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Claims  |
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I claim:
1. A gas sensor construction coupled to an engine producing a gas stream
comprising:
a variably resistive gas and temperature responsive sensing element in
communication with the gas stream for producing a first electrical signal
responsive to the partial pressure of oxygen of the gas stream and to the
temperature of the gas stream;
a measuring means for developing a second electrical signal as a function
of engine speed; and
an electrical circuit means for processing the electrical signals from said
sensing element and said measuring means so that said second electrical
signal is used to reduce any temperature dependence in said first
electrical signal.
2. A gas sensor construction as recited in claim 1 wherein:
said electrical circuit means includes a bias voltage generating means for
producing an output bias voltage, a summing amplifier means for combining
said output bias voltage and said second electrical signal, said summing
amplifier means having sufficient gain so that the combination of said
output biase voltage and said second electrical signal is amplified to a
magnitude which is sufficient to compensate for temperature dependence in
said first electrical signal and has the general form of A (B+X), wherein
A is the magnitude of the gain of said summing amplifier means, B is the
magnitude of the bias voltage, and X is the magnitude of said second
electrical signal.
3. A gas sensor construction as recited in claim 2 wherein:
said summing amplifier means is an operational amplifier with a positive
input for receiving said output bias voltage and said second electrical
signal and a negative input coupled through a variable resistor to the
output of said summing amplifier means to adjust the amplification of the
sum of said output bias voltage and said second electrical signal.
4. A gas sensor construction as recited in claim 1 wherein:
said sensing element has an electrical resistance which varies with the
partial pressure of oxygen of the gas stream and with the temperature of
the gas stream; and
said electrical circuit means is adapted to respond to a voltage drop
across said sensing element, said voltage drop being a function of the
magnitude of said electrical resistance of said sensing element.
5. A gas sensor construction as recited in claim 4 wherein:
said electrical circuit means includes a first comparator having a first
input adapted to receive a signal which is a function of said first
electrical signal and a second input adapted to receive a signal which is
a function of said second electrical signal, said first comparator having
a first output adapted to provide a signal which is a function of the
partial pressure of oxygen and is compensated for temperature of the gas
stream.
6. A gas sensor construction as recited in claim 5 wherein said electrical
circuit means further includes:
a second comparator having a third input adapted to receive a signal which
is a function of said first electrical signal, a fourth input adapted to
receive a signal which is a function of a reference voltage, and a second
output adapted to provide a signal which is a function of the difference
between the reference voltage and said first electrical signal and coupled
to said first input of said first comparator, said second comparator
further comprising a first feedback means for applying the signal provided
by said second output to said third input;
a third comparator having a fifth input adapted to receive a signal which
is a function of engine speed, a sixth input adapted to receive a signal
which has an adjustable voltage level, a third output coupled to said
second input, and a second feedback means coupling said third output and
said sixth input and including a means for varying the magnitude of the
input voltage applied to said sixth input;
a fourth comparator having a fourth output coupled to said fifth input, a
seventh input coupled to said fourth output and an eighth input coupled to
a voltage biasing means, and
a fifth comparator having a fifth output coupled to said fifth input, a
nineth input coupled to said fifth output and a tenth input adapted to
receive an electrical signal changing in magnitude as a function of engine
speed.
7. A system for temperature compensating the resistance variation of an
engine exhaust gas sensor responsive to a partial pressure of oxygen
indicative of the air/fuel ratio of engine operation comprising:
an interface means for establishing a first difference voltage which is a
function of a reference voltage and a function of a sensor voltage which
is related to the resistance of said exhaust gas sensor;
a compensation means for establishing a second difference voltage which is
a function of a tachometer voltage related to engine speed and a function
of a bias voltage which serves to adjust the magnitude of said second
difference voltage; and
a first comparator means for establishing a third difference voltage which
is a function of said first and second difference voltages and adapted to
provide a first output for use in controlling the air/fuel ratio of engine
operation, said first difference voltage being coupled to a first input
and a second difference voltage being coupled to a second input.
8. A system for temperature compensating as recited in claim 7 wherein:
said interface means includes a second comparator having a second output, a
third input coupled to said sensor voltage, a fourth input being coupled
to said reference voltage and a first feedback means coupling said second
output and said third input for stabilizing the electrical signal provided
by said second output.
