JP5115406B2 - Signal input circuit - Google Patents

Description

The present invention relates to a signal input circuit used for checking a valve opening of an electric positioner, and more particularly to a signal input circuit capable of detecting a potentiometer type and a current input type with a single circuit without special operation.

In general, in a control device that adjusts the valve opening with a controller, a potentiometer linked to the valve opening is provided to prevent the discrepancy between the indicated value and the actual valve opening. Check whether the control by the controller is properly performed based on the signal (voltage).

In recent years, in addition to a potentiometer type that generates such a voltage, a type that generates a current has also been distributed. However, these two signals cannot normally be received by the same circuit. Prior art documents of such a potentiometer include the following.

Japanese Patent Laid-Open No. 57-141565

Hereinafter, a conventional signal input circuit will be described with reference to FIG. FIG. 2 shows a potentiometer type signal input circuit. The potentiometer 10 is a friction element linked to a valve opening (not shown), and the resistance value changes depending on the opening of the valve. The potentiometer 10 has a resistance value of about 100Ω, for example.

A terminal a111, a terminal b112, and a terminal c113 are connected to the potentiometer 10, and are used for voltage measurement as follows.

The terminal a <b> 111 is connected to one end of the potentiometer 10, and the reference voltage A is input through the buffer amplifier 200. A voltage corresponding to the opening of the potentiometer 10 is applied to the terminal b112. Therefore, for example, if the reference voltage A is 0.6V and the reference voltage A is divided at a ratio of 50:50 based on the opening of the potentiometer 10, a voltage of 0.3V is applied to the terminal b112. The terminal c is connected to the other end of the potentiometer 10.

In the buffer amplifier 200, the reference voltage A is supplied to the + input terminal by the voltage source 210, and this voltage is output to the terminal a111. The voltage source 210 supplies, for example, a voltage of 0.6 to the + input terminal of the buffer amplifier 200.

The voltage divided by the potentiometer 10 is supplied to the A / D conversion unit 300 via the terminal b112, and this voltage is converted into a digital value. Further, since the resistance value of the potentiometer 10 is normally about 100Ω, the input of the A / D conversion unit 300 needs to have a high impedance.

  Next, a signal input circuit of a current (for example, 4 mA-20 mA) input type will be described with reference to FIG. The terminal d121 is a + side terminal, and the terminal e122 is a − side terminal. Further, in this current input type, a current corresponding to the opening of the valve is output (for this reason, it is represented by the current source 20 in FIG. 3), and the voltage is generated by the current flowing through the resistor 230 via the terminal d121. And the voltage is supplied to the A / D converter 300.

Here, the resistor 230 usually has a resistance value of several tens Ω to 250 Ω. Therefore, when viewed from the terminal d121, the resistance value of the resistor 230 becomes the input impedance of the A / D conversion unit 300.

  Thus, the potentiometer type signal input circuit and the current input type signal input circuit are usually provided separately. A resistor and a switch (not shown) are provided between the terminal b112 and the terminal c113 in FIG. 2, and when the potentiometer is input, the switch is turned off and the terminal b112 and the terminal c113 are disconnected. It is considered that the signal input circuit of FIG. 3 can be configured by one input circuit.

  However, in this configuration, the on-resistance of the switch becomes an error at the time of current input, so that an accurate voltage is not supplied to the A / D conversion unit 300. Therefore, in order to enable these two types of inputs to be received by the same circuit in the past, it is necessary to separately provide at least a terminal dedicated to current as shown in FIG.

  Here, the signal input circuit of FIG. 4 will be described. However, the components already described in FIGS. 2 and 3 are denoted by the same reference numerals and description thereof is omitted. The potentiometer circuit 500 is a circuit in a range surrounded by an alternate long and short dash line, which is similar to the potentiometer type signal input circuit of FIG. When the potentiometer circuit 500 operates, the switch 240 is turned off.

  The current input circuit 510 is a circuit in a range surrounded by a chain line, and is the same as the current input type signal input circuit of FIG. Further, when the current input circuit 510 operates, the switch 240 is turned on.

As described above, in the input circuit of FIG. 4 in which the potentiometer type signal input circuit and the current input type signal input circuit are configured as one circuit, the number of terminals is four.

However, each of the input circuits shown in FIGS. 2 and 3 requires a dedicated input circuit. On the other hand, although the input circuit of FIG. 4 can receive a potentiometer type signal and a current input type signal with one input circuit, four or more terminals are required, and the terminal to be connected must be changed depending on the type of input. Also, switching of the switch 240 is not automatically performed by determining the input type.

The present invention has been made in view of these problems, and an object thereof is to provide a signal input circuit that automatically switches between a potentiometer type input signal and a current type input signal.

