WO2014137994A1

WO2014137994A1 - Thermocouple circuit based temperature sensor

Info

Publication number: WO2014137994A1
Application number: PCT/US2014/020161
Authority: WO
Inventors: Scott Bruce ROSENTHAL
Original assignee: Rosenthal Scott Bruce
Priority date: 2013-03-05
Filing date: 2014-03-04
Publication date: 2014-09-12

Abstract

An apparatus is disclosed that includes a circuit board. The circuit board includes a thermally conductive surface, an A/D converter having an internal temperature sensor, and a thermally conductive pad thermally coupled to the temperature sensor and the thermally conductive surface. The apparatus further includes a thermocouple having a reference junction thermally coupled to the thermally conductive pad and the thermally conductive surface.

Description

THERMOCOUPLE CIRCUIT BASED TEMPERATURE SENSOR

Cross-Reference to Related Applications

[0001] This application claims priority to U.S. Provisional Application No. 61/772,771 titled "THERMOCOUPLE CIRCUIT BASED TEMPERATURE SENSOR", filed March 5, 2013, the disclosure of which is incorporated by reference herein in its entirety.

Field of the Invention

[0002] Embodiments described herein relate to methods and apparatus for fabricating a temperature sensor with high accuracy and stability.

Background

[0003] In many sensor applications, such as in vivo blood analyte measurements, the sensor's measurement can be dependent upon the temperature of the environment in which the measurement is taken, and therefore the accuracy of the sensor measurement can depend on the accuracy with which the associated temperature can be measured. For practical purposes, such temperature sensors need to be small, accurate, easy to manufacture, and inexpensive. At present there are two electrical choices that meet these requirements: thermistors and thermocouples.

[0004] In their most basic form, thermistors require a resistance measurement with just two wires. However, thermistors have several attributes that make them unsuitable for some application, such as in vivo blood analyte measurements. For example, the thermistor bead is too large in diameter to be placed inside many blood vessels and tissues. Additionally, obtaining thermistors that are interchangeable and do not need calibration to the accuracies that are required for satisfactory in-vivo measurements can significantly increase the sensor's cost of manufacture.

[0005] In the case of thermocouples, two major factors affect the accuracy of a thermocouple measurement: the purity of the thermocouple wires and the accuracy of the reference junction temperature. The reference junction has typically been problematic for using thermocouples in high accuracy applications. For example, the reference junction can create errors in thermocouple readings by being at a temperature different than what is expected. Every 1 °C error in the reference junction temperature produces a 1 °C error in the thermocouple temperature reading. Likewise, if the reference junction temperature changes over time, it can introduce a proportionate error in the thermocouple reading. Hence, the main limitation with current thermocouples is accuracy, where system errors of less than 1°C that are routinely required for biological applications can be challenging to achieve.

[0006] Accordingly, a need exists for methods and apparatus for fabricating a simple temperature sensor using a thermocouple that has high accuracy and stability.

Summary

[0007] Disclosed embodiments relate to methods and apparatus for fabricating an accurate and stable temperature sensor that can be placed inside blood vessels and tissues for temperature- correcting in-vivo bio-sensors that detect important biological markers. The disclosed embodiments provide a means for fabricating a temperature sensor by using a Type T thermocouple wire that is accurate to 0.1 °C and a means for measuring the temperature of the reference junction of the thermocouple circuit and, via computational techniques, computing the temperature at the hot junction.

Brief Description Of The Drawings

[0008] FIG. 1 is a schematic of a thermocouple heat measuring circuit with a hot junction, a reference junction and a measuring instrument.

[0009] FIG. 2 is a detailed electrical circuit layout diagram for the thermocouple circuit used in the temperature sensor of FIG. 1.

[0010] FIG. 3 is a printed circuit board (PCB) layout used in the temperature sensor of FIG. 1.

Detailed Description [0011] Thermocouples are formed with two dissimilar conductors in contact, which produce a voltage when heated relative to a reference junction. The magnitude of the voltage is dependent on the difference between the temperature of the hot junction and the reference junction. Thermocouples for practical measurement of temperature are junctions of specific alloys which have a predictable and repeatable relationship between temperature and voltage. Different alloys can be used for different temperature ranges. Properties such as resistance to corrosion may also be important when designing and fabricating a thermocouple. Thermocouples are typically referenced against a temperature of 0°C (i.e., at the "reference" or "cold" junction).

[0012] FIG. 1 is a schematic of a thermocouple heat measuring circuit with a "hot" junction, a reference junction and a measuring instrument. Fundamentally, the process of obtaining the temperature at the thermocouple's measurement point (or hot junction) involves measuring the voltage difference created at the junction of the two dissimilar conductors. Referring to FIG. 1, V_AT is the difference in the voltage generated between points Tl and T2, where Tl is the hot junction of the thermocouple circuit and T2 is the cold or reference junction of the thermocouple circuit. If the temperature at point T2 is known and/or can be determined, then the temperature at Tl can be calculated with the V_AT measurement.

