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Choosing output formats for instrumentation transmitters
May 06, 2011
Dear Control Engineering: The article about new gas flowmeters says that the company is using a more modular transmitter that makes it easier for the user to select a signal format. Why aren’t more companies doing that? Having to change field devices is a major drag on trying to use a more sophisticated networking technology.
One of the more interesting aspects of the flowmeters that FCI has introduced is the modularity of its transmitter. While most devices are hard wired to one specific format (e.g., 4-20 mA with HART or Foundation fieldbus), these new devices offer a plug-in module to allow users to change the output format.
I don’t know if this is the first time somebody has done this, but it certainly is not a common feature. Typically a transmitter is indeed fixed to a given protocol. This fact has made the process of migrating to a more sophisticated networking protocol much more expensive. For example, if all the devices in a process unit use 4-20 mA or some older analog voltage signal, you’ll probably have to replace them all if you want to move to a fieldbus. FCI has promised that its new transmitter is future proof in the sense that if you do need to make such a change, you can change the module (assuming the right format is available) and not the whole device. The company plans on using this approach with larger portions of its product line as time goes on.
To my thinking, a typical flowmeter or pressure sensor ought to come with a universal transmitter that is software configurable or you set a group of DIP switches to give the connectivity you want. Set it yourself for anything from 4-20 mA, Modbus, USB, Profibus, Foundation fieldbus, or any number of industrial Ethernet flavors. Maybe we’ll even throw in wireless. That isn’t quite practical yet, but I doubt it is far away. Some protocols still depend on a dedicated ASIC which is probably why having a plug-in component is still a requirement. My approach is probably still much too expensive, but might be here sooner than you think.
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Peter Welander, pwelander(at)cfemedia.com
Thursday, 05-01-12 11:56
Milltronics (now Siemens) did this with their SmartLinx communications adapter cards for their level tranmitters. It's a great idea for non-hazardous area devices. It cuts manufacturing costs and can make more protocols available to users.
It also makes for replaceable comm cards so that when EMP from a near lightning strike takes out a couple of devices, hopefully the comm card took the hit and isolated the damage from the rest of the electronics; and the comm card is replaceable. Not all comm ports are easily replaced (like many PLC/PAC controllers).
It's my opinion that changing any internal part of a device that has a UL/FM/CSA (whatever) hazardous area approval is a no-no. I won't take the liability. The stainless riveted tag on the device with the approval was given for the configuration it came with from the factory. Maybe others would change out comm cards.