CA1063827A - Exhaust gas sensor output signal control system - Google Patents

Abstract

EXHAUST GAS SENSOR OUTPUT SIGNAL CONTROL SYSTEM
ABSTRACT OF THE DISCLOSURE
A system for generating an output signal from an exhaust gas sensor which output signal is linear for variations in the air/fuel mixture. The measured signal can be used to control the air/fuel mixture at a desired value. The system includes two voltage dividers connected in parallel to a regulated voltage. One voltage divider includes a variable resistance type exhaust gas sensor and generates a signal voltage while the other voltage divider generates a reference voltage, the voltages being received at separate inputs of an amplifier the output of which is rendered linear with respect to changes in the air/fuel ratio of the combustion mixture by a feedback circuit.

Description

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- The present invention is directed to the field of internal combustion engine control systems and primarily to that portion of the above-noted field which is concerned with ~;
the chemical analysis of the composition of the exhaust gases produced by the internal combustion engine. More particularly, the present invention is also concerned with a system for providing an exhaust gas sensor output signal which is linear with respect to changes in the air/fuel ratio of the combustible mixture being provided to the engine and producing the exhaust gas environment of the sensor.
This application is a division of copending appli-- cation Serial No. 228,777 filed June 5, 1975 and entitled "Electrical Control System for an Exhaust Gas Sensor".
Ceramic sensors for electrically reacting to the ` partial pressure of oxygen within the exhaust gases produced -by an internal combustion engine are known. One such sensor is described in U.S. Patent No. 3,593,230. The sensor ;
there described relies upon changes in the electrical resis-... .
tance of titania ceramic material in response to changes in the partial pressure of oxygen in the environment of the -sensor. These sensors operate at elevated temperatures on the order of about, for example, 600 to 900C. Other sensor ceramic materials having a variable electrical resistance are known. Since the resistance of the ceramic material may vary with variations in the temperature within the operating range, `~
as well as with variations in the partial pressure of oxygen, ~ ~-it has become apparent that accurate control of the temperature o~ the sensor is desirable when partial pressure of oxygen .~ .
determination is desired. The parent application Serial No.

228,777 describes and claims a system ~or controlling the

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application of heat to a ceramic exhaust gas sensor to maintain a selected temperature at the sensor along with electrical circuits for generating a predetermined tempera~
ture from an electrical heating winding and for controlling the average electrical power delivered to a heater winding.
In the control of an internal combustion engine to provide an exhaust gas having a precisely controlled chemical ; composition for subsequent treatment by exhaust gas treatment devices, the maintenance of the air/fuel ratio of the combustion mixture at a precisely controlled value is o~
cardinal importance. The known exhaust gas sensor ceramic -materials which demonstrate a variable resistance in response to variations in the partial pressure of oxygen in the exhaust gases and which are compatible in terms of response time and life capacity with an automotive environment show a resistance variation such that the logrithm of resi~tance is approximately linear in terms of variation in air/fuel ratio. In order to provide an output signal for modulating aither the air or the fuel content of the air/fuel mixture it ls desirable to match the resistance variation of the sensor, the output signal, and the response of the modulating mechanism.
The present invention provides an electrical system for generating an exhaust gas sensor output signal which varies approximately linearly with respect to variation of the air/fuel ratio of the combustion mixt~re and thereby convert the normally logrithmic sensor signal to an approximately linear sensor signal. The linear response may be used by a controller for modulating either the air or the fuel content of the combustion mixture.
In accordance with the present inv~ntion, there is provided a system for generating an output signal from an exhaust gas sensor which output signal is linear for variations in the air/fuel mixture comprising in combination:
a variable resistance type exhaus~ gas sensor; mean~ ~or generating a regulated voltage; first voltage divider means, including the sensor, connected to the regulated voltage for generating a signal voltage; second voltage divider means .:
connected to the regulated voltage in parallel with the .~.
first voltage divider means, for generating a reference voltage; amplifier means having an output terminal and .:.
at least two input terminals, the input terminals arranged . to receive the signal and reference voltages, respectively;
and circuit means interconnecting the output terminal and .. ..
one of the input terminals, operative to controllably vary the energization of the sensor whereby the output signal of the amplifier may be rendered linear with respect to changes : in the air/fuel ratio of the combustion mixture forming the exhaust gas environment of the se:nsor. ~
The invention is described further, by way of illus- ~-: ~;
.: 20 tration~ with reference to the accompanying drawings, in which: ~:

