Tips to help you select fieldbus sensors
April 28, 2005
By Ian Verhappen
People always ask me: “What makes fieldbus sensor selection different than flow, level, temperature or pressure sensor selection?” I tell them that fieldbus sensors use less power than conventional analogue sensors, and can do more with less-including sense and transmit multiple readings from a single device. The selection of the sensor itself remains unchanged.
Take a temperature transmitter, for example. Most fieldbus transmitters are capable of accepting, conditioning and transmitting two measurements from one field device. Some manufacturers have a temperature transmitter specifically designed to handle eight signals with a single device. The popular Coriolis meter has multiple signals that can be sent from a single device without modifications, extra wires, or alterations to the device’s design. The biggest advantage of fieldbus sensors is you require fewer devices and associated pipe penetrations, and save on cabling costs.
Before selecting a fieldbus supplier, you should consider a few things. When it comes to how much power a fieldbus system uses, the rule “less is more” applies. It is always better to select a device that requires little current and operates at a relatively low minimum voltage. Doing so will allow you to place more devices on a single network or segment, which will result in larger construction savings in the field.
Another consideration is the capacitance of the device as it impacts its ability to be Fieldbus Intrinsically Safe Concept (FISCO) certified. All things being equal, having a FISCO-certified device is better than an Intrinsically Safe (IS) device because you can install the FISCO-certified one in a wider variety of situations including IS. FISCO-certified devices also tend to be less taxing on your system.
Foundation Fieldbus uses the Interoperability Test Kit (ITK) to confirm device compliance against its specifications. It’s always a good idea to test your device against the latest version of the (usually) annually updated test kit to ensure you are taking advantage of all your device’s features. The current ITK is version 4.6. Version 5.0 is in the works and will likely be available at the end of the year.
ITK 4.6 is the first version that contains transducer-block testing. If you are planning to purchase a pressure transmitter, the transducer block in the device will now have to be compatible with this specification to obtain the Foundation’s approval.
Function blocks are the key to fieldbus technology. Since many host systems have trouble instantiating function blocks, it is important that each device have at least one instantiated function block, such as an AI, AO, DI or DO, to transmit at least a single value between the host and the field.
You also want to think about what function blocks you require, and in what format. A device with one AI generates a single output that can be used for control. A device with multiple AIs (MAI) is capable of a single output consisting of eight readings. Devices with multiple AI blocks can be used for closed loop control, with each block taking on a different value.
The other blocks to think about are the PID and LAS blocks. This is a personal choice, though you should have at least one device on each segment that is LAS capable so it can serve as the back-up LAS to the master or primary LAS in the host.
The last issue to consider is host compliance. If you have already selected your host system, you should check your supplier’s list of approved devices to confirm that the company has documented the operation of the device you want. The Host Interoperability System Test (HIST) and capabilities files allow the host system to support each device with the fieldbus checkmark.
Ian Verhappen is director of ICE-Pros, Inc., an independent instrument and control engineering consulting firm specializing in fieldbus, process analyzer sample systems and oil sands instrumentation/control. E-mail Ian at Ian.Verhappen@ICE-Pros.com or visit www.ICE-Pros.com.