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FLEXO Magazine : February 2011
Technologies & Techniques Get them all! Precision Velocity Control in Coating & Laminating Applications it Can be Achieved; Here’s How By Tom england Inweb handling applications, much of the coating focus is placed on achieving perfect velocity control — and rightly so, as operators know all too well, velocity ripple causes uneven coating and unsightly horizontal bars across the substrate. There is one solution to overcoming these chal- lenges – perfect velocity control. Simply put, when metering of the coating material is perfect, perfect velocity will result in an even application of the coating. As an example, consider film coating, where if one were depositing a dark film onto the substrate material, varying ve- locity would result in a series of dark and light “bars” across the material. But knowing the solution and effectively achieving it are two very different things. As a starting point, there are a number of factors that directly impact the ripple problem, including: • Mechanical transmissions cause tooth or belt chatter, and each gear tooth or timing belt will cause a torque/ve- locity perturbation in the roll, resulting in the “bar ” effect, sometimes referred to as banding. • Machine mechanics, along with imperfections in the mo- tor, feedback device, and drive control loop, combine to foil the notion of perfect velocity. Correcting for these variables will go a long way toward achieving the desired velocity control. Here we’ll focus on overcoming motor, feedback and drive imperfections. Motor The motor’s primary contribution to velocity ripple is its electromagnetic cogging, which is usually at the pole or slot frequency or some multiple of it. The best way to overcome electromagnetic cogging is to begin with a low cogging motor that requires less correction. High performance motors with less than 1percent pk-pk cogging are a very good start, and nearly always superior, compared with standard motor options, due to better elec- tromagnetic design, and simulation of the electromagnetic circuit. Cartridge DDR (direct drive rotary) servomotors, or frame- less direct drive motors, are the best weapons on precision coating lines because they eliminate mechanical transmis- sions. Cartridge DDR motors have the added advantage of being as easy to implement as a conventional full frame motor. FeedbACk Since the feedback device is in the control loop, any error of the device itself is “corrected” by the drive in the servo loop, and ends up as a velocity ripple on the coating roll. No feedback device is perfect, but since encoders offer the best performance for such applications in terms of resolution and accuracy (+/-25 arc-sec is typical), particularly when compared with a resolver at +/- 10 arc-min. As the servo loop gain is increased, the error in the feedback device is ampli- fied and transferred into the coating roll. So, starting with as precise a feedback device as possible is very important to minimize velocity ripple. driVe The drive can add to and correct various velocity ripple components. It adds to the ripple by having uneven current loop gains in each of the phases, i.e . phase A might be 1per- cent higher than phase B or C and this causes a torque error in the motor. Likewise, the amplitudes of each phase might be slightly different, adding an error to the torque as well. The drive can correct these by having digital adjustments in the current loops to take out these errors. One can also reduce the velocity ripple of the system by using harmonic correction in the current waveform. For example, when the characteristic of the cogging in the motor (frequency and amplitude) is known, this can be corrected by applying an anti-phase ripple in the three-phase current from the drive. The same can be done with the feedback device to correct a repeatable error. Both the motor and feedback error frequencies are well known and repeatable, so all we are left to do is adjust phase and amplitude. n About the Author: Tom England is director, Global Product Planning, Kollmorgen. He can be reached at: Tel. 540-633- 3545 or firstname.lastname@example.org. Vanquish Chatter & Other Imperfections • Perfect velocity will result in even application of the coating. • Velocity ripple causes uneven coating and unsightly horizontal bars across the substrate. • Mechanical transmissions and machine mechanics must be addressed. 32 FLeXO FeBruAry 2011 www.flexography.org TROUBLeShOOTING GUIDe FLX_Feb11_mech.indd 32 2/16/11 8:50 AM
Sustainable Winter 2011