Air/Fuel Ratio Sensor Having Oxygen Sensor Cell and Oxygen Pump Cell

The present invention relates to air/fuel ratio sensors used for internal combustion engines or other combustion engines, and more specifically to air/fuel ratio sensors of a type having an oxygen sensor cell and an oxygen pump cell.

Japanese patent provisional publication No. 60-236056 discloses one conventional example of such air/fuel ratio sensors. As shown in FIG. 7, the oxygen sensor cell of this example has a primary element of metal oxide, such as a transition metal oxide, whose electrical resistance changes with change in oxygen concentration. The oxygen pump cell has a member of oxygen ion conductive solid electrolyte.

Japanese patent provisonal publication No. 61-30758 discloses another conventional example which is shown in FIG. 20A. The oxygen sensor cell of this example is composed of a substrate of oxygen ion conductive solid electrolyte, which is sandwiched between a reference electrode in direct contact with a reference gas such as air, and a measuring electrode in direct contact with a gas to be measured.

However, these air/fuel ratio sensors are unsatisfactory in that the burden of the oxygen pump cell is too much, and the tendency to overshoot in a transient response is too high, as explained hereinafter more in detail.

It is an object of the present invention to provide an air/fuel ratio sensing device which is superior in the durability of the pump cell and the transient response characteristic.

According to the present invention, a device for sensing an air/fuel ratio comprises sensing means and controlling means. The sensing means comprises a substrate and a primary measuring element mounted on a first surface of the substrate. The controlling means comprises a first cover mounted on the substrate for enclosing the primary element within a control space formed between the substrate and the first cover. The first cover is formed with at least one hole for allowing a gas to move into and out of the control space. At least a part of the cover is made of an oxygen ion conductive solid electrolyte. The controlling means further comprises inner and outer electrodes forming an oxygen pump cell with the oxygen ion conductive solid electrolyte. The inner electrode is attached to an inner surface of the first cover within the control space. The outer electrode is attached to an outer surface of the first cover outside the control space. The primary element of the present invention is disposed outside a projection of the hole formed by projecting the hole onto the first surface of the substrate along a line along which the hole extends.

The primary element is mounted on the substrate and exposed to the gas in the control space. In some embodiments of the present invention, the primary element is made of a metal oxide whose electrical resistance changes in dependence on an oxygen concentration of a gas to be measured. In the other embodiments, the primary element is a measuring electrode mounted on the substrate of an oxygen conductive solid electrolyte.

In the illustrated embodiments of the present invention, the first surface of the substrate is substantially flat, the hole extends through the first cover along a line substantially perpendicular to the upper surface of the substrate, and the primary element has an opening whose center is situated just below the hole and whose size is larger than that of a cross section of the hole. A boundary surface of the hole and the above-mentioned projection are geometrically related so that the boundary surface of the hole is a surface generated by moving a straight line perpendicular to the first surface of the substrate, and during this motion, the straight line intersects the first surface of the substrate always at a point lying on an outline of the projection.

FIG. 1A is a sectional view, taken on a line I-1 of FIG. 1B, schematically showing an air/fuel ratio sensor of a first embodiment of the present invention,

FIG. 1B is a plan view of the air/fuel ratio sensor of FIG. 1A whose cover is removed to show its interior construction,

FIG. 2 is a diagram of a circuit including the air/fuel sensor of FIG. 1A, arranged to detect the air/fuel ratio,

FIG. 3 is a graph of a characteristic of a pump current ip of the circuit of FIG. 2 versus an excess air factor "lambda" of the gas being measured,

FIG. 4 is a graph showing an output characteristic of the air/fuel ratio sensor of FIG. 1A,

FIG. 5 is a plan view similar to FIG. 1B, showing a low resistivity region (A.sub.5) of a metal oxide element of the air/fuel ratio sensor of FIG. 1A in a rich atmosphere,

FIG. 6A is a plan view of an air/fuel ratio sensor of a second embodiment of the present invention, whose cover is removed,

FIG. 6B is a sectional view taken on a line VIB--VIB of FIG. 6A, showing the air/fuel ratio sensor of the second embodiment including the cover,