[0013] Two primary methods have been used to establish a reference junction. One type of reference junction involves creating a known temperature and using that a priori knowledge, together with the voltage (V_AT) generated at the thermocouple measurement point, to calculate the true thermocouple temperature. One example of this approach is to put junction T2 in an ice bath, which corresponds to the known ice bath temperature of 0°C. Then, the temperature at junction Tl can be calculated as follows:

ΖΊ - 0% 4 /^,{½.) Eq. (l) where , (¾ is a function of V_AT that can be used to calculate the temperature at the hot junction Tl and is a property of the thermocouple material. Note that the temperature of reference junction T2 does not need to be determined by submerging the junction in an ice bath. Rather, the temperature of reference junction T2 can be established by other techniques, such as disposing the junction on a block of metal heated to a known temperature or by measuring directly the junction's temperature using a separate measurement system. [0014] A second type of reference junction can employ a secondary electronic circuit to simulate a known temperature by applying a compensating voltage. The secondary circuit injects a voltage that creates a "reference" baseline temperature at T2 such that measuring V_AT results in computing the actual temperature at T 1. The secondary circuit can involve simulating the actual thermocouple material profile and knowing its own temperature.

[0015] A high accuracy temperature sensor according to an embodiment can be fabricated by using a Type T thermocouple wire (using copper and constantan as the dissimilar metals), which is accurate to 0.1 °C. To be able to use the thermocouple wire to its stated accuracy, the measurement circuit used with the temperature sensor should have an accuracy that is at least as good as, or better, than that of the thermocouple wire. To accomplish this, the temperature of the T2 reference junction is measured and the measured temperature is used in the computation of the temperature at the Tl hot junction. The device uses the temperature sensor built into an analog-digital (A/D) converter, the A D converter ground pad surface, and a metal surface on the printed circuit board (PCB).

[0016] FIG. 2 is an electrical circuit layout diagram for the thermocouple circuit used in the temperature sensor. The Co and Cu connections relate to the thermocouple's constantan and copper wires, respectively. In this embodiment, a Linear Technology LTC2486 A/D converter is used. There are four features of the Linear Technology LTC2486 A/D converter that are being exploited in the temperature sensor:

[0017] The first feature that is exploited is the differential input stage (see FIG. 2). This feature allows the sensor to eliminate most common mode interferences, which can be a problem at the low voltage levels generated by the thermocouple. Examples of such interference can include, but are not limited to, common mode noise, normal mode noise, electrostatic noise, etc.

[0018] The second feature that is exploited is the 24-bit A/D conversion. The 24-bit A/D conversion with a 2.5 volt reference voltage (see FIG. 2), allows resolutions to 0.15 microvolts. Since the Type T thermocouple generates approximately 43 microvolts/°C, this yields high resolution for directly measuring the thermocouple's V_AT without any additional amplification. [0019] The third feature that is exploited is the internal temperature sensor of the A/D converter. The internal temperature sensor is not particularly accurate, but it's behavior is well defined. As confirmed by extensive factory and laboratory testing, it has been shown that although the temperature sensor has an offset, it tracks temperature changes with a linear slope of 1. Hence, after compensating for the offset, the A/D converter can accurately track and report the temperature of the chip. This compensation can be determined during a factory calibration, and the offset value can be stored in a suitable memory from which it can be retrieved for use in making temperature measurements. For example, it can be stored in the serial EEPROM of flash memory of a CPU to which the temperature sensor output is communicated.

[0020] The fourth feature that is exploited is the use of a large ground pad on the bottom surface of the A/D converter chip. The LTC2486 A/D converter chip has a large metal pad at the bottom surface that is connected internally to its ground. The purpose of the large pad is to ensure a good ground for the A/D converter so as to minimize conversion errors. Hence, advantage is taken of this large surface area to thermally couple the internal temperature sensor to the reference junction.

[0021] An additional aspect of this embodiment is the use of an isothermal "block" on the PCB. The isothermal block is formed by the combination of the A/D converter chip's ground pad and the PCB's ground plane. The purpose of the isothermal block is to create an environment such that the connection of the thermocouple's constantan wire to the PCB copper trace is at the same temperature as the A/D converter. The connection of the thermocouple's copper wire to the PCB can occur either within the isothermal block or outside of it since the copper-to-copper connection does not form a new thermocouple junction.

