Straight Talk About Linear Motors

Linear motor technology has been touted as the next killer app for some years now. Industrial machine builders that require precise, reliable linear motion have heard the promise of vast performance improvement over more traditional translated linear motion.

Touted as...? Promise of...? Is that still all there is? As we take another look at the technology, the question is simple: Has linear motor technology broken through to wider acceptance?

Regrettably, the answer remains decidedly mixed. In this article, we detail some of those promising examples of linear motor application, as well as some present-day realities of cost and understanding that have hindered and may continue to hinder widespread use.

It's been two years since a major confectionery customer approached custom machinery manufacturer Tritech Industries (www.tritech-industries.com) with a challenge. The food processor wanted to cut a product at very high speed using ultrasonic cutting technology, but also needed a high-quality cut.

Rather than using cam or screw-driven motors, Tritech engineers used linear servo motors and off-the-shelf ultrasonic blades and peripherals to design a cutter (Figure 1) that tracks the product at the speed of the conveyor on the x-axis while making a perpendicular slice through the product on the y-axis. The Tritech machine controls acceleration and deceleration in both axes and avoids the elliptical cutting motion of other types of cutters (Figure 2). More importantly, the linear motor machine beats standard system throughput rates (about 150-200 cuts per minute) by a significant margin, according to Tritech.

"We focused on the customer's business drivers," says Rick Tkaczyk, vice president of sales for Elk Grove Village, Ill.-based Tritech. "We were able to provide higher speeds, and [the customer] got higher production rates. With linear servos, they potentially also could have lower maintenance costs because there are fewer moving parts. And by using the linear servo technology, we could change the motion profile in our software package, so the customer could cut variable lengths of product on the fly by changing the recipe."

In the notoriously conservative food industry, linear motor technology is relatively new. The industry as a whole is not known to be an early adopter of new technology, and its low profit margins make companies hesitate to invest in technology with a higher capital cost than "tried-and-true" solutions. But Tkaczyk is seeing growing interest in linear servo-driven ultrasonic cutters throughout the food industry, as well as in the pharmaceutical, electronics, and automotive industries.

"It's not for every cutting application," Tkaczyk concedes. "It's meant for applications where speed, quality, maintenance reduction and higher yield are critical to the operation."

Industrial machine builders who have switched to linear motors on cutting tools, medical devices, packaging pick-and-place machines, and similar high-speed equipment have realized dramatic increases in tool positioning speed and acceleration.

"Companies that use linear motors are producing machines that are extremely fast," says Paul Webster, servo product manager for GE Fanuc Automation (www.gefanuc.com). "They have switched because rotary motors cannot provide the acceleration and top speed of a linear motor. These are typically light part weight or very small machines. The speed and acceleration can only be realized by keeping the weight down, substantially lower than traditional machines. Also, a linear motor does not require the space of a traditional motor, so the machine can be made smaller."

Speed was a critical factor for a machine builder that used linear motors in its latest laser metal-cutting tools. Alabama Laser Systems (www.alspi.com) and GE Fanuc collaborated on the design and engineering of CO2 laser CNC cutting systems that are used to make high-speed cuts in sheet metal up to one-inch thick (Figure 3).

"We wanted to take high-speed cutting technology into the future and build machines that exceed the current state of the art," notes Sai Mudiam, vice president of engineering for Alabama Laser Technologies, parent company of ALS in Munford, Ala. "We decided linear motors are the technology of the future for the machining industry. With these motors, our machines are able to move faster with higher acceleration rates."

Using 1,500-W and 3,000-W lasers, the ALS systems are capable of cutting 233 holes per minute, with contouring speeds of 1,000 in./min. using linear motor coils and brake and brakeless servomotors. The motors are mounted on both sides of a lightweight gantry arm and can withstand three Gs of acceleration force with no backlash.

If you believe the vendors, linear motors are finding slow, steady growth in specialized applications where rapid axis acceleration, repeatability, low maintenance downtime, and accurate programmable motion control top the wish lists of industrial OEMs.