FIG. 7 is a sectional view of a conventional air/fuel ratio sensor of the same type as the sensors of the first and second embodiments,

FIG. 8 is a circuit diagram showing a circuit for detecting the air/fuel ratio by using the sensor of FIG. 7,

FIG. 9 is a graph showing resistance versus excess air factor relationships of the air/fuel ratio sensor of FIG. 7,

FIG. 10 is a diagram of an equivalent circuit arranged to measure the excess air factor (air/fuel equivalent ratio) by using the sensor of FIG. 7,

FIG. 11 is a graph showing a pump current versus excess air factor relationship of the equivalent circuit of FIG. 10,

FIG. 12A is a plan view of the sensor of FIG. 7 for showing a low resistivity region (A.sub.1) of the metal oxide element in a lean atmosphere,

FIG. 12B is a plan view of the sensor of FIG. 7 for showing a low registivity region (A.sub.3) of the metal oxide element in a rich atmosphere,

FIG. 13 is a graph showing an output characteristic of the sensor of FIG. 7,

FIG. 14A is a vertical section schematically showing an air/fuel ratio sensor of a third embodiment of the present invention,

FIG. 14B is a plan view of the sensor of FIG. 14A in which a cover is removed,

FIG. 15 is diagram of an air/fuel ratio detecting circuit using the sensor of FIG. 14A,

FIG. 16 is a graph showing a pump current versus excess air factor relationship of the air/fuel ratio sensor of FIG. 14A,

FIG. 17 is a graph showing a response characteristic of the air/fuel ratio sensor of FIG. 14A,

FIGS. 18A and 18B are plan views similar to FIG. 14B, for showing high potential region and low potential region,

FIG. 19A is a vertical section schematically showing an air/fuel ratio sensor of a fourth embodiment of the present invention,

FIG. 19B is a plan view of the air/fuel ratio sensor of FIG. 19A in which a cover is removed,

FIG. 20A is a vertical section of a conventional air/fuel ratio sensor of the same type as the sensors of the third and fourth embodiments of the present invention,

FIG. 20B is a plan view of the conventional sensor of FIG. 20A with its cover removed,

FIG. 21 is a circuit diagram showing a circuit using the conventional air/fuel ratio sensor of FIG. 20A,

FIG. 22 is a graph showing oxygen sensor cell EMF versus excess air factor relationships of the conventional air/fuel ratio sensor of FIG. 20A,

FIG. 23 is a diagram of an equivalent circuit arranged to measure the excess air factor by using the sensor of FIG. 20A,

FIG. 24 is a graph showing a pump current versus excess air factor relationship of the equivalent circuit of FIG. 23,

FIGS. 25A and 25B are schematic plan views of the conventional air/fuel ratio sensor of FIG. 20A, for showing high and low potentional regions in lean and rich atmospheres, and

FIG. 26 is a graph showing a response characteristic of the sensor of FIG. 20A.

To facilitate understanding of first and second embodiments of the present invention, reference is first made to a conventional air/fuel ratio sensor as disclosed in Japanese provisional publication No. 60-236056.

A primary measuring element 103 of an air/fuel ratio sensor 101 shown in FIG. 7 is made of metal oxide, such as an oxide of a transition metal, whose electrical resistance varies in accordance with change in oxygen concentration of a gas to be measured. The metal oxide primary element 103 is formed in the shape of a layer on an upper surface of a flat substrate 102 of insulating material. First and second electrodes 104 and 105 are attached to the metal oxide primary element 103 to measure the electrical resistance of the primary element 103. An oxygen sensor cell 106 composed of the primary element 103 and the first and second electrodes 104 and 105 is enclosed by a cover 107 of an oxygen ion conductive solid electrolyte, within a gas diffusion control space 108 formed between the cover 107 and the oxygen sensor cell 106. The solid electrolyte cover 107 is formed with a small gas diffusion hole 109. Annular third and fourth electrodes 110 and 111 are attached, respectively, to inner and outer surfaces of the solid electrolyte cover 107 around the hole 109 to form an oxygen pump cell 112.

