WO2001091081A1

WO2001091081A1 - Device for uni-directionally or bi-directionally exchanging data

Info

Publication number: WO2001091081A1
Application number: PCT/EP2001/005241
Authority: WO
Inventors: Dietmar Spanke; Rolf Deserno; Hainer Wetteborn
Original assignee: Endress + Hauser Gmbh + Co. Kg.; WETTEBORN, Sabine
Priority date: 2000-05-22
Filing date: 2001-05-09
Publication date: 2001-11-29
Also published as: AU2001260286A1; DE10024959A1; DE10024959B4

Abstract

The invention relates to a device (1) for uni-directionally or bi-directionally exchanging data between a measuring unit (2) for measuring a physical quantity and a control/evaluation unit (3), which determines the physical quantity with a given accuracy by using the measurement data provided by the measuring unit, whereby the measuring unit (2) is connected to the control/evaluation unit (3) via at least one data line (4). The aim of the invention is to optimize the data transmission between a measuring unit (2) and a remote control/evaluation unit (3). To this end, the invention provides that at least one compression unit (6) is provided, which is assigned to the measuring unit (2) and which compresses the measurement data supplied in a predetermined cycle by the measuring unit (2). The measurement data is compressed in such a manner that the information content of the measurement data is transmitted over the data line (4) to the control/evaluation unit (3) completely or partially reduced such that the physical quantity is defined with a given accuracy, and such that the transmitted volume of data is minimal.

Description

Device for the unidirectional or bidirectional exchange of data

The invention relates to a device for unidirectional or bidirectional exchange of data between a measuring unit that measures a physical quantity and a control / evaluation unit that determines the physical quantity with a predetermined accuracy based on the measurement data provided by the measuring unit, and wherein the measuring unit is connected to the control / evaluation unit via at least one data line.

Measuring units for determining / monitoring a physical quantity, such as fill level, pressure, temperature, flow, density, provide, in addition to a wealth of measurement data, also other information which is important for the user and the operating and service personnel. This additional information may e.g. are device parameters or service or diagnostic data.

The time factor comes into play as soon as the measuring unit is physically separated from the control / evaluation unit and both units communicate with each other via a data line. The measurement data is usually transmitted via the data line according to one of the known transmission standards or field protocols. The disadvantage of at least some of the field protocols is that they have a relatively low data transmission rate. This is particularly noticeable when the physical quantity to be measured or controlled is determined on the basis of a quantity of data which is composed of a large number of individual measured data. The time delay until the measured value of the physical quantity is ultimately made available or displayed is, of course, quite troublesome in connection with highly precise measuring and control systems.

The object of the invention is to optimize the data transmission between a measuring unit and a control / evaluation unit that is spatially separate from the measuring unit.

BESTATIGUNGSKOPIE The object is achieved in that at least one compression unit is provided which is assigned to the measurement unit and which compresses the measurement data supplied by the measurement unit in a predetermined cycle in such a way that on the one hand the information content of the measurement data is completely or reduced in this way via the data line to the control unit. / Evaluation unit is transmitted, that the physical size is determined with the specified accuracy, and that on the other hand the amount of data transmitted is minimal.

According to an advantageous development of the device according to the invention, the measuring unit is a fill level measuring device with a transmitting unit and receiving unit, the fill level measuring device determining the fill level of a filling material in a container over the running time of measuring signals which are reflected on the surface of the filling material as echo signals. The measurement signals are electromagnetic signals, e.g. B. microwave signals, or around ultrasonic signals. In the case of microwave signals, these either spread out in the free field or are introduced into the container via a conductive element. The measurement data provided by the fill level measuring device are preferably the amplitude values of the echo signals as a function of the transit time from the measuring unit to the surface of the filling material. The corresponding measurement data are referred to below as the measurement data of the digital envelope.

Devices that check the fill level of a product in a container

Determine the running time of measurement signals, take advantage of the physical laws, according to which the running distance is equal to the product of the running time and the speed of propagation. In the case of level measurement, the running distance corresponds to twice the distance between the antenna and the surface of the product. The actual useful echo signal, i.e. the proportion of the measurement signal that is reflected on the surface of the filling material, and its transit time are determined using the so-called echo function or the digital envelope curve, whereby - as already mentioned - the envelope curve shows the amplitudes of the echo signals as a function of the Distance 'antenna - surface of the product' reproduces. The level itself then results from the difference between the known Distance of the antenna from the bottom of the container and the distance of the surface of the filling material from the antenna determined by the measurement.

