Where linear motors improve designs: Application example in multi-axis stage

Machine-tool applications and the manufacture and assembly of semiconductor components account for more than half of all linear-motor use. That’s because linear motors are precise (albeit costly compared to other linear-motion options). Other applications for these relatively new motion components also include those that need fast and precise positioning or slow and extremely steady strokes.

Linear motor speeds range from a few inches to thousands of inches per second. The designs can deliver unlimited strokes and (with an encoder) accuracy to ±1 μm/100 mm. For this reason, a variety of medical, inspection and material-handling applications use linear motors to boost throughput.

Unlike rotary motors (which need mechanical rotary-to-linear devices to get straight moves) linear motors are direct drive. So, they avoid the gradual wear of traditional rack-and-pinion sets. Linear motors also avoid drawbacks of rotary motors running belts and pulleys … limited thrust because of tensile-strength limits; lengthy settling times; belt stretching, backlash and mechanical windup; and speed limits of 15 ft/sec or so. Plus linear motors avoid lead and ballscrew inefficiencies (about 50 and 90% respectively) as well as whip and vibration. They don’t force designers to sacrifice speed (with higher pitches) for lower resolution, either.

Multi-axis stages that use linear motors on each axis are more compact than traditional set-ups, so fit into smaller spaces. Their lower component count also boosts reliability. Here, the motors connect to regular drives, and (in servo operation) a motion controller closes the position loop.

Linear stepper motors deliver speeds to 70 in./sec, suitable for relatively quick-acting pick-and-place and inspection machines. Other applications include part-transfer stations. Some manufacturers sell twin linear steppers with a common forcer to form X-Y stages. These stages mount in any orientation and have high stiffness and flatness to a few nanometers for every hundred millimeters to output accurate moves.

Some cost-sensitive applications benefit from hybrid linear motors, as they have inexpensive ferromagnetic platens. Much like linear stepper motors, they vary magnetic saturation from the platen to shape opposition to magnetic flow. Feedback plus a PID loop with positioning control helps the motor output servo-grade performance. The only catch is that hybrid motors have limited output and exhibit cogging from coupling between forcer and platen. Two solutions are phase-teeth offset and driving for partial saturation of platen teeth and forcer teeth sections. Some hybrid motors also use external cooling to boost output during continuous operation.

Linear ac induction motors that run to 2,000 in./sec work for people movers, roller coasters and large aerospace applications. General-purpose types can move a few inches to 150 ft/sec or faster. Cylindrical linear motors have steel rods and a moving coil or rods filled with stacked magnets, so work in myriad ma-chines that need quick and accurate strokes.

In a similar way, ironless-core (or air-core) linear motors output up to 3,000 N and speeds exceeding 230 in./sec. These capabilities make this linear-motor subtype indispensable in long-stroke pick-and-place applications, flying-shear setups, and laser and waterjet cutting.

Linear ac synchronous motors can output 7,000 N or more. Some use water cooling to boost force output. Such designs find use in baggage-handling systems and amusement rides. Iron-core motors also work in machine tool and robotics applications.

Linear-motor application example: Researchers use XYZ robot to develop hardier crops

Watching plants grow and develop roots can be tiresome, but tracking plant development reveals how genetically modified plants can give better yields than unaltered versions.

Now, plant-physiology researchers at the University of Wisconsin-Madison are using automated image acquisition to make that easier.

Researchers place plant samples in an LED-illuminated 6×6 grid of Petri plates on a vertical tubular aluminum frame to conduct 36 experiments in parallel. Then robotic camera CCD imagers working off an XYZ vertical gantry create time-lapse movies of plant growth to give researchers better understanding of how genetic manipulations affect development.

The XYZ gantry consists of computer-controlled linear actuators, each integrating a brushless linear servo motor from IntelLiDrives with a high-resolution linear encoder.

The gantry moves the CCD cameras over a 1-m-by-1-m area with positioning resolution of 10 µm.

An extra linear axis lets the researchers change the camera’s field of view to get well-focused images of the seedlings in their Petri plates.

The robotic camera snaps a picture every 30 seconds to capture the curling and twisting motion of germinating seeds putting out new roots.

After capture, the images go through analysis that determines cellular growth rates in the root as well as the angle and curvature of the root tip. Computer-vision algorithms study the camera’s time-lapse videos and measure (among other things) the sizes of seeds, plants’ cellular growth rates, and the angle and curvature of the roots.

Using this XYZ robot with computer vision gives researchers the ability to automate the plant growth time-lapsed image acquisition and processing … and increase throughput of experiments to track the plants growth and development. Now researchers can find the genes that control plant’s root growth and have fundamental importance to crop improvement. It lays the foundation for discoveries that could one day further improve plants for human consumption and other use.