Temperature Detection Through Differential Dual Detectors

Abstract

Disclosed herein is a sensor system including four interconnected resistors, where two of the resistors are photoconductive detectors, where the photoconductive detectors are illuminated with light at least at two different wavelengths, where two of the resistors does not change their resistance due to the illumination, where an external voltage is applicable to the sensor system, where a differential voltage is measurable, which depends on the resistance changes of the illuminated photoconductive detectors, where the differential voltage gives a mathematical ratio of the four respective resistances.

Claims

1. A sensor system comprising four interconnected resistors, wherein at least two of the resistors are photoconductive detectors configured for each exhibiting an electrical resistance dependent on an illumination of its respective light sensitive region, wherein at least two of the photoconductive detectors each respond to electromagnetic energy of a different wavelength, wherein the two other resistors are configured for each exhibiting an electrical resistance essentially constant under illumination, wherein an external voltage is applicable to the sensor system, wherein the sensor system is configured for measuring a differential voltage, wherein the differential voltage is dependent on changes of the electrical resistances of the photoconductive detectors, wherein the differential voltage gives a mathematical ratio of the four respective resistances.

2. The sensor system according to claim 1, wherein the resistors exhibiting an electrical resistance essentially constant under illumination are photoconductive detectors, darkened by a cover.

3. The sensor system according to claim 1, wherein the resistors essentially constant under illumination are thermistors, wherein a change of their resistance as a function of temperature has the same characteristics as of the photoconductive detectors.

4. The sensor system according to claim 1, wherein the photoconductive detectors are covered by filter elements for preparation of light at different wavelengths.

5. The sensor system according to claim 1, wherein the resistors are interconnected by a bridge circuit arrangement.

6. The sensor system according to claim 1, wherein the sensor system is in direct line of sight of a measured object, wherein filters are arranged to be within the wavelength range of the electromagnetic radiation which is in the line of sight.

7. The sensor system according to claim 6, wherein the sensor system and the measured object are separated by a separating object, wherein the separating object is at least partially transparent at the at least two wavelengths, wherein the filters are arranged to be within the wavelength range of the electromagnetic radiation transmitted through the separating object.

8. The sensor system according to claim 1, wherein the four photoconductive resistors are arranged in an array next to another.

9. A method of using a sensor system according to claim 1 as a sensor for temperature measurement.

10. A method of using a sensor system according to claim 1 as a sensor for gas and/or liquid analysis.

11. A method of using a sensor system according to claim 1 as a sensor for concentration of gas and/or liquid or gases and/or liquids.

12. A method of using a sensor system according to claim 1 as a sensor for material classification between to pre-defined material classes.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0058] Further optional details and features of the invention are evident from the description of preferred exemplary embodiments which follows in conjunction with the dependent claims. In this context, the particular features may be implemented alone or with features in combination. The invention is not restricted to the exemplary embodiments. The exemplary embodiments are shown schematically in the figures. Identical reference numerals in the individual figures refer to identical elements or elements with identical function, or elements which correspond to one another with regard to their functions.

[0059] Specifically, in the figures:

[0060] FIG. 1 illustrates a first electrical circuit to measure electrical resistance by a Wheatstone Bridge;

[0061] FIG. 2 illustrates a second electrical circuit to measure electrical resistance by a Wheatstone Bridge;

[0062] FIG. 3 illustrates a third electrical circuit to measure electrical resistance by a Wheatstone Bridge;

[0063] FIG. 4 results of a simulation for two different circuits

EXEMPLARY EMBODIMENTS

[0064] This invention offers a simplified solution to measure the temperature of an object without any knowledge about its emissivity with only one read-out electronics. Dual-wavelength IR measurement is performed by means of the photoconductive detectors at different wavelengths.

[0065] Depending on the temperature of the measurement object, suitable photoconductors and wavelengths should be chosen.

[0066] As an example, the temperature range between 100° C. and 250° C. can be measured with 2 mm×2 mm PbS detectors, while sampling the wavelength ranges between 2.2 and 2.4 um with one detector and the range between 2.6 and 2.8 um with the other. Suitable optical filters can be positioned on top of the optical detectors, thus sampling the chosen wavelength ranges.

[0067] FIG. 1 shows an embodiment of a sensor system 100 according to the present invention. The Sensor system 100 comprises four interconnected resistors 102, 103, 104, 105. At least two of the resistors, in FIG. 1 resistors 104 and 105, are photoconductive detectors R.sub.Photo1 and R.sub.Photo2 configured for each exhibiting an electrical resistance dependent on an illumination of its respective light sensitive region. At least two of the photoconductive detectors each respond to electromagnetic energy of a different wavelength. The photoconductive detectors may be arranged in at least one array of photoconductors, in particular next to each other. The photoconductive detectors may be neighboring detectors of the array. The photoconductive detectors each respond to electromagnetic energy of a different wavelength. The present invention proposes dual-wavelength, in particular infrared measurement, by means of the photoconductive detectors configured for being sensitive at at least two different wavelengths. In particular, the photoconductive detectors each may detect electromagnetic absorption at different wavelengths in the electromagnetic spectrum. The photoconductive detectors of the array may be designed such that each pixel in the array responds to electromagnetic energy of a different wavelength. The photoconductive detectors may be covered by filter elements, also denoted as filters, for preparation of illumination at different wavelengths. For example, at least one filter arrangement may be used. However, other arrangements are possible. This may allow using the array for spectrometer applications.

