Product category:
Industrial Drives/Controls
News Release from: Micromech | Subject: Motion control systems
Edited by the Engineeringtalk Editorial
Team on 17 August 2000
Complex positioning job solved for
Parker
Micromech Systems felt quite confident when Parker Chomerics came to them with a requirement to produce reels of thin copper gaskets for the EMC industry...
Feed-to-length is a common requirement in many automated processes, where acontinuous web is to be indexed in controlled increments of speed and position sothat some activity can then take place on it - cutting, stamping, printing etc The simplest way to provide this control is usually by means of a pair of pinch rollers of known circumference, rotated by stepping or servo motors to perform the required move profile, and most practical systems use some variant of this approach
This article was originally published on Engineeringtalk on 15 Jun 2001 at 8.00am (UK)
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Braintree-based Micromech Systems Ltd have produced many feed-to-length systems for the Printing, Packaging, and Factory Automation markets and therefore felt quite confident when Parker Chomerics came to them with a requirement to produce reels of thin copper gaskets for the EMC industry.
But Motion Control has many pit-falls for the unwary, and what at first may seem to be a simple positioning application, can sometimes turn out to be more complex.
The design called for gaskets to be placed on a Mylar web at 50 per minute with a positioning accuracy of +/- 0.45mm.
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Parker 404XR slides were chosen for the pick and place function, using a vacuum head to lift gaskets from a stack and place them on the web.
The nip rollers which indexed the web would also compress the gaskets as they passed through, securing them onto a Thermotack coating on the web.
The functions to be controlled included Nip, Backing-paper reel, Pick and Place, Off-reel, and three Feed axes, indicating a 7-axis system.
A Parker 6K8 eight axis controller was selected, with Warner high-torque KMT stepping motors for all the axes.
Because there were 10 profile shapes to be handled, a different, purpose-designed vacuum-head, was needed for each gasket variant - this presented no problems for the Micromech team because of the range of auxiliary functions available in the 6K8 controller.
However, all 10 shapes were variations of an asymmetric chevron form,and herein lay the difficulty.
Nip rollers can handle a uniform web thickness with ease, but, in prototype test, itsoon became apparent that with asymmetrical chevron shapes lying on the web the nip rollers were seeing anything but a uniform web - as the chevrons passed between the rollers the effective diameters of the rollers altered along their length.
This caused an uneven draw off the backing-paper reel and skewing of the web,after just a few indexes.
This kind of pseudo-random error proved impossible to programme out, but fortunately, the response time of the 6K8 was fast enough to allow canned-cycle re-zeroing of the nip rollers so that they could be raised and lowered after each index,thus overcoming the inherent non-linearity within the cycle, and preventing the error from becoming accumulative.
Passing chevrons at speed can sometimes be more difficult than it looks...
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