In order to achieve such a problem, the present invention has the following configuration.
(1) In a signal input circuit for receiving a potentiometer type voltage obtained by dividing an applied voltage and a current input type signal having a lower limit value by an A / D converter,
A resistor that converts the current input type signal to a voltage when the current input type signal is input and generates a voltage equal to the applied voltage when the potentiometer type signal is input;
The applied voltage is smaller than a voltage value at which the current of the lower limit value is converted by the resistor;
A signal input circuit that switches a signal supplied to the A / D converter according to a voltage value generated in the resistor .
(2) A signal including an amplifier that outputs a first reference voltage to a potentiometer that divides the voltage, and a resistor that converts a current output from a current source whose current lower limit is not zero into a voltage. In the input circuit,
An A / D converter to which a voltage applied to the resistor or a voltage of the potentiometer is input;
A diode that prevents backflow of current to the amplifier when the voltage across the resistor is greater than the first reference voltage;
A comparator that compares a second reference voltage set higher than the first reference voltage with a voltage applied to the resistor or the first reference voltage, and outputs a signal when the second reference voltage is small; ,
A switch for supplying a voltage applied to the resistor to the A / D converter when a signal is input from the comparator.
(3) The signal input circuit according to claim 2, wherein the resistance value of the resistor is a value for converting the current output from the current source into a voltage higher than the first reference voltage.
(4) The resistor is two resistors connected in series,
4. The signal input circuit according to claim 2, wherein one end of the switch is connected to the A / D converter and the other end is connected to a connection point of the two resistors.
(5) The resistance values of the two resistors are such that (voltage of the potentiometer) and (voltage applied to the connection point of the two resistors when the switch is on) are relatively close to each other. The signal input circuit according to claim 4, wherein the signal input circuit is selected.

The present invention has the following effects. In this way, the voltage range of the potentiometer type and the current input type is divided, and the point that is input to the A / D converter by the comparator is switched using this voltage difference, so that the potentiometer type can be used without any special operation. Each signal of current input type can be received.

Hereinafter, a configuration example of a signal input circuit according to the present invention will be described with reference to FIG. However, the components described in FIGS. 2 to 4 are denoted by the same reference numerals and description thereof is omitted. The resistor 251 and the resistor 252 are connected in series between the terminal a111 and the terminal c113. Further, it is assumed that the resistance value of the resistor 251 is 200Ω, for example, and the resistance value of the resistor 252 is 50Ω, for example.

Further, (a connection point between the resistor 251 and the resistor 252) and the terminal b112 are connected via the switch 241.

The diode 260 has an anode connected to the output of the buffer amplifier 200 and a cathode connected to the terminal a111. The diode 260 is provided to prevent a reverse current flow.

The comparator 270 has a + input terminal connected to the connection point C, and a reference voltage B (for example, 0.8 V) is supplied from the voltage source 211 to the − input terminal. Then, by comparing the reference voltage B and the voltage applied to the + input terminal, a signal is output and the switch 241 is switched.

Next, the operation will be described. The explanation of the operation is
(1) Operation when receiving voltage generated by potentiometer 10 (2) Operation when receiving current generated by current source 20 (3) Switching operation by comparator 270 will be described in this order.

(1) An operation when receiving a voltage generated by the potentiometer 10 will be described. First, the switch 241 is turned off by a signal output from a comparator 270 described later.

The reference voltage A is supplied to the terminal a111 via the buffer amplifier 200. Further, a voltage corresponding to the opening of the potentiometer 10 is applied to the terminal b112.

Therefore, for example, if the reference voltage A is 0.6V and the reference voltage A is divided at a ratio of 50:50 based on the opening of the potentiometer 10, a voltage of 0.3V is applied to the terminal b112. For this reason, a voltage of 0.3 V is input to the A / D converter 300, and the A / D converter 300 converts this voltage into a digital signal.

The reference voltage A is also applied to the resistor 251 and the resistor 252 connected in parallel with the potentiometer 10. However, when the voltage generated by the potentiometer 10 is received by the A / D converter 300, the voltage applied to the terminal b112 becomes a problem, and what voltage is applied to these resistors 251 and 252? When the voltage generated by the potentiometer 10 is received by the A / D converter 300, no problem occurs.

Next, (2) an operation when receiving a current generated by the current source 20 will be described. First, the switch 241 is turned on by a signal output from a comparator 270 described later. In the case of the current input type, since the potentiometer 10 is not used, the connection between the terminal b112 and the potentiometer 10 is open.

The current generated by the current source 20 flows to the terminal c113 via the terminal a111, the resistor 251, and the resistor 252. At that time, the current is converted into a voltage by the resistor 251 and the resistor 252. When the resistance value of the resistor 251 is 200Ω and the resistance value of the resistor 252 is 50Ω, the voltage is divided by 4: 1.

Therefore, for example, if a current of 4 mA flows from the current source 20, a voltage of 1V is applied to the terminal a111, and a voltage of 0.2V is input to the A / D converter 300. The A / D converter 300 converts this voltage into a digital value.

Since the voltage is divided by the resistors 251 and 252 in this way, a value (0.2 V) relatively close to 0.3 V output from the potentiometer 10 can be input to the A / D conversion unit 300, and A / Even if the dynamic range of the D converter is narrow, it can be used without any problem.

Next, (3) the switching operation by the comparator 270 will be described. As described above, the current input type signal input circuit operates at 4 mA to 20 mA. That is, at least 4 mA flows in the current input type.

Therefore, a case where a current of 4 mA which is the lower limit value flows from the current source 20 will be described as an example. First, the reference voltage A is set to 0.6V, and the reference voltage B is set to 0.8V.

