Every distribution system needs to perform five basic functions: receiving, product storage, picking for order fulfillment, sortation, and shipping. Although each sector has experienced improvements resulting from technical advances, sortation is where significant leaps in efficiency are now being made. One example is Dematic’s introduction of its FlexSort® integrated sortation subsystem.
FlexSort is defined by system integration and resulting efficiencies for the high-speed sortation environment. All sortation functions are controlled as if on one unified machine: from the pre-merge to the merge, to the transport conveyors feeding the gapping, to the gapping function itself, to the linear sortation, and finally the take-away conveyors, the entire process operates with single-source control and full integration.
System-wide, distributed, variable-speed control is based on how much product is being fed into the system and how much product is moving out. It will automatically, incrementally adjust its speed as required. The system is further defined by the absence of rollers and extensive use of belts throughout. Package control is lost with rollers, but FlexSort maximizes efficient monitoring, routing, and control of packages throughout the entire system. Finally, high-speed, parallel-divert sliding shoe sortation can provide a 20% increase in efficiency while maintaining comparable throughput volumes.
All of these elements combine to bring a new level of efficiency in product throughput, low operating costs, and system availability to high-speed distribution centers.
Integrated Control
Control of the sortation subsystem resembles a PLC/PC-controlled unified machine more than a typical sortation system. Dematic’s Integrated Sortation System (ISS) treats all of the subsystem components as one machine. As such, the system is constantly adjusting speeds based on how much volume is coming through upstream of the merge, how much volume is coming off the sorter, and how much volume is en route from the pre-merge through the sorter.
From a speed-setting viewpoint, the sorter acts as the master device followed by everything else in the subsystem. FlexSort functions on a closed-loop speed control. The sorter is the variable speed unit, the system watches how fast the sorter is going, and all the other subsystem equipment adjusts to it.
The system is fully integrated from a single control point. Its PLC handles system control, while gapper and sortation equipment is PC controlled to handle the diverse I/Os. 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.
Precision Control of Gapping
The FlexSort system eliminates all roller conveyors from gapping, and between all functions from the pre-merge conveyor through sortation, replacing them with belts. Compared to rollers, belts require a much higher level of speed and operation control -- it would be unacceptable to have one belt stop and another one upstream continue to run, for example. So the system uses variable-frequency ac drives in the gapping function, and throughout the entire sortation subsystem, to provide precision control of its belts.
Traditionally, with high-rate sortation systems, very heavy duty servo drives are used with their own separate servo controllers. Because these drives are costly, systems are typically set up with a main servo drive powering two or three mechanically driven slave belts. Since the same drive powers more than one belt, the ratio between those belts is mechanically fixed and their speed cannot be changed independently; they all operate at a constant speed determined by the servo drive. Thus, speed control of individual belts is inflexible.
FlexSort overcomes this problem by utilizing an independent premium-efficiency drive for each belt in its FlexGap® gapping device, equipped with a high-performance, variable-frequency controller instead of a servo controller. The motor is driving each zone (belt) independently, so there is no gear reduction, giving good control over the ratio from speed of one belt to the next. This means more precise gapping.
FlexGap also employs more belts than conventional gapping devices. Belt size coupled with independent control has appreciably increased the precision of package movement within the gapping system. The belts do not require tracking or tensioning; no adjustment is needed. Direct-drive (no reducer) cog belts are virtually maintenance free. This simplified design has reduced mean repair time to less than 15 minutes.
Typical gapping systems utilize one photo eye and a Pulse Position Indicator (PPI) on each gapping machine belt. The PPI is an electronic wheel that rides on, or otherwise physically monitors, a belt. The wheel sends out a signal, typically a couple hundred times per rotation; when the PPI wheel is turning, the PLC gets pulse signals that indicate the belt has traveled a specific distance. The PLC tells the gapper’s servocontroller 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. It knows how fast the belt is moving and the distance traveled, so the system can track the time and predict where that package will be. When the package gets to the next belt transition it can adjust speeds accordingly to achieve the gap desired.
FlexGap, on the other hand, handles this task by incorporating horizontal array sensors for gapping in place of the traditional 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 eliminating the PPI, it eliminates a common subsystem failure point. If the PPI stops working, which occasionally happens, then the sorter will shut down. FlexGap removes this liability.
