docs: Reword parts of the pressure advance kinematics description.
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
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Kevin O'Connor
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@ -54,10 +54,10 @@ of zero duration (if the end speed is equal to the cruising speed).
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Lookahead
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=========
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Look-ahead
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==========
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The "lookahead" system is used to determine cornering speeds between
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The "look-ahead" system is used to determine cornering speeds between
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moves.
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Consider the following two moves contained on an XY plane:
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@ -70,7 +70,7 @@ but that is not ideal as all that acceleration and deceleration would
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greatly increase the print time and the frequent changes in extruder
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flow would result in poor print quality.
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To solve this, the "lookahead" mechanism queues multiple incoming
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To solve this, the "look-ahead" mechanism queues multiple incoming
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moves and analyzes the angles between moves to determine a reasonable
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speed that can be obtained during the "junction" between two moves. If
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the next move forms an acute angle (the head is going to travel in
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@ -85,17 +85,17 @@ acceleration". Best
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[described](https://onehossshay.wordpress.com/2011/09/24/improving_grbl_cornering_algorithm/)
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by the author.
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Klipper implements lookahead between moves contained in the XY plane
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Klipper implements look-ahead between moves contained in the XY plane
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that have similar extruder flow rates. Other moves are rare and
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implementing lookahead between them is unnecessary.
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implementing look-ahead between them is unnecessary.
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Key formula for lookahead:
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Key formula for look-ahead:
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```
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end_velocity^2 = start_velocity^2 + 2*accel*move_distance
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```
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Smoothed lookahead
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------------------
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Smoothed look-ahead
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-------------------
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Klipper also implements a mechanism for smoothing out the motions of
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short "zig-zag" moves. Consider the following moves:
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@ -124,7 +124,7 @@ top-speed.
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Generating steps
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================
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Once the lookahead process completes, the print head movement for the
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Once the look-ahead process completes, the print head movement for the
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given move is fully known (time, start position, end position,
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velocity at each point) and it is possible to generate the step times
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for the move. This process is done within "kinematic classes" in the
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@ -249,18 +249,20 @@ pressure. Pressure increases when filament is pushed into the extruder
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the pressure necessary to extrude is dominated by the flow rate
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through the nozzle orifice (as in
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[Poiseuille law](https://en.wikipedia.org/wiki/Poiseuille_law)). The
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details of the above physics are not important - only that the
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relationship between pressure and flow rate is linear. See the
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[pressure advance](Pressure_Advance.md) document for information on
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how to find the pressure advance coefficients.
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key idea is that the relationship between filament, pressure, and flow
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rate can be modeled using a linear coefficient:
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```
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extra_filament = pressure_advance_coefficient * extruder_velocity
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```
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See the [pressure advance](Pressure_Advance.md) document for
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information on how to find this pressure advance coefficient.
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Once configured, Klipper will push in an additional amount of filament
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during acceleration and retract that additional filament during
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deceleration. The higher the desired filament flow rate, the more
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filament must be pushed in during acceleration to account for
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pressure. Any additional filament pushed in during head acceleration
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is retracted during head deceleration (the extruder will have a
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negative velocity).
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during acceleration. The higher the desired filament flow rate, the
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more filament must be pushed in during acceleration to account for
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pressure. During head deceleration the extra filament is retracted
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(the extruder will have a negative velocity).
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@ -268,8 +270,8 @@ One may notice that the pressure advance algorithm can cause the
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extruder motor to make sudden velocity changes. This is tolerated
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based on the idea that the majority of the inertia in the system is in
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changing the extruder pressure. As long as the extruder pressure does
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not change rapidly it is okay to make some sudden changes in extruder
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motor velocity.
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not change rapidly the sudden changes in extruder motor velocity are
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tolerated.
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One area where sudden velocity changes become problematic is during
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small changes in head speed due to cornering.
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@ -277,6 +279,8 @@ small changes in head speed due to cornering.
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To prevent this, the Klipper pressure advance code utilizes the move
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lookahead queue to detect intermittent speed changes. In these cases
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the amount of pressure increased and decreased will be reduced or
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eliminated.
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look-ahead queue to detect intermittent speed changes. During a
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deceleration event the code finds the maximum upcoming head speed
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within a configurable time window. The pressure is then only adjusted
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to this found maximum. This can greatly reduce (or even completely
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eliminate) pressure changes during cornering.
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@ -15,7 +15,10 @@
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@ -57,7 +60,7 @@
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@ -125,43 +128,80 @@
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