Title:
ENGINE CONTROL SYSTEMS
United States Patent 3689753
Abstract:
An engine control system includes first transducer means producing a first digital signal representing a first variable parameter and second transducer means for producing a signal representing a second parameter. The first parameter changes in steps of X, and the second parameter in steps of Y. The two signals are fed to a memory store, which produces an output indicating the way in which the engine is to be controlled. In order to obtain better accuracy, at least the first digital signal is varied, for a given value of the first variable parameter, by an amount no greater than X.
US Patent References:
Interpolator devices
Spencer - November 1961 - 3010656
Function generator
Serrell - February 1960 - 2925220
Electrical function generators
Ingham et al. - November 1963 - 3110802
Function generator
Schmid - October 1967 - 3345505
ENGINE TESTING APPARATUS
Cross - October 1969 - 3472066
Interpolator devices
Spencer - November 1961 - 3010656
Function generator
Serrell - February 1960 - 2925220
Electrical function generators
Ingham et al. - November 1963 - 3110802
Function generator
Schmid - October 1967 - 3345505
ENGINE TESTING APPARATUS
Cross - October 1969 - 3472066
Inventors:
Malcolm Williams, 38 Willow Road (Solihull, GB2)
Duncan Barry, Hodgson 90 Heathcate Road
Leamington, Gb2
Michael Murray, Bertioli 38 Court Drive
Lichfield, Gb2
Duncan Barry, Hodgson 90 Heathcate Road
Leamington, Gb2
Michael Murray, Bertioli 38 Court Drive
Lichfield, Gb2
Application Number:
05/072722
Publication Date:
09/05/1972
Filing Date:
09/15/1970
View Patent Images:
Export Citation:
Primary Class:
Other Classes:
701/103, 123/486
International Classes:
F02D9/00; F02D41/24; F02P5/15; G06F1/035; G06F17/17; F02D41/00; G06F1/02; G06J1/00; G06G7/26
Field of Search:
235/197,150.53,150.5,150.21,152 73/116 318/301
US Patent References:
Primary Examiner:
Joseph, Ruggiero F.
Attorney, Agent or Firm:
Holman & Stern
Claims:
1. An engine control system for controlling the quantity of fuel supplied to the engine, said system comprising in combination first transducer means for producing a first digital signal representing the manifold pressure of the engine, said first digital signal having one of a number of values changing in steps of X, second transducer means producing a second digital signal representing engine speed, said second digital signal having one of a number of values changing in steps of Y, a memory store to which the first and second output signals are fed, said memory store being programmed to produce a pre-determined output signal dependent on the values of the two digital signals, control means operated by the output signal from the memory store for controlling the supply of fuel to the engine, and means for varying one of said digital signals, for a given value of the associated variable parameter, so as to sample different
2. An engine control system for controlling the quantity of fuel supplied to the engine, said system comprising in combination first transducer means for producing a first digital signal representing the manifold pressure of the engine, said first digital signal having one of a number of values changing in steps of X, second transducer means producing a second digital signal representing throttle angle, said second digital signal having one of a number of values changing in steps of Y, a memory store to which the first and second output signals are fed, said memory store being programmed to produce a pre-determined output signal dependent on the values of the two digital signals, control means operated by the output signal from the memory store for controlling the supply of fuel to the engine, and means for varying one of said digital signals, for a given value of the associated variable parameter, so as to sample different
3. An engine control system for controlling the quantity of fuel supplied to the engine, said system comprising in combination first transducer means for producing a first digital signal representing the engine speed, said first digital signal having one of a number of values changing in steps of X, second transducer means producing a second digital signal representing throttle angle, said second digital signal having one of a number of values changing in steps of Y, a memory store to which the first and second output signals are fed, said memory store being programmed to produce a pre-determined output signal dependent on the values of the two digital signals, control means operated by the output signal from the memory store for controlling the supply of fuel to the engine, and means for varying one of said digital signals, for a given value of the associated variable parameter, so as to sample different output signals
4. An engine control system for controlling the ignition timing of the engine, said system comprising in combination first transducer means for producing a first digital signal representing the manifold pressure of the engine, said first digital signal having one of a number of values changing in steps of X, second transducer means producing a second digital signal representing engine speed, said second digital signal having one of a number of values changing in steps of Y, a memory store to which the first and second output signals are fed, said memory store being programmed to produce a pre-determined output signal dependent on the values of the two digital signals, control means operated by the output signal from the memory store for controlling the ignition timing of the engine, and means for varying one of said digital signals, for a given value of the associated variable parameter, so as to sample different
