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April 20, 2006
Best Plan for Start-Stop Processes?
What is the best tension control plan for processes that start and stop frequently?
The biggest problem with stopping and starting is inertia and coordinating motor speeds, especially for the unwinding and winding rolls. Inertia time acceleration (or deceleration) creates a torque (at steady speed, there is no accel, so no torque). The inertial torques of rolls or rollers can easily be many time the torque of tension (times radius of the roller) or other torque losses (drag, etc.).
Any tension control plan must answer four questions:
Q1. How will I apply torque to the web to create tension (motor, brake, clutch)?
Q2. How many tension zones will I have in my process?
Q3. Which drive point will be my master or pacer drive (the motor that is in speed control, no tension loop)?
Q4. How will I trim the torque or speed of the follower sections?
Stop start may affect all of these decisions.
A1. Regenerative motors (motors that can function in power or braking mode) can be programmed to respond to predicted inertial loads, reducing tension upsets.
A2. The number of zone you need may go up with high inertial torque from a process with a large number of rollers.
A3. You need to ensure that the pacer section doesn't slip. The inertial loads of stop-start processes can create surprising tension changes within a zone and across drive points. Becare to estimate the tension swings from inertial loads when determine the friction needed at the pacer drive point.
A4. Dancer rollers do a great job of forgiving speed errors between pacer and follower sections. For intermediate tension zones, where the master and follower are controlled by rollers (with constant diameter) and a line speed reference signal is used to set the follower's baseline speed, a transducer roller is usually good enough to feedback web tension. However, on unwinds and winders, where diameters are changing, a dancer roller is a good alternative to forgive the more likely speed errors created by roll diameter uncertainty.
Lastly, for any process, I think it is a good idea to include a 'stall' mode in the process. In this mode, the line first pulls the web taut at zero speed before shifting to accel and run mode. The stall mode ensures a smooth transition from stopped to running conditions.
This all said, the slower you accel/decel, the less these issues are a big deal.
Posted by Tim Walker at April 20, 2006 09:38 AM
