docs: Update Pressure_Advance.md - don't set square_corner_velocity=0

Setting the square corner velocity to zero would cause stuttering
during the test if a bed mesh is in effect.  It's possible to get the
desired effect by reducing the square corner velocity and setting the
pressure advance lookahead time instead.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

1 files changed, 19 insertions, 7 deletions

diff --git a/docs/Pressure_Advance.md b/docs/Pressure_Advance.md
index 7bb694e1..f4cc4b8e 100644
--- a/docs/Pressure_Advance.md
+++ b/docs/Pressure_Advance.md
@@ -22,10 +22,11 @@ Use a slicer to generate g-code for the large hollow square found in
 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%).
 
-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.
+Prepare for the test by issuing the following G-Code commands:
+`SET_VELOCITY_LIMIT SQUARE_CORNER_VELOCITY=1 ACCEL=500` and
+`SET_PRESSURE_ADVANCE ADVANCE_LOOKAHEAD_TIME=0`. These commands make
+the nozzle travel slower through corners and they 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
@@ -71,8 +72,8 @@ 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.
+RESTART command will also return the acceleration, cornering speeds,
+and look-ahead times to their normal values.
 
 Important Notes
 ===============
@@ -93,7 +94,7 @@ Important Notes
 * 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
+  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
@@ -105,6 +106,17 @@ Important Notes
   enough torque to push the required filament. If this occurs, either
   use a lower acceleration value or disable pressure advance.
 
+* The pressure_advance_lookahead_time 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 is
+  recommended to follow the steps above so that it is set to zero
+  during tuning and to use the default (0.010) during normal prints.
+  It is possible to tune this setting - higher values will reduce the
+  amount of pressure change in the nozzle during cornering, but
+  setting it too high can cause blobbing during cornering. (Tuning
+  this value is unlikely to impact ooze.) The default of 10ms should
+  work well on most printers.
+
 * 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