5. An engine control system for controlling the ignition timing of the engine, said system comprising in combination first transducer means for producing a first digital signal representing the manifold pressure of the engine, said first digital signal having one of a number of values changing in steps of X, second transducer means producing a second digital signal representing throttle angle, said second digital signal having one of a number of values changing in steps of Y, a memory store to which the first and second output signals are fed, said memory store being programmed to produce a pre-determined output signal dependent on the values of the two digital signals, control means operated by the output signal from the memory store for controlling the ignition timing of the engine, and means for varying one of said digital signals, for a given value of the associated variable parameter so as to sample different
6. An engine control system for controlling the ignition timing of the engine, said system comprising in combination first transducer means for producing a first digital signal representing the engine speed, said first digital signal having one of a number of values changing in steps of X, second transducer means producing a second digital signal representing throttle angle, said second digital signal having one of a number of values changing in steps of Y, a memory store to which the first and second output signals are fed, said memory store being programmed to produce a pre-determined output signal dependent on the values of the two digital signals, control means operated by the output signal from the memory store for controlling the ignition timing of the engine, and means for varying one of said digital signals for a given value of the associated variable parameter, so as to sample different output signals from the memory store.
2. An engine control system for controlling the quantity of fuel supplied to the engine, said system comprising in combination first transducer means for producing a first digital signal representing the manifold pressure of the engine, said first digital signal having one of a number of values changing in steps of X, second transducer means producing a second digital signal representing throttle angle, said second digital signal having one of a number of values changing in steps of Y, a memory store to which the first and second output signals are fed, said memory store being programmed to produce a pre-determined output signal dependent on the values of the two digital signals, control means operated by the output signal from the memory store for controlling the supply of fuel to the engine, and means for varying one of said digital signals, for a given value of the associated variable parameter, so as to sample different
3. An engine control system for controlling the quantity of fuel supplied to the engine, said system comprising in combination first transducer means for producing a first digital signal representing the engine speed, said first digital signal having one of a number of values changing in steps of X, second transducer means producing a second digital signal representing throttle angle, said second digital signal having one of a number of values changing in steps of Y, a memory store to which the first and second output signals are fed, said memory store being programmed to produce a pre-determined output signal dependent on the values of the two digital signals, control means operated by the output signal from the memory store for controlling the supply of fuel to the engine, and means for varying one of said digital signals, for a given value of the associated variable parameter, so as to sample different output signals
4. An engine control system for controlling the ignition timing of the engine, said system comprising in combination first transducer means for producing a first digital signal representing the manifold pressure of the engine, said first digital signal having one of a number of values changing in steps of X, second transducer means producing a second digital signal representing engine speed, said second digital signal having one of a number of values changing in steps of Y, a memory store to which the first and second output signals are fed, said memory store being programmed to produce a pre-determined output signal dependent on the values of the two digital signals, control means operated by the output signal from the memory store for controlling the ignition timing of the engine, and means for varying one of said digital signals, for a given value of the associated variable parameter, so as to sample different
5. An engine control system for controlling the ignition timing of the engine, said system comprising in combination first transducer means for producing a first digital signal representing the manifold pressure of the engine, said first digital signal having one of a number of values changing in steps of X, second transducer means producing a second digital signal representing throttle angle, said second digital signal having one of a number of values changing in steps of Y, a memory store to which the first and second output signals are fed, said memory store being programmed to produce a pre-determined output signal dependent on the values of the two digital signals, control means operated by the output signal from the memory store for controlling the ignition timing of the engine, and means for varying one of said digital signals, for a given value of the associated variable parameter so as to sample different
6. An engine control system for controlling the ignition timing of the engine, said system comprising in combination first transducer means for producing a first digital signal representing the engine speed, said first digital signal having one of a number of values changing in steps of X, second transducer means producing a second digital signal representing throttle angle, said second digital signal having one of a number of values changing in steps of Y, a memory store to which the first and second output signals are fed, said memory store being programmed to produce a pre-determined output signal dependent on the values of the two digital signals, control means operated by the output signal from the memory store for controlling the ignition timing of the engine, and means for varying one of said digital signals for a given value of the associated variable parameter, so as to sample different output signals from the memory store.
