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SpecMate: ABCs of XYZ Motion

April 14, 2003

Control systems designers who need multi-plane motion in their applications find they have three basic drive mechanisms to choose from for building x-y, x-z, and x-y-z motion solutions.

When converting rotary motion to linear motion, lead screws provide precise, cost-effective solutions for many motion control requirements, say the engineers at Danaher Precision Systems (www.neat.com). Each lead screw is paired with a specially designed anti-backlash friction nut to provide high-level repeatability--typically less than +/- 2 ?m when unloaded--and fast settling.

The friction on the nut is a function of the operating environment, lubrication, load, and duty cycle, explain the engineers at Techno Inc. (www.techno-isel.com), so it's not easy to quantify typical life cycles. They say the advantages of a lead screw are its self-locking capability in back drive mode, making it a good choice in vertical applications; low initial cost; near silent operation; and availability in a choice of materials. The disadvantages include low efficiencies--given its sliding contact operation--that demand larger motor drives, and unpredictable service life.

Ball screws have great similarity to lead screws except they use a ball bearing train that rides between the screw and the nut in a recirculating raceway. Given the increased number of mating and moving parts, matching part tolerances is far more critical. The screw threads have rounded shapes conforming to the shape of the raceway balls. Ball screws are well-suited for applications with high acceleration and deceleration needs, report the Danaher folks. They also advise that a motor-mounted brake is recommended to control backdriving, when ball screws are used in a vertical application.

Ball screws demonstrate efficiencies of 90-95% compared with 30-50% for lead screws, say the Techno engineers. That means lower motor torque needs. Other advantages include a more predictable service life and low wear rates and maintenance costs. Disadvantages include a more limited materials of construction menu, higher initial costs, and the auxiliary braking requirement.

Choice three is a linear motor. Brushless linear servo motors drive the load directly with high-speed, smooth motion, compared with the mechanical solutions noted above. A linear motor is a rotary motor sliced open and rolled out flat to produce a linear force. The two primary components are the sliding coil assembly, which is normally attached to the load, and the stationary magnet way. The Danaher engineers report that linear motors can be commutated digitally or sinusoidally, the latter providing the smoothest motion.

Baldor Electric engineers (www.baldorlinear.com) say the advantages of linear motors include high accuracy, compact, low profile design, long life, high acceleration, rapid response, smooth and reversible travel, high velocities, high resolution, and reliability. Because there are no mechanical linkages, backlash is eliminated, as are oiling and greasing routines.

As with all applications, consideration of the load is important. The larger the load, the less precision is required, while for lighter loads, linear stepper or induction motors can be an effective, economical approach.

Linear servo motors require positioning feedback, which also determines a system's resolution and accuracy. Danaher specifies non-contact linear encoders, believing them to be cost-effective, and they available in a wide range of resolutions and accuracies. They say laser interferometers are a good choice in very demanding applications requiring high accuracy.

E-mail Joe at [email protected].

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