June 14, 2007 by Ian Verhappen
One of the difficulties faced by fieldbus technologies is supporting signal types that they were not originally intended to support. For example, although Foundation Fieldbus supports discrete signals with its discrete input and output blocks, there are few devices that actually make effective use of this capability. Several output devices, such as valves, support the on/off feature, but in the case of limit switches and similar “contact” devices, the present H1 direct options require a field-mounted device that reads the incoming contact or coil status.
This is similar to a nano-PLC needing a local power supply voltage to wet the terminals, as well as those of the discrete device being measured/controlled. The device is not truly standalone because it requires a local power supply in addition to the communication cable.
Similarly, other protocols, such as AS-i, do not support analogue signals well, often because they do not have a large enough data packet to provide the necessary signal resolution. (AS-i now has a mechanism to combine multiple messages to support analogue, but it is not native to the original protocol.)
A larger issue, and one becoming more prominent as we seek more efficient ways to use energy, is collecting data and controlling motors via digital communications. Industry is using more variable speed and frequency drives connected to pumps and other prime movers as a way to maximize the amount of energy input being put to work on the process. (A valve is a variable area orifice that converts the change in pressure or flow to temperature, meaning the energy put into the process is lost as heat to the ambient environment – not a good use of the expensive electricity or steam used to increase the pressure in the first place.)
The most commonly used protocols for communications with motors are Profibus DP and DeviceNet. This is not surprising since each protocol is supported by a major drive manufacturer. However, with the adoption of the various flavours of industrial Ethernet and its associated higher bandwidth, additional protocol possibilities will soon be available.
Profibus has different flavours of ProfiNET to meet the needs of the application based on the response time required. As a result, if the process is controlled by process automation with an interface to DP, the lowest of the ProfiNET speeds can be used. However, in the case of precision control of three-axis robots, the highest Internet Real Time (IRT) version of ProfiNET is required.
The Fieldbus Foundation is developing a series of specifications to standardize the remote I/O capabilities of its protocol as a means of bringing high-speed data ollection of analogue and discrete I/O into the control system. It will include a number of protocol gateways to directly read and “translate” the native motor protocol to high-speed Ethernet (HSE). Once in the HSE format, it is possible to integrate this information directly into the control and asset management/maintenance systems at a facility using e-EDDL and the upcoming OPC-UA enhancements. The expectation is that the first phase of this work to integrate conventional I/O will be completed in late 2008, and that phase two will address the integration of other protocols into HSE as a gateway device.
In the next issue, we’ll talk about some of the misconceptions around field cabling options, especially those related to Foundation Fieldbus H1 installations.
Ian Verhappen is an ISA Fellow, ISA certified automation professional, adjunct professor at Tri-State University and director of industrial networks at MTL Instruments, a global firm specializing in fieldbus and industrial networking technologies. E-mail him at Ian.Verhappen@ICE-Pros.com, or visit his website at www.ICE-Pros.com.
- At a press briefing at the Interkama 2007 Trade Fair and Exhibition in Hannover, Germany, the EDDL Cooperation Team (ECT) and FDT Group announced that they will combine efforts and work towards a single, unified solution for device integration compatible with both groups’ technologies. The agreement lays the groundwork for developing a common device integration technology consolidating the advantages of EDDL and FDT technologies. As part of the agreement, the FDT Group will join the ECT as its newest member. Future technology developments will use a subset of the OPC Unified Architecture within a client-server architecture.
- The HART Communications Foundation released the Wireless HART protocol standard to its members at Interkama 2007. This will make it possible to develop wireless HART communications between field devices and gateways to provide access to additional information, beyond the process variable available from HART devices.
- Rockwell Automation and Cisco Systems recently announced that, as part of Cisco’s “Ethernet to the Factory,” they will be leveraging their participation in the ODVA standards group to develop reference architectures and detailed design guidelines for the use of common networking technologies across the production and enterprise network.
- PLCopen will start a new security working group. The goal is to define a test bed and a set of routines, implement a set of software tools and scripts to perform these tests, and make this available to the public to prove a level of networking security within a broad set of control systems. When completed, PLCopen plans to make the resulting products available to the open market, so other parties can perform these tests.