Description:
This invention relates to engine control systems generally, but is particularly concerned with engines used in road vehicles, where it is often required that the engine should be controlled to obtain some desideratum, for example minimum exhaust emission, maximum fuel economy or peak performance. The control will in most cases be of the injection of fuel to the engine, but can, in the case of an engine having a spark ignition system, be of the ignition timing, or even a combination of timing and fuel injection.
An engine control system according to the invention comprises in combination first transducer means producing a first digital signal representing a first variable parameter of the engine, said first digital signal having one of a number of values changing in steps of X, second transducer means producing a second digital representing a second variable parameter of the engine, said second digital signal having one of a number of values changing in steps of Y, a memory store to which the first and second output signals are fed, said memory store being programmed to produce a predetermined output signal dependent on the values of the two digital signals, control means operated by the output signal from the memory store for controlling a characteristic of the engine, and means for varying said first digital signal, for a given value of the first variable parameter, so as to sample different output signals from the memory store.
An example of the invention is illustrated in the accompanying drawings, in which
FIG. 1 is a schematic circuit diagram illustrating a memory unit and associated circuits used in the example,
FIG. 2 is a truth table indicating possible outputs from the memory unit,
FIG. 3 illustrates a circuit arrangement for varying each of the digital signals and,
FIG. 4 is a circuit diagram of a memory unit used in a second example of the invention.
Referring to FIG. 1, a signal is produced by any convenient transducer 8 representing one of the three variables, engine speed, throttle angle and manifold depression, the signal being produced in the form of a three bit binary word transducers are commercially available which will sense engine speed and produce an electrical output which can be provided in the form of a three bit binary word, and similar transducers are available for giving a three bit binary word representing throttle angle and manifold depression. The exact details of these transducers are not important to an understanding of the invention. It will further be appreciated that in order to effect the required control, the two parameters utilized can be any two parameters chosen from the group of three parameters. Another three bit binary word is produced by another transducer 9 which responds to another of the three parameters, and the two words are fed respectively to a pair of decoders 11 and 12. There are 8 combinations of each digital signal, and for each input signal, the decoder 11 energizes one of eight input lines 13 of a diode matrix. The decoder 12 energizes one of eight output lines 14 depending on its input signal, and the lines 14 control a switching device 15, which can energize any one of eight groups of five lines 16 each of which crosses each line 13, with connections made where required by diodes for simplicity are shown as dots. One output line 21 is connectible to the first line 16 in each group and four further output lines 21 are connectible to the other four lines 16 in each group, the lines 21 being coupled to a device 22 for controlling the supply of fuel to the engine 23. The dots at the crossing points of the various lines within the device 15 represent electrical connections within the device 15 and do not of course represent diodes.
