Web machines should speed up and slow down smoothly and gracefully, just as the family car does. The engine should not cough, hesitate or backfire when you depress the accelerator. Similarly, the brakes should slow the machine predictably, with all four brakes performing in unison.

The web can show symptoms of poor machine dynamics. First, the width or registration of the product may vary during speed changes if tension is allowed to vary. Second, the path of the web through the machine may change when tension variations act on roller misalignment. In both cases, the wound roll would record the resulting edge disturbances as a defect known as an "acceleration offset." Third, the web may break more frequently. It is not unusual for paper coaters to have one or two orders of magnitude greater web breaks during a minute of speed change when compared with a minute of steady state. In extreme cases, the web may go slack or drum tight during the speed ramp.

The machine also gives symptoms of poor machine dynamics. The best measure of drive health is a steady load-cell tension reading. Lacking that, you can look at motor ammeters to make sure they move in concert during the inertial demands of speed changes. Also, the ammeter should move quickly to its new location but without overshoot. Unfortunately, the ubiquitous dancer roll can't be read for dynamic health.

To achieve graceful speed changes, the designer must pay attention to details. To begin with, the speed reference from the operator must be modified to an "S" shape composed of three parts before the drive can use it. First, the speed rounds from the current speed onto the linear ramp over the course of several seconds. Second, the linear acceleration ramp should be programmed to be appropriate for the duty. Third, the speed rounds from the linear ramp onto the new speed over the course of several seconds.

The accel/decel rate may be less than 10 fpm/sec for complex machines and those with poor drives, excessive roller inertia or other compromises. It is very important not to push acceleration rates on machines that nominally run continuously as control quality will suffer. However, some machines, such as rewinders, must accelerate quickly for productivity reasons, sometimes at rates in excess of 100 fpm/sec.

All other drives must follow an appropriate target, be responsive, and be accurate. Briefly, the error (difference between the setpoint and feedback) should always be small. To do this, the drive must be tuned for maximum stable gain. Unwind and rewind drives thus require inertia compensation to vary the gain so that the drive is aggressive with errors at large-roll diameters (because the inertia resists changes) and moderate with errors at small-roll diameters (to avoid instability). Feed-forward is helpful to alert drives to changes in speed or other conditions.

Always keep in mind that machines are electro-mechanical systems, so the mechanicals must also be in good health. In particular, the machine must not have excessive roller counts or inertia. Sensors such as dancers must be low friction, and load cells must be sized appropriately so that you aren't trying to weigh a pea in a dump truck.

Finally, actuators must be sized so that drives are not idling. In other words, pneumatic brakes should be kept above 10-psi pressure, and electric motors should be loaded to a fair fraction of their capacity.