JPH0119065Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION TECHNICAL FIELD OF THE INVENTION This invention relates to linearization circuits, particularly linearization circuits used in combination with resistance thermometer devices or thermocouples to linearize the output of the thermometer device or thermocouple.

Description of the Prior Art Temperature signal emitters typically employ resistance thermometer devices or thermocouples as a means of measuring the temperature of the medium in which the temperature signal emitter is immersed. When using a resistance thermometer device, the temperature signal generator must convert the resistance variation of the device with temperature variations into a voltage. However, when a thermocouple is used, there is no need to change its output since the thermocouple itself generates a voltage directly. The voltage obtained in both cases is non-linear. Because of this non-linearity, errors of several percent of the maximum scale (full scale) occur in currently used temperature signal transmitters.

In many cases, errors caused by nonlinearities such as those mentioned above are not corrected, especially if the equipment involved does not require very high precision. In other cases, the errors must be corrected in subsequent parts of the control device using relatively complex electronic techniques that are very expensive. It would therefore be desirable to develop a simple and inexpensive electronic device that could be used to correct for the non-linear voltage response of resistance thermometer devices and thermocouples.

Purpose and Summary of the Invention The present invention seeks to solve the above-mentioned problem of non-linear response of resistance thermometer devices and thermocouples. By generating a voltage proportional to the square of the nonlinear input voltage and combining this voltage or a reduced magnitude of it with the original nonlinear input voltage, the voltage response curve obtained is very linear. It was found that it was approximated. According to the invention, this linearizing operation is accomplished using an operational amplifier, a pair of matched field effect transistors, each connected to the operational amplifier via feedback means, and a resistor forming a voltage divider with each transistor. achieved. The field effect transistors are matched and their gates are interconnected so that the two voltage dividers are interdependent to produce a voltage at the output of the second operational amplifier that is proportional to the square of the input voltage. This voltage, or a voltage reduced in magnitude, is then combined with the original input voltage to obtain a highly linear output voltage.

DESCRIPTION OF THE EMBODIMENTS Preferred embodiments of the present invention will now be described with reference to the drawings.

Referring to the drawings, FIG. 1 shows a linearization circuit that can be used with either a resistance thermometer device or a thermocouple. In FIG. 1, the temperature signal generator (not shown) is connected to the input terminal 20.
and 22. Temperature signal generators typically transmit a DC signal over a specific temperature range and use a resistance thermometer device or a thermocouple as the temperature sensor. In the case of a resistance thermometer device, the transmitter converts the resistance change of the thermometer device into a voltage signal. However, in the case of a thermocouple, since the thermocouple output itself is already a voltage signal, no change in the form of the thermocouple output signal is required in the transmitter. Temperature signal transmitters generally do not require their own power source and are connected to a two-wire process control loop powered by 24V DC, with both power supply and signal transmission accomplished through the same conductor pair.

The input terminal 20 is the common potential point of the circuit and is connected via a conductor 24 to one end of a resistor R ₁ , the opposite end of which is connected to the positive input of the first operational amplifier 26 as well as to the resistor R ₁ . connected to device _R2 . The opposite end of resistor _R2 is coupled to a constant reference voltage designated as -Vref. The output of the operational amplifier 26 is connected to the gate of a first field effect transistor 28, which constitutes one of a pair of matched field effect transistors. The drain of field effect transistor 28 is connected to input terminal 20 via conductor 24 . The source of transistor 28 is connected via a feedback loop 30 to the negative input of operational amplifier 26 and to one end of resistor R ₃ , the opposite end of which is connected to the input via conductor ₃₂ . terminal 2
Connected to 2.

The conductor 24 is also connected to the drain of a second field effect transistor 34 forming the other of the matched pair of field effect transistors. The gate of transistor 34 is connected to the gate of field effect transistor 28 and thus also to the output of operational amplifier 26. The source of transistor 34 is the second
The output terminal of the operational amplifier 36 is connected to the negative input terminal of the operational amplifier 36, and the output terminal of the operational amplifier 36 is connected to the feedback loop 38.
is connected to its negative input terminal through resistor _R4 . The positive input of operational amplifier 36 is connected to resistors R ₅ and R ₆ , the opposite ends of which are connected to conductors 24 and 32, which are further connected to input terminals 20 and 22, respectively.

