Electronic closed loop control system for internal combustion engine

A clamping circuit is provided between a differential signal generator (such as a differential amplifier or a comparator) and a control signal generator (such as an integrator and/or a proportional amplifier) of an electronic closed loop control system for controlling an air-fuel ratio of the air-fuel mixture supplied to an internal combustion engine. The clamping circuit generates a signal whose maximum and maximum levels are symmetrical with respect to the half value of a potential of a d.c. power source provided for the clamping circuit.

This invention relates in general to an electronic closed loop air-fuel 
ratio control system, and particularly to such a control system which 
controls an air-fuel ratio of the air-fuel mixture supplied to an internal 
combustion engine based on a signal representative of a sensed 
concentration of a component in exhaust gas. 
Various systems have been proposed to optimally control the air-fuel ratio 
of the air-fuel mixture to an internal combustion engine in dependence of 
the modes of engine operation, one of which is to utilize the concept of 
an electronic closed loop control system based on a sensed concentration 
of a component (such as CO, CO.sub.2, HC, NO.sub.x and O.sub.2, etc) in 
exhaust gases from the engine. 
FIG. 1 illustrates schematically an example of a conventional electronic 
closed loop control system. An exhaust gas sensor 3 is provided in an 
exhaust gas pipe 2 extending from an internal combustion engine 1. The 
exhaust gas sensor 3 senses a concentration of a component in exhaust 
gases to generate a signal representative thereof. A differential signal 
generator 4 (a differential amplifier or comparator) is connected to the 
sensor 3 to receive the signal therefrom. The signal thus received is 
compared, in the generator 4, with a reference signal which is fed through 
a terminal 8 to the generator 4 or which is generated in the generator 4 
itself. Thus, the generator 4 generates a signal representative of a 
differential between the signal from the sensor 3 and the reference 
signal. The signal from the generator 4 is then fed to the next stage, 
viz., a control signal generator 5 which usually includes a conventional 
p-i (proportional-integral) controller and a pulse generator. The 
provision of the p-i controller, as is well known in the art, is to 
improve the efficiency of the electrical closed loop control system, in 
other words, to facilitate a rapid transient response of the system in 
question. The p-i controller feeds a signal to the pulse generator which 
generates a pulsating signal in order to control an actuator 6 (for 
example, an electromagnetic valve). Thus, the air-fuel ratio of the 
air-fuel mixture fed to the engine 1 is regulated by controlling the 
amount of fuel and/or air through the actuator 6. 
In the above, the magnitude of the reference signal is previously 
determined in due consideration of optimum air-fuel ratio of the air-fuel 
mixture supplied to the engine 1 for maximizing the efficiency of a 
catalytic converter 7 or a reactor (not shown) provided in the exhaust gas 
pipe 2 downstream of the sensor 3. In this instance, when a so-called 
three way catalytic converter is employed, the air-fuel ratio is 
maintained in the vicinity of stoichiometric air-fuel ratio. This is 
because the efficiency of the three-way catalytic converter is maximized 
at that air-fuel ratio. A three-way catalytic converter, as is well known, 
as a characteristic of deoxidizing NO.sub.x and oxidizing both CO and HC 
at the same time. 
FIG. 2 illustrates an example of a conventional circuit configuration of 
the differential signal generator 4 in FIG. 1. Since the circuit of FIG. 2 
has been known in the art, detailed description thereof will be omitted in 
the following. The signal from the sensor 3 is supplied to the 
differential signal generator 4 through a terminal 9, on the other hand, 
the reference signal is also supplied to the same through a terminal 8. 
The terminal 8 is connected to the base of a transistor 20 the emitter of 
which is connected to the base of a transistor 21 and the collector 
thereof to the ground. The emitter of the transistor 21 is connected 
through a suitable resistor 24 to the d.c. power source (not shown) 
coupled to a terminal 25 and the collector thereof is connected to the 
ground through a forwardly provided diode 26. Whilst the terminal 9 is 
connected to the base of a transistor 22 the collector of which is 
connected to the ground and the emitter thereof is directly connected to 
the base of a transistor 23. The emitter of the transistor 23 is connected 
to the terminal 25 through the resistor 24 and the collector thereof is 
connected to the collector of a transistor 27 whose emitter is connected 
to the ground. The base of the transistor 27 is connected to a junction 
between the collector of the transistor 21 and the anode of the diode 26. 
