EP2225537A2

EP2225537A2 - Field device and method for checking the calibration of a field device

Info

Publication number: EP2225537A2
Application number: EP08864890A
Authority: EP
Inventors: Klaus Korsten
Original assignee: Endress and Hauser Process Solutions AG
Current assignee: Endress and Hauser Process Solutions AG
Priority date: 2007-12-20
Filing date: 2008-12-12
Publication date: 2010-09-08
Also published as: WO2009080549A3; DE102007062394A1; WO2009080549A2

Abstract

A field device of processing automation technology, having an interface for the connection of a field bus, comprises a time generator for generating a time signal and a control module, which is designed for the purpose of comparing a starting time, which can be programmed from the fieldbus, of a measurement to the time signal, to start the measurement upon correspondence, and to store a measurement result of the measurement under an address which can be read out from the fieldbus.

Description

Field device and method for checking the calibration of a feeder

The invention relates to a field device according to the preamble of claim 1 and a field bus system according to the preamble of Anspurchs 3. Furthermore, the invention relates to a method for starting a measurement in a field device according to the preamble of claim 20 and a method for checking the calibration of a field device according to the preamble of claim 21.

In process automation technology, field devices are often used to detect and / or influence process variables. Examples of such field devices are level gauges, mass flowmeters, pressure and temperature measuring devices, etc., which detect the corresponding process variables level, Durchfiuss, pressure or temperature as sensors.

To influence process variables actors, z. B, valves or pumps on the Durchftussuss a liquid in a Rohrieitungsabschnitt or the level can be changed in a container.

In principle, field devices are all devices that are used close to the process and that supply or process process-reliable information.

A variety of such field devices is manufactured and sold by the company Endress + Hauser.

During operation, the field devices are sometimes under aggressive

Environmental conditions operated. As a result, contamination, deposits, corrosion occur and this also affects the accuracy of the measurements provided by the field device. For these reasons, it is necessary to check the measurement accuracy of a field device at intervals with the aid of a reference device of the same type. The reference device is known to be properly calibrated and to provide correct readings.

To carry out a comparison measurement between a test field device and a reference field device, it was previously necessary, each with a Service interface to access the two devices. The service parts were subjected to a time-synchronous measurement by the device under test and the reference device, and then the results thus measured were compared. Disadvantage of this approach is that the calibration can not be checked via the fieldbus itself due to the latency of the fieldbus, but a separate service interface is required.

The object of the invention is to simplify a review of the calibration of a field device.

This object is achieved by the features specified in claims 1, 3, 20 and 21.

Advantageous further developments of the invention are specified in the subclaims,

In order to enable a time-accurate start of a measurement, in the solution according to the invention, a desired start time is written from the fieldbus into the field device. On the field device, a timer is provided which generates a local time signal which may be synchronized with a higher system time. The stored start time is compared with the local time signal, and if coincident, the measurement is automatically started by the field device.

Half of this measurement release mechanism allows a timely start of the measurement. This makes it possible to set up a desired measurement via the fieldbus, but to be independent of the latency times on the Feidbus during the timing of the measurement. If one were to start the measurement by transmitting a start command over the fieldbus, there would be delays caused by the cycle time of the fieldbus.

The use of such a triggering mechanism makes it possible, in particular, to carry out a comparison measurement between a test field device and a reference field device. For this purpose, a common start time is transmitted to the DUT field device and the reference field device via the fieldbus and on the respective device stored. Both on the test field device a! S and on the reference field device, the measurement is started at the scheduled start time. In this way, it is possible to carry out the measurement and the reference measurement exactly synchronously. Since the DUT field device and the reference field device measure the same physical quantity and the measurement is time-synchronized, the measured value determined by the DUT should exactly match the reference value determined by the reference field device. By comparing the measured values supplied by the test object and the reference device, the measurement accuracy of the test device field device can therefore be assessed. This is especially important for quality management in process engineering.

