US20070019704A1 - Temperature sensor apparatus and method - Google Patents

Abstract

A system and method for determining a temperature including: a temperature sensor including: a substrate resistance configured to be primarily responsive to a temperature of a substrate; and a membrane resistance configured to be primarily responsive to a temperature of a membrane. The substrate resistance and the membrane resistance are arranged in a series circuit configured to facilitate measurement of a voltage responsive to a temperature change. The system also includes a controller in operable communication with the temperature sensor. The method includes: receiving a temperature signal, the temperature signal indicative of a composite temperature variation including a first varying portion and a second varying portion; configuring the temperature signal to eliminate the second varying portion; and generating a temperature value based on the configuring wherein the temperature value is substantially based on the first varying portion.

Description

BACKGROUND
• This invention relates to a temperature sensor bridge apparatus and an algorithm for processing signals therefrom. Temperature sensors with fast response-time are desirable for a number of applications. A conventional approach for fast temperature sensing a micromachined MEMS device with a low thermal mass membrane where the temperature detector is located. The detector typically responds very quickly during the initial phase of a thermal event, followed by a much slower phase that is representative of the slower dynamics of the bulk of the silicon device. Unfortunately, the slower response due to the bulk of the silicon impedes detection of the faster dynamics.
• Therefore, it would be beneficial to provide an approach whereby the fast response component is detected and extracted through a novel use of a temperature detector bridge and associated electronics.
BRIEF SUMMARY
• Disclosed herein is a temperature sensor comprising: a first substrate resistance configured to be primarily responsive to a temperature of a substrate; a first membrane resistance configured to be primarily responsive to a temperature of a membrane; and wherein the first substrate resistance and the first membrane resistance are arranged in a series circuit configured to facilitate measurement of a voltage across each responsive to a temperature change.
• Also disclosed herein is a system for determining a temperature comprising: a temperature sensor comprising: a first substrate resistance configured to be primarily responsive to a temperature of a substrate; and a first membrane resistance configured to be primarily responsive to a temperature of a membrane. The first substrate resistance and the first membrane resistance are arranged in a series circuit configured to facilitate measurement of a voltage across each responsive to a temperature change. The system also includes a controller in operable communication with the temperature sensor, the controller configured to receive and condition a signal indicative of a temperature.
• Further disclosed herein is a method of determining a temperature comprising: receiving a temperature signal, the temperature signal indicative of a composite temperature variation including a first varying portion and a second varying portion; configuring the temperature signal to eliminate the second varying portion; and generating a temperature value based on the configuring wherein the temperature value is substantially based on the first varying portion.
• Also disclosed herein, in yet another embodiment, is a system for determining a temperature. The system comprising: a means for receiving a temperature signal, the temperature signal indicative of a composite temperature variation including a fast varying portion and a slowly varying portion; a means for configuring the temperature signal to eliminate the slowly varying portion; and a means for generating a temperature value based on the configuring wherein the temperature value is substantially based on the fast varying portion.
BRIEF DESCRIPTION OF THE DRAWINGS
• The present invention will now be described, by way of an example, with references to the accompanying drawings, wherein like elements are numbered alike in the several figures in which:
• FIG. 1 depicts a simplified block diagram for employing an implementation of an exemplary embodiment;
• FIG. 2A is a diagram depicting an implementation of a temperature sensor;
• FIG. 2B is a selected cross sectional view of the sensor ofFIG. 2A depicting an implementation of a temperature sensor;
• FIG. 3A is a diagram depicting an implementation of a temperature sensor in accordance with an exemplary embodiment;
• FIG. 3B is a selected cross sectional view of the sensor ofFIG. 3A depicting an implementation of a temperature sensor in accordance with an exemplary embodiment;
• FIG. 4 is a simplified schematic diagram depicting a bridge amplifier network utilizing an exemplary temperature sensor in accordance with an exemplary embodiment;
• FIG. 5 is simplified schematic diagram depicting a bridge amplifier network utilizing an exemplary temperature sensor in accordance with an exemplary embodiment; and
• FIG. 6 is a block diagram depicting a flow chart of methodology in accordance with an exemplary embodiment.
DESCRIPTION OF AN EXEMPLARY EMBODIMENT
• Temperature sensors with fast response-time are desirable for a number of applications including, but not limited to automotive engine control and emission control. A conventional approach for fast temperature sensing includes a micro-electro-mechanical systems (MEMS) device. A low thermal mass membrane includes the temperature detector. The detector typically responds very quickly during the initial phase of a thermal event, followed by a much slower phase that is representative of the slower dynamics associated with the bulk of the silicon device. Disclosed herein in an exemplary embodiment is an apparatus and methodology whereby the fast response component is extracted utilizing a temperature detector bridge and associated electronics.
• Referring toFIG. 1,reference numeral10 generally designates a control system suitable for implementation of the disclosed embodiments. It should be noted that although the disclosed embodiments may be described by way of reference to temperature sensors for engine and emission control applications, it will be appreciated that such references are illustrative only and the disclosed embodiments may be applied to any instance where temperature sensing is desired
• Thetemperature sensor20 transmits temperature signal(s)22 to controller16 for processing in accordance with an exemplary embodiment.Controller16 executes a processing algorithm performing a methodology100 (SeeFIG. 5) for processing the temperature signal(s)22. Preferably, the sensing electronics associated with thetemperature sensor20 are configured integral with thetemperature sensor20 orcontroller16. Advantageously, this approach further reduces cost and simplifies the interface electronics. Further details of thealgorithm100 may be found at a later point herein.
