• Comexi PROSLIT gains inspiration from former regulation models to develop a sensorless rewinder control system.
Author: Jordi Prat, Technical Director Slitting & Rewinding Comexi Proslit– Comexi Group
Trends are inclined to follow a cyclic evolution. What we value and use today will be gone and obsolete tomorrow. However, nowadays there is a growing tendency to recover past trends.
In a society such as ours, ever changing and eager for technological progress, the fact that we travel to the past to recover basic regulation principles from the 1960’s is surprising, given that they may seem obsolete to us today.
In the past when it was only possible to control speed with a direct current motor, most machines offered current control in the rewinder motor. The built-in potentiometer limited the power in the armature while a classic needle indicated the level of the current.
The main problem with this system was the tendency of the motor to maintain the power of the current and increase to its maximum speed when the material broke. In addition, the frictions that were generally high and produced by the mechanic transmission system were eliminated.
Therefore, when the use of electronics spread throughout the industrial world and manufacturers were stricken with the desire to seek out increased precision, standardization began on a feedback regulation system via a voltage sensor in the material. In this way, regardless of the type of control used in the motor, a real and apparent voltage could be established and regulated.
This solution has stayed around for many years thanks to its easy application in any type of mechanics system used. However, systems evolve as new technological advances are made. Thus, the introduction of asynchronous motors allows us to precisely adjust voltage in the rewinder without using external sensors while allowing us to take advantage of all the benefits this has to offer.
All systems with regulation inevitably involve oscillation, which is why feedback systems require a series of adjustments to minimize variations. Therefore achieving precision and finding the optimal point in these systems becomes more difficult as it is limited to the abilities of the personnel in charge of making adjustments. In addition, the obligatory strict transfer of material by means of the rollers that contain the voltage reading element and the need to maintain the angle of wrap allow for improvements in this system.
The invasion on the market of three-phase synchronous motors, designed for applications with high levels of coupling at low revolutions, has led to the recovery of a system based on former sensorless versions. As it does not have voltage reading elements, this system is capable of solving problems from the past in terms of material, offering customized adjustment and oscillations from regulation as well as reducing costs.
The project has become a reality in the ComexiProslit S-Turret slitter rewinder. To develop this concept, we used motors that directly couple the extendible shaft of the rewinder to the shaft of the motor, thus creating a solid assembly completely free of loads.
Thanks to this cutting-edge mechanical set-up, the kinematic chain between motor and load is limited to the bearings that support the shaft. This, in turn, achieves a drastic reduction in frictions and greater stability. If we add the large number of electronic aids adopted by innovative motor control equipment to this mechanical revolution, we can record the bearing torque needed in each revolution. The graph resulting from the torque/revolutions enables us to anticipate the torque limit.
Furthermore, electronics gives us the possibility to synchronize real shafts with virtual shafts (perfect in terms of reactions). This synchronization shows real values for the degree of acceleration of the assembly, which is why it is not difficult to calculate the inertia torque of the mass composing the load we must add at the time of acceleration.
We can calculate the voltage torque needed to tauten the material from the required setpoint and the radius of the roll to wind. At this point, it is crucial to obtain a real and stable diameter of the reel to avoid voltage oscillations and working with a voltage other than the one required. In the S-Turret, the rewind shaft moves lengthwise as the diameter increases by means of the two synchronized servomotors. Thanks to this feature, the current position of the servomotors directly expresses the radius, thus conferring absolute stability to the value.
The sum of these three torque values calculated is sent to the motor regulation equipment as the torque limit. If we add a certain overspeed to the speed reference to have a regulation margin, we obtain what we call “Sensorless rewinder” (patent pending), based on the regulation systems from the 1960’s.