Method for safely carrying out a power fail function in a 3-wire or 4-wire field device

Abstract

The invention relates to a method for safely carrying out a power fail function in a 3-wire or 4-wire field device (17) having at least three connection pins (1, 2, 3), of which a first and a second connection pin (1, 2) are used to supply energy to the field device (17) and a third connection pin (3) is used to provide a power output, the method comprising the following steps: - monitoring a voltage (V) applied across the first and third connection pin (1, 3), or a voltage attributable thereto, for a voltage drop; - if a voltage drop is detected, checking whether the voltage drop is attributable to a high resistance being connected to the power output provided at the third connection pin (3); - carrying out a safety measure if a voltage drop is detected at the first and third connection pin (1, 3) and it is also identified that the voltage drop is not attributable to a high resistance being connected to the power output provided at the third connection pin (3).

Description

[0001] Procedure for safely executing a power fail function on a 3- or 4-wire field device

[0002] The invention relates to a method for safely executing a power fail function in a 3- or 4-wire field device, a field device of automation technology and an automation system.

[0003] In automation technology, particularly in process automation technology, field devices are often used to record and / or influence process variables. Sensors such as level gauges, flow meters, pressure and temperature gauges, pH and redox potential meters, conductivity meters, etc., are used to record process variables and record the corresponding process variables: level, flow, pressure, temperature, pH value, or conductivity. Actuators such as valves or pumps, which can be used to change the flow of a liquid in a pipe section or the fill level in a container, are used to influence process variables. In principle, field devices are all devices that are used close to the process and that provide or process-relevant information.In the context of the invention, field devices are understood to include, in particular, remote I / Os, radio adapters or, in general, devices that are arranged at the field level.

[0004] A large number of such field devices are manufactured and distributed by Endress + Hauser.

[0005] The field devices can be connected, for example, via a two-wire line, i.e. a line with two separately designed wires, to a higher-level unit, e.g. a control unit, PLC or control system. The field devices are designed in such a way that the measured or control values ​​are communicated, i.e. transmitted, as process variables via the two-wire line or two-wire cable in analog form in the form of a 4-20 mA loop current or current signal. In this case, a loop current of the two-wire line is set to a specific value by the field device or the higher-level unit according to the recorded process variable. Furthermore, the field devices are also supplied with power via the two-wire line in 2-wire operation. However, in this case only a very limited amount of power is available (usually less than 50 mW). Such field devices are also referred to as two-wire field devices or 2-wire field devices.

[0006] Field devices that require more power than can be transmitted via a two-wire cable are designed as so-called 3- or 4-wire field devices, occasionally also referred to as 3-wire or 4-wire field devices. Such field devices are configured to have at least one additional connection in addition to the two-wire cable used to transmit measured values, so that these field devices can be connected to a separate power supply. This means that such field devices are not supplied with power via the two-wire cable used to transmit the measured value, but rather that the power is transmitted using a three- or four-wire cable on a separate pair of cables, and the measured value or control value is transmitted on a third wire in the form of a 4-20 mA signal.

[0007] To ensure that parameter data from a field device can be safely saved, so-called power fail functions or power fail circuits are implemented. These detect a sudden voltage drop, for example, when the device is unplugged, and implement appropriate safety measures. Such power fail functions are now common in 2-wire field devices. Power fail functions do not currently function reliably in 3- or 4-wire field devices, at least not if the 3- or 4-wire field device has an actively wired current output.

[0008] It is therefore an object of the invention to show a possibility how a power fail function or circuit can be reliably implemented in a 3- or 4-wire field device with a current output.

[0009] The object is achieved according to the invention by the method for safely executing a power fail function in a 3- or 4-wire field device according to patent claim 1, a field device of automation technology according to patent claim 7 and an automation technology system according to patent claim 10.

