US 20060062274 A1
Tympanic temperature measurements are obtained from the output of a radiation sensor mounted in an extension from a housing. The housing has a temperature display and supports electronics for responding to sensed radiation. The sensor is mounted in an improved extension which is shaped to fit into smaller ear canals, such as a child's ear canal or a swollen adult ear canal. Within the extension, the sensor is positioned in a highly conductive environment and receives radiation from an external target through a tube. Electronics determine the target temperature based on the sensor output signal and a temperature sensor signal.
1. A temperature detector comprising:
a surface temperature sensor mounted to sense temperature of a target surface;
an ambient temperature sensor sensing ambient temperature;
an electronic circuit coupled to the surface temperature sensor and ambient temperature sensor and responsive to a signal from the surface temperature sensor and the ambient temperature sensed by the ambient temperature sensor to provide an indication of an internal temperature of the target adjusted for the ambient temperature to which the target is exposed; and
an output providing an indication of the internal temperature.
2. A temperature detector as claimed in
This application is a Continuation of application Ser. No. 10/667,135, filed on Sep. 19, 2003, which is a Continuation of application Ser. No. 10/390,532, filed on Mar. 14, 2003, now abandoned, which is a Continuation of application Ser. No. 09/808,619, filed on Mar. 14, 2001, which is a Continuation of application Ser. No. 09/293,477, filed Apr. 16, 1999, now U.S. Pat. No. 6,219,573, which is a Continuation of Ser. No. 08/682,260, filed Jul. 17, 1996, now U.S. Pat. No. 6,047,205, which is a Continuation of Ser. No. 08/333,205, filed Nov. 2, 1994, now U.S. Pat. No. 5,653,238, which is a Continuation-in-part of application Ser. No. 07/832,109, filed Feb. 6, 1992, now U.S. Pat. No. 5,325,863 and of application Ser. No. 07/889,052, filed May 22, 1992, now U.S. Pat. No. 5,381,796 and of application Ser. No. 07/760,006, filed Sep. 13, 1991, now U.S. Pat. No. 5,445,158, which is a Continuation-in-Part of application Ser. No. 07/646,855, filed Jan. 28, 1991, now U.S. Pat. No. 5,199,436, which is a Divisional of application Ser. No. 07/338,968, filed Apr. 14, 1989, now U.S. Pat. No. 5,012,813, which is a Continuation-in-part of application Ser. No. 07/280,546, filed Dec. 6, 1988, now U.S. Pat. No. 4,993,419.
The entire teachings of the above applications are incorporated herein by reference.
Radiation detectors which utilize thermopiles to detect the heat flux from target surfaces have been used in various applications. An indication of the temperature of a target surface may be provided as a function of the measured heat flux. One such application is the testing of electrical equipment. Another application has been in the scanning of cutaneous tissue to locate injured subcutaneous regions. An injury results in increased blood flow which in turn results in a higher surface temperature. Yet another application is that of ear temperature measurement. More specifically, a tympanic device relies on a measurement of the temperature of the tympanic membrane area in the ear of an animal or human by detection of infrared radiation as an alternative to traditional sublingual thermometers. Other ear temperature measurements may be limited to the outer region of the ear canal.
In one embodiment of the present invention, a radiation detector comprises a radiation sensor such as a thermopile and a thermal mass enclosing the thermopile. The thermal mass includes an elongated thermally conductive tube of a first internal diameter. The tube extends from the distal end of the detector to a rear volume of larger internal diameter in which the sensor is mounted. In one device, the tube is gold plated and is thus highly reflective. In another device the tube is plated with a metal oxide for high absorption of radiation. A rigid window is mounted adjacent to an end of the tube, preferably the distal end where it seals the tube.
In accordance with one feature of the present invention, the portions of the thermal mass forming the tube and rear volume are formed in a unitary structure of high thermal conductivity material. The unitary thermal structure has an outer surface with an outer diameter at its distal end which is less than an outer diameter about the rear volume. The outer surface is tapered about the tube such that a unitary thermal mass of increasing outer diameter is provided about the end of the tube adjacent to the rear volume. The unitary thermal mass maximizes conductance and thermal mass within a limited diameter. To avoid changes in fixtures used in mounting the thermopile within the unitary thermal structure, in one embodiment the thermal structure is of limited diameter and may be supplemented with an additional thermal mass. The additional thermal mass surrounds the rear volume and a portion of the tube and is in close thermal contact with the unitary thermal structure. In another embodiment, the unitary thermal structure extends from the distal end of the detector to a housing such that no additional thermal mass is required.