[0022] FIG. 3 illustrates a portion of a printed circuit board (PCB) layout in which this embodiment can be implemented. As shown in FIG. 3, PCB includes a ground plane with a portion U5 (to which the analog-digital (A/D) converter can be coupled), a portion CO (to which the constantan wire of the thermocouple can be coupled), and a portion CU (to which the copper wire of the thermocouple can be coupled). The A/D converter can be mounted, and closely thermally coupled, to the PCB ground plane at location U5, such as by soldering the ground pad on the bottom of the A/D chip to the ground plane. The ground pad and ground plane collectively form the isothermal block, which acts as a large thermal mass or heat sink. The constantan wire can be connected to the conductive pad CO, which is also closely thermally coupled to the isothermal block. The point at which the constantan wire is connected is the reference junction. With the constantan connection closely thermally coupled to the A/D converter's temperature sensor, the temperature reading from the A/D is essentially identical to the T2 temperature, i.e., the reference junction temperature. Since T2 is accurately measured, the temperature Tl (the temperature at the thermocouple hot junction) calculated according to Eq. 1 based on VAT and T2 is also accurately calculated.

[0023] The isothermal block need not be formed only of copper layers on the A/D converter and/or the ground plane. For example, any material that provides a close thermal coupling between the A/D converter's temperature sensor and the reference junction connection to the constantan wire is suitable. One embodiment could use a pad of an elastomeric thermal interface material, such as Sil-Pad (a silicone rubber / fiberglass composite) sandwiched between the A/D converter and the PCB to conduct heat.

[0024] As noted above, FIG. 3 illustrates a portion of a PCB in which an embodiment of a temperature sensor can be implemented. Not shown are portions of the PCB on which other components of a system that includes the temperature sensor can be disposed, such as a CPU to which the temperature sensor output is communicated and within a memory of which a calibration offset value can be stored.

[0025] The temperature sensor described in FIGS. 1-3 can meet the needs of high stability and can significantly improve the simplicity of including a thermocouple device in a device such as, for example, a bio-sensor. Hence, the methods and apparatus described in FIGS. 1-3 illustrate the simplicity of using a thermocouple with this arrangement (i.e., the simplicity can approach that of using a thermistor). Such a temperature sensor can be placed inside blood vessels and tissues for temperature-correcting in-vivo bio-sensors that detect important biological markers. In essence, this temperature sensor can make the use of a thermocouple as easy to use as a thermistor, but with the added benefits of lower sensor cost and easy sensor interchangeability.

[0026] Some embodiments described herein relate to a computer storage product with a non- transitory computer-readable medium (also can be referred to as a non-transitory processor- readable medium) having instructions or computer code thereon for performing various computer-implemented operations. The computer-readable medium (or processor-readable medium) is non-transitory in the sense that it does not include transitory propagating signals per se (e.g., a propagating electromagnetic wave carrying information on a transmission medium such as space or a cable). The media and computer code (also can be referred to as code) may be those designed and constructed for the specific purpose or purposes. Examples of non- transitory computer-readable media include, but are not limited to: magnetic storage media such as hard disks, floppy disks, and magnetic tape; optical storage media such as Compact Disc/Digital Video Discs (CD/DVDs), Compact Disc-Read Only Memories (CD-ROMs), and holographic devices; magneto-optical storage media such as optical disks; carrier wave signal processing modules; and hardware devices that are specially configured to store and execute program code, such as Application-Specific Integrated Circuits (ASICs), Programmable Logic Devices (PLDs), Read-Only Memory (ROM) and Random- Access Memory (RAM) devices.

[0027] Examples of computer code include, but are not limited to, micro-code or microinstructions, machine instructions, such as produced by a compiler, code used to produce a web service, and files containing higher-level instructions that are executed by a computer using an interpreter. For example, embodiments may be implemented using imperative programming languages (e.g., C, Fortran, etc.), functional programming languages (Haskell, Erlang, etc.), logical programming languages (e.g., Prolog), object-oriented programming languages (e.g., Java, C++, etc.) or other suitable programming languages and/or development tools. Additional examples of computer code include, but are not limited to, control signals, encrypted code, and compressed code.

[0028] While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation, and as such, various changes in form and/or detail may be made. Any portion of the apparatus and/or methods described herein may be combined in any suitable combination, unless explicitly expressed otherwise. Where methods and/or schematics described above indicate certain events and/or flow patterns occurring in certain order, the ordering of certain events and/or flow patterns may be modified. Additionally certain events may be performed concurrently in parallel processes when possible, as well as performed sequentially. [0029] The various embodiments described herein should not to be construed as limiting this disclosure in scope or spirit. It is to be understood that no limitation to the scope of the disclosure is intended thereby. It is to be further understood that resort may be had to various other embodiments, modifications, and equivalents thereof which may suggest themselves to those skilled in the art without departing from the spirit of the present disclosure and/or scope of the appended claims.

[0030] Those skilled in the art will recognize, or be able to ascertain, using no more than routine experimentation, numerous equivalents to the specific embodiments described specifically herein. Such equivalents are intended to be encompassed in the scope of the following claims.

Claims

What is claimed is:

1. An apparatus comprising:

a circuit board including:

a thermally conductive surface;

an A D converter having an internal temperature sensor;

a thermally conductive pad thermally coupled to the temperature sensor and the thermally conductive surface; and

a thermocouple having a reference junction thermally coupled to the thermally conductive pad and the thermally conductive surface.