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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.
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.
Closed Loop Web Tension Control Pathway
