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Application Article

Digital Servo Drives for Winding/Unwinding Applications

Dedicated to tension control applications, the SMTBD DI/I servo amplifier is a model of the standard INFRANOR SMTBD D/A servo amplifier. The SMTBD DI/I servo amplifier is a DSP based amplifier implementing a torque ripple compensation method that has reduced torque ripple from 14% to just 1% using standard brushless motors.

Tension control is required in many industrial continuous processes such as web cutting, slicing, printing and winding/unwinding applications. As machine performances have increased servo drives have replaced mechanical solutions such as gears, linkages, clutches, etc. giving cost benefits and lower maintenance. Looking just at the winding/unwinding end of the machine for this article some of the problems and solutions are examined.

Brushless servo motors are well suited for continuous process applications thanks to their superior mechanical, thermal, and dynamic performances with regard to their DC counterparts. Furthermore, maintenance problems Figure 2associated with brushes and commutators are eliminated in brushless technology. However, AC servomotors contain many new sources of torque ripple that are not prevalent with DC servomotors; they are summarized in Fig. 1.

These problems of torque ripple reduce dramatically the material tension accuracy because of the force fluctuations. This has made brushless motors unsuitable for many applications. For this reason INFRANOR has implemented a torque ripple compensation in a DSP based servo drive dedicated to tension control applications. This has reduced the ripple from 14% to 1%. This allows for much improved performance with open loop tension control. Fig. 2 shows the general layout.

The diameter sensor inputs a signal into the INFRANOR Digital drive that allows the amplifier gains to be adjusted Figure 3to cope with the changing inertia of the rewind reel. A much more accurate method can be implemented by the addition of an encoder and load cell (Fig. 3). In this configuration the SMTBD DI/I amplifier can control the rewind roller in such a way as to maintain a constant tension in the material independent of the line speed and rewind diameter. The tension of the material is regulated by the signal from the load sensor. The diameter measurement of the reel is likewise received by the amplifier. The line speed of the material is measured by the encoder. The gains of the speed regulation are adapted automatically as a function of the reel diameter to improve the dynamic stability under changing load conditions.

When using this system, a large improvement in the consistency in the rewound material is noted in comparison to other control methods. Using this technology, a whole line of servo motors may be controlled throughout slitting and rewind to maintain correct tension throughout the entire process.