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More than 20 years have already passed since the first light of Antu, the Very Large Telescope (VLT) Unit 1 at Paranal Observatory on 25th of May 1998. Encoders are key parts of the control of axes like Altitude, Azimuth, Adapters and Rotators in all the telescopes, where absolute encoders are used to get an accurate measurement of the position, at the level of micrometer. The intention of this document is to summarize and show some problems we had in these years, some modifications and suggestion to improve the design in order to avoid those problems. We also present some new applications of encoders to measure speed instead of position. A typical problem in the Adapters is analyzed, given that the encoder tape is located too close to the bearing it easily gets dirty, forcing our Maintenance Support and Engineering Department (MSE) to dismount the attached instrument and the whole Adapter Rotator getting access to clean its encoder system, with the consequent risk of losing operation time. Same issue is shown in Auxiliary telescopes, GRAAL Co-rotator and the European antenna in ALMA. All the encoder heads in the Adapters and Rotators were modified to avoid damage in the encoder tape, a change of a damaged tape was needed in 2005 and was done for the first time at ESO. A new design of the encoder head support is shown for both Altitude and Azimuth, which provides a fine and accurate way to adjust them. They have been installed in UT4 Azimuth and UT3 Altitude axes. A new design for Azimuth encoder tape is suggested to improve its robustness against variations of height in the hydraulic bearing system. Preventive, Condition-based, Corrective, and “Predictive” Maintenance applied to encoder systems are commented. The Obsolescence is analyzed. And a special case related with Calibration of encoders is also discussed. The integration of encoder signals to the axes plays a pivotal role in the deployment of a cutting-edge control system for the primary axes, Altitude and Azimuth. These encoders not only accurately gauge position but also serve to measure velocity during the encoder initialization phase and provide high-precision commutation information for the power amplifiers. This further enhances precision and efficiency.
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