It has been found that a narrow field of view radiation detector provides a more accurate and reliable reading of tympanic temperature. In the detector of the present invention, that field of view is obtained by controlling the reflectance of the inner surface of the tube, the length and diameter of the tube and the position of the thermopile behind the tube. In one embodiment, the tube has a reflective inner surface providing a field of view from the thermopile of about sixty degrees or less. A field of view of less than about sixty degrees allows for viewing of only a portion of the ear canal within less than about 1 centimeter of the tympanic membrane. In another embodiment, the tube has a nonreflective inner surface which produces a field of view from the thermopile of about thirty degrees or less. In either embodiment, the thermopile response may be fine tuned by changing the position of the thermopile behind the tube which changes the field of view and alters the thermopile response signal level.
In accordance with another aspect of the present invention, the infrared radiation sensor is mounted in the rear volume within the unitary thermal mass. The sensor has an active area influenced by radiation from an external target and a reference area of known temperature which is substantially unaffected by radiation. The sensor is preferably a thermopile having its cold junction reference area thermally coupled to the thermal mass but it may be a pyroelectric device. The thermally conductive tube is thermally coupled to the thermal mass and passes radiation to the sensor from the external target. A thermal barrier surrounds the thermal mass and tube. The temperature of the thermal mass, and thus of the sensor reference area, is allowed to float with ambient. A temperature measurement of the thermal mass is made to compensate the sensor output.
Temperature differences between the tube and sensor reference area would lead to inaccurate readings. To avoid those differences, the large unitary thermal mass minimizes temperature changes from heat which passes through the thermal barrier, and good conductivity within the mass increases conductance and minimizes temperature gradients. The outer thermal RC time constant for thermal conduction through the thermal barrier to the thermal mass and tube is at least two, and preferably at least three orders of magnitude greater than the inner thermal RC time constant for the temperature response of the reference area to heat transferred to the tube and thermal mass. For prompt readings, the inner RC time constant should be about ˝ second or less.
Preferably, the thermally conductive tube is thermally coupled to the sensor by a thermally conductive material such as epoxy. In accordance with the present invention, the amount of thermally conductive material is tuned to the detector to minimize the response of the sensor to undesired thermal perturbations of the tube. Providing an insufficient amount of material causes a positive error response from the sensor for thermal perturbations, while too much material causes a negative error response from the sensor for thermal perturbations. By providing the proper amount of material between the sensor and the tube, the added thermal conductance from the material tunes the reference area and the active area of the sensor to respond in phase to thermal perturbations such that the sensor response is substantially unaffected by said perturbations.
In the radiation detector of the present invention, the radiation sensor and the tube are positioned in an extension which is particularly suited for obtaining tympanic temperature measurements. To accomplish this, the extension is inserted into a subject's ear, and preferably into the ear canal. Once inserted, the extension is pivoted and the sensor scans the ear canal and senses the emitted radiation. The detector employs electronics which detects the peak radiation sensed by the sensor and converts it to a tympanic temperature indication.
The probe extension which supports the radiation sensor extends from a housing which displays the tympanic temperature. The housing extends along a first axis and the extension preferably extends at an angle of about 75 degrees from the first axis. This housing supports the battery powered electronics for converting radiation sensed by the sensor to tympanic temperature displayed by the display. The electronics included a processor for providing the displayed temperature based on radiation received from the tympanic membrane. If the sensor receives radiation from the cooler outer ear instead of the tympanic membrane, the processor determines the displayed temperature as a function of the received radiation compensated by an indication of ambient temperature to produce a core temperature approximation. The entire instrument is housed in a single hand-held package. The small additional weight of the electronics in the hand-held unit is acceptable because readings can be made quickly.
In accordance with another aspect of the present invention, the probe extension is adapted to be inserted into an ear canal. More specifically, the diameter of the distal tip as well as the shape and taper of the extension may be set to provide a detector useful in normal adult ear canals or a pediatric detector useful in small ear canals, especially children's ear canals, and swollen adult ear canals. To that end, the extension has a diameter of about 3-8 mm about its distal end and a substantially conical shape increasing in diameter along its length from its distal end and characterized by an included angle of about 25-60 degrees. As such, the extension is capable of being inserted into an ear canal up to one-third of the length of the ear canal.
In a pediatric detector embodiment the conical shape of the extension has an included angle of about forty degrees. Further, the diameter of the tip of the distal end of the extension is preferably in the range of 3-6 min. As such, the pediatric detector is particularly useful on subjects having small ear canals but is also useful on adult subjects. In another embodiment the conical portion of the extension has an included angle of about thirty degrees. The diameter of the tip of the distal end of the extension is no more than about 7 min. As such, the detector is particularly useful on adult subjects having normal ear canals, but it may also be used on children.
The radiation sensor assembly of a preferred embodiment includes a sensing device which is mounted within a rigid structure of high thermal conductivity such as beryllium oxide and has its reference area thermally coupled thereto. The passage through a thermally conductive tube passes thermal radiation from the external target, such as a tympanic membrane, to the thermopile. A window is mounted on the rigid structure such that it is in close thermal contact with the structure.