In a circuit shown in FIG. 8, the air/fuel ratio sensor 101 of FIG. 7 is connected with a resistance meter 113 and a power supply 114 for the pump cell 112. The resistance meter 113 is connected between the first and second electrodes 104 and 105 of the oxygen sensor cell 106 through leads 104a and 105a. The power supply 114 is connected between the third and fourth electrodes 110 and 111 of the pump cell 112 through leads 110a and 111a.

When no pump current is supplied from the power supply 114, the resistance of the metal oxide element 103 measured by the resistance meter 113 varies in dependence on the oxygen concentration of the gas flowing into the control space 108 through the hole 109 according to a characteristic A shown in FIG. 9, which has a large and sharp change of the resistance at or near the A/F equivalent ratio (excess air factor) "lambda"=1.

When a pump current in a direction i.sub.1 shown in FIG. 8 is supplied from the power supply 114, oxygen ions flow through the solid electrolyte cover 107 from the third inner electrode 110 to the fourth outer electrode 111, so that oxygen is consumed near the third inner electrode 110. In the case that the movement of the gas in the gas diffusion control space 108 and/or the gas diffusion hole 109 is a rate-determining step, the oxygen concentration becomes low near the metal oxide element 103 especially in a region remote from the gas diffusion hole 109 even if the ambient gas contains a high percentage of oxygen (that is, the ambient gas is lean.). Therefore, the air fuel ratio sensor 101 provides a characteristic B shown in FIG. 9, so that the air/fuel ratio control becomes possible on the lean side.

When the pump current in a direction i.sub.2 shown in FIG. 8 is supplied from the power supply 114, oxygen ions flow in the solid electrolyte cover 107 from the fourth outer electrode 111 to the third inner electrode 110, so that oxygen is supplied into the control space 108 near the third electrode 110. Therefore, even if the ambient gas contains only a low percentage of oxygen (that is, the gas is rich.), the oxygen concentration becomes high near the metal oxide element 103 especially in the region remote from the gas diffusion hole 109. In this case, a characteristic C of FIG. 9 is obtained, and the air/fuel ratio sensor 101 enables the air/fuel ratio control on the rich side.

An equivalent circuit shown in FIG. 10 is one example used in actual systems for measuring the excess air factor of exhaust gases. In the circuit of FIG. 10, the oxygen sensor cell 106 and a resistor 120 are connected in series to form a series combination, to which a constant voltage V.sub.1 is applied. The resistance of the metal oxide element 103 of the oxygen sensor cell 106 is measured from an output voltage V.sub.2 obtained from an intermediate tap between the sensor cell 106 and the resistor 120. The power supply 114 is arranged to supply the pump current ip so as to maintain the output voltage V.sub.2 at a predetermined value Va. Therefore, the pump current ip varies in dependence on the excess air factor "lambda" of the gas, as shown by a characteristic of FIG. 11.

However, the conventional air/fuel ratio sensor 101 is still unsatisfactory in the following points. FIG. 12A schematically shows the metal oxide element 103 in the case that the gas to be measured is lean. As shown by a two-dot chain line in FIG. 12A, the metal oxide element 103 is held at a low resistivity state in a region A.sub.1 remote from a projection 109a of the gas diffusion hole 109. When the gas is rich, the metal oxide element 103 is held at the low resistivity state only in a region A.sub.3 closest to the projection 109a, and the remaining region A.sub.4 of the element 103 is held at a high resistivity state. Therefore, in order to maintain the output voltage V.sub.2 at the predetermined value Va, it is necessary to maintain the oxygen concentration sufficiently high in most of the gas diffusion control space 108 by supplying excessive pump current more on the rich side, as compared with on the lean side. This promotes degradation of the oxygen pump cell performance. Furthermore, overshoots tend to occur in a transient response of the pump current ip resulting from changes of the gas from rich to lean and from lean to rich, as shown in FIG. 13, because too much pump current is required to remove an excess of oxygen in the diffusion control space 108 especially on the rich side.

