Temperature sensor

A temperature sensor includes a ceramic substrate, and a sensing resistor containing platinum is embedded in the ceramic substrate. Electric current can be applied to the sensing resistor through the lead. The voltage of the sensing resistor can be detected through the second lead. Another resistor for dividing voltage is electrically connected to the sensing resistor, the resistor is trimmed by laser irradiation so as to adjust its electrical resistance value such that upon applying electric current having a certain value onto the sensing resistor an output voltage having a predetermined value is generated. Heat generated by the laser irradiation to the resistor does not affect an electrical resistance value of the resistor as much as that of the sensing resistor, thereby improving precision of the temperature sensor. The sensing resistor avoids contact with the atmosphere.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a temperature sensor or a resistance 
thermometer using platinum or a cermet of platinum and a metal as a 
resistor. The temperature sensor of the present invention improves 
reliability at high temperatures and precision. 
2. Description of Related Art 
A temperature sensor or a resistance thermometer has a sensing resistor 
having an electrical resistance varying with temperature. The temperature 
sensor is driven by a constant current source in order to develop voltages 
across the changing resistance of the sensing resistor. A temperature 
sensor having a platinum film is disclosed in a preprint "Platinum Film 
Temperature Sensors" by G. S. Iles for Automotive Engineering Congress and 
Exposition, Detroit, Mich., Feb. 24-28, 1975, held by Society of 
Automotive Engineers. 
Japanese Patent Application Laid-Open No. 4-279831 discloses a temperature 
sensor including a ceramic substrate, a platinum resistor coated thereon, 
and a glass layer coated onto the platinum resistor. Before being coated 
by the glass layer, the platinum resistor on the substrate is trimmed by 
laser irradiation so as to have electrical resistance of a predetermined 
value. 
However, in the step of trimming the platinum resistor, the laser 
irradiation heats the resistor thereby changing its electrical resistance. 
Consequently, it is not easy to improve precision in an ohmic resistance 
value of the resistor, and variance of the ohmic resistance value tends to 
increase. Moreover, the glass layer may not withstand high temperatures, 
and the temperature sensor may not be used at high temperatures. 
In some of applications, the temperature sensor is used under demanding 
conditions, for example, at high temperatures as well as in an oxidizing 
atmosphere, a reducing atmosphere, and an atmosphere containing a 
corrosive gas. For example, temperatures of an exhaust gas from an 
internal combustion engine and an exhaust gas from a plant may need to be 
monitored. Under these conditions, the resistor of the temperature sensor 
may gradually change its electrical resistance value over a long period. 
SUMMARY OF THE INVENTION 
An object of the present invention is to solve the aforementioned problem. 
The present invention provides a temperature sensor comprising a ceramic 
substrate; a sensing resistor, being embedded in the ceramic substrate, 
having a positive temperature coefficient of resistance; a first lead 
connected to the sensing resistor, wherein electric current can be applied 
to the sensing resistor through the first lead; a second lead connected to 
the sensing resistor, wherein a voltage of the sensing resistor can be 
detected through the second lead; a second resistor electrically connected 
to the sensing resistor, the second resistor having an electrical 
resistance value such that upon applying electric current having a certain 
value the sensing resistor has output voltage having a predetermined 
value. 
In the temperature sensor of the present invention, a sensing resistor (2) 
is embedded in a ceramic substrate (1, 11, 18) so that the sensing 
resistor avoids contact with an atmosphere to be sensed thereby the 
sensing resistor does not deteriorate. Consequently, the temperature 
sensor of the present invention remains stable and reliable even under 
demanding conditions, such as an oxidizing atmosphere at high 
temperatures, a reducing atmosphere at high temperatures, an atmosphere 
containing a corrosive gas, etc. 
A ceramic material for the ceramic substrate preferably insulates 
electricity. The ceramic material preferably has low thermal conductivity 
so as to decrease the heat flow between the sensing resistor and another 
resistor through the ceramic substrate, thereby improving the precision of 
the temperature sensor. The ceramic substrate may be composed of, for 
example, alumina, stearite, mullite, etc. Preferably, the entirety of the 
ceramic substrate may consist of the same ceramic material. However, part 
of the ceramic substrate may be composed of a different ceramic material 
from the other parts of the ceramic substrate. The ceramic substrate may 
be dense so as to prevent gas molecules from permeating therethrough. 