[0010] The method according to the invention for safely executing a power fail function in a 3- or 4-wire field device having at least three connection pins, of which a first and a second connection pin serve to supply power to the field device and a third connection pin serves to provide a current output, provides the following steps:

[0011] - monitoring a voltage across the first and third terminal pins or a voltage attributable thereto for a voltage drop;

[0012] - Check whether the voltage drop is due to a high-impedance circuit of the current output provided on the third connection pin, in the event that a voltage drop is detected;

[0013] - Carrying out a safety measure in the event that a voltage drop is detected at the first and third connection pin and it is further determined that the voltage drop is not due to a high-impedance circuit of the current output provided at the third connection pin.

[0014] This invention proposes a method for safely executing a power fail function in a 3- or 4-wire field device with a current output. This method implements a corresponding safety measure in the event of a sudden voltage drop in the voltage applied to the first and third connection pins or a voltage attributable to this voltage. To this end, the method not only monitors the voltage applied to the first and third connection pins or the voltage attributable to this voltage for a sudden voltage drop, but also checks whether the voltage drop is attributable to a high-impedance current output of the field device. A safety measure is only implemented if the voltage drop is not attributable to a high-impedance current output.A high-impedance circuit is understood to mean a circuit of the current output such that an input resistance of the current output is greater than 100 ohms, preferably greater than 500 ohms at a voltage of a few volts (approx. 5-40 V).

[0015] An advantageous embodiment of the method according to the invention provides that in order to check whether the voltage drop is due to a high-impedance circuit of the current output provided at the third connection pin, the current output is deactivated, in particular briefly.In particular, the embodiment can provide that, in order to check whether the voltage drop is due to a high-impedance circuit of the current output provided at the third connection pin, the voltage applied across the first and third connection pin or the voltage attributable thereto is determined before and after deactivation of the current output and, based on this, a check is made to determine whether the voltage drop is due to a high-impedance circuit of the current output and / or that the voltage drop is attributed to a high-impedance circuit of the current output if it is determined that the voltage after deactivation is greater than it was before deactivation of the voltage drop.

[0016] A further advantageous embodiment of the method according to the invention provides that in the event that it is determined that the voltage drop is due to a high-impedance circuit of the current output provided at the third connection pin, no safety measure is carried out.

[0017] A further advantageous embodiment of the method according to the invention provides that, as a security measure, data and / or parameters of the field device are stored in a persistent memory of the field device electronics.

[0018] The invention further relates to a field device in automation technology, comprising: - at least three connection pins, of which a first and a second connection pin serve to connect the field device to an external supply;

[0019] - field device electronics configured to provide a current output at a third connection pin, wherein the field device electronics is further configured to carry out the method according to at least one of the previously described embodiments.

[0020] An advantageous embodiment of the field device according to the invention provides that the field device electronics are further configured to be able to deactivate the current output provided at the third connection pin, especially temporarily. In particular, the embodiment can provide that the field device electronics have an auxiliary circuit configured to route a current supplied via the first connection pin to the second connection pin instead of via the current output provided at the third connection pin, so that the current output is or will be deactivated.

[0021] The invention further relates to an automation system comprising:

[0022] - a field device according to one of the previously described embodiments;

[0023] - an external supply unit, in particular an external power supply, which is electrically connected to the first and second connection pins, so that a supply voltage is applied to the first and second connection pins;

[0024] - an external burden connected to the provided current output.

[0025] The invention is explained in more detail with reference to the following drawings. It shows:

[0026] Fig. 1: a block diagram of a 3-wire field device with internal field device electronics that is designed to execute a power fail function in the event of a sudden voltage drop in the supply voltage, and Fig. 2: a block diagram of the 3-wire field device with the internal field device electronics in which the power fail function is executed.

[0027] Figures 1 and 2 show a 3-wire field device 17, each with identically designed internal field device electronics 16. The field device electronics comprises at least three connection pins 1, 2 and 3, via which the field device 17 can be connected accordingly in an automation system, an EMC filter 5, which is connected downstream of the connection pins 1, 2 and 3 to protect the field device 17 from possible EMC interference, an electronics supply unit 6 for the internal power supply, a microprocessor or microcontroller 7 for the internal control of the field device electronics 16, a sensor or actuator element 8 for detecting or setting a measured or set value, a load resistor 9 and a 4-20 mA current control unit 10, which together with the load resistor 9 sets a current value I.