9. A system for temperature compensating as recited in claim 7 wherein said
compensation means includes:
a third comparator having a fifth input coupled to a fourth output of a
fourth comparator and a fifth output of a fifth comparator, said third
comparator having a sixth input and a third output coupled by a second
feedback means for adjusting the magnitude of the signal carried by said
third output;
said fourth comparator having a seventh input coupled to said fourth output
and an eighth input coupled to said sensor voltage; and
said fifth comparator having a nineth input coupled to said fifth output
and a tenth input coupled to said tachometer voltage.
10. A system for temperature compensating as recited in claim 9 wherein:
said second feedback means includes a first resistor coupling said sixth
input to ground and a second resistor coupling said third output to said
sixth input, said second resistor being adjustable in magnitude so that
the amplification gain at said third comparator means can be adjusted.
11. A system for temperature compensating as recited in claim 10 wherein:
said eighth input is coupled to a third resistor, said third resistor being
adjustable so as to adjust the magnitude of the input voltage as said
eighth input.
12. A system for temperature compensating as recited in claim 9 wherein:
said compensation means provides an output of the form A (B+X) where A is
an amplification factor provided by said third comparator and said second
feedback means, B is a function of the bias voltage and X is a function of
the tachometer voltage.
13. A method for temperature compensating a gas sensing element including
the step of:
developing a first signal dependent upon the temperature and partial
pressure of oxygen of an exhaust gas stream produced by an engine;
generating a second signal dependent upon the speed of the engine; and
combining the first and second signals so that a third signal is generated
which is substantially independent of the effect of the temperature of the
gas stream and substantially dependent upon the partial pressure of oxygen
of the gas stream.
14. A method as recited in claim 13 wherein generating said second signal
dependent includes having the signal magnitude obey the equation A (B+X)
where A is an amplification constant, B is a biasing constant and X is a
function of the engine speed.
15. A method as recited in claim 14 wherein generating said second signal
includes:
utilizing the voltage drop across a variable resistance to obtain the
biasing constant B;
applying signals representing B and X to the input of summing amplifier;
and
utilizing a variable resistance coupled between the output and input of the
summing amplifier to obtain the constant A. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to the field of electrochemical gas
analyzers. More particularly, the present invention is directed to that
portion of the above-noted field which is concerned with the generation of
an electrical signal indicative of a gas chemistry. More specifically
still, the present invention is directed to that portion of the
above-noted field which is concerned with electrochemical gas sensors
responsive to the partial pressure of oxygen in gaseous samples. More
particularly still, the present invention is directed to that portion of
the above-noted field which is concerned with the generation of an
electrical signal indicative of the partial pressure of oxygen within the
heated gaseous combustion by-products generated by an internal combustion
engine. More particularly still, the present invention is directed to that
portion of the above-noted field which is concerned with the generation of
an electrical signal which may be rendered relatively insensitive to
changes in the temperature of the gaseous combustion by-products while
responding rapidly to variations in the partial pressure of oxygen in the
gaseous combustion by-products.
2. Description of the Prior Art
It has been determined that the operation of a conventional automotive
internal combustion engine produces gaseous combustion by-products
including hydrocarbons, carbon monoxide and various oxides of nitrogen.
Various efforts are being made to reduce the quantity of such by-products.
Extensive investigation into the combustion process, examination of
alternative combustion processes and detailed studies of exhaust gas
treatment devices have lead to the conclusion that the use of a catalytic
converter within the exhaust system of an internal combustion engine
provides a practical and effective technique for substantially reducing
the emission of the gaseous combustion by-products into the atmosphere. A
catalytic exhaust treatment device or converter which is capable of
substantially simultaneously converting all three of the aforementioned
combustion by-products into water, carbon dioxide, and gaseous nitrogen is
referred to as a "three-way catalyst." However, for the known three-way
catalyst devices to be most effective, the gaseous by-products introduced
into the converter must be the by-products of combustion of a
substantially stoichiometric air/fuel mixture. Such three-way catalysts
are said to have a very narrow "window" of air/fuel ratios at which the
device is most efficiently operative on these three combustion
by-products. By way of example if .lambda. is the air/fuel ratio
normalized to stoichiometry, the window may extend from about 0.99.lambda.
to about 1.01.lambda.. Such a three-way catalyst converter is described,
for example, in U.S. Pat. No. 3,895,093 issued to Weidenback et al. on
July 15, 1975, assigned to KaliChemi Aktiengesellschaft and titled
Catalytic Removal of Carbon Monoxide Unburned Hydrocarbons and Nitrogen
Oxides From Automotive Exhaust Gas. For air/fuel ratios of the combustion
mixture on either side of the window, one or two of the aforementioned
combustion by-products will be converted in only very small percentages.
