Tension Control: Choose Open- or Closed-Loop?
Dover Flexo Electronics, Inc (DFE), a leading tension controls manufacturer based in the USA, offers open-loop and closed-loop tension controls to meet the application needs and budgets of almost all web printers, converters and finishers. The SteadyWeb5™ Tension Controller and the EasyWeb™ Torque Controller are both popular solutions for managing web tension in winding and unwinding applications. But what factors determine which one is better for your application?
Both the SteadyWeb5™ Tension Controller and the EasyWeb Torque Controller are designed to manage running web tension in winding and unwinding applications. They look similar and are housed in identical protective enclosures. So how does a production team determine which one is better for their application?
The SteadyWeb5 Tension Controller is a closed loop automatic tension controller with multiple capabilities to accurately control the tension of any flexible material in a web-fed process. To function the SteadyWeb5 Controller requires a feedback signal from tension transducers mounted in the machine’s web path.
The EasyWeb Torque Controller is an open loop controller used to adjust the torque applied to a roll of material as it is wound or unwound at the beginning or end of a process. While the tension of the material is affected by the torque adjustment, the EasyWeb is not a tension controller per se. To be a full tension controller the EasyWeb would need some means of measuring tension; and that’s what it’s missing: direct tension measurement as feedback.
How They Work
The SteadyWeb5 Tension Controller has multiple special control features. It can be used as an unwind, rewind, or intermediate zone controller. The basic unwind/rewind control format would be to use the feedback from tension transducers mounted in the web path to monitor the tension applied to the web material with the controller automatically adjusting the output to a brake or drive to maintain material tension at the operator-selected set point.
This is the same control scheme used by the cruise control of your automobile. You set the desired speed and the control system adjusts the output to maintain the speed at that desired level. The automatic control function of the SteadyWeb5 is based on motor speed or brake/clutch torque to maintain the material tension.
In addition to the basic closed-loop transducer feedback mode, the SteadyWeb5 can be set to operate in a speed follow tension-trim mode. In this mode the controller receives a line speed signal from the process and the motor follows this speed. The tension transducers provide a feedback signal to the controller, and the controller ‘trims’ the motor’s speed signal so that tension always remains at the operator-selected set point. This mode of operation is used in an intermediate nip application or in a surface rewind application.
The EasyWeb Torque Controller functions based on the speed of the material passing through the process, and on the diameter of the roll being controlled. The line speed is used as a basis for the controller to provide the torque desired to operate the brake, clutch, or drive being used. The controller takes the line speed signal (DC Volts or encoder pulses) and a roll signal (can be a roll speed signal or a measured roll diameter) and calculates an output to set the torque of the controlled device. In the case of a calculated diameter the controller calculates the diameter of the roll based on the line and roll speed signals received. If a direct diameter measurement is being made, the controller adjusts the torque based on this diameter. The controller puts out a decreasing torque signal in the case of unwind control, and an increasing torque signal for a rewind.
While these torque outputs do create tension in the material, the control is not based on material tension as there is no feedback (tension transducers) to create a closed loop control system. The Easy Web can provide a tapered output for rewind applications similar to the SteadyWeb5 but again it is a calculated taper and torque with no means of registering web tension.
Which controller one chooses for a given application depends on machinery space constraints, criticality of control, and cost. The SteadyWeb5 is typically the best performing controller in most applications due to the simple fact that it is an automatic closed-loop controller providing precise tension control and displayed results.
However, in cases where there is no space for tension transducers to be mounted, the Easy Web is the best alternative. The EasyWeb may be less expensive to install since it doesn’t require the purchase of tension transducers and cables. Its indirect, open-loop web control may be perfectly adequate for many applications that don’t demand precision tension constancy.