If high-frequency microwave signals are used as measurement signals, the echo signals are usually transformed into the low-frequency range using a sequential scanning method before the evaluation. The intermediate frequency signal created by the transformation is then evaluated. It is characterized by the fact that it has the same course as the envelope curve; however, it is stretched by a defined time delay factor. The advantage of transformation on the

Intermediate frequency can primarily be seen in the fact that relatively slow and therefore inexpensive electronic components can be used for signal detection and / or signal evaluation. An embodiment of a method for sequential sampling of echo signals is described in DE 31 07 444 A1.

In order to determine the fill level of a filling material in a container, the envelope curve, which consists of a large number of measurement data, or a part of the envelope curve can be transmitted via a data line. Each measured value is composed of a predetermined number of digital characters. Since the probability of the occurrence of a digital character depends on the previous character, it is possible to perform data compression. The data compression does not reduce the amount of information contained in the data. In the device according to the invention, the principle known from information transmission is used in the device according to the invention, according to which a coherent series of data only in the limit case that it consists of random and completely independent values, the maximum number of digital characters per value actually available needed. If this is not the case, lossless compression of the

Data are made. Further details can be found in the book 'Nachrichtenentechnik' by Steinbuch, Rupprecht, published by Springer Verlag, 1973. On page 359 of this book, reference is made to the publication of Shannon: 'Mathematical Theory of Information Transfer' from 1948. In this publication, the Requirements that must be met for a complete information transfer, explained.

In connection with the present invention, it is particularly advantageous if the measuring unit or the compression unit is assigned a computing unit which calculates the difference from two successive measured values or from two successive amplitude values of the digital envelope curve and the measurement data thus obtained for the purpose of compressing the Compression unit provides. In this way, the amount of data that must be evaluated to determine the fill level can be reduced by half.

An embodiment of the device according to the invention provides that the compression unit reduces the number of measured values. For example, only every nth measured value is compressed, where n c N and N> 1. This results in a slight loss in terms of measuring accuracy, but this minus of information is largely compensated for by the accelerated provision of measured values.

According to an advantageous alternative of the device according to the invention, it is further provided that the computing unit / compression unit selects or determines and compresses one from a predetermined number m of successive measured values, with mc N, with the selected or determined and subsequently compressed Measured value either around the first measured value, the last measured value, the maximum

Measured value, the minimum measured value, the median (the values are sorted and the minimum and maximum values are discarded) or the mean of the m successive measured values. The compression unit preferably compresses and transmits only the most significant bits of each measured value.

A preferred embodiment of the device according to the invention proposes a decompression unit which is assigned to the control / evaluation unit. In particular, this configuration makes it possible to specify the same fixed coding table for the compression unit and the decompression unit, according to which both compress the measurement data or decompress. It is then unnecessary to transmit the coding table regularly over the data line, which is necessary if an optimal coding table is determined in the measuring unit for each data record. According to the theory of information transmission, there is an optimal coding table for each data record, which is composed, for example, of a large number of individual measured values, i.e. a coding table in which on the one hand the information content is retained as completely as possible and on the other hand the amount of data is minimal. An algorithm according to which this optimal coding table can be created has become known as the Huffmann algorithm under the name of its discoverer. The Huffmann algorithm, as well as other known algorithms for data compression, for example run length coding, which will be described below as a preferred embodiment of the device according to the invention, are in the book 'Algorithms in C by Robert Sedgewick, Addison-Wesley Publishing Company - Verlag, chapter 22).

A further development of the device according to the invention proposes that the computing unit / compression unit compress the measurement data according to a variable coding table; the coding table is synchronized to the cycle with which the measurement data are made available; Furthermore, the coding table is determined in such a way that it is optimally adapted to the measurement data to be transmitted in one cycle, for example to the measurement data of a digital envelope.

An alternative embodiment of the device according to the invention to the aforementioned embodiment proposes that the computing unit / compression unit compresses the measurement data after a run length coding. Further information can be found in the book 'Algorithms in C. The run length coding or the coding with variable length can always be used advantageously if a data record has a large number of identical values. An example is a data record with the string AAAAÄBBBBBBBB. In the case of run length coding, the number of subsequent characters is now written after a special character, for example a% character. In the example selected, the above thirteen-character data record can be made as follows represent:% 5A% 8B - so without loss of information it can be replaced by a data record consisting of six characters.

A device has proven to be particularly favorable, the individual components of which are configured as follows: the computing unit / compression unit compresses, for example, the measurement data in each case using an optimized coding table and additionally according to the run length; the computing unit / compression unit then determines the number of measurement data to be transmitted for each of the two encodings and then selects the variant in which the amount of data to be transmitted is minimal. Of course, more than two compression methods can also be used to determine the cheapest option.

