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Sensorless rewinder control

Inspired by earlier systems, Comexi has developed a sensorless rewinder control method, explains Jordi Prat, technical director (Slitting & Rewinding) for Comexi Proslit.  

In today’s ever-changing world, eager for technological progress, the fact that we’ve travelled back to recover basic regulation principles from the 1960s is surprising!  

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 was a tendency for the motor to maintain power and increase to its maximum speed when material broke.  

When the use of electronics spread throughout the industrial world and manufacturers began to seek increased precision, standardisation 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 concept was in use for many years thanks to its easy application in any type of mechanical system. However, systems evolve as new technological advances are made; and the introduction of asynchronous motors allowed precise adjustment of the voltage in the rewinder without using external sensors.  

All systems with regulation inevitably involve oscillation, which is why feedback systems require a series of adjustments to minimise variations. As a result, achieving precision and finding the optimal point in these systems is difficult as it’s limited to the ability of the personnel in charge to make adjustments. In addition, the strict transfer of material by means of rollers containing the voltage reading element and the need to maintain the angle of wrap allowed for improvements in this system.  

The advent of three-phase synchronous motors, designed for applications with high levels of coupling at low revolutions, led to the recovery of a system based on former sensorless versions. As it doesn’t have voltage reading elements, this system offers customised adjustment and reduces costs.  

Project becomes reality

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, 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 friction 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 torque limit.

Furthermore, electronics allow synchronisation of real shafts with virtual shafts (perfect in terms of reactions). This synchronisation shows real values for the degree of acceleration of the assembly, which is why it’s not difficult to calculate the inertia torque of the mass composing the load added 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 synchronised servomotors. Thanks to this feature, the current position of the servomotors directly expresses the radius, conferring absolute stability to the value. The sum of these three torque values 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 a ‘sensorless rewinder’ (patent pending), based on regulation systems from the 1960s.

More information is available locally from Advanced Packaging Technology.

www.comexigroup.com

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