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Article | September 30, 2000
Ocean Spray rides a new bus
On a new Craisins line at Ocean Spray, point-to-point wiring between machine devices and their master PLC has been replaced on some machines by an open fieldbus called AS-i. Installation time is slashed, while in-plant flexibility is maximized.
The programmable logic controllers that operate so many packaging machines today are only as good as their ability to receive input signals and respond with outputs. Until recently these signals were typically carried on dedicated point-to-point wiring. But now another alternative is available and for the engineers at Ocean Spray Cranberry’s Tomah WI plant it has meant shorter installation time and undreamed-of flexibility in adding or relocating I/O devices. Most important it means PLCs from a wide variety of makers can be seamlessly integrated along the same communications line or bus.
Called AS-Interface or actuator-sensor-interface this new communications network is most commonly called AS-i. Whatever name it goes by it’s an open vendor-neutral fieldbus supported by many established manufacturers. It connects photoeyes proximity switches push-buttons valves relays and any other sensing or actuating device on a packaging machine with that machine’s PLC. It replaces traditional point-to-point hard wiring between each of a machine’s slave devices and the master PLC they serve. And it does this says the AS-i Trade Organization (Scottsdale AZ) with no sacrifice in speed of data transmission. According to AS-i in a configuration with 31 separate devices the scan cycle time is 5 milliseconds. With 62 different devices scan cycle time is 10 milliseconds.
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At the Tomah plant of Middleton MA-based Ocean Spray AS-i is the device-to-PLC communications link on two of the five machines in a new packaging line. Commissioned last May the line packs dried and sweetened cranberries or Craisins® into 10- and 25-lb boxes for food processors who use Craisins as an ingredient. The line consists of a case erector bag-in-case inserter combination scale/filler bag uncuffer/flap-closer and case taper. Ocean Spray’s Dave Frenz senior process engineer was influential in bringing AS-i to the Tomah plant.
Also enthusiastic about specifying AS-i as the fieldbus in its individual machines was Pearson Packaging Systems (Spokane WA). “When I told each machine vendor that I’d need an AS-i block in each of their machines to handle machine-to-machine communications the Pearson people said they wanted their bag inserter and bag uncuffer to also rely on AS-i as the slave device-to-PLC communications link” says Frenz. “They’re going in that direction anyway and they saw this as a great application opportunity.”
The simplicity of the AS-i bus is among the things that impresses Frenz the most.
“Take a look at a packaging machine control panel with an AS-i bus line compared to one without” says Frenz. “The AS-i version is so much cleaner. There are no big wire trains just one two-wire cable that wraps around the whole machine.”
Masters and slaves
The two-wire cable Frenz refers to is what connects sensors and actuators with their master PLC. This electrical connection is made using contacts that pierce the cable’s insulation thus contacting the two wires inside. This eliminates any need to strip the cable and wire it to screw terminals. That saves time during initial installation and when replacing worn components. As Frenz puts it “You just clamp on anywhere along the cable that you want.”
Frenz views this nearly instant connectivity as a huge benefit.
“If you want a photoeye somewhere or an air valve or some other actuator you just clamp it right on. You don’t have any wires to run back to the PLC so you don’t have to call the electrician.”
Also says Frenz the cable material is what he describes as “self-sealing.” “I can unclamp and move a device any time I want without having to somehow reseal the cable” he says.
This “clamp-and-go” feature came in handy a few months after the line went into production. What Frenz and his colleagues noticed is that if a case jammed at the filler the act of removing the jammed case often triggered a photoeye that signaled the next dump of product before the next case could reach its proper position beneath the filler.
“So I clamped onto the AS-i cable a two-button push-button device that the operator can use to disable the dump” says Frenz. “When the jam is cleared and the incoming case is in place the other push-button is pushed to signal the main PLC to signal the filler that it’s okay to dump.” It took just 25 minutes says Frenz to clamp the push-button device into place and perform the necessary ladder-logic programming. If hard wiring were in place the same task might have taken anywhere from five hours to a couple of days says Frenz.
