WO2011029699A1

WO2011029699A1 - Device for transmitting a control signal

Info

Publication number: WO2011029699A1
Application number: PCT/EP2010/061954
Authority: WO
Inventors: Stephan Konrad; Rainer Pöllath
Original assignee: Endress+Hauser Wetzer Gmbh+Co. Kg
Priority date: 2009-09-09
Filing date: 2010-08-17
Publication date: 2011-03-17
Also published as: DE102009029307A1

Abstract

The invention relates to a device for the metallically isolated transmission of a control signal from a primary unit (1) to a secondary unit (2), wherein the primary side (1) generates two complementary control signals S+, S- for controlling a switching contact (3), wherein the secondary unit (2) comprises the switching contact (3) and a control unit (4), wherein there is at least one first capacitor (51) and one second capacitor (52), wherein the first capacitor (51) transmits the control signal S+ for closing the switching contact (3) to the control unit (4), wherein the second capacitor (52) transmits the control signal S- for opening the switching contact (3) to the control unit (4), and wherein the control unit (4) closes or opens the switch contact (3) depending on the control signal S+, S- that is present.

Description

Device for transmitting a control signal

The present invention relates to a device for galvanically isolated

Transmission of a control signal from a primary unit to a secondary unit. The device is integrated, for example, in a field device of process automation, which monitors a process parameter and provides a switching signal at a signal output for transmission to an actuator, for example a valve. The primary unit comprises, for example, a sensor element and a control / evaluation unit and the secondary side represents, for example, the interface to a higher-level control unit.

Often it is desirable for safety reasons that the

Signal transmission from a primary side to a secondary side via a

galvanic isolated interface is possible. This is the case, for example, if a high voltage is applied to the primary side, but the secondary side may only have a low voltage, for example because it is in

explosive environment is located and the danger is to be prevented that from the secondary side contact to the high voltage of the primary side can be made.

Various configurations of galvanically separated interfaces are known from the prior art.

In the case of an optocoupler, a light signal is emitted from the primary side to the secondary side, as a result of which the switching process is activated on the secondary side. With this principle, a signal can be transmitted, but no energy, so that in the event that on the secondary side energy is needed, it must be provided via a separate interface. In addition, an optocoupler requires a lot of energy, so that these systems are only used when the high

Energy requirement of the optical transmission system plays no role. If not much energy is available, alternative methods for galvanically separated signal transmission are preferred. One of these alternative implementations of a galvanically isolated switch includes a transformer. Transformers or inductive interfaces require little energy. However, they have some disadvantages. For one, they are expensive and take up a relatively large amount of space. The latter is a major disadvantage in many systems. On the other hand, additional voltage-limiting components are usually necessary, which limit voltage peaks that occur when switching off or in the event of a fault. This is especially true in applications in

Hazardous areas inevitable. Another conceivable method is the use of a capacitor for

Transmission of a control signal. However, this has the considerable disadvantage that electromagnetic interference influences the control signal in the transmission such that the state of the switch, which by the

Control signal is controlled, can not be reliably adjusted. For example, low voltages induced by interfering fields on the capacitor lead to unwanted signals, which cause an undesirable change in the switching state.

The object of the invention is to provide a device for galvanically isolated signal transmission, which is inexpensive and robust against electromagnetic interference.

The object is achieved by a device for galvanically isolated

Transmission of a control signal from a primary unit to a secondary unit, wherein the primary side generates two complementary control signals S +, S. for controlling a switching contact, wherein the secondary unit comprises the switching contact and a control unit, wherein at least a first capacitor and a second capacitor are provided, wherein the first capacitor transmits the control signal S + to close the switch contact to the control unit, wherein the second capacitor, the control signal S to open the switch contact to the

Transmits control unit and wherein the control unit in dependence of the applied control signal S +, S. closes or opens the switching contact. By concern of a control signal S +, S. is here to be understood that a non-zero voltage value is applied. If the voltage value of the control signal is zero, this is equivalent to the fact that no control signal is present. The device according to the invention consists of a few components, which are also standard components. This makes the device inexpensive and space-saving.

The fact that both the closing of the switch and the opening of the

Switch is controlled by an active signal, the device is robust

against electromagnetic interference and thus works very reliable.

Another advantage of the device according to the invention is that it has only a low energy requirement. This leads to the lower costs in addition to

Energy supply of the device to the advantage that they are also in

Explosive environments can be used.