Flgure 1 is a schematic diagram illustrating an : internal combustion engine having an exhaust responsive : feedbacX fuel control mechanism with which the present ~;

. invention is of utility; `~.

Figure-2 illustrates, in a block diagram form, the -~ electronic fuel control feedback circuit of the present ..

invention; . ; ~::

Figure 3 is an electronic circuit diagram illustra- :

ting a sensor temperature controller; .

- 30 Figure 4 is an electronic circuit diagram illustra-ting a sensor.signal generator; and : :
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Figure 5 is an electronic circuit diagram o~ an air/fuel ratio modulation means usable with the present invention.
Referring now to Figure 1, an internal combustion engine 10 is illustrated. Internal combustion engine 10 is provided with an intake manifold 12 and an exhaust manifold ; 14. Exhaust manifold 14 communicates with an exhaust gas conduit 16. A fuel metering and delivery device 18, which may be for example a fuel injection system or a carburetor is illustrated schematically communicating with the intake end of intake manifold 12. Fuel metering and delivery device 18 is provided with an air cleaner 20 such that air injected by engine 10 through intake manifold 12 may be drawn from the atmosphere through air cleaner 20 and through at least a portion of the fuel metering and delivery device 18. Fuel metering and delivery device 18 is further provided with an air/fuel ratio modulator means 22~ Air/fuel ratio modulator means 22 may be for example, in the case of an electronic fuel injection system, a variable resistor arranged to control the quantity of fuel delivered to engine 10 in relation to a given quantity of air or, in the case of a carburetor, may be a variably positionable metering orifice arranged to control the quantity of fuel delivered to englne 10 in respect of a given quantity of air. The air/fuel ratio modulator means 22 may alternatively be arranged to control a variably positionable air valve so that the quantity of air injected by engine 10 in respect of a given quantity of fuel delivered by fuel metering and delivery device 18 may be modulated.
Exhaust yas conduit 16 is provided with an exhaust gas sensor 24 which is mounted to conduit 16 so as to place '' ' '~:

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3t3~'7 an exhaust gas chemistry responsive element within the stream of exhaust gases flowing through conduit 16. The presently preferred form of this device is that of a variably resistive ceramic exhaust gas sensor formed of, for example, titania or cobalt monoxide. Other forms of this device are suitable and are contemplated. Electronic control means 26 -communicates with exhaust gas sensor 24 through a plurality of sensing leads 28, 30, 34 and heater power lead 32. Elec- -tronic control means also communicates with the air/fuel ratio modulator means 22 over conductive lead 36. As described hereinbelow, the electronic control means 26 is arranged to respond to changes in the exhaust gas chemistry sensed by exhaust gas sensor 24 to provide a control signal ~ for recelpt by air/fuel ratio modulator means 22 which -, control signal may be arranged to modulate either the fuel or the air content of the air/fuel ratio mixture being provided to internal combustion engine 10 to maintain a desired exhaust gas chemistry. , ~;
Referring now to Figure 2, electronic control means ~-26 is shown in a block diagram form. Electronic control ; ~ -means 26 is comprised of sensor temperature controller 38, the sensor signal generator 40 and the modulator control signal generator 42. These devices are energized by a source of electrical energy which may be for example battery 44 or any convenient source of electrical energy. As here illustra-ted, battery ~4 is electrically connected so that its negative terminal is grounded and this corresponds to the present conventional automotive implementation wherein the chassis and body of the vehicle provide a negative common ground.
Sensor temperature controller 38 communicates with e~haust gas sensor 24 through leads 28, 30 and 32 while sensor signal - 6 - ;

