JPH0443791Y2 - - Google Patents

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a resistance temperature detector input circuit that handles signals from a plurality of resistance temperature detectors.

(Prior art) A resistance temperature detector is a temperature detection element that uses platinum as its material, and its resistance value changes depending on the temperature. It is generally obtained as a temperature signal.

FIG. 2 is a connection diagram of a conventional resistance temperature detector input circuit that supplies a constant current to a plurality of resistance temperature detectors and inputs signals therefrom. In the figure, Rt ¹ ~ _{R to}
(n is the same below integer) is a 3-wire resistance temperature detector, CI ₁
~CI _o is a constant current supply circuit that supplies a constant current to the resistance temperature detector, and S ₁ ~ _{S o} is a changeover switch that selects and takes out the signal from each resistance temperature detector Rt ¹ ~ R _to . Each switch constitutes a multiplexer MX. CK is a signal processing circuit that inputs the signal selected by the multiplexer MX, performs predetermined signal processing, and outputs a temperature signal.

In the constant current supply circuit CI ₁ , A ₁ is an operational amplifier, and Q ₁ is an output transistor controlled by the output of this operational amplifier. The first reference voltage VR ₁ is applied to one input terminal of the operational amplifier A ₁ , and the collector of the output transistor Q ₁ is connected to the other input terminal, and the second reference voltage VR 1 is applied via the resistor Ra to the other input terminal. A reference voltage VR ₂ is applied, and the output transistor Q ₁ has a constant current I expressed by the following equation.
is supplied to the resistance temperature detector.

I = (V _r1 - V _r2 )/Ra (Problem to be solved by the invention) In the conventional circuit with such a configuration, multiple constant current supply circuits are used depending on the number of resistance temperature detectors. It will be done. Here, in order to improve the accuracy of the constant current output, the first and second reference voltages VR ₁ , VR ₂ and the resistance
It is necessary to use a highly accurate Ra,
Further, there was a problem that the operational amplifier _A1 had to be one without an offset, making it extremely expensive.

The present invention was developed in view of these problems, and its purpose is to achieve high precision and overall low cost by sharing a constant current supply circuit for each resistance temperature detector. This is an attempt to realize a resistance temperature detector input circuit.

(Means for Solving the Problems) The present invention that solves the above-mentioned problems includes a plurality of resistance temperature detectors and a function that turns off when the resistance temperature detector is selected for each of these resistance temperature detectors. a DC voltage source connected via a switch; a first multiplexer that selects signals from the plurality of resistance temperature sensors;
a signal processing circuit that inputs the signal selected by the first multiplexer to obtain a temperature signal; a first operational amplifier having a high-precision resistor connected between the input terminals; Constant voltage source with high output and accuracy
a second operational amplifier that outputs a difference from Vref; a second multiplexer connected between the DC voltage line and one input terminal of the first operational amplifier; a third multiplexer connected between the temperature resistance element and the other input terminal of the first operational amplifier; and a third multiplexer connected between the second operational amplifier and the output terminal and the plurality of temperature resistance elements; and a fourth multiplexer connected thereto.

(Embodiment) FIG. 1 is a connection diagram showing an embodiment of the present invention. Components corresponding to those in FIG. 2 are designated by the same reference numerals. In the figure, V _cc is a rough voltage source, and when the relevant RTD is selected,
Switch off S ₁₁ ~ S _o1 , rough resistance r ₁ ~ _{r o}
are connected to the corresponding resistance temperature detectors R _t1 to _{R to} via. F ₁ to _{F o} represent the resistance temperature detectors R _t1 to R _to and the switches S ₁ to _{S o} that constitute the first multiplexer MX1.
This is a noise removal filter inserted in between.

