docs: Update Pressure_Advance.md document
Update the document to recommend tuning with decreased acceleration and with square_corner_velocity disabled. Using this approach makes it possible to test with significantly higher pressure advance values. Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
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Kevin O'Connor
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This document provides information on tuning the "pressure advance"
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configuration variables for a particular nozzle and filament. The
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configuration variable for a particular nozzle and filament. The
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pressure advance feature can be helpful in reducing ooze. For more
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information on how pressure advance is implemented see the
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[kinematics](Kinematics.md) document.
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Prerequisites
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=============
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In order to tune the pressure advance setting the printer must be
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configured and operational. The tuning test involves printing objects
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and inspecting the differences between objects. In particular, the
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extruder
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[E steps](http://reprap.org/wiki/Triffid_Hunter%27s_Calibration_Guide#E_steps)
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and
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[nozzle temperature](http://reprap.org/wiki/Triffid_Hunter%27s_Calibration_Guide#Nozzle_Temperature)
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should be tuned prior to tuning pressure advance.
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Tuning pressure advance
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=======================
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Pressure advance does two useful things - it reduces ooze during
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non-extrude moves and it reduces blobbing during cornering. This guide
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uses the second feature (reducing blobbing during cornering) as a
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mechanism for measuring and tuning the pressure advance configuration.
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mechanism for tuning.
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Start by changing the extruder section of the config file so that
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pressure_advance is set to 0.0. (Make sure to issue a RESTART command
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after each update to the config file so that the new configuration
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takes effect.) Then print at least 10 layers of a large hollow square
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at high speed (eg, 100mm/s). See
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[docs/prints/square.stl](prints/square.stl) file for an STL file that
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one may use. While the object is printing, make a note of which
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In order to calibrate pressure advance the printer must be configured
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and operational. The tuning test involves printing objects and
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inspecting the differences between objects. It is a good idea to read
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this document in full prior to running the test.
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Use a slicer to generate g-code for the large hollow square found in
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[docs/prints/square.stl](prints/square.stl). Use a high speed (eg,
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100mm/s) and a coarse layer height (the layer height should be around
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75% of the nozzle diameter). It is fine to use a low infill (eg, 10%).
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Use a low cornering speed during the test by issuing the following
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G-Code command: `SET_VELOCITY_LIMIT SQUARE_CORNER_VELOCITY=0
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ACCEL=500`. This will make the nozzle travel slower through corners
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and thus emphasize the effects of extruder pressure.
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For the first print use a pressure advance of zero by running
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`SET_PRESSURE_ADVANCE ADVANCE=0.000`. Then print at least 10 layers of
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the test object. While the object is printing, make a note of which
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direction the head is moving during external perimeters. What many
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people see here is blobbing occurring at the corners - extra filament
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at the corner in the direction the head travels followed by a possible
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@ -41,15 +40,13 @@ lack of filament on the side immediately after that corner:
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This blobbing is the result of pressure in the extruder being released
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as a blob when the head slows down to corner.
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The next step is to set pressure_advance_lookahead_time to 0.0, slowly
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increase pressure_advance (eg, start with 0.05), and reprint the test
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object. (Be sure to issue RESTART between each config change.) The
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goal is to attempt to eliminate the blobbing during cornering. (With
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pressure advance, the extruder will retract when the head slows down,
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thus countering the pressure buildup and ideally eliminate the
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blobbing.)
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The next step is to increase pressure advance (start with
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`SET_PRESSURE_ADVANCE ADVANCE=0.050`) and reprint the test object.
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With pressure advance, the extruder will retract when the head slows
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down, thus countering the pressure buildup and ideally eliminate the
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blobbing.
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If a test run is done with a pressure_advance setting that is too
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If a test run is done with a pressure advance setting that is too
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high, one typically sees a dimple in the corner followed by possible
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blobbing after the corner (too much filament is retracted during slow
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down and then too much filament is extruded during the following speed
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@ -57,41 +54,61 @@ up after cornering):
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The goal is to find the smallest pressure_advance value that results
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The goal is to find the smallest pressure advance value that results
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in good quality corners:
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Typical pressure_advance values are between 0.05 and 0.20 (the high
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Typical pressure advance values are between 0.050 and 1.000 (the high
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end usually only with bowden extruders). If there is no significant
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improvement seen after increasing pressure_advance to 0.20, then
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improvement after gradually increasing pressure advance to 1.000, then
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pressure advance is unlikely to improve the quality of prints. Return
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to a default configuration with pressure_advance disabled.
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It is not unusual for one corner of the test print to be consistently
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different than the other three corners. This typically occurs when the
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slicer arranges to always change Z height at that corner. If this
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occurs, then ignore that corner and tune pressure_advance using the
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other three corners.
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Once a good pressure_advance value is found, return
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pressure_advance_lookahead_time to its default (0.010). This parameter
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controls how far in advance to check if a head slow-down is
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immediately followed by a speed-up - it reduces pointless pressure
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changes in the head. It's possible to tune this - higher values will
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decrease the number of pressure changes in the nozzle at the expense
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of permitting more blobbing during cornering. (Tuning this value is
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unlikely to impact ooze.) The default of 10ms should work well on most
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printers.
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to a default configuration with pressure advance disabled.
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Although this tuning exercise directly improves the quality of
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corners, it's worth remembering that a good pressure advance
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configuration can reduce ooze throughout the print.
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configuration also reduces ooze throughout the print.
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Finally, once pressure_advance is tuned in Klipper, it may still be
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useful to configure a small retract value in the slicer (eg, 0.75mm)
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and to utilize the slicer's "wipe on retract option" if available.
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These slicer settings may help counteract ooze caused by filament
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cohesion (filament pulled out of the nozzle due to the stickiness of
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the plastic). It is recommended to disable the slicer's "z-lift on
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At the completion of this test, update the extruder's pressure_advance
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setting in the configuration file and issue a RESTART command. The
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RESTART command will also return the acceleration and cornering speeds
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to their normal values.
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Important Notes
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===============
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* The pressure advance value is dependent on the extruder, the nozzle,
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and the filament. It is common for filament from different
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manufactures or with different pigments to require significantly
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different pressure advance values. Therefore, one should calibrate
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pressure advance on each printer and with each spool of filament.
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* Printing temperature and extrusion rates can impact pressure
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advance. Be sure to tune the extruder
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[E steps](http://reprap.org/wiki/Triffid_Hunter%27s_Calibration_Guide#E_steps)
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and
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[nozzle temperature](http://reprap.org/wiki/Triffid_Hunter%27s_Calibration_Guide#Nozzle_Temperature)
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prior to tuning pressure advance.
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* It is not unusual for one corner of the test print to be
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consistently different than the other three corners. This typically
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occurs when the slicer arranges to always change Z height at that
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corner. If this occurs, then ignore that corner and tune pressure
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advance using the other three corners.
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* If a high pressure advance value (eg, over 0.200) is used then one
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may find that the extruder skips when returning to the printer's
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normal acceleration. The pressure advance system accounts for
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pressure by pushing in extra filament during acceleration and
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retracting that filament during deceleration. With a high
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acceleration and high pressure advance the extruder may not have
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enough torque to push the required filament. If this occurs, either
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use a lower acceleration value or disable pressure advance.
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* Once pressure advance is tuned in Klipper, it may still be useful to
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configure a small retract value in the slicer (eg, 0.75mm) and to
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utilize the slicer's "wipe on retract option" if available. These
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slicer settings may help counteract ooze caused by filament cohesion
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(filament pulled out of the nozzle due to the stickiness of the
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plastic). It is recommended to disable the slicer's "z-lift on
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retract" option.
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