Thermal radiation sensor

Details Thermal radiation sensor Thermal radiation sensor

1989-08-18

1991-04-23

Lateef, Marvin M.

Electrical resistors

Resistance value temperature-compensated

338 22R, 338 25, 338 18, 250352, H01C 706

Patent

active

050103156

DESCRIPTION:

BRIEF SUMMARY
PRIOR ART

The invention is based on a thermal radiation sensor according to the generic part of the main claim. A thermal radiation sensor is known from the DE-OS 35 36 133 and comprises two receiver surfaces which are exposed to the radiation, one of which receiver surfaces has a high absorption capacity with respect to the thermal radiation, e.g. by means of blackening, the other receiver surface having a low absorption capacity, e.g. by means of a covering reflecting the thermal radiation. These two receiver surfaces consist of a NTC resistive material and are combined in a bridge circuit together with two temperature-independent cermet resistors. The four resistors are applied to a ceramic substrate and connected with conductor paths which, in turn, end in the necessary connections. However, because of the convective heat transfer at the sensor surfaces, this arrangement has the disadvantage that the output signal depends heavily on the ambient temperature. Thus, in one and the same thermal radiation sensor, as indicated above, at a radiation intensity of 1000 W/m.sup.2 an output voltage of 900 mV is obtained at -30.degree. C. and an output voltage of 550 mV is obtained at 0.degree. C. and at 30.degree. C. In many cases such a dependency of the measurements on the ambient temperature is disadvantageous.


ADVANTAGES OF THE INVENTION

The thermal radiation sensor, according to the invention, with the characterizing features of the main claim has the advantage that the measurement signals no longer show any dependency on the ambient temperature, so that the same measured value can always be obtained at the same radiation intensity regardless of the temperature.
Advantageous developments and improvements of the radiation sensor indicated in the main claim are made possible by means of the steps indicated in the subclaims. It is particularly advantageous if the heating layers consist of a cermet thick film with platinum and have the shape of a meander, so that they can easily be balanced.
In can be shown in physico-mathematical terms that the measurement for the occurring radiation output is in fact independent from the ambient temperature in a substraction between the two sensors, one sensor having a high absorption capacity and the other having a low absorption capacity with respect to the thermal radiation.
The following applies to the first sensor (per surface unit): .multidot.M+A(T.sub.1 -T.sub.u) (1)
Values with index number 1 refer to the first sensor.
In the same way, the following applies to the second sensor: M+A(T.sub.2 -T.sub.u) (2)
Values with index number 2 refer to the second sensor.
The following results from the subtraction of (1) and (2): ##EQU1##
For T.sub.1 =T.sub.2 =T, the equation simplifies to ##EQU2## i.e. the measured quantity M is independent of the ambient temperature.


BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment example of the invention is shown in the drawing and explained in more detail in the following description.
FIG. 1 shows a section through the layer construction of a sensor, according to the invention,
FIG. 2 shows an electric circuit which indicates how the individual layers of the sensor are electrically connected with one another and
FIG. 3 shows a plan view of the sensor in accordance with the present invention.


DESCRIPTION OF THE EMBODIMENT EXAMPLE

According to FIG. 1, the sensor comprises a ceramic substrate in the form of a thin plate 5 which preferably consists of aluminum oxide. The heating conductor layers 6 and 7 are first printed on this substrate in the shape of a meander as shown in FIG. 3 and fired at approximately 1530.degree. C., the heating conductor layers 6 and 7 consisting of a platinum thick film, possibly with a proportion of 20% by volume of a ceramic supporting framework material such as aluminum oxide. These heating resistors have a resistance of approximately 30.OMEGA. in the finished state. The insulating layers 8 and 9, which consist of a crystallizing glass, e.g. of type 9105HT from the Heraeus company, are then printed on the hea

REFERENCES:
patent: 3287976 (1966-11-01), Euser et al.
patent: 3622901 (1971-11-01), Ledran et al.
patent: 4531110 (1985-07-01), Johnson, Jr. et al.
patent: 4770541 (1988-09-01), Fedter et al.

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