• Thetemperature signal22, among others, is applied to thecontroller16. Thecontroller16 processes all input signals to generate values corresponding to each of the signals resulting in a temperature value being made available for processing in various algorithms as prescribed herein. Measurement signals, such as the abovementioned are also commonly linearized, compensated, and filtered as desired or necessary to enhance the characteristics or eliminate undesirable characteristics of the acquired signal. For example, the signals may be linearized to improve processing speed, or to address a large dynamic range of the signal. In addition, frequency or time based compensation and filtering may be employed to eliminate noise or avoid undesirable spectral characteristics.
• In order to perform the prescribed functions and desired processing, as well as the computations therefore (e.g., the temperature sensor signal processing, control algorithm(s), and the like),controller16 may include, but not be limited to, a processor(s), computer(s), DSP(s), memory, storage, register(s), timing, interrupt(s), communication interface(s), and input/output signal(s) interfaces, and the like, as well as combinations comprising at least one of the foregoing. Moreover,controller16 may include or be implemented with various processors, controllers, microcontrollers, logic/gate arrays, programmable logic arrays (PLA), programmable logic devices, and the like, as well as combinations including any of the foregoing. For example,controller16 may include input signal processing and filtering to enable accurate sampling and conversion or acquisitions of such signals from communications interfaces. Additional features ofcontroller16 and certain processes therein are thoroughly discussed at a later point herein.
• As exemplified herein and disclosed above one such process may be determining a temperature value from temperature signal(s)22.Controller16 receives various input signals including, but not limited to, those identified above, to facilitate such processing and may provide one or more output signals in response. Also received by thecontroller16 are a variety of implementation specific parameters, signals and values for initialization and characterization of the prescribed processes and to identify various states of the processes herein.
• Continuing now withFIGS. 2A and 2B as well, a simple MEMS temperature sensor is shown generally employing a membrane with a resistance exhibiting a high thermal coefficient on a silicon substrate. Two lead traces with low resistance for interconnection are also depicted. Turning now toFIGS. 3A and 3B, aMEMS temperature sensor20 in accordance with an exemplary embodiment is depicted. A membrane with aresistance24, also exhibiting a high thermal coefficient denoted R_m, is depicted. In addition, anotherresistance26 also denoted R_s, in this instance mounted primarily on asilicon substrate28 is depicted. A plurality oftraces30, which facilitate interconnection are also depicted. It will be appreciated that in an exemplary embodiment two membrane resistances,R_m24 and two substrate resistances,R_s26, are employed to facilitate implementation with a Wheatstone bridge circuit. It will be appreciated that the exemplary embodiments as disclosed herein are equally applicable employing a plurality of resistances as the membrane resistances,R_m24 and substrate resistances,R_s26, or even a single membrane resistance,R_m24 and a singlesubstrate resistance R_s26. Implementations with other resistance configurations would, of course, require varied electrical circuit interfaces.
• A basic understanding of the mathematical background will be beneficial for appreciation of the exemplary embodiments as disclosed herein. Let R_mand R_sbe the resistances of two thin film temperature detectors, the first one on a thin, lowthermal mass membrane24, and the second one on a high thermalmass silicon substrate26. Preferably, but not necessarily, these resistors are made of the same material that exhibits a high temperature coefficient of resistance. In an exemplary embodiment, a temperature coefficient of resistance in excess of 3000 parts per million (ppm) was employed. However, a temperature coefficient of resistance in excess in excess of about 1200-1500 parts per million (ppm) is satisfactory.
• Let the two resistances be equal when stabilized at an ambient temperature T_α:
  R_m(T_α)=R_s(T_α)=R_α  (1)
  When subjected to a temperature step ΔT, the two resistances R_mand R_sincrease with different dynamic responses, with the resistance of the detector on the membrane R_mreaching its final value at a faster rate than the detector on bulk silicon R_s. The final values at temperature T=T_α+ΔT are:
  R_m(T)=R_s(T)=R_α+ΔR  (2)
  where
  ΔR=Rα(1+cΔT)   (3)
  where “c” is the temperature coefficient of resistance for the material.
• Let the response-time of the temperature detector on a “free-standing”, thermally isolated membrane,R_m24 be τ_f, and the response-time of the temperature detector on a bulksilicon device R_s26 be τ_s. Both time constants τ_fand τ_sdepend significantly on the geometrical design as well as the material thermal properties of the membrane and the bulk silicon device. For a practical configuration though, it will be appreciated that the membrane cannot be entirely freestanding. Often, it is part of a silicon device as illustrated inFIG. 2, with physical and thermal anchoring to the surroundingbulk silicon28. AlthoughFIG. 3A shows asilicon substrate28, other substrate materials may be suitable as well, including, but not limited to, ceramic or silicon carbide substrates.
• For such a configuration, as the response of the detector is primarily a fast rising/varying portion representative of the membrane dynamics denoted as τ_f, followed by a slowly varying response that is representative of the bulk silicon substrate dynamics τ_s. It will be appreciated then, that a composite response-time depends primarily on the degree of thermal coupling between the membrane and the bulk silicon substrate. In an exemplary embodiment, a first varying response corresponding to the fast varying response may be on the order of milliseconds or tens of milliseconds, while a second varying response corresponding to the slowly varying response may be on the order of hundreds of milliseconds or seconds. Therefore, for a first approximation, the response-times of a detector on a practical membrane detector,R_m24 and a sensor onbulk silicon R_s26 are given by:
  R_m(t)=R_α+ΔR{(1−α)[1−exp(−t/τ_f)]+α[1−exp(−t/τ_s)]}  (4)