The drawing shows only the diodes associated with the lines 13 and the first two groups of input lines 16, but by way of example it will be seen that, assuming 1 represents the interconnection of two lines 13,16, then if the first group of lines 16 are energized and the eight lines 13 are energized in turn, the following output signals will be obtained: 10000, 00100, 01001, 01100, 10100, 10010, 11111 and 01000. Similarly, if the group is energized the output as the lines 13 are energized in turn will be 01100, 11001, 10101, 10000, 11001, 01101, 01111 and 11000. It will of course be appreciated that the term "energised" as applied to a group of lines 16 means simply that the group of lines is in operation, that is to say is connected to the device 22. This can be achieved in numerous ways; for example, the switching device 15 can, on receipt of an input on a line 14 provide base current to five transistors serving through their collector-emitter paths to connect the appropriate group of five lines 16 to the lines 21.
It will be appreciated that although a five bit output is obtained in the example, the value n can be from two upwards, depending on the accuracy required. Some outputs may be the same, indicating that the same fuel requirement is necessary for different operating conditions. The exact determination of the output for different inputs is made experimentally for a given engine, after which the same matrix can be used for all such engines. The signal, or a separately obtained signal, can be used to control the ignition timing, and in some instances the ignition timing along can be controlled using as parameters engine speed and throttle angle. The way in which the quantity of fuel and/or the timing is controlled depends on the purpose of the control system, but the arrangement is particularly intended for minimizing exhaust emission.
The system described provides an empirically determined stepped output signal in response to the two input signals, which are also stepped in value. Thus, if the inputs are designated a and b and change in steps of X and Y respectively, then considering only three values for each parameter, a truth table for the system can be written the truth table indicating various possible outputs from the memory unit. Paramater a Parameter b Output Signal. ____________________________________________________________ _____________ _ a b A a b+Y B a b+2Y C a+X b D a+X b+Y E a+X b+2Y F a+2X b G a+2X b+Y H a+2X b+2Y I ____________________________________________________________ _____________ _ It will be appreciated that the value A to I, which are determined for the engine under control empirically, may vary substantially in dependence upon the absolute values of the parameters a and b. Because the system is digital, the output signal is incrementally responsive to changes in engine conditions as a parameter changes. Thus, suppose a is constant at value a, but the engine parameter controlling b is varying, but for the moment has the value b. The output signal is A, but as b increases the output signal stays at A until b + Y is reached, then changes suddenly to B. For many engines control of this nature is sufficiently fine, but for some engines more accurate control is required. For this reason, there is imposed on one or both parameters a waveform of amplitude equal to one-half the step (1/2X or 1/2Y), the waveform having no d.c. component, and in a particular example being triangular waveforms associated with both parameters, the waveforms being 90° out of phase with each other and of the same frequency. The effect of this can be seen from FIG. 2, which represents the truth table mentioned earlier. The output signal will, by virtue or the impressed waveforms, be a summation of up to four of the values A to I. For example, let the parameters have value a + (3/2)X and b + (3/2)Y, then without the impressed waveforms the output signal is E. It is also E with the impressed waveforms because the output signal values of F,B,D, and H exist for zero time. However, consider now the case where the a parameter changes to a + (7/4)X. Without the modification, the signal is E. With the modification, there is a sampling of value H for one quarter of the time, and the output signal is 3/4E + 1/4H. If the b parameter now changes to b + (7/4)Y, the output signal will be 1/2E + 1/4F + 1/4H. It will be seen that in this way a far more accurate control is achieved.
In some circumstances, only one of the parameters need have an impressed voltage. This could be useful where variations in one parameter are far more important than variations in the other.
It will of course be appreciated that there are numerous ways of varying the digital signals in the required manner. By way of example, FIG. 3 shows an arrangement in which an analogue voltage produced by a transducer is fed by way of a resistor 31 to a summing amplifier 32, the output from which is fed to a pair of discriminating circuits 33,34. The circuits 33 and 34 only respond to an input having a predetermined amplitude, and operate respectively in response to positive and negative signals from the amplifier 32. The outputs from the circuits 33 and 34 are fed respectively to a pair of AND gates 35 and 36, each of which also receives an input from a clock source 37. The gates 35 and 36 provide outputs to a reversible binary counter 38, the output from which provides the input to the decoder 11 in FIG. 1.
