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Moving Materials

POSTED 05/25/2011  | By: Kristin Lewotsky, Contributing Editor

Servo motor control increases throughput and accuracy in materials handling applications.

Distribution centers feature complex, highway-like arrangements of conveyor belts to route inventory.Motion control is about getting objects from point A to point B, a task nowhere more apparent than in materials handling. Materials handling encompasses everything from moving raw materials into the processing environment, to loading carton blanks into converting equipment, to shuffling boxes around warehouses. For decades, conveyor belts powered by fixed- or variable-speed motors dominated the application. Today’s high throughput operations increasingly leverage correlated motion, for example synchronizing a robot arm to pick up a carton riding along a conveyor. Induction motors can’t deliver the speed control and flexibility required by the application.  As repetition rates go higher and higher, motion control provides the only avenue to successfully perform these tasks.

Large discount retailers carry tens of thousands of products per location. Keeping the shelves stocked requires sophisticated distribution centers that can supply as many as 100 stores apiece. These centers are highly automated; leveraging RFID tagging and literally miles of conveyor belts configured in complex traffic patterns to continuously sort packages. In the days of fixed-speed conveyors, items could be moved quickly but flexibility was limited. Using motion control, materials handling systems alter the routing on a package by package basis to deliver the right inventory to the correct loading dock as rapidly as possible.
Servo motors provide smooth, high-accuracy motion in conveyor belts like these. Courtesy of: Schneider Electric
Distribution centers mimic highway structures, with cartons moving along parallel lanes, turns, on ramps, and off ramps (see figure 1). The boxes need to be spaced widely enough on the conveyors to allow the machine vision systems to read the ID tags and the motion system to switch the box to the appropriate path, yet closely enough to maximize the number of boxes being handled at any one time. Operations have to take place at the highest possible speed, yet acceleration changes must be smooth enough that items don’t fall from the conveyors or create a jam. Add to that the wide variety box size shapes and you create serious challenge.

The solution is to replace long, fixed-speed conveyor belts with a series of shorter belts that provide near-real-time speed adjustment (see figure 2). By routing packages over these incremental belts, systems can precisely adjust gaps between them, allowing items to be diverted to an adjacent conveyor belt quickly and accurately. The application requires correlated motion between the box and the pick-and-place robot arm. With the aid of closed-loop feedback, the conveyor belt can provide exactly the right motion. “The issue is how smoothly the motor can ramp down from running speed to zero speed and how well it can go back to the application speed without jitter," says Sambhu Banerjee, Motion-centric OEM Solutions, Schneider Electric (Raleigh, North Carolina). "The smoothness of the motion becomes more important. If the [motor provides enough power], a servomotor can do a better job in place of an induction motor and a variable-frequency drive.”

Of course, monitoring and routing as many as 1 million items at a time is a computationally intensive process. To accomplish the tasks without impacting throughput, system designers take a two-pronged approach of using a super-computer for centralized routing but a series of PLCs and motion controllers on each module to handle path planning. By daisy-chaining the controllers together and tying them to the computer in a master-slave Configuration, they can leverage network communications to keep everything running smoothly.

In addition to robot arms, systems incorporate pushers to move items from one lane to another. For many years, hydraulic cylinders performed this task. Today, increasingly, servo-motor-driven rod-style actuators are taking over. The motion control provides greater flexibility and faster changeover than conventional systems. For heavier packages, high-moment rodless actuators use multiple bearings to distribute moment loading, allowing the device to move larger objects without the need for robotic arms and assemblies. The result is a simpler, more economical, more robust solution. Conveyor belts move packages around.  Courtesy of: Schneider Electric

Environmental Challenges
As in most industrial applications, environmental conditions present a challenge. The environment may include dust and lint, oil, even hydraulic fluid. Components can be protected from contamination by enclosures but that requires cabling to transfer motion from the cabinet to the motion axis, which introduces points of failure. Smart components provide a useful alternative, so long as they are robust enough to survive the application (see figure 3).

In the case of rotary motion, just adding ball seals or shaft seals is often enough to take care of the problem. Linear actuators present a greater challenge. Rod type actuators are the easiest to design for protection and are used in most applications that only need thrust-type actuation.  When long strokes are required, rodless actuators are the solution of choice, but the larger egresses on these products make them more vulnerable to contamination. The most common solution is to apply a magnetic strip-seal band over the opening with wiper seals for additional protection. In environments with fine particles this may not be enough, in which case adding a few pounds-per-square-inch (PSI) of positive pressure can significantly reduce contamination inside the actuator.

Smart components bring big benefits in terms of reduced cabling and ease of maintenance, but they need to be affordable and robust enough for the industrial environment. As manufacturers develop hardened components for other applications like automotive, the industrial sector can benefit from economies of scale. “If you can put a packaged electronic drive on the bottom of the bus or a car, it can surely survive an industrial environment," says Jim Monnich, Division Manager for EMN at Parker Hannifin (Irwin, Pennsylvania). Smart, hardened components are expensive, but the automotive market increases volumes, driving down cost so that the technology can move into the materials and product handling market. The result is better performance, more robust designs, and streamlined maintenance. “Instead of putting your servo electronics into a cabinet it will be able to survive on the factory floor, reducing cabling complexity and system footprint. It has been too expensive to do in industrial environments for the most part; however I could see these technologies migrating in the next three to five years.” 

In addition to the usual issues of industrial applications, materials handling presents some surprising challenges. Increasingly, audible noise is an issue governed by occupational health and safety regulations. “In the late 1990s, noise wasn’t that big a consideration, but now we do a lot of selection there,” says Monnich.  “We generally want to get the noise down below 70 dB or so.”

For best results, noise control is an issue that needs to be tackled from the beginning of a project, but that's not always the case. "It tends to be an afterthought," says Tom Baric, engineering manager of the custom products group at Parker Hannifin. “People tend to design the system around the performance they need and then focus in on the noise after the fact.”

Tackling noise from the beginning allows for top to bottom modification, from choice of gearbox to specification on the gear teeth to adjustment in the materials. Mechatronic design techniques can allow engineering teams to minimize resonances, eliminating noise sources from the beginning. "Many products are based on aluminum extrusions and sound generated at one end by a gearbox can resonate through the entire extrusion," says Baric. "By adding dampening materials, you can deaden the vibrations that cause the sound.  Or you can change the natural frequency of the body by applying masses or altering the body configuration.”

Going forward, the manufacturing and retail environment will only grow more competitive. Motion control provides powerful, reliable, economical tools to provide manufacturers the advantages they need to succeed.