Motion System Design

Just the sort

Sorting is the act of merging, identifying, inducting, and separating products to be conveyed to specific destinations. This discussion focuses on the sorting of products packaged in containers, such as cartons, boxes, and plastic bags. The sorting of bulk materials is ignored because it's usually accomplished with little motion control.

Sorting systems typically consist of four subsystems. Each subsystem performs a specific operation: merge, induct, sort, and post-sort or takeaway. In the merge process, packaged products are moved from the picking area on an accumulation conveyor line and consolidated for presentation at the induct area.

The induct process involves determining the destination of each product (by inspection) and then creating the proper gap between them as they are released to the sorter. Common inspection methods include visual, automatic identification, bar code, weight, and machine vision.

In the sort process, products are diverted to the outbound post-sort or takeaway subsystem (sort lines). It would be easy to equate the entire sorting operation to just this step in the process, but without prior and subsequent steps, the sort step would be viewed simply as separation or diversion.

The final operation in a sorting system, post-sort or takeaway, moves the product to shipping, palletizing, staging, or on for additional sorting.

Sort it out

Merge and sort operations typically incorporate diverters, which require a fair amount of motion control. Diverters shift products from one path (or conveyor) to another. They have to be very fast in order to alter the path of selected products without disturbing others.

The challenge can be likened to switching trains from one track to another. Some trains are diverted to the side track, while others are permitted to pass through on the main track. In high-speed sorting machines, however, “trains” (products) are comparatively close together and moving very fast. Diverters therefore must be equally fast, acting and then returning to the starting position, so that only the paths of selected products are altered.

In most high-speed systems, diverter gates are driven by servo actuators commanded with triangular motion profiles. Triangular profiles have no dwell time. This minimizes the interval during which the diverter deflects, then returns to its starting position, where it waits for the next operation.

Induct operations, likewise, involve motion control. High-rate induction systems typically employ servo actuators to adjust the gaps between products. Here, using the train analogy, let's assume a series of trains (packages) are on one main track. The goal is to switch the lead train to a side track, while allowing for the possibility that other trains might already be there (perhaps accumulated packages waiting to be transferred from a sort line). If we maintain a constant gap between the main-track trains, we may not be able to switch the lead train. So we slow it down, as well as its follower, to increase the follower's gap until the side track is clear.

Motion profiles used in such timing operations are typically trapezoidal because adjusting the product gap is a matter of adjusting the dwell time during which the product moves at a particular speed. In many cases, some degree of S-curving is also added to smooth the acceleration/deceleration reactions on the product. Reducing or eliminating jerk in trapezoidal motion profiles can be vital to maintaining the product in its proper orientation, and has the added benefit of being easier on machine/system mechanics.

Facing challenges

One challenge when designing a sorting system is knowing how to select the appropriate technology for each subsystem. Depending on the required speeds, the technologies may run the gamut from ac induction motors to steppers and servos to mechanical devices such as clutch/brakes to pneumatic cylinders. The increasing use of servos for sorting operations reflects the upward trend in system speed as well as the desire to create “all electric” systems, eliminating the need for shop air and other ancillaries.

Another challenge is having to manage several products on a sorting line, requiring different speeds and motion profiles. In many cases, sorting machines are asked to manage these changing products on-the-fly. This makes electrically based servo and stepper systems preferable because they can be configured to make changes quickly and automatically. Information from sensors out on the sorting line can identify the product, telling the motion controller how to operate the servo amplifier and motor.

Sorting system designers also are tasked with optimizing performance, for example, maximizing the speed for a given product while handling it properly. This requires selecting and implementing the appropriate motion profiles for each subsystem and each product to be handled.

Making a difference

Replacing mechanical or pneumatic diverters with servo-controlled diverters is one way to increase speed and throughput. It also eliminates the need for shop air (pneumatics) and offers greater flexibility in operation, including on-the-fly response to changing product makeup.

Induct subsystems can also benefit from smoother operation, through the use of sophisticated motion profiles (like S-curving), while maximizing system speed and maintaining accurate product placement. Not only do products move better, but the machine mechanics see less stress which boosts overall system reliability.

For more information contact the author, Chris Radley, at (866) 993-2624 or via e-mail at [email protected].

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