  R_s(t)=R_α+ΔR [1−exp(−t/τ_s)]  (5)
  where “α” is a coefficient that depends on the degree of thermal coupling between the membrane and the surrounding bulk silicon substrate. For a membrane with a very high level of thermal coupling, α˜1, and for a membrane with very good thermal isolation, α˜0. Therefore, for a practical situation in an actual physical implementation, α is between 1 and 0.
• It will be appreciated that the slower response-time of the sensor on the silicon substrate, τ_smay be more than one or two orders of magnitude larger than the fast response-time τ_f. Accordingly, for a fast response-time sensor e.g.,membrane resistance24, it is desirable to maximize the fast response component in equation (4). This may be achieved by increasing the isolation between the membrane and thebulk silicon28. One possible means of increasing the isolation is by increasing the membrane area. However, a large membrane may not be desirable because it becomes more fragile and easily subject to fatigue and breakage, and it would therefore significantly reduce the durability of the device. Another methodology, such as that disclosed herein in an exemplary embodiment is to take advantage of the distinctions between the membrane response and the response due to thesilicon substrate28.
• Disclosed herein in an exemplary embodiment is an approach where both types of detectors are used with corresponding processing to extract the fast response component from the total response. In an exemplary embodiment as depicted inFIGS. 3A and 3B, as well asFIGS. 4 and 5, two pairs ofresistances R_m24 andR_s26 are employed in a configuration of a bridge as illustrated and shown generally as40. The bridge voltage V_band the differential voltage ΔV_brespond differently to a temperature step ΔT. As demonstrated later in this disclosure, these two signals may be combined in a novel and unique way to extract the fast response component, thereby resulting in a very fast response-time sensing scheme. Advantageously, employing four resistances in abridge40 configuration facilitates elimination of common errors and variations associated with theresistances R_m24 andR_s26. Moreover, the bridge configuration facilitates amplification of the differential voltage generated by the two legs of thebridge40.
• When thebridge40 ofFIG. 4 is excited with a constant bias current, denoted as I₀, it may be shown that the bridge voltage V_band the differential bridge voltage ΔV_bare given by the following equations:
  V_b=0.5I₀(R_m+R_s)   (6)
  ΔV_b=V₁−V₂=0.5I₀(R_m−R_s)   (7)
• The step responses of these two signals are obtained by replacing equations (4) and (5) in equations (6) and (7):
  V_b(b)=I₀R_α+0.5I₀ΔR{(1−α[1−exp(−t/τ_f)]+(1+α)[1−exp(−t/τ_s)]}  (8)
  ΔV_b(t)=0.5I₀ΔR(1−α){[1−exp(−t/τ_f)]−[1−exp(−t/τ_s)]}  (9)
  Through a linear combination of equations (8) and (9), it may be noted that it is possible to cancel out the slowly varying signal. Elimination of the slowly varying signal yields the fast responding signal as desired. One implementation to facilitate elimination of the slowly varying signal may be realized physically with an electronic circuit as illustrated inFIG. 5 that sums the bridge voltage V_band the differential voltage ΔV_bafter the latter has been amplified through anamplifier50 exhibiting a gain G. The resulting sensor output voltage V₀of thesummer60 is given by:$\begin{array}{cc}\begin{array}{c}{V}_0\left(t\right)=V_b\left(t\right)+G\Delta V_b\left(t\right)\\ =I_0\mathrm{Ra}+0.5I_0\Delta R\left\{K_1\left[1-\exp \left(-t/{\tau }_f\right)\right]+{\mathrm{<figure-callout\; label="K"\; filenames="US20070019704A1-20070125-D00001.png"\; state="\{\{state\}\}">K</figure-callout>}}_2\left[1-\exp \left(-t/{\tau }_s\right)\right]\right\}\end{array}& \left(10\right)\end{array}$
  where
  K₁=(1−α)(1+G),K₂=[2−(1−α)(1+G)]  (11)
• It is now shown that with an appropriate value for the gain G, the slow varying term K₂in equation (10) vanishes. The optimal value for the gain, G_opt, is:
  G_opt=(1+α)/(1−α)   (12)
  Using the optimal value for the gain, equation (10) reduces to:
  V_0opt(t)=I₀Rα+I₀ΔR [1−exp(−t/τ_f)]  (13)
• It will be appreciated that the functionality depicted inFIG. 5 may also be readily computed with a processor such as withcontroller16. In another exemplary embodiment,controller16 optionally receives the voltages from thebridge40 for computation in accordance with the teachings herein. It will be appreciated thatcontroller40 may also receive signals indicative of the voltages at nodes V₁and V₂in advance of the buffer depicted for internal computation of ΔV_b.
• Turning now toFIG. 6 as well, a flowchart depicting amethodology100 in accordance with an exemplary embodiment is provided. Atprocess block102, the temperature signals22 (FIG. 1) are received from the bridge40 (e.g., temperature sensor20). Optionally, a value for the voltage change for a given temperature change, ΔV_bis computed from values for the voltages on each leg of the bridge as depicted atoptional process block104. Atprocess block106, the value for ΔV_bis scaled by gain G, where G is preferably equal to G_optto facilitate elimination of the slowly varying portion of the signal. Finally, the optimal value V_0opt(t) for the temperature including only the rapidly varying portion may readily be computed by summing the scaled value of ΔV_bwith the bridge voltage V_bin accordance withEquation 13 as depicted atprocess block108.
• The disclosed invention can be embodied in the form of computer or controller implemented processes and apparatuses for practicing those processes. The present invention can also be embodied in the form of computer program code containing instructions embodied intangible media13, such as floppy diskettes, CD-ROMs, hard drives, or any other computer-readable storage medium, wherein, when the computer program code is loaded into and executed by a computer or controller, the computer becomes an apparatus for practicing the invention. The present invention may also be embodied in the form of computer program code as adata signal15, for example, whether stored in a storage medium, loaded into and/or executed by a computer or controller, or transmitted over some transmission medium, such as over electrical wiring or cabling, through fiber optics, or via electromagnetic radiation, wherein, when the computer program code is loaded into and executed by a computer, the computer becomes an apparatus for practicing the invention. When implemented on a general-purpose microprocessor, the computer program code segments configure the microprocessor to create specific logic circuits.
• It will be appreciated that the use of first and second or other similar nomenclature for denoting similar items is not intended to specify or imply any particular order unless otherwise stated.
• While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims (8)

