Gantry-Style Automated Accumulation
Feed rates and material flow can be challenging with any automated line. Worthington Industries’ plant in Wisconsin worked through those challenges recently with the help of robotics, new controllers, and touchscreens, among other integrated technologies. The company, a global supplier of pressure cylinders, produces LPG (liquefied petroleum gas) cylinders used to hold fuel for gas barbecue grills, camping equipment, handheld torches, and other uses.
The Wisconsin plant had a conveyor line that ran between a valving station and a filling station. For the filling station to work effectively, a constant backlog of cylinders must be maintained. Multiple operators managed the backlog prior to the filling station by adding or removing cylinders manually to the main conveyor. When the filling station was unable to keep up with the valving station, the cylinders were manually removed from the line. When the rate of filling was greater than the valving station, the operators added cylinders to the production line.
|Shown is the loading of empty trays and unloading of full tray stacks. One surprise was the extent of stack height variation, the result of unforseen pallet and tray height variability. Source: HiTech|
Worthington Industries sought the help of HiTech Manufacturing Solutions Inc. in Green Bay, WI, which specializes in designing and building custom automation machinery. Scott Schroeder, HiTech project manager, worked on the concept for Worthington Industries, which led to HiTech being contracted to automate this manually intensive and repetitive process.
Integration, few alterations
HiTech designed and built a gantry-style cell to maintain the necessary backlog prior to the filling station. HiTech’s approach was cost effective, since minimal alterations were required to the existing conveyor and there was very little maintenance required.
The work cell used a two-axis Yamaha robot to load cylinders onto the production conveyor line, remove and accumulate cylinders from the production conveyor line, and add or remove cylinders as required. Accumulated cylinders are stored on plastic donnage trays. The machine is controlled by a Rockwell MicroLogix 1500 PLC and Yamaha RCX222 robot controller. The operator interface is shared between a Allen-Bradley PanelView Plus 600 Touchscreen and the RCX222 Teach Pendant. A zoned safety circuit used light curtains, area scanners, door interlocks, and a pull rope, in conjunction with standard e-stop pushbuttons to safeguard the machine.
Through the stations
There are five stations involved in the process. Each has a dedicated conveyor. The conveyors move the product and dunnage throughout the work cell.
Station 1 (dunnage loading/unloading): A reversing roller chain conveyor is used to move pallets full of dunnage into the work area (station 2) and to move pallets full of dunnage out of the work area;
Station 2 (dunnage pick and place): A reversing roller chain conveyor mounted to a scissors lift is used to move pallets with dunnage into the dunnage loading/unloading station. The scissor lift is used to maintain a consistent height of dunnage for the vacuum pick and place between station 2 and station 3;
Station 3 (loading/unloading cylinders onto production conveyor): A roller conveyor with a pop-up roller chain conveyor is used to make a 90-degree transfer; this conveyor is mounted to a scissor lift used to move pallets with cylinders from station 3 to station 4. The scissor lift is used to maintain a constant elevation of the cylinders for the pick and place between station 3 and the production conveyor. When the production conveyor requires cylinders, the overhead gantry picks up 10 cylinders from station 3 and places them onto the production conveyor. If excess cylinders are being accumulated on the production conveyor, the gantry will pick up 10 cylinders from the production conveyor and move them to station 3;
Station 4 (pre-staging/accumulation of pallets with cylinders for station 3). The roller conveyor, with a pop-up roller chain conveyor transfer mounted to a scissor lift, is used to move pallets with cylinders from station 4 to station 3 and from station 3 to station 4. This station is capable of holding one pallet full of cylinders for accumulation.
Station 5 (loading of pallets with cylinders for station 4): A forklift operator loads or unloads a pallet full of cylinders at this station. A roller chain conveyor is located at this station.
HiTech’s solution required dunnage that held 120 cylinders (10 across and 12 deep). The gantry has a dual head end effector. The front side of the end effector has a pair of grippers capable of picking up 10 cylinders (one row) at a time. The back side of the end effector is capable of picking up an empty dunnage tray with vacuum.
“The biggest challenge in designing this machine,” Schroeder said, “was gripping a row of 10 bottles from the conveyor on their necks and holding them square for placement on a tray. Gripped bottles would then be placed along the side of rows of product already on a tray. Any out-of-square bottles would hit existing bottles on the tray and create a jam.”
|A Cartesian robot picks a row of bottles. The biggest machine design challenge was expected to be gripping a row of ten bottles on their necks and holding them square for placement on a tray, but that turned out to be easier than expected. Source: HiTech|
Unexpected challenges arose during startup and commissioning with the variability in tray stack heights. Variability in heights of the wood pallet and five to six tray layers caused inconsistent picking of bottles from a tray and placing them on the product conveyor. During the picking of bottles, the gripper would grip the neck higher or lower because of the pallet and tray height variability. This would cause the bottles to drop or push bottles onto the conveyor surface while reloading the conveyor line. It became evident that flexible gripper bars were required to account for the stack height variations. Approximately an inch of float was added to the gripper bar to allow for picking and gripping of bottles at a consistent height on the neck thus dropping them off at a fixed height.
In custom automation, one-of-a-kind machines there are always unexpected challenges, said Schroeder. “It’s how you deal with these things to achieve the same end result” that provides an advantage along the way, he said.
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“The automation upgrades provided Worthington with a more effective use of labor… improved quality and increased production,” said Gary Rano, Worthington Industries operation manager. Team members were proud to overcome project challenges, he added.