A first embodiment of the present invention is shown in FIGS. 1A and 1B. An air/fuel ratio sensor 1 of FIGS. 1A and 1B has an insulating substrate 3 in the shape of a flat plate. The insulating substrate 3 of an insulating material has a heating element 2 embedded therein. A first electrode 4 and a second electrode 5 are mounted on an upper surface of the substrate 3. The upper surface of the substrate 3 of this embodiment is substantially flat. Each of the first and second electrodes 4 and 5 of this embodiment is shaped like a strip, and placed in parallel to the other on the upper surface of the substrate 3. An oxygen sensing primary element 6 is placed on the first and second electrodes 4 and 5. The sensing element 6 is made of a metal oxide such as an oxide of a transition metal (or an oxide of a non-transition metal such as Sn or Zn). The sensing metal oxide element 6 is thin and substantiallty flat, and has a circular opening 6a, which is located in a central part of the metal oxide element 6. In this embodiment, the metal oxide element 6 is substantially square, and the center of the opening 6a coincides with the center of the square metal oxide element 6. An oxygen sensor cell 7 is constituted by the first and second electrodes 4 and 5 and the metal oxide element 6. The first and second electrodes 4 and 5 are attached to the metal oxide element 6 to measure change in the electrical resistance of the metal oxide element 6.

A cover 8 is placed on the substrate 3 so as to cover the oxygen sensor cell 7, and to form a gas diffusion control space 9 between the substrate 3 and the cover 8. The oxygen sensor cell 7 is enclosed within the control space 9 by the cover 8. The cover 8 is made of an oxygen ion conductive solid electrolyte. The cover 8 is formed with a gas diffusion hole 11 located at a center of the cover 8. A gas of an ambient atmosphere can flow into and out of the gas diffusion control space 9 through the gas diffusion hole 11. The cover 8 has a flat top wall and a side wall extending from a perphery of the top wall to the substrate 3. The flat top wall of the cover 8 of this embodiment is substantially square, and substantially parallel to the substrate 3. The gas diffusion hole 11 is located at the center of the top wall of the cover 8. The hole 11 of this embodiment is circular, and extends through the top wall of the cover 8 in a direction substantially perpendicular to the top surface of the substrate 3. A circular projection 11a shown in FIG. 1B is produced by projecting the hole 11 of the cover 8 onto the top surface of the substrate 3 in the direction in which the hole 11 extends through the top wall of the cover 8. As shown in FIG. 1B, the projection 11a of the hole 11 is concentric with the circular opening 6a of the oxygen sensing metal oxide element 6. The diameter of the circular opening 6a is larger than that of the projection 11a. That is, the projection 11a is centered within the circular opening 6a. In this way, the oxygen sensing element 6 is placed outside the projection 11a.

The air/fuel ratio sensor 1 further has a third inner electrode 12 and a fourth outer electrode 13 which form an oxygen pump cell 14 together with the solid electrolyte of the cover 8. Each of the third and fourth electrodes 12 and 13 is shaped like a ring, and disposed around the gas diffusion hole 11. The third electrode 12 is attached to an inner surface of the cover 8 within the gas diffusion control space 9, and the fourth electrode 13 is attached to an outer surface of the cover 8 outside the control space 9.

FIG. 2 shows an example of an air/fuel ratio control circuit using the air/fuel ratio sensor 1 of this embodiment. The circuit of FIG. 2 is arranged to determine the air/fuel ratio from an oxygen pump current of the air/fuel ratio sensor 1.

The circuit of FIG. 2 includes a differential amplifier 15. A first input (minus side) of the differential amplifier 15 is held at a reference voltage Vc. A resistor R.sub.1 and the metal oxide element 6 are connected in series to form a voltage divider, to which a constant voltage V.sub.1 is applied. An output voltage V.sub.2 obtained from an intermediate tap between the resitor R.sub.1 and the element 6 is applied to a second input terminal (plus side) of the differential amplifier 15. The differential amplifier 15 supplies the pump current ip to the oxygen pump cell 14 so as to make the output voltage V.sub.2 equal to the reference voltage V.sub.c.

