I/O Choices for Machine Builders Enhance Flexibility

Baggage Handling Systems Often Have Long Conveyors With Many Sensors Distributed Among Them

By Sophie Hennion, Cofely Services

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CONSIDER THE CASE OF A MACHINE OR SYSTEM BUILDER THAT has a large installed base: Some of the machines and systems are on controller platforms supplied by one automation vendor, while others are controlled via other vendors' products. Any new projects and/or upgrades envisioned probably will have to accommodate those disparate systems and machines because of end-user insistence and the knowledge already invested in those machines.
 
Cofely Services' Airport Group can identify with this situation. It operates in more than 25 airports, including Boston-Logan, Louisville, Kentucky, Montreal Pierre-Elliott Trudeau, Paris Charles de Gaulle, Athens, Brussels, London Gatwick and others.

Dual Platform Support

When Yan Le Meur, automation and technology watch expert with Cofely Services, began to evaluate I/O solutions for a baggage-handling system (BHS) project, he indeed defined a specification that required both an EtherNet/IP- and Profibus-compatible device that could communicate with Allen-Bradley and Siemens PLCs.

"We didn't want to move away from the Siemens and Rockwell platforms due to end-user requirements, our own investments in supporting current solutions and employee/user comfort level," Le Meur says.

The bag room improvement project at Halifax Airport, Nova Scotia, was designed to transport baggage from check-in to carousel (Figure 1). About 100 conveyors are used to transport the bags. Each conveyor is equipped with encoders and infrared sensors with deflectors used to track bags or to detect a bag jam; a selector to start/stop the conveyor in automatic or maintenance mode; and motors from ½ to 2 hp, depending of the size of the conveyor. All this equipment is controlled by a safety PLC, and safety logic is handled by the software. Conveyor speed range is 0.7 to 1.5 m/s. During peak periods, each line can carry about 600 bags per hour into a secure area. Lines must be ready to operate from 3 a.m. until 10 p.m., 7 days per week and 52 weeks per year.

Also Read: Infographic: What Type(s) of I/O System Do You Use?

Transport time is critical to guarantee that bags arrive in time to be loaded onto an airplane, so every component must be reliable, and every fault must be identified and fixed as soon as possible to have a very low mean time to repair (MTTR). Each line also is equipped with an automatic tag reader (ATR) that sends bag data to high-level software, Bagstage, which was developed by Cofely Services. This software is used to track bags into the system and to inform the PLC as to which line must be used to send the bag to the final destination. All alarms and status are displayed on seven redundant SCADA viewers in different rooms and areas or on mobiles, and also could be accessible remotely via a VPN connection. For maintenance, alarms are logged to a historian in order to retain operating data.

Improve the I/O Scheme

Baggage handling systems often have long conveyors with many sensors distributed among them. In a traditional, centralized solution, the sensor cable would be tied back to a main control cabinet, resulting in substantial labor and cable cost.

Another possible solution would be to use remote or distributed IP20-rated I/O. This type of solution can reduce cable lengths, but multiple junction boxes are required on the machine. "The cost of junction boxes can add up quickly, so it's important to evaluate the cost of cable, junction boxes and I/O," Le Meur explains. "Cofely Services was well aware of the additional cost associated with traditional solutions. Our knowledge and experience prompted us to specify machine-mountable I/O with IP67-rated protection."

The length of the conveyors had a lot of influence on the selection criteria for this application, Le Meur adds, saying, "Flexible topology and the ability to carry signals long distances between I/O modules therefore became a key differentiator in supplier selection."

Cofely Services wanted to integrate advanced diagnostics in order to make troubleshooting the system easier for the end user. Implementing advanced diagnostics in the system would let the end user identify faults and errors much sooner and result in less downtime and more overall productivity. "LED indicators on each module indicate general and detailed errors in the SCADA system," Le Meur explains. "And because this information is easily accessible by the web server included in each module to provide a completed detailed diagnostic and status, we don't need to develop it on our side. On each conveyor, we can detect faults such as motor overload, encoder faults and bag jams."

The scan time of the network was an important consideration, too. About 2-millisecond (ms) scan rates are needed in order to connect the square signal encoder on a standard I/O module, and move this signal into the PLC's Fast Task. Cofely Services wanted to use an encoder directly on classic I/O, so there would be no need to have a high-speed card. In addition, the company wanted to have the flexibility to set up I/O on devices to connect a motor or a sensor. Conveyors are standard for I/O, but sometimes we need to locate some extra pushbuttons, lamps or stack lights close to this equipment.

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