This invention pertains to a circuit for multiplying one variable by another and is particularly concerned with a circuit which is useful in a number of applications one of which is in an electronic engine control circuit for use with an internal combustion engine spark timing system.
In U.S. Pat. No. 3,978,833, dated Sept. 7, 1976, and assigned to the same assignee as the present application, there is disclosed a circuit for providing a programmed engine control function signal which is specifically disclosed as a programmed vacuum advance signal. This programmed vacuum advance signal is developed from engine manifold vacuum and from an accumulation function which is representative of the duration for which the engine has been operating in selected operating modes. Within its control range between maximum and minimum limits of vacuum spark advance, the programmed vacuum advance signal is developed as a function of the product of these two variables, namely (1) instantaneous engine manifold vacuum, as monitored by a vacuum transducer and associated transducer circuit, and (2) the accumulation function which increases in magnitude as a function of time when the engine is in a non-idle operating condition and which decreases in magnitude as a function of time when the engine is operating at idle condition. The vacuum transducer and associated transducer circuit disclosed in that patent develop a constant frequency rectangular waveform wherein the pulse width of the pulses comprising the waveform is representative of engine manifold vacuum. However, the relationship is such that at zero manifold vacuum the pulses have a definite non-zero pulse width. Thus, the product of the pulse width of the rectangular waveform signal by the accumulation function would yield an incorrect result where it is desired to obtain the product of manifold vacuum by the accumulation function. In order to obtain the correct product of manifold vacuum and accumulation function signal, it becomes necessary to make allowance for the non-zero width of the pulses which exists at zero manifold vacuum. Accordingly, a special circuit which makes this allowance is provided in that patent.
The present invention is concerned with an improvement upon the scheme disclosed in the foregoing patent. The advantages of the improvement afforded by the present invention are as follows.
The gain of the output amplifier is much easier to set. The previous system uses a more complex vacuum amplifier requireing proper choice of four interactive resistors. It is often difficult to find four appropriate resistors among standard values in order to achieve a desired gain while staying within impedance limits imposed by the system. With the present invention, development of engine control systems can be speeded up as systems can now be provided with a variable gain adjusted by a potentiometer. This capability was not available in the previous system as simultaneous adjustment of four interacting resistors was not practical. Additional advantages of the invention will be seen as the description proceeds.