Within the window, the three by-products will be converted at very high
percent efficiencies approaching 90% in some cases. In view of the
narrowness of the catalytic converter window, it has been determined that
the associated internal combustion engine should be operated with a
combustible mixture having an air/fuel ratio as close as possible to
stoichiometry.
The most satisfactory technique for assuring continuous or substantially
continuous operation at the desired air/fuel ratio is through the
utilization of an appropriate feedback control mechanism. In implementing
suitable feedback control systems, it has been proposed to employ sensors
responsive to the chemistry of the exhaust gases, that is, the hot gaseous
by-products of combustion, in order to control the precise air content
and/or fuel content of the air/fuel mixture being provided to the engine.
One type of electrochemical exhaust gas sensor employs a ceramic material
which demonstrates a predictable electrical resistance change when the
partial pressure of the oxygen of its environment changes. An example of
such a material is titania (titanium dioxide having a general formula
TiO.sub.2). Such sensors can be fabricated generally in accordance with
the teachings of U.S. Pat. No. 3,886,785 issued to Stadler et al., titled
Gas Sensor and Method of Manufacture and assigned to the assignee hereof.
Tests of such devices have shown that at elevated and substantially
constant temperatures, the devices will demonstrate a virtual step change
in resistance for rich-to-lean and lean-to-rich excursions of the air/fuel
ratio of the combustion mixture producing the exhaust gas environment of
the device.
A principal difficulty which has been encountered with such variable
resistive devices resides in the fact that such devices will demonstrate a
measurable resistance change which is also a function of change of the
temperature of the ceramic material, for example a change of about
500.degree. F. produces a measurable resistance change on the order of
magnitude of a sensed rich-to-lean or lean-to-rich air/fuel mixture
change. Such a temperature variation can be encountered, depending of
course to some extent on the location of placement of the sensor within an
exhaust system, during acceleration of the associated engine from idle
speed to highway speeds. Heretofore, exhaust gas sensors which employed a
variable resistance sensor ceramic have required that the temperature of
the material be relatively closely controlled for reliable use in a
feedback system intended to provide an internal combustion engine with
very precise air/fuel ratio control.
Temperature control of the associated sensor has required the addition of
expensive electronic temperature sensing and heating control systems
external to the exhaust conduit and the addition of a heater element per
se situated internally of, or in close proximity to, the sensor element.
In order to narrow the operational range of temperature of the sensor, the
sensor has been operated at the higher end of the predictable range of
exhaust gas temperatures thus requiring substantially continuous
application of heat energy for most of the operating cycles of the
associated engine. While such devices have continued to be of rugged
construction, the addition of the heater and associated electronics
devoted to temperature control have increased cost and have increased
statistical failure problems. An additional problem which has been
encountered is a ceramic fracture problem believed to be associated with
thermal shock caused by the rapid heating of the ceramic material by the
heater element. For less precise operation, such devices have been
required to be installed at a location in an exhaust gas environment where
the temperature of the exhaust gases will not vary substantially for
variation in the operating cycle of the associated engine.
Since variable resistance exhaust gas sensor devices are of substantially
greater mechanical strength and ruggedness than are other known types of
exhaust gas sensor and are not subject to the temperature gradient which
is inherent in operation of a galvanic cell type of exhaust gas sensor, it
is an object of the present invention to provide a variable resistance
exhaust gas sensor construction which is relatively temperature
insensitive. With greater particularity, it is an object of the present
invention to provide a titania exhaust gas sensor construction which is
capable of producing an output signal which is rendered relatively
insensitive to the temperature of the surrounding environment. With
greater particularity still, it is a further and particular object of the
present invention to provide a variable resistance ceramic exhaust gas
sensor construction which is relatively insensitive to the temperature of
the surrounding medium and which need not be maintained at a substantially
constant temperature. With the foregoing objective in mind, it is a
further object of the present invention to provide an exhaust gas sensor
which does not require the application of external heating energy. It is
also a further and particular objective of the present invention to
provide a means of temperature compensation for a variable resistance
ceramic exhaust gas sensor whereby sensor performance over a relatively
wide range of operating temperatures will be relatively temperature
insensitive. In furtherance of the foregoing objectives, it is a further
and particular objective of the present invention to provide a variable
resistance ceramic exhaust gas sensor with temperature compensation in the
form of a high temperature thermistor in a construction which is rugged in
use and which does not require expensive manufacturing techniques or
equipment.
