Machine Design

Linear Motors Offer Precise Positioning and Highly Dynamic Response for Many Motion-Control Task


Linear motors offer precise positioning and highly dynamic response for many motion-control tasks. For machine tools, these include not only rapid traverse, but slow, constant-speed movement of machine heads, spindle slides, tool-management systems, and part-handling devices, according to officials at Siemens Energy & Automation, Motion Control Business, Elk Grove Village, Ill.

Despite their capabilities, however, linear motors have not played a significant part in the progression of modern machine design that has seen quantum leaps in control technology. Rather, modern machines still, for the most part, use decades-old slide-propulsion techniques, according to Siemens officials. Machines have gone from tape-driven NC of years ago, driven by servomotors and ball screws, to sophisticated CNC controls of today that take CAD files and generate machine programs at the touch of a button. But slides on today’s machines are still, for the most part, driven by servomotors and ball screws.

Linear motors are proven and economical, says Siemens, and it is time for mechanical systems on these machines to catch up to control technology. For instance, replacing mechanical components with linear motors can result in considerable cost savings, according to company officials. The motors provide a total drive system, offering reliability, precision, high dynamic stability, low maintenance, and faster production, they say.

One advantage is that linear motors are simple. Two main components, the primary containing electromagnets and the secondary either with permanent magnets or magnet-free, drive the moving member. This eliminates servomotors, resolvers, tachometers, couplings, pulleys, timing belts, ball screws and nuts, support bearings, lubrication systems, and cooling systems.

Other advantages include high accelerations and decelerations, high velocities over long distances at constant speeds, backlash-free positioning, contactless operation with no mechanical wear, and design flexibility, as primary sections can be stationary or moving.

This makes linear motors viable candidates to replace: • Hollow ball screws with coolant systems for thermal stabilization. • Rack-and-pinion drives with expensive torque motors and gearboxes. • Chain drives requiring high-torque hydraulic motors and hydraulic power units.

One linear-motor stationary track (with or without magnets) can support several primary sections moving either the same slide in a master-slave configuration or moving separate slides independently at different rates and in different directions. This lets designers consolidate drives on multislide machines to lower costs and improve productivity. For example, a laser, water jet, or router with two heads on the gantry run by linear motors can simultaneously cut two symmetrical or mirror image parts, thus saving considerable raw material.

When moving large and heavy gantry-style slides, multiple primary sections mounted on either side of the gantry provide the force necessary to accelerate and decelerate the slide. In addition, multiple secondary tracks installed side by side can increase force capacity.

On moving slides where long cables pose problems, one or more primary sections can be fixed to a stationary base and the secondary sections attached to the moving member. This lightens the load on the slide and allows cycles with high oscillation rates that might otherwise be impossible with conventional mechanical drives. It also allows for shorter cables with less flexing.

Leading manufacturers offer a range of linear motors to suit a wide array of applications. Siemens, for instance, offers three models that work with all Sinumerik or Simotion controls and Sinamics drives. Linear scales for position and velocity feedback are available from third-party suppliers. 1FN3 peak-load motors have high acceleration/deceleration and velocity rates and can be used for horizontal or compensated vertical axes. Nominal force ratings are as high as 8,100 N, maximum force is 20,700 N, and maximum velocity is 253 m/min with liquid cooling. Typical applications include machine tools with highly dynamic movements, laser machining, and material-handling equipment.

1FN3 continuous-load motors deliver power over long durations. Nominal force is 10,375 N, maximum force is 17,610 N, and maximum velocity is 129 m/min with liquid cooling. Applications include noncircular machining, vertical axes without counterweights, and Cartesian robots. Note that both types of 1FN3 motors can run with air-convection cooling, but it reduces ratings by 50%.

1FN6 magnet-free secondary motors are suited for long traverse lengths at high accelerations and velocities. They use air-convection cooling. Nominal force is 2,110 N, maximum force is 8,080 N, and maximum velocity 532 m/min. Applications include transporting tools and material in assembly and production lines, and water-jet and laser-cutting machines.

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