Thermal measuring and testing – Temperature measurement – In spaced noncontact relationship to specimen
Reexamination Certificate
1999-04-01
2001-07-31
Gutierrez, Diego (Department: 2859)
Thermal measuring and testing
Temperature measurement
In spaced noncontact relationship to specimen
C374S128000, C374S129000, C374S133000, C374S002000, C250S349000, C348S164000, C348S243000, C348S250000, C348S298000
Reexamination Certificate
active
06267501
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a system and method for control and operation of an ambient temperature detector, principally used in conjunction with detection of infrared radiation.
BACKGROUND AND BRIEF DESCRIPTION OF THE PRIOR ART
Prior art detectors, primarily for detection of infrared radiation, generally fall into the classes of cooled detectors (those which are generally cryogenically cooled to 77 degrees Kelvin) and ambient temperature detectors which generally operate at a particular temperature which is in the vicinity of normal room temperature. The prior art ambient temperature detectors generally require a very accurate thermoelectric cooler (TEC) servo controller to maintain a very accurate single temperature of the detector. The detector is then calibrated at this temperature and the responsivity and offsets of the detector are also corrected at this temperature, resulting in a much more predictable situation. A problem with maintenance of the detector at a constant ambient temperature is the requirement of large amounts of power by the servo loop to maintain this constant temperature and costly operation in general. Other prior art detectors use a chopper, or method of interrupting scene information to compensate for drifts. This approach has cost and size penalties.
SUMMARY OF THE INVENTION
In accordance with the present invention, the above described deficiencies of the prior art are minimized and there is provided a detector, primarily for use in conjunction with infrared radiation, which is capable of operation over a wide temperature and resistance range without the use of thermoelectric coolers (TECs) or choppers. The detector in accordance with the present invention can process the data to permit a reasonable picture to be obtained with non-uniformities corrected and also with the capability of making radiometric measurements from each of the pixels despite the fact that nothing is controlled as to actual temperature at each pixel. This allows for a relatively inexpensive video chain based upon software rather than hardware and without the necessity of customized integrated circuits.
In accordance with the present invention, the servo of the prior art ambient temperature detector is eliminated and there is provided a temperature controlled chamber wherein the ambient temperature in which the detector and the electronics are operating are controlled and which allows the detector to operate over a wide temperature range and resistance range. The system looks through the wall of the chamber to a scene which is isothermal across the scene and this temperature is presented to all of the detectors simultaneously. A two dimensional array of scene temperatures and ambient temperatures is provided and, for each point, a measurement is made at each pixel. For offset correction, points are taken where the scene and the environmental temperature are the same and an equation is provided for each pixel which allows for offset correction for each pixel when the ambient temperature is the same as that measured. For responsivity correction, points are taken where the scene temperature and the environmental temperature are different. An equation is provided for each pixel which allows for responsivity correction for each pixel.
Since the sensing element in the detector is a resistor whose value changes with temperature, it is possible to apply the control, calibration and interface techniques used in this invention to other areas requiring the precise measurement of resistance values. Specifically, the techniques described in accordance with the present invention permit simultaneous sampling of multiple resistance values (or related parameters) with a minimum of dedicated hardware per resistive element. Furthermore, the calibration and control techniques permit operation over a wide parametric, operational and environmental range while maintaining calibration and without saturating the sampling and readout circuitry. The resistive element is preferably based upon amorphous silicon microbolometer-based technology which provides high sensitivity without the use of TECs for temperature stabilization and without the use of a chopping mechanism.
Each pixel of the array comprises a resistor, preferably of amorphous silicon, whose value changes with temperature, and circuitry to sample the resistor value at the time of sampling through a capacitive discharge technique. The data from the pixel array is then read out through an n:1 multiplexer. The task is to control analog sampling of the pixel data (resistance value), read the value and correct the data for offset and gain to ultimately compute the temperature. In the process, calibration data relating raw pixel data to temperature in a controlled environment is collected and post-processed and then used in real-time by the electronics to translate the raw pixel data into temperature in a dynamic operational environment.
In operation, the calibration takes place by initially operating the system detector such that a reference pixel(s) observes an ambient reference temperature source and the sampling circuit associated with each reference pixel maintains a constant voltage output from the reference pixel(s) (this voltage can be from a single pixel or the average voltage from a number of pixels) by varying the integration time of the sampling circuit. The integration time is the input variable that will be used in the equations for offset and responsivity correction. The detector of the system is then exposed to various ambient temperatures and various scene temperatures and the data is recorded from each pixel for the integration time associated with each set of data at each ambient temperature and scene temperature. The integration time is always controlled by a servo controller. An equation is then provided for each pixel that relates integration time to pixel value when the ambient temperature and the scene temperature are the same which is used for correction of offsets. An equation is provided for each pixel that relates integration time and offset corrected pixel value to the difference between the ambient (substrate) temperature and the scene temperature when the ambient temperature and the scene temperature are different and is used for correction of responsivity. An equation is provided that relates integration time to ambient temperature and is used for calculating substrate temperature. Each of these equations is determined and stored (or the coefficients of these equations are determined and stored) into the system for use during operation.
To improve the quantization, the integration time for all cycles is not necessarily equal, but rather can switch between the two quantization steps between which the floating point integration time value falls.
If the voltage accumulated from each of the integration cycles is equally weighted in its effect on the resulting pixel voltage, the following scenario is valid:
1. The desired integration time is n+m clock cycles where n is an integer and 0<=m<1. The time represented by n clock cycles is T
n
, The time represented by n+1 clock cycles is T
n+1
.
2. The total number of integration cycles is i.
3. Compute the switch point, s, as the integration cycle where the switch is made between n and n+1 clock cycles. s=integer(i*(1−m)).
4. Integrate for n clock cycles, s times.
5. Integrate for n+1 cycles, i-s times.
τ
inj
execute
ideal
=
1
i
(
∑
j
=
l
s
T
n
+
∑
j
=
t
+
l
l
T
n
+
l
)
It has been determined that the first integration cycle and the i
th
integration cycle are not equally weighted in their effect on the pixel data in the above scenario. It is important whether the n-clock cycles come first or last and whether the different length integration cycles are intermixed. In other words, the order of the integration cycles is very important. Regardless of the order, the value of s is not a simple linear relationship as expressed above.
Even though the relationship is no
Brady John F.
Rachels Kenneth
Ratcliff David D.
Wand Martin A.
Weinstein Michael
Baker & Botts L.L.P.
Gutierrez Diego
Pruchnic Jr. Stanley J.
Raytheon Company
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