One attempt known in the prior art for satisfying the above requirements is
to provide a pair of electrically series connected variable resistance
ceramic sensor elements. One of the ceramic sensor elements is a variably
resistive partial pressure of oxygen responsive and temperature responsive
ceramic such as, for example, titania. The other of the ceramic sensor
members is a variably resistive temperature responsive thermistor. The
ceramic sensor members are connected electrically in series and are
arranged to define a voltage divider network. When a reference voltage is
applied across the voltage divider network, the voltage appearing at the
junction between the ceramic sensor members may define the output voltage
of the exhaust gas sensor construction. The voltage appearing at the
junction of the sensor elements and the voltage divider network will be
relatively temperature independent since temperature effects on the
ceramic members will be electrically complementary. By comparing the
output voltage to the voltage level at either end of the voltage divider
network a useful output signal may be derived. By selectively referencing
to define the output signal as either the voltage drop across the partial
pressure of oxygen responsive member or the voltage drop across the
thermistor sensor member, the resulting output signal can be rendered to
be responsive to the air/fuel ratio. The use of two such elements in the
exhaust gas sensor construction increases the statistical chances of
failure. Further, it is desirable to obstruct the flow of exhaust gases as
little as possible and the use of two such sensors obstructs it more than
the use of a signal sensor.
SUMMARY OF THE INVENTION
This invention recognizes that a temperature compensated electrical sensor
for determining the air/fuel ratio can be accomplished by electrically
processing the output of a single sensor which is both responsive to
changes in temperature as well as air/fuel ratio. More particularly, this
invention recognizes that it is possible to compensate for the variation
in a sensor's characteristics with temperature through use of a signal
voltage related to engine speed. As a result, there is no need to provide
a controlled power supply for a heater adjacent the gas sensor to make
sure the gas sensor operates at a standard temperature. Further, there is
no need to place a temperature sensor resistance in the exhaust stream, in
addition to the gas sensor, to provide temperature compensation for the
gas sensor.
In particular, this invention includes a gas sensor construction comprising
a variably resistive gas and temperature sensing element adapted to be
mounted in a gas stream for producing a first electrical signal responsive
to the partial pressure of oxygen of the gas stream and to the temperature
of the gas stream. A measuring means develops a second electrical signal
as a function of engine speed. An electrical circuit means processes the
electrical signals from the sensing element and the measuring means so
that the second electrical signal is used to reduce the temperature
dependency of the first electrical signal.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram illustrating an internal combustion engine
having an exhaust responsive feedback fuel control mechanism in accordance
with an embodiment of this invention;
FIG. 2 is a circuit diagram, partly in block form, the electronic fuel
control feedback and temperature compensation circuit in accordance with
an embodiment of this invention; and
FIG. 3 is a graph of exhaust gas sensor resistance magnitude verses air to
fuel ratio and the resultant shift in operating point due to speed changes
from idle to 60 mph.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, an internal combustion engine 10 includes 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 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 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
positioning metering orifice arranged to control the quantity of fuel
delivered to engine 10 with respect to a given quantity of air. The
air/fuel ratio modulator means 22 may alternatively be arranged to control
a variable positionable air valve so that the quantity of air injected by
engine 10 with respect to a given quantity of fuel delivered by fuel
metering and delivery device 18 may be modulated.
Exhaust gas conduit 16 is provided with an exhaust gas sensor 24 which is
mounted to conduit 16 so as to place an exhaust gas chemistry responsive
element with the stream of exhaust gases flowing through conduit 16. A
variety of forms of this device are suitable and include a variably
resistive ceramic exhaust gas sensor form of, for example, titania or
cobalt monoxide. Electronic control means 26 communicates with exhaust gas
sensor 24 through sensing leads 28 and 30. Electronic control means also
communicates with the air/fuel ratio modulator means 22 over a conductive
lead 36. An engine tachometer 31 generates electrical voltage proportional
to the engine speed or revolutions per minute of engine 10. Electronic
control means 26 communicates with engine tachometer 31 over a conductive
lead 32. As described hereinbelow, 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 receipt 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. Additionally, electronic control means 26 compensates for
changes in exhaust gas sensor 24 due to the temperature of the exhaust gas
stream. An input from engine tachometer 31 is used to provide a
temperature compensation which is based on the relationship between the
temperature of the exhaust gas stream and the revolution per minute of the
engine. That is, it can easily be appreciated that at very low revolutions
of the engine the temperature is lower than at very high revolutions of
the engine.
