The SteadyWeb5 Tension Controller is a closed loop automatic tension controller with multiple capabilities to accurately control the tension of any material in any web fed process. To function the SteadyWeb5 Controller requires a feedback signal from tension transducers mounted in the 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/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. To be a tension controller the EasyWeb would need some means of measuring tension. It's missing the direct tension measurement as feedback.
How they work
The SteadyWeb5 Tension Controller has multiple capabilities for control. It can be used as an unwind controller, a rewind controller, or as an intermediate controller. The basic unwind/rewind control format would be to use the feedback from the tension transducers mounted in the web path to monitor the tension applied to the web material and automatically adjust the output to the brake or drive to maintain the 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 the desired point. 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 system 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 it adjusts (trims) the speed signal based on the tension feedback 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.
A variation of this is the speed follow diameter compensated tension trim mode. In this instance the controller is used in a center driven unwind or rewind application. The controller is provided with a line speed signal that is trimmed by the roll diameter (can be a measured or calculated diameter) that modifies the speed of the roll as the roll changes in diameter, and the transducer feedback is used to trim this modified speed signal to maintain precise tension in the material. In addition to these control modalities the controller can also provide a tapered tension function for rewinding materials. In a driven application for unwind, rewind, or intermediate use the controller functions by adjusting the speed of the motor to maintain the desired tension in the web.
The EasyWeb Torque Controller functions based on the speed of the material passing through the process and 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 measurement, the controller adjusts the torque based on this diameter. The controller provides a decreasing torque signal for an unwind and an increasing torque signal for a rewind.
While these torque outputs do create tension in the material the control is not and can not be 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.
The tension meter graphic in the above illustration is a calculated estimation only. This is taken from a screenshot of a simulation we run to demonstrate the principle of the EasyWeb Torque controller.
The choice as to which controller to use in a given application is dependent on individual desires, criticality of control, and in some cases simply cost. It is safe to say that the SteadyWeb5 is the best choice in almost all applications due to the simple fact that it is a closed-loop controller thus providing precise control and displayed results. However in the case where there is no space for tension transducers the Easy Web is the best alternative.
For the price-sensitive customer, the EasyWeb will be somewhat less expensive to add due to the simple elimination of tension transducers and cables. Overall, if accuracy is desired it is always best to select the closed-loop option. However, if space for transducers is not available then the Easy Web open loop controller is an option that will provide dependable torque based web control.
Don't hesitate to contact us with questions or for a free analysis of your application.
For further discussion on the subject of open loop torque control, see this article from web handling expert, Tim Walker, of TJ Walker and Associates. The article was published in Tim's recent Web Lines column on PFFC-online.com.