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>

1 files changed, 74 insertions, 57 deletions

diff --git a/docs/Pressure_Advance.md b/docs/Pressure_Advance.md
index 9afee00b..7bb694e1 100644
--- a/docs/Pressure_Advance.md
+++ b/docs/Pressure_Advance.md
@@ -1,36 +1,35 @@
 This document provides information on tuning the "pressure advance"
-configuration variables for a particular nozzle and filament. The
+configuration variable for a particular nozzle and filament. The
 pressure advance feature can be helpful in reducing ooze. For more
 information on how pressure advance is implemented see the
 [kinematics](Kinematics.md) document.
 
-Prerequisites
-=============
-
-In order to tune the pressure advance setting the printer must be
-configured and operational. The tuning test involves printing objects
-and inspecting the differences between objects. In particular, the
-extruder
-[E steps](http://reprap.org/wiki/Triffid_Hunter%27s_Calibration_Guide#E_steps)
-and
-[nozzle temperature](http://reprap.org/wiki/Triffid_Hunter%27s_Calibration_Guide#Nozzle_Temperature)
-should be tuned prior to tuning pressure advance.
-
 Tuning pressure advance
 =======================
 
 Pressure advance does two useful things - it reduces ooze during
 non-extrude moves and it reduces blobbing during cornering. This guide
 uses the second feature (reducing blobbing during cornering) as a
-mechanism for measuring and tuning the pressure advance configuration.
+mechanism for tuning.
+
+In order to calibrate pressure advance the printer must be configured
+and operational. The tuning test involves printing objects and
+inspecting the differences between objects. It is a good idea to read
+this document in full prior to running the test.
+
+Use a slicer to generate g-code for the large hollow square found in
+[docs/prints/square.stl](prints/square.stl). Use a high speed (eg,
+100mm/s) and a coarse layer height (the layer height should be around
+75% of the nozzle diameter). It is fine to use a low infill (eg, 10%).
 
-Start by changing the extruder section of the config file so that
-pressure_advance is set to 0.0. (Make sure to issue a RESTART command
-after each update to the config file so that the new configuration
-takes effect.) Then print at least 10 layers of a large hollow square
-at high speed (eg, 100mm/s). See
-[docs/prints/square.stl](prints/square.stl) file for an STL file that
-one may use. While the object is printing, make a note of which
+Use a low cornering speed during the test by issuing the following
+G-Code command: `SET_VELOCITY_LIMIT SQUARE_CORNER_VELOCITY=0
+ACCEL=500`. This will make the nozzle travel slower through corners
+and thus emphasize the effects of extruder pressure.
+
+For the first print use a pressure advance of zero by running
+`SET_PRESSURE_ADVANCE ADVANCE=0.000`. Then print at least 10 layers of
+the test object. While the object is printing, make a note of which
 direction the head is moving during external perimeters. What many
 people see here is blobbing occurring at the corners - extra filament
 at the corner in the direction the head travels followed by a possible
@@ -41,15 +40,13 @@ lack of filament on the side immediately after that corner:
 This blobbing is the result of pressure in the extruder being released
 as a blob when the head slows down to corner.
 
-The next step is to set pressure_advance_lookahead_time to 0.0, slowly
-increase pressure_advance (eg, start with 0.05), and reprint the test
-object. (Be sure to issue RESTART between each config change.) The
-goal is to attempt to eliminate the blobbing during cornering. (With
-pressure advance, the extruder will retract when the head slows down,
-thus countering the pressure buildup and ideally eliminate the
-blobbing.)
+The next step is to increase pressure advance (start with
+`SET_PRESSURE_ADVANCE ADVANCE=0.050`) and reprint the test object.
+With pressure advance, the extruder will retract when the head slows
+down, thus countering the pressure buildup and ideally eliminate the
+blobbing.
 
