Daniel J. Roberts
Applications engineer
Galil Motion Control Inc.
Rocklin, Calif.

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.