[0068] The sensor system 100, in particular the photoconductive detectors, more particular their light sensitive regions, may be arranged in direct line of sight of an object to be measured. The filter elements may be arranged to be within the wavelength range of the electromagnetic radiation which is in the line of sight. The sensor system 100 and the measured object may be separated by a separating object, such as a separating objected comprised by the sensor system. The separating object may be at least partially transparent at the at least two wavelengths to which the two photoconductive detectors are responsible. The filters may be arranged to be within the wavelength range of the electromagnetic radiation transmitted through the separating object.

[0069] The sensor system 100 may comprise at least one bias voltage source configured for applying at least one bias voltage 106 and 107 to the photoconductive detectors. The photoconductive detectors may be electrically connected with the bias voltage source. The bias voltage may be the voltage applied across the photoconductor material. The photoconductive detectors each may be connected to the bias voltage source such that the bias voltage source can apply the bias voltage 106 and 107 to the photoconductive detectors.

[0070] The two other resistors R.sub.1 and R.sub.3, in FIG. 1 resistors 102 and 103, are configured for each exhibiting an electrical resistance essentially constant under illumination. The resistors exhibiting an electrical resistance essentially constant under illumination are not responding to the illumination. For example, the resistors exhibiting an electrical resistance essentially constant under illumination may be photoconductive detectors darkened by a cover. Thus, the resistors may be covered so they don't see any irradiation. A change on their output signal may depend on their temperature drift.

[0071] An external voltage, in particular a supply voltage, is applicable to the sensor system 100. The sensor system 100 may comprise the supply voltage source configured for applying the supply voltage V.sub.s, such as a direct current (DC) voltage or an alternating current (AC) voltage, to resistors. Therefore, the resistors may be connected to the supply voltage source.

[0072] The sensor system 100 is configured for measuring a differential voltage. The differential voltage is dependent on changes of the electrical resistances of the photoconductive detectors. The differential voltage gives a mathematical ratio of the four respective resistances.

[0073] For example, the resistors may be interconnected by a bridge circuit arrangement. For example, the bridge circuit arrangement may comprise at least one Wheatstone bridge. The Wheatstone bridge may be or may comprise an electrical circuit configured for determining an unknown electrical resistance by balancing two legs of a bridge circuit, wherein one of the legs comprises the unknown electrical resistance. For example, the Wheatstone bridge may comprise the four interconnected resistors, the photoconductive detectors R.sub.photo1 and R.sub.photo2 configured for each exhibiting an electrical resistance dependent on an illumination of its respective light sensitive region, and two other resistors R.sub.3 and R.sub.4.

[0074] The quotient, in particular the differential voltage V.sub.Diff, is calculated directly by means of the Wheatstone bridge as given in the following equation for the Circuit as shown in FIG. 1, resulting in a differential voltage V.sub.Diff (108):

[00002]$V_{\mathrm{Diff}}=V_S\frac{R_{P\mathrm{hoto}1}.Math.R_3-R_{P\mathrm{hoto}2}.Math.R_1}{\left(R_{P\mathrm{hoto}1}+R1\right).Math.\left(R_{P\mathrm{hoto}2}+R3\right)}$

[0075] Sourced by the supply voltage V.sub.s (101), the circuit has two symmetric legs of a bridge consisting of two non-photosensitive resistors R.sub.1 or R.sub.3 (102, 103) and one photosensitive resistor R.sub.Photo1 or R.sub.Photo2 (104, 105). Thus, the measurement of V.sub.Diff (108) is used to calculate the quotient of the values, measured at different wavelengths on an analog basis.

[0076] The resistors R.sub.1 and R.sub.3 may be darkened photoconductors, which means they are covered so they don't see any irradiation. The change on their output signal depends on their temperature drift. With the proposed circuit, any temperature drift of the detectors automatically corrected by the Wheatstone bridge, as long as the detectors, darkened or illuminated, exhibit same temperature behavior.

[0077] The Wheatstone bridge can be driven also with AC voltage, which means the bias voltage applied on the photoconductors is modulated. The modulation can be unipolar or bipolar. The frequency of the modulation can be chosen freely, but higher frequencies are recommended for low 1/f noise.

