Multi-axis control

Returning to the papermaking example, the first step in designing a successful solution is to determine the number of axes required. For simplicity, subordinate axes such as feed travelers will be omitted. The master feed roll will constitute one axis, the rotary shear another, and the separation cam and final stacking phase two more, for a total of four.

A four-axis motion controller is ideal for this application — provided it offers plenty of I/O for machine synchronization and 100baseT Ethernet for fast communication to a monitoring station. To save space and simplify design, some controllers also integrate their own drives. For this example, internal servo drives that generate 500 W or more would be adequate for the separator and stacking phases, while larger external drives would be needed for the feeding roller and flying shear.

Managing teamwork

To understand how the whole machine works together, it's important to define what each axis will be responsible for, determine the various I/O points, and establish a rough idea of the required mechanics to accomplish the application.

Axis A — Feed rollers and conveyors: Two take-up rollers will feed stock paper into the machine. Paper feed speed is the determining factor for all motion in the system. If the feeding rate changes, the rest of the processes must follow. This is accomplished by setting the feed roller as the master axis in all electronic gearing and coordination. Subordinate axes such as the stock paper spool and the paper-processing conveyor can be slaved to this axis through electronic gearing with a 1:1 ratio.

Static paper strip cutters: Compared to the dynamic rotary shear, the static strip cutter is easier to work with from a motion control perspective, since only the paper is moving. The strip cutter slices the paper into long, equal-width strips and can be controlled by a single digital output, which drives the cutting blade mechanism. Cutting can be stopped for safety or loading events, and can be started again as needed. The operation is static, as it is not a function of paper feed rate.

Axis B — Dynamic rotary shear: Paper columns are cut into equallength sheets at a rate equal to the feed rate. If the feed rate increases, the cutting rate must also increase or the sheets of paper would be too large. This is accomplished by using a rotary shear. A drum is equipped with cutting blades along its circumference with an arc length equal to the

desired cut length. The drum is electronically geared to the feed axis so that the cuts will occur at the appropriate place on the paper stock. The ability to superimpose independent movements on top of the gearing allows the rotating shear to cut different lengths without changing the mechanics of the cutting drum.

Axis C — Column separator: As paper columns exit the rotary shear, they are separated by a mechanism that allows space for stacking. This is accomplished through an electronic cam. As each group of pages leaves the rotary shear, the cam widens the spacing between them. Because the cam profile is geared to the master feeder, the separation occurs at a given distance (one page) regardless of feed velocity.

Axis D — Paper stacker and stack sled: In the stacking phase, consisting of a page-counting sensor and a hopper, paper is gathered in 500page stacks, then moved by sled to an exit conveyor. The mechanism is not directly geared to the feeder speed. Instead, page sensor events are counted and the sled axis waits until the total reaches 500. The speed of the stack drop onto the exit conveyor is fast enough to accommodate the feeder's maximum speed.

Convert and conquer

Designing a machine for converting applications may at first appear to be a daunting task. The motion control requirements are many, but today's technology can handle the various modes of motion required by even the most demanding application. Working closely with a motion control company to fully define the requirements of the application and choosing the right controller are vital milestones along the path to conquering any converting application.

For more information, e-mail the editor at frichards@penton.com or visit galilmc.com