Comin' up around the bend

Oct. 7, 2004
Curvilinear servomotors eliminate mechanical transmissions and provide independent control of multiple carriages.

Keith Jacobs
Jacobs Automation LLC
Loveland, Ohio

The PackTrak system is a linear-motor system that lets multiple carriages, or movers, ride on a single curved-rail system.

Cutaway of mover-carriage assembly

The mover carriage is mounted on four sets of track rollers that ride on opposing rails. Current to each coil is controlled separately and sensors provide position feedback.

Linear servomotors, serving up straight back-and-forth motion, have been around for years. But standard linear motors can only work in a straight line, typically controlling only a single moving carriage. Many applications require independent control over multiple moving elements and the ability to travel around curves. Now, new servomotor designs let motion-control engineers design straight and curved motion paths.

The PackTrak system is a direct-drive, movingmagnet linear servomotor. The motor path can be curvilinear which means it isn't limited to straightline motion. The mover carriage mounts on four sets of track rollers that ride on opposing rails. Two sets of drive magnets mount opposite each other on the inside surface of the carriage. Energizing stationary armature coils close to each mover provide force. Current to each coil is controlled independently letting each mover be separately controlled. This also differs from traditional three-phase motor controls. An array of sensor elements mounted on the stator provides position feedback. A position magnet mounted on the carriage activates the sensor elements. This position is then fed to the servodrive to close the loop.

The motor can independently control multiple movers on the same track. Each mover is passive, meaning it has no external attachments such as a power or communications cable or tether. Absolute position of each mover is determined at power up, eliminating homing. Systems that require a homing operation at power up or after a reset waste time. The motor surfaces withstand washdown and contain no crevices or undrained cavities that could create sanitation problems.

The system falls in line with a general trend in motion-control technology: the reduction of mechanical transmission parts such as chains, belts, and gears and a matching increase of electronic parts and software.-It is driven by the need for greater flexibility, reliability, and speed. For example, in the packaging industry, line-shaft-driven machines are being replaced by rotary-servomotor-driven machines. A rotary servomotor directly coupled to each subsystem eliminates the line shaft. This improves flexibility through electronic cams that let arbitrary motions be defined in software and electronic gears that allow two or more motors axes to be synchronized based on software-defined ratios. Again, reducing part count improves reliability. The speed of these new machines is often faster due to the reduced inertia, compliance, and wear of the mechanical transmission.

For applications that require frequent changeovers, it provides the flexibility to adjust machine pitch without any hardware changes because software defines the motion profiles. For applications where reliability is paramount, machine downtime and maintenance can be reduced by eliminating wear-prone mechanical transmission parts. Similarly, machine speeds often can be increased because inertia and compliance of the mechanical transmission is gone. In applications where size is important, PackTrak offers the potential to reduce the machine footprint.

The biggest payoff for the system is that it could dramatically reduce end-user costs. Analyses show the potential to reduce the total cost of packaging by 25% or more. With such cost savings potential, PackTrak should be considered for any packaging application that requires independent control of multiple discrete items and/or curvilinear motion.

Ring around the filler

Curvilinear motion systems used in packaging applications can bring separately controlled carriages on a racetrack-type setup.

Vertical form, fill, and seal (VFFS) machines are used to package dry goods such as snack foods and liquids like aseptic juices.

The basic sequence of operation is:

  • Packaging material on a web roll is fed into a machine and formed around a vertical tube.
  • The packaged product is fed down the tube.
  • The tube is sealed transversely and cut to create discrete packages.
  • Individual packages are then discharged from the machine, ready for the next step in the process

Today's machines use a rotary servomotor to drive a mechanical linkage attached to opposing seal heads. A PackTrak system could replace the mechanical transmission. Using PackTrak's servo capabilities, each seal head is synchronized with its partner. Further, the motion of each seal head pair is adjusted based on the registration mark found on each package to keep the machine in register with the tube.

The speeds of today's VFFS machines are limited by the inertia and wear of mechanical cams and linkages and the ability to seal only one package at a time. PackTrak overcomes these limitations and increases operating speeds. It does this by providing nearly unlimited sealing dwell time along its straight section and the ability to engage multiple packages simultaneously.

Eliminating the mechanical transmission would require a racetrack with an independently controlled mover driving each seal head. Further, applications like this also require the ability to register to each individual package to keep the machine in register and, in some cases, to provide for adequate package formation.

Jacobs Automation LLC,

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