Electronic-cam profilers improve accuracy and repeatabilityElectronic-cam profilers improve accuracy and repeatability
April 7, 1997
Chatsworth, CA--While servos smooth operation and increase flexibility, they have generally fallen short of anticipated accuracy and repeatability improvements in the converting industry. The reason: Before DSPs, microcontrollers lacked the computational power to make on-the-fly electronic-cam profile changes. And most cam profilers available in the last decade require that the machine be shut down so the controller can compute new cam positions.
Most converting manufacturers operate continuously, so web motion continues during repeat-value changes, and valuable raw material is wasted. As a result, most rotary knives, perforators, sealers, and even flying shears implement ratio controllers that vary the input/output encoder counts to make on-the-fly electronic gear ratio changes. Ratio-control algorithms assume a constant line speed and make no corrections for line-speed variation during the correction angle.
"This means that line-speed changes turn up as repeat error," says Westamp Inc.'s president Brad Landseadel. Most converting manufacturers using high line speeds and frequent repeat changes have decided it's more economical to live with repeatability error than to waste raw material.
This conclusion is no longer inevitable. PECO Inc. (Fairfield, NJ), a machine builder for the converting industry, recently gave Westamp the nod to upgrade a bag perforator/sealer machine from clutch/brake to servo controls with electronic-cam profilers. Westamp's SP series 6000 motion controllers implement 32-bit, floating point DSPs that compute all parabolic or S-curve profile positions on the slave servo in less than 2 mseconds. "Now the operator can enter a change and watch it happen in one machine cycle, with zero waste," explains PECO President Herbert Baier.
PECO's in-line bag machine receives film continuously from an extruder. A 150-lb rotary knife perforates the bag-film to a specific length, then heated bars make a seal just beside the perforation. Previously, a 12-inch-diameter, single-turn, vacuum-operated, clutch/brake unit powered the perforator and seal sections as the web moved continuously through the machine. An encoder on the infeed nip section provides count signals to a programmable counter, where bag length is specified in inches. When the correct count is reached, the clutch/brake fires to release the brake and engage the clutch.
A 60-degree infeed dancer synchronizes web speed and tension control, while a shaft-mounted speed potentiometer trims the main-potentiometer signal to accelerate the variable-speed main-machine drive from a virtual standstill to full line speed. Maximum cycling rate was about 100 cycles/minute or 200 ft/minute. "Accelerating a 200-lb load from virtual standstill to 200 ft/minute in 20 mseconds, then bringing it to a screeching halt, produces severe mechanical stress," says Baier. "Just leaning against the machine lets you feel the forces acting on the chains, sprockets, belts, pulleys, and fasteners."
Integrating Westamp's operator interface, brushless servo motors, and intelligent and slave drives on PECO's existing design reduced scrap, increased line speed by 50% (300 ft/minute), and decreased repeatability error. "The controller ramps up to line speed and then ramps back down to a smooth stop. Less mechanical stress translates into higher reliability for the end-user," explains Baier.
Developing the machine-control code was a formidable challenge, says Landseadel. The joint development program re-quired several trips out East by Westamp engineers. Their motion control expertise, combined with PECO's knowledge of web control, resulted in an algorithm that can handle web speeds to 700 ft/minute. "Maximum cycling is now limited only by product type and machine-sealing capacity, not by the servo drive," says Baier.
Westamp approched the print registration scheme in a unique way, enabling the end-user to choose between traditional auto-averaging or one-cycle adjustments. "Westamp servo drives on our rotary perforators can operate at speeds of 500 ft/minute, something that was simply not possible with clutch/brake-driven systems," Baier asserts. "Five years ago, price prohibited upgrading this bag machine, but today, these advantages justify the costs."
Additional details...Contact Jennifer Flynn, Westamp Inc., 9006 Fullbright Ave., Chatsworth, CA, (818) 709-5000, Ext. 201
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