The constant current supply circuit CI is shared by each resistance temperature detector R _t1 to R _to , and includes a first operational amplifier A ₁ and a highly accurate resistor R ₁ connected between the input terminals. ,
It is configured to include a second operational amplifier A ₂ that outputs the difference between the output of this operational amplifier A ₁ and a reference voltage from a highly accurate constant voltage source Vref. 1st
One input terminal of the operational amplifier A ₁ is connected to the switch S ₁₂
~S _o2 is connected to the voltage source _Vcc connected to each resistance temperature detector R _t1 ~R _to , and the other input terminal is
Through switches S ₁₃ to S _o3 , resistance temperature detectors R _t1 to R _to
It is connected to one end P (one end of the resistors r ₁ to r _o ).
Further, the output terminal of the second operational amplifier _A2 is connected to one end P of the resistance temperature detector R _t1 to R _to via the switches S ₁₄ to S _o4 .
It is connected to the.

Here, switches S ₁₂ to S _o2 are multiplexers
The switches S ₁₃ to _{S o3} constitute the third multiplexer MX3, and the switches S ₁₄ to _{S o4} constitute the _fourth multiplexer MX4.
It is designed to work in sync with S _o .

The operation of the circuit configured in this way will be explained next.

Each resistance temperature detector R _t1 ~ R _to and rough DC power supply V _cc
Switches S ₁₁ to _{S o1} are on when the resistance temperature detector for which they are responsible is not selected, and are supplied with DC voltage from the DC power supply V _cc .
When the resistance thermometer you are in charge of is selected,
It is working so that it is turned off.

Now, when resistance temperature detector R _t1 is selected, switch S ₁₁ is turned off, and the first to fourth multiplexers MX
Switches S ₁ , S ₁₂ , S ₁₃ , and S ₁₄ of No. 1 to MX4 are all turned on, and a DC voltage from the DC power supply V _cc is applied to the resistance temperature detector R _t1 via the resistor R ₁₁ . A highly accurate constant current as shown by the following equation flows through the temperature resistor R _t1 .

I=Vref/R ₁₁ At this time, if the line resistance is ra at one input terminal A and the other input terminal B of the signal processing circuit CK,
Voltages V _A and V _B as shown in the following equations are applied.

V _A = (2ra + R _t1 ) I ... (1) V _B = (ra + R _t1 ) I ... (2) The signal processing circuit CK uses these signals to
By performing calculations such as the following formula, the line resistance can be determined by calculating the line resistance.
A temperature signal based on the resistance of the resistance temperature detector R _t1 is obtained without being affected by ra.

2V _B −V _A =I・R _t1 −2ra+2ra=I・R _t1 Note that in this example, the zero suppression resistor is omitted to simplify the explanation, but in the actual circuit, A zero suppression resistor is inserted in the signal line from the resistance temperature detector.

Thereafter, temperature signals are obtained for other resistance temperature detectors R _t2 to _{R to} by the same operation. Here, before each RTD is selected, a rough DC current is given to it, and after it is selected, a highly accurate constant current is given to it, so there is no influence from filter response delays, etc. Each multiplexer MX1 to MX4
can be switched at high speed.

(Effects of the invention) As explained above, according to the invention, it is possible to share a high-precision constant current supply circuit for each resistance temperature detector, and it is possible to perform high-precision and inexpensive measurement as a whole. A temperature resistor input circuit can be realized.

[Brief explanation of the drawing]

FIG. 1 is an electrical connection diagram showing an example of the circuit of the present invention, and FIG. 2 is an electrical connection diagram of a conventional circuit. R _t1 ~R _to ...Resistance temperature detector, MX1~MX4...
Multiplexer, _Vcc ...DC power supply, CK...signal processing circuit, CI...constant current supply circuit.

Claims (1)

[Scope of utility model registration request] A DC voltage source connected to each of these resistance temperature sensors via a switch that turns off when the resistance temperature sensor is selected; a first multiplexer that selects a signal; a signal processing circuit that inputs the signal selected by the first multiplexer to obtain a temperature signal;
a second operational amplifier that outputs the difference between the output of the operational amplifier and a highly accurate constant voltage source Vref; and the DC voltage source and the first operational amplifier. a second multiplexer connected between one input terminal of the plurality of resistance temperature detectors and the other input terminal of the first operational amplifier; A resistance temperature detector input circuit comprising an output terminal of a second operational amplifier and a fourth multiplexer connected to the plurality of resistance temperature detectors.