The closer the measured value determined by the test specimen to the reference value, the better the measuring accuracy of the test specimen. On the other hand, if the difference between the measurement result of the device under test and the reference value of the reference device exceeds a certain limit, then the measurement accuracy of the device under test is no longer sufficient. In this case recalibration of the device under test is required. By redetermining the calibration parameters of the test specimen, the measurement accuracy can be restored to its former level,

In an advantageous embodiment, in the event of a poor match between the values measured by the device under test and the reference device, a recalibration of the device under test is automatically carried out. For this purpose, the measurements required for recalibration are triggered by a superordinate control unit, for example by a master. The newly determined calibration parameters are then transmitted via the fieldbus to the DUT field device, stored and used from then on.

In the solution according to the invention, both the comparison measurement and the recalibration can be carried out exclusively via the fieldbus. Additional service parts and complex calibration tools are no longer required.

The invention is explained in more detail with reference to an embodiment shown in the drawing. Show it;

Flg. 1 field device for the timely execution of a measurement; Fig. 2A Vergieichsmessung according to the prior art; Fig. 2B comparative measurement according to an embodiment of the invention; Fig. 3 block diagram of a measuring arrangement.

1 shows a field device 1 according to an embodiment of the present invention, which is connected to a higher-level control unit 4 via a fieldbus interface 2 and a fieldbus 3. In the higher-level control unit 4, it may be z. For example, it may be a class 2 master. The field device 1 serves to detect a process variable relevant to the process, for example one or more of the following: volume flow, mass flow, pressure, temperature, differential pressure, fill level, pH, etc.

When transmitting commands and data via the Feidbus 3, there are latency times, which are primarily caused by the cycle time of the feeder bus. Further delays arise due to device cycle times and system cycle times. Despite these latencies, the field device 1 according to the invention makes it possible to start a measurement precisely to a start time which can be programmed by the superordinate control unit 4. For this purpose, a local timer 5 is arranged on the field device 1, which is synchronized with a system-wide existing system time. This system time is generated by a system timer 6, which may be arranged on the side of the higher-level control unit 4, for example.

The manner in which the synchronization between the local timer 5 and the system timer 6 is established differs depending on the type of fieldbus used. If a Foundation Fieldbus is used as fieldbus 3, the synchronization can be accomplished by means of a broadcast command. The system timer 6 sends a broadcast command at a certain time! to all the local timers 5 and thus allows synchronization of the local time with the system time. For other fieldbus standards, such as the Profibus, there is no broadcast command. In such standards, it is necessary for the synchronization of the timer, from the higher-level control unit 4 each of the local timer 5 to send a telegram one after the other with the respective current system time of the system timer 6. In this way, even if no broadcast command! is available, a synchronization between the local timer 5 and the system timer 6 can be achieved.

The field device 1 further comprises a plurality of memory cells 7, which can be addressed via the fieldbus 3. In the case of a Profibus device, the data can usually be addressed via the "physical block", whereas the data in a Foundation Fieldbus device can be addressed via the "Transducer Block".

in the memory cells 7 parameters of the measurement to be performed are stored. In particular, parameters such as a start time 8, a measurement duration 9 and a measurement mode 10 can be stored in the memory cells 7. These parameters of the measurement to be carried out are written from the higher-level unit 4 via the acyclic data traffic of the fieldbus 3 into corresponding memory lines 7.

The performance of the measurement is controlled by a control module 11. The control module 11 is designed to control the performance of the measurement in dependence on the parameters stored in the memory lines 7. The control module 1 1 compares the time provided by the local timer 5 with the start time 8, and if coincident, the measurement is started. To carry out the measurement, the field device 1 comprises a sensor 12 which supplies a measurement signal, for example a current signal. Calibration parameters are stored on the field device 1 in order to convert the measurement signal into a physical measured variable 13. Measurements such. As a pH measurement, pressure measurement or temperature measurement are performed at a given time. In Voiumenfluss- or mass flow measurements, however, the volume or mass of the liquid flowing past the sensor 12 is integrated over a predetermined measurement period 9 away. The physical measured variable 13 determined in this way is stored in one of the memory cells 7 and can be read out from the higher-order control unit 4 via the fieldbus 3 in the acyclic data traffic. The mechanism described with reference to FIG. 1 for the timely execution of a measurement is particularly suitable for checking the calibration of a field device with the aid of a reference field device and possibly recalibrating the field device.

