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Overcoming obstacles to use of linear motors

Posted: 16 Mar 2006 ?? ?Print Version ?Bookmark and Share

Keywords:george procter? copley controls corp.? direct-drive linear motor? ServoTube linear motor?

By eliminating the linear conversion mechanics and its associated inertia, direct-drive linear motors have improved speed, acceleration and settling characteristics. Freedom from ball screw backlash and slop means better accuracy and reliability. Linear motors also run quietly and provide very long travel distances. Nor is there a fast-rotating lead screw to spray contaminating lubricant.

In terms of bandwidth, linear motors provide high performance in applications involving rapid short-distance increments. This is particularly true for repetitive X-Y positioning systems, where the combination of speed, acceleration and settling time brings unmatched throughput. But linear motors do have their application impediments, causing some designers to shy away from them, such as:
? Form factor is radically different from conventional electric motors. Automation equipment must be designed at the outset to accommodate the shape and operating characteristics.
? Rotary-to-linear conversion mechanisms provide a mechanical advantage not achievable with direct-drive linear motors.
? External encoders used for servo feedback often cost nearly as much as the motor itself, with alignment complexities adding insult to injury.
? Standard U-channel linear motors use an exposed permanent-magnet track, which introduces handling difficulties and requires protection from contaminants. Environmental sealing is also a problem; nooks and crannies accumulate particles, creating hot spots. The U-channel motor's drive coil travels inside the permanent-magnet track, creating thermal difficulties and, often, the need for forced-air cooling.
? Platen-type motors with exposed track produce magnetic attraction between coil assembly and track. The unwanted force creates a retarding load.

To overcome these issues, the ServoTube linear motor's design inverts coil and magnet configuration and encloses the permanent magnets in a hermetically sealed stainless-steel tube. A magnet coil forcer assembly surrounds the thrust tube's encapsulated magnets, creating a symmetrical magnetic field free from lateral force-reducing attraction. The forcer moves in free air, creating an efficient thermal design that can dispense with fan cooling. Its other advantages are:
? No encoder!A patented magnetic-field design raises Hall-effect accuracy almost tenfold, enabling an integral Hall position sensor to provide 20?m repeatability. The motor's relatively simple design, freed from the external encoder's cost and fragility, brings competitive pricing and meets international IP67 ruggedness specs.
? Multiple forcers!The motor's simplicity introduces application ingenuity impossible with lead screw and belt drives. For example, independent control of two forcers on a single thrust rod doubles productivity!one forcer loads while the other unloads. Multiple forcers can also act in unison to multiply drive force. Navy contractor VSSL has developed a simulator using six long-travel linear motors with four forcers each to quadruple drive force.
? Actuator function!The ServoTube's thrust rod is ordinarily supported at both ends, with the forcer positioning the load. However, Copley has reversed the forcer and thrust rod functions to create a high-speed direct-drive actuator suited for push/pull/lift material-handling applications. With industry-standard mounting, the motor can be a drop-in replacement for conventional closed-loop pneumatic positioning mechanisms.
? Control versatility!A wide selection of controllers!offering sophisticated camming and indexing capabilities as well as CANopen networking!permits seamless linear motor integration into PLC-based control systems.

- George Procter
VP for Motion Systems Division
Copley Controls Corp.

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