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Case study: Ice cream factory's ergonomic challenge solved with food grade robots
By Adam Carreau
JMP Engineering's customer had an ergonomic issue in their ice cream factory. Plant workers manually grouped ice cream bars on a high-speed, flat-belt conveyor and placed them in trays on a conveyor. The trays had five different pack patterns: 1-pack, 3-packs, 4-packs, 6-packs, and 9-packs. The line ran at approximately 150 bars per minute (bpm).
The main issue was the high-speed, repetitive task of packing the bars in the 1-pack trays. Workers stood in one position, with their heads bent over the conveyor, reaching in front of them to orient the ice cream bars on the flat-belt conveyor and then pull them into the pockets of the tray conveyor. The workers were packing bars onto a conveyor moving at a blistering 30 inches per second and could stay at that position for only 15 minutes before rotating to a different job.
Robots to the rescue
JMP designed a solution that used three food-grade robots rated for high-speed, continuous-duty picking and placing. Simulations of the proposed cell were developed, and results showed the concept was feasible and operable at 99 percent overall equipment effectiveness (OEE). This target meant the work cell could miss only one bar per 100 coming down the line.
Once the work cell was shipped, the project team confronted many issues that had been either unforeseen or assumed to be manageable onsite. The fundamental challenge was the speed of the product coupled with the variability in product flow. This particularly affected the robots on two pack patterns: the 1-pack pattern, where the tray conveyor was moving at 30 inches per second, and the 4-packs, where the robots were restricted to picking only two bars at a time instead of the usual three. Because the robots were picking only two bars at a time, this effectively forced a reduction in the robots' efficiency.
The variability in the flow caused by mis-wrapped bars, slight delays, inconsistent feeding from upstream, and so on, caused major problems for the robots. Bars that were one-half inch closer than they should be resulted in a momentary, instantaneous spike in flow rate to 180 bars per minute. Conversely, gaps in product flow resulted in momentary dips in flow rate down to 100 bars per minute. For the process to work, the robots needed to be able to handle these instant spikes and fill in the gaps to ensure that the right number of product went into the box every time. The variability resulted in the robots sometimes missing bars (sending them off the end) and sometimes not having enough to pack the trays, resulting in partially filled boxes.
Another issue was placement accuracy on the 1-pack pattern. The speed of the tray conveyor, coupled with the scan time limitations of the robots, made it difficult for the robots to be consistent. The scan time of the robots was approximately 12 milliseconds, with the speed of the conveyor being 1 millisecond of time equating to approximately 1 millimeter of travel. Therefore, the drop position of the ice cream bar could vary by approximately 12 mm. This resulted in some partial or complete misplaces by the robot. Due to the above issues, the performance of the cell was far from reaching the desired design performance.
Dealing with the ebb and flow
To deal with the variability in flow, JMP replaced the entire in-feed conveyor system with a unique conveying and buffering system. This buffering consisted of a series of independent motor drives that allowed the product to be conveyed at different speeds on different sections of the conveyor. This conveyor would decelerate the bars, forcing them to group together and align in the process. The conveyor would then accelerate them before presenting them to the robots, which would create a uniform, consistent gap between them. Product flow became smooth and consistent; robots could run continuously steadily, without instantaneous spikes or dips in flow.
One final hurdle remained. The drop accuracy on the 1-pack pattern threatened the project's success. A dedicated motion controller was added to the system to specifically control the robot grippers. JMP worked with the robot vendor to integrate the motion controller with the robot software and other parts of the system. The motion-controller's scan time was approximately 100 times faster than that of the robot's to insure proper drop precision, dramatically improving accuracy over performance of the robot by itself.
After implementing the new motion controller and eliminating the last performance issue, the system ran flawlessly and met the desired performance targets.
ABOUT THE AUTHOR
Adam Carreau, B.E.Sc., M.Eng, is a controls specialist with JMP Engineering's automation group based in London, Ontario. He is responsible for designing, testing, and commissioning solutions using robotics, machine vision, and controls technologies. JMP Engineering is a certified member of the Control System Integrators Association. To learn more about CSIA and CSIA Certification, visit the association's website at www.controlsys.org.