Web Tension Control

What is Web Tension Control?

Web Tension Control means control of motors and sometimes brakes for the purpose of maintaining a sheet of material (the web) at a constant tension during processing. Web tensioning is used in a large array of machines and industries including paper processing, packaging equipment, consumables fabrication, composite fabrication, textile equipment, and more.

Design Considerations for Web Tension Control

The diagram below shows a representative flow path for web tension control. There are actually many ways to 'thread' a web, but the key task is to maintain the web at a controlled tension and speed, particularly in the zone where the material will be processed. In this article our focus will be on closed loop web tension control, where motors such as DC or Brushless motors are actively commanded to maintain the tension at a high level of constancy. 

In web tension control the processing zone is called the internal zone (also sometimes called the intermediate zone). The other zones are the unwind zone, which unwinds the raw material to be processed, and the rewind zone, which re-spools the processed web material.

Depending on the requirements of the web tensioning application, one of a few control options can be selected. In this diagram the internal zone uses a dancer, which is essentially a spring-loaded arm with a position sensor. The sensor can derive the web tension very accurately by measuring the deflection of the dancer arm and adjusting for the arm geometry.

The feedback from the sensor is input to a controller which continually alters the torque being commanded to the slave drive motor while the master drive motor is commanded with a velocity, thereby determining the feed rate of the material through the internal zone. Instead of a dancer some applications use a strain gauge to measure tension, but the overall control approach is the same.

Key capabilities of the motor controller in this application are use of a velocity loop, and control techniques that minimize the tendency of the web tension to oscillate. These include electronic damping in the servo filter and frequency-based compensation using digital filters such as biquads.

Another challenge for some web tension control applications occurs if the web must accelerate or decelerate rather than move at a steady speed. This complicates tension control because the inertia of the web stock itself as well as the rollers in the web path will contribute to the effective tension seen in the web. To remedy this feedforward compensation from the acceleration term of the profile generator to the servo compensation scheme is used.

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Vital Web Tension Control Techniques

  • Oscillation damping algorithms
  • Velocity loop control
  • Biquad filters
  • Downloadable user software
  • Multi-sensor input
  • Serial digital network support
  • Analog signal input
  • Feedforward

For very high performance systems the ability to host application-specific software code may be needed in the controller. This software can be used to compensate for non-standard geometries of the web path, or special temperature, tension, or deflection-dependent qualities of the web material as it travels through the web path and engages at the internal zone. Layering of multiple composite sheet webs is an example of an application that sometimes benefits from fully user customizable control software.

Finally, especially for tension control of the unwind and rewind zones, the ability of the controller to input a variety of sensor data is a useful controller feature. For example to measure the diameter of the spooling web material ultrasound sensors or mechanical follower arms may be used. These inputs may provide analog output signals or digital encoder signals, or even use a serial digital bus such as CANOpen.


Closed Loop Web Tension Control Diagram

The diagram below shows a generalized pathway for web tension control applications. There are many variations, and both the unwind and rewind tension zones, which in this diagram use just a torque motor or brake to maintain tension, may also utilize a closed loop tension controller in connection with a dancer or strain gauge for tension measurement.

web-tension-control-motion-application-diagram-part2

Closed Loop Web Tension Control Pathway


Web Tension Control Solutions From PMD

Performance Motion Devices motor control drives and intelligent amplifiers are used in a wide variety of web tensioning applications due to their unique combination of ruggedness, high performance, and affordable cost. PMD’s N-Series ION Digital Drives are PCB-mountable modules which support multiple sensor input and full customizability via downloadable user-written software. PMD’s ION/CME 500 drives provide a similar capability in a cable connected module format. For IC-based solutions PMD’s MC73112 and MC71112 motor control ICs drive BLDC and DC Brush motors respectively and are ideal for web tension control applications.

 


Machines That Frequently Use Web Tension Control

  • Paper processing equipment
  • Paper converting machinery
  • Packaging equipment
  • Consumables packaging equipment
  • Pharmaceutical processing equipment
  • Composite layering equipment
  • Composite fabrication machinery
  • Laminating equipment
  • Coating equipment
  • Film processing equipment
  • Foil processing machinery
  • Synthetic fabric production equipment
  • Plastic film manufacturing machinery
  • Winding machinery
  • Slitting machinery
  • Thin film application equipment
  • Unwinding machinery
  • Polymer film production machinery
  • Nylon fabrication equipment

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