FIG. 2A shows how the verification of the calibration of a field device is carried out in the solutions of the prior art. The respective measured variable, for example a volume flow 14 in a pipeline 15, is measured simultaneously by a test field device 16 and by a reference field device 17. The reference field device 17 is known to be calibrated correctly and to provide correct measurements. The PrüfSings field device 16 may be connected to a fieldbus 18. However, the calibration measurement is not initiated by the fieldbus 18 from. Instead, both the DUT field device 16 and the reference field device 17 are each equipped with a service interface 19, 20. At these service interfaces 19 and 20, a service device 21, such as a laptop, connected. Under the control of the service device 21, the volume flow is measured synchronously by the test device field device 16 and the reference field device 17. After the measurement, the measured value determined by the specimen testing device 16 is compared with the reference value determined by the reference field device 17. If the measured value determined by the test device field device 16 deviates only slightly from the reference value, no recalibration of the test device field device 16 is necessary. If the deviations are greater, it is necessary to redetermine the calibration parameters stored in the test device field device 16.

FIG. 2B shows a system according to the invention for carrying out a calibration measurement with a test field device 22 and a reference field device 23. The

DUT field device 22 and reference fiducial device 23 are designed to perform the

Flow 24 in a pipe 25 to measure. Both the device under test field device 22 and the reference field device 23 correspond to the embodiment according to the invention shown in FIG. 1 and are therefore able to perform a measurement automatically at a freely programmable start time.

The device under test field device 22 and the reference field device 23 are connected to a master 27 via a field bus 26. To carry out the comparison measurement, the master 27 writes a start time for the start of the measurement in the two field devices 22 and 23. The DUT field device 22 and the Refθrenz field device 23 perform from the specified start time in absolute synchronous flow measurement, the measurement results obtained are respectively stored in the DUT field device 22 and the reference field device 23. After completion of the measurement, the measured values determined by the two field devices 22, 23 are interrogated by the master 27 and compared with one another. If the measured value determined by the test device field device 22 is sufficiently close to the measured value determined by the reference field device 23, then the test device field device 22 still operates with sufficient accuracy and need not be recalibrated. On the other hand, if the measured value determined by the test longitudinal field device 22 lies outside of a tolerance window around the reference value, a recalibration of the test device field device 22 is required. For recalibration, the UUT field device 22 and the reference field device 23 perform a number of different measurements. For example, measurements are made at various flow rates, pressures, temperatures, etc. Based on the obtained measured values and reference measured values, the calibration parameters of the test device field device 22 can be redefined.

It can be seen on the basis of FIG. 2B that the comparison measurement can be controlled via the fieldbus 26 through the use of the inventive feeder devices 22, 23. With the aid of the measurement triggering mechanism according to the invention, if the respective local timers are synchronized, the required measurements can be carried out absolutely synchronously. It is therefore no longer necessary to equip the DUT 22 and the reference field device 23 each with separate service interfaces.

In Fig. 3, the measuring system according to the invention is shown in a more detailed block diagram. The measuring arrangement comprises a device under test device 28 which is connected to a master 31 via a fieldbus interface 29 and a fieldbus 30, and a reference field device 32 which is connected to the master 31 via a fieldbus interface 33 and the fieldbus 30 is. A first timer 34 is arranged on the test field device 28, and a second timer 35 is located on the reference field device 32. Both the first timer 34 and the second timer 35 are synchronized with a system timer 36 arranged on the master 31 side , Of the System timer 36 provides the system time for the entire measurement setup shown in FIG.

The test field device 28 also includes a first sensor 37, a first control module 38 that controls the performance of the measurement, and memory cells 39 in which parameters of the measurement to be performed, measurement results and calibration parameters can be stored. Accordingly, the reference field device 32 comprises a second sensor 40, a second control module 41 for controlling the reference measurement, and memory cells 42 in which parameters of the reference measurement, measurement results and calibration parameters can be stored.

To carry out a comparison measurement between the test field device 28 and the reference field device 32, a start time 43 and a measurement duration 44 are written from the master 31 into corresponding addresses of the memory cells 39, 42. This writing operation takes place in the acyclic data traffic of the fieldbus 30. In the first control module 38, the start time 43 is compared with the time generated by the first timer 34. If there is a match, the test field device 28 performs a measurement, the measurement duration 44 being predetermined. The measurement result 45 thus obtained is stored under a memory address of the memory cells 39 provided for this purpose. The second control module 41 in the test field device 32 also performs a comparison between the time generated by the second sequencer 35 and the start time 43. When the start time is reached, the reference measurement is started, the duration of which is specified by the measurement duration 44. The thus determined reference measurement result 46 is stored under an address of the memory cells 42 provided for this purpose.

