Linear motion: Novel design expands conveyor system flexibility
Chicago, IL — Visitors at the Pack Expo 2008 conference and expo saw the results of a collaboration between Siemens Energy and Automation (SEA) and Jacobs Automation (JA) to produce a novel conveyor system, called PackTrak, built on linear motor technology. Material is moved along a curvilinear track on small, magnetized, independently controllable carriers called “movers.” A Siemens Simotion P350 PC-based controller running Linux real-time operating system keeps track of
|The PackTrak mover wraps around the track to allow installation in any orientation.|
All linear motors operate the same, based on magnetic forces between magnets installed in tracks and in carriers. In the PackTrak system, each carrier incorporates a bar magnet made of neodymium-iron permanent magnet material mounted underneath each mover, and separately energized electromagnet coils built into the track. A roller/guide-rail arrangement provides mechanical forces to support the carriers and keep them running true on the track, while allowing them free movement along it.
To move a carrier, the controller sends commands to a DSP-based drive, which applies current to track electromagnets at the carrier’s location. Depending on the current sense, magnetic forces will push the carrier forward or backward along the track.
The DSP drive knows each carrier’s location through commutation signals provided by a unique encoder system. Sensors built into the track use the giant magnetoresistive (GMR) effect to sense the location of small permanent magnets attached to each mover. The GMR sensors operate in saturation mode, sensing the position-magnet field’s angular orientation at the nearest position sensor. From that angle and knowledge of the track geometry, the drive can calculate the position magnet’s location to a fraction of the distance between sensors. This commutation system provides absolute position encoding, making it unnecessary to “home” the system on power up, even if carriers have moved.
The drive thus can provide appropriate currents to hold each carrier still or move it in a direction at a speed commanded by the controller. The controller treats each carrier as a separate degree of freedom with absolute-encoder feedback. The drive, on the other hand, views each carrier as an N-pole motor, where N is the number of electromagnets in the complete track.
Having the Siemens controller allows the system to interface to the outside world as if it were any other motion-control system. Expanding the number of carriers controlled means expanding the number of degrees of freedom for the system.
Movers wrap around the track so they can be oriented horizontally, vertically, or even upside down. Propulsion magnets mount on both sides of the track, so that magnetic forces in the plane normal to the motion direction are always balanced.
Tracks come in 50 mm segments that can be straight or curved. Each segment includes 24 electromagnets, along with mover position sensors. Adding a segment requires only mechanically fixturing it and plugging in a cable from the drive. Entire conveying systems typically use a closed-circuit track to allow motion in one direction for numerous movers, but open-ended tracks, where carriers move back and forth between ends, are possible as well.
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— C.G. Masi , senior editor
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