This air/fuel ratio sensor 1 has a characteristic between the pump current ip and the excess air factor "lambda" of the gas to be measured, shown by a solid line in FIG. 3. A broken line in FIG. 3 shows a characteristic of the conventional air/fuel ratio sensor 101 shown in FIG. 7. As shown in FIG. 3, the value of the pump current of the sensor 1 is lower than that of the conventional sensor 101 especially on the rich side. Therefore, the design of the air/fuel ratio sensor 1 of the present invention can reduce the burden of the oxygen pump cell 14, and limit the undesired aging of the output characteristic to a low level.

FIG. 4 is a graph of an output characteristic of the air/fuel ratio sensor 1 according to the present invention. The characteristic of FIG. 4 is a response characteristic resulting from a change of the gas from lean to rich and vice versa. According to the present invention, the metal oxide primary element 6 is shaped and positioned so that the metal oxide does not exist inside the projection 11a of the hole 11. Especially in the rich atmosphere, the metal oxide element of the present invention 6 is held at a low resistivity state in a region A.sub.5, which is not situated just below the gas diffusion hole 11, as shown in FIG. 5. Therefore, the oxygen concentration becomes excessively high only in a small part of the gas diffusion space 9, so that the gas in the diffusion space 9 can be renewed with a small amount of the pump current during a change between rich and lean. As a result, the air/fuel ratio sensor 1 of the present invention provides a satisfactory transient response characteristic having no overshoot, as shown in FIG. 4.

A second embodiment of the present invention is shown in FIGS. 6A and 6B. In an air/fuel ratio sensor 1 of FIGS. 6A and 6B, the oxygen ion conductive solid electrolyte cover 8 is designed so that the rate of the gas diffusion is determined predominantly in the gas diffusion control space 9 instead of in the gas diffusion hole 11. For example, the diameter of the gas diffusion hole 11 is equal to 0.6 mm, and the height of the gas diffusion control space 9 (which is a distance between the inner surface of the top wall of the cover 8 and the upper surface of the substrate 3) is equal to 20 micrometers. In this embodiment, the surfaces of the first and second electrodes 4 and are flush with the surface of the substrate 5, and the metal oxide element 6 is divided in two pieces 6a and 6b, as shown in FIGS. 6A and 6B. Each piece of the metal oxide element 6 is shaped like a bar, and placed so that both ends are in contact with the first and second electrodes 4 and 5, respectively. Each piece of the metal oxide element 6 is spaced from the projection 11a of the hole 11 at a distance such that the rate of the gas diffusion is determined in a course between the position immediately below the gas diffusion hole 11 and the position of each piece 6a or 6b of the metal oxide element 6.

In the air/fuel ratio sensor of the second embodiment, the volume of the gas diffusion control space 9 is further reduced. That is, the amount of the gas to be replaced during a change between rich and lean is reduced. Therefore, the tendency to overshoot is further decreased, and the speed of response of the pump current ip is increased significantly (For example, the speed of response is more than five times as high as that of the conventional sensor).

To facilitate understanding of third and fourth embodiments of the present invention, reference is made to a conventional air/fuel ratio sensor shown in FIG. 20A. (A similar sensor is disclosed in Japanese patent provisional publication No. 61-30758.)

An air/fuel ratio sensor 200 of FIG. 20A has a substrate 201 of an oxygen ion conductive solid electrolyte, which is sandwiched between first and second electrodes 202 and 203 to form an oxygen sensor cell 204. The first electrode 202 is enclosed by a lower cover 205 within a reference space 206 formed between the lower cover 205 and the substrate 201. A reference gas such as air is introduced into the reference space 206. The first electrode 202 in the reference space 206 serves as a reference electrode. The second electrode 203 is enclosed by an upper cover 207 of an oxygen ion conductive solid electrolyte, within a gas diffusion control space 208 formed between the solid electrolyte upper cover 207 and the solid electrolyte substrate 201. The upper cover 207 is formed with a small gas diffusion hole 209, through which a gas to be measured is introduced into the control space 208. Annular third and fourth electrodes 210 and 211 are attached to inner and outer surfaces of the upper cover 207 around the hole 209 to form an oxygen pump cell 212.

