Recently, I was called in to a paper mill to look at a problem following a drive upgrade. This was an old machine that had one section running in tension control for about 40 years. The old drive used a dc generator. The new drive was a previously enjoyed digital dc drive from another location.
After the upgrade, the tension regulator was unstable and was left in manual (draw) control. Things remained in this state for several months. Additionally there were problems with stability of the draw and slack take up. The operators were frustrated, and not at all pleased with the upgrade. Threading was difficult and the draw required constant monitoring while running. This demonstrates that sometimes replacing old for new is not the same as making an improvement.
The tension and draw problems were solved with a filter on the analog speed reference. The slack take up was restored with a small programming change in the drive.