RU2569170C1 - Calibration of thermal imager built around microbolometric matrix and device to this end - Google Patents

Abstract

FIELD: instrumentation.

SUBSTANCE: invention relates to instruments and can be used for remote temperature determination at surfaces and structural elements. Claimed process consists switching on of the thermal imager to be kept in ON state for temperature control and registration of signals from microbolometric matrix sensors. Said signals are digitized, inverted and written into imager controller memory. Then, said signals are summed up with digitized signals of appropriate sensors of said matrix. Opaque shutter absorbing the radiation of the wavelength working range is intermittently set ahead of thermal imager objective and close thereto. Thereafter, signals from microbolometric matrix sensors are registered. Claimed device comprises controller built in thermal imager connected with microbolometric matrix, 1^st, 2^nd and 3^rd timers, shutter arranged ahead of and outside the imager objective shutter provided with appropriate drive with limit switch and logical element AND.

EFFECT: higher accuracy of calibration.

2 cl, 1 dwg

Description

The described technical solution relates to the field of devices for visualizing temperature fields and remote sensing of temperatures of surfaces and elements of objects of technology, as well as biological objects, and more specifically to the field of creating thermal imaging devices (TVP) for remote sensing of human body temperature.

TVPs are increasingly used in science and technology. One of the known directions of their use is the creation of devices for remote observation of temperature fields in order to detect the distribution of temperatures on the surface of various, including biological, objects. In particular, TBPs are known that are used in medicine and allow detecting areas on the human body that have an abnormally high temperature [1-3]. Modern TVPs contain, as a rule, a microbolometric matrix (MBM), which provides the conversion of the temperature distribution formed on its surface into an electrical signal. A characteristic feature of TVP on MBM is the spread in the sensitivity of their elements, as well as a small fraction of the useful component in the signal from each element of the MBM due to the significant radiation of the structural elements of the TVP (the so-called background illumination), which leads to a violation of the accuracy of reproduction (visualization) thermal field (distortion of the grayscale image characteristics), and makes it difficult to accurately determine the temperature of the observed object at its specific point. To compensate for this phenomenon, the so-called operation is used. calibration, which can significantly eliminate the influence of background illumination and compensate for the non-uniformity of the sensitivity of the MBM field (frame).

Known thermal imager [4], which provides a calibration operation to compensate for the spread of sensitivity of MBM elements. To do this, it is equipped with a “curtain” located between the lens and the MBM, which is opaque in the range of working wavelengths of the TVP, with its drive mechanism driven by an electric motor. To calibrate the TVP, the curtain using the electric motor is set to the position at which the MBM input window is closed from incident radiation.

- TVP are turned on and kept on for thermostating. As a result of heating, the internal volume of the TVP comes to a steady-state (stable in time) temperature state. In practice, thermostating time can be tens of minutes;

- a shutter is introduced between the TVP lens and the MBM, which is opaque and non-reflective in the MBM operating wavelength range (usually from 3 to 14 μm). At the same time, the radiation flux passing through the TVP lens is blocked, and it is believed that the MBM receiving platform is uniformly darkened and the temperature of all its elements is the same;

- register the values of signals from each of the sensitive elements of the MBM (the so-called dark currents);

- signals from each of the sensitive elements of the MBM are digitized, inverted, and recorded in the memory of the TVP controller;

- the signals recorded in the memory of the TVP controller are summed with the digitized signals from the corresponding sensitive elements of the MBM.

Thus, upon completion of calibration, zero signals will be generated at the output of the controller integrated in the TVP, in which the dark currents of each MBM element will be fully compensated.

After the calibration operation is completed, the curtain is taken outside the MBM working platform and the TVP is considered ready for operation.

A detailed analysis of the calibration method described above showed that the use of an internal curtain covering the receiving area of the MBM does not allow correct calibration of the TVP. This statement can be justified by the following obvious provisions:

1. The TVP lens projects the thermal field of the object in its field of view onto the curtain, heats it, and as a result of radiation from the curtain, the radiation uneven in area enters the MBM receiving area. An additional contribution to the unevenness of this radiation is made by the uneven transmission of the lens across the field.

2. The radiation coming through the lens is reflected inside the TVP, partially reflected from the curtain, and again reflected by the TVP design elements. The total re-reflected radiation hits the curtain, is absorbed by it, it heats up and re-radiates to the receiving area of the MBM, and the distribution of this radiation over the area is random.

3. The shutter blocks radiation from internal elements of the design of the TVP. Despite thermostating, inside the TVP there are elements with different temperatures, different from the average temperature of the TVP. Due to the emission of these elements, the curtain is heated indefinitely and re-radiates to the receiving area of the MBM.

4. There is always a constructive gap between the curtain and the receiving area of the MBM through which the internal radiation of the TVP will reach the receiving area of the MBM.