Without temperature compensation, the change in the resistance of the gas
sensor, e.g., a TiO.sub.2 material sensor, can be seen in FIG. 3. As shown
in FIG. 3, the air to fuel ratio can shift as much as 0.46 from an idle
condition to a 60 mph condition. In tabular form the change in voltage
output from the gas sensor as measured at an interface circuit 40
described below is:
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Operating Pt. of
Vehicle Speed
Temp. Sensor at Interface
M.P.H. .degree. F.
Stochiometry Output
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IDLE 820 120K .43V
20 900 80K .48V
40 1040 8.5K .9V
50 1160 5K .98V
60 1280 2K 1.08V
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Ideally it would be desirable to have the interface output remain constant
as at stoichiometry even if there is variation in the temperature of the
exhaust gas. This invention recognizes that such temperature compensation
can take place using a parameter based upon engine speed.
Particular parameters which have been experimentally found indicate that
the temperature correction is of the form A(B+X) where A and B are
constants and X is a function of engine speed. For example, when X is a
voltage of the magnitude 0.123 volts/1000 rpm the correction can be 1.9
(0.105+X). Experimental data indicates that an assumption of a linear
relationship between vehicle speed and exhaust temperature is justified.
Referring to FIG. 2, electronic control means 26 includes an interface 40
coupled to exhaust gas sensor 24 by sensing leads 28 and 30, and a
compensation circuit 50 coupled to engine tachometer 31 by conductive lead
32. A comparator 70 has an output connected by conductive lead 36 to
air/fuel ratio modulator means 22 which includes an interface such as a
motor drive circuit utilizing an electrical signal carried by conductive
lead 36 to adjust the air/fuel ratio. A first positive input of comparator
70 is connected by a conductive lead 71 to the output of interface circuit
40, and from a second negative input, by a conductive lead 72 to the
output of compensation circuit 50.
Interface circuit 40 acts to convert exhaust gas sensor 24 resistance
change to voltage change and includes a resistor 41 coupled between a
voltage V.sub.c and sensing lead 28. Sensing lead 30 is coupled between
exhaust gas sensor 24 and ground. Accordingly, resistor 41 and exhaust gas
sensor 24 act in combination as a voltage divider with the relative
resistance of exhaust gas sensor 24 and resistor 41 determining the
fraction of voltage V.sub.c across exhaust gas sensor 24. Voltage V.sub.c
can be obtained from any convenient source such as, for example, an
automobile battery. A resistor 42 acts as an input resistor to couple
sensing lead 28 to the negative input of an operational amplifier or
comparator 43. The positive input to operational amplifier 43 is connected
to a junction between resistors 44 and 45, which are connected between
voltage V.sub.c and ground, thus establishing the reference voltage. A
resistor 46 is connected between the output of operational amplifier 43
and the negative input terminal thus providing a feedback path which tends
to stabilize operation. A resistor 47 is connected in series between the
output of operational amplifier 43 and the positive input of comparator
70. A resistor 48 is connected between the positive input of comparator 70
and ground. Resistors 47 and 48 tend to act as a voltage divider for the
output of operational amplifier 43 with respect to the positive input of
comparator 70.
Compensation circuit 50 includes summing amplifier 51 having an output
connected to the negative input of comparator 70 by conductive lead 72.
The positive input of summing amplifier 51 is connected to the output of
an operational amplifier or comparator 52 by a resistor 53 and to the
output of an operational amplifier or comparator 54 by a resistor 55. The
negative input of summing amplifier 51 is connected to ground through a
resistor 56 and to the output of summing amplifier 51 by a variable
resistor 57 thus varying amplification and permitting adjustment of the
output of summing amplifier 51 with respect to the input of summing
amplifier 51. The positive of input of operational amplifier 52 is
connected to the curser of a variable resistance 58 which in turn is
connected between ground and V.sub.c thus providing a bias voltage for the
positive input of operational amplifier 52. The negative input of
operational amplifier 52 is connected to the output of operational
amplifier 52 to provide a stabilizing feedback. Analogously, the negative
input of operational amplifier 54 is connected to the output of
operational amplifier 54. The positive input of operational amplifier 54
is connected by conductive lead 32 to engine tachometer 31 and thus
provides a buffer for the tachometer voltage.