In one embodiment of the present invention, a detector comprises a substantially conical extension employing the above-described radiation sensor assembly. Preferably, the sensor assembly includes a thermopile sensor. In this embodiment, the tube provides a field of view from the thermopile of about thirty degrees or less. A thermal mass of high thermal conductivity material surrounds the tube and encloses the rigid structure in a rear volume. The thermal mass has a region within the rear volume which is defined between a rearwardly facing surface of the thermal mass and forward a face of the window. The region is preferably filled with air, providing a low thermal conductivity environment therein. The high thermal conductivity mass provides close thermal contact among the tube, the rigid structure, the thermopile cold junction and the ends of the window. As such, a continuous low thermal resistance path is formed from the tube to the cold junction of the thermopile and the window is held to the cold junction temperature.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.
A description of preferred embodiments of the invention follows.
In one embodiment of the present invention, the radiation detector 12 of
A cross-sectional view of the extension of the detector of
A preferred disposable element to be used over the extension 18 is presented in the '419 patent and will not be discussed here.
The edge at the end of the probe is rounded so that when the probe is inserted into the ear it can be pivoted somewhat without discomfort to the patient. The probe is also curved like an otoscope to avoid interference with the ear. By thus pivoting the probe, the ear canal is scanned and, at some orientation of the probe during that scan, one can be assured that the maximum temperature is viewed. Since the ear canal cavity leading to the tympanic area is the area of highest temperature, the instrument is set in a peak detection mode, and the peak detected during the scan is taken as the tympanic temperature.
An improved assembly within the extension 18 is illustrated in
The thermopile views the tympanic membrane area through a radiation guide 32. The radiation guide 32 is gold plated to minimize oxidation. It is closed at its forward end by a germanium window 35. The rigid germanium window assures that the radiation guide is sealed from contamination and is itself easily cleaned. Germanium is less fragile than silicon and passes higher wavelengths. To minimize expense, the window is square with each side slightly longer than the diameter of the radiation guide 32. The window is cemented with epoxy within a counterbore in a flange 37 at the end of the radiation guide. The epoxy serves as a gas seal and mechanical support for the somewhat brittle germanium window. The flange serves to protect the germanium window should the detector be dropped. The diagonal of the window is less than the diameter of the counterbore, and its thickness is less than the depth of the counterbore. Therefore, if the detector is dropped, any force which presses the plastic housing toward the window is absorbed by the flange. The germanium need only withstand the forces due to its own inertia.
From the perspective of the thermopile flake 28, the radiation guide 32 shifts the front aperture at the window 35 back to the proximal end of the radiation guide at 46. Thus, the field of view of the device is determined by the diameter of the aperture 46 and its distance from the flake 28. There are, however, stray rays which, though not being directed to the flake from the aperture, may ultimately strike the flake after reflection within the volume at 31. Such reflections effectively increase the field of view and are thus undesirable. The frustoconical surface 44 surrounding the aperture 46 reflects most of those stray rays toward the rear of the volume 31 rather than toward the thermopile flake. As shown in
The angled surface surrounding the aperture can be applied to more conventional thermopile cans as illustrated in
Whereas the detector disclosed in the '419 patent had a field of view of about 120°, it has been determined that a narrower field of view of about sixty degrees or less provides the user with an easier and more accurate indication of tympanic temperature. With a narrower field of view, the thermopile flake, when directly viewing the tympanic membrane, also views less than about one centimeter along the ear canal wherein the tissue is at substantially the same temperature as the tympanic membrane. A better view of the tympanic membrane also results from the cylindrical extension 43 beyond the conical portion of the extension 18. With the ear canal straightened by the probe, the extension 43 can extend well into the ear canal beyond any hair at the canal opening.
The tympanic membrane is about 2.5 centimeters from the opening of the ear canal. The ear canal for an adult subject is typically about 8 mm wide, so the diameter of the tip of the extension is no more than about 8 mm wide. The conical portion of the extension 18 prevents the tip of the extension from extending more than about eight millimeters into the ear canal. Beyond that depth, the patient suffers noticeable discomfort. With a field of view of less than about sixty degrees, the ear canal is viewed more than about eight millimeters from the tip of the extension 18. Thus, only the ear canal within less than 9 millimeters of the tympanic membrane is viewed as the radiation guide is directed toward the membrane. The result is a more accurate and reliable reading of the tympanic temperature which is essentially core temperature.
With the present instrument, the narrow field of view is obtained by extending the enlarged rear volume between the flake and the radiation guide. Radiation which enters the radiation guide at greater angles, yet travels through the radiation guide, leaves the guide at greater angles and is thus unlikely to be viewed by the flake. The length of the radiation guide is another parameter which affects the field of view. By using a planoconvex lens as the window 35, the field of view can be further limited.
Decreasing the field of view increases the amount of heat which is absorbed by the can in which the thermopile is mounted. The added heat load adds to the importance that the can, including the radiation guide, have a large thermal mass and high thermal conductivity as discussed below.