Furthermore, if the total resistance value of the resistor 251 and the resistor 252 is 250Ω, a voltage of 4 mA * 250Ω = 1V is generated at the connection point C. Note that since the diode 260 is provided at the output of the buffer amplifier 200 that outputs the reference voltage A, even if the voltage applied to the terminal a111 is 1V, which is higher than the reference voltage A, the current does not flow backward.

Therefore, the potential at the connection point C is 1V. The comparator 270 compares the potential of 1V with the reference voltage B of 0.8V, determines that it is higher than the reference voltage B, and outputs a signal for turning on the switch 241.

Needless to say, when a current of 20 mA flows from the current source 20, the potential at the connection point C becomes higher than the reference voltage B, so that the switch 241 is similarly turned on.

On the other hand, when the current input type is not used (that is, the potentiometer type), no current is generated from the current source 20. Accordingly, the potential at the connection point C in this case is the reference voltage A supplied via the buffer amplifier 200.

Since the reference voltage B is set to 0.8 V and the reference voltage A is set to 0.6 V as described above, the potential at the connection point C does not exceed the reference voltage B, and the potentiometer type In this case, a signal for turning on the switch 241 is not output from the comparator 270.

In this embodiment, the case where the reference voltage A is 0.6 V and the reference voltage B is 0.8 V has been described as an example. However, when a lower limit current (4 mA) flows, the potential of the connection point C> reference The reference voltage A and the reference voltage B can be set within a range satisfying the relationship of voltage B> reference voltage A.

Further, in this embodiment, the case where the resistance value of the resistor 251 is 200Ω and the resistance value of the resistor 252 is 50Ω has been described as an example, but when the lower limit current (4 mA) flows, the potential at the connection point C If a resistance value such that the reference voltage A is selected is selected, a diode 260 for preventing backflow is provided, so that (a terminal a111 for applying the voltage of the potentiometer 10) and (a positive side terminal of the current source 20, that is, a terminal) Even if a111) is shared, a current input type can be used without being influenced by the reference voltage A.

When the dynamic range of the A / D converter 300 is wide, the resistor 252 may be 250Ω and the resistor 251 may not be connected (that is, the connection point C and one end of the resistor 252 are short-circuited). .

According to the example described above, a voltage of 1 V is supplied to the A / D converter 300 in the case of the current input type, and a voltage of 0.3 V is supplied to the A / D converter 300 in the case of the potentiometer type. Therefore, the input level of the A / D conversion unit 300 differs depending on the input type.

In this way, the voltage range of the potentiometer 10 type and the current input type is divided, and the point to be input to the A / D converter 300 is switched by the comparator 270 using this voltage difference, so that there is no special operation. The potentiometer 10 type and current input type signals can be received. Further, the number of terminals does not increase as in the conventional example of FIG. 4, and it is not necessary to use the terminals properly.

It is a block diagram of the signal input circuit of this invention. It is a block diagram of the conventional potentiometer type signal input circuit. It is a block diagram of a conventional current input type signal input circuit. This is a conventional signal input circuit capable of receiving both potentiometer type and current input type signals.

Explanation of symbols

10 Potentiometer 20 Current source 111 Terminal a
112 Terminal b
113 Terminal c
200 Buffer Amplifier 210 Voltage Source 211 Voltage Source 241 Switch 251 Resistor 252 Resistor 260 Diode 270 Comparator 300 A / D Converter

Claims (5)

In a signal input circuit that receives a potentiometer type voltage obtained by dividing an applied voltage and a current input type signal having a lower limit value by an A / D converter,
A resistor that converts the current input type signal to a voltage when the current input type signal is input and generates a voltage equal to the applied voltage when the potentiometer type signal is input;
The applied voltage is smaller than a voltage value at which the current of the lower limit value is converted by the resistor;
A signal input circuit that switches a signal supplied to the A / D converter according to a voltage value generated in the resistor . In a signal input circuit comprising: an amplifier that outputs a first reference voltage to a potentiometer that divides a voltage; and a resistor that converts a current output from a current source whose current lower limit value is not zero into a voltage. ,
An A / D converter to which a voltage applied to the resistor or a voltage of the potentiometer is input;
A diode that prevents backflow of current to the amplifier when the voltage across the resistor is greater than the first reference voltage;
A comparator that compares a second reference voltage set higher than the first reference voltage with a voltage applied to the resistor or the first reference voltage, and outputs a signal when the second reference voltage is small; ,
A switch for supplying a voltage applied to the resistor to the A / D converter when a signal is input from the comparator.   The signal input circuit according to claim 2, wherein the resistance value of the resistor is a value for converting a current output from the current source into a voltage higher than the first reference voltage. The resistors are two resistors connected in series,
4. The signal input circuit according to claim 2, wherein one end of the switch is connected to the A / D converter and the other end is connected to a connection point of the two resistors. The resistance values of the two resistors are selected so that (voltage of the potentiometer) and (voltage applied to the connection point of the two resistors when the switch is on) are relatively close to each other. 5. The signal input circuit according to claim 4, wherein:

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