Scanning Infrared Thermometer With DC Offset and Emissivity Correction

Scanning infrared thermometers which scan a target for infrared radiation and average a plurality of infrared radiation samples from a plurality of points along a scan line on a target to determine the average radiation from a target are known. An optical system including a rotating reflective member collects light, such as infrared light, from the target, and a photoelectric detector converts the collected infrared radiation to an analog radiation signal representative of the magnitude of the radiation. After the analog radiation signals for a number of samples associated with the selected target are averaged, the average radiation for the selected target is converted to a representation of temperature.

In such scanners, it is also known to provide one or more reference sources of known temperatures, such as a relative cold reference at case or ambient temperature and a relatively hot reference source, also located in the scanning path. The radiation signal developed from these reference sources are used to make corrections in the conversion circuitry for DC offset variations and emissivity variations.

Accurate temperature measurement in infrared thermometry involves solving a complex equation. The output signal from the infrared detector is a function of not only the target temperature but also the wavelength of the energy reaching the detector, the emissivity of the target, the temperature of the detector, background energy reflections and other second order effects. The equation describing the detector output is defined by equation (1), below: ##EQU1## where, E is target emissivity;

.lambda. is wavelength;

K is power factor (5=thermal, 4=quantum);

T is target temperature;

C.sub.2 is a constant (14388 for degrees in Kelvin);

C.sub.1 is a detector scaling constant; and

T.sub.B is background temperatures reflecting off the target.

When T.sub.B is much lower than T the second term of equation (1) can be ignored. The problem faced by any system is to solve for T after measuring V.

Equation (1) can be restated in an abbreviated form as equation (2), below:

and then rearranged to form equation (3), below:

Prior systems have solved equation (3) with multisegment linearizer circuits or even with look-up tables. These schemes have assumed fixed values for E, C and C.sub.o. C.sub.1 and C.sub.o depend on hot and cold reference targets which are not necessarily held constant. In a scanning system, the target emissivity E can change as a function of the scan angle, and product characteristics may cause the emissivity E to be different at different positions across the product. Regardless of the cause or reason for different values for emissivity E, it must be corrected in order to obtain an accurate solution to equation (3).

Systems are known which have used computers to calculate T for a specific spot in the product. However, in such computerized systems the calculations were so slow that multiple points could not be calculated in real time. Additionally, these digital computerized systems suffer from inaccuracy due to the fact that they correct for emissivity and circuit gain errors only after conversion of the analog radiation signal from the detector by an A/D converter. Consequently, for low emissivity values, the full range of the A/D converter is not utilized.

Another disadvantage of known scanning infrared thermometers is that the detector and conversion circuitry are not provided in proximity to each other, but are located in separate spaced housings which can cause substantial noise difficulties which precludes sending corrections for emissivity and DC offset errors back to the detector circuitry. Instead, correction is performed in the housing of the conversion circuitry which is connected to the detection circuitry by means of an elongated cable which is prone to pick up noise along its length.

It is therefore a principal object of the present invention to provide a scanning infrared thermometer with a conversion circuit which overcomes the aforementioned problems of known scanning infrared thermometers and more accurately corrects for DC offset and emissivity errors by providing all elements together in proximity in a single housing assembly and by correcting the DC offset and emissivity errors prior to conversion of the analog detection signal to digital form by providing a multitarget detector.

Specifically, it is an objective to provide a scanning infrared thermometer having an optical system for collecting light from a target and means including a photoelectrical detector for converting the collected light to an analog radiation temperature signal having a magnitude related to temperature with a correction circuit comprising means for altering the analog temperature signal to produce a corrected analog temperature signal in which errors due to emissivity are reduced including a reference and means for correcting for emissivity, and means responsive to the reference for automatically correcting for DC fluctuations prior to corrections by said emissivity correcting means, means including an analog to digital converter for converting the corrected analog temperature signal to generate a corresponding corrected digital temperature signal representative of the magnitude of the corrected analog temperature signal, and means responsive to the corrected digital temperature signal for displaying the temperature of the target.

