Are Networked Process Sensors In Your Future?
The latest fashion in machine controls is to network temperature and pressure sensors with fieldbus protocols for faster, more accurate data acquisition, built-in sensor diagnostics, and better machine uptime. The first distributed control networks are just beginning to appear in a few extrusion plants. But they are limited so far to large resin producers, fiber spinners, and a handful of major processors with extremely complicated lines, expensive products, and big R&D budgets.
Networked sensors may be something to consider for new plant construction but are not readily retrofittable. Networked or “distributed” systems often require software licenses and sensors with expensive interface devices. They also need more sophisticated troubleshooting and maintenance capabilities than are necessary for traditional machine controls, which have inputs and outputs from devices wired directly to a control system’s I/O panel.
“It’s not actually a pricing issue,” notes Mark Caldwell, v.p. of Gefran ISI, a maker of pressure sensors. “Most processors aren’t staffed to evaluate and test distributed systems.” In Europe—but not yet in the U.S.—Gefran has extrusion customers using CANopen networks of pressure sensors, which can monitor a line remotely and correct problems more quickly, Caldwell notes.
Dynisco LLC, which supplies pressure sensors and controls, is leading the charge for networked controls in this country because of the simpler wiring and installation, faster and more accurate data collection, and faster device response time. Digitally networked pressure sensors send data back and forth continuously through a single fieldbus cable, providing closed-loop, real-time control of all the devices in the network by an industrial PC, which may double as the operator interface.
In a traditional machine control system with a PLC, data is processed sequentially with ladder logic, which imposes delays. And with the older PLC-controlled approach, “if you had 900 two-wire pressure transmitters, that’s 1800 wire terminations to hook up,” notes Dynisco v.p. of product marketing Douglas Joy. PLCs are still used in a networked system, but for simple on/off jobs like controlling switches and valves.
Dynisco offers pressure sensors adapted to most commercially available fieldbus protocols: Foundation Fieldbus, developed by Emerson Electric; CANopen, developed by Bosch in Europe; DeviceNet, developed by Allen-Bradley; and the older HART (highway-addressable remote transmitter) system, also developed by Bosch. Dynisco doesn’t offer sensors for Profibus PA, developed by Siemens.
There are only a handful of fieldbus networked systems in extrusion and compounding so far, mostly at large resin companies. Dynisco has some extrusion installations with networked pressure sensors using Foundation Fieldbus protocols at resin companies and one at a film processor on a pilot polyvinyl butyral line. That processor already had temperature sensors networked via DeviceNet protocols and was looking to network pressure sensors on its R&D line for ease of use, reduced wiring, and greater accuracy and diagnostic capability. One additional advantage it obtains with Foundation Fieldbus is that networked control loops can continue to run the extruder independently if the central PC fails.
Another extrusion installation of distributed controls has pressure sensors networked through discrete controllers with Profibus protocols. It was engineered by the processor using Dynisco sensors and Eurotherm controls on multiple coextruders producing optical film with hundreds of microlayers. The advantage is that faster access to pressure data from the extremely complex feedblock is critical to maintaining product quality.
MPI Melt Pressure Transducers in Canada, which builds temperature and pressure sensors, believes that if processors install networked systems at all, they will use Profibus PA because of its lower installation cost. MPI, which has quoted one such networked system, notes that Profibus requires no license fee for software and uses less expensive interfaces than other protocols.
A melt-pressure transducer measures pressure and gives a raw signal in millivolt output. The millivolt transducer signal can be paired with an amplifier (signal conditioner). The amplifier can either provide an analog or a digital signal (such as Foundation Fieldbus) and can be used for display or transmission of the pressure signal.
“The sensor signal isn’t usually digital to begin with. The sooner you convert the data, the more accurate it is,” notes Al Betz, v.p. of sales at Eurotherm, which offers networkable temperature controls using DeviceNet and Profibus and has many injection molding applications and some extrusion installations using these protocols.
Dynisco notes its DeviceNet and Foundation Fieldbus networks are more accurate because they are truly digital from the initial sensor signal. Dynisco’s networkable pressure sensors and transmitters incorporate the electrical signal conditioner in the sensor itself.
Additionally, its Foundation Fieldbus devices can be programmed to communicate pressure data either to the controlling PC, or if the PC fails, directly to devices like valves that need the data. This allows the smart devices to take over machine control if needed. But the extra functions also make these sensors more expensive when they need to be replaced.
In the future, processors may encounter their first networked sensors when they buy a new extrusion line. Some European OEMs are looking into networked controls to reduce the size of control cabinets and improve the speed and accuracy of data collection, especially on complex coex cast film lines.
But networked controls could raise maintenance and spare-parts issues. Sensors from different manufacturers, may not be completely interchangeable, even if nominally designed to be compatible with the same network protocol, warns Eurotherm’s Betz.
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