The ceramic substrate may have a planar shape. However, the ceramic 
substrate may have any shape including a tubular shape and a cylindrical 
shape. The ceramic substrate favorably has a shape such that heat that is 
experienced at the sensing resistor does not affect the temperature of the 
voltage-dividing resistor. 
The sensing resistor contains a metal having a positive temperature 
coefficient of resistance. The positive temperature coefficient of 
resistance is preferably large. The metal may include, for example, 
platinum, rhodium, nickel, tungsten, etc., and especially platinum is 
favorable. The sensing resistor may be composed of any of these metals, an 
alloy including any of these metals, or a cermet consisting of a ceramic 
material and any of these metals. The temperature sensor of the present 
invention can measure temperature by properties that the sensing resistor 
changes its electrical resistance depending on temperature. 
A resistor for dividing voltage is electrically connected to the sensing 
resistor by means of, for example, a pair of leads. In contrast to the 
sensing resistor, the voltage-dividing resistor preferably has a small 
temperature coefficient of resistance in terms of absolute value. In other 
words, upon varying temperatures, a voltage-dividing resistor preferably 
does not change its electrical resistance much. The voltage-dividing 
resistor may be made by printing a metal or metal oxide. Alternatively, 
the voltage-dividing resistor may be made of a glass matrix and particles 
dispersed therein, and the particles are made of a metal or a metal oxide. 
Alternatively, the voltage-dividing resistor may include a film made of a 
metal or metal oxide and a metal wire. 
The voltage-dividing resistor may preferably be coated onto a surface of 
the ceramic substrate so that the voltage-dividing resistor is easily 
trimmed by laser irradiation so as to adjust output voltage. Upon applying 
electric current the output voltage is generated. During the laser 
irradiation, electric current having a certain value is applied to the 
sensing resistor and the voltage-dividing resistor at a certain 
temperature, for example, at 25.degree. C. while output voltage is 
detected. The voltage-dividing resistor is trimmed such that the backward 
voltage of the sensing resistor has a predetermined value, which 
corresponds to the electric current and an electric resistance value. 
In the present invention, the output voltage of the temperature sensor has 
a small variance. In the present invention, the voltage-dividing resistor 
having a small temperature coefficient of resistance is trimmed by laser 
irradiation instead of the sensing resistor having a large temperature 
coefficient of resistance, and heat generated by the laser irradiation 
does not affect an electrical resistance value of the voltage-dividing 
resistor as much, thereby reducing error in the output voltage. In the 
present invention, the backward voltage of the sensing resistor is 
monitored during the trimming step so as to substantially decrease the 
effects of electrical resistance changes in the leads due to temperature 
changes, thereby decreasing a variance of the output voltage. 
Preferably at 25.degree. C. the second resistor has electrical resistance 
larger than one hundred times as much as electrical resistance of the 
sensing resistor so as to improve the precision of the temperature sensor. 
Further preferably at 25.degree. C. the second resistor has electrical 
resistance larger than one thousand times as much as electrical resistance 
of the sensing resistor. 
In the present invention, the voltage-dividing resistor may be disposed at 
a position that the voltage-dividing resistor is not exposed to an 
atmosphere to be sensed. In this arrangement, the voltage-dividing 
resistor remains reliable, and an electrical resistance value of the 
voltage-dividing resistor does not change much over a long period. 
Preferably the ceramic substrate may have two ends, the sensing resistor 
may be disposed in one of the two ends, and the voltage-dividing resistor 
may be disposed in the other end. 
The voltage-dividing resistor is preferably disposed at a position that has 
a certain distance from the sensing resistor so as to reduce the heat flow 
between the voltage-dividing resistor and the sensing resistor. This is 
especially the case when the temperature sensor detects high temperatures. 
While the sensing resistor is exposed to high temperatures, for example, 
at 1000.degree. C., the voltage-dividing resistor is kept at much lower 
temperatures, for example, at about 300.degree. C., resulting in a longer 
service life of the voltage-dividing resistor. 
