JPS6231397B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a resistance/electrical signal converter that converts the electrical resistance value of a resistor into a voltage or current signal corresponding to the resistance value and outputs the signal, and particularly relates to a resistor and a conversion circuit. This invention relates to a resistance/electrical signal converter that automatically compensates for the influence of wiring resistance when two wires are used to connect the two.

In the field of process measurement and control, as a means of detecting and measuring process physical quantities such as temperature and displacement, current is passed through a resistor to be measured such as a resistance temperature detector or strain gauge, and the voltage generated across the resistor is detected. things will be done. The electrical signal converter, which is generally installed at a remote location from the process quantity measurement point where the resistor to be measured is placed, and the resistor to be measured are connected by several wires.

If the connection between the resistor to be measured and the electrical signal converter is made using two-wire wiring that both supplies current and detects voltage, the resistance of the wiring will be in series with the resistor to be measured. Therefore, when the wiring resistance becomes too large to be ignored relative to the resistance value of the resistor to be measured, an error occurs in the measurement result, and it has been difficult to automatically compensate for this error. In order to cope with this problem, three-wire wiring in which a separate line for detecting the resistance of the wiring is provided has been put into practical use and in widespread use. In this three-wire wiring, methods include using a bridge circuit with the resistor to be measured as one side, or detecting voltages proportional to the resistor and wiring resistance, and adding and subtracting these using operational amplifiers.
Compensates for the influence of wiring resistance. However, in any case, when using a 3-wire system, the resistance values of the two wirings that supply current to the resistor among the three wirings connecting the resistor to be measured and the electrical signal converter must be equal. is a necessary condition. For this reason, the same wire type, same wire diameter, and same wire length are used for the wiring. However, the wire diameter (wire cross-sectional area)
Due to the tolerance, it is often not possible to maintain the same resistance between two wires, such as when the wiring distance is several hundred meters or more, or when the wiring route is complicated due to obstacles such as terrain or plants. ,
This method has the disadvantage that it cannot be applied in cases where high precision is required for resistance/electrical signal conversion.

In such cases, conventionally, a four-wire wiring system has been adopted in which the current supply line and the voltage detection line are completely separated using four wiring lines. This 4-wire system is
Since it is possible to measure resistance without any current flowing through the two voltage detection lines, a resistance/electrical signal converter that is not affected by the wiring resistance even if the resistance values of the four wiring lines are different can be obtained. be able to. However, when using a 4-wire system, not only does the configuration of the conversion circuit become complicated, but the large number of wires makes the wiring process extremely complicated, especially when there are many measurement points, and the wiring work cost is relatively high. There were some inconveniences.

An object of the present invention is to provide a resistance/electrical signal converter that can compensate for the influence of wiring resistance in two-wire wiring with a simple circuit configuration, which has been considered difficult in the past, while eliminating the disadvantages of the prior art described above. It is on offer.

In order to achieve the above object, the features of the present invention are as follows:
A parallel connection circuit in which a capacitor with an impedance value smaller than the resistance value is connected in parallel to both ends of the resistor to be measured is placed at the resistance measurement point, and one wire is connected to each end of this parallel connection circuit. Then, a constant current is supplied from a DC constant current source and a square wave constant current source, respectively, and the voltage generated in the parallel connection circuit obtained through the wiring is turned on and off in synchronization with the frequency of the square wave constant current source. The structure is such that an electric signal corresponding to the resistance value of the resistor is obtained from the switching circuit by inputting the signal to the switching circuit which is turned off.

The present invention will be explained below with reference to the drawings.

FIG. 1 is a circuit diagram of an embodiment of the present invention, and FIGS. 2 and 3 are time charts of various signals for explaining its operation. In the embodiment, a temperature measuring resistor whose resistance value Rt changes depending on the temperature is used as the resistor. In Fig. 1, 1 is a two-wire resistance temperature detector, with a capacitor C connected in parallel to both ends of it.
This parallel connection circuit is installed at the measurement point A, and the electrical signal conversion section B is placed at a remote location from the measurement point A. From the parallel circuit of the DC constant current source 2 and the square wave constant current source 3 of the electric signal converter B section, a constant current is connected to both ends of the parallel connection circuit of the resistance temperature detector 1 and the capacitor C via wiring L1 and L2, respectively. Current is supplied. r1 and r2 are equivalent resistances of the wirings L1 and L2, respectively. Wiring L1, L by passing current
The resistance temperature detector 1 obtained through 2 and the capacitor C
The voltage e _R generated across the parallel connected circuit is input to a switching circuit 4 which operates on and off in synchronization with the frequency of the square wave constant current source 3. The switching circuit 4 is composed of, for example, a series-parallel circuit of semiconductor switches 5 and 6 which are alternately turned on and off as shown in the figure.

The operation of the embodiment circuit having such a configuration will be explained using the time chart shown in FIG. First, if we focus only on the DC constant current source 2, a constant current i1 as shown in FIG. 2A flows through the resistance temperature detector 1 as shown by the solid arrow in FIG.
_R1 is e _R1 = (r1 + Rt + r2) · r1 (1) with point b as the reference. Here, Rt is the resistance value of the resistance temperature detector 1. FIG. 2C shows a time chart of this generated voltage e _R1 . Moreover, if we focus only on the square wave constant current source 3, as shown in FIG.
outputs an intermittent square wave constant current that becomes zero. Here, the relationship between the resistance value Rt of the resistance temperature detector 1, the frequency of the square wave constant current source 3, and the capacitance of the capacitor C (represented by the same sign) is expressed as Rt≫1/(2πC) (2) If selected, the current (-i2) from the square wave constant current source 3 as shown in FIG. flows. That is, by making the impedance value of the capacitor C extremely small compared to the resistance value Rt of the resistance temperature detector 1, the square wave current (-i2) from the square wave constant current source 3 flows only through the capacitor C. Therefore, the voltage e _R2 generated at point a with reference to point b is as shown in Figure 2 D: In section T1, e _R2(T1) =-(r1+r2)・r2 In section T2, e _R2(T2) = 0 …(3) becomes. Thus, the voltage e actually generated at point a
Since _R is a composite value of the voltages generated by the current i1 and the current i2, e _R =e _R1 +e _R2 (4). From equations (1), (3), and (4), in section T1, e _R(T1) = (r1+Rt+r2)・i1 −(r1+r2)・i2...(5) In section T2, e _R(T2) = ( r1+Rt+r2)・i1 (6) The time chart at this time is shown in Figure 2E.