When a signal is received by way of the resistor 31, it is fed through the amplifier 32 to the circuit 33, and assuming that the signal is greater than the minimum amplitude set by the circuit 33, an output is produced turning on the gate 35, so that the clock source 37 is connected to the binary counter to cause the counter 38 to operate. Operation of the counter 38 turns on successively transistors 41, 42, 43 each of which is connected to the input to the amplifier 32. The arrangement is such that for a given analogue voltage fed through resistor 31, the counter 38 will assume a stable condition in which one or more of the transistors 41 to 43 is conducting and the gate 35 turns off. Thus, the stable condition of the counter gives a digital signal representing the analogue voltage.
In order to generate the required triangular waveform, there is provided a schmitt trigger circuit 44 the output from which is fed to an integrater 45, which in turn has its output connected through a resistor 46 to the input terminal of the amplifier 32. The circuit 44 has two stable conditions, in which it has two different levels of voltage output. In one condition, the integrater 45 produces an increasing voltage which is fed back by way of a resistor 47a to the circuit 44 until the circuit 44 triggers to its alternative state. The integrater 45 then produces a decreasing voltage which again is fed back to the circuit 44 until the circuit 44 resumes its first state. The effect of this is that a triangular waveform is produced and fed through the resistor 46 to the amplifier 32. The output from the amplifier 32 is therefore stepped with an amplitude which is arranged to be equal to the level set by the circuits 33 and 34, which in turn is equal to the voltage level required to step the counter 38 one position. Thus, the effect will be that described with reference to FIG. 2.
There is of course a second amplifier 32 and associated components for the other parameter, and it is necessary to produce another triangular waveform 90° out of phase with the waveform described above, to be fed to the amplifier 22 associated with the other parameter. For this purpose, the output from the integrater 45 is also fed to a comparater 47, the output from which is fed to a further integrater 48 connected to the amplifier 32 associated with the other parameters. The comparater 47 detects the waveform produced by the integrater 45 and compares it with an earth potential, and since this output is then integrated by the integrater 48, a waveform is produced which is 90° out of phase with the original waveform, but is identical in frequency.
It will of course be appreciated that there are numerous other ways in which the digital signals can be varied in the required manner.
Although in FIG. 1 one form of matrix is shown, it will be appreciated that all that is required is some form of memory store which is designed to produce an output dependent on the two inputs it receives. This memory store can take a variety of forms, and merely by way of example a second form is shown in FIG. 4 This particular form of memory unit is particularly useful in controlling ignition timing.
The memory shown in FIG. 4 consists of a number of units 51, 52, 53, it being appreciated that the number of units can be considerably greater than the three shown. Each unit consists of a plurality of parallel resistor and switch combinations, and transducer means 54 operable by one of the parameters, for example engine speed, produces a digital signal acting through a switching device 55 to close one switch in each of the units 51, 52 and 53. The same switch is closed in each of the units, so that for a given value of the parameter, the first switch in each unit may be closed, but as the value increases, the second switch in each unit is closed instead of the first switch, and then the third switch and so on. The outputs from the units 51 to 53 are fed to a unit 56, which contains a plurality of switches, one for each of the units 53, these switches being controlled by a switching device 57 which in turn is controlled by a digital signal produced by transducer means 58 operable by the other parameter, conveniently manifold pressure. Thus, the first parameter chooses which switch in each of the units 51 to 53 is closed, and so selects one of a plurality of resistors to be connected to the unit 56. The other parameter closes one only of the switches in the unit 56, to determine which unit 51, 52, 53 is to be operative, and the resistor in use in that unit is then connected to an amplifier 59 which operates a control device 61 for varying the ignition timing.