The voltage-dividing resistor is preferably coated with the glass layer so 
as to improve durability. In the embodiment that the voltage-dividing 
resistor has a certain distance from the sensing resistor to reduce heat 
conduction, the glass layer may withstand higher temperatures. 
A method of coating the glass layer onto the voltage-dividing resistor may 
include the steps of making a slurry including glass (for example, 
borosilicate lead) powder, applying the slurry onto the surfaces of the 
voltage-dividing resistor, drying the slurry thereon, and firing the 
slurry thereon. The slurry applying step can be carried out by immersion, 
blade coating, spray coating, etc. 
In the temperature sensor of the present invention, upon applying electric 
current to the sensing resistor, the sensing resistor generates voltage, 
and the voltage signal is detected. The voltage signal may be then 
converted, for example, by a central processing unit to show temperature. 
Even when the leads, terminal pads, and side connections have some 
electrical resistance, the temperature sensor keeps the precision to 
detect temperatures. A lead may be used to transmit electric signals, such 
as voltage, from the sensing resistor, and the lead may be connected to 
the sensing resistor. Alternatively, electric power having a certain 
voltage may be applied to the sensing resistor while an electric current 
value is detected. 
The sensing resistor, the second resistor, the leads, and the terminal pads 
may preferably be printed so as to form a film. However, these elements 
also may be formed by blade coating, spray coating, and so on.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows the first embodiment of the temperature sensor of the present 
invention, and the ceramic substrate 1, 11, 18 is separated into three 
sheets for clarity. FIG. 1 can be seen as showing a process of making the 
first embodiment also. FIG. 7 is an expanded view of a part of FIG. 1. 
In FIG. 1, a sensing resistor 2, which is composed of a paste mixture of 
platinum and alumina, is printed onto a surface of a ceramic sheet 1. 
During the printing step, the ceramic sheet is green alumina that is not 
fired yet. 
A pair of current leads 3, 4 are also printed onto a surface of the ceramic 
sheet 1. One end of both of the current leads 3, 4 connects to the ends of 
the resistor 2. The other ends of the current leads 3, 4 connect to 
connecting pads 9, 10 respectively, for ensuring connection with a side 
surface 1s. 
A pair of leads 5, 6 is printed onto the surface of the ceramic sheet 1 
also. Voltage generated at the resistor 2 can be detected through the 
second leads 5, 6. One end of both of the leads 5, 6 connects to the ends 
of the resistor 2. The other ends of the second leads 5, 6 connect to 
connecting pads 7, 8, respectively, for ensuring connection with a side 
surface 1s. One end 2a of the resistor 2 diverges into the current lead 3 
and the lead 5 (see FIG. 7), while the other end 2b of the resistor 2 
diverges into the current lead 4 and the lead 6. 
Current leads 3, 4, leads 5, 6, and connecting pads 9, 10 may be composed 
of, for example, a paste mixture of platinum and alumina. They are 
preferably printed in the step of printing the resistor 2. However, 
materials for current leads 3, 4, leads 5, 6, and connecting pads 7, 8 may 
not be the same as that for the resistor 2. 
A ceramic substrate 11 is preferably made of the same material for the 
ceramic substrate 1. Side connections 12, 13 for electrically connecting 
the connecting pad 9, 10 are printed onto side surface 11s of the ceramic 
substrate 11. 
The side connection 12 electrically connects to a terminal pad 15. A pad 16 
for dividing voltage is printed between the side connections 12, 13. The 
pad 16 connects to another terminal pad 17. The terminal pad 15 does not 
cross the terminal pad 17. 
The terminal pads 15, 17 and the pad 16 are printed onto a surface of an 
end 11a of the ceramic substrate. The terminal pads 15, 17 and the pad 16 
may be composed of a paste mixture of platinum and alumina. However, 
materials for the terminal pads 15, 17 and the pad 16 may not be the same 
as that for the resistor 2. 