The voltage e generated at point a shown by equations (5) and (6)
_R is input to the switching circuit 4. In synchronization with the frequency of the square wave constant current source 3, the switching circuit 4 turns on the switch 5 and turns off the switch 6 in the interval T1, and turns the switch 5 off in the interval T2.
The switch 6 is turned on and off alternately. Therefore, the output e _O of the switching circuit 4, that is, the voltage e _O across the switch 6, is the second
As shown in the figure, in section T1, the voltage expressed by equation (5) is output as is. In interval T2, the voltage expressed by equation (6) is input to the switching circuit 4, but since the switch 6 is short-circuited, the output e _O is zero. That is, in section T1, e _O(T1) = (r1+Rt+r2)・i1 −(r1+r2)・i2 (7), and in section T2, e _O(T2) =0 (8). Here, the output current i1 of the DC constant current source 2
and the output current i2 of the square wave constant current source 3 are set to be equal, i1=i2=i, then equation (7) becomes the following equation (9).

e _O(T1) = Rt・i (9) That is, as the output of the switching circuit 4, the resistance of the resistance temperature detector 1 is compensated for the influence of the wiring resistances r1 and r2, as shown in equation (9). An intermittent square wave voltage proportional to the value Rt only can be obtained.

FIG. 3 is a time chart for another embodiment of the invention, which in FIG.
In contrast to the case where the output is an intermittent square wave current as shown in Figure 2 (b),
In FIG. 3, an alternating square wave current (±i2) as shown in FIG. 3B is used. In this case, the voltage generated at point a by flowing an alternating square wave current (±i2) is e _R2 is: In section T1, e _R2(T1) =-(r1+r2)・i2 In section T2, e _R2(T2) = (r1+r2)・i2…(10). The time chart at this time is shown in FIG. 3D. Therefore, current i1 and current (±i
2) The composite voltage e _R at point a generated by
The waveform will be as shown in Figure E. That is, equation (1) and (10)
Using the formula, in interval T1, e _R(T1) = (r1+Rt+r2)・i1 − (r1+r2)・i2 …(11) In interval T2, e _R(T2) = (r1+Rt+r2)・i1 + (r1+r2)・i2 …( 12) becomes. Since this voltage e _R is outputted by the switching circuit 4, only the voltage e _{R (T1)} of T1 shown in equation (11) is output, so the output e _O of the switching circuit 4 is the same as the time chart shown in FIG. It becomes like this. That is,
Equation (11) is the same as the above-mentioned equation (5), and the output e _O is the same as the above-mentioned equations (7) and (8). Here, if the relationship between the current i1 and the current (±i2) is selected as i1=i2=i, the output e _O will be the same as equation (9).

In addition, in the above-mentioned embodiment, the switching circuit 4
Although it has been explained that the switch is composed of a series-parallel circuit of semiconductor switches 5 and 6, it is not limited to this, and depending on the configuration of an amplifier or smoothing circuit (not shown) connected to the output side of the switching circuit 4, the switch 6 may be is not necessarily required.

In addition, there is a parasitic capacitance in the resistance temperature detector 1, and the value of the parasitic capacitance is extremely large, especially when it contains a filler such as paraffin.
By selecting the frequency of the square wave constant current source 3 that satisfies equation (2), it is possible to omit the capacitor C in FIG. 1 in some cases.

Further, although the embodiment has been described with respect to a temperature measuring resistor, it is of course applicable to other resistors such as a strain gauge.

As described above, according to the present invention, it is possible to automatically compensate for the influence of wiring resistance by employing two-wire wiring, and there is no need to equalize the resistance values of the two wirings. , wiring work costs can be significantly reduced, wiring processing is easier, measurement accuracy is improved, and the circuit configuration is simple, consisting of only a DC constant current source, a square wave constant current source, a switching circuit, and a capacitor. , can be realized at low cost.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of an embodiment of the present invention, and FIGS. 2 and 3 are time charts of various signals for explaining the operation of the present invention. DESCRIPTION OF SYMBOLS 1...Resistance temperature sensor, 2...DC constant current source, 3...Square wave constant current source, 4...Switching circuit, 5, 6...Switch, L1, L2...Wiring, C...Capacitor.

Claims (1)

[Claims] 1 In a resistance/electrical signal converter that converts the resistance value of a resistor into an electrical signal corresponding to the resistance value and outputs it, a capacitor with an impedance value smaller than the resistance value of the resistor is connected in parallel to both ends of the resistor. A parallel connection circuit of resistors and capacitors that are connected to each other and placed at the resistance measurement point, and a DC constant power supply and a rectangular DC power supply that supply a constant current through one wire each to both ends of the upper and front parallel connection circuits. A constant wave current source receives the generated voltage of the parallel connection circuit obtained through the wiring as an input, and operates on and off in synchronization with the frequency of the square wave constant current source to determine the resistance value of the resistor. A resistance/electrical signal converter comprising a switching circuit that outputs an electrical signal corresponding to the above.