Motion Control Resources
by Kristin Lewotsky, Contributing Editor
Motion Control & Motor Association Posted 06/22/2010
Motion control brings synchronization, fast changeover, and reliability to textile manufacturing.
Looms and spinning wheels were among the first complex machines made. Their mechanization helped usher in the Industrial Revolution. It makes sense, then, that textile manufacturing would constitute a strong market for motion control.
The scope of textiles goes far beyond clothing to encompass furnishings like drapes and carpets, consumer goods like automotive upholstery, and industrial textiles like those used for filters and mats. Textile manufacturing takes place in long, multi-step, production lines. For years, mechanical shafting held sway in the textile industry. By converting from mechanical to electronic shafting, machine builders were able to speed changeover, simplify maintenance, and reduce cost. Perhaps more important, the technology allowed machines to support the high degree of synchronization required by web processing.
When the fabric leaves the loom it must be prepped, washed, and dyed. After coloring, the textile undergoes additional washing and heat treatments before being cut into sections. Each module of the web process requires precise synchronization of the outfeed with the infeed of the next module. That’s where motion control comes in. "We have to synchronize the speeds of each line precisely to get the proper tension because we don't want to stretch the material or distort it," says Curt Feldman, Value-Stream Manager at the Parker-Hannifin Corp. SSD Drives Division (Charlotte, North Carolina).
At each step, the material must be repeatedly unwound from a spool and fed into a machine for a processing step, then rewound onto a spool using winding heads known as sleds (see figure1). Typically, a sled consists of a rotary motor in combination with a tension-feedback device like a dancer bar. Unwind and rewind functions impose a controlled tension on the web infeed and take-up, even in the face of changing parameters such as web diameter or fabric give. In the case of elastic fabrics, for example, unwind/rewind can introduce a controlled stretch. Length counters monitor footage, while registration control allows textiles with repeat patterns to be synchronized.
In the 1980s, factories began the switch to electronic shafting, generally choosing AC motors and drives. The choice of vector motors over servo motors was driven by practicality. “They’re more economical [than servo motors],” says Feldman. “Textile machines don't really require the power density that a permanent magnet servo motor can give you.” As the adage goes, the technology doesn't need to be good; it merely needs to be good enough.
“We had a big fiber winder that was using inverters and a hodgepodge of controls,” says Michael Miller, Supervisor, Field Application Engineering for Yaskawa America Inc. (Waukegan, Illinois). The design featured a 40-hp motor on the winder and a 15-hp motor on the feeder. Changeovers took too much time, though, and the customer had difficulty finding replacement parts. “They ended up using vector drives and single machine controller," says Miller. "Servo motors in that size are so expensive, and for that process they don't need capabilities of a servo.” Vector motors provided an economical solution, especially when the design team added regenerative capabilities. “When you're pulling the web out from the unwind it’s basically generating energy, and if you don't have any place to put it, it's going into a resistor,” he says. “With line regeneration, you’re looking at about a third of the energy that you would have just put into a resistor going back to the line.”
Staying In Sync
A multistage machine called a dye range, which contains anywhere from 18 to 20 axes, washes and pigments the fabric. After the textile web exits the dye range, it passes into a machine called a tenter frame, which applies heat to pre-shrink the cloth. In the tenter frame, long chains on either side of the web grab the cloth and convey it through a long dryer. To keep the fabric from being stretched or distorted, the two chains must move at the exact same speed. Feedback monitors the position of one motor relative to the other. A high-speed communications channel between the drives keeps the two motors electrically locked together so that the tenter frame doesn’t skew the cloth front to back, to ensure the weave is still at 90° when it comes out of the machine.
The systems typically use distributed architectures with smart inverters capable of performing path planning on their own. Because they have memory, they are capable of plug-and-play operation, with the parameters downloaded from PC to the component via a USB port. This capability allows them to communicate from machine to machine, shop floor to top floor, or even from an outside integrator into the factory. “That's a huge value proposition, especially for textile machines that have been moved to emerging countries where they don't have the whole maintenance structure," says Feldman. "They rely on OEMs and integrators to jump onto the internet and log onto the machine to see what's going on.”
Textile factories can be hostile places for electronic components. The many wash and drive steps subject the hardware to heat and humidity. The presence of corrosive chemicals further complicates matters. To keep the thread from tangling during weaving, for example, it is covered with a wax-like coating. The first step in preparing it to accept dye is to remove the wax with a caustic bath. Many of the motion components used boast stainless steel housings to survive the effects of harsh chemicals used on the dye ranges. Although most of the motors are epoxy painted, few are traditional IP-rated motors. Cooled enclosures keep electronic components clean and dry, while communications over noise-immune fiber-optic cables prevent electrical interference and cross-talk.
Once the fabric has been woven, dyed, and heat shrunk, it must be sewn together into product. These are not your mother's sewing machines, though. At the Parker Hannifin Gmbh Automation Group (Offenburg, Germany), they combine servo motors with centralized control for automated, industrial sewing machines with as many as seven axes of motion (see figure 2). “There are many reasons we use servo motors,” says Henry Claussnitzer, Marketing Manager. “There is robustness, dynamics, high-speed applications, reliability.” A PC with a high-speed communication card acts as both motion controller and HMI. The motors themselves range from 60 to 155 mm, with powers as high as 10 kW per axis. Working with resolvers rather than encoders for feedback provides a more robust solution, Claussnitzer adds.
As motion control technology becomes smarter and more comprehensive, machine builders continue to get more for their investment. Advances in drives, for example, provide multi-function performance, allowing them to run induction motors, vector motors, or servo motors. The commonality not only reduces inventory, it increases operator familiarity, speeding programming and installation. Whether installed as retrofits or new factory builds, motion control will play an increasingly important role in textile manufacturing.