A ceramic substrate 18 is preferably made of the same material for the 
ceramic substrate 1. Side connections 19, 20 extending to a back surface 
of the ceramic substrate are printed onto side surface 18s of the ceramic 
substrate 18. The side connections 19, 20 connect to terminal pads 21, 22, 
respectively printed onto a back surface of the ceramic substrate. The 
side connections 19, 20 and terminal pads 21, 22 may be composed of a 
paste mixture of platinum and alumina. However, materials for the side 
connections 19, 20 and terminal pads 21, 22 may not be the same as that 
for the resistor 2. 
The three green ceramic substrates 1, 11, 18 are laminated together, 
pressed, and then fired at 1,600.degree. C. so as to form a unitary piece. 
When the resistor 2 contains tungsten or nickel, the atmosphere in the 
firing step may be a reducing atmosphere. When the resistor 2 contains 
platinum or rhodium, the atmosphere may be either a reducing atmosphere or 
an oxidizing atmosphere. 
A resistor 14 for dividing voltage, which is composed of a mixture of 
ruthenium oxide and glass is printed and fired so as to connect side 
connections 12, 13. The resistor 14 coats at least a part of the pad 16 
for dividing voltage so as to connect to the pad 16. For example, the 
resistor 14 may have electrical resistance of 50 kiloohms, and the sensing 
resistor may have electrical resistance of 20 ohms. 
Electric current having a certain value is applied to the resistor 2 
through the terminal pads 21, 22 while an output voltage from the resistor 
2 is detected through the terminal pads 15, 17. Under these conditions, 
the resistor 14 for dividing voltage is trimmed by laser irradiation so as 
to give an output voltage having a certain value corresponding to the 
electric current. After trimming, the resistor 14 may be coated by a glass 
layer so as to protect the resistor. 
In the trimming step, an infrared laser or an ultraviolet laser may be 
used. For example, a yttrium aluminum garnet laser generates a ray having 
a diameter of 50 .mu.m onto the resistor 14 moving at a rate of 0.25 mm 
per second. The laser may have an oscillating frequency of one kilohertz 
and a power of 600 milliwatts. 
The electrical circuit is illustrated in FIG. 8. The electrical circuit has 
temperature sensitive resistor 2 and voltage-dividing resistor 14, which 
are in parallel. Electric current is applied to the sensing resistor 2 by 
input terminals 21, 22. Voltage generated at the sensing resistor 2 is 
divided by the resistor 14 so as to give output voltage at output 
terminals 15, 17. 
As the temperature of the sensing resistor 2 varies, upon applying constant 
electric current, the electrical resistance of the sensing resistor 2 
varies so as to change the voltage generated at the sensing resistor 2, 
thereby the output voltage at output terminals 15, 17 changes accordingly. 
FIG. 2 shows a manner of using a temperature sensor 30 in the exhaust 
system of an automobile. The temperature sensor 30 is attached to an 
exhaust pipe 31 by means of a housing 32. One end 30a, which the resistor 
2 is embedded in, is inserted into the inside of the exhaust pipe 31, 
while the other end 30b having the resistor 14 is disposed on the outside 
of the exhaust pipe 31. The housing 32 is threadedly engaged to the 
exhaust pipe 31, and a member 33 for absorbing shock is disposed between 
the housing 32 and the temperature sensor 30. 
The end 30a is covered by a protection cover 34 so as to avoid an impact on 
the end since the ceramic substrate of the temperature sensor 30 may be 
vulnerable to the impact. The protection cover 34 is perforated so as to 
introduce an exhaust gas into the inside of the protection cover 34. 
The other end 30b is connected to a connector 36 so that electric signals 
at terminal pads 15, 17, 21, 22 are transmitted to lines 37. The end 30b 
and the connector 36 are installed in a casing 35. 
In a method of making the first embodiment, three green ceramic sheets 1, 
11, 18 are laminated together, pressed, and then fired so as to form a 
unitary piece. However, three green ceramic sheets may not be required to 
make the first embodiment. For example, without the green ceramic sheet 
18, terminal pads electrically connected to connection pads 9, 10 may be 
printed onto a back surface of the green ceramic sheet 1, which is 
opposite to the surface that the resistor 2 is printed onto. 
