JP2012242294A - Three-wire system resistance measurement instrument - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a three-wire system resistance measurement instrument capable of canceling a measurement error due to a relative error between two current sources.SOLUTION: A resistance measurement instrument is constituted such that a first current source 41 is connected to one end of a measurement object resistance 10 via a first wire, a second current source 42 is connected to the other end of the measurement object resistance via a second wire, the other end of the measurement object resistance 10 is connected to reference potential via a third wire, and a reference resistance 50 is inserted to the current circuit of the second current source 42. The instrument includes: a signal processing part 200 for calculating the value of the measurement object resistance 10 on the basis of a first voltage signal, which indicates an output potential difference between the first current source 41 and the second current source 42, and a second voltage signal, which indicates a voltage drop between the terminals of the reference resistance; and current switching means 100 for switching connection to the first wire and second wire of the first current source 41 and second current source 42. The signal processing part 200 calculates the value of the measurement object resistance on the basis of first voltage signals and second voltage signals before and after the switching.

Description

  In the present invention, a first current source is connected to one end of the measured resistance via a first wiring, and a second current source is connected to the other end of the measured resistance via a second wiring, and the measured resistance Is connected to a reference potential via a third wiring, a reference resistor is inserted and connected to the current circuit of the second current source, and indicates the output potential difference between the first current source and the second current source. The present invention relates to a three-wire resistance measuring device including a signal processing unit that calculates a value of the measured resistance based on a first voltage signal and a second voltage signal indicating a voltage drop between terminals of the reference resistor.

  A three-wire resistance measuring device using one current source is disclosed in Patent Document 1. A system using two current sources that can easily incorporate a burnout circuit that warns of a resistance to be measured or a disconnection of a field wiring has been proposed (such as ADS 1248 of TI), both of which are well known.

  FIG. 5 is a functional block diagram showing a configuration example of a conventional three-wire resistance measuring apparatus using two current sources. Three terminals P1, P2, and P3 are provided on the interface F between the field side (A) and the measuring instrument side (B).

  One end of the resistance temperature detector (resistance value Rrtd) 10 that is a resistance to be measured is connected to the first current source 41 via the terminal P1 via the first wiring 31 having the wiring resistance 21 (resistance value Ra). , Current Ia flows.

  The other end of the resistance temperature detector 10 is connected to the second current source 42 via the terminal P2 and the reference resistance (resistance value Rref) 50 via the second wiring 32 having the wiring resistance 22 (resistance value Rb). , Current Ib flows.

  The other end of the resistance temperature detector 10 is connected to a reference potential via a third wiring 33 having a wiring resistance 23 (resistance value Rc), a terminal P3, and a bias resistance (resistance value Rbias) 60, and merges. Current Ia + Ib flows.

  The first voltage signal Vab (= Va−Vb) indicating the potential difference between the output potential Va of the first current source 41 and the output potential Vb of the second current source 42 is converted into a digital value by the AD converter 70 and is then sent to the signal processing unit 90. Entered. A second voltage signal Vbc (= Vb−Vc) indicating a potential difference (voltage drop due to the reference resistor 50) between the output potential Vb of the second current source 42 and the terminal P2 via the reference resistor 50 is digitally converted by the AD converter 80. It is converted into a value and input to the signal processing unit 90.

  Based on the input first voltage signal Vab (= Va−Vb) and second voltage signal Vbc (= Vb−Vc) and the resistance value Rref of the reference resistor 50, the signal processing unit 90 performs resistance of the resistance temperature detector 10. The value Rrtd is calculated by the following formula.

  Rrtd = Rref (Vab / Vbc + 1) (1)

Here, the equation (1) is obtained by solving the following equation for Rrtd.
Va = (Ra + Rrtd) Ia + (Rc + Rbias) (Ia + Ib)
Vb = (Rb + Rref) Ib + (Rc + Rbias) (Ia + Ib)
Vc = RbIb + (Rc + Rbias) (Ia + Ib)
Ra = Rb = Rc
Ia = Ib

  Here, the wiring resistances 21, 22, and 23 of the three wirings 31, 32, and 33 are equal, and the output current values Ia and Ib of the two current sources 41 and 42 are also equal. All the drops are canceled out, and the resistance value of the resistance temperature detector is obtained without error from the differential voltage value and the reference resistance value. This resistance value is converted into a temperature measurement value based on a conversion table defined in JIS C 1604.

JP 2010-48733 A

  When there is a relative error between the output currents Ia and Ib of the two current sources 41 and 42, the resistance value Rrtd of the resistance temperature detector can be obtained from the following equation, but the measurement error caused by the relative error of the two current sources ε will be included.

