U.S. patent application number 12/187452 was filed with the patent office on 2009-02-12 for gas sensor, air-fuel ratio controller, and transportation apparatus.
This patent application is currently assigned to YAMAHA HATSUDOKI KABUSHIKI KAISHA. Invention is credited to Noriko OH-HORI, Toshio SUZUKI.
Application Number | 20090038289 12/187452 |
Document ID | / |
Family ID | 40029304 |
Filed Date | 2009-02-12 |
United States Patent
Application |
20090038289 |
Kind Code |
A1 |
OH-HORI; Noriko ; et
al. |
February 12, 2009 |
GAS SENSOR, AIR-FUEL RATIO CONTROLLER, AND TRANSPORTATION
APPARATUS
Abstract
A gas sensor includes a gas detection section, a heater whose
electrical resistance value changes in accordance with temperature,
and a control section arranged to control an operation of the
heater and to perform powering of the heater for heating. The
control section includes a current supply section to supply a
current to the heater in a period during which powering of the
heater for heating is stopped, a voltage detection section arranged
to detect an end-to-end voltage of the heater while a current is
being supplied to the heater from the current supply section, and a
current adjustment section arranged to adjust the level of the
current, during a cold period of the heater, which is supplied from
the current supply section based on an ambient temperature and the
end-to-end voltage of the heater as detected by the voltage
detection section.
Inventors: |
OH-HORI; Noriko; (Shizuoka,
JP) ; SUZUKI; Toshio; (Shizuoka, JP) |
Correspondence
Address: |
YAMAHA HATSUDOKI KABUSHIKI KAISHA;C/O KEATING & BENNETT, LLP
1800 Alexander Bell Drive, SUITE 200
Reston
VA
20191
US
|
Assignee: |
YAMAHA HATSUDOKI KABUSHIKI
KAISHA
Iwata-shi
JP
|
Family ID: |
40029304 |
Appl. No.: |
12/187452 |
Filed: |
August 7, 2008 |
Current U.S.
Class: |
60/285 |
Current CPC
Class: |
G01N 27/122
20130101 |
Class at
Publication: |
60/285 |
International
Class: |
F01N 3/00 20060101
F01N003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 8, 2007 |
JP |
2007-206674 |
Claims
1. A gas sensor comprising: a gas detection section; a heater
having a resistance value that changes in accordance with
temperature; and a control section arranged to control an operation
of an power supplied to the heater; wherein the control section
includes: a current supply section arranged to supply a
predetermined level of current to the heater in a period of time
during which powering of the heater is stopped; a voltage detection
section arranged to detect an end-to-end voltage of the heater
while a current is being supplied to the heater from the current
supply section; and a current adjustment section arranged to,
during a cold period of the heater, adjust the predetermined level
of the current which is supplied from the current supply section
based on an ambient temperature and the end-to-end voltage of the
heater detected by the voltage detection section.
2. The gas sensor of claim 1, wherein the current adjustment
section adjusts the predetermined level of the current supplied
from the current supply section so that the end-to-end voltage of
the heater substantially equals a target voltage which is
determined in accordance with the ambient temperature.
3. The gas sensor of claim 1, wherein the control section further
includes an ambient temperature detection section arranged to
detect the ambient temperature.
4. The gas sensor of claim 1, wherein the control section
determines a temperature of the heater based on an end-to-end
voltage of the heater which is detected while the level of current
adjusted by the current adjustment section is being supplied to the
heater, and adjusts a powering state of the heater so that the
temperature of the heater has a value in a predetermined range.
5. The gas sensor of claim 4, wherein the control section
determines the temperature of the heater by using a correction
formula which includes a quadratic or higher-order temperature
coefficient of resistance of the heater.
6. The gas sensor of claim 5, wherein the correction formula
includes a correction factor used to realize a substantially equal
distribution of positive and negative values of temperature
errors.
7. The gas sensor of claim 1, wherein the gas detection section is
arranged to detect oxygen.
8. An air-fuel ratio controller comprising the gas sensor of claim
1.
9. The air-fuel ratio controller of claim 8, further comprising an
electrical control unit connected to the gas sensor, wherein, the
electrical control unit also functions as the control section of
the gas sensor.
10. A transportation apparatus comprising: an internal combustion
engine; and the air-fuel ratio controller of claim 8 arranged to
control an air-fuel ratio of the internal combustion engine.
11. The transportation apparatus of claim 10, wherein the heater of
the gas sensor is positioned so as to be exposed to an exhaust gas
from the internal combustion engine.
12. The transportation apparatus of claim 10, wherein the current
adjustment section executes a current adjustment at a start of the
internal combustion engine.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a gas sensor, and in
particular to a gas sensor having a heater arranged to elevate the
temperature of a gas detection section. The present invention also
relates to an air-fuel ratio controller and a transportation
apparatus incorporating such a gas sensor.
[0003] 2. Description of the Related Art
[0004] From the standpoint of environmental and energy issues, it
has been desired to improve the fuel consumption of internal
combustion engines, and reduce the emission amount of regulated
substances (e.g., NO.sub.x) that are contained within exhaust gas
from internal combustion engines. In order to meet these needs, it
is necessary to appropriately control the ratio between fuel and
air in accordance with the state of combustion, so that fuel
combustion will occur always under optimum conditions. The ratio of
air to fuel is called an "air-fuel ratio" (A/F). In the case where
a ternary catalyst is employed, the optimum air-fuel ratio would be
the stoichiometric air-fuel ratio. The "stoichiometric air-fuel
ratio" is an air-fuel ratio at which air and fuel will just
sufficiently combust.
[0005] When fuel is combusting at the stoichiometric air-fuel
ratio, a certain amount of oxygen is contained within the exhaust
gas. When the air-fuel ratio is smaller than the stoichiometric
air-fuel ratio (i.e., the fuel concentration is high), the oxygen
amount in the exhaust gas decreases relative to that under the
stoichiometric air-fuel ratio. On the other hand, when the air-fuel
ratio is greater than the stoichiometric air-fuel ratio (i.e., the
fuel concentration is low), the oxygen amount in the exhaust gas
increases. Therefore, by measuring the oxygen amount or oxygen
concentration in the exhaust gas, it is possible to estimate how
much deviation the air-fuel ratio has relative to the
stoichiometric air-fuel ratio. This makes it possible to adjust the
air-fuel ratio and control the fuel combustion so as to occur under
the optimum conditions.
[0006] In order to measure the oxygen concentration within exhaust
gas, an oxygen sensor is used. Since a high temperature of about
500.degree. C. or above is required in order for the oxygen sensor
to operate suitably, a heater is provided in the oxygen sensor. An
example of an oxygen sensor having a heater is shown in FIG. 14. In
FIG. 14, the oxygen sensor 510 is shown exploded for clarity.
[0007] The oxygen sensor 510 includes a substrate 531 which is
composed of an insulator, such as alumina, and a gas detection
section 501 which is provided on a principal surface 531a of the
substrate 531. The gas detection section 501 is composed of an
oxide semiconductor, whose resistance value changes in accordance
with the partial pressure of oxygen which is contained in the
atmosphere. On the principal surface 531a of the substrate 531,
electrodes 532 arranged to detect the resistance value of the gas
detection section 501 are provided so as to be in contact with the
gas detection section 501. Instead of a resistance-type as
mentioned above, the gas detection section 501 may be an
electromotive-force type in which a solid electrolyte is used.
[0008] On a rear surface 531b side of the substrate 531, a heater
502 is provided in a position corresponding to the gas detection
section 501. The heater 502 is a heating element of a
resistance-heating type, which achieves heating by utilizing a
resistance loss when a current flows through the resistor. When a
predetermined voltage is applied to electrodes 533 which extend
from the heater 502, a current flows through the resistor having a
predetermined shape, whereby the resistor generates heat and
achieves heating. The heater 502 is formed by a screen printing
technique or the like, using a metal material such as platinum. By
elevating the temperature of the gas detection section 501 with the
heater 502 to promptly activate the gas detection section 501, the
detection accuracy at the start of the internal combustion engine
can be improved.
[0009] Since the resistance value of the heater 502 changes with
temperature, by measuring the resistance value of the heater 502,
it becomes possible to infer: the temperature of the heater 502;
and the temperature of the gas detection section 501 (hereinafter
also referred to as the "sensor temperature") which is in thermal
contact with the heater 502 via a thin insulator (i.e., the
substrate 531). By controlling the temperature of the heater 502
based on the inferred sensor temperature, the sensor temperature
can be controlled to be within an appropriate range.