As distinguished from the structure presented in the '419 patent, the volume 31 surrounding the thermopile and the radiation guide are formed of a single piece of high conductivity copper. This unitary construction eliminates any thermal barriers between the foremost end of the radiation guide and the portion of the can surrounding the thermopile which serves as the cold junction of the thermopile. Further, at least a portion of added thermal mass which surrounds the radiation guide is unitary with the can as well. Specifically, a taper 39 results in an enlarged region 41 which serves as a thermal mass in accordance with the principals of the parent application. The taper 39 continues along a conductive thermal mass 34 which surrounds the can and a conductive plug 36. Both the mass 34 and plug 36 are of copper and are in close thermal contact with the can 30.
The outer sleeve 38 of the extension 18 and the intermediate extension 20 are of plastic material of low thermal conductivity. The sleeve 38 is separated from the can 30 and thermal mass 34 by an insulating air space 40. The taper of the can 30 and thermal mass 34 permits the insulating space to the end of the extension while minimizing the thermal resistance from the end of the tube 32 to the thermopile, a parameter discussed in detail below. The inner surface of the plastic sleeve 38 may be coated with a good thermal conductor to distribute across the entire sleeve any heat received from contact with the ear. Twenty mils of copper coating would be suitable.
In contrast with the prior design, the portion of the sleeve 38 at the foremost end of extension 18 has a region 43 of constant outer diameter before a tapered region 45. The region of constant outer diameter reduces the outer diameter at the distal end and minimizes interference when pivoting the extension in the ear to view the tympanic membrane area. The tapered region is spaced six millimeters from the end of the extension to allow penetration of the extension into the ear canal by no more than about eight millimeters.
One of the design goals of the device was that it always be in proper calibration without requiring a warm-up time. This precluded the use of a heated target in a chopper unit or heating of the cold junction of the thermopile as was suggested in the O'Hara et al. U.S. Pat. No. 4,602,642. To accomplish this design goal, it is necessary that the system be able to operate with the thermopile at any of a wide range of ambient temperatures and that the thermopile output have very low sensitivity to any thermal perturbations.
The output of the thermopile is a function of the difference in temperature between its warm junction, heated by radiation, and its cold junction which is in close thermal contact with the can 30. In order that the hot junction respond only to radiation viewed through the window 35, it is important that the radiation guide 32 be, throughout a measurement, at the same temperature as the cold junction. To that end, changes in temperature in the guide 32 must be held to a minimum, and any such changes should be distributed rapidly to the cold junction to avoid any thermal gradients. To minimize temperature changes, the tube 32 and the can 30 are, of course, well insulated by means of the volume of air 40. Further, a high conductance thermal path is provided to the cold junction. This conductance is enhanced by the unitary construction. Further, the can 30 is in close thermal communication with the thermal masses 34 and 36, and the high conductivity and thickness of the thermal masses increase the thermal conductance. A high thermal conductivity epoxy, solder or the like joins the can and thermal masses. The solder or epoxy provides a significant reduction in thermal resistance. Where solder is used, to avoid damage to the thermopile which is rated to temperatures of 125° C., a low temperature solder of indium-tin alloy which flows at 100° C. is allowed to flow into the annular mass 34 to provide good thermal coupling between all elements.
The thermal resistance from the outer surface of the plastic sleeve 38 to the conductive thermal mass is high to minimize thermal perturbations to the inner thermal mass. To minimize changes in temperature of the guide 32 with any heat transfer to the can which does occur, the thermal mass of the can 30, annular mass 34 and plug 36 should be large. To minimize thermal gradients where there is some temperature change in the tube during measurement, the thermal resistance between any two points of the thermal mass should be low.
Thus, due to the large time constant of the thermal barrier, any external thermal disturbances, such as when the extension contacts skin, only reach the conductive thermal mass at extremely low levels during a measurement period of a few seconds; due to the large thermal mass of the material in contact with the cold junction, any such heat transfer only causes small changes in temperature; and due to the good thermal conductance throughout the thermal mass, any changes in temperature are distributed quickly and are reflected in the cold junction temperature quickly so that they do not affect temperature readings.
The thermal RC time constant for thermal conduction through the thermal barrier to the thermal mass and tube should be at least two orders of magnitude greater than the thermal RC time constant for the temperature response of the cold junction to heat transferred to the tube and thermal mass. The RC time constant for conduction through the thermal barrier is made large by the large thermal resistance through the thermal barrier and by the large thermal capacitance of the thermal mass. The RC time constant for response of the cold junction is made low by the low resistance path to the cold junction through the highly conductive copper can and thermal mass, and the low thermal capacitance of the stack of beryllium oxide rings and pin conductors to the thermopile.
Although the cold junction capacitance is naturally low, there are size constraints in optimizing the thermal capacitance of the thermal mass, the thermal resistance through the thermal barrier and the internal thermal resistance. Specifically, the external thermal resistance can be increased by increased radial dimensions, the capacitance of the thermal mass can be increased by increasing its size, and the thermal resistance through the longitudinal thermal path through the tube can be decreased by increasing its size. On the other hand, the size must be limited to permit the extension to be readily positioned and manipulated within the ear.