The transistors 20, 21, 22, and 23 form a differential amplifier, so that 
these transistors are selected to have substantially equal characteristic 
with one another. As shown, the collector of the transistor 27 is 
connected to the base of a transistor 29. The collector of the transistor 
29 is connected to the ground and the emitter thereof to both the base of 
a transistor 35 and the terminal 25 through a resistor 28. The collector 
of the transistor 35 is connected to the terminal 25 and the emitter 
thereof directly to the base of a transistor 36. The emitter of the 
transistor 36 is connected to the ground and the collector thereof to the 
bases of transistors 39, 42, and also connected to the terminal 25 through 
a resistor 37. The transistors 29, 35 and 36 are provided to amplify the 
signal current. The collector of the transistor 39 is connected to the 
terminal 25 and the emitter thereof to the base of a transistor 40. The 
collector of the transistor 40 is connected to the terminal 25 and the 
emitter thereof to the emitter of a transistor 42 through a resistor 41. 
The transistors 39 and 40 form a so-called Darlington amplifier. The 
collector of the transistor 42 is connected to the ground. An output 
terminal 10 is connected to the emitter of the transistor 42, from which 
terminal the control signal generator 5 derives the signal indicative of 
the differential between the two signals supplied to the generator 4 
through the terminals 8 and 9. 
In such a differential signal generator as shown by reference numeral 4, 
usually only one d.c. power supply (the potential of which is assumed to 
be V.sub.cc) is employed, in the case of which the potential V.sub.cc is 
determined to be the maximum of the output signal from the generator and 
the potential "zero" to be the minimum and the half valve of V.sub.cc is 
determined to correspond to the case where a signal fed to the 
differential signal generator is equal to a reference one. 
However, in the circuit of FIG. 2, the maximum potential of its output 
signal is not equal to V.sub.cc but to V.sub.cc -2V.sub.BE, and the 
minimum potential becomes V.sub.BE, where V.sub.BE is a voltage drop 
between the base and the emitter of each of the transistors 39, 40 and 42. 
Therefore, as shown in FIG. 3, the potential difference between the 
maximum and 1/2V.sub.cc is no longer identical to that between 1/2V.sub.cc 
and the minimum. This unbalance with respect to the half value of V.sub.cc 
may invite an undesirable phenomenon in a stage which follows the control 
signal generator 5. That is, when an integrating circuit of FIG. 4, which 
consists of a resistor 5' and a capacitor 5", is connected to the 
differential signal generator 4, the charging and the discharging time of 
the integrating circuit 5 is not equal to each other, so that the change 
in signal magnitude from the sensor 3 is no longer employed for precise 
control of the air-fuel ratio of the air-fuel mixture. 
In order to avoid the above described defect, a circuit as shown in FIG. 5 
has been proposed which makes equal the charging and the discharging time 
in question. In FIG. 5, four resistors 50 to 53 are connected in series 
between the d.c. power source (V.sub.cc) and the ground, supplying the 
bases of transistors 54 and 55 with predetermined potential in order that 
the transistor 54 and 55 are respectively rendered conductive when the 
signal from the differential signal generator 4 is lower and higher than 
the half value of V.sub.cc. As shown, two variable resistors 56 and 57 are 
respectively connected to the collectors of the transistors 54 and 55. The 
resistances of the variable resistors 56 and 57 are adjusted to make equal 
the charging and the discharging time of a capacitor 58, Thus, a properly 
integrated signal is derived from a terminal 59. 
However, the circuit of FIG. 5 is not preferable in that, due to the 
provision of the two variable resistors to be precisely adjusted, the 
circuit is not suitable for mass production and complicated in 
configuration. 
In order to remove the above described defect, in accordance with the 
present invention, an improved circuit is provided between the 
differential signal generator 4 and the control signal generator 5. This 
circuit clamps the signal from the generator 4 to supply the generator 5 
with a signal the maximum and the minimum levels of which are symmetrical 
with respect to the half value of the potential of a d.c. power source 
provided for the circuit. 