Compared with an optical interface, as known from the prior art, the device offers a further advantage. Due to the capacitors, in addition to the signal transmission, the transmission of energy to the secondary side is possible, so that it does not require a separate power supply. This saves space and is very cost effective.

In a first embodiment of the solution according to the invention, the primary side has a processing unit for generating the control signals S +, S from a primary switching signal Si. The primary switching signal is for example one

Square-wave voltage. In the simplest case, the conditioning unit inverts the primary switching signal Si and forwards the unchanged primary switching signal Si as the control signal S ₊ to the first capacitor and the inverted primary switching signal Si as the control signal S to the second capacitor. As a result, the one capacitor is charged to which a non-zero voltage value is applied, while no voltage is applied to the other capacitor. According to one embodiment of the invention, at least one third capacitor is connected in series with the first capacitor and is at least a fourth

Capacitor connected in series with the second capacitor, so that their respective insulation voltages add up to a total voltage U. In this way, a higher electrical isolation can be achieved.

In a further development of the invention, a feedback device is provided on the secondary side, which serves to control the switching state of the switching contact. In this way, a diagnostic function is feasible, which is for devices that must meet certain safety specifications, a great advantage.

In one embodiment, the feedback device transmits the information about the switching state of the switching contact via a capacitive interface to the primary side. As a result, the information transmission of the feedback device is also galvanically separated. The capacitive interface can also be constructed of several capacitors.

In an advantageous embodiment of the invention, the capacitors in addition to the control signal S +, S. also transfer energy to the secondary side.

The control signals S ₊ , S. can be superimposed in this case, for example, a fundamental voltage as modulations. This embodiment is particularly advantageous since no further interface for energy transmission has to be provided. A development of the solution according to the invention is that of

Switching contact has the function of a pulse generator for transmitting the information contained in the primary switching signal Si.

The device is part of a measuring device, for example. In this case, the switching contact serves, for example, to pass on a measuring signal determined by a measuring instrument to a field device for displaying the measured value. According to one embodiment, the switching contact is connected to a device which is switched on or off depending on the switching state of the switching contact.

In this way, e.g. an actuator in response to a process variable, which forms the basis of the primary switching signal control. An application example is a limit switch with a valve.

In an advantageous embodiment, the device according to the invention is part of a measuring device for determining and / or monitoring a physical, biological or chemical process variable, a transmitter or a

Field scoreboard. Such measuring and field devices are offered and distributed by the applicant in a wide variety.

The invention will be explained in more detail with reference to the following figures.

Fig. 1 shows a galvanically isolated switch with two capacitors;

Fig. 2 shows a galvanically isolated switch with two capacitors and a feedback device.

In Fig. 1, a galvanically isolated switch with two capacitors 51, 52 is shown. This consists of a primary side 1, where a switching signal is generated, and a secondary side 2 with a switching contact 3, whose switching state is regulated according to the switching signal. The switch is part of, for example, a process automation or monitoring field device, such as a transmitter, a display element, or a limit switch.

On the primary side 1 there are, for example, a sensor for determining a process variable and an evaluation unit which compares the determined process variable with a desired value and generates a primary switching signal Si in dependence thereon. In response to this primary switching signal Si changes

Switching contact 3 on the secondary side 2 its switching state. The switching contact 3 can in this case have the function of an on / off switch or a pulse generator. In one exemplary embodiment, the current supply of a device connected to the secondary side 2 is regulated by the switching contact 3, so that the device can be switched on and off by the switching contact 3. In an alternative

Embodiment, the switching contact 3 is a pulse generator, which forwards the primary switching signal Si in the form of a pulse to an interface, and thus makes available, for example, a further field device connected to the interface.

The primary side 1 is galvanically isolated from the secondary side 2, wherein the

Interface for transmitting the primary switching signal Si from the primary side 1 to the secondary side 2 by two capacitors 51, 52 is realized.

If a control signal S + is present in the form of a voltage at the first capacitor 51, then the switching contact 3 is closed. Is located on the second capacitor 52 a

Control signal S. in the form of a voltage, then the switching contact 3 is opened. Because an active signal is required both for closing and for opening the switching contact 3, the switching device is insensitive to electromagnetic interference fields. An accidental switching of the switching contact 3 is thereby avoided.

In order to increase the galvanic isolation, the separation voltage can be increased by connecting the first capacitor 51 and the second capacitor 52 in series with third and fourth capacitors, respectively. The number of capacitors connected in series can also be further increased in this case.