[0010] However, since the resistor of the heater 502 may vary in
line width and/or thickness depending on the dimensional accuracy
during the production process, the resistance value of the heater
502 may deviate or vary from its design value. Moreover, the
resistance value of the heater 502 may also fluctuate through
deterioration of the material of the resistor over time. Thus, the
resistance value of the heater 502 contains an error (deviation
from the design value).
[0011] Therefore, when a resistance value which is the same as the
design value is used for control, the inferred sensor temperature
may deviate from the actual sensor temperature because variations
in resistance value-sensor temperature characteristics may occur
with respect to each gas sensor (i.e., for each heater), or with
time of usage. This may lead to being unable to control the sensor
temperature to be within the desired range. In the case where a
resistance-type gas detection section is used, the sensor output
has high temperature dependence, and therefore this problem would
make it difficult to control the air-fuel ratio with a high
precision. Moreover, in recent years, since there is a need for
controlling the air-fuel ratio with a higher precision, it is
desired to accurately infer the sensor temperature also when using
an electromotive-force type gas detection section. Furthermore,
there is a possibility that the life of the gas sensor may be
reduced when the actual sensor temperature is higher than the
inferred sensor temperature because, even if the sensor temperature
is controlled to constantly stay at about 700.degree. C., for
example, the actual sensor temperature will be higher than
that.
[0012] For example, a temperature T and a resistance value R of the
heater 502 at the temperature T can be expressed as eq. (1), by
using a resistance value R.sub.0 of the heater 502 at 0.degree. C.,
a temperature coefficient of resistance .alpha. (which is a
coefficient specific to the resistor material) of the heater 502,
and an error .delta. in the resistance value.
R=R.sub.0(1+.delta.)(1+.alpha.T) eq. (1)
[0013] Therefore, the temperature T when the error .delta. is zero
(.delta.=0), i.e., true temperature, is expressed as eq. (2).
T={(R/R.sub.0)-1}/.alpha. eq. (2)
[0014] On the other hand, the temperature T' when the error .delta.
is not zero (.delta..noteq.0), i.e., incorrectly inferred
temperature, is expressed as eq. (3).
T ' = { R 0 ( 1 + .delta. ) ( 1 + .alpha. T ) / R 0 - 1 } / .alpha.
= { ( 1 + .delta. ) ( 1 + .alpha. T ) - 1 } / .alpha. = T + .delta.
( 1 / .alpha. + T ) eq . ( 3 ) ##EQU00001##
[0015] Therefore, the temperature error .DELTA.T is expressed as
eq. (4).
.DELTA.T=T'-T=.delta.(1/.alpha.+T) eq. (4)
[0016] For example, when the material of the heater 502 is
platinum, the temperature coefficient of resistance .alpha. is
0.4%/.degree. C. Thus, when the true temperature T is 700.degree.
C., eq. (4) shows that the temperature error .DELTA.T will be so
large as .+-.35.degree. C. even if the error .delta. (variation due
to e.g. the production process; i.e., individual difference of the
heater 502) is only about .+-.5%.
[0017] In order to reduce the aforementioned deviation between the
inferred temperature and the actual temperature, Japanese Laid-Open
Patent Publication No. 2000-180406 discloses a technique which
involves providing a correction resistance in series or parallel to
the heater, or laser-trimming the heater in order to ensure that
the resistance value of the heater equals the design value.
[0018] On the other hand, Japanese Laid-Open Patent Publication No.
2000-2678 discloses a technique of calculating the resistance value
of a heater at room temperature from an inrush current which flows
into the heater immediately after a voltage is applied to the
heater and from the applied voltage, and performing control based
on the calculated resistance value.
[0019] However, performing a correction for each gas sensor as
disclosed in Japanese Laid-Open Patent Publication No. 2000-180406
would complicate the production steps, thus adding to the
production cost. In the case of adopting the technique disclosed in
Japanese Laid-Open Patent Publication No. 2000-2678, it is
necessary to provide both a means for measuring the inrush current
and a means for measuring the applied voltage; and, in practice,
the heater temperature will keep increasing in the short time
during which these measurements are taken, which makes it difficult
to accurately calculate the heater resistance value at room
temperature.
[0020] It might be possible to implement a calculation program in a
microcomputer that supports a heater resistance value (resistance
value at room temperature) which differs from gas sensor to gas
sensor, however, such a technique would be very complicated and
unpractical.
SUMMARY OF THE INVENTION
[0021] In order to overcome the problems described above, preferred
embodiments of the present invention provide a gas sensor which can
compensate for an error in the resistance value of a heater in an
accurate and simple manner.
[0022] A gas sensor according to a preferred embodiment of the
present invention includes a gas detection section, a heater whose
resistance value changes in accordance with temperature, and a
control section arranged to control an operation of the heater and
performing powering of the heater. The control section includes a
current supply section arranged to supply a predetermined level of
current to the heater in a period during which powering of the
heater is stopped, a voltage detection section arranged to detect
an end-to-end voltage of the heater while a current is being
supplied to the heater from the current supply section, and a
current adjustment section arranged to adjust, during a cold period
of the heater, the level of the current which is supplied from the
current supply section based on an ambient temperature and the
end-to-end voltage of the heater as detected by the voltage
detection section.
[0023] In a preferred embodiment, the current adjustment section
adjusts the level of the current supplied from the current supply
section so that the end-to-end voltage of the heater substantially
equals a target voltage which is determined in accordance with the
ambient temperature.
[0024] In a preferred embodiment, the control section further
includes an ambient temperature detection section arranged to
detect the ambient temperature.
[0025] In a preferred embodiment, the control section determines a
temperature of the heater based on an end-to-end voltage of the
heater which is detected while the level of current adjusted by the
current adjustment section is being supplied to the heater, and
adjusts a powering state of the heater so that the temperature of
the heater has a value in a predetermined range.
[0026] In a preferred embodiment, the control section determines
the temperature of the heater by using a correction formula which
includes a quadratic or higher-order temperature coefficient of
resistance of the heater.
[0027] In a preferred embodiment, the correction formula includes a
correction factor for realizing a substantially equal distribution
of positive and negative values of temperature errors.
[0028] In a preferred embodiment, the gas detection section detects
oxygen.
[0029] An air-fuel ratio controller according to a preferred
embodiment of the present invention includes a gas sensor of the
aforementioned construction.
[0030] In a preferred embodiment, the air-fuel ratio controller
according to the present invention further comprises an electrical
control unit connected to the gas sensor. The electrical control
unit also functions as the control section of the gas sensor.
[0031] A transportation apparatus according to another preferred
embodiment of the present invention includes an internal combustion
engine, and the air-fuel ratio controller of the aforementioned
construction arranged to control an air-fuel ratio of the internal
combustion engine.
[0032] In a preferred embodiment, the heater of the gas sensor is
positioned so as to be exposed to an exhaust gas from the internal
combustion engine.
[0033] In a preferred embodiment, the current adjustment section
executes a current adjustment at a start of the internal combustion
engine.
[0034] A gas sensor according to a preferred embodiment of the
present invention includes a control section arranged to control
the operation of a heater and to perform powering of the heater for
heating. Powering of the heater for heating is intermittently
performed by alternately repeating an ON operation and an OFF
operation. The control section includes a current supply section
arranged to supply a predetermined level of current to the heater
in a period during which powering of the heater for heating is
stopped, and a voltage detection section arranged to detect an
end-to-end voltage of the heater while a current is being supplied
to the heater from the current supply section. This makes it
possible to control the temperature of the heater in accordance
with a voltage which is detected by the voltage detection section
(which varies depending on the resistance value of the heater and
therefore takes a value corresponding to the heater temperature).
The control section further includes a current adjustment section
arranged to adjust, during a cold period of the heater, the level
of the current which is supplied from the current supply section
based on an ambient temperature and the end-to-end voltage of the
heater as detected by the voltage detection section. Thus, with
this current adjustment section, the level of the current which is
supplied from the current supply section is adjusted so as to
counteract errors in the resistance value (i.e., so that a constant
relationship exists between the temperature and the end-to-end
voltage regardless of the magnitude of the error in the resistance
value). In other words, the current adjustment section compensates
for errors in the resistance value of the heater (any deviation
from a design value associated with productional variations and
fluctuations due to deterioration over time). Thus, it is possible
to perform an accurate temperature measurement independent from
errors in the resistance value of the heater.
[0035] Typically, the current adjustment section determines a
certain target voltage in accordance with the ambient temperature,
and adjusts the level of the current which is supplied from the
current supply section so that the end-to-end voltage heater
substantially equals the target voltage.
[0036] The control section may include an ambient temperature
detection section arranged to detect the ambient temperature.
Specifically, the ambient temperature detection section (e.g., an
ambient temperature detection circuit) can detect the ambient
temperature by measuring the temperature of a substrate supporting
the gas detection section, the temperature of air suctioned into
the internal combustion engine, or the like.