Besides the transfer of heat from the environment, another significant heat flow path to the conductive thermal mass is through leads to the system. To minimize heat transfer through that path, the leads are kept to small diameters. Further, they are embedded in the plug 36 through bores 70; thus, any heat brought into the system through those leads is quickly distributed throughout the thermal mass, and only small changes in temperature and small gradients result.
Because the temperature of the thermal mass is not controlled, and the response of the thermopile 28 is a function of its cold junction temperature, the cold junction temperature must be monitored. To that end, a thermistor is positioned at the end of a central bore 72 in the plug 36.
Another embodiment of the present invention is illustrated in
As previously discussed, the other embodiment provided an extension with a constant diameter tip which works well in ear canals of about the same diameter. However, this tip does not fit within smaller ear canals, and subjects with larger diameter ear canals will suffer discomfort as the constant diameter tip of the extension contacts the walls of their ear canals during pivoting to scan the ear canal. In accordance with the present invention, the substantially conical shaped extension 218 has an increasing diameter along a portion of its length from its distal end such that the extension may be inserted into an ear canal without causing discomfort. Once inserted, the extension 218 is pivoted to scan the ear canal adjacent to and including the tympanic membrane. The conical shape of the extension 218 ensures that the edge of the tip of the extension is unable to contact the walls of the ear canal. The thermopile 28 senses radiation within the ear canal during the pivotal rotation of the extension 218. The detector 212 employs electronics in the housing 14 for detecting the peak radiation sensed by the sensor 28 and converting it to a tympanic temperature indication. Further, the electronics may also provide an audible tone indicating that peak radiation has been detected for a particular measurement period. The variable tone or variable pulse signal allows a user to know when to stop pivoting the extension for a given subject.
The diameter of the distal tip of the probe extension as well as its taper may be selected to provide a detector useful in normal ear canals or a pediatric detector useful in small ear canals. In one configuration, as shown in
In another configuration, indicated by dashed lines in
An improved assembly within the extension 218 is shown in
It has been determined that a significantly narrower field of view provides the user with an easier and more accurate tympanic temperature indications. The detector disclosed in the '419 patent had a wide field of view of about 120° and the detectors disclosed in the '813 patent and described in the other embodiment have a field of view of about 60° or less. Thus, one object of this embodiment was to reduce the field of view to obtain a narrower field of view of about thirty degrees or less. To that end, the narrower field of view is obtained by plating the inner surface of the tube 232 with a layer of non-reflective material. Preferably, the non-reflective layer is a metal oxide such as nickel oxide or aluminum oxide. A metal oxide layer is employed because metal oxides are durable and will not change in properties if the inner surface of the tube is cleaned. Further, the metal oxide layer should be thin (a few tenths of thousandths of an inch) such that virtually no temperature gradient exists across the layer. The metal oxide surface absorbs substantially all radiation which strikes the tube 232 and allows radiation passing directly through the tube to reach the thermopile 28.
The dimensions of the tube 232 are chosen such that radiation entering the tube at angles of only up to fifteen degrees from the longitudinal axis of the tube passes directly to the thermopile. With the thirty degree field of view, the probe can easily be positioned such that substantially only the tympanic membrane may be viewed.
The above approach to decreasing the radiation gathering aperture size to about 3 mm and reducing the field of view to about thirty degrees significantly increases the noise level at the thermopile relative to the signal level. Further, this approach increases the amount of radiation which is absorbed by the thermal mass in which the thermopile is mounted. These two effects add to the importance that the thermal mass, including the tube, provide a large thermal mass and high thermal conductivity.
The thermal mass 230 is of unitary construction which eliminates thermal barriers between the tube 232 and the portion 241 of the thermal mass surrounding the thermopile 28. Further, a plug 272 of high thermal conductivity material positioned behind the thermopile 28 is in close thermal contact with the mass 230. The outer sleeve 238 is formed of low thermal conductivity plastic and is separated from the mass 230 by an insulating air space 240. The taper 239 of the mass 230 increases the insulating air space adjacent to the end of the extension 226 while minimizing thermal resistance from the tube 232 to the thermopile. The inner surface of the plastic sleeve 238 may be coated with a good thermal conductor to distribute across the entire sleeve any heat received from contact with the ear.
In order that the hot junction respond only to radiation viewed through the window 235, it is important that the tube 232 and the window 235 be, throughout a measurement, at the same temperature as the cold junction. The thermopile 28 acts as a thermal amplifier having a gain based on the number of junctions and the Seebeck coefficient. Thus, temperature gradients sensed by the thermopile are amplified by a factor of about 100. To minimize errors, changes in temperature in the tube 232 must be held to a minimum, and any such changes should be distributed rapidly to the cold junction to avoid any thermal gradients. To minimize temperature changes, the tube 232 and the mass 230 are well insulated by means of the volume of air 240. To avoid thermal gradients, the tube 232 is plated with a thin layer of high conductance non-reflectance metal oxide which minimizes temperature gradients across the layer. Further, the thermal mass 230 is thermally coupled to the rings 250 and 252 with high conductivity thermal epoxy 255 such that a high conductance thermal path is provided from the tube 232 to the cold junction. This conductance is enhanced by the unitary construction of the mass 230.
