01 September 2004
Think outside the zone
Conveyors look to smart sensors for simple solution.
By Peter Cicchetto
When Roach Conveyors' vice president of sales, Charlie Parks, and electrical engineering manager, LaRoy McCann, began designing a new series of zero pressure accumulation conveyors, they looked for technology to meet the industry's demand for transporting lighter packages at high speeds while streamlining the control system. "We've had accumulation conveyors for years," Parks said, "and we always used pneumatic logic or mechanical linkages in our zone control." But such technology wasn't befitting of the trends taking place in today's market.
Trumann, Ark.-based Roach Conveyors is a conveyor systems producer specializing in turnkey solutions and 24-hour shipment of standard conveyors. They use a combination of the latest manufacturing processes and hard work to build material-handling products, from belt and roller conveyors to high-speed diverters and sorters.
End users like Roach have been looking for ways to reduce the material and labor costs associated with traditional programmable logic controller (PLC)-based accumulation-conveying applications. Roach specifically wanted to combine sensor and logic in a simple solution—suitable for their full range of conveyors—light- and heavy-duty models alike.
Photoelectric sensors have emerged in the last five years as the predominant technology in the industry, replacing the mechanical sensor rollers most commonly found in accumulation conveyors. Yet the problem with most photoelectric sensors is that you still need to wire them back to the controller, where the necessary conveyor control logic occurs. As Parks and McCann investigated the options on the market, they found the available solutions involving sensors with embedded logic weren't rugged enough for industrial applications. "Everything out there involved running ribbon cable along the conveyor and tapping off of it with little squeeze-type connectors—that didn't look industrial duty to us," Parks said.
After evaluating products that didn't fit the bill, Parks and McCann asked Jim Berry at Arkansas Industrial Electric for help. The solution was a zone-control photoelectric sensor featuring built-in accumulation zone control in a stand-alone package. Outfitted with multiple cable leads, each sensor has connections for an actuator and the two neighboring sensors immediately upstream and downstream. The leads carry 24V direct current power while allowing communication between sensors for effective control of up to 50 conveyor zones (25 zones powered on each side of the power supply). So each sensor is able to talk to the sensors in adjacent zones, informing each other when to turn their respective roller mechanisms on and off based on zone status.
Using photoelectric sensing technology eliminates the need for mechanical switches, whose minimum package weight restrictions have hindered applications with light packages or empty cartons. The polarized retroreflective sensing mode of this new sensor allows it to sense even shiny packages at distances up to 16 feet. It also has on-board variable time delay, selectable singulation mode/slug release mode, and dual zone logic function for the minimization of gaps between packages.
Since Roach ships its conveyors in sections, the new sensor's quick-connect leads offered plug-and-play capability. So they could connect conveyors at the joints without a licensed electrician. The sensor eliminated loads of hard wiring—a previous difficulty and expense in installation and maintenance. "In addition to building conveyors, we also install them," Parks said. "The modular nature of the control system saves you a minimum of 15–20% of the total equipment cost when compared to other photoelectric logic installations, mainly because you don't need an electrician."
The built-in logic of this smart sensor allowed Roach to eliminate the controller and associated wiring while maintaining the flexibility and control smart zone conveyors were designed to provide. The sensor's internal timing functions would allow them to customize box spacing to best suit the application while facilitating loading and unloading of the conveyor. They could also adjust settings via a potentiometer right on the sensor, as opposed to having to employ and configure an external timer. Plus, they could select slug release and singulation modes using external contact closures. During tests of the smart zone conveyors at the plant, Roach successfully conveyed and accumulated boxes, catalogs, and finally individual sheets of paper.
Behind the byline
Peter Cicchetto is a senior product marketing engineer with Rockwell Automation in Chelsford, Mass.
Accumulation conveyors primer
Generally used in distribution centers to move materials from the warehouse to shipping, accumulation conveyors control product flow and optimize throughput while preventing packages from colliding. By preventing contact between packages, accumulation conveyors not only prevent damage to the items they are transporting, but also prevent jams along the line. The conveyor consists of sections, or zones, each containing one sensor and one actuator typically wired to a central controller (PLC) that provides the desired control logic. That logic dictates each zone's behavior, and may include timing functions to allow for various operations such as counting and sorting.
The sensor is generally of the photoelectric type, but mechanical sensors are still quite common. Mechanically actuated sensors, however, have minimum weight restrictions that make them ineffective for the lightest packages. Actuators can be pneumatic or power rollers that initiate conveyor movement, turning on or off based on the presence of packages in other zones as reported by the sensors. To put it simply, each conveyor zone operates based on feedback from the other zones.
The PLC typically performs conveyor logic—designed to optimize product flow while preventing collisions. The logic may include system-wide or local zone timer functions to maximize or minimize gaps between products. The controller establishes the method of release at the discharge end of the conveyor. Once product accumulates at the discharge end of the conveyor, release can occur singly or as a group, known in the industry as a slug. You might need additional advanced control functions—at high-speed distribution hubs—but the bulk of conveyor installations require only the most basic zone control.