Description
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. Eowever, 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 Regtien - 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 of 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-current 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.
Figure 3 is a circuit diagram of a third embodiment of the linearization circuit of the present invention.
Figure 4 is a circuit diagram of a 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 IX 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: IL = Io + K·E (1)
In the latter equation, Io is an offset current, K is the voltage-to-current conversion factor in amperes/volts, and " · " signifies multiplication.
The output current IL 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 Iχ). The excitation current is given by the following equation: IX = IR - IF (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
T of resistor 18. The value of current I
F is given by the following equation:
where E = VR + RT · I X
The resistance of the temperature-sensitive element 18 is related to the temperature by the following equation:
RT = RTo (1 + α ·t + β·t2) (4) where RTo is the resistance value at zero degrees Celsius, α = 3.9 ×10-3K-1 and β = -5.8×10-7 K-2.
Consequently, the following relationship for the voltage E at the input of converter 14 can be established:
The relationship (5) , which is a rational polynomial, can be rewritten as :
r1 = α · (b - a·c) (12) r2 = (b-a·c)-[β - (a+1)·a·α2] (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.
By substituting equation (7) into equation (14), an expression for the feedback resistance RF can be established:
B = 2·RA·A - RTo.(1-K·RA) (17) C = A·RA 2 - RTo·RA·(1-K·RA)-RTo 2·(1-K·RA)2 (18)
Therefore, given a voltage-to-current conversion factor K, a resistance value RA, and the zero-degree Celsius value of RT, the feedback resistance RF is calculated from equation (15) to
establish the linearization of the output current of the apparatus. For example, where K = 0.175 amperes/volts, RTo = 100 ohms and RA = 50 ohms, then
RF = 19.5 Kohms.
Figure 2 is a 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 E1 between source 16 and resistor 18, and also detects the voltage E2 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
E1 between source 16 and resistor 18, and also detects the voltage E2 at the positive terminal of voltage source 42.
Figure 4 is a circuit diagram of 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 4.
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, IF, is only a small fraction of the excitation current Iχ.
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, IF, is only a small fraction of the excitation current
IX.
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 preferred 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.