The data transmission via the data line is preferably carried out according to a known transmission standard. Examples include the HART protocol, the Fieldbus Foundation standard or the Profibus PA standard.

The invention is illustrated by the following drawings. It shows:

1: a schematic representation of a preferred embodiment of the device according to the invention,

2: a representation of two schematic envelopes at different fill levels and

3: a flowchart which describes the cooperation between the measuring point and the control / evaluation unit for the purpose of finding and transmitting a minimized amount of data.

Fig. 1 shows a schematic representation of a preferred embodiment of the device according to the invention. A filling material 13 is stored in a container 15. The fill level measuring device 8 serves to determine the fill level L. which is mounted in an opening 18 in the lid 19 of the container 15. Transmitted signals, in particular microwaves, generated in the signal generating / transmitting unit 9 are emitted in the direction of the surface 14 of the filling material 13 via the antenna 12. The transmitted signals are reflected as echo signals on the surface 14. These echo signals are received in the receiving unit 10 and, if necessary, transformed to the intermediate frequency. In particular, as already described at the previous point, the time-stretched digital envelope curve is formed, which describes the amplitude values of the echo signals as a function of the transit time between antenna 12 and surface 14 of the filling material 13. Incidentally, the correct timing of transmission of the transmission signals and reception of the echo signals takes place via the transmission / reception switch 11. Of course, a transmission antenna and a separate reception antenna can also be used.

The measurement data of each data record, that is to say the measurement data of a digital envelope curve in the preferred and shown case, are compressed in the computing unit 7 and / or in the compression unit 6 according to a fixed or variable coding table. The compressed measurement data are then forwarded via the data line 4, 5 to the control / evaluation unit 3 and decompressed in the decompression unit 17 in accordance with the coding table used in each case. The measured value is displayed or output on the display / output unit 16.

2 shows a representation of two schematic envelopes at different fill levels L1, L2. Digital envelopes have the following properties regardless of the level: After a pronounced peak that occurs in the vicinity of the zero point and that is caused by reflections in the antenna area of the level measuring device 8, the curve falls over a distance of more or less depending on the level almost constant amplitude value. A second peak is based on the reflection of the measurement signals on the surface 14 of the filling material 13 at level L1 or at level L2. The less filling material 13 is present in the container 15, the further the peak moves in the direction of the positive x-axis, the amplitude of the peak becoming slightly smaller. Due to the data compression, the information contained in a coherent data record, for example in the digital envelope, can be transmitted over the data line 4, 5 in a shortened period of time. Measured values can therefore be updated at relatively short intervals, which is of course extremely important for fast control processes; or - alternatively and from a different point of view - an extended amount of data and consequently more information can be transmitted via the data line within a defined period of time.

3 shows a flowchart which illustrates the interaction of the measuring point and the control / evaluation unit with regard to the data exchange according to a preferred embodiment of the device according to the invention. This embodiment is preferably used in connection with the transmission of the data of the digital envelope curves, which are used for determining the fill level L of a filling material 13 in a container 15.

The compression unit 6 and the decompression unit 17 have direct access to the same coding table; a fixed coding table is therefore stored in both units 6, 17. This coding table is preferably designed so that the number of measured values contained in each data record in the form of bytes and / or the number of digital characters (bits) which uniquely characterize each measured value is kept to a minimum. The former can be implemented, for example, by coding the difference between two successive measured values. Furthermore, it is possible to use only every nth value or one of m successive values for the measurement value determination. Since the measured values or the difference values of two successive measured values of an envelope curve are partly the same or have a high degree of similarity, B. the run length coding effectively reduce the amount of data to be transmitted.

As already mentioned above, it is also possible to determine the amount of data to be transmitted with the different compression algorithms. The computing unit 7 then selects the method for compressing the measurement data in which the amount of data to be transmitted is minimal. It should be noted, however, that a variable coding table together with the data contained in the data set, via the data line 4; 5 must also be transmitted. In order to correctly determine the actual transmission time, the data in the coding table must always be taken into account in the amount of data to be transmitted.

To achieve a further optimization, the amount of data to be transmitted can be calculated using at least two different algorithms. The transmission ultimately takes place according to the algorithm in which the minimum amount of data is to be transmitted. As already mentioned, this is composed of the compressed measurement data and the data from the coding table.

Let us now come back to the concrete example shown in FIG. 3. The coding table and the number P of measured values that are present in a data record and for the purpose are specified

Evaluation of the physical quantity, here the level L, are to be used.