One other feature of the AS-i technology that Frenz greatly appreciates is that it makes I/O devices self-diagnostic. A photoeye for example on the AS-i bus constantly reports four pieces of information to the line’s central PLC. First is it blocked or clear? Second is it positioned too high or too low too close or too far to perform its function? Third is the device itself experiencing an internal failure? And fourth is there a power failure?
“Normally you don’t know a photoeye is defective until you get a jam on your packaging line because the backup logic didn’t work due to the photoeye’s failure” says Frenz. “But on this line if an eye is defective I get a message right away on the HMI panel. So I can go out and replace the eye before it has a chance to fail and cause a jam.”
Frenz isn’t the only one who values the AS-i fieldbus. Don Parker of Pearson Packaging Systems which supplied the bag-inserter bag uncuffer and top taper says AS-i simplifies machine manufacturing and shortens machine installation.
“Because it is a truly open system we can complete the wiring of our machines with an AS-i configuration before we even know what PLC the customer prefers” says Parker. “Then when the PLC is specified we put in whatever brand the customer wants.”
Installation is simplified too says Parker.
“With push-buttons brakes indicator lights photoeyes and so on there might be 64 devices on the bag inserter alone” says Parker. “If they have to be hard-wired to the PLC installation might require five days. With AS-i that installation time is more like one day.
“Other fieldbuses” Parker continues “like DeviceNet for example would be overkill in this kind of application. DeviceNet delivers I/O to and from variable-frequency drives servo drives motion controllers and so on. It’s much more sophisticated than a binary system like AS-i. On/off present/absent blocked/clear—that’s all the information AS-i is designed to carry to the PLC. Now if you want to know the temperature of a proximity switch or if you want to know how much dust has collected on a photo eye that’s what DeviceNet is good for. In many instances that level of information is overkill” says Parker.
In addition to delivering I/O within single machines AS-i also can act as a real-time bus permitting the line’s central PLC a Simatic 555 from Siemens (Alpharetta GA) to perform machine-to-machine communications or “interlocks” as it’s sometimes called. Suppose for instance there’s a case jam at the filler and the upstream case erector must be “told” to pause. The jam at the filler is detected by a photoeye which signals that information over the AS-i bus to the Siemens PLC. (The filler is controlled by this PLC.) The Siemens PLC then sends a signal again over the AS-i cable to the case erector’s PLC thus triggering an actuator that causes the case erector to pause.
What’s significant here is that while the line’s central PLC is made by Siemens the case erector has an Allen-Bradley PLC from Rockwell Automation (Milwaukee WI). Also supplied by Rockwell are the PLCs in the Pearson bag inserter and bag uncuffer.
“With AS-i you can use a mix of PLCs if you want because it’s an open protocol” says Frenz. “My main controller doing the ‘traffic copping ’ around the line is made by Siemens. But individual machines have Allen-Bradley PLCs.”
Frenz notes that one additional component must be included in a machine’s control panel if an Allen-Bradley PLC is to communicate along an AS-i fieldbus. Supplied by SST (Waterloo Ontario Canada) it’s a communications module that converts AS-i signals into I/O that an Allen-Bradley PLC can interpret correctly. The SST module is shown in the photo on page 66.
A similar module or gateway is required to permit the Siemens PLC to communicate along AS-i. The module in this case is supplied by Siemens.
Pleased with his AS-i experience on the Tomah Craisins line Frenz now hopes to continue moving Ocean Spray away from proprietary I/O systems on a corporate-wide basis. In addition to the advantages outlined above such a move could also save some big bucks.
“If you have a Siemens rack in a machine and you want to switch to an Allen-Bradley PLC you have to replace all the Siemens I/O cards and wiring” says Frenz. “That’s costly. If we can rely on I/O that’s vendor-independent we can avoid those costs.”
See sidebar to this article: Controls package illustrated
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