It is therefore an object of the present invention to provide an improved 
circuit which can remove the above described defect with a simple 
configuration and which is suitable for mass production.

FIG. 6a illustrates a circuit 4' embodying the present invention, together 
with the conventional differential signal generator 4. A d.c. power source 
(not shown, the potential of which is V.sub.cc ') is connected to the 
cathode of a diode 71 through a terminal 71. The potential V.sub.cc ' is 
determined to be lower than the potential V.sub.cc - 2V.sub.BE. This 
potential V.sub.cc ' can be readily obtained by dividing V.sub.cc by means 
of a suitable voltage divider. The anode of the diode 71 is connected to 
the terminal 25 through a resistor 72 and to a junction 75 between the 
anode of a diode 73 and the base of a transistor 76. The cathode of the 
diode 73 is connected to both the output terminal 10 of the differential 
signal generator 4 and one end of a resistor 74. The other end of the 
resistor 74 is connected to the ground. The collector of the transistor 76 
is directly connected to the terminal 25 and the emitter thereof to the 
ground through a resistor 77. An output terminal 78 is connected to the 
emitter of the transistor 76 for deriving the output signal from the 
circuit 4'. 
With this arrangement, when the potential appearing at the terminal 10 is 
larger than V.sub.cc ', the diode 71 is rendered conductive with the diode 
73 nonconductive. As a result, the potential at the junction 75 is 
V.sub.cc ' + V.sub.D1, where V.sub.D1 is a forward voltage drop across the 
diode 71. This means that the potential at the terminal 78 is equal to the 
potential V.sub.cc ', because the voltage drop between the base and the 
emitter of the transistor 76 is substantially equal to V.sub.D1 so that 
V.sub.D1 and V.sub.BE are cancelled each other. Thus, the maximum of the 
signal from the terminal 78 is V.sub.cc ' as shown in FIG. 7. 
On the other hand, when the potential at the terminal 10 is lower than 
V.sub.cc ', the diode 73 is in turn rendered conductive with the diode 71 
nonconductive. Therefore, the potential at the junction 75 is the 
potential at the teminal 10 plus V.sub.D2, where V.sub.D2 is a forward 
voltage drop across the diode 73. This means that the potential at the 
terminal 78 is equal to the potential at the terminal 10, because the 
voltage drop between the base and the emitter of the transistor 76 
(V.sub.BE) is substantially equal to V.sub.D2, so that V.sub.D2 and 
V.sub.BE cancel each other. 
In the circuit of FIG. 6a, the purpose of the resistor 74 is to lower the 
minumum value of the signal from the terminal 78 to about zero. To this 
end, the resistance of the resistor 74 is determined to be much smaller 
than that of the resistor 72 (for example, 1:100), and also smaller than 
impedence of the transistor 40 when it is nonconductive. 
If the resistor 74 is not provided, the minimum value of the signal from 
the terminal 78 is about V.sub.BE. This is because, when the diode 73 is 
conductive (that is, the potential at the terminal 10 is lower than 
V.sub.cc '), a current flows through the resistor 72, the diode 73, and 
the transistor 42, with the result that the potential at the terminal 10 
is about V.sub.BE. 
FIG. 7 illustrates the potential appearing at the terminal 78 as a function 
of the potential of the signal from the sensor 3. As shown, the maximum 
potential of the signal is V.sub.cc ', and the minimum is substantially 
zero, and the maximum and the minimum levels are symmetrical with respect 
to 1/2V.sub.cc '. 
Therefore, due to the provision of the circuit 4' of FIG. 6a, the simple 
integrating circuit in FIG. 4 can be utilized without adversely affecting 
the precise control of the electronic closed loop control system in 
question. 
In the circuit of FIG. 6a, the transistor 76 can be replaced by a diode 80 
as shown in FIG. 6b. 
It is understood from the foregoing that, in accordance with the present 
invention, the control signal generator 5 can receive a signal, the 
maximum and the minimum values of which are symmetrical with respect to 
the half value of the potential of the d.c. power source provided for the 
clamping circuit. As a result, as the control signal generator 5, a simple 
circuit can be used thereby to simplify the control system with the 
advantage of suitability for mass production.