Distribution Center – Automated Product Stacking

Project Scope: A distribution center had an automated system to de-stack totes and then stack them again as they come out of the washing machine. Replacement controls for the stacking operations were no longer available so a retrofit with modern controls was decided upon.

Previous System: The existing system implemented two stand-alone variable frequency drives—one drive for the de-stacker and one drive for the stacking operation. All of the controls were operating via digital inputs and outputs.

New System Design and Function: The application called for drives with the capability to provide full torque at zero speed in order to vertically lift the totes and hold them in position during stacking. Allen Bradley 755 drives were chosen for their ability to provide torque proving, accurate positioning with encoder feedback, and EtherNet/IP communications for seamless integration with Allen Bradley CompactLogix PLC’s. A CompactLogix PLC was added to this project as well to control all of the existing digital inputs and outputs.

CompactLogixThe customer wanted the functions to remain the same from the operator’s viewpoint. To accomplish this, the digital inputs and outputs that used to be connected to the drives are now connected to one PLC. In the logic of the PLC, the controls are programmed to function the same on the outside while still properly interacting with new components in the control cabinet.

PLC LadderAll of the logic programming for homing, starting, stopping, motor speed, and setting lifting heights is managed in one location. The PLC then communicates all of this to the drives over Ethernet. With only an Ethernet connection to the system, a technician can gain access to both drives for simpler system troubleshooting and adjustments. The added benefit of using Ethernet communications via the PLC is that, in the event of drive failure in the future, all of the programs are backed up. Rather than typing each parameter into the drive and removing all of the digital IO wires, this system allows the technician to simply download the program to the replacement drive and reconnect the Ethernet cable, avoiding costly downtime.

Project Results: This retrofit provides a state-of-the-art control solution on the inside, while maintaining consistency from the outside. Service on both parts of the process can now be accessed from one convenient location via Ethernet. Most importantly, replacement parts are now locally accessible, in stock, and simple to install, which leads to greater reliability and less machine downtime.