SOC signal analysis
TECHNICAL FIELD OF THE INVENTION 
     The invention relates to processing the Start of Combustion (SOC) signal from a SOC sensor, as applied to fuel injection control. 
     BACKGROUND OF THE INVENTION 
     Commonly-owned U.S. Pat. No. 4,463,729 (Bullis et al, 1984) discloses a start of combustion (SOC) sensor that provides a SOC signal indicative of the onset of combustion event. The SOC sensor is an optical device disposed in the combustion chamber. 
     The SOC signal is advantageously supplied to a timing control circuit which delivers a timing control signal to a fuel delivery device, such as the controller associated with a fuel pump. The control circuit stores one or more start of combustion values which indicate the desired timing, relative to an engine cycle, for the start of combustion event as a function of speed and load. One or more adjustment signals are stored and applied as a function of speed and load to adjust the desired signal such that the control signal is corrected for delays. The actual SOC signal is compared with the desired signal to generate an error signal which may be used to finely adjust the stored SOC signal for particular speed and load conditions. This patent is incorporated by reference herein. 
     DISCLOSURE OF THE INVENTION 
     It is an object of this invention to provide a technique for analyzing the SOC signal to determine the actual time that combustion starts within the cylinder of an engine. 
     According to the invention, the actual time for start of combustion in an engine cylinder is provided by disposing a sensor in the combustion chamber of the cylinder, said sensor providing a signal indicative of the amplitude/time of the combustion event within the cylinder. Two signal levels are measured, a first level which is above the noise in the signal and a second level which is below the maximum amplitude of the signal which may be attenuated by carbon buildup on the sensor. By extrapolating backwards from these two sensed levels, the actual time for the start of combustion is calculated. 
     Other objects, features and advantages of the invention will become apparent in light of the following description thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph of weak and strong SOC sensor signals. Particular points thereon illustrate the method of this invention. 
     FIG. 2 is a block diagram of apparatus for implementing the invention shown with respect to FIG. 1. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     FIG. 1 shows signals 10A and 10B that may be provided by a SOC sensor. The signal 10A is a strong signal. The signal 10B is a weak signal. The signal slope and amplitude varies with the attenuation of the optical output of the SOC sensor due to carbon buildup on the SOC sensor in the combustion chamber. 
     A portion 12A,12B of the signals 10A,10B represents noise. The critical point to be determined on the signals 10A and 10B is the actual start of combustion. A rise in the signal amplitude occurs as a result of combustion, but the actual movement that this rise occurs is indeterminate. 
     In this invention, the actual start of combustion is determined indirectly by measuring two points on the signal 10A,10B. A first point 14A,14B is measured at a predetermined lower trigger level which is about the electrical noise and background noise level 12A,12B in the signal 10A,10B. A second point 16A,16B is measured at a predetermined upper trigger level which is above the lower trigger level and below the maximum amplitude of the weakest signals (10B). Since the level of the upper and lower trigger levels is fixed, by measuring the time interval between these two events it is possible to extrapolate (linearly) backwards in time from the points 14A,14B and 16A,16B (as indicated by the dashed lines 17A,17B) to a reference level, i.e., to the &#34;hypothetical&#34; (calculated, not directly measured) time for the rising edge of the SOC signal. This time, which is represented by the points 18A,18B and which is indicative of the moment at which combustion started in the cylinder is converted to an engine crankshaft angle and is used in the closed loop control of the aforementioned U.S. Pat. No. 4,463,729. 
     It should be noted that the signal levels 14A,14B and 16A,16B have been carefully chosen so that the backwards extrapolations from either signal 10A or 10B are convergent in most cases, and hence so that the points 18A and 18B are coincident. This establishes a valid calculation for the hypothetical time which is consistently related to the actual start of combustion time over a range of SOC signal strengths. 
     FIG. 2 shows a circuit for sensing the two points 14A,14B and 16A,16B. The SOC sensor 20 provides an optical signal to a photodetector 22, the output of which is amplified by an amplifier 24 to increase the signal strength. The output of the amplifier is provided to two comparators 26 and 28, for comparing the amplified SOC signal against two predetermined reference levels 30 and 32, respectively. 
     The comparator 26 provides an output on a line 34 when the amplified SOC signal is at the lower trigger level (14A,14B), and the comparator 28 provides an output on a line 36 when the amplified SOC signal is at the upper threshold level (16A,16B). The signals on the lines 34 and 36 are provided to the interrupt inputs of a microprocessor that is suited to receive two such inputs for performing the aforementioned extrapolation. 
     In the event that the particular microprocessor employed has only one interrupt input, the signals on the lines 34 and 36 are provided to the two inputs of a flip-flop circuit 40, the subsequent output of which will be a pulse in a line 42 representative of the measured time between the points 14A,14B and the points 16A,16B. The line 42 is connected to the signal input of the microprocessor. 
     It has been deemed useful to configure the control software in the microprocessor so that if the higher trigger level (16A,16B) is not received, or if the time interval between the lower trigger level (14A,14B) and the higher trigger level (16A,16B) is greater than a preset maximum, to ignore the present data and revert to a suitable backup for fuel pump timing control, such as the latest good value for start of combustion or a control algorithm that is independent of SOC. This eliminates the use of SOC signals outside of the normal operating range and may also be used to aid in system self-health diagnostics. 
     Similarly, the microprocessor software should be configured such that spurious signals occuring outside of a specified engine operating range (i.e., 10 degrees BTDC to 10 degrees ATDC) are ignored.