It will be appreciated that there are several waveforms that can be used to obtain an interpolation effect. A further specific example is the use of a triangular waveform, with equal rising and falling slopes, with no d.c. component, of amplitude equal to half of the step, applied to an input parameter, and a sawtooth waveform of twice the frequency, with the waveform returning suddenly to zero at points coincident with the peaks of the triangular waveform, and amplitude equal to one half the step, and no d.c. component applied to the other input parameter.
Although it is preferred to vary each input signal by an amount no greater than the step, it is possible to vary each input signal by an amount greater than the step, provided that the output signals from the memory store are suitably selected.
It will be understood that the memory store must be arranged so that when an input signal is varied, for example while it is in square A in FIG. 2, the memory store will still provide an output when the signal leaves the square. In other words, the memory store must have a periphery beyond which the signal does not move.
An engine control system according to the invention comprises in combination first transducer means producing a first digital signal representing a first variable parameter of the engine, said first digital signal having one of a number of values changing in steps of X, second transducer means producing a second digital representing a second variable parameter of the engine, said second digital signal having one of a number of values changing in steps of Y, a memory store to which the first and second output signals are fed, said memory store being programmed to produce a predetermined output signal dependent on the values of the two digital signals, control means operated by the output signal from the memory store for controlling a characteristic of the engine, and means for varying said first digital signal, for a given value of the first variable parameter, so as to sample different output signals from the memory store.
An example of the invention is illustrated in the accompanying drawings, in which
FIG. 1 is a schematic circuit diagram illustrating a memory unit and associated circuits used in the example,
FIG. 2 is a truth table indicating possible outputs from the memory unit,
FIG. 3 illustrates a circuit arrangement for varying each of the digital signals and,
FIG. 4 is a circuit diagram of a memory unit used in a second example of the invention.
Referring to FIG. 1, a signal is produced by any convenient transducer 8 representing one of the three variables, engine speed, throttle angle and manifold depression, the signal being produced in the form of a three bit binary word transducers are commercially available which will sense engine speed and produce an electrical output which can be provided in the form of a three bit binary word, and similar transducers are available for giving a three bit binary word representing throttle angle and manifold depression. The exact details of these transducers are not important to an understanding of the invention. It will further be appreciated that in order to effect the required control, the two parameters utilized can be any two parameters chosen from the group of three parameters. Another three bit binary word is produced by another transducer 9 which responds to another of the three parameters, and the two words are fed respectively to a pair of decoders 11 and 12. There are 8 combinations of each digital signal, and for each input signal, the decoder 11 energizes one of eight input lines 13 of a diode matrix. The decoder 12 energizes one of eight output lines 14 depending on its input signal, and the lines 14 control a switching device 15, which can energize any one of eight groups of five lines 16 each of which crosses each line 13, with connections made where required by diodes for simplicity are shown as dots. One output line 21 is connectible to the first line 16 in each group and four further output lines 21 are connectible to the other four lines 16 in each group, the lines 21 being coupled to a device 22 for controlling the supply of fuel to the engine 23. The dots at the crossing points of the various lines within the device 15 represent electrical connections within the device 15 and do not of course represent diodes.
The drawing shows only the diodes associated with the lines 13 and the first two groups of input lines 16, but by way of example it will be seen that, assuming 1 represents the interconnection of two lines 13,16, then if the first group of lines 16 are energized and the eight lines 13 are energized in turn, the following output signals will be obtained: 10000, 00100, 01001, 01100, 10100, 10010, 11111 and 01000. Similarly, if the group is energized the output as the lines 13 are energized in turn will be 01100, 11001, 10101, 10000, 11001, 01101, 01111 and 11000. It will of course be appreciated that the term "energised" as applied to a group of lines 16 means simply that the group of lines is in operation, that is to say is connected to the device 22. This can be achieved in numerous ways; for example, the switching device 15 can, on receipt of an input on a line 14 provide base current to five transistors serving through their collector-emitter paths to connect the appropriate group of five lines 16 to the lines 21.