5. After completing the calibration process, the curtain extends beyond the receiving area of the MBM. In this case, the structure of the temperature fields on the surface of the receiving platform, at which the calibration was carried out, due to the influence of the factors discussed above (paragraphs 1-4), the signals recorded (as discussed above in the description of the known calibration method) in the memory of the TVP controller are significantly distorted, will not correspond to the actual conditions of observation.

These circumstances lead to a decrease in calibration accuracy and distortion of the observed thermal picture, and also make it difficult to accurately determine the absolute temperature of the observed object.

The purpose of the proposed method for calibrating TVP is to increase its accuracy by eliminating the factors noted above in paragraphs. 1-4.

The stated objective of the invention is achieved by the fact that after exposure of the TVP in the on state for thermostating in front of the lens of the thermal imaging device, an opaque and absorbing radiation (in the operating range of MBM wavelengths) is installed shutter periodically, at certain intervals experimentally determined, and then the values are recorded signals from each of the sensitive elements of the microbolometric matrix.

The claimed method of calibration is as follows.

TVPs are turned on and kept on for temperature control. Signals from each of the sensitive elements of the MBM are recorded in the electronic path of the TVP. For the convenience of working with signals, they are digitized, after which the digitized signals are inverted and recorded in the memory of the TVP controller. After that, the digital signals recorded in the memory of the TVP controller are summed with the digitized signals from the corresponding sensitive elements of the MBM. The specified summation leads to zeroing of signals from each element of the MBM in the absence of external (incoming through the lens of the TVP) thermal radiation. The described operation is repeated periodically, after a certain time, during which the heat balance inside the TVP changes.

The frequency of the calibration operation is determined empirically as follows. TVPs are turned on and kept on for temperature control. Signals from each of the sensitive elements of the MBM are recorded in the electronic path of the TVP. For the convenience of working with signals, they are digitized, after which the digitized signals are inverted and recorded in the memory of the TVP controller. After that, the digital signals recorded in the memory of the TVP controller are summed with the digitized signals from the corresponding sensitive elements of the MBM. The specified summation leads to zeroing of signals from each element of the MBM in the absence of external (incoming through the lens of the TVP) thermal radiation. After this, the TVP is left in the on state and the signals from each MBM element are monitored. The calibration periodicity time is defined as the time during which these signals are changed to a predetermined value, based on the required measurement accuracy, for example, a value corresponding to 0.5 ° C. The use of an external curtain with respect to the TVP lens provides the following advantages that determine an increase in the accuracy of the calibration operation:

1. The TVP lens does not project the thermal field of the object in its field of view onto the curtain, which prevents its uncontrolled heating and the corresponding uncontrolled radiation from the curtain to the MBM receiving area.

2. Due to the absence of radiation coming through the lens, there is no re-reflection inside the TVP, its partial reflection from the curtain, and re-reflection by the structural elements of the TVP. Therefore, there is no effect of the indicated re-reflected radiation on the curtain, its additional uncontrolled heating and re-emission of undetermined thermal radiation to the receiving area of the MBM.

3. The absence of an internal curtain prevents the distortion of radiation from internal, unevenly heated, structural elements of the TVP, that is, no distortion is introduced into the thermal fields compared to the measurement mode (in the absence of an internal curtain).

4. There are no distortions of the thermal field arising from the constructive gap between the "inner" curtain and the receiving area of the MBM.

5. After the calibration process is completed, the structure of the temperature fields on the surface of the receiving area during calibration due to internal (inside the PVP) heating sources is not violated.

The described method for calibrating TVP on MBM is implemented in a device whose structural diagram is shown in the figure.

When realizing the purpose of the invention, the device shown in the figure contains a controller 2 integrated in the TVP 1, connected to the MBM 3, a first timer 4, a second timer 5 and a third timer 6. There is also an opaque and absorbing radiation installed outside the TVP 1 in front of its lens 7 (in the working the wavelength range of the microbolometric matrix) curtain 8, equipped with a drive 9 for its movement with a limit switch 10, and a logic element “I” 11, while the input of the first timer 4 is connected to the power button of the TVP 1, and its output is connected to the input of the second timer and 5 and the input of the third timer 6, the output of the second timer 5 is connected to the drive 9, while the limit switch 10 and the output of the second timer 5 are connected to the controller 2 through the logic element “11”, and the output of the third timer 6 is connected to the input of the second timer 5 .

The following technical solutions can be used in the device. Controller 2 can be implemented on a microcomputer. Timers 4-6 are most conveniently built on NE555N chips. Lens 7 is realized on the basis of spherical or aspherical lenses from optical germanium. The shutter 8 is made of metal with an absorbing coating or of another material that absorbs heat radiation well in the range from 3 to 14 microns. It has been experimentally established that good results can be obtained using plain paper. The drive 9 is based on an electric motor with a gearbox equipped with a limit switch 10 (for example, based on the micro button KM-1). The logical element "And" 11 - 530LA1 chip.

When implementing the purpose of the invention, a device whose structural diagram is shown in the figure, operates as follows.