[0037] Typically, the control section determines (infers) the
temperature of the heater based on an end-to-end voltage which is
detected while the level of current adjusted by the current
adjustment section is being supplied to the heater, and adjusts a
powering state of the heater so that the temperature of the heater
has a value in a predetermined range. In order to perform a more
accurate temperature measurement, it is preferable that the control
section determines the temperature of the heater by using a
correction formula which includes a quadratic or higher-order
temperature coefficient of resistance of the heater, and it is
further preferable that the correction formula includes a
correction factor used to realize a substantially equal
distribution of positive and negative values of temperature
errors.
[0038] The gas sensor according to a preferred embodiment of the
present invention is suitably used as an oxygen sensor whose gas
detection section detects oxygen, for example.
[0039] The gas sensor according to a preferred embodiment of the
present invention is preferably used for an air-fuel ratio
controller which controls an air-fuel ratio of an internal
combustion engine. In this case, it may be possible to adopt a
construction where an electrical control unit of the air-fuel ratio
controller functions also as the control section of the gas
sensor.
[0040] An air-fuel ratio controller having the gas sensor according
to preferred embodiments of the present invention is suitably used
in various types of transportation apparatuses. In a construction
where the heater of the gas sensor is exposed to the exhaust gas
from the internal combustion engine, the heater will experience a
severe deterioration over time, thus adding to the significance of
applying preferred embodiments of the present invention. Moreover,
the current adjustment section may execute a current adjustment at
the start of the internal combustion engine, for example.
[0041] According to various preferred embodiments of the present
invention, there is provided a gas sensor which can compensate for
variations in the resistance value of a heater in an accurate and
simple manner.
[0042] Other features, elements, processes, steps, characteristics
and advantages of the present invention will become more apparent
from the following detailed description of preferred embodiments of
the present invention with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0043] FIG. 1 is a block diagram schematically showing an oxygen
sensor (gas sensor) 10 according to a preferred embodiment of the
present invention.
[0044] FIG. 2 is an exploded perspective view showing a gas
detection section and a heater included in the oxygen sensor 10
together with their neighboring structure.
[0045] FIG. 3 is a flowchart for explaining the timing with which
to execute a current adjustment operation.
[0046] FIG. 4 is a diagram for specifically describing a current
adjustment operation.
[0047] FIG. 5 is a circuit diagram showing an example of a specific
construction of a V-I conversion circuit.
[0048] FIG. 6 is a flowchart showing a procedure of setting a
target voltage in a current adjustment operation.
[0049] FIG. 7 is a flowchart showing a procedure of executing a
current adjustment operation.
[0050] FIG. 8 is a graph showing a relationship between an output
voltage from a differential amplifier and temperature.
[0051] FIG. 9 is a graph showing an error in the case where the
heater temperature is inferred by employing preferable correction
formulas (correction formulas including higher-order temperature
coefficients of resistance of the heater).
[0052] FIG. 10 is a graph showing errors in the case of employing a
correction formula which includes a correction factor for realizing
a substantially equal distribution of positive and negative values
of temperature errors.
[0053] FIG. 11 is a flowchart showing a procedure of executing a
temperature error correction.
[0054] FIG. 12 is a diagram schematically showing an exemplary
motorcycle incorporating the oxygen sensor 10.
[0055] FIG. 13 is a diagram schematically showing a control system
of an engine in the motorcycle shown in FIG. 12.
[0056] FIG. 14 is an exploded perspective view schematically
showing a conventional oxygen sensor 500.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0057] Hereinafter, preferred embodiments of the present invention
will be described with reference to the accompanying drawings.
Although an oxygen sensor for detecting oxygen preferably is
exemplified below, the present invention is not limited to an
oxygen sensor, but can be suitably used for gas sensors including a
heater.
[0058] FIG. 1 is a circuit diagram showing the construction of an
oxygen sensor 10 according to a preferred embodiment of the present
embodiment. As shown in FIG. 1, the oxygen sensor 10 includes a gas
detection section 1, a heater 2 for elevating the temperature of
the gas detection section 1, and a control section 3 for
controlling the operation of the heater 2.
[0059] The gas detection section 1 detects the concentration and/or
amount of a predetermined gas that is contained in an atmosphere
which is in contact with the gas detection section 1. The gas
detection section 1 of the present preferred embodiment is a
so-called resistance type, whose resistance value changes in
accordance with the partial pressure of a predetermined gas (which
herein is preferably oxygen) that is contained in the
atmosphere.
[0060] The resistance-type gas detection section 1 can suitably be
composed of an oxide semiconductor, for example. An oxide
semiconductor having a porous structure releases or absorbs oxygen
in accordance with an oxygen partial pressure in the atmosphere. As
a result, the oxygen concentration in the oxide semiconductor
changes, whereby the resistance value of the oxide semiconductor
changes. As the oxide semiconductor, titania (titanium dioxide) or
ceria (cerium oxide) can be used, for example. It is preferable
that the oxide semiconductor contains 50 wt % or more of ceria.
Note that an electromotive-force type gas detection section in
which a solid electrolyte is used may be adopted as the gas
detection section 1. An electromotive-force type gas detection
section is disclosed in Japanese Laid-Open Patent Publication No.
8-114571, for example.
[0061] The heater 2 is a resistance-heating type heating element
which achieves heating by utilizing a resistance loss.
Specifically, the heater 2 is preferably constructed from a
resistor which is composed of a metal material such as platinum or
tungsten, or an oxide conductor such as rhenium oxide. By using the
heater 2 to elevate the temperature of the gas detection section 1,
the gas detection section 1 can be promptly activated.
[0062] Moreover, the heater 2 changes its resistance value
(electrical resistance value) with temperature. Therefore, by
measuring the resistance value of the heater 2, the temperature of
the heater 2 can be detected. Since the heater 2 and the gas
detection section 1 are in thermal contact via a thin insulator (a
substrate as described later), the temperature of the gas detection
section 1 can be detected by detecting the temperature of the
heater 2. In other words, the heater 2 is used not only as a
"heating element" for elevating the temperature of the gas
detection section 1, but also as a "temperature detection element"
for detecting the temperature of the heater 2 and the gas detection
section 1.
[0063] FIG. 2 shows an example of the gas detection section 1 and
the heater 2, together with their neighboring structure. As shown
in FIG. 2, the gas detection section 1 is supported by a substrate
31. The substrate 31 is composed of an insulator (preferably a
ceramic material) such as alumina or magnesia. The substrate 31 has
a principal surface 31aand a rear surface 31b opposing each other,
and a gas detection section (oxide semiconductor layer) 1 is
provided on the principal surface 31a.
[0064] On the principal surface 31a, electrodes 32 for detecting
the resistance value of the gas detection section 1 are defined.
The electrodes 32 are composed of an electrically conductive
material, such as a metal material, e.g., platinum,
platinum-rhodium alloy, or gold. Preferably, the electrodes 32 are
formed in a combteeth or interdigitated arrangement so as to be
able to efficiently measure changes in the resistance value of the
gas detection section 1.
[0065] Although not illustrated in the figures, a catalyst layer is
provided on the gas detection section 1. The catalyst layer
preferably includes a catalytic metal. Due to the catalytic action
of the catalytic metal, at least one kind of substance other than
the gas to be detected (i.e., oxygen) is decomposed. Specifically,
any gas or microparticles (e.g., in the case where the atmosphere
is exhaust gas, the hydrocarbon which has failed to completely
combust, carbon, and nitrogen oxide) which may unfavorably affect
the oxygen detection by the gas detection section 1 is decomposed,
such gas or microparticles are prevented from attaching to the
surface of the gas detection section 1. As a catalytic metal,
platinum may be used, for example.
[0066] The heater 2 is provided on the rear surface 31b side of the
substrate 31. Ends of the heater 2 are connected to electrodes 33a
and 33b, as shown in FIG. 2. The electrodes 33a and 33b are used
for supplying power to the heater 2 ("powering for heating"), and
also for detecting the temperature of the heater 2 by measuring the
resistance value of the heater 2. Preferably, the electrodes 33a
and 33b are formed integrally with the heater 2. As will be
described later, when the oxygen sensor 10 is provided in a
transportation apparatus, the heater 2 will be exposed to an
exhaust gas atmosphere, thus suffering from an even more severe
deterioration over time.