Examples of particular values for elements in electronic circuit are:
V.sub.cc =15 volts D.C.
resistor 53=100 k .OMEGA.
resistor 55=100 k .OMEGA.
resistor 57=500 k .OMEGA.
resistor 56=47 k .OMEGA.
resistor 58=100 k .OMEGA.
comparator 70=LM 339 National Semiconductor comparators or operational
amplifiers
43, 52, 54=LM 324 National Semiconductor
OPERATION
Electronic control means 26 compensates for the temperature dependence of
titania exhaust gas sensor 24. That is, compensation of the variation in
exhaust sensor 24 characteristics with temperature is done by engine speed
dependent programming of the sensor's operating point. In brief, the
exhaust sensor 24 operates as a stoichiometric exhaust air/fuel sensor.
Ideally, the sensors is assumed to undergo a step change in some
electrical characteristics, independent of temperature, as the exhaust air
to fuel ratio changes through stoichiometry. The sensor signal is
continually compared to a reference value which represents the sensor
characteristics corresponding to the exhaust air to fuel ratio changing
through the exact stoichiometric fuel value. An error signal is generated
from this comparison to signal a need for correction to be made at the
engine intake, for example, the carburetor.
In the case of a titania sensor such as exhaust gas sensor 24, a change in
the exhaust air to fuel ratio through the stoichiometric value produces an
abrupt change in the sensor's electrical resistance. Exhaust gas sensor 24
behaves as a variable resistance in the presence of hot gases having
varying oxygen pressure so that the voltage across gas sensor 24 will be
indicative of the instantaneous oxygen partial pressure. By communicating
the voltage at sensor 24 to the electronic control circuit, a command
signal may be generated for application by conductor 36 to air/fuel ratio
modulator means to maintain the combustible mixture provided to engine 10
at a preselected, for example, stoichiometric air/fuel ratio. The value of
resistance corresponding to the exact stoichiometric air to fuel ratio
value is also a function of the exhaust gas temperature. To compensate for
this dependence, a variable reference which is proportional to engine
speed is used since a linear relationship has been found to exist between
vehicle speed and the exhaust temperature. The circuit to implement this
concept is shown in FIG. 2.
Generally, interface circuit 40 converts the resistance change of exhaust
gas sensor 24 to a voltage. Further, interface circuit 40 acts as a buffer
and generally provides an output voltage level which is compatible with
the voltage operation level of comparator 70. That is, the two inputs to
operational amplifier 43 are at an appropriate level to establish a
difference voltage which has an appropriate magnitude for multiplication
by operational amplifier 43. An output resistance divider including
resistors 47 and 48 establishes an appropriate voltage level for the input
of comparator 70.
In an analageous manner, compensation circuit 50 acts to provide a
difference voltage with respect to the tachometer voltage which is an
appropriate level for being an input to summing amplifier 51. Compensation
circuit 50 sums the tachometer voltage available on conductive lead 32
with a bias voltage available at bias resistor 58 to yield a variable
reference voltage available at the output of summing amplifier 51.
Operational amplifier 54 acts as a buffer amplifier for the input of the
tachometer voltage. Similarly, operational amplifier 52 acts as an
operational buffer amplifier for the offset provided by the bias voltage.
The output of operational amplifier 52 and 54 are coupled to resistors of
equal magnitude to the positive input of summing amplifier 51. Then, this
input is amplified by a desired gain to get the desired output voltage.
The amplification is accomplished by the adjustment of variable resistor
57 in combination with resistor 56. The output of summing amplifier 51 is
applied to the negative input of comparator 70 and provides a set point
which is adjusted with respect to the voltage output of the titania sensor
so that the output which is due to temperature is compensated. The gain of
summing amplifier 51 and the bias voltage are determined from experimental
data of exhaust gas sensor 24 electrical resistance versus the air to fuel
ratio as a function of temperature. It has been shown on one test vehicle
that the variable reference gives good results with only a 0.08 air to
fuel ratio spread from idle to 60 mph while a fixed reference gives a 0.29
air to fuel ratio spread over the same speed range. Thus when the air fuel
ratio is at a stoichiometric setting, the voltage output of the titania
sensor is due to the temperature of the exhaust gases and is balanced by
the temperature compensated output of the compensation circuit and the
output of comparator 70 is a signal indicating no change in the air fuel
ratio is necessary.
Various modifications and variations will no doubt occur to those skilled
in the various arts to which this invention pertains. For example, a
particular means for generating a bias voltage or given a difference
between voltages may be varied from the apparatus disclosed herein. These
and all other variations which basically rely on the teachings through
which this disclosure has advanced the art are properly considered within
the scope of this invention.
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