LINEARIZATION CIRCUIT AND METHOD

Abstract: A linearization circuit and method for a condition-sensitive element involves the application of excitation current via a common junction to the condition-sensitive element, detection of the voltage at the common junction, conversion of the detected voltage to current, and feeding back of at least a portion of the current resulting from conversion to the common junction. As a result, the excitation current applied to the condition-sensitive element is altered as the sensed condition changes, resulting in linearization of an electrical characteristic of the condition-sensitive element. The linearization circuit comprises a constant current source connected to the condition-sensitive element, a voltage-to-current converter, and a feedback resistor. Further features include: connection of a further constant current source/resistor combination to an input of the converter; connection of a voltage source between the condition-sensitive element and an input of the converter; and provision of a power supply for powering the circuit.

Description: Descrintion Linearization Circuit and Method Technical Field The present invention relates to a linearization circuit and method for signal conditioners of remote sensors. More specifically, the linearization circuit and method are applicable to a condition-sensitive element, such as a temperature-sensitive resistor. Background Art As is the case with most types of impedance transducers, condition-sensitive elements such as temperature-sensitive resistance transducers have a resistance which is dependent upon temperature, and have an electrical characteristic which is non¬ linear. That is to say, changes in the temperature of the element do not give rise to changes in the resistance of the element in direct proportion to the temperature changes. Such transducers are often employed in arrangements wherein it is extremely desirable to obtain an output signal which is linearly related to the particular condition sensed, such as temperature, and to which the transducer is responsive. This is due to the fact that any non-linearity inherent in a condition-sensing element tends to amplify the non- linearity of the overall arrangement in which the condition-sensitive element is employed. Various solutions to this problem have been attempted, one of the more common methods involving straight-line approximation within predetermined intervals of the range of values of the electrical signal. Eowevεr, such a method is expensive and, in any event, gives rise to approximation errors which increase with the size of the approximation intervals employed, as well as the degree of curvature of the electrical characteristic. Other attempts at obtaining a solution to the problem of non-linearity are disclosed in the following U.S. patents: Braki - 4,000,454 and egtien - 4,556,330. However, the arrangements disclosed in these patents are still burdened by certain inefficiencies and disadvantages, and it is the purpose of the present invention to overcome or eliminate such inefficiencies and disadvantages. Disclosure of Invention The present invention relates to a linearization circuit and method for signal conditioners αf remote sensors, and more particularly to a linearization circuit and method for application to condition-sensitive elements such as temperature- sensitive resistors. In accordance with the present invention, an excitation current is applied to a condition- sensitive element by a constant current source via a common junction therebetween. The voltage at the common junction is converted, in a voltage-to-current converter, to a current which flows in the two-wire line connecting the remotely located signal conditioner to a power supply. The output of the converter is connected via a feedback resistor to the common junction, so that changes in the electrical characteristic of the condition-sensitive element result in changes in the converter output, and this results in adjustment of the voltage at the common junction with corresponding adjustment of the excitation current applied to the condition-sensitive element. In this manner, a voltage having a linear function relationship to the condition sensed by the condition-sensitive element is maintained across the condition-sensitive element. A primary advantage of the arrangement just described resides ' in the fact that it provides simple and continuous linearization for a condition-sensitive element which is excited by a constant current source to produce a voltage signal which is converted into current by means of a linear voltage-to-current converter. In a first embodiment of the invention described above, the constant current source, the condition-sensitive element and a voltage source are connected in series across the positive and negative terminals of a power supply, and the converter is connected to the positive terminal of the power supply. In this manner, the constant current source and the converter are commonly biased. In addition, the junction between the output of the converter and the feedback resistor is connected via a further resistor to the negative terminal of the power supply. In a second embodiment of the invention, an additional constant