JPH0140397B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a signal conversion device suitable for constructing a non-isolated process control system using a two-wire transmitter.

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

When configuring multiple process control systems using the current transmission/voltage reception method using this two-wire transmitter, the circuit configuration of the devices that make up each control system is simple and inexpensive, so these devices can be easily configured. A method is used in which a direct current power supply is commonly supplied to a plurality of control systems as a power source, and the control system is directly driven without using a direct current isolation means.

FIG. 1 shows a conventional example of a non-insulated process control system.

In FIG. 1, an input resistor Ri is connected in series with a two-wire transmitter TR connected to input terminals 4 and 5. Both ends of this series circuit are used as power supply terminals 2 and 3,
A DC power supply 1 common to a plurality of control systems via a positive power supply line l1 and a negative power supply line l2, respectively.
are connected in parallel. A two-wire transmitter compatible with process physical quantities that operates by supplying, for example, 24 VDC as the power supply voltage Es from this DC power supply 1.
The output current Ii of the TR, for example 4 to 20 mADC, flows through the input resistor Ri as shown by the arrow in the figure. If the resistance value is 250Ω, for example, then the output current Ii, for example 4 to 20 mADC, flows between the output terminals 6 and 7, which are connected to both ends of the input resistor Ri. A voltage of 1-5VDC is generated. Here, terminals 2 to 7 and input resistance
The signal conversion device A is surrounded by a dashed-dotted line and constituted by Ri.

The receiving instrument group PC, which uses the generated voltage obtained from the output terminals 6 and 7 of the signal converter A as the process input signal PV, is also connected in parallel to the DC power supply 1 via the power lines l1 and l2, and is dependent on the power supply voltage Es. It is directly driven. Note that the receiving instrument group PC is composed of a plurality of devices necessary for configuring a control system, such as a PID controller, an alarm meter, and various arithmetic units (not shown). Control calculations for these devices are performed using a unified signal of 1 to 5 VDC, which is finally converted to a current signal Io of, for example, 4 to 20 mADC.
Send as shown by the arrow. The process output signal Io from the receiving instrument group PC provides control operations to operating devices VP such as control valves.

One control system is constituted by the two-wire transmitter TR, signal converter A, receiving instrument group PC, and controller VP described above. A similar control system is shown in the figure.
A plurality of process control systems are configured by connecting a plurality of them in parallel to the DC power supply 1 via power supply lines l1 and l2. And in order to determine the potential of the entire control system,
The power supply line l2 is grounded at a point P on the power terminal side of the DC power supply 1. In addition, in the figure, the plurality of control systems are illustrated with terminals of the signal conversion device omitted.

The process control system shown in FIG. 1 constructed using the conventional signal converter has the following disadvantages.

In other words, the two-wire transmitter TR is generally
It is installed in a remote location outdoors along with VP. For this reason,
Accidents such as ground faults and short circuits due to induced lightning or cross contact may occur on the two-wire transmitter side.

In Figure 1, for example, if there is a short circuit between a○ input terminals 4 and 5; an excessive current of Es/Ri = 24V/250Ω = 96mA will flow through the resistor Ri, but the power rating of the resistor Ri is 3 watts. If you do this, there will be no damage, and once it is restored it will work normally. Note that there is no problem with other control systems.

b○ In the case of a ground fault on the input terminal 5 side; Since both ends of the input resistor Ri are grounded and short-circuited, the process input signal PV will drop to zero, but once it returns, it will operate normally. Note that this does not affect other control systems.

c○ If there is a ground fault on the input terminal 4 side; both ends of the DC power supply 1 are grounded and short-circuited, causing a power down. All control systems stopped, and the DC power supply was damaged and did not recover even after it was restored.

Incidentally, by inserting a fuse between the terminals 2 and 4, it is possible to prevent a power down due to the blowing of the fuse, and to prevent other control systems from being affected. However, as is well known, the deterioration lifespan of fuses is short, so periodic replacement is required, which is undesirable as it reduces the reliability of the entire control system.

d○ If input terminals 4 and 5 are grounded at the same time; Same as item c○ above.

SUMMARY OF THE INVENTION An object of the present invention is to provide a signal conversion device that eliminates the drawbacks of the prior art described above and can improve safety and reliability when configuring a non-insulated process control system.

The gist of the present invention is to connect a resistor in series with the two-wire transmitter in a non-isolated process control system in which the two-wire transmitter is directly driven by a DC power supply common to a plurality of control systems. The voltage generated by passing the output current of the transmitter, which operates by connecting the DC power supply in parallel with the series circuit, through the resistor is converted back into a proportional current. The present invention is characterized in that the signal conversion device is configured to level shift the potential and send out an output signal having the same potential as the output of the transmitter.

