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Tension control methods

Depending on the requirements, economic efficiency and technical possibilities, there are various methods of controlling the tensile force in a production process. The tensile force of the yard goods or the coiled material must be precisely adapted for a machining process which, for example wires, stranded or coated, for example. The basic requirements of a tension control system are highlighted here using an application example in the form of a simple rewinder for wires.

On the left side, the upper figure shows an unwinder, on the right side a rewinder.

The tensile force must be such as to ensure optimum guidance of the wire. If the tensile force of the wire is too low, this will lead to poor winding positions or to the wire slipping off the guide rollers and thus to a process standstill. On the other hand, if the wire is stretched until it breaks, the tensile force is too high.

These two extremes illustrate that the tensile force must be adapted not only to the processing operation but also to the physical properties of the product used. To ensure that the product being wound is processed optimally, however, the requirements for tensile force control are much higher: the attenuation properties of an optical waveguide change as the tensile stress changes.

To ensure consistent quality of the optical fiber, the tensile stress must be kept constant during processing.

Fluctuations in tensile stress lead to poor quality of the wound material and must therefore be prevented.

During the rewinding process, various parameters change, such as that of the coil diameter and with it the number of revolutions of the unwinder. Regardless of the changing parameters, the tension control system should be able to keep the tension constant. Another requirement is determined by the web speed of the wire. The faster the web speed is changed, the more dynamically the tension control must be able to work.

The methods of active tension control are:

• Drive control with tensile force sensors
• Dancer position regulation
• Torque control

Tensile force sensors

The tensile force can be determined with the aid of tensile force sensors and controlled via a

drive control. The measurement is made directly on the product, for example by means of rollers connected to a force measuring device. Depending on the accuracy required and the different methods are used for measuring the force, depending on the accuracy are used. These include pressure sensors and strain gauges, as well as piezoelectric and electromagnetic methods, which are referred to respectively as force and pressure sensors, or transducers, or as load cells.

In principle, a very precise measurement of the tensile force can be achieved with a force sensor can be achieved. The demand on the drive control is very high, since a control deviation of the drive leads directly to a change in the tensile force.

Dancer position regulation

Drive control with a dancer position is based on sag control with the important difference that the tensile force is not is not controlled by the product’s own mass, but by an external force acting on the product. 

Dancer position control is a proven method for active tension control of coiled products. Active tension control uses drive motors for general transport of the wound product, for example in the form of a coil drive instead of braking systems. In the principle circuit diagram, a rewinding process of a winding material from the unwinder to the rewinder with dancer position control is outlined. In the principle circuit diagram, it is assumed that the rewinder winds up the product at a given speed. The unwinder must follow the rewinder so that a desired tension is achieved.

This is achieved by applying a force to the product being wound via a movable deflection roller which corresponds to the tensile stress in a fixed ratio. In the schematic diagram shown, a positive acceleration of the winder would result in an upward movement of the movable deflection roller. A negative acceleration would cause the deflection roller to move downwards accordingly. The up and down movement is also called “prancing”, which gave rise to the synonym dancer. According to the pulley principle, the traction force is proportional to the dancer force. The dancer position is in turn used to control the speed of the drive motor, preferably with a PID controller. If the dancer moves upwards, the control deviation leads to positive acceleration of the unwinder and vice versa. To ensure that the traction force does not change in the event of a control deviation, the dancer force must always be kept constant, even regardless of the dancer position.

With conventional dancer systems consisting of weights, pneumatic cylinders and (or) springs, only relatively coarse tension force controls with corresponding tension force fluctuations can be implemented. This is due to the physical property of the dancer force. For dancers with weights, the force according to F = ma already changes due to the dancer movement. Mass inertia inevitably causes a change in the tensile force. Springs always have a different force proportional to the spring length and contradict the need for a constant dancer force, which is difficult to get a grip on even with special pneumatic cylinders.

The conventional methods have reached their technical limits, especially for the tension control of sensitive products. For this reason, Supertek GmbH has developed an electromagnetic dancer based on an electromagnetic force method patented by Supertek, which can generate a highly dynamic and dancer position-independent constant dancer force. With the EDL 60 electromagnetic dancer, tensile stresses in the range of 10 cN to 600 cN can be controlled with a resolution of 0.1 cN. Where other systems fail, the use of the electromagnetic dancer means that even the most sensitive winding materials can now be processed very quickly, precisely and without problems.

The change of the dancer position should not change the tractive force, but only serve to regulate the drive motor.

Torque regulation

Torque control is used to control the torque of the unwinder or rewinder drive. Tension control via torque control appears to be a suitable method with today’s drive technology, but it has certain limitations. Since the circumference of the spool changes during winding or unwinding, this results in a change in the tensile force of the wire if the torque is constant. Torque control can be used for small changes in circumference or for permissibly wide tensile force tolerances, especially at high tensile strengths. In addition, the torque can be adapted to the circumference of the coil. This requires a measuring and control device that permanently determines the circumference. The torque must therefore be adapted to the circumference and continuously calculated. Tension control based on torque control is complicated by acceleration processes. Torque control is therefore rather unsuitable for very precise tension requirements or very small forces.

The biggest disadvantage of torque control in contrast to dancer position control is that a control deviation of the motor has a direct effect on the tensile stress. A buffer for external disturbances is also not available with torque control.

Metrology

For controlled rewinding of wires and fibers, it is necessary to determine various physical quantities such as wire length, winding speed or tensile force.

Measurement of length and speed

To measure the physical quantities, length, speed and force, pulleys with which the wire is guided can be used.

The force can be measured with tensile force sensors or load cells. To measure the speed and length, rotary encoders or encoders mounted on a deflection pulley are suitable.

The length is calculated as follows:

The speed n of the deflection roller, the radius rU of the deflection roller to the running surface and half the wire diameter rD, or the radius of the wire, must be taken into account.

The wire diameter influences the measurement of the length and the measurement of the speed and should be parameterizable for the HMI.

The speed v is calculated accordingly:

The resolution and accuracy of the length and speed determination depends on several factors such as the design of the pulley, the type of speed measurement or resolution of the encoder, the friction force (slip) between the pulley and the material to be measured and the signal processing and calculation time (real time). For speed measurement, Supertek offers different idler pulleys with encoders. The calculation of the length and speed is done with the Winding Controller MCU in real time. Thus, a target length of a material can be wound accurately. This is required, for example, in the production of precise coils.

Tensile force measurement

If the tension control is implemented with our electromagnetic dancers, an additional measurement of the force with tension measuring devices is not necessary, because the dancer force of the electromagnetic dancer works equivalent to a precision balance with electromagnetic force compensation.