The output terminal of the operational amplifier 36 is connected to the feedback loop 38, and in addition, one of the variable resistors _R7 is connected to the feedback loop 38.
connected to the end. This variable resistor R ₇ may be a potentiometer, the opposite end of which is connected to the input terminal 40 of a two-position switch, indicated generally by the reference numeral 42. The two-position switch 42 has output terminals 44 and 46 connected to the negative and positive inputs of a summing amplifier 48, respectively. The negative input of summing amplifier 48 is also connected to conductor 24 via resistors _R8 and _R9 ;
Therefore, it is connected to the input terminal 20 which is a common potential point. On the other hand, the positive input of summing amplifier 48 is connected to conductor 32 through resistor R ₁₀ and thus to input terminal 22 . The output terminal of summing amplifier 48 is connected to the base of transistor 50;
The emitter of transistor 50 is connected to the junction of resistors R ₈ and R ₉ so that resistor R ₈ is inserted into a feedback loop from the emitter of transistor 50 to the negative input of summing amplifier 48. It turns out. The output of the linearization circuit is taken from the collector of transistor 50 and supplied to the next stage of the temperature control process.

Referring now to FIG. 2, there is shown a voltage-temperature response characteristic curve for a resistance thermometer device. As shown in the figure, the measured value curve shown by a solid line is non-linear and has a downwardly concave shape. When this response curve is transformed into a quadratic, the resulting curve becomes upwardly concave as shown by the dashed line.
And if we add the measured voltage represented by this curve, or the measured voltage whose magnitude has been reduced, to the original voltage response curve, the resulting curve will closely approximate a straight line, and in this way the original non-linear voltage curve can be combined with its squared value to obtain a linear output.

Similar results can be obtained with respect to the voltage response curve of the thermocouple, as shown in FIG. FIG. 3 shows the voltage-temperature response curve of the thermocouple. In this case, the resulting curve is upwardly concave, as shown by the solid line. When this curve is converted to quadratic, the resulting curve is also upwardly concave, as shown by the dotted line. Therefore, subtracting the voltage represented by this curve or a voltage with a reduced magnitude from the original voltage response curve will still yield a result that closely approximates a straight line,
It is clear from this that even if a resistance thermometer device or a thermocouple is used as the temperature sensing means, a linear output can be obtained by appropriately combining the original nonlinear voltage curve with its squared curve. Obtainable.

Next, circuit operation will be explained.

Referring again to FIG. 1, a non-linear output voltage ei of a temperature signal generator using either a resistance thermometer device or a thermocouple as the temperature sensing means is applied to input terminals 20,22. a series resistor connected between a conductor 24 connected to the input terminal 20, which is the common potential point of the circuit, and a reference voltage -Vref;
R ₁ and R ₂ constitute a first resistive voltage divider, and a constant divided voltage e ₁ is generated across the resistor R ₁ by the reference voltage −Vref. The first operational amplifier 26 adjusts the circuit so that a voltage equal to this constant divided voltage e ₁ is also generated at the source of the first field effect transistor 28 and is held constant. Since this voltage e ₁ is applied via the feedback loop 30 to the negative input of the operational amplifier 26, this first operational amplifier 26 will control the circuit so that its two input voltages are equal. This circuit configuration constitutes a voltage divider that performs a voltage dividing operation expressed by the following equation.

e ₁ =R _q1 /R _q1 +R ₃ ei (1) In the above formula, R _q1 is the drain-source resistance of the field effect transistor 28.

Similarly, in the case of the second operational amplifier 36, the resistor R 5 is controlled by a second resistive voltage divider constituted by the resistors R ₅ and R ₆ connected in series between the input terminals ₂₀ and 22. The voltage generated across e ₂
is applied to the positive input of this operational amplifier 36. Operational amplifier 36 adjusts its output voltage e ₀₂ as follows. That is, a voltage equal to e ₂ is applied via the feedback loop 38 to the negative input of the operational amplifier 36 as well as to the source of the field effect transistor 34. The output voltage e ₀₂ of the operational amplifier 36 under these conditions can be expressed by the following equation.

e ₀₂ =R ₄ +R _q2 /R _q2 e ₂ (2) In the above formula, R _q2 is the drain-source resistance of the field effect transistor 36. As long as the voltage e ₂ is obtained by the voltage divider circuit formed by the resistors R ₅ and R ₆ , equation (2) can be rewritten as follows.

e ₀₂ =R ₄ +R _q2 /R _q2 eiR ₅ /R ₅ +R ₆ ...(3) As mentioned earlier, the field effect transistor 28,
34 are in matched pair topology, so the drain-source resistance R _q1 is equal to the drain-source resistance R _q2
be equivalent to. Furthermore, resistors R ₃ and R ₄ are chosen to be equal to each other. Therefore, by substituting R _q2 in place of R _q1 and R ₄ in place of R ₃ in equation (1), the following equation is obtained.

e ₁ = R _q2 / R _q2 + R ₄ ei ...(4) The above equation can be rewritten as follows.

