FEATURE MOTION CONTROL An alternative control solution
A new motion control solution has been developed that improves closed track transport systems, resulting in reduced cost and increased performance. The solution, which is proven and implemented in the field, is the result of a joint effort by the motion control provider ACS Motion Control (controller and drives), linear motors manufacturer Tecnotion, and intensive design and system integration work by E-tech. Ofer Ben-Ari, ACS Motion Control; Eldad Lapidot, E-tech; and Arno Winkel, Tecnotion, explain
M
achine throughput and production rates are key factors for efficient
manufacturing, and it is well known that building machines that move back and forth results in lower efficiency than unidirectional movements. In closed track transport systems, combining unidirectional motion and high accuracy introduces a mechanical barrier resulting from the nature of the linear motor characteristics, as linear motors are typically only used for linear motion. Due to this limitation, machine designers have been forced to build a machine with a linear motor for the straight section (usually the ‘production zone’), and use other drive technology such as conveyor belts in order to bring the moving carriage back to the beginning of the straight section. This concept introduces many design obstacles such as engaging and disengaging linear bearings from their guidance, cabling issues, position feedback (encoder) limitations, parts gripping and many others. As a result, such a solution is rarely used in accurate automation systems. Moving magnet motion solutions require complex commutation control, multiplexing between sensors and several (non-standard) drives, one for each section of track - meaning the number of drives required increases with the length of track. Moving magnet control solutions usually implement a 6-step trapezoidal commutation and hall-sensor based feedback, which have limited resolution and accuracy. Plus, with limited force constants, moving magnet solutions are not suitable for many high performance applications, e.g. printing machines.
20 MAY 2015 | DESIGN SOLUTIONS
THE SOLUTION A new closed track technology allows machine designers to benefit from the dirt immunity and mechanical tolerances of the magnetic technology without the need to compromise on measuring accuracy. The motion control solution fully supports on-the-fly switching from analogue Hall feedback to linear encoder feedback, while maintaining the same engineering units defined by the user regardless of the feedback in use. The new concept
combines a control algorithm from ACS Motion Control with curved magnet tracks that directly join to linear magnet tracks from Tecnotion. The ACS advanced motion control algorithms are designed for its SPiiPlusNT family of
motion controllers and drives. The system offers improved performance at a fraction of the cost when compared with traditional moving magnet based closed track transport systems. Now, manufacturers and users of linear motor based closed track transportation systems have an off- the-shelf control solution, and users of moving magnet based systems, limited by motion performance, have an alternative solution. Although some engineering efforts are
still needed in order to manage proper guidance, including bearings, cabling and other design factors, the advanced motion control solution provides significant advantages over moving magnet transport systems. The algorithms adaptively compensate for the changes in commutation period and motor force constant that result from curved shape magnets in the curved linear motor track. The algorithms also provide seamless
transitions between feedback devices during motion, which allows high- resolution optical encoders to be used along certain regions of the track. Additional advanced control
algorithms, such as ‘ServoBoost’ may be used to further improve performance. A powerful multi-tasking application layer enables robust anti-collision, where anti-collision zones can be modified on the fly to meet application needs. Featuring a simple control scheme
that treats each motor forcer on the track like a standard linear motor axis, the motion control solution for closed track transportation systems provides higher performance via: • Sinusoidal commutation for considerably smoother motion
• High resolution optical encoders may be used at the required regions (i.e. printing zone)
• Lower cogging that can be completely compensated by algorithms • Position error of a few microns or less during motion. The solution only requires one drive for each moving forcer and is not dependent on the track length. In addition, it is significantly more cost-effective, and provides lower cogging, better performance and higher motor force constants (up to 1900N continuous). The motion control
algorithms are already being used in high- throughput digital printing, 3D printing, medical and packaging applications.
LINEAR MOTORS ON A CURVED TRACK The curved magnet plates have a smart design that compensates for force losses in the curves due to commutation error, which occur due to straight coils on curved tracks. The motors
/ DESIGNSOLUTIONS
The new motion control solution delivers many advantages over moving magnet transport
systems, including higher accuracy and faster response time because there are no issues with latency due to direct feedback of encoder to the drive
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