As shown in FIG. 21, a potentiometer 213 is connected between the first and second electrodes 202 and 203 of the sensor cell 204 through leads 202a and 203a, and a power suply 214 is connected between the third and fourth electrodes 210 and 211 of the pump cell 212 through leads 210a and 211a.

When there is no pump current from the power supply 214, the difference in electric potential between the first and second electrodes 202 and 203 of the oxygen sensor cell 204, measured by the potentiometer 213 varies in dependence on the oxygen concentration of the gas introduced to the space 208 as shown by a characteristic A of FIG. 22. In this case, the potential difference, that is an oxygen sensor cell electromotive force, changes sharply at or near the A/F equivalent ratio (excess air factor) equal to 1. Like the sensor 101 shown in FIG. 7, the sensor 200 provides an oxygen sensor cell EMF versus excess air factor characteristic B of FIG. 22 when the pump current is supplied in the direction i.sub.1 shown in FIG. 21, and a characteristic C of FIG. 22 when the pump current is supplied in the opposite direction i.sub.2.

An equivalent circuit shown in FIG. 23 is one example used in actual systems for measuring the excess air factor of exhaust gases. In the circuit of FIG. 23, the potentiometer 213 and the power supply 214 are connected to supply the pump current ip such that the electromotive force Vs of the oxygen sensor cell 104 is held at a predetermined value Va. FIG. 24 shows a characteristic of the pump current ip versus the excess air factor, obtained by the arrangement of FIG. 23.

FIGS. 25A and 25B show the second electrode 203 of the sensor 200. When the gas to be measured is lean, the second electrode 203 is held at a high potential state with respect to the reference electrode in a region A.sub.1 which is outside a two dot chain line circle L.sub.1 and remote from a projection 209a of the hole 209, as shown in FIG. 25A. A region A.sub.2 inside the circle L.sub.1 is held at a low potential state. Therefore, a large part of the gas diffusion control space 208 must be held at a low oxygen concentration state by imposing an excessive load on the oxygen pump cell 212. When, on the other hand, the gas is rich, the second electrode 203 is held at the high potential state with respect to the reference electrode in a region A.sub.3 lying near the projection 209a of the hole 209 inside a two dot chain line circle L.sub.2 around the projection 209a, and at the low potential state in a remaining region A.sub.4 outside the circle L.sub.2, as shown in FIG. 25B. Therefore, a large part of the gas diffusion control space 208 must be held at a high oxygen concentration by imposing an excessive load on the oxygen pump cell 212. Because of the excessive load, the oxygen pump cell 212 of the conventional sensor 200 is subject to degradation in characteristic due to its use.

Furthermore, in changes in gas composition between rich and lean, the conventional sensor 200 requires a large amount of the pump current ip to remove an excess of oxygen due to a rich operation from the diffusion control space 208, and to supply a lack of oxygen due to a lean operation. Therefore, overshoots appear in a transient response characteristic of the pump current ip, as shown in FIG. 26.

The third embodiment of the present invention is shown in FIGS. 14A-18B.

An air/fuel ratio sensor 21 shown in FIG. 14A has an oxygen ion conductive solid electrolyte substrate 22 of a plate shape, which is sandwiched between first and second electrodes 23 and 24 to form an oxygen sensor cell 25. The first electrode 23 is enclosed by a lower cover 26 within a reference gas space 27 formed between the lower cover 26 and the substrate 22. A reference gas such as air is introduced into the space 27, and the first electrode 23 is used as a reference electrode. The lower cover 26 has a heating element 29 which is electrically insulated by an insulator 28 and embedded in the cover 26.