-If a test run is done with a pressure_advance setting that is too
+If a test run is done with a pressure advance setting that is too
 high, one typically sees a dimple in the corner followed by possible
 blobbing after the corner (too much filament is retracted during slow
 down and then too much filament is extruded during the following speed
@@ -57,41 +54,61 @@ up after cornering):
 
 ![corner-dimple](img/corner-dimple.jpg)
 
-The goal is to find the smallest pressure_advance value that results
+The goal is to find the smallest pressure advance value that results
 in good quality corners:
 
 ![corner-good](img/corner-good.jpg)
 
-Typical pressure_advance values are between 0.05 and 0.20 (the high
+Typical pressure advance values are between 0.050 and 1.000 (the high
 end usually only with bowden extruders). If there is no significant
-improvement seen after increasing pressure_advance to 0.20, then
+improvement after gradually increasing pressure advance to 1.000, then
 pressure advance is unlikely to improve the quality of prints. Return
-to a default configuration with pressure_advance disabled.
-
-It is not unusual for one corner of the test print to be consistently
-different than the other three corners. This typically occurs when the
-slicer arranges to always change Z height at that corner.  If this
-occurs, then ignore that corner and tune pressure_advance using the
-other three corners.
-
-Once a good pressure_advance value is found, return
-pressure_advance_lookahead_time to its default (0.010). This parameter
-controls how far in advance to check if a head slow-down is
-immediately followed by a speed-up - it reduces pointless pressure
-changes in the head. It's possible to tune this - higher values will
-decrease the number of pressure changes in the nozzle at the expense
-of permitting more blobbing during cornering. (Tuning this value is
-unlikely to impact ooze.) The default of 10ms should work well on most
-printers.
+to a default configuration with pressure advance disabled.
 
 Although this tuning exercise directly improves the quality of
 corners, it's worth remembering that a good pressure advance
-configuration can reduce ooze throughout the print.
+configuration also reduces ooze throughout the print.
+
+At the completion of this test, update the extruder's pressure_advance
+setting in the configuration file and issue a RESTART command. The
+RESTART command will also return the acceleration and cornering speeds
+to their normal values.
+
+Important Notes
+===============
+
+* The pressure advance value is dependent on the extruder, the nozzle,
+  and the filament. It is common for filament from different
+  manufactures or with different pigments to require significantly
+  different pressure advance values. Therefore, one should calibrate
+  pressure advance on each printer and with each spool of filament.
+
+* Printing temperature and extrusion rates can impact pressure
+  advance.  Be sure to tune the extruder
+  [E steps](http://reprap.org/wiki/Triffid_Hunter%27s_Calibration_Guide#E_steps)
+  and
+  [nozzle temperature](http://reprap.org/wiki/Triffid_Hunter%27s_Calibration_Guide#Nozzle_Temperature)
+  prior to tuning pressure advance.
+
+* It is not unusual for one corner of the test print to be
+  consistently different than the other three corners. This typically
+  occurs when the slicer arranges to always change Z height at that
+  corner.  If this occurs, then ignore that corner and tune pressure
+  advance using the other three corners.
+
+* If a high pressure advance value (eg, over 0.200) is used then one
+  may find that the extruder skips when returning to the printer's
+  normal acceleration. The pressure advance system accounts for
+  pressure by pushing in extra filament during acceleration and
+  retracting that filament during deceleration. With a high
+  acceleration and high pressure advance the extruder may not have
+  enough torque to push the required filament. If this occurs, either
+  use a lower acceleration value or disable pressure advance.
 
-Finally, once pressure_advance is tuned in Klipper, it may still be
-useful to configure a small retract value in the slicer (eg, 0.75mm)
-and to utilize the slicer's "wipe on retract option" if available.
-These slicer settings may help counteract ooze caused by filament
-cohesion (filament pulled out of the nozzle due to the stickiness of
-the plastic). It is recommended to disable the slicer's "z-lift on
-retract" option.
+* Once pressure advance is tuned in Klipper, it may still be useful to
+  configure a small retract value in the slicer (eg, 0.75mm) and to
+  utilize the slicer's "wipe on retract option" if available. These
+  slicer settings may help counteract ooze caused by filament cohesion
+  (filament pulled out of the nozzle due to the stickiness of the
+  plastic). It is recommended to disable the slicer's "z-lift on
+  retract" option.