[0028] In addition, the field device electronics can include a display and / or input unit 11, for example in the form of a display, especially a touchscreen. The display and / or input unit 11 can be used, for example, to display measured or manipulated values ​​or to configure the field device 17.

[0029] To clarify or explain the functioning of the field device electronics 16 when executing a power fail function, the field device electronics 16 is shown in Figure 1 in proper operation and in Figure 2 in improper operation. Proper operation is understood to be the situation in which a resistance for the external burden 18 is within a range specified for the current output and there is no sudden voltage drop in the supply voltage. A range specified for the current output can, for example, be less than 1 kOhm (kQ), preferably less than 500 Q, more preferably less than 100 Q, most preferably less than 30 Q. In contrast, improper operation is understood to be the situation in which the current output is connected to a high-resistance burden 18 and there is a sudden voltage drop in the supply voltage.Furthermore, for clarity, the inactive electrical connections are marked with dashed lines, the electrical connections used for data transmission are marked with a thin line, the electrical connections used for energy transmission are marked with a medium line or a line of medium thickness, and the electrical connections used for energy and data transmission are marked with a thick line.

[0030] The field device electronics 16 are configured to be supplied with power via the first and second connection pins 1, 2 with a supply voltage from an external power supply 14. As a supply voltage, for example, a voltage of greater than 10 V, preferably greater than 20 V, particularly preferably of approximately 24 V can be applied to the first and second connection pins 1, 2. In this case, a current I for the power supply is supplied to the electronics supply unit 6 via a first data and power supply path 6a, which is connected downstream of the EMC filter 5 and connects the first connection pin 1 to the power supply unit 6. Furthermore, the field device electronics are configured to provide a current output via the third connection pin 3. As can be seen from Figure 1, an external burden is connected to the third connection pin 3.

[0031] The field device electronics 16 are configured, during proper operation, to conduct the current I via an internal output path 6b, which connects the electronics supply unit 6 to the third connection pin 3 via the internal burden 9 and the EMC filter 5. Furthermore, the field device electronics 16 is configured according to the invention to monitor the voltage V applied across the first and third connection pins 1, 3 or a voltage attributable thereto for a voltage drop and, if a voltage drop is detected, to check whether the voltage drop is attributable to a high-impedance circuit of the current output provided at the third connection pin 3. This can be done, for example,This can be done in such a way that the field device electronics 16 internally measures the voltage V or the voltage attributable to it at regular intervals and compares it with a previously measured value. If the two values ​​differ from each other by a predetermined value, which can be less than 10 V, for example, a sudden voltage drop can be detected. In this case, the field device electronics 16 can further be configured to be able to conduct the current to the second connection pin 2 via an auxiliary circuit in order to be able to deactivate the current output provided at the third connection pin 3. For this purpose, the auxiliary circuit 12 can have an auxiliary path 12a that can be activated, for example by the microprocessor, which connects the internal load via the EMC filter 5 to the second connection pin 2, so that the current I flows via the auxiliary path 12a to the second connection pin 2 and not to the third connection pin 3.To control the auxiliary circuit 12 and activate the auxiliary path 12a, a control line 7a can be provided from the microprocessor 7 to the auxiliary circuit. The voltage can be measured directly at the first and third connection pins 1, 3. Alternatively, however, a voltage can also be used that is attributable to the voltage applied across the first and third connection pins 1, 3. This can, for example, be a voltage applied between the first data and power supply path 6a and the internal output path 6b or a voltage applied between the first data and power supply path 6a and the path located between the internal load 9 and the electronics supply unit 6.

[0032] Furthermore, the field device electronics 16 can be configured to measure the voltage again after deactivation of the current output and to compare it with the value before deactivation of the current output and, in the event that the voltage V increases or has increased after deactivation of the current output, to attribute this to a high-impedance circuit of the switching output.