[0067] Next, the functions of the control section 3 will be
described. The control section 3 performs powering of the heater 2
for heating. More specifically, the control section 3 selectively
executes an ON operation connecting power to the heater 2, or an
OFF operation for stopping power to the heater 2. By executing the
ON operation, the temperature of the heater 2 is increased (heating
mode). By executing the OFF operation, the temperature of the
heater 2 is decreased (cooling mode). However, according to the
present preferred embodiment, not only the ON operation is
performed but also the OFF operation is performed in the heating
mode, in regularly-occurring short periods. In other words, in the
heating mode for increasing the temperature of the heater 2,
switching between the ON operation and the OFF operation
periodically occurs. In the cooling mode for decreasing the
temperature of the heater 2, the OFF operation is executed. The
execution time of the ON operation and the execution time of the
OFF operation preferably are each about 5 msec to about 50 msec,
for example. Note that the adjustment of the temperature of the
heater 2 does not need to be made through a single ON/OFF control
(binary control), the applied voltage to the heater 2 may be
controlled in multiple steps, for example.
[0068] The OFF operation in the heating mode is executed in order
to measure the temperature of the heater 2 during the heating mode.
The control section 3 in the present preferred embodiment includes
a current supply section for supplying a predetermined level of
current to the heater 2 in a period during which the OFF operation
is being executed, and a voltage detection section for detecting an
end-to-end voltage of the heater 2 while a current is being
supplied to the heater 2 from the current supply section.
Therefore, by supplying a current to the heater 2, the control
section 3 performs an operation of detecting an end-to-end voltage
of the heater 2 which changes depending on the resistance value of
the heater 2 (hereinafter referred to as a "voltage detection
operation"), and is able to control the temperature of the heater 2
in accordance with the voltage detected by the voltage detection
operation. More specifically, based on the voltage detected by the
voltage detection operation, the control section 3 is able to
determine the temperature of the heater 2, and adjust the powering
state of the heater 2 so that the temperature of the heater 2 has a
value in a predetermined range.
[0069] The control section 3 in the present preferred embodiment
preferably further includes a compensation section arranged to
perform, in a period from when the heater 2 is at room temperature
to when the ON operation is begun (i.e., during a cold period of
the heater 2), an operation of compensating for an error in the
resistance value of the heater 2 (i.e., any deviation from a design
value associated with productional variations and fluctuations due
to deterioration over time). Specifically, this compensation
section is a current adjustment section which adjusts the level of
the current supplied from the current supply section based on the
ambient temperature and the end-to-end voltage of the heater
detected by the voltage detection section. For example, the current
adjustment section adjusts the level of the current supplied from
the current supply section so that the end-to-end voltage of the
heater 2 substantially equals a target voltage which is determined
in accordance with the ambient temperature. By performing a voltage
detection operation by using the level of current having been
adjusted through the above-described operation of the current
adjustment section, it becomes possible to realize an accurate
temperature measurement which is free from an error in the
resistance value of the heater 2. Hereinafter, the reason why an
error in the resistance value of the heater 2 is compensated for by
the aforementioned current adjustment operation will be
described.
[0070] First, an end-to-end voltage V.sub.R of the heater 2 when a
weak current I (which typically is about 1/30 or less of the
current for heating the heater 2) is flowing through the heater 2
is expressed by eq. (5), by using a temperature T of the heater 2,
a resistance value R.sub.0 of the heater 2 at about 0.degree. C., a
temperature coefficient (which is a coefficient specific to the
resistor material) .alpha. of resistance of the heater 2, and an
error .delta. in the resistance value of the heater 2 (i.e.,
deviation from a design value of the resistance value).
V.sub.R=IR.sub.0(1+.delta.)(1+.alpha.T) eq. (5)
[0071] In the current adjustment operation, the level of the
current I flowing through the heater 2 is adjusted so that the
end-to-end voltage V.sub.R equals a target voltage V.sub.set which
is set in accordance with the ambient temperature (i.e.,
temperature during a cold period) T.sub.a. The target voltage
V.sub.set is expressed as eq. (6).
V.sub.set=V.sub.0+V.sub.a eq. (6)
[0072] Herein, V.sub.0 is a target voltage when the ambient
temperature T.sub.a is about 0.degree. C., and is set to an
appropriate value that makes the current I sufficiently weak (e.g.,
about 0.1 V). V.sub.a is an increment in the target voltage which
is in accordance with the level of the ambient temperature T.sub.a
(i.e., an increment from when the ambient temperature T.sub.a is
about 0.degree. C.).
[0073] When the ambient temperature T.sub.a is about 0.degree. C.,
if the end-to-end voltage V.sub.R is set to the target voltage
V.sub.set=V.sub.0, the target voltage V.sub.0 when the ambient
temperature T.sub.a is about 0.degree. C. is expressed as eq. (7),
from eq. (5).
V.sub.0=IR.sub.0(1+.delta.) eq. (7)
[0074] On the other hand, by substituting the end-to-end voltage
V.sub.R expressed by eq. (5) for the target voltage V.sub.set
expressed by eq. (6), eq. (8) is obtained, utilizing eq. (7).
V 0 + V a = I R 0 ( 1 + .delta. ) ( 1 + .alpha. T a ) = V 0 ( 1 +
.alpha. T a ) eq . ( 8 ) ##EQU00002##
[0075] Therefore, as shown in eq. (9), the increment V.sub.a is
expressed as a value obtained by multiplying V.sub.0 by the
temperature coefficient of resistance .alpha. and the ambient
temperature T.sub.a.
V.sub.a=V.sub.0.alpha.T.sub.a eq. (9)
[0076] Therefore, as shown in eq. (10), the target voltage
V.sub.set which is set in accordance with the ambient temperature
T.sub.a is expressed in terms of V.sub.0, the temperature
coefficient of resistance .alpha., and the ambient temperature
T.sub.a.
V.sub.set=V.sub.0+V.sub.a=V.sub.0+V.sub.0.alpha.T.sub.a=V.sub.0(1+.alpha-
.T.sub.a) eq. (10)
[0077] From the above equations, ultimately, the end-to-end voltage
V.sub.R of the heater 2 is expressed by eq. (11), which does not
contain the error .delta. and whose only parameters are the target
voltage V.sub.0 corresponding to the ambient temperature T.sub.a=0
(which is a predetermined value regardless of the actual resistance
value of the heater 2), the temperature coefficient of resistance
.alpha., and the temperature T.
V.sub.R=V.sub.0(1+.alpha.T) eq. (11)
[0078] As described above, by performing a voltage detection
operation by using a level of current having been adjusted through
the current adjustment operation, it becomes possible to realize an
accurate temperature measurement, independent from errors in the
resistance value of the heater 2. The error in the resistance value
of the heater 2 is compensated for by the current adjustment
operation because the current adjustment operation adjusts the
level of the current used in the voltage detection operation so as
to counteract the error .delta. in the resistance value (i.e., so
that the temperature T and the end-to-end voltage V.sub.R have a
constant relationship regardless of the magnitude of the error
.delta. in the resistance value).
[0079] Next, the timing with which to execute the aforementioned
current adjustment operation will be described.
[0080] For example, in the case where the oxygen sensor 10 is
mounted in a transportation apparatus having an engine, the current
adjustment operation may be executed in response to an engine
starting operation which is performed by the operator, i.e.,
immediately after the main switch of the engine is turned ON, while
the ambient temperature T.sub.a (i.e., the temperature in the
exhaust pipe) is as low as approximately room temperature. The
current adjustment operation does not need to be executed when the
ambient temperature T.sub.a is high, e.g., immediately after the
main switch of the engine is turned OFF, or when the engine is
restarted promptly thereafter.
[0081] FIG. 3 is a flowchart for explaining the timing with which
to execute the current adjustment operation. When the engine is
started, the ambient temperature T.sub.a is measured first (step
S1). Next, a comparison is made between the measured ambient
temperature T.sub.a and the predetermined value A (e.g. about
50.degree. C.) (step S2). If the ambient temperature T.sub.a is
less than a predetermined value A (T.sub.a<A), the current
adjustment operation is executed (step S3), and the powering
current value for the heater 2 is updated (step S4). On the other
hand, if the ambient temperature T.sub.a is equal to or greater
than the predetermined value A (T.sub.a.gtoreq.A), the current
adjustment operation is not executed.
[0082] Note that the current adjustment operation does not need to
be executed every time the engine is started. The current
adjustment operation may be executed at an engine start when a
predetermined time has elapsed since the previous run of the
current adjustment operation, or may be executed on a regular
basis, e.g., with a frequency of once a week or once a month. When
the current adjustment operation is not executed at an engine
start, the setting value from the previous run (or a predefined
value) may be adopted as the current value when executing the
voltage detection operation. Through the first run of the current
adjustment operation, variations in the resistance value of the
heater 2 associated with the production process are compensated
for. Through the second run of the current adjustment operation and
onwards, fluctuations in the resistance value of the heater 2
associated with deterioration over time will be compensated
for.