  Rrtd = Rref (1 + ε) (Vab / Vbc + 1) − {Ra−Rb (1 + ε)} (2)

Here, equation (2) is obtained by solving the following equation for Rrtd, and ε is a relative error between the output currents of the two current sources 41 and 42.
Va = (Ra + Rrtd) Ia + (Rc + Rbias) (Ia + Ib)
Vb = (Rb + Rref) Ib + (Rc + Rbias) (Ia + Ib)
Vc = RbIb + (Rc + Rbias) (Ia + Ib)
Ra = Rb = Rc
Ia = I0, Ib = I0 (1 + ε)

  The error in the first term on the right side of equation (2) above can be corrected by adjustment during manufacturing, but the error in the second term on the right side is an amount proportional to the wiring resistance value, so the installation environment (the length and resistance of the wiring) The correction is not easy.

  An object of the present invention is to realize a three-wire resistance measuring apparatus that can cancel a measurement error caused by a relative error between two current sources.

In order to achieve such a subject, the present invention has the following configuration.
(1) A first current source is connected to one end of the measured resistance via a first wiring, and a second current source is connected to the other end of the measured resistance via a second wiring. The other end is connected to a reference potential via a third wiring, a reference resistor is inserted and connected to the current circuit of the second current source, and a second output potential difference between the first current source and the second current source is shown. In the three-wire resistance measuring device including a signal processing unit that calculates a value of the measured resistance based on one voltage signal and a second voltage signal indicating a voltage drop between terminals of the reference resistor,
Current switching means for switching connection of the first current source and the second current source to the first wiring and the second wiring;
The signal processing unit calculates a value of the measured resistance based on the first voltage signal and the second voltage signal before switching, and the first voltage signal and the second voltage signal after switching. A characteristic three-wire resistance measuring device.

(2) The three-wire resistance measuring device according to (1), wherein the reference resistance is connected between an output of the second current source and the second wiring.

(3) The three-wire resistance measuring device according to (1), wherein the reference resistance is connected between the reference potential and the third wiring.

(4) The three-wire resistance measurement according to any one of (1) to (3), wherein the current switching means connects the output of the first current source to the output of the second current source. apparatus.

(5) The three-wire resistance measurement according to any one of (1) to (4), wherein the signal processing unit inputs the first voltage signal and the second voltage signal in time series. apparatus.

(6) The three-wire resistance measuring device according to any one of (1) to (5), wherein the resistance to be measured is a resistance temperature detector.

  According to the present invention, in a three-wire resistance measuring apparatus using two current sources, an error caused by a relative error between the two current sources affects the resistance value measurement result of the resistance to be measured. It can be avoided.

It is a functional block diagram which shows one Example of the 3-wire type resistance measuring apparatus to which this invention is applied. It is a functional block diagram which shows the other Example of the 3-wire type resistance measuring apparatus to which this invention is applied. It is a functional block diagram which shows further another Example of the 3-wire type resistance measuring apparatus to which this invention is applied. It is a functional block diagram which shows further another Example of the 3-wire type resistance measuring apparatus to which this invention is applied. It is a functional block diagram which shows the structural example of the conventional 3-wire type resistance measuring apparatus.

  Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a functional block diagram showing an embodiment of a three-wire resistance measuring apparatus to which the present invention is applied. The same elements as those of the conventional configuration described with reference to FIG.

  The feature of the present invention added to the conventional configuration shown in FIG. 5 is that the current switching means 100 for exchanging (swapping) the output destinations of the first current source 41 and the second current source 42 and the current switching means 100 In this configuration, a signal processing unit 200 that inputs and calculates a differential voltage before switching and a differential voltage after switching is provided.

  When the current switching means 100 is in a state before switching (solid arrow: pattern 1), the configuration is the same as the conventional configuration of FIG. 5, the current Ia from the first current source 41 flows through the first wiring 31, and the second wiring 32. In addition, the current Ib from the second current source 42 flows through the reference resistor 50, and the combined current Ia + Ib flows through the third wiring 33.

  In the state after the switching by the current switching means 100 (dotted line arrow: pattern 2), the current Ib flows through the first wiring 31, the current Ia flows through the second wiring 32 and the reference resistor 50, and the third wiring 33 passes through. The combined current Ia + Ib flows.

  Two differential voltages obtained from the AD converters 70 and 80 when the state of the current switching means 100 is a solid line (pattern 1) are Vab1 (= Va1−Vb1) and Vbc1 (= Vb1−Vc1), and a dotted line (pattern) Assuming that the two differential voltages in case 2) are Vab2 (= Va2-Vb2) and Vbc2 (= Vb2-Vc2), the resistance value of the resistance temperature detector Rrtd is the four differential voltage values and the reference resistance value Rref. Is obtained from the following equation.