The second electrode 24 is a primary measuring element in direct contact with the gas to be measured, and serves as a measuring electrode of the sensor cell 25. The second electrode 24 is formed with a circular opening 24a lying in a central part of the electrode 24. In this embodiment, the second electrode 24 is substantially square, and the center of the opening 24a coincides with the center of the second electrode 24, as shown in FIG. 14B. The second electrode 24 is enclosed by an upper cover 31 of an oxygen ion conductive solid electrolyte within a gas diffusion control space 32 formed between the upper cover 31 and the substrate 22. A circular gas diffusion hole 33 is formed in a central part of the upper cover 31. The upper cover 31 of this embodiment has a flat top wall which is substantially parallel to the substrate 22. The hole 33 of this embodiment is located at the center of the top wall of the upper cover 31, and extends through the top wall of the upper cover 31 in a direction substantially perpendicular to the top surface of the substrate 22. A two dot chain line small circle 33a of FIG. 14B is a projection produced by projecting the hole 33 onto the top surface of the substrate 22 in the direction in which the hole 33 extends through the upper cover 31. The circular opening 24a of the second electrode 24 is sized and located so that the circular opening 24a is substantially concentric with the circle 33a, and the diameter of the circular opening 24a is larger than that of the circle 33a. The second electrode 24 is shaped and located so that the second electrode 24 does not exist within the circle 33a.

Annular third and fourth electrodes 34 and 35 are attached, around the hole 33, to inner and outer surfaces of the top wall of the upper cover 31, respectively, to form an oxygen pump cell 36.

In a circuit of FIG. 15, an electromotive force Vs of the oxygen sensor cell 25 of the sensor 21 is inputted to a first input terminal (minus side) of a differential amplifier 41 whose second input terminal (plus side) is held at a reference voltage Vc. The differential amplifier 41 supplies the oxygen pump cell 36 with the pump current ip so that the electromotive force Vs of the sensor cell 25 is maintained equal to the reference voltage Vc.

The air/fuel ratio sensor 21 of the third embodiment exhibits a pump curent versus excess air factor characteristic shown by a solid line in FIG. 16. A broken line in FIG. 16 shows a characteristic of the conventional sensor 200 of FIG. 20A. In the sensor of the third embodiment, the absolute value of the pump curent is reduced on both of the rich and lean sides, as compared with the conventional sensor 200. Thus, the load on the pump cell 36 is reduced, and the undesired aging of the sensor output characteristic is limited.

The air/fuel ratio sensor 21 exhibits a satisfactory transient response characteristic having no overshoot, as shown in FIG. 17. The second electrode 24 is held at a high potential state in a region A.sub.5 shown in FIG. 18A in the case of a lean atmosphere, and at a low potential state in a region A.sub.6 shown in FIG. 18B in the case of a rich atmosphere. Each of the regions A.sub.5 of FIG. 18A and the region A.sub.6 of FIG. 18B is situated apart from the position right below the gas diffusion hole 33a, so that the oxygen concentration is held at an excessively high or excessively low value only in a small volume of the gas diffusion space 32. Therefore, the gas in the control space 32 can be easily renewed with a small amount of the pump current, and overshoot is prevented as shown in FIG. 17.

A fourth embodiment of the present invention is shown in FIGS. 19A and 19B. An air/fuel ratio sensor 21 of the fourth embodiment is basically identical to the sensor of the third embodiment. As in the second embodiment, the upper cover 31 of the fourth embodiment is designed so that the rate of the gas diffusion is determined predominantly in the gas diffusion control space 32 instead of in the gas diffusion hole 33. For example, the diameter of the gas diffusion hole 33 is equal to 0.6 mm, and the height of the control space 32 (which is a distance between the substrate 22 and the top wall of the upper cover 31) is equal to 20 micrometers. The second electrode 24 of the fourth embodiment is narrow, and shaped like a ring as shown in FIG. 19B. The ring-shaped second electrode 24 is placed around the projection 33a of the hole 33. In the fourth embodiment, the second electrode 24 is situated at a considerable distance from the projection 33a, and the rate of the gas diffusion is determined in the interval between the point immediately below the hole 33 and the second electrode 24.

Like the second embodiment, the sensor of the fourth embodiment can further reduce the tendency to overshoot, and increase the speed of response approximately to a value five times as high as that of the conventional example.

There are known in the art various oxygen conductive solid electrolytes usable in the present invention, various metal oxides usable as the metal oxide element 6 of the first and second embodiments, and various materials usable as the electrodes of the sensor cell and the pump cell of the present invention.