[0083] Next, referring back to FIG. 1, the specific construction of
the control section 3 will be described. In the present preferred
embodiment, an engine controller of the transportation apparatus
also functions as the control section 3 of the oxygen sensor 10. As
will be appreciated, however, the present invention is not to be
limited to such a construction. The gas detection section 1 is
electrically connected to the control section 3 via the electrodes
32, and the heater 2 is electrically connected to the control
section 3 via the electrodes 33a and 33b.
[0084] As shown in FIG. 1, the control section 3 includes an
ambient temperature detection circuit 4, a resistance-voltage
conversion circuit 5 which is connected to the gas detection
section 1, a constant-current circuit 6 and an end-to-end voltage
detection circuit 7 which are connected to the heater 2, and a
controller 8 which receives the outputs from the ambient
temperature detection circuit 4, the resistance-voltage conversion
circuit 5, and the end-to-end voltage detection circuit 7. The
controller 8 in the present preferred embodiment is preferably
implemented as a single-chip microcomputer, for example. The
constant-current circuit 6, the end-to-end voltage detection
circuit 7, and the controller 8 function respectively as the
aforementioned current supply section, the voltage detection
section, and the current adjustment section.
[0085] The control section 3 further includes a sensor input
circuit 9 which is connected to various sensors (not shown; a
throttle sensor, a water temperature sensor, etc.), such that the
output from the sensor input circuit 9 is also input to the
controller 8. Moreover, an actuator output circuit 11 is connected
to the controller 8, and the operation of various elements of the
engine are controlled by the output from the actuator output
circuit 11.
[0086] The ambient temperature detection circuit 4 detects the
ambient temperature T.sub.a around the oxygen sensor 10. The
ambient temperature detection circuit 4 preferably includes a
thermistor, for example, and is able to detect the ambient
temperature T.sub.a by measuring the temperature of the substrate
31, the temperature of air suctioned into the internal combustion
engine, or the like. Note that it is not necessary to employ a
thermistor to detect the ambient temperature T.sub.a. The ambient
temperature T.sub.a may be detected by using the output from a
water temperature sensor which measures the temperature of cooling
water of the internal combustion engine, or the output from an
exhaust temperature sensor which measures the temperature of
exhaust gas (i.e., these elements may be allowed to function as the
ambient temperature detection section).
[0087] The output of the ambient temperature detection circuit 4 is
input to an analog-digital converter (ADC) 13 via a selector 12 of
the controller 8. From the ADC 13, a digital value (a value
indicating the ambient temperature T.sub.a) corresponding to the
output (analog value) from the ambient temperature detection
circuit 4 is output to a data bus line 14 in the controller 8. As
has already been described, the controller 8 preferably includes a
single-chip microcomputer, which includes a CPU (central processing
unit) 15, a ROM (read only memory) 16, a RAM (random access memory)
17, as well as a timer 18, a sensor interface (SIF) circuit 19, an
actuator interface (AIF) circuit 20, etc. Exchange of commands from
the CPU 15, data having been read from the ROM 16, and the like is
performed via the data bus line 14. The SIF circuit 19, which
includes an ADC, a timer, a port, etc., is connected to the sensor
input circuit 9. The AIF circuit 20, which includes a DAC, a timer,
a port, etc., is connected to the actuator output circuit 11.
[0088] The resistance-voltage conversion circuit 5 measures the
resistance value Rg of the gas detection section 1, and outputs a
voltage which is in accordance with the measured resistance value
Rg (resistance-voltage conversion). The resistance-voltage
conversion circuit 5 is controlled by data (e.g., 2-bit data) which
is generated inside the controller 8 and fed from a port 21. By
measuring the resistance value Rg of the gas detection section 1
with the resistance-voltage conversion circuit 5, an oxygen
concentration within the ambient gas can be determined. The output
(voltage) from the resistance-voltage conversion circuit 5
connected to the gas detection section 1 is input to the ADC 13 via
the selector 12. From the ADC 13, a digital value (a value
indicating oxygen concentration) corresponding to the output
(analog value) from the resistance-voltage conversion circuit 5 is
output to the data bus line 14.
[0089] The end-to-end voltage detection circuit 7 detects a voltage
(end-to-end voltage) V.sub.R which is being applied to both ends of
the heater 2 when a predetermined level of current I is supplied to
the heater 2. Since the end-to-end voltage V.sub.R of the heater 2
depends on temperature, as also shown by eq. (11), the temperature
of the heater 2 can be determined from the detected voltage value.
Since the heater 2 is in thermal contact with the gas detection
section 1 via a thin insulating layer (i.e., the substrate 31), by
detecting the temperature of the heater 2 and controlling the
temperature of the heater 2 to be within a predetermined range, it
becomes possible to also control the temperature of the gas
detection section 1 to be within an appropriate range.
[0090] In addition to the constant-current circuit (current supply
section) 6 for supplying a current to the heater 2, the control
section 3 includes a power circuit 22 for generating a power
voltage which is necessary for the operation of each electronic
circuit within the control section 3, the power circuit 22 being
preferably connected to a +12 V power supply (battery). Whereas the
current which is supplied from the power supply to the heater 2 is
used for heating the heater 2, the current which is supplied from
the constant-current circuit 6 to the heater 2 is used for
measuring the resistance value of the heater 2.
[0091] As the port 23 of the controller 8 switches a semiconductor
switching element 25 to be ON or OFF via a gate drive 24, the
heater 2 is selectively connected to the +12 V power supply or to
the constant-current circuit 6, via the electrodes 33a and 33b
shown in FIG. 2. When heating the heater 2, the heater 2 and the
power supply are connected. When measuring the temperature of the
heater 2, the semiconductor switching element 25 operates so that
the destination to which the heater 2 is connected is switched from
the power supply to the constant-current circuit 6. Note that a
diode 26 is provided to ensure that the current from the power
supply will not flow into the constant-current circuit 6 when
heating the heater 2, thus imparting directionality to the current.
The level of the weak current which is supplied from the
constant-current circuit 6 is controlled by the output voltage from
the digital-analog converter (DAC) 27. Unless the level of the
current is adjusted by the current adjustment operation, the
constant-current circuit 6 supplies a constant level of current to
the heater 2.
[0092] When the heater 2 is connected to the constant-current
circuit 6, a current I of a predetermined level flows from the
constant-current circuit 6 to the heater 2 via the electrodes 33b
and 33c, so that the end-to-end voltage detection circuit 7
connected to the electrodes 33b and 33c detects the end-to-end
voltage V.sub.R of the heater 2. The current I supplied from the
constant-current circuit 6 is weak, and at a level such that the
heater 2 is substantially unheated (e.g., no less than about 10 mA
and no more than about 50 mA). Detection of the end-to-end voltage
V.sub.R occurs in a short period of time (e.g., about 1 ms to about
5 ms). Since a predetermined relationship exists between the
end-to-end voltage V.sub.R of the heater 2 and temperature, it is
possible to infer the temperature of the heater 2 (which also
corresponds to the temperature of the gas detection section 1)
based on the value of the detected voltage.
[0093] Specifically, a voltage Vh which is obtained through a
differential amplification (.times.A) of the end-to-end voltage
V.sub.R of the heater 2 and the end-to-end voltage V.sub.T at about
0.degree. C. is subjected to an analog-digital conversion by the
ADC 13, and the temperature is calculated by a program in the
controller 8 (single-chip microcomputer). The end-to-end voltage
V.sub.T at about 0.degree. C. is generated by subjecting the output
voltage from the port 28 of the controller 8 to a voltage
division.
[0094] Note that, in the present preferred embodiment, the single
ADC 13 is preferably used for applying an analog-digital conversion
to all outputs from the ambient temperature detection circuit 4,
the resistance-voltage conversion circuit 5, and the end-to-end
voltage detection circuit 7. The timing with which the ambient
temperature T.sub.a is detected, the timing with which the
resistance value Rg of the gas detection section 1 is measured, and
the timing with which the end-to-end voltage V.sub.R of the heater
2 is detected are all shifted relative to one another. Therefore,
through the switching operation by the selector 12, it is possible
to efficiently perform various analog-digital conversions by using
the single ADC 13.
[0095] Now, referring back to FIG. 3, the timing with which to
execute the current adjustment operation will be described more
specifically. When the engine is started, first, a measurement of
the ambient temperature T.sub.a is taken by the ambient temperature
detection circuit 4 (step S1). The output from the ambient
temperature detection circuit 4 is input to the ADC 13 via the
selector 12, and a digital signal indicating the ambient
temperature T.sub.a is output from the ADC 13 to the data bus line
14 in the controller 8. Next, the CPU 15 performs a comparison
between the measured ambient temperature T.sub.a and a
predetermined value A (e.g., about 50.degree. C.; stored in the ROM
16) (step S2). When the CPU 15 determines that the ambient
temperature T.sub.a is less than the predetermined value A
(T.sub.a<A), a current adjustment operation by the controller 8
is executed (step S3). When the current adjustment operation is
executed, a powering current value for the heater 2 which has been
stored in the RAM 17 is rewritten, thus, updating the powering
current value (step S4). On the other hand, when the CPU 15
determines that the ambient temperature T.sub.a is equal to or
greater than the predetermined value A (T.sub.a.gtoreq.A), the
current adjustment operation is not executed.