  Rrtd = Rref ((Vab1 + Vab2) / (Vbc1 + Vbc2) +1) (3)

The above equation (3) is obtained by solving the following equation for Rrtd, and it can be seen that there is no term including the relative error ε of the two current sources on the right side.
Va1 = (Ra + Rrtd) Ia + (Rc + Rbias) (Ia + Ib)
Vb1 = (Rb + Rref) Ib + (Rc + Rbias) (Ia + Ib)
Vc1 = RbIb + (Rc + Rbias) (Ia + Ib)
Va2 = (Ra + Rrtd) Ib + (Rc + Rbias)) (Ia + Ib)
Vb2 = (Rb + Rref) Ia + (Rc + Rbias) (Ia + Ib)
Vc2 = RbIa + (Rc + Rbias) (Ia + Ib)
Ra = Rb = Rc
Ia = I0, Ib = I0 (1 + ε)

  Here, the potential with the subscript “1” indicates the potential when the state of the current switching means 100 is a solid line (pattern 1), and the potential with the subscript “2” indicates the state of the current switching means 100 with a dotted line (pattern 2). Represents the potential in the case of.

  FIG. 2 is a functional block diagram showing another embodiment of a three-wire resistance measuring apparatus to which the present invention is applied. The difference from the embodiment shown in FIG. 1 is that the voltage between the terminals of the reference resistor 50 is not included in the differential voltage Vab. The differential voltage Vab in FIG. 2 can be either a bipolar value or a monopolar value depending on the magnitude relationship between the resistance values of the reference resistor 50 and the resistance temperature detector 10, but the differential voltage Vab in FIG. Don't be.

  FIG. 3 is a functional block diagram showing still another embodiment of the three-wire resistance measuring apparatus to which the present invention is applied. The difference from the embodiment shown in FIGS. 1 and 2 is that the bias resistor 60 is omitted and the reference resistor 50 is connected between the terminal P3 and the reference potential.

  In this configuration, regardless of the switching pattern of the current switching means 100, the voltage Vc between the terminals of the reference resistor 50 becomes Vref (Ia + Ib), and this voltage Vc between the terminals is input to the AD converter 80.

  With this configuration, the differential voltage sum (Vbc1 + Vbc2) between both ends of the reference resistor 50 can be acquired by one AD conversion. In the case of adopting this configuration, the input of the AD converter 80 needs to allow zero potential.

  FIG. 4 is a functional block diagram showing still another embodiment of the three-wire resistance measuring apparatus to which the present invention is applied. The feature of this embodiment is indicated by a chain line in which the current switching means 100 adds two constant current source outputs to the reference resistor 50 in addition to the patterns 1 and 2 shown in FIGS. The pattern 3 is provided.

  In the pattern 3, the current flowing through the reference resistor 50 is Ia + Ib. Therefore, the input voltage of the AD converter 80 is Vref (Ia + Ib), which is the same as in FIG. 3, and the differential voltage sum (Vbc1 + Vbc2) between the both ends of the reference resistor 50 can be obtained by one AD conversion. Become. The present invention can be applied to a configuration in which the input voltage of the AD converter 80 does not allow zero potential and the bias resistor 60 is required.

  In the embodiment shown in FIG. 1 to FIG. 4, two configurations of the AD converter 70 and the AD converter 80 are illustrated. However, an input voltage switching unit is provided in the preceding stage of one AD converter, and a differential voltage is provided. A configuration can be adopted in which the input is taken into the signal processing unit in time series and processed.

  In the embodiment shown in FIG. 1 to FIG. 4, the resistance temperature detector 10 is illustrated as an object to be measured, but is not limited to this, and a three-wire resistance measuring device using two current sources is used. It is possible to apply in general.

DESCRIPTION OF SYMBOLS 10 Resistance thermometer 21, 22, 23 Wiring resistance 31, 32, 33 1st, 2nd, 3rd wiring 41 1st current source 42 2nd current source 50 Reference resistance 60 Bias resistance 70, 80 AD converter 100 Current Switching means 200 Signal processor

Claims (6)

A first current source is connected to one end of the measured resistance via a first wiring, a second current source is connected to the other end of the measured resistance via a second wiring, and the other end of the measured resistance is A first voltage signal is connected to a reference potential via a third wiring, a reference resistor is inserted and connected to the current circuit of the second current source, and indicates a difference in output potential between the first current source and the second current source. And a three-wire resistance measuring device including a signal processing unit that calculates a value of the resistance to be measured based on a second voltage signal indicating a voltage drop between the terminals of the reference resistor,
Current switching means for switching connection of the first current source and the second current source to the first wiring and the second wiring;
The signal processing unit calculates a value of the measured resistance based on the first voltage signal and the second voltage signal before switching, and the first voltage signal and the second voltage signal after switching. A characteristic three-wire resistance measuring device.   2. The three-wire resistance measuring apparatus according to claim 1, wherein the reference resistor is connected between an output of the second current source and the second wiring.   The three-wire resistance measuring apparatus according to claim 1, wherein the reference resistance is connected between the reference potential and the third wiring.   4. The three-wire resistance measuring apparatus according to claim 1, wherein the current switching unit connects the output of the first current source to the output of the second current source. 5.   5. The three-wire resistance measuring apparatus according to claim 1, wherein the signal processing unit inputs the first voltage signal and the second voltage signal in time series.   The three-wire resistance measuring device according to claim 1, wherein the resistance to be measured is a resistance temperature detector.

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