Pinking detection systems are being used more and more frequently on internal combustion engines as they enable, for a reference engine, the optimum adjustment of one or more functional parameters, in particular the ignition advance, the richness of the mixture and the supercharging pressure in the case of a supercharged engine, without having to use a safety margin due to the problems of deviations between engines, ageing or change of environment (ambient temperature, humidity, etc.).
Known pinking detection systems generally include a pinking sensor, such as an accelerometer mounted on the cylinder head of the engine, a processing and shaping sequence for the output signal of the accelerometric sensor, and an electronic circuit, such as a digital computer, which compares the useful signal coming from the processing sequence with a reference signal and delivers a pinking detection signal when the useful signal exceeds the reference signal by a predetermined value.
On the other hand, it is known that the pinking of an engine is likely to appear at the moment at which the pressure in the combustion chamber is maximum and that this phenomenon can only occur within a restricted angular window of the cycle depending on the engine and on its adjustment parameters.
This is the reason why the processing sequence of such a pinking system is adapted in order to take account of the output signal of the accelerometric sensor within an optimized angular window in order to give the best signal to noise ratio between an operating condition with pinking and an operating condition without pinking and, in particular, this window is chosen so that the valve closures which generate vibrations which could be interpreted incorrectly as pinking occur outside the window.
U.S. Pat. No. 4,300,503 issued Nov. 17, 1981, issued to Deleris et. al. and entitled "Process and System for Computation and Adjustment of Optimum Ignition Advance", describes such a processing sequence of analog type and is shown in block diagram form in FIG. 1 of the appended drawings. This processing sequence is inserted between a stage 2 of amplification and filtering of the output signal of an accelerometric sensor 1 and a digital computer 3 which provides the actual pinking detection. The processing sequence 4 includes an analog switch 5 controlled such that the output signal a of stage 2 is applied to a full-wave rectifying stage 6 during the measuring window. The signal a' applied to the input of stage 6 gives rise at the output of this stage to a signal b which is integrated by a stage 7 producing an integrated signal c. The integrated analog signal c is converted to a digital form d in an analog-digital converter 8 receiving a reference voltage Vref and it is this digital value d which is read by the computer 3. The signals a, a', b and c are shown in FIG. 2 of the appended drawings.
The transfer function of such an analog processing sequence can be expressed in the following way: ##EQU1## X is the result, converted into digital form, of the analog processing of the output signal of the accelerometric sensor,
.xi. is the time constant of the integrator, PA1 TFAC is the duration of the measuring or analysis window, PA1 V.sub.ref is the reference voltage of the analog-digital converter, PA1 N is the number of significant bits of the converter and therefore of the result of the processing sequence, PA1 ABS (Ve) is the absolute value of the output signal of the accelerometric sensor 1 after shaping in the amplification and filtering stage 2, PA1 Mo is a constant associated with the integer part function INT [ ].
However, the discrete or integrated analog embodiment of a processing sequence, such as defined above, raises numerous problems.
First of all, it is difficult to produce an ultralinear full-wave rectifier because, as shown in FIG. 3 of the appended drawings, the rectification stage introduces, with respect to an ideal transfer function shown in dotted and dashed line, an offset error EO, a gain error Eg and a linearity error El when approaching saturation. In addition, the transient response of the rectifier is very dependent on the performance of the operational amplifiers used.
A second source of difficulties is associated with the integrator of which the accuracy of the time constant .xi.=RC depends upon that of the R and C components used, an accuracy which is very difficult to guarantee to better than 10%. In addition, the variations in these components due to temperature and ageing do not allow this accuracy to be maintained throughout the operational lifetime. Finally, it is necessary to use a self-zeroing system in order to cancel the errors due to the integration of offset voltages and polarization currents in the amplifiers used, which complicates the implemented circuits.
A third problem encountered is that of determining the conversion time of the analog-digital converter in relation to the development of the integrated values stored in the capacity of the integrator.
Finally, the design of a traditional analog processing sequence is very delicate, considering the number of variables to be brought together and taken account of in each component in the sequence. Now it is important to produce such a design as the effect of temperature variations, in the context of the climatic environment specifications of automobile electronic, equipment has very great consequences in the permanence of the accuracy of the processing sequence.