Conversion excursion: Following the paper trail

June 1, 2007
Converting is the process of taking a raw medium and transforming it into a functional end product. This could be as simple as converting a roll of paper

Converting is the process of taking a raw medium and transforming it into a functional end product. This could be as simple as converting a roll of paper stock into measure-cut and stacked reams, which could be converted once more in a printing, coating, creping, or embossing process, resulting in stationery, literature, or paper napkins.

Converting processes are found in almost all industries, from textiles to metalworking to plastic forming. Although they are involved in vastly different operations, they share a common need for speed and precise timing which, in most cases, can be met with a commercially available motion controller.

Paper production

The converting application that will be discussed here is a process often employed in papermaking, where a roll of raw stock is cut into measured sheets and stacked. The process begins by feeding the paper into a cutting apparatus that slices it into strips of equal width. A rotating shear then cuts the strips into singles of proper length. Next, a separation mechanism spaces the columns of singles, which are reduced and stacked in 500-sheet reams.

It may sound simple enough, but this dynamic application poses several motion control challenges. For example, the rotation of the cutting shear, the separation mechanism, and the stacking discriminator are functions of the speed of the initial paper feed and conveyance. This requires multi-axis coordinated motion in the motion controller.

Controller considerations

To begin developing the converting machinery, it's important to understand the fundamental operation and structure of a typical motion control system.

The primary converting logic and all motion-related computations are performed in the motion controller. Although digital controllers can typically run background application code, respond to I/O, and service communications, their primary responsibility is controlling the servomotor, a task managed in steps by two independent modules.

The first module, the motion profiler, calculates a trajectory or acceleration-deceleration curve for the servomotor that would best fit the given move. In discrete intervals, the profiler calculates where the servo is supposed to be, which is often referred to as the “commanded position.” The commanded position can be generated in various ways, one of which is to follow the position of another axis (or encoder) according to a given ratio.

In the converting application, this sort of following (or electronic gearing) technique is used to keep all axes coordinated with the desired through-rate. A slightly more complicated mode, electronic camming, is also used, which has the effect of “gearing” a periodic movement to a master axis. The cyclic action achieved through electronic camming is employed in the separator.

The second module in a digital motion controller is tasked with analyzing feedback, interpreting where the motor currently is and how to adjust the command signal to minimize position error. This is typically accomplished with a microprocessor running a digital filter that ensures motor stability, responsiveness, and accuracy. Working with the motion profiler, the digital feedback module and filter combine to synthesize the command signal that's fed into the amplifier.

The servo amplifier transforms this low-power command signal into a high-power motor signal. A common amplifier is a torque-mode transconductance amplifier, which converts the command signal voltage to a proportional motor current. The servomotor uses this current to produce motion, which is monitored by an internal encoder that sends a position-dependent, periodic pulse train back to the controller.

Some motion control systems also include a host, a computer or HMI (human-machine interface) that issues high-level directives such as begin, pause, interrogate status, and other commands. It's important to remember that the host is not responsible for motion control. In fact, many motion control applications perform without a regular connection to a host.

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 equal-length 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 500-page 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.

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