Any extruder can build pressure rather quickly but it takes a bit before it

NathanielStenzel · May 31, 2017, 7:51pm

Any extruder can build pressure rather quickly but it takes a bit before it stops extruding. Shouldn’t there be a deceleration setting and an acceleration setting in firmware for extruders? You only tend to see a single acceleration setting.
Do any slicers accommodate that notion?

Stephanie_A · May 31, 2017, 7:55pm

Some slicers do, but it doesn’t work very well. I know marlin has an extruder advance feature. I’ll be using it once I get my extruder perfectly calibrated.

Ishaan_Gov · May 31, 2017, 8:23pm

I think @Stephanie_A is on the right track with the EXTRUDER_ADVANCE stuff;
Advance bumps up extrusion velocity on acceleration moves to build nozzle back pressure and the opposite on deceleration moves; I’ll be taking a closer look at advance settings sometime in the near future

Ryan_Carlyle · May 31, 2017, 9:26pm

Pressure advance functions compensate for flow lag by advancing/retarding the extruder motion relative to the XYZ motion. Sailfish, MachineKit, and RepRapFirmware have fully-functioning pressure advance. Smoothie has a fork in the wild that implements it for segmented kinematics only (Deltas and SCARA). Marlin has it but it is a massive performance hit on 8bit processors, so most people only get it to work well when they print really slow or simple geometry… and then you don’t need pressure advance very much…

Getting Sailfish pressure advance to work on an 8bit processor was a mammoth code-optimization effort… it’s not going to happen on 8bit Marlin. The Smoothie fork may or may not get merged. I would personally do RepRapFirmware if it’s something you want to play with.

Sidenote… the elastic effects in the flow lag (bowden tube stretch, filament compression and shear at the drive hob, stepper rotor springiness) are symmetrical for acceleration and deceleration. There are also some thermal lag effects that are not symmetrical, like the cap zone moving up and down (where solid filament enters the melt pool) as extrusion speed changes. And there’s also a slower thermal degradation oozing effect where steam foaming or polymer breakdown causes the melt pool to slowly expand in volume as it heat-soaks. It’s quite complicated.

Daid_Braam · June 1, 2017, 5:16am

Acceleration per axis is there?

github.com

Ultimaker/Marlin/blob/Marlin_v1/Marlin/Configuration.h#L460


//#define MANUAL_Z_HOME_POS 402 // For delta: Distance between nozzle and print surface after homing.

//// MOVEMENT SETTINGS
#define NUM_AXIS 4 // The axis order in all axis related arrays is X, Y, Z, E
#define HOMING_FEEDRATE {50*60, 50*60, 4*60, 0}  // set the homing speeds (mm/min)

// default settings

#define DEFAULT_AXIS_STEPS_PER_UNIT   {78.7402,78.7402,200.0*8/3,760*1.1}  // default steps per unit for Ultimaker
#define DEFAULT_MAX_FEEDRATE          {500, 500, 5, 25}    // (mm/sec)
#define DEFAULT_MAX_ACCELERATION      {9000,9000,100,10000}    // X, Y, Z, E maximum start speed for accelerated moves. E default values are good for Skeinforge 40+, for older versions raise them a lot.

#define DEFAULT_ACCELERATION          4000    // X, Y, Z and E max acceleration in mm/s^2 for printing moves
#define DEFAULT_RETRACT_ACCELERATION  3000   // X, Y, Z and E max acceleration in mm/s^2 for retracts

// Offset of the extruders (uncomment if using more than one and relying on firmware to position when changing).
// The offset has to be X=0, Y=0 for the extruder 0 hotend (default extruder).
// For the other hotends it is their distance from the extruder 0 hotend.
// #define EXTRUDER_OFFSET_X {0.0, 20.00} // (in mm) for each extruder, offset of the hotend on the X axis
// #define EXTRUDER_OFFSET_Y {0.0, 5.00}  // (in mm) for each extruder, offset of the hotend on the Y axis

It’s just that setting it low would mess up retractions a lot.

Cura has a feature these days to stop extruding slightly before the end of the path to help in this area.

NathanielStenzel · June 1, 2017, 7:42pm

Okay. So the firmware just cares about the motor and the slicer cares about the flow and pressure. Do any of the slicers do a test where it tries to extrude and then retracts and then has you measure the filament that came out of the hotend? I have only heard of measuring the filament pushed into the hotend and that is for e-steps. Measuring the filament that came out and when it stopped could help calibrate retraction and flow calculations. More measuring and tests makes it more science and less guesswork.

Panayiotis_Savva · June 1, 2017, 7:49pm

In Marlin, there is the LIN_ADVANCE feature which is a pressure control algorithm.
You need to set a K value, depending on your setup and Filament.
I’ve tested and used it extensively and it works great.

I suggest not to use it with CURA as Cura users different layer widths, which is not great with the advance algorithm. There is an override in the Firmware to cater for it though.