Tensile Force Control
A tensile force control system is necessary in production of strands of materials including wire, textiles, tubing, and fiber optics. The physical properties of these various materials require variation in the tensile force that is applied in the winding process for each different material. The trend towards miniaturization of devices and components increases the need to be able to process finer and finer material making precise tensile force control a must to avoid material breakage, or an improperly layered wind.
Tensile Force Control Methods
The tensile force must be adjusted to accommodate the winding process itself and the attributes of the material being wound. The current methods include the following:
- Sagging control
- Torque control
- Braking control
- Force measuring
- Dancer position control
As the name suggests this type of tensile force control uses the material’s own weight to control the tension by allowing it to sag between the payoff and take up spools. This system is sometimes controlled with optical sensors that measure the position of the sagging material and adjusts the speed of the payoff or take up spools to regulate the sag. This method is ineffective for fine materials due to obvious reasons of the material being too light and easily disturbed by external forces such as moving air.
Torque control is achieved using the payoff or take up spool drive motors and is the most commonly used method. However this method is limited due to the need to vary the torque as the circumference of the wind on both the payoff and take up changes. As with sagging control external forces in addition to changes in process speed affect the amount of torque control required and the amount must be constantly changed through the entire production run. This method while possible is inaccurate in the winding of fine materials which have low tensile strength and can be easily broken.
Braking control is another method described by its name in that a brake is placed between the payoff and take up spools to control the tensile force. A tensile force parameter is set and when reached the brake is activated to maintain the prescribed tensile force to achieve the correct winding layer. Different methods of braking exist such as mechanical using weights, springs, or magnets, or electro-mechanical that uses electromagnets, or motor driven brakes. Precision is still limited in this method due to twisting requiring a sagging control system to be used in conjunction for flat material which again prevents this method being used for very fine material.
Force measuring heads directly measure the product using rollers connected to the measuring device. Very accurate measurement can be achieved using pressure sensors, expansion measurement strips, and piezo electric sensors depending on the requirements for the product. The requirement for precision is very high due to deviation in the control directly leads to an expansion or relaxing of the product yielding a poor quality wind.
Dancer Position Control
A dancer pulley is placed between the pay off and take up spools and its position is adjusted to apply or reduce tension in the product. This method most closely resembles Supertek’s own design, but more commonly uses springs, weights or pneumatic cylinders to provide position control where as Supertek’s own EDL-60 and EDR-10 models use electromagnets.