In accordance with another aspect of the invention, the amount of thermal epoxy 255 between the rings 250 and 252 and the mass 230 is tuned to the assembly to minimize the response of the thermopile 28 to undesired thermal perturbations at the end of the mass. Referring to
Accordingly, the epoxy 255 may be incrementally added to adjust the high conductivity thermal path to the cold junction to bring the hot and cold junction thermal responses in phase. An insufficient amount of epoxy 255 causes a positive error response as the hot junction responds to thermal variations faster than the cold junction. Alternatively, too much epoxy 255 causes a negative error response as the cold junction responds faster to thermal variations than the hot junction. When the proper amount of epoxy has been provided, the tuned assembly produces no more than 0.1° thermopile response for up to 20° thermal variations during a test.
It has been determined in previous devices that a significant source of thermal gradients is caused by radiation from the window. To minimize these thermal gradients, the window 235 is mounted on the ring 250 with high thermal conductivity epoxy 255 such that it is thermally coupled to the cold junction. The epoxy provides a significant reduction in thermal resistance and provides good thermal coupling between all elements. On the other hand, conductance to the viewing region of the window should not be less than that to the cold junction. Thus, the window 235 is spaced from a rear face 237 of the mass 230 and its ends are spaced from the inner volume 231 by a low thermal conductivity air region. The region ensures that temperature gradients are distributed to the cold junction via the thermal mass and not directly through the window causing thermal gradients.
The thermal resistance from the outer surface of the plastic sleeve 238 to the conductive thermal mass 230 is high to minimize thermal perturbations to the inner thermal mass. The thermal mass is large to minimize changes in temperature of the tube 232 with any heat transfer to the mass which does occur. Further, the thermal resistance between any two points of the thermal mass 230, the tube 232, the window 235 or the rings 250 and 252 is low to minimize thermal gradients where there is some temperature change in the tube during measurement.
Thus, due to the large time constant of the thermal barrier 238, any external thermal disturbances, such as when the extension contacts skin, only reach the conductive thermal mass 230 at extremely low levels during a measurement period of a few seconds. Due to the large thermal mass of the materials in contact with the cold junction, any such heat transfer only causes small changes in temperature. Also, due to the good thermal conductance throughout the thermal mass, tube, window and rings any changes in temperature are distributed quickly and are reflected in the cold junction temperature quickly so that they do not affect temperature readings.
The thermal RC time constant for thermal conduction through the thermal barrier 238 to the thermal mass 230 and tube 232 is at least two orders of magnitude greater than the thermal RC time constant for the temperature response of the cold junction to heat transferred to the tube and thermal mass. The RC time constant for conduction through the thermal barrier 238 is made large by the large thermal resistance through the thermal barrier and by the large thermal capacitance of the thermal mass. The RC time constant for response of the cold junction is made low by the low resistance path to the cold junction through the highly conductive thermal mass, and the low thermal capacitance of the stack of beryllium oxide rings to the thermopile.
Besides the transfer of heat from the environment, another significant heat flow path in the system is through the leads 260. To minimize heat transfer through that path, the lead diameters are kept small and the leads 260 are trimmed off in the region 246. A pair of 40 gauge wires 262 are soldered to the shortened leads 260. The wires 262 extend from the region 246 through the plug 272 and provide thermopile signals to the electronics.
Yet another potential heat flow path in the system is through the header 256 to the plug 272. Since the header is in close thermal contact with the thermopile cold junction, any thermal gradients through the header 256 would be amplified 100 to 1000 times by the thermopile producing large error signals. In the present invention, the insulating region 246 of air is provided behind the header 256 to minimize heat transfer through that path. Thus, any thermal gradients in the plug would be forced to travel through the mass 230 and would be substantially dissipated without affecting the thermopile.
Because the temperature of the thermal mass 230 is not controlled and the response of the thermopile 28 is a function of its cold junction temperature, the cold junction temperature must be monitored. To that end, a thermistor 271 is positioned adjacent to the region 246 in the plug 272. The plug 272 is in thermal contact with the mass 230 such that the thermistor 271 is thermally coupled to the cold junction of the thermopile 28. However, the thermal path between the thermopile 28 and the thermistor has some thermal resistance. This resistance produces a temperature difference between the cold junction temperature and the sensed temperature which is not amplified. Therefore such error is not as significant as gradient errors amplified by the thermopile.