Since the first timer 34 and the second timer 35 are both synchronized with the system timer 36 and the respective start time and duration of the measurements made by the device under test field device 28 and the reference field device 32 match, the two measurements are performed synchronously.

From the master 31, the measurement result 45 and the reference measurement result 46 can be retrieved from the test field device 28 and from the reference field device 32 via the fieldbus 30, wherein the data transmission takes place again in the acyclic data traffic. It can now be judged on the master 31 side whether the device under test field device 28 measures with sufficient accuracy or not. For this purpose, the measurement result 45 is compared with the reference measurement result 46. If the deviation between the two measurement results lies below a predefined threshold value, then the test field device 28 operates with sufficient accuracy. In this case, no recalibration of the DUT field device 28 is necessary. On the other hand, if the measurement result 45 is outside a tolerance range defined by the reference measurement result 46, recalibration of the test gauge 28 is required.

According to a preferred embodiment of the invention, this recalibration is also performed under the control of the master 31. For example, a number of measurements can be performed both by the test device field device 28 and by the reference field device 32, wherein the physical variable to be measured is varied. For example, readings can be taken at different pressures, temperatures and at different flow rates. In the case of a flow measurement different liquids with different density can be measured. From the measured values determined in this way, new calibration parameters for the test object feeder 28 can be determined. These calibration parameters determine how the measurement signal delivered by the measurement sensor 37 can be converted into a physical measurement variable. The calibration parameters are determined by a calculation unit 47 on the master 31 side and subsequently transferred to the device under test field device 28 via the fieldbus 30 in acyclic operation. There, the calibration parameters are stored and henceforth used for the conversion of the measurement signal into the measured variable.

The calibration method according to the invention offers the advantage that the calibration measurement can be controlled and programmed via the fieldbus 30. As a result of the triggering mechanism according to the invention, the latency of the field bus does not adversely affect the temporal accuracy of the measurements. The calibration measurement may be performed by the master 31 in accordance with a schedule at regular time intervals, such as at weekly or monthly intervals, to thereby monitor the measurement accuracy of the test-length feeder 28 over extended periods of time. The respective results of the calibration measurements can be documented and archived. This is particularly important in the context of quality management. By the Regular checking of the field devices used, deterioration of the measuring accuracy as a result of contamination or wear can be detected and corrected at an early stage in order to be able to guarantee a defined measuring accuracy of the field devices used.

Claims

claims

1. A field device (1) of the process automation technology, with an interface (2) for connecting a fieldbus (3), characterized by

a timer (5) for generating a time signal,

a control module (1 1) adapted to compare a starting time (8) of a measurement programmable from the fieldbus (3) with the time signal, start the measurement if coincident, and a measurement result (13) of the measurement under one from the fieldbus (3) from readable address store.

2. Field device according to claim 1, characterized in that the time signal is synchronized with an external system time.

3. A fieldbus system having a first field device (28), which is connected via a fieldbus (30) to a higher-order unit (31), characterized in that the first field device (28) comprises:

a first timer (34) for generating a first time signal,

a first control module (38) which is designed to compensate a first start time of a first measurement programmable from the fieldbus (30) with the first time signal, to start the first measurement if coincident, and a first measurement result (45) of the first one Measurement under a read from the fieldbus (30) from read address, wherein the superordinated unit (31) is adapted to transmit the first start time (43) of the first measurement via the fieldbus (30) to the first field device (28) and the from first field device (28) determined first measurement result (45) from the first

Call field device (28).

4. Fieldbus system according to claim 3, characterized by a reference field device (40), which is connected to the field bus, with

a second timer (35) for generating a second time signal,

a second control module (41) which is designed to compare a second start time of a reference measurement programmable from the fieldbus (30) with the second time signal, to start the reference measurement in case of agreement, and store a reference measurement result (46) of the reference measurement under an address readable from the fieldbus (30).

