X-Y motion: Meters to nanometers

Contributing columnist Wayne Labs takes a look at how multi-axis motion applications at both the macroscopic and microscopic levels are benefiting from the latest advances in x-y positioning systems.

Wayne LabsBy Wayne Labs, Contributing Editor

 

THE U.S. Army’s unexploded ordnance/countermine test stand has an x-y positioning surface area of 5.5x6.1 m, and achieves accuracy to 1 millimeter. Likewise, ICS Robotics and Automation’s x-y positioning system for car fascia panels carries a weld head over an area of 2.0x0.8 m at a linear speed of 1 m/s, achieving an overall cycle time of 20 sec for 20 weld locations.

Similarly, Queensgate Instruments Ltd.’s nano-positioning stage has greater than 100-µm travel along each axis with sub-nanometer resolution, and can be used in various microscopy applications. This nano-positioning stage uses integrated capacitance position sensors to achieve accuracy, but a stage also can use laser interferometers to achieve better than 1-nm resolution.

For example, Renishaw has a fiber-optic laser with a resolution of 38 picometers (micromicrometers) at velocities to 1 m/s, which is typically better than encoders. Jacob Tal, chairman and cofounder of Galil Motion Control, says encoders with sinusoidal outputs and interpolation can approach 1-nm resolution, but won’t have absolute accuracy because an imperfect sine signal can create periodic errors in each cycle.

 

”Encoders with sinusoidal outputs and interpolation can approach 1-nm resolution, but won’t have absolute accuracy because an imperfect sine signal can create periodic errors in each cycle.”

 

 

Tal sees integration as the motivation for shrinking x-y equipment, such as the controller and drives for all axes integrated in one piece of electronics. Alternatively, some companies offer an integrated axis, which includes a motor, drive and controller. “More interesting developments,” says Tal, “include tuning programs, dual-loop feedback, tools to compensate for the non-orthogonality of the table, and 2-D correction tables for mapping position errors.”

Robert Novotnak, advanced automation division manager at Aerotech, says much tighter integration of motion, PLC and vision systems is becoming available, as well as controllers with adaptive or learning capabilities.

Todd Kanipe, precision mechanical product manager at Parker Hannifin, says the big trend is miniaturization, but adds, “Unfortunately, many suppliers still offer stages too large to fit the application.” Parker’s miniature stage series is a little larger than a deck of cards, and maintains sub-micron precision. “There’s an ongoing migration from screw drives to linear-motor drives as their costs decrease,” says Kanipe. “But rotary motors aren’t finished yet. Integration of the [rotary] motor directly into the screw-driven table offers users several advantages, including improved bandwidth, higher system stiffness and reduced package size.”

Achieving highly accurate positioning with rotary motors requires low, constant friction throughout the stroke, including the start, according to Yoshiro Oishi, manager, standard product engineering, THK America. He says one reason for inconsistent friction is ball-to-ball contact. THK cages the balls in specially designed guides to eliminate this contact, applies the same technology to its ball screws, and uses a tangential ball pick-up, allowing the ball screws to achieve a maximum nut rpm.

“The industry is moving toward the integrated motor/slide and an integrated drive and controller package, which means ball-screw and belt-drive mechanisms will be used less, especially if the cost of linear-motor magnets comes down,” says Bashar Abdo, product manager for Bosch Linear Motion and Assembly Technologies. “However, plug-and-play components will be a must in the next five years.”

Pat Berkner, Parker’s product manager for servo motors, says one advance toward plug-and-play is IEC mounting. This means one vendor’s motor can mechanically replace another vendor’s motor. Other system improvements include using absolute encoders to eliminate homing; lighter, smaller, shorter motors; high-flex cables; segmented-lamination technology (especially “kit” motors); and using wireless feedback devices—although noise and safety are still big concerns.

Abdo suggests that programming systems need to become so easy to use that a non-technical operator can program and maintain motion and robotics systems. While software performance is improving, other issues must still be addressed, such as complicated selection/sizing, time-consuming engineering/modeling, multi-vendor product incompatibility, logistic/outsourcing costs, and debug/setup/startup costs.

“Other requirements include the ability to handle last-nanosecond—not last-minute—changes in a commanded trajectory; motion control with algorithms beyond traditional model-based PID control; and high-speed servo update rates up to 200 kHz,” says Rahul Kulkarni, National Instruments' proudct manager for data acquisition and control.

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