A schematic illustration of the electronics in the housing 14 of both embodiments of the present invention (
The voltage generated across the thermopile 28 due to a temperature differential between the hot and cold junctions is amplified in an operational amplifier 74. For the detector of
Another input to the multiplexer 76 is a voltage taken from a voltage divider R1, R2 which is indicative of the potential V+ from the power supply 78. A third input to the multiplexer 76 is the potential across a thermistor RT1 mounted in the bore 72 of block 36. The thermistor RT1 is coupled in a voltage divider circuit with R3 across a reference potential VRef. The final input to the multiplexer is a potential taken from a potentiometer R4 which may be adjusted by a user. The system may be programmed to respond to that input in any of a number of way. In particular, the potentiometer may be used as a gain control or as a DC offset control.
At any time during the software routine of the microprocessor 75, one of the four inputs may be selected by the select lines 78. The selected analog signal is applied to a multiple slope analog system 80 used by the microprocessor in an integrating analog-to-digital conversion 80. The subsystem 80 may be a TSC500A sold by Teledyne. It utilizes the reference voltage VRef from a reference source 82. The microprocessor 75 responds to the output from the converter 80 to generate a count indicative of the analog input to the convertor.
The microprocessor drives four 7-segment LED displays 82 in a multiplexed fashion. Individual displays are selected sequentially through a column driver 84, and within each selected display the seven segments are controlled through segment drivers 86.
When the switch 22 on the housing is pressed, it closes the circuit from the battery through resistors R5 and R6 and diode D1 to ground. The capacitor C1 is quickly charged, and field effect transistor T1 is turned on. Through transistor T1, the V+potential from the storage cell is applied to a voltage regulator 88. The regulator 88 provides the regulated +5 volts to the system. It also provides a reset signal to the microprocessor. The reset signal is low until the +5 volt reference is available and thus holds the microprocessor in a reset state. When the +5 volts is available, the reset signal goes high, and the microprocessor begins its programmed routine.
When the switch 22 is released, it opens its circuit, but a charge is maintained on capacitor C1 to keep transistor T1 on. Thus, the system continues to operate. However, the capacitor C1 and transistor T1 provide a very simple watchdog circuit. Periodically, the microprocessor applies a signal through driver 84 to the capacitor C1 to recharge the capacitor and thus keep the transistor T1 on. If the microprocessor should fail to continue its programmed routine, it fails to charge the capacitor C1 within a predetermined time during which the charge on C1 leaks to a level at which transistor T1 turns off. Thus, the microprocessor must continue in its programmed routine or the system shuts down. This prevents spurious readings when the processor is not operating properly.
With transistor T1 on, the switch 22 can be used as an input through diode D2 to the microprocessor to initiate any programmed action of the processor.
In addition to the display, the system has a sound output 90 which is driven through the driver 84 by the microprocessor.
In order to provide an analog output from the detector, a digital-to-analog convertor 92 is provided. When selected by line 94, the convertor converts serial data on line 96 to an analog output made available to a user.
Both calibration and characterization data required for processing by the microprocessor may be stored in an electrically erasable programmable read only memory (EEPROM) 100. The EEPROM may, for example, be a 93c46 sold by International CMOS Technologies, Inc. The data may be stored in the EEPROM by the microprocessor when the EEPROM is selected by line 102. Once stored in the EEPROM, the data is retained even after power down. Thus, though electrically programmable, once programmed the EEPROM serves as a virtually nonvolatile memory.
Prior to shipment, the EEPROM may be programmed through the microprocessor to store calibration data for calibrating the thermistor and thermopile. Further, characterization data which defines the personality of the infrared detector may be stored. For example, the same electronics hardware, including the microprocessor 73 and its internal program, may be used for a tympanic temperature detector in which the output is accurate in the target temperature range of about 60° F. to a 110° F. or it may be used as an industrial detector in which the target temperature range would be from about −100° F. to 5000° F. Further, different modes of operation may be programmed into the system. For example, several different uses of the sound source 90 are available.
Proper calibration of the detector is readily determined and the EEPROM is readily programmed by means of an optical communication link which includes a transistor T2 associated with the display. A communication boot may be placed over the end of the detector during a calibration/characterization procedure. A photodiode in the boot generates a digitally encoded optical signal which is filtered and applied to the detector T2 to provide an input to the microprocessor 73. In a reverse direction, the microprocessor may communicate optically to a detector in the boot by flashing specific segments of the digital display 82. Through that communication link, an outside computer 106 can monitor the outputs from the thermistor and thermopile and perform a calibration of the devices. A unit to be calibrated is pointed at each of two black body radiation sources while the microprocessor 73 converts the signals and sends the values to the external computer. The computer is provided with the actual black body temperatures and ambient temperature in the controlled environment of the detector, computes calibration variables and returns those variable to be stored in the detector EEPROM. Similarly, data which characterizes a particular radiation detector may be communicated to the microprocessor for storage in the EEPROM.
