In spite of all modifications I have made there were still some unwanted properties of the printer. Although the uplift (caused by pulling filament from the spool) of the beam which holds the extruder was minimized ( viewtopic.php?f=53&t=16375 ) there could still be noticed some uplift caused by torsional behavior of the extruder beam (because the extruder is not in the center) . Another frustrating phenomenon is that the filament is breaking after some time if no precautions are taken. Normally I prevent breaking to take some windings from the spool. However, sometimes I forget to do this and when printing in the night the filament is sometimes broken in the morning. Some filaments are very sensitive for breaking when stretched for a while.
The solution of both problems is to take some windings from the spool all the time.
In this case the filament does not break and no forces are subjected to the beam.
However, to keep the situation stable the spool has to be unwinded actively. This has been achieved by an engine with a reduction unit.
The engine is controlled (more or less like a step motor) by a pulse switch. The switch is activated by the small gearwheel of the extruder motor. The switch is too slow to follow the fast movements of (re)traction.
A simple electronical cirquit has been designed which allows adjustment of the width of the pulse and the voltage of the engine.
Since there is not a fixed connection between spool and driving axis slip prevents to much suppletion of filament. Although the adjustment of the system is not so critical I am still looking for a sensor system which controls the feeding of the filament more precisely.