[0096] Next, with reference to FIG. 4, the current adjustment
operation of the control section 3 will be described more
specifically.
[0097] First, when the main switch of the engine is turned ON,
before beginning powering of the heater 2 for heating (i.e., during
a cold period of the heater 2), the output voltage from the port 28
of the controller 8 is set to about 0 V, and the output voltage
V.sub.T from a voltage divider 29 in the end-to-end voltage
detection circuit 7 is set to about 0 V. At this time, the output
voltage Vh from the differential amplifier 30 in the end-to-end
voltage detection circuit 7 is expressed as eq. (12), by using the
weak current I flowing in the heater 2, the resistance value Rh of
the heater 2 during a cold period, and the gain A of the
differential amplifier 30.
Vh=AIRh eq. (12)
[0098] Herein, the resistance value Rh of the heater 2 during a
cold period is expressed as eq. (13), by using the temperature
coefficient of resistance .alpha. of the heater 2, the resistance
value R.sub.0 of the heater 2 at about 0.degree. C., and the
ambient temperature T.sub.a.
Rh=R.sub.0(1+.alpha.T.sub.a) eq. (13)
[0099] Therefore, the output voltage Vh from the differential
amplifier 30 is expressed as eq. (14).
Vh=AIR.sub.0(1+.alpha.T.sub.a) eq. (14)
[0100] Through adjustment of the output from the DAC 27, the weak
current I (which is the output from the V-I conversion circuit
(constant-current circuit) 6) is changed so that the output voltage
Vh from the differential amplifier 30 equals the target voltage
V.sub.set (i.e., Vh=V.sub.set). The current I at this time is
expressed as eq. (15).
I=V.sub.set/{AR.sub.0(1+.alpha.T.sub.a)} eq. (15)
[0101] While maintaining the weak current I thus set, the output
voltage from the port 28 of the controller 8 is set to the power
voltage (V.sub.DD), and powering of the heater 2 for heating is
begun in such a manner that Vh at about 0.degree. C. equals about 0
V, thus performing temperature control and temperature measurement
(voltage detection operation).
[0102] The output voltage Vh from the differential amplifier 30
during the voltage detection operation is expressed as eq.
(16).
Vh = A ( I Rh - V T ) = A { I R 0 ( 1 + .alpha. T ) - V T } eq . (
16 ) ##EQU00003##
[0103] In order to ensure that the Vh at about 0.degree. C. equals
about 0 V, a ratio of voltage division D of the voltage divider 29
needs to satisfy the relationship of eq. (17).
V.sub.T=V.sub.DDD=IR.sub.0 eq. (17)
[0104] Accordingly, eq. (16) is transformed into eq. (18).
Vh = A { I R 0 ( 1 + .alpha. T ) - I R 0 } = A I { R 0 ( 1 +
.alpha. T ) - R 0 } = [ V set { R 0 ( 1 + .alpha. T ) - R 0 } ] / {
R 0 ( 1 + .alpha. T a ) } eq . ( 18 ) ##EQU00004##
[0105] The target voltage V.sub.set is expressed as eq. (10) by
using the target voltage V.sub.0 corresponding to the ambient
temperature T.sub.a=0, the temperature coefficient of resistance
.alpha., and the ambient temperature T.sub.a during a cold period.
Therefore, as is shown by eq. (19), the output voltage Vh from the
differential amplifier 30 contains the V.sub.0, the temperature
coefficient of resistance .alpha., and the temperature T as its
only parameters.
Vh = [ V 0 ( 1 + .alpha. T a ) { R 0 ( 1 + .alpha. T ) - R 0 } ] /
{ R 0 ( 1 + .alpha. T a ) } = V 0 .alpha. T eq . ( 19 )
##EQU00005##
[0106] Thus, with the oxygen sensor 10 of the present preferred
embodiment, errors in the resistance value of the heater 2 are
compensated for, through a current adjustment operation which is
performed by the current adjustment section during a cold period of
the heater 2. Therefore, through a voltage detection operation, an
accurate temperature not influenced by the errors in the resistance
value of the heater 2 can be inferred (determined). Note that, a
"cold period" of the heater 2 literally means a period during which
the heater 2 is sufficiently cool, and refers to any period when
the temperature of the heater 2 is equal to or less than a
predetermined temperature which is sufficiently lower than the
operating temperature (a temperature at which the gas detection
section 1 is sufficiently activated and detection of an oxygen
concentration is actually performed) of the oxygen sensor 10 (e.g.,
about 50.degree. C. or less, as has already been exemplified).
[0107] An example of a specific construction of the V-I conversion
circuit (constant-current circuit) 6 is shown in FIG. 5. The V-I
conversion circuit 6 shown in FIG. 5 preferably includes a
plurality of resistors R1 to R9, a capacitor C1, an amplifying
element A1, and a transistor T1. With these circuit elements, the
V-I conversion circuit 6 converts an input voltage from the DAC 27
to a predetermined level of current, and outputs it to the heater 2
via the diode 26. It will be appreciated that the types, number,
positioning, etc. of circuit elements composing the V-I conversion
circuit 6 are not limited to what is exemplified in FIG. 5.
[0108] FIG. 6 and FIG. 7 are flowcharts showing a procedure of
setting the target voltage V.sub.set and a procedure of executing a
current adjustment operation, respectively.
[0109] When setting the target voltage V.sub.set, as shown in FIG.
6, a detection of the ambient temperature T.sub.a is performed
first (step S11). Next, from the detected ambient temperature
T.sub.a, a value of the target voltage V.sub.set is determined
based on a target voltage adjustment formula (e.g., eq. (10)) (step
S12). Thereafter, the determined value of the target voltage
V.sub.set is retained (step S13).
[0110] When performing a current adjustment operation, as shown in
FIG. 7, the output voltage V.sub.T from the voltage divider 29 is
first set to about 0 V (step S21), and the count of number of
adjustments N is set to 1 (step S22). Next, powering for the heater
2 is performed (step S23). The level .DELTA.I of current at this
time is a half of the maximum value I.sub.MAX (i.e.,
I.sub.MAX/2).
[0111] Next, an output voltage Vh from the differential amplifier
30 (hereinafter also referred to as the "heater voltage") is
measured (step S24), and a comparison is made between the heater
voltage Vh and the target voltage V.sub.set (step S25). If the
heater voltage Vh is less than the target voltage V.sub.set
(Vh<V.sub.set), the level .DELTA.I of powering current is
increased by I.sub.MAX/(2N) (step S26). On the other hand, if the
heater voltage Vh is equal to or greater than the target voltage
V.sub.set (Vh.gtoreq.V.sub.set), the level .DELTA.I of powering
current is decreased by I.sub.MAX/(2N) (step S27).
[0112] Next, the count of the number of adjustments N is
incremented by one (step S28), and it is determined whether the
number of adjustments N is equal to or greater than a predetermined
value N.sub.set (step S29). If the number of adjustments N is less
than the predetermined value N.sub.set (N<N.sub.set), a series
of steps from the measurement of the heater voltage Vh (step S24)
to the comparison between the number of adjustments N and the
predetermined value N.sub.set (step S29) are repeated. If the
number of adjustments N is equal to or greater than the
predetermined value N.sub.set (N.gtoreq.N.sub.set), the present
level .DELTA.I of powering current is retained (step S30), and the
output voltage V.sub.T from the voltage divider 29 is set to a
predetermined value (a value that causes the Vh at about 0.degree.
C. to be about 0 V) (step S31).
[0113] By performing the adjustment of the level .DELTA.I of
powering current (increase or decrease of step S26 or step S27)
eight times, for example, (i.e., by setting the predetermined value
N.sub.set to 8), the heater voltage Vh can be brought sufficiently
close to the target voltage V.sub.set. As a result, the current
adjustment operation can be completed in a very short period of
time of about several msec to about several dozen msec.
[0114] The oxygen sensor 10 of the present preferred embodiment
compensates for errors in the resistance value of the heater 2
through the current adjustment operation as described above. This
makes it unnecessary to individually perform a correction for each
gas sensor (a correction to be made in accordance with the
resistance value of an actually-produced heater), unlike in the
technique disclosed in Japanese Laid-Open Patent Publication No.
