9.5 Closed-Loop Control

Instead, let me talk about some of the issues here. I’m most familiar with the speed-trimmed tension loops that are probably the most common closed-loop tension control method in use today.

These systems get tuned with a PI loop (usually no D or derivative in the PID loops). The proportional variable is a function of the web and tension zone length. If you want to increase the tension from 1 PLI to 2 PLI, this may represent a small speed change (0.01 percent in Aluminum), a modest speed change (0.1 percent in many papers or polyester), or a significant speed change (1 to 5 percent in stretchy film and nonwovens). So the tension loop must be set up for a given web spring constant. If the web is set up and tuned for one material, you may find it is unresponsive or hyper-responsive for another material. In these wide ranging processes, the solution is either to change the tension loop tuning for extreme product changes or switch to a torque-based tension control that doesn’t care about speeds.

When I first asked this question in the early 1990s, the answer was about 2 Hz or updating 2 times per second. This goes along with how the closed-loop control is set up. The 2 Hz answer came from a consideration of the three nested loops in tension control. The first loop is the motor’s internal torque loop, operating at 200 Hz. The next loop is the motor speed control. Since the speed loop is outside the torque loop, it must be slower, running around 20 Hz. Lastly, the tension loop is outside the speed loop and must run slower yet at 2 Hz.

Since the early 1990s, motors and drive technology has changed and I’m told tension loops are running at closer to 20 Hz or an update every 50 ms.

Is 2 Hz or 20 Hz fast enough? Consider the tension upset from an out-of-round roller or unwinding roll. If the roll or roller diameter is 150mm (6″), the circumference is about 0.5 m (20 inches). At 60 m/min (200 fpm or 40 in/s), the upset frequency is 2 Hz. At 600 m/min (2000 fpm), the upset frequency is 20 Hz. Like any control loop, there is a high-frequency limit where the control loop is too slow to respond.

Moral of this analysis: Don’t expect your tension control loop to eliminate tension upsets from out-of-round roll or rollers, bad bearings, gear teeth, or other high-frequency problems.