current source and a series- connected resistor are connected in parallel with the constant current source and series-connected condition-sensitive element, the junction between the additional constant current source and the resistor being connected to an additional input of the voltage-to-current converter. In a third embodiment of the invention, a further voltage source is connected between an additional input of the voltage-to-currεnt converter, on the one hand, and the junction between the condition-sensitive element and its series-connected voltage source, on the other hand. In a fourth embodiment of the invention, the voltage source of the first embodiment described above is replaced by a fixed-value resistor. As discussed in further detail below, this embodiment is especially suitable in cases where the amount of correction current or feedback current is only a small fraction of the excitation current. Therefore, it is a primary object of the present invention to provide a linearization circuit and method for signal conditioners of remote sensors. It is an additional object of the present invention to provide a linearization circuit and method for condition-sensitive elements, such as temperature-sensitive resistors. It is an additional object of the present invention to provide a linearization circuit and method wherein a constant current source applies an excitation current to a condition-sensitive element via a junction therebetween. It is an additional object of the present invention to provide a linearization circuit and method wherein the voltage at the junction between the constant current source and the condition- sensitive element is detected and converted to current. It is an additional object of the present invention to provide a linearization circuit ^and method wherein the current output, of a voltage-to- current converter is used to provide a feedback current to the junction between the constant current source and the condition-sensitive element. It is an additional object of the present invention to provide a linearization circuit and method wherein an additional constant current source and a series-connected resistor are connected in parallel with the constant-current source and the condition-sensitive element. It is an additional object of the present invention to provide a linearization circuit and method wherein a further voltage source is connected between an additional input of the voltage-to-current converter, on the one hand, and* the junction between the condition-sensitive element and its series- connected voltage source, on the other hand. It is an additional object of the present invention to provide a linearization circuit and method wherein a voltage source or a fixed-value resistor is connected in series with the condition- sensitive element. The above and other objects, as well hereinafter appear, will be ' more fully understood by reference to the following detailed description, the appended claims, and the accompanying drawings. Brief Description of Drawing Figures Figure 1 is a circuit diagram of a first embodiment of the linearization circuit of the present invention. \ Figure 2 is a circuit diagram of a second embodiment of the linearization circuit of the present invention. Figurε 3 is a circuit diagram of a third embodiment of the linearization circuit of the present invention. Figure 4 is a circuit diagram of ε fourth embodiment of the linearization circuit of the present invention. Description of Preferred Embodiments The invention will now be described in more detail with reference to the accompanying drawings. Figure 1 is a circuit diagram of a first embodiment of the linearization circuit of the present invention. As seen therein, the linearization circuit 10 comprises a power supply 12, voltage-to-current converter 14, constant current source 16, temperature-sensitive resistor 18, feedback resistor 19, return path resistor 20, and voltage source 22. In general, the aforementioned circuit elements form an arrangement which is connected to power supply 12 via its positive and negative terminals. The circuit 10 serves as a signal conditioner for a remote sensor incorporating the temperature-sensitive resistor 18, and provides a current output signal via the two-wire arrangement, which current output signal is linearly related to the temperature being experienced by the temperature- sensitive resistor 18. Constant current source 16 acts as an excitation current source or current sink for applying current I χ to the temperature-sensitive resistor 18. The junction 21 between the source 16 and resistor 18 is connected to the input of converter 14, as well as to the feedback resistor 19. The converter 14 and source 16 are, preferably, commonly biased by power supply 12 via its positive terminal. Converter 14 provides, at its output, a current signal having a linear function relationship to the voltage E appearing at the input of converter 14, and this function can be expressed by the following equation: I L = I 0 + K'E (1) In the latter equation, I 0 is an offset current, K is the voltage-to-current conversion factor in amperes/volts, and " • " signifies multiplication. The output current I L of converter 14 is split into two portions, one