed up the processing of the digital information or, in other words, reduce the time required for ramp counter 50 to attain its ultimate usable count value.

Claims: A method of developing a compensated digital electrical signal for use in determining dwell time in a dwell control circuit for an ignition apparatus for an internal combustion engine (20), the ignition apparatus having an ignition coil (24) including primary and secondary windings (26,22), and switching means (30) switchable to provide current to the ignition coil up to a sensed current limit value and to maintain the current at the sensed current limit value until ignition; the method including the steps of applying constant frequency clock pulses (46) to an up-counter (50) for a period of time beginning with energization of the primary winding (26) of the ignition coil (24) and ending when primary winding current increases to the sensed current limit value whereby the count magnitude attained by the up-counter is a function of the duration of said period of time, and then processing said count magnitude attained by the up-counter during said period of time to produce the digital signal; characterised in that the magnitude of the digital signal is equal to said count magnitude added to a fixed percentage of said count magnitude. A method as claimed in claim 1, comprising loading a down-counter (60) with a count magnitude that is equal to the count magnitude attained by the up-counter (50) during the period of time divided by a predetermined constant factor and then causing the up-counter to count-up and the down counter to count-down at a constant frequency until the count in the down-counter is counted down to zero. A dwell control circuit for developing a compensated digital electrical signal for use in determining dwell time in an ignition apparatus of an internal combustion engine (20), the ignition apparatus comprising an ignition coil (24) having primary and secondary windings (26,22), and transistor switching means (30) connected in series with the primary winding, the dwell control circuit comprising biasing means (38) for biasing the transistor switching means (30) periodically conductive and nonconductive, current sensing means (38) connectable to the primary winding (26) for sensing primary winding current, developing means (38) coupled to the current sensing means for developing a current limit signal and for causing the transistor switching means to operate in a current limit mode when primary winding current attains a current limit magnitude, an up-counter (50), a source (46) of constant frequency clock pulses, applying means (54) for causing said clock pulses to be applied to the up-counter (50) for a period of time beginning when the transistor switching means (30) is biased conductive and ending when the current limit signal is developed whereby the up-counter (50) attains a count magnitude that is related to the duration of said period of time, and processing means (60) for processing the count magnitude to obtain the digital signal; characterised in that the magnitude of the digital signal is equal to the count magnitude added to a fixed percentage of the count magnitude. A dwell control circuit according to claim 3, wherein the up-counter (50) is a multi-bit counter, and wherein the processing means comprises a multi-bit down-counter (60) that is adapted to be loaded with the most significant bits of the count attained by the up-counter during said period of time. A dwell control circuit as claimed in claim 4, comprising means (54) for causing the up-counter (50) to count up at a constant frequency for a time period beginning with energization of the primary winding (26) and ending when primary winding current attains a current limit value, means (82) operative to load the most significant bits of the count attained by the up-counter during said time period into the down-counter (60) when the primary winding current attains the current limit value whereby the down-counter is loaded with a count magnitude that is a divided representation of the count attained by the up-counter, means (58,62) operative after the down-counter has been loaded for causing the up-counter to count up and for causing the down-counter to count down at a constant frequency, and means (102) for causing the up-counting of the up-counter and the down counting of the down-counter to terminate when the count in the down-counter is counted down to zero. A dwell control circuit according to claim 5, comprising first and second clock pulse sources (46,48), the frequency of the second clock pulse source (48) being higher than the frequency of the first clock pulse source (46), and wherein the up-counter (50) is up-counted by the first clock pulse source during said time period and wherein the up-counting of the up-counter and simultaneous down-counting of the down-counter (60) is by the second clock pulse source.

Inventors: HANSEN MARK C, KOSIAK WALTER K, HANSEN, MARK C., KOSIAK, WALTER K.
Applicants: DELCO ELECTRONICS CORP, DELCO ELECTRONICS CORPORATION (A DELAWARE CORP.)
Application year: 1989
Main classification: F02P3
IPC codes: F02P3, H01F38