Another objective is to provide a scanning infrared thermometer having a scanning optical system for collecting light from a target and means including a photoelectrical element for converting the collected light to an analog temperature signal with a magnitude related to temperature with a correcting circuit comprising means for altering the analog temperature signal to correct for DC offset errors from the detector to produce a corrected analog temperature signal, means including an analog to digital converter for converting the corrected analog temperature signal to generate a corresponding corrected digital temperature signal representative of the magnitude of the corrected analog temperature signal, and means responsive to the corrected digital temperature signal for displaying the temperature of the target.

Further, it is an objective to provide a scanning infrared thermometer having a scanning optical system for collecting light from a target and means including a photoptical detector for converting the collected light to an analog temperature signal with a magnitude related to the temperature of the target with a correcting circuit responsive to the analog temperature signal circuitry for producing a corrected analog temperature signal corrected for errors due to at least one of (a) variations in DC offset of the light converting means and (b) errors due to target emissivity, at least once per complete scan of the scanning optical system, a linearizing circuit responsive to the corrected analog temperature signal for producing a linearized, corrected temperature signal, a controller responsive to the linearized, corrected temperature signal for controlling the correcting circuit, a housing assembly for protectively enclosing together in proximity the correcting circuit, linearizing circuit and the controller, and an output interface. The output interface includes an output terminal mounted to and accessibly from the outside of the housing and means for producing a representation of the linearized, corrected temperature signal at said output terminal.

Yet, another objective of the invention is to provide a scanning infrared thermometer having a scanning optical system for collecting light from a target and a detector circuit including a photoelectrical device for converting the collected light to an analog temperature signal having a magnitude related to temperature with a correcting circuit comprising means for altering the analog temperature signal to correct for DC offset errors from the detector circuit to produce a corrected analog temperature signal, means for altering the magnitude of the analog temperature signal to correct for errors caused by at least one of emissivity effects, circuit stability and component variations, means including an analog to digital converter for converting the corrected analog temperature signal to a corresponding corrected digital temperature signal representative of the magnitude of the corrected analog temperature signal and means responsive to the corrected digital temperature signal for outputting the signal in digital format to at least one of a display, a recorder and process controller.

The foregoing objects and advantageous features of the invention will be explained in greater detail and others will be made apparent from the detailed description of the preferred embodiment of the present invention which is given with reference to the several figures of the drawing, in which:

FIG. 1 is a perspective view of a preferred embodiment of the scanning infrared thermometer of the present invention;

FIG. 2 is a general functional block diagram of the circuitry enclosed within the housing of the sensor of FIG. 1;

FIG. 3 is a circuit schematic of the converter circuit functional block of FIG. 2; and

FIG. 4 illustrates comparative waveforms of some of the signals applied to the conversion circuit of FIG. 3.

Referring to FIG. 1, the preferred embodiment of the scanning infrared thermometer 10 of the present invention is scanning an exemplary target 12, such as a section of hot steel, plastic or glass being produced from an extrusion process, for infrared radiation along a sensing line or scanning line 16 on the target 12.

In the spot mode of operation, the temperature of each of a series of spot targets 16', or relatively short line segments along line 16, is individually determined. In a continuous mode of operation, the average temperature or temperature profile of the entire line 16 is determined.

The sensor has a single housing assembly 18 which is pivotally mounted on a vertical swivel assembly connected to a base 19 and mounted on a horizontal swivel assembly 25. The sensor 10 is moved on these swivels during aiming of the sensor 10. An alignment laser marks the target 12 with light at the precise location, or locations, on line 16 from which it is desired to sense the radiation of target 12 and determine the temperature. Control inputs, such as manual control inputs (not shown), enable an operator to select the number, length and relative location of each of the spot targets 16' after the line has been positioned on the target 12. These control inputs determine which of a plurality of radiation samples collected over the entire length of line 16 will be selected to individually determine the average radiation and average temperature of each of the spots 16'.