A switch 108 is positioned behind a hole 110 (
A switch 106 may be provided either internally or through the housing to the user to set a mode of operation of the detector. By positioning the switch at either the lock position, the scan position or a neutral position, any of three modes may be selected. The first mode is the normal scan mode where the display is updated continuously. A second mode is a lock mode where the display locks after a selectable delay and then remains frozen until power is cycled or, optionally, the power-on button is pushed. The sound source may be caused to sound at the time of lock. The third mode is the peak mode where the display reads the maximum value found since power-on until power is cycled or, optionally, the power-on button is pushed.
The processor determines when the voltage from the divider R1, R2 drops below each of two thresholds. Below the higher threshold, the processor periodically enables the sound source to indicate that the battery is low and should be replaced but allows continued readout from the display. Below the lower threshold, the processor determines that any output would be unreliable and no longer displays temperature readings. The unit would then shut down upon release of the power button.
In the present system, the target temperature is computed from the relationship
The gain calibration factor Kh is temperature compensated by the relationship
As noted, the relationship by which the target temperature is determined includes the hot junction temperature as the second term rather than the cold junction temperature. Hot junction temperature is computed from the relationship
The actual sensor output VS can be determined from the digital value available to the processor from the equation:
Reference to the hot junction temperature rather than the cold junction temperature in each term of the relationship for determining the target temperature provides for much greater accuracy over a wide range of ambient temperatures and/or target temperatures.
To provide a temperature readout, the microprocessor makes the following computations: First the signal from thermistor RT1 is converted to temperature using a linear approximation. Temperature is defined by a set of linear equations
The hot junction temperature is computed. A fourth power representation of the hot junction temperature is then obtained by a lookup table in the processor ROM.
The sensed radiation may be corrected using the gain calibration factor Kh, the sensor gain temperature coefficient Tco, the average of the hot and cold junction temperatures and a calibration temperature Tz stored in the EEPROM. The corrected radiation signal and the fourth power of the hot junction temperature are summed, and the fourth root is taken. The fourth root calculation is also based on a linear approximation which is selected according to the temperature range of interest for a particular unit. Again, the break points and coefficients for each linear approximation are stored in the EEPROM and are selected as required.
An additional facgtor based on ambient temperature may also be included as an adjustment. The temperature of the ear Te is sensed instead of the temperature of the tympanic membrane, the temperature sensed by the thermopile is not actually the core temperature Tcr. There is thermal resistance between Tcr and Te. Further, there is thermal resistance between the sensed ear temperature and the ambient temperature. The result is a sense temperature Te which is a function of the core temperature of interest and the ambient temperature. Based on an assumed constant KC which is a measure of the ratio of thermal resistances between Tcr, Te and Ta, Tcr and Ta core temperature can be computed as
A similar compensation can be made in other applications. For example, in differential cutaneous temperature scanning, the significance of a given differential reading may be ambient temperature dependent.
The actual computations performed by the processor are as follows, where:
The following is a list of the information which may be contained in the EEPROM and therefore be programmable at the time of calibration:
The system then enters a loop in which outputs dictated by the mode switch are maintained. First the timers are updated at 122 and the switches are again read at 124. When the power is switched off, from 126 the system enters a power down loop at 128 until the system is fully down. At 130, the mode switch is checked and if changed the system is reset. Although not in the tympanic temperature detector, some detectors have a mode switch available to the user so that the mode of operation can be changed within a loop.
At 132, 136 and 140, the system determines its mode of operation and enters the appropriate scan process 134, lock process 138 or peak process 142. In a scan process, the system updates the output to the current reading in each loop. In a lock process, the system updates the output but locks onto an output after some period of time. In the peak process, the system output is the highest indication noted during a scan. In each of these processes, the system may respond to the programming from the EEPROM to perform any number of functions as discussed above. In the peak process which is selected for the tympanic temperature measurement, the system locks onto a peak measurement after a preset period of time. During assembly, the system may be set at a test mode 144 which will be described with respect to
In any of the above-mentioned modes, an output is calculated at 146. Then the system loops back to step 122. The calculation 146 is illustrated in
At 148 in
Analog-to-Digital conversion is performed periodically during an interrupt to the loop of
Where the 100 millisecond flag is noted at 172, an A-to-D conversion is to be performed. The system first determines at 182 whether a count indicates there should be a conversion of the thermopile output at 184 or a conversion of the thermistor output at 186. The thermopile sensor conversion is performed nine out of ten cycles through the conversion loop. At 188, the system checks to determine whether a conversion is made from the potentiometer R4 or from the battery voltage divider R1, R2 at 192. These conversions are made alternately.
While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. For example, most features of the invention may be applied to a device having a pyroelectric radiation sensor. Also certain features such as the low reflectance, high thermal conductivity tube may provide stable response and narrow field of view even where the tube is thermally isolated from the sensor. In that case, a second temperature sensor would be provided for the tube to compensate for temperature differences between the tube and sensor cold junction.