The signal conversion device according to the present invention will be explained in detail using the embodiment shown in FIG. Note that in FIG. 2, the same parts as in FIG. 1 are indicated using the same reference numerals to avoid duplication of explanation.

In Fig. 2, the differences from Fig. 1 are the circuit configuration of signal converter B and the series circuit of the two-wire transmitter TR and input resistor Ri; It is connected to the line l2 side. Other components are the same as in FIG.

That is, the input resistor Ri is connected in series with the two-wire transmitter connected to the input terminals 4 and 5. Both ends of this first series circuit are used as power supply terminals 2 and 3,
positive power supply line l1 and negative power supply line l, respectively.
A DC power supply 1 common to a plurality of control systems is connected in parallel via 2. Also, connected to the power supply terminals 2 and 3 is a second series circuit consisting of a voltage-current conversion circuit 8 whose input is the voltage generated across the input resistor Ri and an output resistor Ro. Both ends of the output resistor Ro are connected to output terminals 6 and 7, respectively. That is, a parallel circuit of the first series circuit, the second series circuit, and the DC power supply 1 is formed. Here, one end of the first series circuit, that is, one end of the input terminal 5 to which one end of the transmitter TR is connected, and one end of the second series circuit, that is, one end of the output terminal to which one end of the output resistor Ro is connected. 7 and one of the power supply terminals 3 are connected in common, and the negative pole side of the DC power supply 1 is connected via the negative pole power line l2. The voltage-current conversion circuit 8 described above, the input resistance Ri, the output resistance Ro,
The signal conversion device B is the part surrounded by the dashed line and constituted by the terminals 2 to 7.

The operation of the signal conversion device B configured in this way will be explained.

For example, the output current Ii of the two-wire transmitter TR corresponding to the process physical quantity operated by the supply of 24 VDC as the power supply voltage Es from the DC power supply 1 connected to the power supply terminals 2 and 3 is 4 to 20 mADC.
For example, as the input low resistance Ri as shown by the arrow shown in the figure,
It flows through 250Ω and generates a voltage of 1 to 5VDC across it. With this generated voltage as input, the voltage/current conversion circuit 8 directly driven by the DC power supply 1 which operates with the power supply terminal 2 side as a reference potential converts it again into a current signal Is, for example, 4 to 20 mADC, as shown by the arrow in the figure. For example, it flows through 250Ω as the output resistor Ro, and a voltage of 1 to 5 VDC is generated across it. This generated voltage, whose potential level is shifted so that the power supply terminal 3, that is, the negative power supply line 12 side, is set as a reference potential, is inputted to the receiving instrument group PC as a process input signal PV via output terminals 6 and 7. In other respects, control calculations are performed in the same manner as in the embodiment shown in FIG. 1, and control operations are applied to the operating device VP.
In the embodiment shown in FIG. 2 as well, the plurality of control systems are illustrated with the terminals of the signal conversion device omitted.

In this way, in the process control system shown in Fig. 2 using the signal conversion device of the present invention configured to be able to send a process input signal of the same potential as the output of a two-wire transmitter, the two-wire Let's take a closer look at the case where a ground fault or short circuit similar to the example in Figure 1 occurs on the transmitter side.

If there is a short circuit between input terminals 4 and 5; Same as item a○ in Figure 1.

In the case of a ground fault on the input terminal 5 side; the same control operation as in normal times continues without causing any trouble to the process control system.

The PV signal does not drop to zero as in the example of section b○ in FIG.

If there is a ground fault on the input terminal 4 side; the mode will equivalently be the same as in item 1, and it will be the same as the example in item a○ in Figure 1.

Therefore, power down does not occur as in the example of item c in FIG. 1.

If the input terminals 4 and 5 are grounded at the same time, the mode will equivalently be the same as the above items and items, and the same as the example in item a○ in Figure 1.

Therefore, power down does not occur as in the example of item d○ in FIG. 1.

As described above, if the signal conversion device according to the present invention is applied to a process control system, even if a ground fault or short circuit accident occurs on the two-wire transmitter side, the DC power source 1
This prevents the DC power supply 1 from being short-circuited, causing a power down, stopping all of the multiple control systems, and damaging the DC power supply 1, so there is no need to use preventative fuses with short deterioration lifespans. ,
The reliability of the entire control system is improved. In addition, even if there is a ground fault on the input terminal 5 side as described in section
Since the PV signal does not drop to zero and the control operation can continue normally, it has the effect of improving safety compared to the conventional method.

In the embodiment shown in FIG. 2, the output terminals 6,
The PV signal from 7 was a voltage signal, but the output resistance
If Ro is removed from the circuit, it can also be sent as a current signal.