Referring to FIG. 2, in keeping with one objective of the invention, it is seen that all the elements of the sensor 10 are contained together in proximity within the housing 18. This includes a detector 32 having a photoconductor and a preamplifier which obtains the infrared light from a scanner 21. The scanner 21 has an optical system including a rotating mirror and a lens which focuses the light on the detector 32. The detector 32 converts the infrared light to an analog radiation temperature signal on output 37 having a magnitude related to temperature. The sensor 10 also includes within housing assembly 18 a scanner control 29 with an encoder for providing a sync signal on an output 39 which applied to a converter 35. The control 29 also provides a control signal on an output 33 to the scanner and on/off control signals on an output 31 to an alignment laser 40. The alignment laser 40 is directed through the scanner 21 to mark the spot targets 16' when aiming the sensor 10. A sighting apparatus 41 such as a filtered camera is used to enhance viewability of the light marks.

As seen in FIG. 4, the sync signal 39' is generated once during each revolution of the scanner 21, preferably when the scanner is receiving infrared light form, or "looking at" , a hot reference 34, FIG. 2. Referring to FIG. 4, when this sync signal 39' is being generated, the detector 32 is producing an analog hot reference signal 34' having a magnitude related to the temperature of the hot reference, or hot reference source, 34, FIG. 2. Likewise, when the scanner 21 is looking at a cold reference, or cold reference source, 36, a cold reference signal 36' is generated having a magnitude related to the cold reference 36. Still referring to FIG. 4, when the scanner 21 is looking at the target 12 and collecting infrared light from along line 16, an analog radiation temperature signal 16" with a magnitude related to the temperature of the target 12 is produced on output 37 of the detector 32. The temperature of both the hot reference 34 and the cold reference 36 are independently measured by a reference temperature measurement circuit 45 which respectively provides those two reference temperature to the converter 35 on outputs 42 and 44. The converter 35 has a plurality of outputs, or output ports, 26 for providing digital signals representative of the temperature of the plurality of spot targets 16', respectively. Each of these output ports 26 are selectively connected to one or more of a multiple target temperature display 27, a target temperature recorder 28 and a process controller 30 for controlling the process which produces target 12. An emissivity settings circuit 41 provides emissivity settings on an output 43 to the converter 35 for each of the selected spot targets 16'.

Reference should be made to U.S. patent application Ser. No. 07/600643 of Kalley et al. entitled "Scanning Radiation Sensor For Multiple Spots And Apparatus And Methods Of Averaging Radiation Samples Of Same" filed contemporaneously herewith and assigned to the assignee of this application if further details of the sensor structure and means of correcting samples of radiation temperature are desired.

Referring now to FIG. 3, the converter 35 receives the analog temperature radiation signal on 37 and applies a correcting signal to a nulling circuit 46 when the scanner 21 is viewing the cold reference 36. The DC offset corrected analog radiation signal produced on the output 48 of the nulling circuit 46 is then applied to a programmable gain circuit 50 at which emissivity corrections are made. The DC offset corrected and emissivity corrected analog temperature radiation signal on an output 52 of the programmable gain circuit 50 is then applied through a sampling switch 54 which produces analog samples at a rate of approximately eighty kHz on an output 55. The samples on output 55 of the sampling switch 54 is applied to an input of an analog to digital, or A/D, converter 56 of a linearizing circuit 58. A D/A converter 60 of a bias circuit 62 applies an analog bias signal to another input 64 of the A/D converter 56. This bias signal is first provided to the bias circuit 62 in digital form on an output 66 of a microprocessor 68 of a control circuit 70. The A/D converter 56 produces a corrected digital temperature signal for each sample on its output 72 which is passed through a read only memory, or ROM, 74 which converts the signal to linearized temperature made available on output 76. The microprocessor 68 stores and makes available selected ones of these digital representations of temperature on the plurality of outputs 26. The microprocessor 68 also generates a DC offset correction signal on its output 78 which is applied to an input 80 of a D/A converter 82 of nulling circuit 46.