of which flows through resistor 20, the other flowing through feedback resistor 19. The voltage E at the input of converter 14 results from the addition of the voltage provided by voltage source 22 and the voltage developed across the temperature-sensitive resistor 18 itself (by virtue of excitation current I χ ) . The excitation current is given by the following equation: L X - τ R ~ (2) In the latter equation, I R is a constant current from the source 16, while I F varies in accordance with the current resistance value R j , of resistor 18. The value of current Ip is given by the following equation: I = [E-(1- 'R A ) - 'I 0 ] (3) (*R A + ) where E = V R + R- j , • I X The resistance of the temperature-sensitive element 18 is related to the temperature by the following equation: Rp β £ , (1 + c * t + β't 2 ) (4) where R-ι Q is the resistance value at zero degrees Celsius, o = 3.9 xio "3 ^ 1 and β = -5.8xl0 ~7 K~ 2 . Consequently, the following relationship for the voltage E at the input of converter 14 can be established: (b+c) + j -b't + β'b-t- * where The relationship (5) , which is a rational polynomial, can be rewritten as : = r Q + r^t + r 2 - t < (10) J R \ where: b + c r Λ = (11) C • (b - a-c) (12) • = (b-a-c)-[β - (a÷l)-a-cc ] (13) As the output current of the converter 14 is linearly related to the input voltage E, it is necessary to cancel the second-order term in equation (10) to achieve linearization of this parameter. That is: β = a * (a+1) (14) * 2 By substituting equation (7) into equation (14) , an expression for the feedback resistance F can be established: where: β A •= (16) oC 2 B = 2-R A 'A - R To .(l-K-R A ) (17) C = A'R A 2 - R To -R A -(l- 'R A )-R To 2 -(l-K-R A ) 2 (18) Therefore, given a voltage-to-current conversion factor K, a resistance value R A , and the zero-degree Celsius value of R j ., the feedback resistance R F is calculated from equation (15) to establish the linearization of the output current of the apparatus. For example, where = 0.175 amperes/volts, R- j , 0 = 100 ohms and R A = 50 ohms, then F = 19.5 Kohms. Figure 2 is ε circuit diagram of a second embodiment of the linearization circuit of the present invention. Where appropriate reference numerals identical to those used in Figure 1 have been retained in Figure 2. In the linearization circuit 10 ! of Figure 2, an additional constant current source 32 is connected in series with a resistor 34, and the latter series-connected circuit elements are connected in parallel with source 16 and temperature- sensitive resistor 18, resistors 18 and 34 and voltage source 22 being interconnected at junction 27. A junction 33 between source 32 and resistor 34 is connected to an additional input of converter 14. Thus, converter 14 has a differential input; that is, converter 14 detects the voltage E- j _ between source 16 and resistor 18, and also detects the voltage E 2 between source 32 and resistor 34. Figure 3 is a circuit diagram of .a third embodiment of the linearization circuit of the present invention. Where appropriate, reference numerals identical to those employed in previous Figure 1 have been retained in Figure 3. In the linearization circuit 10" of Figure 3, a voltage source 42 is connected between an additional input of converter 14, on the one* hand, and the junction 27 between temperature-sensitive resistor 18 and voltage source 22, on the other hand. Again, this amounts to a differential arrangement of converter 14 in that converter 14 detects the voltage E-, between source 16 and resistor IS, and also detects the voltage E 2 at the positive terminal of voltage source 42. '' Figure 4 is a circuit diagram cf a fourth embodiment of the linearization circuit of the present invention. Where appropriate, reference numerals identical to those employed in previous Figure 1 have been retained in Figure . In the linearization circuit 10 - ■ ' of Figure 4, the fixed-value resistor 50 replaces the voltage source 22 employed in the embodiment of Figure 1. Otherwise, the embodiment of Figure 4 is identical to the embodiment of Figure 1. The embodiment of Figure 4 is especially suitable in cases where the amount of correction current or feedback current, I F , is only a small fraction of the excitation current I χ . Further referring to the embodiment of Figure 4, it should be noted that the substitution of a fixed-value resistor 50 for the voltage source 22 of Figure 1 is equally valid for the embodiments of Figures 2 and 3. That is to say, in Figures 2 and 3, the voltage sources 22 can be replaced by a fixed- value resistor 50, especially in cases where the amount of correction current or feedback current, l F , is only a small fraction of the excitation current ∑x- It should be noted that power supply 12 in Figures 1-4 discussed above can be any conventional power supply. Similarly, voltage-to-current converter 14 can be any conventional voltage-to- current converter, and constant current sources 16 and 32 can be any conventional constant current sources known to those of skill in the art. While preferrεd forms and arrangements have been shown in illustrating the invention, it is to be understood that various changes may be made without departing from the spirit and scope of this disclosure.

