Controlling web tension is essential for applications like printing, packaging, and textile processing. For a high-quality process and to ensure rapid throughput, it is essential to have precise control over the material’s tension and speed. This article describes the typical architecture of a web tension control application and emphasizes the essential elements needed for its motion coordination.
Processing sheets of materials like textiles or paper requires an application for controlling web tension. When arranged in the machine, the material looks like a web, interlaced between unwinding and rewinding spools along a route that includes the processing station. No matter the goal of the processing element, be it printing, cutting, or laminating, maintaining the ideal tension of the material is essential for ensuring processing quality and safeguarding the material’s integrity and shape
A closed loop system, typically involving DC or brushless motors, is essential to maintain precise control of the material’s tension. As the unwind motor feeds the material from the unwinding spool, its torque must decrease in line with the spool’s continually reducing diameter. Conversely, at the rewind motor, the terminus of the operation, the torque must increase to match the growing diameter of the rewind spool as the material is wound on.
While these two axes can maintain the general tension of the material, they cannot alone control the material’s speed with a sufficient level of precision. Any mismatch between these two axes could cause the material to stretch or jerk, presenting processing challenges at the critical stage.
Constant speed control
To achieve the constant speed required to process the material with precision, independent speed control must be applied through the web’s internal zone. Positioned after the unwinding spool motor, a lead drive sets the line speed as the reference for the synchronisation of the process stages, such as cutters or printers, including the feed speed to the downstream stages.
After the processing stage, the follower drive axis controls material tension within the process zone by modulating torque based on tension sensor feedback, while the master drive maintains the line speed. Without this follower axis, tension fluctuations could arise, leading to disruption, misregistration, or even breakages in the material. However, even very small discrepancies between the lead drive’s velocity and the follower’s torque response can create tension fluctuations that impact the quality of the processing stage. As a result, a compliance device, such as a dancer, which is a spring-loaded arm, is required to smooth-out this difference.
Integral to the control of the web tension machine, an additional component of the compliance device is a position sensor. The position sensor provides accurate web tension feedback by measuring the deflection of the dancer arm and adjusting for the arm geometry. Instead of a dancer, a sensor device like a strain gauge could also perform an equivalent feedback signal.
Managing speed and torque
While the lead drive is commanded with a velocity provided by the motion controller, feedback from the tension sensor is used to continually modulate the torque commanded by the controller to the follower drive. As a result, key control capabilities include precise command of the lead drive’s velocity loop and the follower’s current (torque) loop, which helps smooth tension during material processing. Techniques that minimise tension oscillation are also important, such as electronic damping in the servo filter, and frequency-based compensation using digital filters like biquads.
A challenge in web tension control arises when the web must accelerate or decelerate instead of moving at a constant speed. This could happen during machine start up, as well as part of intermittent or indexing processes. During these speed changes, the inertia of the web material and the rollers can also change the tension. To resolve this, a feedforward compensation signal, derived from the acceleration profile of the motion controller, can be applied, helping to anticipate and counteract tension changes before they occur.
For processes that require the highest levels of precision, application-specific software code might also be required. Bespoke programming could compensate for non-standard geometries of the web path, or specific properties of the material, such as temperature, tension, or deflection tolerance. For example, the increased complexity of layering multiple composite sheet webs can benefit from user-customisable control software.