One of the objectives of the inventions is achieved by virtue of the microcomputer 68 responding to inputs from the emissivity settings circuit 41 which indicates the selected emissivity settings for each of the spot targets 16' to alter the preamplified analog radiation signal on output 37 from the detector 32, FIG. 2. The sync signal 39' indicates to the control circuit 70 when to sample the hot references signal 34' and the cold reference signal 36', FIG. 4. The control circuit 70 uses the reference signals 34' and 36' along with the desired emissivity settings on output 43 from the emissivity settings circuit 41 to adjust the nulling circuit 46 and the programmable gain circuit 50. The nulling circuit 46 is adjusted for each measurement sample, and the emissivity settings circuit 41 can provide each sample point with its own unique emissivity setting.

The microprocessor 68 controls biasing circuit 62 in response to the cold reference signal 36' to provide a cold reference temperature correction bias signal on its output 66 to the bias circuit 62. Advantageously, the ROM 74 is encoded with the linearizing, or temperature conversion, data in a direct look-up table format in a pair of look-up tables respectively associated with temperatures represented in Celsius and Fahrenheit degrees. The control circuit 70 selects which table to use on an output 84 according to an operator input (not shown). This provides the ability to get readings directly in Celsius or in Fahrenheit degrees. Preferably, a pass-through table in ROM 74 gives direct voltage readings which are needed for the reference signals 34' and 36'.

The purpose of the nulling circuit 46 is to make the output 48 of a preamp 86 be zero when the scanner 21 and, thus, the detector 32 is looking at the cold reference 36. The nulling voltage is preferably integrated over time to reduce noise effects according to the algorithm of equation (4):

where N is the response time factor given by equation (5):

Seconds

The nulling voltage on output 78 is provided through the voltage output D/A converter 82 feeding a summing input 88 of the inverting opamp stage 86. The D/A converter 82 is preferably a conventional voltage output D/A converter, such as a DAC707 made by Burr Brown. The opamp 86 is preferably an OPA627 opamp made by Burr Brown.

Advantageously, the bias circuit 62 injects the ideal cold reference signal 34' back into the circuit independently of the emissivity settings. The actual cold reference temperature is measured with an independent sensor (not shown) of the reference temperature measurement circuit 42 which provides a representative signal on input 44 to the control circuit 70. The control circuit 70 uses the cold reference temperature to look up in a separate table in ROM 74 the desired voltage for the corresponding cold reference temperature. This signal is fed to the voltage output D/A converter 60 to provide the bias voltage at 64.

The programmable gain circuit 50 is used to inject both a hot reference correction signal and the emissivity setting for the sample point. The gain circuit 50 is adjusted for every sample point. The circuit is comprised of a multiplying D/A converter 90, preferably a DAC7541A converter made by Burr Brown and a high speed opamp 92, preferably an OPA627 opamp made by Burr Brown.

The hot reference temperature is measured with an independent sensor (not shown) which sends a signal to the control circuit 70 at input 42. The microprocessor 68 uses this hot reference signal to index a table in ROM 74 which gives the desired hot reference signal for the preamp 86. The ratio of the desired preamp signal and the actual preamp signal yields the necessary gain correction. The calculated gain correction is filtered with each successive scan to reduce noise effects. The algorithm for the correction factor (CF) is given by equation (6), as follows:

where

HREF is the desired hot reference signal;

HVAL is the actual hot reference signal; and

N is the response time factor.

The programmable gain setting is calculated by equation (7) as follows:

where E.sub.s is the selected emissivity of the sample.

The A/D converter 56 is a fast (100 kHz) A/D converter with built in track and hold such as the AD779 made by Analog Devices and the ROM 74 is preferably a word wide ROM, such as a 27C210 made by Intel. The microprocessor 68 is preferably an 87C196 single chip microcomputer made by Intel. The nulling circuit 46, programmable gain circuit 50 and the bias circuit 62 circuitry prior to the A/D converter 56 is necessary for the direct look-up in the ROM 74 following the A/D converter 56 to yield the correct result. Advantageously, because the signal at 54 is always normalized before the A/D converter 56 the full resolution of the A/D converter 56 is maintained to yield higher accuracy than that which could be otherwise obtained.

While a detailed description of the preferred embodiment of the invention has been given, it should be appreciated that many variations can be made thereto without departing from the scope of the invention as set forth in the appended claims.