Ultimaker Original (1) - EXTRUDER POSITION

[ultiprint 0]

Hi guys,

I know that the limit switches can check if the extruder has reached the limit of the axis (X or Y). What I do not understand is how do the stepper drivers check the positioning of the extruder at any given moment, in order to understand how much it needs to move in each direction. Are there any other sensors that check the position of the extruder?

Thanks!

 

[IRobertI 520]

There is no feedback. Generally you will use motors that are stronger than you need so that you can assume that when you tell the motor to move Xmm it will move Xmm. So after the firmware has established the home position it just assumes that the physical position of the head matches the "digital" position so to speak.

 

[anon4321 16]

No, steppers are open loop control.

The firmware establishes "home" or 0, 0, 0 using the switches. From then on, the firmware tracks the steps away from 0, 0, 0 to know the nozzle's location.

 

[anon4321 16]

From the firmware source -

#define DEFAULT_AXIS_STEPS_PER_UNIT {78.7402,78.7402,200.0*8/3,760*1.1} // default steps per unit for Ultimaker

These values represent the X, Y, Z and extruder stepper motor steps per mm.

So if the gcode calls for the head to move in the X direction 100mm, the firmware will issue 7874 steps. If the gcode then has the head move back 50mm in teh X direction, the firmware assumes it is starting at 100mm and so steps in the reverse direction 3937 steps.

This works as long as what IRobertI mentioned is true. The steppers must be strong enough so that steps occur when requested. If the steppers "miss a step", the print will be off as there is no feedback that the miss has occurred.