Further, in the embodiment shown in FIG. 2, the reference potential of the signal is set to the power line l2 side, that is, the negative electrode side of the DC power supply 1, but as shown in the embodiment shown in FIG. It can also be configured as the positive electrode side of. Note that in FIG. 3, the same parts as in FIG. 2 are indicated by the same reference numerals. Furthermore, in the figure, terminals of the signal converter and a plurality of control systems are omitted from illustration. Here, the functions and effects of FIG. 3 are the same as those of the embodiment of FIG. 2, and will be easily understood by those skilled in the art, so the explanation will be omitted.

FIG. 4 shows a specific example of the circuit configuration of the signal conversion device B according to the present invention shown in FIG. In FIG. 4, the same parts as in FIG. 2 are designated by the same reference numerals.
In Fig. 4, 2 connected to input terminals 4 and 5
An input resistor Ri and a zener diode ZD are connected in series with the wire transmitter TR, and both ends of this series circuit are used as power supply terminals 2 and 3, and the power supply voltage Es is supplied from the DC power supply 1 via wiring l1 and l2. for example
24VDC is supplied. As a result, 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, and the zener voltage Vz is applied across the zener diode ZD, and the zener voltage Vz is applied across the input resistor Ri.
The voltages Ri and Ii are generated respectively. Input resistance Ri
Input to the positive phase input terminals of operational amplifiers OP on both ends.
The output of the operational amplifier OP is connected to the base of the transistor Q, and the emitter is branched and connected to the intersection of the zener diode ZD and the input resistor Ri via the negative phase input terminal of the operational amplifier OP and the emitter resistor Re. , an output resistor Ro is connected to the collector. Here, voltage-current conversion circuit 8 is constructed with operational amplifier OP, transistor Q, and emitter resistor Re.
is configured. As a result, a current Is proportional to the voltage generated across the input resistor Ri is obtained from the collector of the transistor Q, and the output resistor
A level-shifted voltage signal can be obtained from the output terminals 6 and 7, which flow to Ro and are connected at both ends. In the embodiment of FIG. 4 configured in this way, the output current Ii of the transmitter TR
4 to 20mADC, input resistance Ri, emitter resistance Re,
All output resistances Ro are equal Ri=Re=Ro=250Ω
If selected, the output current Is of the voltage-current conversion circuit 8
4 to 20 mADC, and a voltage signal of 1 to 5 VDC, which has the same potential as the transmitter output, can be obtained from the output terminals 6 and 7. Here, the positive and negative power supply terminals of the operational amplifier OP are connected to power supply terminals 2 and 3, and the power supply lines l1 and l
Although it is directly driven by the DC power supply 1 via 2,
Operational amplifier OP is Zener diode ZD and input resistor
Since it operates using the intersection point of Ri as a reference potential, it is equivalent to driving with two positive and negative power supplies, a positive voltage Vz and a negative voltage - (Es - Vz). Although a Zener diode was used to obtain the voltage Vz in the embodiment shown in Fig. 4, an operational amplifier that operates on a low-level power supply was selected, and the constant bias level of the signal was set to the lower limit power supply operating level of the operational amplifier. If a stable operating level of the operational amplifier OP can be ensured by selecting the values of the current Ii and the resistance Ri so as to be larger than the zener diode ZD, the zener diode ZD is not necessarily required.

As described above, the signal conversion device according to the present invention can be realized at low cost with a simple circuit configuration, and when applied to a non-isolated process control system using a two-wire transmitter, It has improved reliability and safety, achieving all of the initial objectives, and is useful for industrial measurement.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment in which a control system is configured by applying a conventional signal conversion device, and FIGS. 2 and 3 are examples in which a control system is configured by applying a signal conversion device according to the present invention. FIG. 4 is a diagram showing a specific embodiment of the signal conversion device according to the present invention. 1... DC power supply, 2, 3... Power terminal, 4, 5... Input terminal, 6, 7... Output terminal, 8... Voltage/current conversion circuit, TR... 2-wire transmitter, Ri... Input resistance, Ro... Output resistance , Re...emitter resistance, PC...receiving instrument group,
VP...actuator, PV...process input signal, Io...process output signal, Es...power supply voltage, OP...operational amplifier, Q...transistor, ZD...zener diode,
l1...Positive side power line, l2...Negative side power line, P...Grounding point, B...Signal conversion device.

Claims (1)

[Claims] 1. In a process control system configured using a two-wire transmitter, the two-wire transmitter is connected between the input terminals, a resistor is connected in series to this two-wire transmitter, and the two-wire transmitter is A DC power supply is connected in parallel to a series circuit of a transmitter and a resistor, a voltage-current conversion circuit that converts the voltage across the resistor into a current is connected to the resistor, and the output end of this voltage-current conversion circuit is connected to the DC power supply. A signal conversion device characterized in that: