JPH0124705Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a signal conversion device particularly suitable for process control systems with large pulsations.

2. Description of the Related Art In industrial measuring equipment, there is a two-wire transmitter that converts a process physical quantity into an electrical signal, and uses two wires for both power supply and electrical signal transmission.

When using this two-wire transmitter to configure a process control system, the pulsation is extremely large when measuring the pressure and flow rate of systems that include compressors, pumps, and blowers, making the control system unstable if left as is. This not only causes fatigue to the measuring equipment, but also makes control itself impossible.

Possible methods for suppressing this are a mechanical method using an aperture mechanism, etc., and an electrical method using a time constant circuit. However, the former is a technical problem, and the latter is an intrinsically safe and explosion-proof problem.
In either method, it is currently difficult to incorporate a satisfactory suppression function into a two-wire transmitter.

Conventionally, a countermeasure against this problem has been to add a first-order delay calculator using an electrical method to the control system. This method had the following disadvantages.

That is, the pulsation process occurs not only in the above-mentioned system but also due to plant installation conditions, piping conditions, etc., and is often unspecified, and cannot be determined unless confirmed by a test run. However, during trial operation, the design of the instrumentation panel that makes up the control system, equipment installation, wiring, etc., have all been completed, and adding a new primary delay calculator on-site will cause problems such as space and modification wiring. , it is often not possible to add them easily. Furthermore, adding first-order delay calculators to all anticipated control systems in advance would be expensive, and is not a special solution.

On the other hand, when configuring a process control system using a two-wire transmitter, a power source is required to drive the two-wire transmitter. Further, since the circuit configuration of the receiving device constituting the control system is simple and can be realized at low cost, and it is easy to exchange signals with a computer, a voltage signal of, for example, 1 to 5 VDC is used as the control signal.
Therefore, a signal conversion function is required to convert a current signal from a two-wire transmitter, for example, 4 to 20 mADC, into a voltage signal.

An object of the present invention is to eliminate the drawbacks of the prior art described above and to provide a signal conversion device having a pulsation suppressing function and a power supply function.

Therefore, the present invention converts the signal from a two-wire transmitter into a single voltage signal, passes it through a first-order delay time constant circuit, and then converts it back into a current signal using a voltage-current conversion circuit consisting of an operational amplifier and a transistor. These circuits are configured to operate with a single power source.

Hereinafter, one embodiment of the present invention will be described in detail based on the drawings.

In FIG. 1, 1 and 2 are input terminals, to which a two-wire transmitter TR is connected. Then, the Zener diode ZD, the resistor Ri, and the transmitter TR are connected in series in this order, and one end of the Zener diode ZD is connected to the positive side of the DC power supply 3 via the wiring l1, and the transmitter
One end of the TR is connected to the negative electrode side of the DC power supply 3 via the input terminal 2 and the wiring 12. With this, for example, the DC voltage Es from the DC power supply 3 is
24VDC is applied to the series circuit, and the output current Ii from the transmitter TR corresponding to the physical quantity flows through the series circuit as shown by the arrow in the figure, so that a zener voltage Vz is applied across the zener diode ZD, and Ri is applied across the resistor Ri.・Ii
Each voltage is generated. The voltage generated across the resistor Ri is transmitted through a first-order delay time constant circuit for suppressing pulsation, which consists of a variable resistor Rv and a capacitor C.
Reference potential is the intersection of Zener diode ZD and resistor Ri
Input to the positive phase input terminal of operational amplifier OP, which operates as CM. Here, in order to prevent errors from occurring due to signal attenuation due to changes in the resistance of the variable resistor Rv, it is desirable that the operational amplifier OP be an IC that uses a field effect transistor with a high input resistance in the first stage.
Further, the output of the operational amplifier OP is connected to the base electrode of the PNP transistor Q1, and the emitter electrode is branched and connected to the reference potential CM via the negative phase input terminal of the operational amplifier OP and the resistor Re. Further, the collector electrode is connected to the wiring 12 via a resistor Ro, and both ends of the resistor Ro are connected to output terminals 4 and 5, respectively. Here, the input terminal 2 and the output terminal 5 are commonly connected by the wiring 12, and the above-mentioned [Requirement 4] is achieved. In addition, the positive and negative power supply terminals of the operational amplifier OP are directly connected to the DC power supply 3 via the wiring l1 and l2, but since the operational amplifier OP operates based on the reference potential CM, the positive voltage Vz
This is equivalent to driving with two positive and negative power supplies with negative voltage - (Es - Vz).