R _q2 + R ₄ / R _q2 = ei/e ₁ ... (5) If the above equation is substituted for (R _q2 + R ₄ )/R _q2 in equation (3), the following equation is obtained.

e ₀₂ = ei/e ₁ eiR ₅ /R ₅ +R ₆ The above equation can be simplified as follows.

e ₀₂ =ei2k In the above formula, k is a constant.

Thus, the output voltage e ₀₂ of operational amplifier 36 is proportional to the square of the input voltage ei.

Referring again to FIG. 1, the output voltage e ₀₂ of operational amplifier 36 is then significantly reduced by variable resistor R ₇ and applied to input terminal 40 of switch 42 . Depending on whether a resistance thermometer device or a thermocouple is used, the output voltage e ₀₂ is added to or subtracted from the original input voltage ei. When a resistance thermometer device is used, switch 42 is operated in the lower position shown, switch input 40 is connected to switch output 46, and output voltage e ₀₂ is summed by summing amplifier 48 to input voltage ei. added to.
Conversely, if a thermocouple is used, switch 4
2 is actuated to the upper position shown so that switch input 40 is connected to switch output 44 and output voltage e ₀₂ is subtracted by summing amplifier 48 from input voltage ei. In either case, the operational amplifier 48 provides an output that closely approximates a straight line. Even though the original input voltage is non-linear, the final result is an output current that is very close to linear flowing through the transistor 50.
This current is supplied to subsequent parts of the control device.

Having described preferred embodiments of the invention, those skilled in the art will recognize that numerous modifications and changes can be made without departing from the spirit and scope of the invention.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of a linearization circuit according to the present invention, Figure 2 is a typical voltage response curve for a resistance thermometer device, a secondary voltage response curve for a resistance thermometer device, and a combination of the above two curves. Figure 3 shows a typical voltage response curve for a thermocouple, a secondary voltage response curve for a thermocouple, and a characteristic diagram that can be obtained when the above two curves are combined. FIG. 3 is a characteristic diagram showing a voltage response curve. 26, 36... operational amplifier, 28, 34, 36
... Field effect transistor, 38 ... Feedback loop, 48 ... Summing amplifier, 50 ... Transistor.

Claims (1)

[Claims for Utility Model Registration] (1) First and second inputs to which a nonlinear input voltage ei supplied from a temperature signal transmitter using a resistance thermometer device or a thermocouple as a temperature sensor A first operational amplifier 26 has a gate connected to the output of the operational amplifier, a drain connected to one of the input terminals 20 which is a common potential point of the circuit, and a source connected to the first operational amplifier 26. a first field effect transistor 28 connected to the other input terminal 22 via a resistor R ₃ and a reference voltage source −Vref;
a first resistive voltage divider R 1 , R ₂ connected between one of the input terminals 20 and supplying _a constant divided voltage e ₁ to one input of the operational amplifier;
the other input of said operational amplifier is connected to the source of said field effect transistor, and a first adjusting the circuit such that the voltage supplied to the other input of said operational amplifier is equal to said constant divided voltage e ₁ ; a second operational amplifier 36 having a drain-source resistance substantially equal to the drain-source resistance of the first field effect transistor 28, the gate of which is connected to the output of the first operational amplifier 26; a second field effect transistor 34 connected to the second operational amplifier 36, whose drain is connected to one of the input terminals 20, which is a common potential point of the circuit, and whose source is connected to one input of the second operational amplifier 36; is connected between the input terminals and supplies the divided voltage e ₂ to the second
a second resistive voltage divider _R5 , _R6 supplied to the other input of the operational amplifier, the output of the second operational amplifier having a resistance value approximately equal to that of the first resistor _R3 ; a second voltage dividing means connected to the one input of the operational amplifier via a resistor R ₄ of 2 and generating an output voltage e ₀₂ proportional to the square of the non-linear input voltage ei; An output voltage e ₀₂ proportional to the square of the non-linear input voltage ei is added to said non-linear input voltage ei when a resistance thermometer device is used as a temperature sensor, and a thermocouple as a temperature sensor. coupling means 42, 48, 50 for generating an approximately linear output signal by subtracting from said non-linear input voltage ei when
A voltage linearization circuit comprising R ₈ and R ₉ . (2) The voltage linearization circuit according to claim 1, wherein the coupling means comprises a switching means 42 and an amplifying means 48 electrically connected thereto. (3) Output voltage proportional to the square of the input voltage ei
The voltage linearization circuit according to claim 1, which comprises means _R7 for changing the magnitude of _e02 .