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i,3~ ;t generator 40 communicates with sensor 24 through lead 34.
The signal from sensor signal generator 40 is communicated to the modulator control signal generator 42 by conducting lead 46.
As illustrated in Figure 2, exhaust gas sensor 24 is arranged to support a wafer 48 of ceramic sensor material within the exhaust gas stream flowing within exhaust conduit 16. Such structure is disclosed, for example, in our co-pending Canadian patent application Serial No. 228,786 filed June 5, 1975. The sensor temperature control-ler 38, sensor signal generator 40 and modulator control signal generator 42 are illustrated as receiving positive voltage from battery 44 through positive bus or conductor 50.
Referring now to Figure 3, the sensor temperature controller 38 is illustrated in its presently preferred electronic embodiment. This circuit forms one embodiment of the invention described in the aforementioned parent application Serial No. 228,777.
In view of the fact that resistive type ceramic exhaust gas sensors have a resistance value which varies both as a function of temperature and as a function of the partial pressure of oxygen within the yas of the environment of the sensor it is necessary to maintain the sensor at a substantially constant temperature so that resistance varia-tions of the sensor are indicative solely of the partial pressure of oxygen within the exhaust gases and hence of the :
complete chemistry of the exhaust gases. To this end~ a heating means usually in the form of a heater wire is arranged in close proximity to the sensor element per se in order to establish and maintain a fixed level of temperature.

The above noted co-pending Canadian patent application Serial ~., ~ ;.
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Number 228,786 illustrates one construction wherein the :
heater wire comprises a coil of platinum resistance wire in surrounding relation to the active ceramic sensor element. .:
In Figure 3, the heater means is deno~ed as the winding 52 of resistance wire. Power is applied to the circuit of Figure 3 from the positive bus 50 through transis- :
tor switch 54. Transistor switch 54 is illustrated as a PNP -.
transistor having its emitter connected to positive bus 50 .
and its collector connected to a common conductor 56. Common .
conductor 56 is provided with a first voltage divider network ':
comprising resistances 58, 60 and having a common point 62.
Common conductor 56 lS also connected to a second voltage divider co~prised o~ adjustable resistance 64 and heater ~ ' winding 52. A junction 66 is formed intermediate variable resistance 64 and heater winding 52. Common point 62 is connected to the negative terminal of amplifier 68 through ':
resistanc~, 70. Junction 66 is connected to the positive ~ :
terminal o~ amplifier 70 through a pair of fixed trim resistors 72, 74 and adjustable trim resistor 76. The ou~put of amplifier 68 is communicated to the base of - -transistor switch 54 through diode output network 78.
Oscillator amplifier 80 is arranged to have its :
output connected to the diode output network 78. The negative .' terminal of amplifier 80 is communicated to the junction 83 of a voltage dividèr network comprising resistances 82, 84 connected in series and resistance 86 connected between ~unction 83 and common conductor 56. The positive terminal of amplifier 80 is connected to one plate of capacitor 88 and, through resistance 90 to common conductor 56.
In addition, the circuit of Figure 3 illustrates :
various electronic components such as zener diode 92 whlch ~' . '' '' ~ - 8 -.
';' , ,, is operative to turn on tran,istor switch 54 in order to pass voltage spikes and capacitor 94 in series with indicator lamp 96 connected between the common conductor 56 and grouna which may be operative to give an indication that the circuit is operational. Further re, various other components are illustrated, though not identified, in order to assure that the circuit will turn on and turn off properly and th~t various voltage levels which may exist within the circuitry will not -~ be of sufficient magnitude to cause damage to the delicate electronic components connected thereto.
The circuit of ~igure 3 main$ains control of the temperature of heater 52 by regulating heater resistance.
The heater, which may be for example platinum conductive ~ wire, has a high temperature coefficient of resistance, which ; makes its resistance change to temperature a convenient control variable. With transistor switch 54 in the on, or conductive, mode full bus voltage is applied to the series connection of adjustable resistance 64 and heater winding 52. Transistor switch 54 is maintained in the on or conductive mode by amplifier 6B which draws current through the base lead 54b of transistor 54. As current continues to flow through winding 52, the resistance of winding 52 will increase with ; increasing temperature causing the voltage appearing at junction 66 to increase relative to ground. When the heater resistance rises to the value corresponding to the set value, that is the value when the voltage at junction 66 equals to that at common point 62, the voltage at the positive terminal of amplifier 68 will equal the voltage at the negative ter- -minal of the amplifier and the voltage at the output terminal -of the amplifier will rise to a high value approximating that on bus SO. Thus, base current will not be allowed to flow .. ~ ~ ;,'"' _ g _ ',,: :' ; .
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tii3~7 out of the base terminal of the transistor 54 and the voltage on common conductor 56 will approach the ground or zero value.
The voltage established by voltage divider 98, 100 operating through diode 102 will maintain the bias of the positive terminal of amplifier 68 sufficiently high to hold the switch 54 in the of~ or non-conducting mode.
Resistances 82, 84, 86 provide a voltage divider applied to the negative input of amplifier 80. This voltage may be for example about 8 volts with 12 volts applied across the bus and ground and with common conductor 56 also at a ;~ -positive 12 voltage through transistor switch 54. Conversely, ~ -,-as common conductor 56 approaches ground potential, the vol-tage appearing at the negative input of amplifier 80 will drop to approximately 4 volts under the same conditions.
Capacitor 88 in conjunction with resistances 90 and 104 forms an ~C oscillatory circuit such that the positive terminal of ampIifier 80 will be held at about 10 volts which value will decay to approximately 4 volts as capacitor 88 is charged through resistance 90. The output of amplifier 80 will `^
- 20 remain high during this time period and will switch to a low value when the voltage at the positive terminal goes below the