It will be appreciated that although a five bit output is obtained in the example, the value n can be from two upwards, depending on the accuracy required. Some outputs may be the same, indicating that the same fuel requirement is necessary for different operating conditions. The exact determination of the output for different inputs is made experimentally for a given engine, after which the same matrix can be used for all such engines. The signal, or a separately obtained signal, can be used to control the ignition timing, and in some instances the ignition timing along can be controlled using as parameters engine speed and throttle angle. The way in which the quantity of fuel and/or the timing is controlled depends on the purpose of the control system, but the arrangement is particularly intended for minimizing exhaust emission.
The system described provides an empirically determined stepped output signal in response to the two input signals, which are also stepped in value. Thus, if the inputs are designated a and b and change in steps of X and Y respectively, then considering only three values for each parameter, a truth table for the system can be written the truth table indicating various possible outputs from the memory unit. Paramater a Parameter b Output Signal. ____________________________________________________________ _____________ _ a b A a b+Y B a b+2Y C a+X b D a+X b+Y E a+X b+2Y F a+2X b G a+2X b+Y H a+2X b+2Y I ____________________________________________________________ _____________ _ It will be appreciated that the value A to I, which are determined for the engine under control empirically, may vary substantially in dependence upon the absolute values of the parameters a and b. Because the system is digital, the output signal is incrementally responsive to changes in engine conditions as a parameter changes. Thus, suppose a is constant at value a, but the engine parameter controlling b is varying, but for the moment has the value b. The output signal is A, but as b increases the output signal stays at A until b + Y is reached, then changes suddenly to B. For many engines control of this nature is sufficiently fine, but for some engines more accurate control is required. For this reason, there is imposed on one or both parameters a waveform of amplitude equal to one-half the step (1/2X or 1/2Y), the waveform having no d.c. component, and in a particular example being triangular waveforms associated with both parameters, the waveforms being 90° out of phase with each other and of the same frequency. The effect of this can be seen from FIG. 2, which represents the truth table mentioned earlier. The output signal will, by virtue or the impressed waveforms, be a summation of up to four of the values A to I. For example, let the parameters have value a + (3/2)X and b + (3/2)Y, then without the impressed waveforms the output signal is E. It is also E with the impressed waveforms because the output signal values of F,B,D, and H exist for zero time. However, consider now the case where the a parameter changes to a + (7/4)X. Without the modification, the signal is E. With the modification, there is a sampling of value H for one quarter of the time, and the output signal is 3/4E + 1/4H. If the b parameter now changes to b + (7/4)Y, the output signal will be 1/2E + 1/4F + 1/4H. It will be seen that in this way a far more accurate control is achieved.
In some circumstances, only one of the parameters need have an impressed voltage. This could be useful where variations in one parameter are far more important than variations in the other.
It will of course be appreciated that there are numerous ways of varying the digital signals in the required manner. By way of example, FIG. 3 shows an arrangement in which an analogue voltage produced by a transducer is fed by way of a resistor 31 to a summing amplifier 32, the output from which is fed to a pair of discriminating circuits 33,34. The circuits 33 and 34 only respond to an input having a predetermined amplitude, and operate respectively in response to positive and negative signals from the amplifier 32. The outputs from the circuits 33 and 34 are fed respectively to a pair of AND gates 35 and 36, each of which also receives an input from a clock source 37. The gates 35 and 36 provide outputs to a reversible binary counter 38, the output from which provides the input to the decoder 11 in FIG. 1.
When a signal is received by way of the resistor 31, it is fed through the amplifier 32 to the circuit 33, and assuming that the signal is greater than the minimum amplitude set by the circuit 33, an output is produced turning on the gate 35, so that the clock source 37 is connected to the binary counter to cause the counter 38 to operate. Operation of the counter 38 turns on successively transistors 41, 42, 43 each of which is connected to the input to the amplifier 32. The arrangement is such that for a given analogue voltage fed through resistor 31, the counter 38 will assume a stable condition in which one or more of the transistors 41 to 43 is conducting and the gate 35 turns off. Thus, the stable condition of the counter gives a digital signal representing the analogue voltage.