To adapt the switching signal to the interfaces so that an active

Control signal S + for opening and a second active control signal S is generated for closing, which are suitable to be transmitted by the two capacitors 51, 52, the capacitors 51, 52 is preceded by a conditioning unit 6. This generates two complementary control signals S +, S. and acts on the capacitors 51, 52 with selbigen. By way of example, the original primary switching signal Si is a rectangular signal. This is forwarded unchanged or with a certain amplification factor amplified to the first capacitor 51. The second capacitor 52 receives the inverted control signal S .. On the secondary side 2 is a control unit 4, which processes the voltages applied to the two capacitors 51, 52, generates a secondary switching signal S2 therefrom and transmits the secondary switching signal S2 to the switching contact 3. Fig. 2 discloses a feedback device 7 for checking the switching state of the switching contact 3. By such a device, it is possible that

Primary page 1 to make the information about the switching state of the switch contact 3, so that it can be checked whether the control signals S +, S. were correctly transmitted from the primary side 1 to the secondary side 2. This functional test is advantageous, for example, if the device is part of a device which has to meet certain safety requirements. An example of this is compliance with the specifications of a safety level according to the standard IEC 61508 or IEC 62061, a so-called SIL (Safety Integrity Level).

For reading out the switching state of the switching contact 3, a measuring shunt 9 is connected to the switching contact 3.

A transmission unit 10, which is connected to the measuring shunt 9 and the control unit 4, reads out the switching state of the switching contact 3 and generates a feedback signal. The feedback signal is preferably transmitted to the primary side 1 via a further capacitive interface 8. On the primary side 1 is a

Microcontroller arranged which receives the feedback signal and evaluates, for example by the primary switching signal S1 with the

Feedback signal compares. In this case, the microcontroller can be the same component which gives the command for switching over the switching contact 3. If it results in the evaluation of the switching state of the switching contact 3, that this is in the wrong state, for example, an error message is generated and output. Preferably, the measuring device, in which the galvanically isolated switch is integrated, additionally converted into a safe state defined by the manufacturer. Particularly advantageous is a redundant configuration of the switching contact 3, so that in case of failure of the first switching contact 3, an identical switching contact 3 'can be switched. LIST OF REFERENCE NUMBERS

1 primary page

2 secondary side

3 switching contact

4 control unit

51 First capacitor

52 second capacitor

53 Third capacitor

54 fourth capacitor

6 processing unit

7 feedback device

8 capacitive interface

9 measuring shunt

10 transmission unit

1 1 microcontroller

Claims

Patent claims

Device for the galvanically isolated transmission of a control signal from a primary unit (1) to a secondary unit (2),

wherein the primary side (1) generates two complementary control signals S+, S. for controlling a switching contact (3),

where the secondary unit

(2) has the switching contact (3) and a control unit (4),

wherein at least a first capacitor (51) and a second capacitor (52) are provided,

wherein the first capacitor (51) receives the control signal S+ to close the

Switch contact (3) transmits to the control unit (4),

wherein the second capacitor (52) receives the control signal S. to open the

switching contact

(3) transmits to the control unit (4) and

where the control unit

(4) depending on the applied control signal S+,

S- the switching contact (3) closes or opens.

Device according to claim 1,

wherein the primary side has a processing unit (6) for generating the

Control signals S+, S. from a primary switching signal Si.

Device according to claim 1 or 2,

wherein at least a third capacitor (53) is connected in series with the first capacitor (51) and at least a fourth capacitor (54) is connected in series with the second capacitor (52), so that their insulation voltages each add up to a total voltage U .

Device according to at least one of the preceding claims, wherein a feedback device (7) is provided on the secondary side (2), which serves to control the switching state of the switching contact (3).

5. Device according to claim 4, wherein the feedback device (7) transmits the information about the switching state of the switching contact (3) to the primary side (1) via a capacitive interface (8).

6. Device according to at least one of the preceding claims,

wherein the capacitors (51, 52) also transmit energy to the secondary side (2) in addition to the control signal S+, S.

7. Device according to at least one of the preceding claims,

wherein the switching contact (3) has the function of a pulse generator for transmitting the information contained in the primary switching signal Si.

8. Device according to at least one of the preceding claims,

wherein the switching contact (3) is connected to a device which is switched on or off depending on the switching state of the switching contact (3).

9. Device according to at least one of the preceding claims,

wherein the device is part of a measuring device for determining and/or monitoring a physical, biological or chemical

process variable, a transmitter or a field indicator.