4 volt value established at the negative terminal of amplifier 80. When the output of amplifier 80 goes to a low value base current will be drawn from the base 54b of transistor switch 54 and transistor switch 54 will be turned to the on or conducting mode. At this time, current will again flow through the first and second voltage dividers and amplifier 68 will operate to compare the voltage at junction 66 with the voltage appearing at common point 62. Assuming that the temperature of the heater winding 52 has drifted from the set value, the amplifier 68 will provide a relatively low :' . ' :"
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voltage output at its output terminal holding transistor switch 54 in the on or conducting mode until such time as the temperature has risen to the desired value.
Resistance 64 is here shown to be an adjustable resistance since it is desired to accurately match the ratio of resistance 54 to the resistance of the winding 52 to equal the ratio of resistance 58 and resistance 60 to provide for accurate control. In one forml adjustable resi~tance 64 may be a piece of manganin wire or other very low temperature coefficient of resistance resistor. The wire may be covered with an electrically insulating sleeve and may be made physically a part of the cable of wires extending from sensor ; temperature controller 38 to exhaust gas sensor 24. The temperature of the environment of winding 52 may be set at ~ the desired value and the terminal of variable resistance ;' 64 may be varied by use o~ a brass ferrule. The resistance '~
of variable resistor 64 may be adjusted while the heater is powered from a suitable source. The two sensing wires 28, 30 which extend from the heater allow the controller to respond to heater resistance only with negligible temperature dependence on the resistance of the various leads extending to the heater 52. This is important as the heater leads may have a resistance which is on the same order of magnitude as resistanre of the heater itself~ The voltage drop from the heater winding to ground is eliminated by sensing lead 28 which places this lead voltage drop outside the comparison ;-~ bridge. The voltage drop to the heater itself is compensated for by the potential divider composed of resistances 72, 74 and 76.
Referring now to Figure 4, the sensor signal generator 40 is illustrated. Sensor signal generator 40 , ~:.::
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~ 'f' receives voltage from common conductor 50. 10ad resi tance 106 is arranged to be in series between common conductor 50 -~
and the remainder of the portion of sensor signal generator 40. The voltage divider comprise resistances 108 and 110 in conjunction with amplifier 112 is operative to provide a ~-regulated voltage vus 114 at about for example 9 volts. A
comparison voltage divider comprised of resistances 116, 118 such that the junction 120 of resistances 116 and 118 is communicated to the positive input terminal of amplifier 122.
Sensor 48 is arranged to be-excited through lead 34 from variable resistance 1~3 so that the junction 124 intermediate resistance 123 and sensor 48 is communicated to the negative input terminal of amplifier 122. Amplifier 122 is provided with a linearizing feedback n~twork 126 which interconnects the output of amplifier 122 with the negative input of amplifier 122.
Referring now to Figure 5, the modulator control signal generator 42 is illustrated in a presently preferred embodiment. Signal generator 42 is arranged to receive the signal generated by the sensor signal generator 40 over conductor 46 and is also arranged to receive the positive voltage on ~us 50. Potentiometer 128 is connected between .:, . . :,: :
bus 50 and ground and is arranged so that its center tab ~ i communicates with the positive terminal of amplifier 130. ~ ~ ;
The signal appearing on conductor 46 is applied, through ~-;
variable resistance 132 to the negative input terminal of - amplifier 130. Potentiometer 128 is adjusted so that, when -the exhaust gas mixture contains a partial prassure of oxygen indicative of engine operation at the proper air/fuel ratio 30 of signals applied to the positive input terminal of amplifier 130 and on signal lead 46 will be equal in magnitude.

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Modulator control signal generator 42 is provided with limit signal means 138, 140, which include resistances 134, 136 and which are operative to saturate amplifier 130 for extreme signal conditions and to prevent the application of control signals to the air/fuel ratio modulator means 22 which are of excessive magnitude~ Amplifier 130 is also provided with the integrating feedback means 138 which modulates the speed of response of the amplifier 130 to avoid a transient oscil-lation from building and which integrates air/fuel ratio to provide effectively zero error after sufficient time.
Feedback means 142 may be adjusted by use of suitably sized components to provide an integrating time period approaching the internal combustion engine transport time to permit the effects of an error correction signal to be seen by sensor ~4.

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The present invention, therefore, provides an .` electronic circuit for generating a sensor output signal ;:
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which is substantially linear with respect to variations in :~

, the air/fuel ratio of the mixture being provided to the ~.1" . . ,: .
:~ 20 internal combustion angine and which is generating the exhaust ~: :
.: . -gases of the environment of the sensor.

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Claims (2)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A system for generating an output signal from an exhaust gas sensor which output signal is linear for varia-tions in the air/fuel mixture comprising in combination:
a variable resistance type exhaust gas sensor;
means for generating a regulated voltage;
first voltage divider means, including said sensor, connected to said regulated voltage for generating a signal voltage;
second voltage divider means connected to said regulated voltage in parallel with said first voltage divider means, for generating a reference voltage:
amplifier means having an output terminal and at least two input terminals, said input terminals arranged to receive said signal and reference voltages, respectively; and circuit means interconnecting said output terminal and one of said input terminals, operative to controllably vary the energization of said sensor whereby the output signal of said amplifier may be rendered linear with respect to changes in the air/fuel ratio of the combustion mixture forming the exhaust gas environment of the sensor.

2. The system of claim 1, wherein said circuit means comprise means interconnecting said amplifier output terminal and the amplifier input terminal to which the sensor is communicated operative to vary the current flow through said first voltage divider means in response to the voltage at the amplifier output terminal.