After the program starts at point 20, the measurement data P of a data record is compressed successively under program points 21 to 25. For this purpose, the corresponding coding is determined for each value p, with p = 1, 2,... P, using the predetermined coding table (point 22); the coded value is then stored in a buffer. If the coded, compressed data of the data set are completely available, the data are transmitted from the measuring unit 2 to the control / evaluation unit 3 via the data line 4, 5 in accordance with the field protocol used in each case. This transmission is characterized by program point 26.

In the control / evaluation unit 3, the data is decompressed under program points 27 to 31. In particular, the decoding is successively sought for each of the values p = 1, 2,..., P using the predetermined coding table; the corresponding decoded value of p is saved. If all decoded values P of the data set are available, then the current measured value represented by the data set is determined under point 32. The current measured value is displayed or output on the display / output unit. LIST OF REFERENCE NUMBERS

device according to the invention

measuring unit

Control / evaluation unit

data line

compression unit

computer unit

level meter

Signal generation / transmission unit

receiver unit

Transmit / receive switch

antenna

filling

Surface of the product

container

Display / output unit

decompressor

opening

cover

Claims

claims

1. Device for unidirectional or bidirectional exchange of data between a measuring unit (2) for measuring a physical quantity and a control / evaluation unit (3) which determines the physical quantity with a predetermined accuracy based on the measurement data provided by the measuring unit, the Measuring unit (2) is connected to the control / evaluation unit (3) via at least one data line (4), at least one compression unit (6) being provided, which is assigned to the measuring unit (2) and which is connected to the measuring unit (2). Measurement data supplied in a predetermined cycle is compressed in such a way that on the one hand the information content of the measurement data is transmitted completely or in such a reduced manner to the control / evaluation unit (3) via the data line (4) that the physical quantity is determined with the specified accuracy, and that on the other hand, the amount of data transferred is minimal.

2. Device according to claim 1, wherein the measuring unit (2) is a level measuring device (8) with

Transmitting unit (9) and receiving unit (10), the fill level measuring device (8) determining the fill level (L) of a filling material (13) in a container (15) over the transit time of measurement signals which are on the surface (14) of the filling material ( 13) are reflected as echo signals.

3. Apparatus according to claim 2, wherein the measurement data is the amplitude values of the echo signals as a function of the distance from the measuring unit (2) to the surface (14) of the filling material (13), that is to say the so-called digital envelope curve or at least a partial area of the digital envelope.

4. Apparatus according to claim 2 or 3, wherein the measuring unit (2) or the compression unit (6) is assigned a computing unit (7) which consists of two successive measured values or the two successive amplitude values of the digital envelope, the difference is calculated and the measurement data obtained in this way are made available for compression by the compression unit (6).

5. The device according to claim 1 or 4, wherein the compression unit (6) compresses only every n-th measured value, where nc is N ^' and N> 1.

6. The device according to claim 5, wherein the computing unit (7) / compression unit (6) selects or determines and compresses one from a predetermined number m of successive measured values, with m <= N, with the selected or determined and subsequently compressed measured value is either the first measured value, the last measured value, the maximum measured value, the minimum measured value, the median or the mean of the m successive measured values.

7. The device according to claim 1, 5 or 6, that the compression unit (6) compresses only the most significant bits of each measured value.

8. The device according to one or more of the preceding claims, wherein a decompression unit (17) is provided, which is assigned to the control / evaluation unit (3).

9. The device according to claim 1, 2, 3, 4, 5, 6, 7 or 8, wherein the compression unit (6) and the decompression unit (17) compress or decompress the measurement data according to a fixed coding table.

10. The apparatus of claim 1, 2, 3, 4, 5, 6, 7 or 8, wherein the computing unit (7) / compression unit (6) compresses the measurement data according to a variable coding table, the coding table to the cycle with which the Measurement data are made available, is synchronized and the coding table being determined in such a way that it is optimally adapted to the measurement data to be transmitted in a cycle, for example to the measurement data of a digital envelope.

11. The device according to claim 2, 3 or 4, wherein the computing unit (7) / compression unit (6) compresses the measurement data after a run length coding.

12. The apparatus of claim 2, 3 or 4, wherein the computing unit (7) / compression unit (6) each the measurement data using an optimized coding table and according to

Run length compressed, the computing unit (7) / compression unit (6) determining the number of measurement data to be transmitted in each of the two cases, and wherein the computing unit (7) / compression unit (6) in each case selects the variant in which the number of the to amount of data transferred is minimal.

13. The device according to one or more of the preceding claims, wherein the data transmission via the data line (4) takes place according to a transmission standard.

14. The apparatus of claim 13, wherein the transmission standard is, for example, the HART protocol, the Fieldbus Foundation standard or the Profibus PA standard.