When turning on the TVP 1, the first timer 4 is simultaneously turned on, which sets the heating time of the TVP 1 after switching on. During this time, heating of the TVP 1 for thermostating is carried out. The time (duration) of heating is indicated in the instructions for use of the TVP and is determined experimentally during its testing. At the end of the heating time, the output of the first timer 4 generates a voltage of "logical 1", which starts the second timer 5, forming a U-shaped pulse with a duration of 5 ... 10 seconds. The specified pulse is fed to the control input of the actuator 9, which brings the shutter 8 to the operating position - in front of the lens 7 of the TVP 1 close to the lens. As a result of this, the thermal field on the surface of MBM 3 is leveled. In controller 2, the information that the TVP 1 is in calibration mode comes from the output of the logic element “I” 11, which is triggered when voltage arrives at its control inputs from the output of the second timer 5 (U-shaped pulse that starts the calibration mode) and limit switch 10, triggered when the curtain 8 reaches a predetermined position in front of the lens 7 of the TVP 1, in which the radiation flux at the input of the lens 7 is reliably blocked. After the logical element “AND” 11 is triggered, the controller 2 enters the recording mode of the digitized and inverted signals from the elements of the MBM 3. When the TVP 1 operates, these signals will be summed with the corresponding signals from the elements of the MBM 3, which will compensate for the uneven sensitivity of the elements of the MBM 3.

After the calibration time determined by the exposure time of the second timer 5:

- the voltage at the output of the second timer 5 is reduced to zero;

- the drive 9 of the shutter 8 comes to its original position and the input window of the lens 7 opens;

- the signals at the inputs of the logical element "AND" 11 are reset and it is closed;

- after zeroing the voltage at the output of the logic element “AND” 11, the controller 2 enters the operating state and starts processing video signals from the sensitive elements of the MBM 3, adding the signals recorded in its memory with the current values of the corresponding signals from the sensitive elements of the MBM 3, thereby compensating for the unevenness dark currents of sensitive elements.

The calibration process occurs periodically through the time of stable operation of the TVP 1, which is determined empirically. This time, when setting up the device, it is introduced into the third timer 6. When the device is operating, by connecting the input of the third timer 6 with the output of the first timer 4, the third timer 6 starts simultaneously with the first start of the second timer 5. After a specified steady-state time has passed, the pulse from the output of the third timer 6 arrives at the input of the second timer 5 and starts the calibration process, which will be performed as described above. Due to the fact that the third timer 6 operates in the mode of continuous (with a given periodicity) generation of triggering timer 5 pulses, such a start will occur repeatedly with a given frequency while the TVP 1 is in the on (working) state, which ensures high accuracy of temperature field measurements for all the time of the operation of the TVP 1.

Pilot operation of the claimed device confirmed its performance and demonstrated a real increase in measurement accuracy.

1. Lloyd J. Thermal imaging systems. M .: Mir, 1978, 416 pp., Ill.

2. Miroshnikov M.M. Theoretical foundations of optoelectronic devices: Textbook. manual for instrument-making universities. - 2nd ed. supervisor and ext. - L.: Engineering, Leningrad Branch, 1983. - 696 p., Ill.

3. Sagaidachny A. A. Thermal biomedical diagnostics: Textbook. allowance for students. Fak. nano and biomed. technologies trained in special. “Medical Physics” and the direction “Biomedical Engineering” / A.A. Sagaidachny, A.V. Skripal, D.A. Usanov. Saratov. 2009.118 p., Ill.

4. RF patent for utility model No. 49664, cl. IPC H04N.

Claims (2)

1. A method of calibrating a thermal imaging device on a microbolometric matrix, which is that the thermal imaging device is turned on, held in the on state for thermostating, the signal values from each of the sensitive elements of the microbolometric matrix are recorded, these signals are digitized, inverted and recorded in the memory of the controller of the thermal imaging device, after which they are summed with digitized signals from the corresponding sensitive elements of the microbolometric matrix, different t we note that after holding the thermal imaging device in the on state for thermostating in front of the lens of the thermal imaging device, periodically, at the intervals determined by experience, periodically, opaque and absorbing radiation is established in the operating wavelength range of the microbolometric matrix, then the signal values from each of the sensitive elements of the microbolometric matrix. 2. A device for calibrating a thermal imaging device on a microbolometric matrix, containing a controller integrated in the TVP connected to a microbolometric matrix, characterized in that there are first, second and third timers installed outside the thermal imaging device in front of its lens and are opaque and absorbing radiation in the operating wavelength range microbolometric matrix of the curtain, equipped with a drive for its movement with a limit switch, and a logic element "AND", while the input of the first timer is connected to the button the first time the thermal imaging device is turned on, and its output is connected to the inputs of the second and third timers, the output of the second timer is connected to the drive, while the limit switch and the output of the second timer are connected to the controller through the logical element “I”, and the output of the third timer is connected to the input of the second timer .