[0033] In addition, the field device electronics 10 is configured to subsequently execute a safety measure if a voltage drop was previously detected at the first and third connection pins 1, 2 and it was further determined that the voltage drop is not due to a high-impedance circuit of the current output provided at the third connection pin 3. Such a safety measure can, for example, be the storage of data and / or parameters of the field device 17 in a persistent memory 7b. The persistent memory 7b can, for example, be designed as part of the microprocessor or separately therefrom. Furthermore, the field device electronics 16 is configured not to execute a safety measure if it is determined that the voltage drop is due to a high-impedance circuit of the current output provided at the third connection pin 3.

[0034] List of reference symbols

[0035] 1 First connection pin

[0036] 2 Second connection pin

[0037] 3 Third connection pin

[0038] 5 EMC filters

[0039] 6 Electronics supply unit

[0040] 6a First data and / or power supply path / 4-20mA path

[0041] 6b Internal output path

[0042] 7 microprocessor

[0043] 7a Control line

[0044] 7b Persistent memory

[0045] 8 Sensor or actuator element

[0046] 9 Load resistance

[0047] 10 4-20mA current control unit

[0048] 11 Display and / or output unit

[0049] 12 Auxiliary circuit

[0050] 12a First auxiliary path

[0051] 14 Power supply

[0052] 16 Field device electronics

[0053] 17 field device

[0054] 18 External Burden

[0055] I Current

[0056] V voltage io

Claims

Patent claims 1 . A method for safely executing a power fail function in a 3- or 4-wire field device (17) having at least three connection pins (1, 2, 3), of which a first and a second connection pin (1, 2) serve to supply power to the field device (17) and a third connection pin (3) serves to provide a current output, comprising the following steps: - monitoring a voltage (V) across the first and third connection pins (1, 3) or a voltage attributable thereto for a voltage drop; - Check whether the voltage drop is due to a high-impedance circuit of the current output provided on the third connection pin (3), in the event that a voltage drop is detected; - Carrying out a safety measure in the event that a voltage drop is detected at the first and third connection pins (1, 3) and it is further determined that the voltage drop is not due to a high-impedance circuit of the current output provided at the third connection pin (3).

2. Method according to claim 1, wherein, in order to check whether the voltage drop is due to a high-impedance circuit of the current output provided at the third connection pin (3), the current output is deactivated, in particular briefly.

3. Method according to the preceding claim, wherein, in order to check whether the voltage drop is due to a high-impedance wiring of the current output provided at the third connection pin (3), the voltage (V) applied across the first and third connection pin (1, 3) or the voltage attributable thereto is determined before and after the deactivation of the current output and, based on this, it is checked whether the voltage drop is due to a high-impedance wiring of the current output.

4. Method according to one of claims 2 or 3, wherein the voltage drop is attributed to a high-impedance circuit of the current output if it is determined that the voltage (V) or the voltage attributable thereto is greater after deactivation than it was before deactivation of the voltage drop.

5. Method according to one or more of the preceding claims, wherein in the event that it is determined that the voltage drop is due to a high-impedance circuit of the current output provided at the third connection pin (3), no safety measure is carried out.

6. Method according to one or more of the preceding claims, wherein, as a security measure, data and / or parameters of the field device (17) are stored in a persistent memory (7b) of the field device electronics.

7. Field device of automation technology, comprising: - at least three connection pins (1, 2, 3), of which a first and a second connection pin (1, 2) serve to connect the field device to an external supply; - field device electronics (16) which are configured to provide a current output at a third connection pin (3), wherein the field device electronics (16) are further configured to carry out the method according to at least one of the preceding claims.

8. Field device of automation technology according to the preceding claim, wherein the field device electronics (16) is further configured to be able to deactivate the current output provided at the third connection pin (3), in particular for a short time.

9. Field device of automation technology according to the preceding claim, wherein the field device electronics (16) has an auxiliary circuit (12) which is designed to supply a current via the first connection pin (1) instead of via the current supplied to the third connection pin (3) provided current output to the second connection pin (2) so that the current output is or will be deactivated.

10. Automation system comprising: - a field device according to the preceding claim; - an external supply unit (14), in particular an external power supply, which is electrically connected to the first and second connection pins, so that a supply voltage is applied to the first and second connection pins; - an external load (18) which is connected to the provided current output.

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