Filament sync drive
When there is a large distance between extruder drive gear and filament stock (roll of filament, or filament stock is very heavy, then the extruder motor is both pushing/feeding the extruder and pulling the filament stock.
The power used to pull the filament stock should be used to push the filament.
To use all of the power of the extruder motor to push the filament through the nozzle, another motor (filament drive) will be used to pull the filament of the filament stock and push it to the extruder drive.
These 2 motors can not run in parallel as there will be a deviation in drive gear diameter.
Method of solution
The filament is being fed to the extruder drive through a tube.
The tube is not locked at the extruder drive side, and is locked at the filament stock side.
As the filament is being fed into the extruder the tube will get closer to the extruder drive. Once the tube is at the extruder drive, a sensor is triggered and the filament drive will push an amount of filament toward the extruder drive.
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