Smart machine control creates unique sortation system

Sortation subsystem benefits from integrated automation to redefine efficiency in high-speed distribution centers.
By Jim McMahon for Dematic March 1, 2010
FlexSort system is defined by its systemwide, distributed, variable-speed control; it automatically adjusts speed incrementally as required.

FlexSort system is defined by its systemwide, distributed, variable-speed control; it automatically adjusts speed incrementally as required.

In a high-volume distribution environment, the speed of package flow through the system as determined by the sortation equipment often determines production speed. It has been a standard for some time that to increase throughput one must increase the speed at which the sortation equipment is running. But, running a sorter faster increases vector stresses on the equipment and on the packages, which means less package control and more maintenance effort. It also decreases the life of the equipment and ultimately increases the equipment cost of ownership.

Dematic Corp. (formerly Rapistan) changed this scenario with its FlexSort SL Parallel Diverting sorter system, which benefits from smart machine control. The system is capable of processing 300 cartons per minute at the same speed that contemporary high-speed sorters can process 250 cartons. Conversely, it can process the same number of cartons at 590 feet per minute (FPM) compared to the industry standard of 650 FPM.

Typical sortation equipment with angled diverters rotate the boxes to facilitate discharge to the takeaway conveyors. When a package is rotated, essentially it becomes longer because one corner falls back, so a bigger gap is required between packages. FlexSort Sl does not need to rotate boxes, allowing for tighter gaps between conveyed products (1 to 3 inches instead of 6 to 12 inches). Also, FlexSort runs slower, subjecting packages to lower dynamic loads (the effects on packages when they change direction or speed on the system).

FlexSort is defined by its unique system integration. The logic on how the system is controlled is quite different in that all sortation functions are controlled as if it is one unified machine. From pre-merge to merge, to the transport conveyors feeding the gapping, to the gapping function itself, to the linear sortation, and finally to the take-away conveyors, the process operates with single-source control and full integration. The FlexSort system is also defined by its system-wide, distributed, variable-speed control; it will automatically adjust its speed incrementally as required.

Dematic FlexSort uses Demag linear induction motors for non-contact movement.

Dematic FlexSort uses Demag linear induction motors for non-contact movement.

Automation makes it possible

Automation inside the FlexSort sortation system makes these advances possible.

  • PLC hardware. Rockwell Automation Allen-Bradley ControlLogix or CompactLogix PLC. The PLC provides overall supervisory control of the sortation system. The PLC is controlling the variable speed of the sorter. It has inputs monitoring the amount of product coming into the system and going off. It controls all the VFDs and the speeds. Allen-Bradley was selected because it is the vendor predominantly accepted by its customers in the U.S.

  • PC-based controller. The FlexSort PC controller is an Intel Pentium class, industrial-hardened chassis to Dematic specification. The computer is assembled and provided to Dematic by Systel ( ). This runs only the sorter and gapping devices in the sortation subsystem.

  • Programming software. For the PLCs, ladder logic for all system control. The operating system on the PC is QNX 6.4. It is a real-time operating system. The program was developed by Dematic in C++.

  • Photo eye and pulse position indicator (PPI). For the photo eyes, Dematic is using Allen-Bradley RightSight photoelectric sensors. Outside of the U.S., Dematic uses Sick as an alternate. The PPI is made by Photocraft Encoders. Dematic has stayed with them for about 20 years because of reliability.

  • Electromagnetic divert switches. These were designed and are patented by Dematic. No one else on the market has electromagnetic divert switches.

  • Variable frequency ac drives (VFDs). Very precise control is required for the gapping function. The VFD on the gapper is used to make high frequency adjustments to the belt speed to modify the gap. After evaluating numerous suppliers, Dematic chose a Mitsubishi VFD for the gapper device because of its latency performance (how fast the drive responds when you tell it to do something). For the other VFDs on the sorter and transport conveyor, Dematic is using Allen-Bradley PowerFlex drives. Rockwell Automation was selected because it is the vendor predominantly accepted by Dematic customers in the U.S.

  • Linear induction motors. Currently Dematic is using a linear induction motor made by Demag. The use of linear induction motors — electro-magnetic drives that move a magnetic field across the face of the drive — is a unique feature of the sorter. They move the aluminum slats of the sorter without mechanical contact. (An air gap exists between the electromagnet and the slats, which have steel inserts inside.)

System design delivers unique functions

Most sorters use a mechanical divert switch. FlexSort uses electromagnetic divert switches that eliminate pneumatics and significantly reduce component wear.

Typical sorters provide a divert confirmation, but do not track individual slat IDs, which limits specific tracking of packages. FlexSort has small magnets on designated reference slats that are read by a sensor. Data is constantly sent to and updated by the controller to determine slat location, speed and operational status.

From a speed-setting viewpoint, the sorter acts as the master device that everything else in the subsystem follows. FlexSort functions on a closed-loop speed control. The sorter is the variable speed unit, the system is watching how fast the sorter is going, and all the other subsystem equipment adjusts to it.

The system is fully integrated from one point of control. Its PLC handles the system control, while the gapper and sortation equipment is PC controlled to handle diverse I/O types. Dematic’s SortDirector program interfaces with the host system and the system scanners to tell it what to do with the packages once they are identified.

Typical gapping systems use one photo eye and a PPI on each belt of the gapping machine. The PPI is an electronic wheel that physically monitors the belt and sends out a signal, typically a couple hundred times per rotation. When the PPI wheel is turning, the PLC is getting pulse signals that indicate specific distance travelled by the belt. The PLC then tells the gapper’s servo-controller what gap is needed between packages. The controller, via the photo eye, knows where each package is located on the belt and, using the PPI, it can then predict where that package is located after it has been seen by the photo eye. When the package gets to the next belt transition, the controller adjusts speed to achieve the desired gap.

FlexGap handles this task differently and more precisely. It incorporates horizontal array sensors for gapping in place of the PPI and discrete photo eyes. It uses a solid state photo eye array with a very small 2 mm pitch between photo eyes for the length of the gapping device. Because the system is tracking the packages and tracking the gap between packages in real time, it can adjust the speed of the belts to manipulate the gap much more precisely. By not using the PPI, it has eliminated a common failure point in traditional subsystems.



Author Information
Jim McMahon ( ) is a writer for Dematic Corp., Grand Rapids, MI. Dematic provides integrated material handling solutions that incorporate process improvements, material flow technologies, controls, and software.

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