2000-180406. On the other hand, in the technique disclosed in
Japanese Laid-Open Patent Publication No. 2000-2678, the heater
temperature might increase during the measurement of an inrush
current and an applied voltage, thus making it difficult to perform
an accurate compensation. However, with the oxygen sensor 10 of the
present preferred embodiment, the temperature of the heater 2
undergoes hardly any increase associated with the current
adjustment operation which is performed for the purpose of
compensation; thus, the aforementioned problem is avoided. Thus, in
accordance with the oxygen sensor 10 of the present preferred
embodiment, errors (productional variations and fluctuations due to
deterioration over time) in the resistance value of the heater 2
can be compensated for in an accurate and simple manner.
[0115] Next, a technique for further reducing the inference error
of the temperature of the heater 2 will be described. As has
already been described, by performing the current adjustment
operation, the oxygen sensor 10 of the present preferred embodiment
is able to reduce inference errors that are caused by errors in the
resistance value of the heater 2. However, depending on how the
relationship between the resistance value of the heater 2 and
temperature is approximated (i.e., what sort of relational
expression is used for the approximation), some errors may occur
between the temperature and the actual temperature. Such inference
errors can be reduced by a technique described below.
[0116] Since the temperature coefficient of resistance of the
heater 2 relies on temperature, the relationship between the
resistance value of the heater 2 and temperature is not linear.
Therefore, simply approximating the resistance value of the heater
2 with a linear equation of temperature (i.e., linear
approximation) may possibly result in large inference errors. Some
conceivable methods for solving this problem might be: using
polygonal-line characteristics for approximating the relationship
between the resistance value of the heater 2 and temperature (in
the case of an analog circuit); or providing a detailed mapping of
the relationship between the resistance value of the heater 2 and
temperature on a non-volatile memory (in the case of a digital
circuit). However, inference errors can be reduced even more simply
by using the technique described below.
[0117] Specifically, it is preferable that the control section 3
determines the temperature of the heater 2 by using a correction
formula which includes a quadratic or higher-order temperature
coefficient of resistance of the heater 2. Through such a
determination of temperature, the difference between the actual
temperature and the inferred temperature can be made small (e.g.,
to about .+-.5.degree. C. or less). Hereinafter, this will be
described more specifically.
[0118] The relationship between the temperature T of the heater 2
and the resistance value Rh is expressed by a polynomial including
a linear term and a quadratic term of the temperature T, such as
eq. (20). In eq. (20), .alpha. is a linear temperature coefficient
of resistance of the heater 2, and .beta. is a quadratic
temperature coefficient of resistance of the heater 2. As .alpha.
and .beta., either theoretical values or measured values may be
used.
Rh=R.sub.0(1+.alpha.T-.beta.T.sup.2) eq. (20)
[0119] Now, eq. (20) is converted into a relational expression
between the output voltage Vh from the differential amplifier 30
and the temperature T, thus giving eq. (21).
Vh=IA{R.sub.0(1+.alpha.T-.beta.T.sup.2)-R.sub.0}=V.sub.0(.alpha.T-.beta.-
T.sup.2) eq. (21)
[0120] Note that V.sub.0 in eq. (21) is the output voltage at
T=0.degree. C., which is expressed by eq. (22).
V.sub.0=IAR.sub.0 eq. (22)
[0121] Based on eq. (21), an inverse function for converting the
output voltage Vh to the temperature T is created. Specifically, as
shown in FIG. 8, the output voltage Vh at a given
sufficiently-large temperature Tp is defined as Vp, and a line
connecting the point Vh=0 and the point Vh=Vp is imagined. Then, a
gradient k of this hypothetical line is expressed as eq. (23). Note
that the actual relationship between the output voltage Vh and the
temperature T is non-linear, as shown in FIG. 8.
k = Vp / Tp = { V 0 ( .alpha. Tp - .beta. Tp 2 ) } / Tp = V 0 (
.alpha. - .beta. Tp ) eq . ( 23 ) ##EQU00006##
[0122] Moreover, this hypothetical line can be expressed by eq.
(24), assuming an inferred temperature T.sub.E when a linear
equation is used to express the relationship between the output
voltage Vh and the temperature T.
Vh=kT.sub.E eq. (24)
[0123] Based on eq. (21), eq. (23), and eq. (24), an equation for
deriving T from T.sub.E will be created, which first gives eq. (25)
below.
V.sub.0(.alpha.-.beta.Tp)T.sub.E=V.sub.0(.alpha.T-.beta.T.sup.2)
eq. (25)
[0124] Dividing both sides of eq. (25) by V.sub.0 (eq. (26)), a
further transformation gives eq. (27).
( .alpha. - .beta. Tp ) T E = ( .alpha. T - .beta. T 2 ) eq . ( 26
) T = [ .alpha. - { .alpha. 2 - 4 .beta. ( .alpha. - .beta. Tp ) T
E } ] / ( 2 .beta. ) = [ 1 - { 1 - 4 ( .beta. / .alpha. ) ( 1 - (
.beta. / .alpha. ) Tp ) T E } ] / { 2 ( .beta. / .alpha. ) } eq . (
27 ) ##EQU00007##
(27)
[0125] From eq. (27), T can be expressed by a polynomial of
T.sub.E, as shown in eq. (28).
T .apprxeq. [ 2 ( .beta. / .alpha. ) { 1 - ( .beta. / .alpha. ) Tp
} / { 2 ( .beta. / .alpha. ) } ] T E + [ 2 ( .beta. / .alpha. ) 2 {
1 - ( .beta. / .alpha. ) Tp } 2 / { 2 ( .beta. / .alpha. ) } ] T E
2 + [ 4 ( .beta. / .alpha. ) 3 { 1 - ( .beta. / .alpha. ) Tp } 3 /
{ 2 ( .beta. / .alpha. ) } ] T E 2 + = { 1 - ( .beta. / .alpha. )
Tp } T E + ( .beta. / .alpha. ) { 1 - ( .beta. / .alpha. ) Tp } 2 T
E 2 + 2 ( .beta. / .alpha. ) 2 { 1 - ( .beta. / .alpha. ) Tp } 3 T
E 3 + = T E - ( .beta. / .alpha. ) ( Tp - T E ) T E - 2 ( .beta. /
.alpha. ) 2 ( Tp - T E ) T E 2 + .delta. eq . ( 28 )
##EQU00008##
[0126] On the right-hand side of eq. (28), the first term T.sub.E
is a temperature which is inferred from a simple linear equation
(hypothetical line), and the second and subsequent terms are
correction terms. Eq. (28) implies that insufficiencies in the
correction provided by the second term are remedied by the
correction provided by the third term. Note that the hypothetical
line and the actual characteristic curve intersect when T=0 and
when T=Tp, so that the error becomes zero. This is the reason why
(Tp-T.sub.E)T.sub.E and (Tp-T.sub.E)T.sub.E.sup.2 are included as
variables in the polynomial.
[0127] The effects (simulation results) that are obtained by using
a correction formula as shown by eq. (28) are illustrated in FIG.
9. It can be seen from FIG. 9 that, as compared to the case where
no correction is performed (i.e., the temperature is inferred from
a linear equation), the errors are reduced by adding a quadratic
correction term (i.e., the second term on the right-hand side of
eq. (28)), and further reduced (specifically, to 5.degree. C. or
less) by adding a cubic correction term (i.e., the third term on
the right-hand side of eq. (28)).
[0128] As described above, by determining the temperature of the
heater 2 with a correction formula which includes a quadratic or
higher-order temperature coefficient of resistance of the heater 2,
the control section 3 is able to further reduce the inference
errors. Although the above description illustrates an example where
the correction formula includes a quadratic temperature coefficient
of resistance .beta. in addition to a linear temperature
coefficient of resistance .alpha., it is also possible to use a
correction formula which further includes a cubic or higher-order
temperature coefficient of resistance.
[0129] Moreover, as shown in FIG. 9, the errors after correction
are all positive. Therefore, the errors can be further reduced by
distributing the errors after correction to both positive and
negative sides. For example, by ensuring a substantially equal
distribution of positive and negative values of error, the errors
can be reduced to about a half.
[0130] In order to distribute errors to both positive and negative
sides, the coefficients of the quadratic correction term and the
cubic correction term may be made greater than necessary, so as to
cause an overcompensation. For example, by using a correction
formula which includes up to a cubic correction term, and setting
the coefficient of the cubic correction term to be about 1.22 times
as large, errors can be significantly reduced to about
.+-.2.degree. C. or less in the range from about 0 to about
1000.degree. C., as shown in FIG. 10.