[0078] FIG. 2 shows a calculation for an example for an isotropic radiator with a 1 mm×1 mm area and with an emissivity of 1, the detectors R.sub.Photo1 and R.sub.Photo2 with bandpass filters in the above-mentioned wavelength ranges will change their resistance values differently. With 1 MΩ dark resistance for all detectors, two illuminated and two darkened, and in a distance of 10 cm from the isotropic radiator, the differential voltage can be measured as a function of temperature. V.sub.s for this calculation is set 1 V. The calculated values may vary depending on the distance, emissivity of the radiator, spectral detectivity and responsivity of the detectors, the transmission properties of the used filters and many other parameters. The circuit of FIG. 1 is not the only possible solution but should serve as an example.

[0079] The lower curve represents the electrical circuit represented in FIG. 1. A second simulation referring the circuit of FIG. 3 is represented in the upper curve.

[0080] As long as the resistance of the photoconductors changes with the same factor due to the temperature, the differential voltage curve remains the same. Alternatively, temperature sensitive resistors can be employed as R.sub.1 and R.sub.3, as long as their temperature-resistance behavior is identical to that of photoconductors. It is a known fact that not only the resistance but also the responsivity of the photoconductors depends on the temperature. In this case, (if the change in the differential voltage is unacceptable high) a contact temperature sensor, such as a cheap PT100 or PT1000, can be used to correct the look-up table to convert the differential voltage into temperature.

[0081] FIG. 3 gives an alternative setting for the circuit, where the sensitivity of the circuit on the irradiance can be improved by positioning the both darkened and illuminated photoconductive detectors diagonally. Alternatively, temperature sensitive resistors can be employed as R.sub.1 and R.sub.3. The comparison of the resulting differential voltages can be seen in FIG. 2 upper line.

[00003]$V_{\mathrm{Diff}}=V_S\frac{R_{P\mathrm{hoto}1}.Math.R_3-R_{P\mathrm{hoto}2}.Math.R_1}{\left(R_{P\mathrm{hoto}1}+R1\right).Math.\left(R_{P\mathrm{hoto}2}+R3\right)}$

[0082] The third alternative is shown in FIG. 4. The robustness of the circuit on the resistance changes may be improved by positioning the both darkened and illuminated photoconductive detectors on the same leg, respectively. The illuminated detectors can change their resistance with the same factor, while the darkened detectors, or alternatively temperature sensitive resistors, have the same change factor. The resulting differential voltage remains the same.

[00004]$V_{\mathrm{Diff}}=V_S\frac{R_{P\mathrm{hoto}1}.Math.R_3-R_{P\mathrm{hoto}2}.Math.R_1}{\left(R_{P\mathrm{hoto}1}+R1\right).Math.\left(R_{P\mathrm{hoto}2}+R3\right)}$

[0083] Fluctuations on the supply voltage are balanced out since both arms of Wheatstone bridge are connected to the same potential and fluctuate the same, thus differential voltage remains constant.

[0084] By measuring the differential voltage V.sub.Diff only, a detector-temperature independent, emissivity independent dual-wavelength temperature measurement can be achieved with high resolution and with minimum numbers of components for the read-out electronics. The differential voltage can then be amplified and converted into digital values by means of an ADC. There are off-the-shelf amplifiers, analog front ends and analog-digital converters available for the measurement of differential voltages for both DC and AC supply voltages V.sub.s.

[0085] The sensor system 100 can be employed for emissivity independent temperature measurement. The sensor system 100 may be in the direct line of sight of the measured object or the sensor can measure the temperature of the object through another object, which is transparent at the sampled wavelengths. This is possible for the example of ceramic cooktops which are transparent for some specific infrared frequencies.

[0086] Alternatively, the sensor system 100 can be employed for gas analyses. The concentration of a gas can be determined by measuring the decrease of the light intensity from a light source through a gas filled optical path according to Lambert-Beer law, whereas the wavelengths to be sampled should be chosen depending on the gas to be measured. Generally, two wavelengths are chosen in such a way, that the measured gas absorbs at one wavelength and transmits at the other wavelength without absorption losses, thus the latter serves as the reference. The quotient of both signals depends on the measured gas concentration. In an analogous manner, liquids can also be monitored.

[0087] The sensor system 100 can be employed for measuring the diffuse reflection from a solid, illuminated with a light source. By sampling the diffuse reflection at two wavelengths concentration of known materials can be determined, or material classification between two pre-defined classes can be performed, like human skin or not, plastic or glass etc. Such measurements are common for optical sorting tasks for the recycling of plastics, glasses etc.

LIST OF REFERENCE NUMBERS

[0088] 100 Sensor system [0089] 101 Voltage supply V.sub.s [0090] 102 Resistor R.sub.1 [0091] 103 Resistor R.sub.3 [0092] 104 Photo Resistor R.sub.Photo1 [0093] 105 Photo Resistor R.sub.Photo2 [0094] 106 V.sub.Bias1 [0095] 107 V.sub.Bias2 [0096] 108 V.sub.Diff