1-24. (canceled)

25. A method of determining a temperature comprising:

receiving a temperature signal, said temperature signal indicative of a composite temperature variation including a first varying portion and a second varying portion;

configuring said temperature signal to eliminate said second varying portion; and

generating a temperature value based on said configuring wherein said temperature value is substantially based on said first varying portion.

26. The method ofclaim 25 wherein said temperature signal comprises a differential voltage.

27. The method ofclaim 25 wherein said configuring includes scaling said temperature signal and combining said scaled temperature signal with an additional signal to cancel said second varying portion.

28. The method ofclaim 25 wherein said scaling includes a gain based on a thermal coupling and indicative of a relative proportion of said first varying portion and said second varying portion.

29. The method ofclaim 25 wherein said temperature signal comprises a voltage from a bridge, said bridge formed with two pairs of resistances, a first pair corresponding to a substrate resistance, and a second pair corresponding to a membrane resistance.

30. The method ofclaim 25 wherein said first varying portion corresponds to variations of about 1 to 100 milliseconds and said second varying portion corresponds to variations in excess of about 200 milliseconds.

31. A system for determining a temperature comprising:

a means for receiving a temperature signal, said temperature signal indicative of a composite temperature variation including a first varying portion and a second varying portion;

a means for configuring said temperature signal to eliminate said second varying portion; and

a means for generating a temperature value based on said configuring wherein said temperature value is substantially based on said first varying portion.

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