[0131] Thus, when the correction formula includes correction
factors for realizing a substantially equal distribution of
positive and negative values of temperature errors, the inference
errors of temperature can be further reduced. When eq. (28) is
expressed as eq. (29), it is preferable that the correction factor
p included in the cubic correction term is about 2 to about 2.5,
specifically.
T.apprxeq.T.sub.E-(.beta./.alpha.)(Tp-T.sub.E)T.sub.E-p(.beta./.alpha.).-
sup.2(Tp-T.sub.E)T.sub.E.sup.2 eq. (29)
[0132] Although the minuscule portion .delta. from eq. (28) is
omitted in eq. (29) for simplicity, this omission can be
compensated for by adjusting the correction factor p.
[0133] FIG. 11 shows a flowchart of a procedure of executing the
above-described temperature error correction. First, the data "0 "
of the heater voltage (output voltage from the differential
amplifier 30) V.sub.0 when T=0.degree. C. is acquired (step S41),
and then the data "p" of the heater voltage Vp when T=Tp.degree. C.
is acquired (step S42).
[0134] Next, from the acquired data 0 and p, an equation
T.sub.E=Vh/k (which is a transformation of eq. (24)) for inferring
the temperature through linear approximation is generated (step
S43). Then, the heater voltage Vh at a given temperature is
detected (step S44), and the temperature T.sub.E at that time is
inferred (calculated) from the equation T.sub.E=Vh/k (step
S45).
[0135] Thereafter, a temperature correction is executed from
T.sub.E, Tp, and the temperature coefficients of resistance .alpha.
and .beta., thus calculating the actual temperature T (step S46).
In this manner, a correction for the temperature error can be
performed.
[0136] Next, a vehicle which incorporates the oxygen sensor 10
according to the present preferred embodiment and which employs an
internal combustion engine as a driving source will be described.
FIG. 12 schematically shows a motorcycle 300 incorporating the
oxygen sensor 10.
[0137] As shown in FIG. 12, the motorcycle 300 includes a body
frame 301 and an engine (for example, an internal combustion
engine) 100. A head pipe 302 is provided at the front end of the
body frame 301. To the head pipe 302, a front fork 303 is attached
to be capable of swinging in the right-left direction. At the lower
end of the front fork 303, a front wheel 304 is supported so as to
be capable of rotating. Handle bars 305 are attached to the upper
end of the head pipe 302.
[0138] A seat rail 306 is attached at an upper portion of the rear
end of the body frame 301 so as to extend in the rear direction. A
fuel tank 307 is provided above the body frame 301, and a main seat
308a and a tandem seat 308b are provided on the seat rail 306.
Moreover, rear arms 309 extending in the rear direction are
attached to the rear end of the body frame 301. At the rear end of
the rear arms 309, a rear wheel 310 is supported so as to be
capable of rotating.
[0139] The engine 100 is held at the central portion of the body
frame 301. A radiator 311 is provided in front of the engine 100.
An exhaust pipe 312 is connected to an exhaust port of the engine
100. As will be specifically described below, an oxygen sensor 10,
a catalyst 104, and a muffler 106 are provided on the exhaust pipe
(in an ascending order of distance from the engine 100). The top
end of the oxygen sensor 10 is exposed in a passage within the
exhaust pipe 312 in which exhaust gas travels. Thus, the oxygen
sensor 10 detects oxygen within the exhaust gas. The oxygen sensor
10 has the heater 2 as shown in FIG. 2, etc., attached thereto. As
the temperature of the gas detection section 1 is elevated by the
heater 2 at the start of the engine 100 (e.g., elevated to about
700.degree. C. in about 5 seconds), the detection sensitivity of
the gas detection section 1 composed of an oxide semiconductor is
enhanced.
[0140] A transmission 315 is linked to the engine 100. Driving
sprockets 317 are attached on an output axis 316 of the
transmission 315. Via a chain 318, the driving sprockets 317 are
linked to rear wheel sprockets 319 of the rear wheel 310.
[0141] FIG. 13 shows main component elements of a control system of
the engine 100. On a cylinder 101 of the engine 100, an intake
valve 110, an exhaust valve 106, and a spark plug 108 are provided.
There is also provided a water temperature sensor 116 for measuring
the water temperature of the cooling water with which to cool the
engine. The intake valve 110 is connected to an intake manifold
122, which has an air intake. On the intake manifold 122, an
airflow meter 112, a throttle valve 114, a throttle sensor 114a,
and a fuel injector 111 are provided. Instead of the airflow meter
112, a vacuum sensor may be provided between the throttle valve 114
and the intake valve 110, and the intake amount may be
measured.
[0142] The airflow meter 112, the throttle sensor 114a, the fuel
injector 111, the water temperature sensor 116, the spark plug 108,
and the oxygen sensor 10 are connected to an ECU (electrical
control unit) 118. A vehicle velocity signal 120, which represents
the velocity of the motorcycle 300, is also input to the ECU
118.
[0143] When a rider starts the engine 100 by using a self-starting
motor (not shown), the ECU 118 calculates an optimum fuel amount
based on detection signals obtained from the airflow meter 112, the
throttle sensor 114a and the water temperature sensor 116, and the
vehicle velocity signal 120. Based on the result of this
calculation, the ECU outputs a control signal to the fuel injector
111. The fuel which is injected from the fuel injector 111 is mixed
with the air which is supplied from the intake manifold 122, and
injected into the cylinder 101 via the intake valve 110, which is
opened or closed with appropriate timing. The fuel which is
injected in the cylinder 101 combusts to become exhaust gas, which
is led to the exhaust pipe 312 via the exhaust valve 106.
[0144] The oxygen sensor 10 detects the oxygen in the exhaust gas,
and outputs a detection signal to the ECU 118. Based on the signal
from the oxygen sensor 10, the ECU 118 determines the amount of
deviation of the air-fuel ratio from an ideal air-fuel ratio. Then,
the amount of fuel which is injected from the fuel injector 111 is
controlled so as to attain the ideal air-fuel ratio relative to the
air amount which is known from the signals obtained from the
airflow meter 112 and the throttle sensor 114a. Thus, an air-fuel
ratio controller which includes the oxygen sensor 10 and the ECU
118 connected to the oxygen sensor 10 appropriately controls the
air-fuel ratio of the internal combustion engine.
[0145] Note that the ECU 118 may also function as the control
section 3 of the oxygen sensor 10. That is, the constituent
elements composing the control section 3 (e.g., a microcomputer
implementing the controller 8) shown in FIG. 1 and other figures
may be those which are mounted on the ECU 118.
[0146] In the motorcycle 300 incorporating the oxygen sensor 10
according to the present preferred embodiment, the sensor
temperature can be accurately measured, and thus the sensor
temperature can be suitably controlled to be in a desired range.
Therefore, the oxygen sensor 10 has a long life, thus allowing fuel
and air to be mixed at an appropriate air-fuel ratio for a long
period of time, so that fuel can be combusted under optimum
conditions. Since the sensor temperature can be controlled within a
narrow range, even in the case where a gas detection section whose
sensor characteristics have a large temperature dependence (e.g.,
the resistance-type gas detection section 1) is used, the influence
which temperature fluctuations exert on the sensor characteristics
can be reduced, and an accurate air-fuel ratio can be detected.
[0147] Although a motorcycle has been illustrated for instance, the
present invention can also be suitably used for any other
transportation apparatus, e.g., a four-wheeled automobile.
Moreover, the internal combustion engine is not limited to a
gasoline engine, but may alternatively be a diesel engine or other
type of engine.
[0148] Furthermore, the present invention can be used for various
types of gas sensors, without being limited to oxygen sensors. For
example, the present invention can be suitably used for hydrogen
sensors, NO.sub.x sensors, hydrocarbon sensors, organic compound
sensors, and the like.
[0149] According to various preferred embodiments of the present
invention, there is provided a gas sensor which is capable of
compensating for variations in the electrical resistance value of a
heater in an accurate and simple manner. The present invention is
suitably used for various types of gas sensors. A gas sensor
according to a preferred embodiment of the present invention is
suitably used in an air-fuel ratio controller for various
transportation apparatuses, e.g., a car, a bus, a truck, a
motorbike, a tractor, an airplane, a motorboat, a vehicle for civil
engineering use, or the like.
[0150] While the present invention has been described with respect
to preferred embodiments thereof, it will be apparent to those
skilled in the art that the disclosed invention may be modified in
numerous ways and may assume many embodiments other than those
specifically described above. Accordingly, it is intended by the
appended claims to cover all modifications of the invention that
fall within the true spirit and scope of the invention.
[0151] This application is based on Japanese Patent Application No.
2007-206674 filed on Aug. 8, 2007, the entire contents of which are
hereby incorporated by reference.
* * * * *