In FIG. 3, the second embodiment is similar to the first embodiment except 
that one end of the ceramic sheet 41a is coated by a coating layer 41b 
made of a ceramic material so that a sensing resistor 42 is embedded in 
the ceramic substrate 41 while the other end of the ceramic sheet 41a is 
not coated by the coating layer 41b. In the second embodiment, the ceramic 
substrate 41 has the ceramic sheet 41a and the coating layer 41b, and the 
sensing resistor 42 is disposed between the ceramic sheet 41a and the 
coating layer 41b. Leads are connected to the sensing resistor 42, and 
parts of the leads are coated by the coating layer 41b also. The other 
parts of the leads and a resistor for dividing voltage are coated by a 
glass layer. 
In a method of making the second embodiment of the present invention, a 
ceramic paste is coated onto a surface of one end of the green ceramic 
sheet 1 so that the resistor 2 is coated by the ceramic paste. However, 
the other end is not coated by the ceramic paste. The green ceramic sheet 
is fired with the paste so as to form a unitary piece. 
In FIG. 4, the third embodiment is similar to the first embodiment except 
that one end of the ceramic sheet 46a is coated by a coating layer 46b 
made of a ceramic material so that a sensing resistor 47 is embedded in 
the ceramic substrate 46 while the other end of the ceramic sheet 46a is 
not coated by the coating layer 46b. In the third embodiment, the coating 
layer 46b covers all the surfaces of the one end of the ceramic sheet 46a 
having the resistor 47, while in the second embodiment the coating layer 
42 covers only the surface having the resistor 42 thereon. Similar to the 
second embodiment, leads are connected to the sensing resistor, and parts 
of the leads are coated by the coating layer 46b also. The other parts of 
the leads and a resistor for dividing voltage are coated by a glass layer. 
In a method of making the third embodiment of the present invention, one 
end of the green ceramic sheet 1 is dipped into a ceramic slurry so that 
the end having the resistor 2 is coated by the ceramic coating. The green 
ceramic sheet is fired with the ceramic coating so as to form a unitary 
piece. 
In FIG. 5, the fourth embodiment of the present invention is similar to the 
first embodiment except that the ceramic substrate 51 of temperature 
sensor 50 is formed with a through hole 58 between a sensing resistor 52 
in one end of the ceramic substrate and a resistor 54 for dividing voltage 
in the other end of the ceramic substrate so as to reduce the heat flow 
between the sensing resistor 52 and the voltage-dividing resistor 54 
through the ceramic substrate. When the sensing resistor 52 is exposed to 
high temperatures, the voltage-dividing resistor 54 is kept at much lower 
temperatures, resulting in a longer service life of the resistor 54. As 
shown in FIG. 5, temperature sensor 50 also includes side connection 53, 
terminal pads 55 and 57, and pad 56 for dividing voltages. These 
components correspond to those described above in connection with FIG. 1. 
Preferably, the through hole 58 is formed close to the sensing resistor 52. 
The through hole 58 extends in the thickness direction of the ceramic 
substrate 51. The resistor 52 is embedded in an end of the substrate 51. 
In FIG. 6, the fifth embodiment of the present invention is similar to the 
first embodiment except that an end 61a of the ceramic substrate 61 of 
temperature sensor 60 is thinner than the other parts of the ceramic 
substrate 61 so as to reduce the heat flow between the sensing resistor 62 
embedded in the thin end 61a and the voltage-dividing resistor 64 in the 
other end through the ceramic substrate 61. When the sensing resistor 62 
is exposed to high temperatures, the voltage-dividing resistor 64 is kept 
at much lower temperatures, resulting in a longer service life of the 
resistor 64. As shown in FIG. 6, temperature sensor 60 also includes side 
connection 63, terminal pads 65 and 67, and pad 66 for dividing voltages. 
These components correspond to those described above in connection with 
FIG. 1. 
In the temperature sensor of the present invention, the sensing resistor is 
embedded in the ceramic substrate thereby the sensing resistor does not 
easily deteriorate. An electrical resistance value of the voltage-dividing 
resistor is adjusted so as to decrease variation in the output voltage of 
the temperature sensor. Moreover, where the voltage-dividing resistor has 
larger electrical resistance than the sensing resistor has, the 
temperature sensor has improved precision in determining temperatures.