5. fieldbus system according to claim 4, characterized in that it is the first measurement and the reference measurement is one of: Voiumenfi-uss-

Measurements, mass flow measurements, pressure measurements, temperature measurements, differential pressure measurements, level measurements, pH measurements, integrated volume flow measurements, integrated mass flow measurements.

6. fieldbus system according to claim 4 or claim 5, characterized in that the first Zeitsigna! and the second time signal is synchronized with a system time.

7. Fieldbus system according to one of claims 4 to 6, characterized in that the superordinate unit comprises a system timer (36) for generating a system time, wherein the first timer and the second timer are synchronized with the system time generated by the system timer.

8. Fieldbus system according to claim 6 or claim 7, characterized in that the superordinated unit is designed to synchronize the first timer and the second timer with the system time each time the current state of the system time successively to the first timer and to the second timer to transfer.

9. Fieldbus system according to one of claims 4 to 8, characterized in that the superordinated unit is designed to transmit the same starting time via the fieldbus to the first field device and to the reference fiddle device for carrying out a comparison measurement.

10, fieldbus system according to one of claims 4 to 9, characterized in that the first measurement is performed synchronously with the reference measurement.

1 1. Fieldbus system according to one of claims 4 to 10, characterized in that the superordinated unit is adapted to transmit additional measurement parameters via the fieldbus to the first feeder and to the reference feeder.

12. Fieldbus system according to claim 1 1, characterized in that the additional measurement parameters include one or more of the following: measurement duration, measurement mode, measurement method, type of a measured liquid, setpoint specification.

13. Fieldbus system according to one of claims 4 to 12, characterized in that the superordinated unit is adapted to read the first measurement result via the field bus from the first field device and read the reference measurement result via the field bus from the reference field device.

14. Fieldbus system according to claim 13, characterized in that the higher-order unit is designed to determine whether the first measurement result is within a tolerance range around the reference measurement result.

15. Fieldbus system according to claim 13 or claim 14, characterized in that the higher-level unit is designed to determine whether a recalibration of the first field device is required.

16. Feidbussystem according to one of claims 4 to 15, characterized in that the superordinated unit is adapted to automatically perform a recalibration of the first field device, if a recalibration of the first field device is required.

17. Fieldbus system according to one of claims 4 to 16, characterized in that the higher-order unit comprises a calibration unit (47) which is adapted to redetermine one or more calibration parameters of the first field device.

18. Fieldbus system according to one of claims 4 to 17, characterized in that the superordinate unit is designed to transmit newly determined Kaiibrierparameter over the fieldbus to the first feeder.

19. Fieldbus system according to one of claims 4 to 18, characterized in that the superordinated unit is designed to automatically perform comparative measurements of the first field device and the reference Id device in accordance with a schedule at predetermined times.

20. A method for starting a measurement in a Feidgerät (1) of the process automation technology, which has an interface (2) for connecting a Feidbusses (3), characterized by the following steps:

Generating a time signal,

- comparing a programmable starting time (8) of the measurement with the time signal,

Starting the measurement if the start time (8) matches the time signal,

Determining a measurement result (13),

- Store the measurement result (13) under an address readable from the fieldbus (3).

21. Method for checking the calibration of a first field device (28) by means of a reference field device (32), wherein the first FeJdgerät (28) and the reference field device (32) via a fieldbus (30) with a superordinated unit (31) are connected characterized by the following steps: - starting a first measurement by the first field device (28) to one from the fieldbus

(30) programmable first start time,

Determining a first measurement result (45),

- Storing the first measurement result (45) under an ausiesbaren from the fieldbus address of the first field device (28), - Starting a reference measurement by the reference field device (32) to one of

Fieldbus from programmable second start time,

Determining a reference measurement result (46),

Storing the reference measurement result (46) under an address of the reference field device (32) that can be read out from the fieldbus (30), - Reading the first measurement result (45) from the first field device (28) and the reference measurement result (46) from the reference measuring device (32) by the parent unit (31).

22. The method according to claim 21, characterized by the following further step:

Compare the first measurement result with the reference measurement result and determine whether a recalibration of the first field device is required.

23. The method of claim 21 or claim 22, characterized by the following further step: if a recalibration of the first field device is required, automatically determining new Kaiibrierparametern for the first field device, and transmitting the new calibration parameters via the fieldbus to the first field device.