In the embodiment of FIG. 1 constructed in this manner, the collector current Io of the transistor Q1 is given by equation (1).

Io=[1/1+TSRi・Ii]/Re...(1) Here, T is a time constant, T=Rv・C, and S is a Laplace transform operator. Further, the output voltage Eo of the output terminals 4 and 5 is given by equation (2).

Eo=Ro・Io...(2) In equation (1), the values of resistance Ri and resistance Re are set equal to Ri
If =Re is selected, equation (3) is obtained.

Io=[1/(1+TS)]Ii…(3) Therefore, we obtain equation (4) from equations (2) and (3).

Eo = [1/(1+TS)] Ro・Ii...(4) In equations (3) and (4), for example, the output current Ii of the two-wire transmitter TR is 4 to 20 mADC, and the values of the resistors Ri, Re, and Ro are Ri = Re = Ro = 250Ω, the value of variable resistor Rv is 200kΩ, and the value of capacitor C is 100μF.
If selected, the time constant 0 to 20 from output terminals 4 and 5
1 with first-order delay calculation that can be continuously variable up to seconds
You can get an output voltage Eo of ~5VDC. The value of this time constant is a value that can sufficiently suppress pulsations in the process. Note that the value of the time constant can be freely selected by changing the constant of the variable resistor Rv or the capacitor C. Furthermore, if the resistor Ro is removed from the circuit, an output current Io of 4 to 20 mADC, which is equal to the output current Ii of the transmitter, can be obtained at the same potential.

In the embodiment shown in FIG. 1, a zener diode ZD is used to obtain the voltage Vz, but the invention is not limited to this, as long as the power supply voltage level that allows the operational amplifier OP to operate stably can be secured. A voltage generating element such as a diode or a resistor that generates a voltage by flowing current can be used.

In addition, a variable resistor Rv was used to set the time constant, but a fixed resistor or a wire (chiyoke coil)
Any device that can set the time constant of the first-order lag may be used.

In addition, in the embodiment shown in FIG. 1, the input/output reference potential is configured to be on the wiring l2, that is, on the negative electrode side of the DC power supply 3, but as shown in the embodiment shown in FIG. It is also possible to configure the positive electrode side of the input/output reference potential to be the input/output reference potential. Note that in FIG. 2, the same parts as in FIG. 1 are indicated by the same symbols. In the figure, it is only necessary to change the transistor to NPN transistor Q2 as the transistor for converting into current, and the other operations are the same as in Figure 1, and will be easily understood by those skilled in the art, so the explanation will be omitted. .

As described above, according to the present invention, it is possible to obtain a signal conversion device having a pulsation suppressing function and a power supply function. Further, since one of the input terminal and the output terminal is commonly connected, no problem occurs even if the detection end (two-wire transmitter) and the reception end or operation end of the process control system are grounded at two points at the same time.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of a signal converting device according to the present invention, and FIG. 2 is a diagram showing another embodiment of the signal converting device according to the present invention. 1, 2... Input terminal, 3... DC power supply, 4, 5
...Output terminal, TR ...2-wire transmitter, Ri, Re,
Ro...Resistor, Rv...Variable resistor, C...Capacitor, ZD...Zener diode, OP...Operation amplifier, Q1, Q2...Transistor, l1...
Positive side power wiring, l2... Negative side power wiring, CM...
Reference potential of OP.

Claims (1)

[Scope of utility model registration request] In a process control system configured using a two-wire transmitter, the two-wire transmitter is connected between input terminals, and a series circuit of a first resistor and a voltage generating element is connected to the two-wire transmitter. a DC power supply is connected in parallel to this series circuit, a time constant circuit is connected to the first resistor, and an operational amplifier is connected to the time constant circuit, the positive and negative power supply terminals of which are connected to both ends of the DC power supply. A positive phase input terminal is connected to the operational amplifier, an intersection point between the first resistor and the voltage generating element is connected via a second resistor to the negative phase input terminal of the operational amplifier, and a transistor is connected to the output terminal of the operational amplifier. Connect the base electrode,
A signal converting device characterized in that the emitter of the transistor is connected to the negative phase input terminal of the operational amplifier and the second resistor, and the collector of the transistor is connected to the DC power supply.