Abstract:
Field devices are commonly used in unprotected environments, which means that these field devices and the conductor pairs connecting the field devices to the input modules/output modules of an automation installation are exposed to significant climatic and environmental factors. These factors have a detrimental effect on the transmission quality of the signals to be transferred between field devices and modules, which may result in erroneous measurements and dangerous operating states during control of a technical process. Measures to simplify monitoring of the transmission medium are proposed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of European application No. 05016231.2 EP filed Jul. 26, 2005, which is incorporated by reference herein in its entirety. 
     FIELD OF INVENTION 
     The invention relates to a method for monitoring a transmission medium, via which a first device can be connected to a second device, and via which can be transferred a current signal and a digital communications signal superimposed on this current signal. In addition, the invention relates to an arrangement suitable for implementing such a method, and a device for such an arrangement. 
     SUMMARY OF INVENTION 
     A method of the type cited in the introduction is known, for example, from the Siemens SIMATIC PCS7 catalog, chapter 12, 2005 edition, where a “HART” communications procedure is described (HART=Highway Addressable Remote Transducer) which allows flexible parameterization of field devices via conventional 4 to 20 mA technology. A feature of this technology is that both the analog 4 to 20 mA current signal and the digital communications signal can be transferred on one conductor pair, with an “FSK” technique (Frequency Shift Keying) being used for superimposing the digital communications signal on the analog 4 to 20 mA signal and for the bit coding. An analog signal having a frequency of 2200 Hz is assigned to a logic “0”, and an analog signal having a frequency of 1200 Hz is assigned to a logic “1”. 
     Field devices are commonly used in unprotected environments, which means that these field devices and the conductor pairs connecting the field devices to the input modules/output modules of an automation installation are exposed to significant climatic and environmental factors. These factors have a detrimental effect on the transmission quality of the signals to be transferred between field devices and modules, which may result in erroneous measurements and dangerous operating states during control of a technical process. 
     In order to prevent this, it is necessary to check the quality of the transmission path e.g. by measuring manually at the input module/output module a current indicated by a sensor and comparing it with the indicated current. 
     An object of the present invention is to define a method of the type cited in the introduction, which is used to simplify monitoring of a transmission medium. In addition, an arrangement shall be created that is suitable for implementing such a method, and also a device for such an arrangement. 
     The object is achieved by the independent claims. 
     It is advantageous that the transmission medium is checked automatically. The comparison result can undergo further processing in a higher-level “Asset Management System” in order to provide the user with information on the degradation level of the transmission medium in a diagnostics and maintenance view, where this Asset Management System also suggests to the user appropriate maintenance actions. 
     The invention is based on the idea that the digital communications signal representing a measured value or setpoint value is largely independent of the quality of a transmission medium. If the measured value or setpoint value determined from the communications signal differs from the current signal representing a measured value or setpoint value, then this indicates a fault in the transmission medium. 
     Further advantageous embodiments of the invention follow from the subsequent dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, its embodiments and advantages are explained in greater detail below with reference to the drawing which illustrates an exemplary embodiment of the invention. 
         FIG. 1  shows a remote I/O unit, and 
         FIG. 2  shows a process control system. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Reference is made to  FIG. 2  first, which shows a process control system  1  known per se. This process control system  1  has an engineering system  2 , a control and monitoring system  3  and a programmed and configured automation installation  4 , where the engineering system  2  is provided for programming and/or configuring the automation installation  4 . The automation installation  4 , which is connected via a bus system  5  and bus connections (not shown here) to the engineering system  2  and the control and monitoring system  3 , comprises various automation devices  6 ,  6   a , plus actuators  7  and sensors  8  and other automation components  9  required for controlling a technical installation. The automation devices  6 , which are connected together via the bus system  5  and/or other suitable bus systems  10 , can have different designs. Thus automation devices for performing small, medium-scale and larger automation tasks are usually provided within the automation installation  4 , where micro programmable logic controllers can be used to perform small automation tasks, mini programmable logic controllers for performing medium-scale automation tasks and powerful programmable logic controllers for performing more complex automation tasks. The process control system is also provided with what is known as a remote I/O unit  11 , which is connected to a master module of the automation device  6   a , and is also connected to a sensor  14  and an actuator  15  via a respective I/O module and a conductor pair  12 ,  13  respectively, where the 4 to 20 mA technology based on the HART specification, known per se, is used. 
     Which hardware and software components the automation installation  4  is to be provided with, i.e. which automation devices, control and monitoring devices, bus systems, actuators and sensors, and which control programs it is to be provided with, depends on the complexity of the technical installation to be controlled and on the control task, where the required hardware and software components of the automation installation  4  can be programmed and/or configured by the engineering system  2 . 
     Climatic and environmental factors may now have a detrimental effect on the transmission quality of the current to be transferred via the conductor pair  12  from the sensor  14  to the master module of the automation device  6   a . Measures are therefore provided to monitor the quality of the conductor pair  12 . These measures are described in greater detail below with reference to  FIG. 1 , which shows the remote I/O unit  11  of  FIG. 2 . The remote I/O unit  11  is provided with a master module  16 , via which the remote I/O unit  11  is connected to the automation device  6   a . In addition, the remote I/O unit  11  comprises an analog input module  17   a  and an analog output module  17   b , with the sensor  14  being connected to the analog input module  17   a  via the conductor pair  12 , and the actuator  15  being connected to the analog output module  17   b  via the conductor pair  13 . The remote I/O unit  11  can obviously comprise other analog input modules and/or analog output modules for connecting additional sensors and/or actuators. 
     It is assumed below that the analog input module  17   a  supplies the sensor  14 , e.g. a sensor in the form of a temperature transducer, with a constant voltage. As a result of this supply voltage and as a result of a measured temperature of a heating boiler, a current representing a measured value flows from the temperature transducer to the analog input module  17   a , and a microcontroller  18  of the temperature transducer determines the measured value from said current. The temperature transducer also transfers to the analog input module  17   a  via the conductor pair  12  a digital communications signal representing this measured value and superimposed on the current, where periodically or at definable times an evaluation unit  19  of the analog input module  17   a  determines the measured values from this communications signal and from the current, and compares them. In the event that the comparison result reaches a definable value, the evaluation unit  19  produces a fault signal and transfers it e.g. to the control and monitoring system  3  ( FIG. 1 ), which displays a suitable fault message and/or initiates appropriate maintenance actions in a diagnostics and/or maintenance view. Of course the evaluation unit  19  can be designed such that it not only reports a fault signal in the event of a fault, but reports the transmission quality in the form of a status signal whatever the situation. In addition, the evaluation unit  19  need not necessarily be part of the analog input module  17   a , but can be part of an additional device of the automation installation that is connected to the analog input module  17   a.    
     It is assumed below that the conductor pair  13 , which connects the analog output module  17   b  to the actuator  15 , is to be monitored. For this purpose, the analog output module  17   b  transfers to the actuator  15  both a current signal representing a setpoint value and a digital communications signal representing this setpoint value and superimposed on the current signal. The actuator determines the setpoint values from these signals, compares them and in the event that the comparison result reaches a definable value, the actuator  15  reports a fault. 
     The actuator can also be designed such that it merely transfers to the analog output module  17   b  a digital communications signal representing the comparison value, where the analog output module  17   b  or a device of the automation installation connected to this analog output module  17   b  reports a fault if the comparison result reaches a definable value.