Variable-Speed, Parallel Diverting Sortation
In a high-volume distribution environment, speed of package flow through the system, as determined by the sortation equipment, is usually focused upon as key to production. It is generally accepted that to increase throughput one must increase the speed at which the sortation equipment is running. Of course, running a sorter faster increases vector stresses on the equipment and on the packages flowing through the system, which means less package control and more maintenance effort. It decreases the life of the equipment and ultimately increases the equipment cost of ownership.
With FlexSort, increases in throughput can be achieved without increases in system speed thanks to the FlexSort SL Parallel Diverting sorter. Typical sortation equipment with angled diverts rotates the boxes to facilitate discharge to the takeaway conveyors. When a package is rotated, it essentially becomes longer because one corner falls back. This means a bigger gap is required between packages. FlexSort SL does not need to rotate the boxes, so tighter gaps are possible between the conveyed products. Instead of the common 6 to 12 in. between packages, the system requires only 1 to 3 in. gaps. The take-away is designed to complete the rotation to the correct orientation, using gravity wheels or powered rollers that rotate and realign the packages.
Because the sorter runs at a slower speed, it can handle packages more positively than contemporary sortation equipment, which runs 20 – 30% faster and creates 35 – 50% higher dynamic loads.
When packages tip over or slide on the conveyor and lose their intended position, the potential for side-by-sides, jams, and mis-diverts increases, resulting in reduced accuracy, reduced productivity per man-hour due to downtime, and reduced throughput. The overall effectiveness of the system suffers in quality and performance -- not due to improper operation, but because of the inherent inefficiencies within the system’s design and application.
A package traveling in a straight line at a constant speed does not experience forces that would cause it to topple or move on the conveyor. But when packages transition from the merge inputs onto the merge (both a directional change and a speed change, from a slow-moving input to a fast-moving sorter feed), travel around a curve, speed up and slow down for gap adjustments, and divert off of the sorter, they experience forces that vary in proportion to the square of their velocity.
FlexSort ISS technology automatically varies the system speed with its real-time throughput requirements. The system provides not just a selection of predetermined speed modes, but rather a gradient spectrum of automated variable speeds.
Because the system runs at a lower average speed, packages will experience significantly lower average forces when they change direction or speed. A system that is designed to handle a high peak rate, for example, but on most days is only utilizing 60% of that peak realizes a net speed reduction of 46% with FlexSort ISS vs a fixed-speed system. Force is not linearly proportional to velocity, so in this example the forces on the packages and components of the system are reduced by 70%.
The Dematic FlexSort SL can process 300 cartons per minute (CPM) at the same speed that contemporary high-speed sorters can process 250 cartons. Conversely, it can process the same number of cartons at 590 fpm compared to the industry standard of 650.
A key feature of the sorter is its use of linear induction motors – electromagnetic drives that move a magnetic field across the face of the drive and 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. The shoes are also electromechanically directed into divert rails and pushed across the sorter.
On a slat-shoe sorter, the slats are typically joined by a chain that is driven by a sprocket with a conventional drive. The FlexSort SL sorter has no belts, drive chains, gears, sprockets, or divert switches, thus eliminating the need for lubrication.
Whereas most sorters use a mechanical divert switch, the new system uses electromagnetic divert switches that eliminate pneumatics and significantly reduce component wear, as the show pins are pulled into the divert rail rather than pushed, with no mechanical impact.
Typical sorters provide a divert confirmation, but do not track individual slat IDs, limiting specific tracking of packages within the system. The FlexSort SL employs small magnets on designated reference slats, which are read by a sensor, the data is constantly sent to and updated by the controller to determine slat location, speed, and operational status. This allows exact real-time location of any package in the sorter.
Sortation System Uptime
Sortation subsystem uptime is impacted by how long the equipment lasts, how much time it takes to repair, and how easy it is to perform preventive maintenance. It is also impacted by system availability in terms of redundancy. Most sortation subsystems use drive trains, which can act as single points of failure, bringing down the entire process. Because its drives are distributed throughout its system, FlexSort SL can withstand losing up to 10% of its drives and still maintain operation and production for the facility. And because it can handle more packages within the same unit of time compared to conventional systems, the sorter can reduce cost of ownership by lowering the operating cost.
Distribution managers seeking more efficient systems may wish to consider the ways in which FlexSort can upgrade efficiencies in sortation systems within high-volume distribution centers.