Interfacing main axis encoders to the control system of the Gemini 8m telescopes

The Gemini Telescopes project is building two 8 m optical/IR telescopes. One will be in Hawaii and one in Chile. The mount control system is, amongst other things, responsible for moving and tracking the two axes of the telescope. The servos that control the axes are implemented on PMAC cards which are proprietary VME cards that use the Motorola 56000 digital signal processor. The encoding system that is used for position feedback is based around a Heidenhain optical tape encoding system. The firmware supplied on the PMAC cards is sufficient for most servo applications, but the outputs from the Heidenhain read heads are not supported by standard PMAC inputs. These signals (two quadrature sinusoids with a period of one tape pitch) require a pitch interpolation algorithm that has to be implemented as user written code on the PMAC card. In addition, the raw Heidenhain signals suffer from repeatable errors due to head misalignment and hardware channel inequalities. These errors can be corrected by using Heydemann compensation, this also has to be implemented as part of the tape interface code. Besides the main tape encoders, there are other devices; all are interfaced to PMAC and the resulting values are combined in additional custom written code, known as the virtual encoder, to produce a single absolute position for feedback to the servo loops. This paper will present an overview of the Gemini project and the place of the mount control system within it