Claims: Claims 1. A linearization circuit for a condition- sensitive element, comprising: excitation current means connected to said condition-sensitive element at a common junction for applying an excitation current to said condition- sensitive element, said condition-sensitive element exhibiting an electrical characteristic in response thereto; convertεr means having an input connected to said excitation current means and to said condition-sensitive element at said common junction for detecting a voltage at said common junction, said voltage at said common junction varying as the electrical characteristic of said condition-sensitive element varies, said converter means converting said voltage at said common junction to a current provided at an output of said converter means; and feedback means for connecting said output of said converter means to said common junction and having a feedback current flowing therethrough, said feedback means being responsive to said current at the output of said converter means for adjusting said feedback current so as to adjust the voltage at said common junction and said excitation current applied to said condition-sensitive element, thereby maintaining across said ' condition-εensitivε element a voltage which is a linear function of a condition sensed by said condition-sensitive element. 2. The circuit of claim 1, wherein said condition-sensitive element comprises a temperature- sensitive element. 3. The circuit of claim 2, wherein said temperature-sensitive elemεnt comprises a temperature-sensitive resistor. . The circuit of claim 3, wherein said excitation current means comprises a constant current source. 5. The circuit of claim 4, wherein said feedback means comprises a feedback resistor. 6. The circuit of claim 1, further comprising power supply means having a first terminal connect to said excitation current means and a second terminal connected to said condition-sensitive element for supplying power therεto. 7. Thε circuit of claim 6, whεrein said condition-εensitivε εlement is connectεd to said second terminal of said power supply means via a voltage source. 8. The circuit of claim 6, wherein said condition-sensitive elemεnt is connεcted to said second terminal of said powεr supply means via a fixed value resistor. 9. The circuit of claim 8, wherein said feedback means is connected to said output of said convertεr means at a further junction, said further junction being connεctεd to said εεcond terminal of said power supply means via a rεsistor. 10. The circuit of claim 7, wherein said feedback means is connεcted to said output of said converter means at a further junction, said further junction being connεctεd to said εεcond terminal of said power supply mεans via a rεsiεtor. 11. The circuit of claim 6, wherein said first terminal of said power supply means is connected to said converter means for supplying power thereto. 12. The circuit of claim 11, wherein said excitation current means and said converter means are voltage-biased in common by said power supply means. 13. The circuit of claim 1, further comprising a circuit connectεd electrically in parallel with said excitation current means and said condition-sensitive elemεnt, said circuit comprising a resistor and additional excitation current means connectεd to said resistor at an additional common junction for applying an additional excitation current to said resistor, said additional common junction being connected to an additional_ input of said converter means. 14. The circuit of claim 13, wherein said condition-sensitive elemεnt comprises a tempεraturε- εεnsitivε elε ent. 15. The circuit of claim 14, whεrεin said tεmpεraturε-sεnsitivε εlement comprises a tempεraturε-sεnsitive resistor. 16. The circuit of claim 15, wherεin at least onε of said excitation control means and said additional excitation control means comprisεs a constant currεnt εourcε. 17. The circuit of claim 16, wherein said feedback means compriseε a feεdback rεsistor. 18. The circuit of claim 13, furthεr comprising power supply means having a first terminal •■ connectεd in common to said εxcitation currεnt means and said additional excitation currεnt means, and a second tεr inal connεctεd in common to said condition-sensitive element and said resistor. 19. The circuit of claim 18, wherein said condition-sensitive elεmεnt and said rεsistor arε connεctεd to said εεcond terminal of said power supply means via a voltage source. 20. The circuit of claim 18, wherein said condition-sensitive elemεnt and said rεsistor are connected to said second terminal of said power supply means via a fixed-valuε rεsiεtor. 