With its bidirectional communication between intelligent field devices and host systems, HART has become a global standard. The example of valves shows how important the data provided by field devices can be for processes. Often, it is not only a question of how often the valve is operated, but also how long it takes to open. If this time period gets longer, it may be an indication of wear.
The correct times for maintenance or replacement are relevant. Whereas a premature replacement increases costs, a date scheduled too late may lead to a total system failure. The fact that HART is also useful with Industry 4.0 is shown in the solutions for two very different requirements: the essential differences are whether a plant is modernised toward 4.0 by means of a new control system, or whether the operator only wants to prepare more information for the targeted implementation with 4.0.
When modernising a plant, a remote I/O system is the ideal solution since new control systems work with bus protocols such as Profibus, Profinet, Modbus RTU or Modbus TCP. The remote I/O system replaces the point-to-point connection between field devices and control levels, thus enabling migration to a bus protocol. However, only the connection from the control system to the remote I/O is a bus; the existing cabling from the remote I/O to the field devices remains. The benefit of this is that plant operators can retain the existing structure with HART and 4-20 mA, and all of its advantages. In parallel to Profinet or Modbus TCP, there is an Ethernet-based communication with the control system, such as HART IP via Profinet. Direct access to the field device is also possible via HART IP. In addition, an OPC UA server can be implemented into a remote I/O gateway. The advantage of OPC UA is standardised communication according to IEC 62541 with a high data throughput for connecting different systems. The operator can then choose whether data shall be accessed via HART or other protocols.
However, if the operator only requires more information from the field devices for Industry 4.0, without a modernisation, it can be conveniently implemented via a HART multiplexer. This is connected with the control system via an RS-485 interface. The multiplexer can integrate in up to 32 different loops in the field and request HART data from each of the devices. The multiplexer can also ensure that requests from the asset management system are forwarded. This variant can be used with OPC UA to access data via the serial interface and distribute it as needed.
What are the advantages of HART-enabled devices to the operator?
The simplification of processes using HART is reflected in three different areas: the unified integration of device software; the reduced complexity with simple functions; and easy diagnostic options.
The reduced complexity is an important aspect because modern devices offer an ever greater number of higher quality functions.
Standards facilitate easy handling
More standards and fewer proprietary solutions facilitate easy handling. This includes NAMUR NE 107 for the self-monitoring and diagnosis of field devices. With a unified colour scheme and symbols, this standard ensures that five classified states can be quickly recognised by the operator. In addition, a standard such as OPC UA allows the user to exchange the actual data as well as the descriptions.
Standardised information models make it possible to interpret the data and thus reduce the configuration work. The next generation of device integration builds on this exact concept with FDI (field device integration), which specifies how the information model can provide clients via an OPC-UA interface according to IEC 62541. Previously, when a new measuring instrument or an actuator was integrated into the process, the user deployed EDDL or FDT. This meant either a text-based parameter description or a standardised, multi-vendor software interface that allowed the programming and description of complex functions and displays.
However, neither variant alone meets the current requirements when it comes to integrating functions of field devices. In contrast, FDI provides the full description of a device in an FDI device package. This is supplied by the device manufacturer and contains all the essential information for integration.
The device parameters are described via an EDDL interpreter (electronic device description language) and supplemented by complex graphic elements in freely programmable user plug-ins, just as with FDT. A further advantage of FDI is that it can be easily integrated into existing system architecture. An FDI-enabled host must therefore be available for new plants. In the case of existing plants with FDI devices in the field, the control systems must be able to understand EDDL and FDT.
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