In order to generate the required triangular waveform, there is provided a schmitt trigger circuit 44 the output from which is fed to an integrater 45, which in turn has its output connected through a resistor 46 to the input terminal of the amplifier 32. The circuit 44 has two stable conditions, in which it has two different levels of voltage output. In one condition, the integrater 45 produces an increasing voltage which is fed back by way of a resistor 47a to the circuit 44 until the circuit 44 triggers to its alternative state. The integrater 45 then produces a decreasing voltage which again is fed back to the circuit 44 until the circuit 44 resumes its first state. The effect of this is that a triangular waveform is produced and fed through the resistor 46 to the amplifier 32. The output from the amplifier 32 is therefore stepped with an amplitude which is arranged to be equal to the level set by the circuits 33 and 34, which in turn is equal to the voltage level required to step the counter 38 one position. Thus, the effect will be that described with reference to FIG. 2.
There is of course a second amplifier 32 and associated components for the other parameter, and it is necessary to produce another triangular waveform 90° out of phase with the waveform described above, to be fed to the amplifier 22 associated with the other parameter. For this purpose, the output from the integrater 45 is also fed to a comparater 47, the output from which is fed to a further integrater 48 connected to the amplifier 32 associated with the other parameters. The comparater 47 detects the waveform produced by the integrater 45 and compares it with an earth potential, and since this output is then integrated by the integrater 48, a waveform is produced which is 90° out of phase with the original waveform, but is identical in frequency.
It will of course be appreciated that there are numerous other ways in which the digital signals can be varied in the required manner.
Although in FIG. 1 one form of matrix is shown, it will be appreciated that all that is required is some form of memory store which is designed to produce an output dependent on the two inputs it receives. This memory store can take a variety of forms, and merely by way of example a second form is shown in FIG. 4 This particular form of memory unit is particularly useful in controlling ignition timing.
The memory shown in FIG. 4 consists of a number of units 51, 52, 53, it being appreciated that the number of units can be considerably greater than the three shown. Each unit consists of a plurality of parallel resistor and switch combinations, and transducer means 54 operable by one of the parameters, for example engine speed, produces a digital signal acting through a switching device 55 to close one switch in each of the units 51, 52 and 53. The same switch is closed in each of the units, so that for a given value of the parameter, the first switch in each unit may be closed, but as the value increases, the second switch in each unit is closed instead of the first switch, and then the third switch and so on. The outputs from the units 51 to 53 are fed to a unit 56, which contains a plurality of switches, one for each of the units 53, these switches being controlled by a switching device 57 which in turn is controlled by a digital signal produced by transducer means 58 operable by the other parameter, conveniently manifold pressure. Thus, the first parameter chooses which switch in each of the units 51 to 53 is closed, and so selects one of a plurality of resistors to be connected to the unit 56. The other parameter closes one only of the switches in the unit 56, to determine which unit 51, 52, 53 is to be operative, and the resistor in use in that unit is then connected to an amplifier 59 which operates a control device 61 for varying the ignition timing.
It will be appreciated that there are several waveforms that can be used to obtain an interpolation effect. A further specific example is the use of a triangular waveform, with equal rising and falling slopes, with no d.c. component, of amplitude equal to half of the step, applied to an input parameter, and a sawtooth waveform of twice the frequency, with the waveform returning suddenly to zero at points coincident with the peaks of the triangular waveform, and amplitude equal to one half the step, and no d.c. component applied to the other input parameter.
Although it is preferred to vary each input signal by an amount no greater than the step, it is possible to vary each input signal by an amount greater than the step, provided that the output signals from the memory store are suitably selected.
It will be understood that the memory store must be arranged so that when an input signal is varied, for example while it is in square A in FIG. 2, the memory store will still provide an output when the signal leaves the square. In other words, the memory store must have a periphery beyond which the signal does not move.