21. Thε circuit of claim 20, whεrεin said feedback means is connεcted to said output of said converter eanε at a furthεr junction, εaid furthεr junction bεing connεctεd to εaid εεcond tεrminal of εaid powεr εupply mεanε via a furthεr reεiεtor. 22. Thε circuit of claim 19, whεrein εaid feedback meanε iε connεctεd to εaid output of said convεrtεr means at a further junction, εaid further junction being connectεd to εaid εεcond tεrminal of εaid powεr εupply εans via a further resistor. 23. The circuit of claim 18, wherein εaid firεt terminal of εaid power εupply means is connected to said convertεr means for supplying power thereto. 24. The circuit of claim 23, wherein εaid excitation current means, εaid additional excitation current means and εaid convertεr means are voltage- biaεεd in common by εaid powεr εupply means. 25.- The circuit of claim 1, further comprising a voltage εourcε connεcted bεtwεεn an additional input of εaid converter meanε and εaid condition-sensitive elemεnt. 26. Thε circuit of claim 25, wherein εaid condition-εenεitivε elεmεnt compriεεε a temperaturε- εensitive element. 27. The circuit of claim 26, wherein εaid temperature-sensitive element comprises a tempεrature-εεnεitive rεεistor. 28. The circuit of claim 27, wherein said excitation current means comprises a constant currεnt source. 29. The circuit of claim 28, whεrεin εaid feedback means comprises a feedback resistor. 30. The circuit of claim 25, further comprising power supply mεanε having a first tεrminal connect to εaid excitation current means and a second terminal connected to εaid condition-sensitivε elεment for supplying power therεto. 31. Thε circuit of claim 30, whεrεin said voltagε εource has a first end connected to εaid additional input of εaid convertεr means and a second end connectεd to εaid condition-sensitive element, said second end of εaid voltage εource also being connected to said second terminal of εaid powεr εupply means. 32. The circuit of claim 31, whεrεin εaid condition-sensitive element and εaid resistor are connectεd to εaid sεcond tεrminal of εaid powεr supply means via a voltage source. 33. The circuit of claim 31, wherεin εaid condition-sensitive element and εaid resistor arε connεctεd to said εεcond tεrminal of εaid powεr εupply means via a fixed-valuε resiεtor. 34. Thε circuit of claim 33, whεrεin said fεεdback mεans is connected to εaid output of εaid convertεr mεanε at a further junction, εaid further junction bεing connεctεd to said sεcond tεrminal of said power εupply means via a further reεiεtor. 35. Thε circuit of claim 32, wherein εaid feεdback means is connεctεd to said output of εaid convertεr means at a further junction, εaid further junction being connected to said second terminal of εaid power εupply means via a further rεεiεtor. 36. Thε circuit of claim 30, whεrεin εaid first tεrminal of εaid powεr εupply mεans is connεctεd to εaid converter means for supplying power therεto. 37. The circuit of claim 36, wherεin εaid excitation currεnt meanε and εaid convertεr means are voltage-biasεd in common by εaid power εupply means. 38. A linearization method for a condition- sensitive elεment having a first end and a second end, the method comprising the steps of: applying an excitation current to εaid first end of εaid condition-εenεitivε elεmεnt, εaid condition-εεnεitivε elεmεnt εxhibiting an elεctrical charactεriεtic in rεεponεε therεto; dεtεcting a voltagε at εaid first εnd of εaid condition-sεnsitivε εlεmεnt, εaid voltagε at said firεt εnd of εaid condition-εεnεitivε elemεnt varying aε thε εlectrical characteristic of said condition-senεitivε εlεmεnt variεs; convεrting εaid voltagε at εaid firεt εnd of said condition-εεnεitivε elεmεnt to a currεnt; and fεεding back to εaid first end of said condition-sεnεitive elemεnt at lεast a portion of said currεnt rεεulting from εaid convεrting εtep. 39. The method of claim 38, compriεing the further εtεp of adjuεting thε voltagε at εaid firεt end of εaid condition-εensitivε element in accordance with variation in εaid current fed back during εaid feeding ba ,k εtep. 40. Thε method of claim 39, comprising the further step of adjusting εaid excitation current applied to εaid first end of said condition-sensitive element in accordance with adjustment of the voltage at εaid firεt end of εaid con ition-εensitive εlεmεnt, thεreby maintaining across εaid condition- sensitive element a voltage which is a linear function of a condition εensεd by said condition- εenεitive elεment.

Inventors: REIS CARLOS ALBERTO DOS, REIS, CARLOS ALBERTO DOS
Applicants: THERMO ELECTRIC INT, THERMO ELECTRIC INTERNATIONAAL B.V.
Application year: 1989
Main classification: G01K7
IPC codes: G01D3, G01K7, G01L1