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docs: Reword parts of the pressure advance kinematics description.

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Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
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Kevin O'Connor 2017-04-19 20:28:37 -04:00
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54
docs/Kinematics.md
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@ -54,10 +54,10 @@ of zero duration (if the end speed is equal to the cruising speed).
![trapezoids](img/trapezoids.svg.png)
Lookahead
=========
Look-ahead
==========
The "lookahead" system is used to determine cornering speeds between
The "look-ahead" system is used to determine cornering speeds between
moves.
Consider the following two moves contained on an XY plane:
@ -70,7 +70,7 @@ but that is not ideal as all that acceleration and deceleration would
greatly increase the print time and the frequent changes in extruder
flow would result in poor print quality.
To solve this, the "lookahead" mechanism queues multiple incoming
To solve this, the "look-ahead" mechanism queues multiple incoming
moves and analyzes the angles between moves to determine a reasonable
speed that can be obtained during the "junction" between two moves. If
the next move forms an acute angle (the head is going to travel in
@ -85,17 +85,17 @@ acceleration". Best
[described](https://onehossshay.wordpress.com/2011/09/24/improving_grbl_cornering_algorithm/)
by the author.
Klipper implements lookahead between moves contained in the XY plane
Klipper implements look-ahead between moves contained in the XY plane
that have similar extruder flow rates. Other moves are rare and
implementing lookahead between them is unnecessary.
implementing look-ahead between them is unnecessary.
Key formula for lookahead:
Key formula for look-ahead:
```
end_velocity^2 = start_velocity^2 + 2*accel*move_distance
```
Smoothed lookahead
------------------
Smoothed look-ahead
-------------------
Klipper also implements a mechanism for smoothing out the motions of
short "zig-zag" moves. Consider the following moves:
@ -124,7 +124,7 @@ top-speed.
Generating steps
================
Once the lookahead process completes, the print head movement for the
Once the look-ahead process completes, the print head movement for the
given move is fully known (time, start position, end position,
velocity at each point) and it is possible to generate the step times
for the move. This process is done within "kinematic classes" in the
@ -249,18 +249,20 @@ pressure. Pressure increases when filament is pushed into the extruder
the pressure necessary to extrude is dominated by the flow rate
through the nozzle orifice (as in
[Poiseuille law](https://en.wikipedia.org/wiki/Poiseuille_law)). The
details of the above physics are not important - only that the
relationship between pressure and flow rate is linear. See the
[pressure advance](Pressure_Advance.md) document for information on
how to find the pressure advance coefficients.
key idea is that the relationship between filament, pressure, and flow
rate can be modeled using a linear coefficient:
```
extra_filament = pressure_advance_coefficient * extruder_velocity
```
See the [pressure advance](Pressure_Advance.md) document for
information on how to find this pressure advance coefficient.
Once configured, Klipper will push in an additional amount of filament
during acceleration and retract that additional filament during
deceleration. The higher the desired filament flow rate, the more
filament must be pushed in during acceleration to account for
pressure. Any additional filament pushed in during head acceleration
is retracted during head deceleration (the extruder will have a
negative velocity).
during acceleration. The higher the desired filament flow rate, the
more filament must be pushed in during acceleration to account for
pressure. During head deceleration the extra filament is retracted
(the extruder will have a negative velocity).
![pressure-advance](img/pressure-advance.svg.png)
@ -268,8 +270,8 @@ One may notice that the pressure advance algorithm can cause the
extruder motor to make sudden velocity changes. This is tolerated
based on the idea that the majority of the inertia in the system is in
changing the extruder pressure. As long as the extruder pressure does
not change rapidly it is okay to make some sudden changes in extruder
motor velocity.
not change rapidly the sudden changes in extruder motor velocity are
tolerated.
One area where sudden velocity changes become problematic is during
small changes in head speed due to cornering.
@ -277,6 +279,8 @@ small changes in head speed due to cornering.
![pressure-cornering](img/pressure-cornering.svg.png)
To prevent this, the Klipper pressure advance code utilizes the move
lookahead queue to detect intermittent speed changes. In these cases
the amount of pressure increased and decreased will be reduced or
eliminated.
look-ahead queue to detect intermittent speed changes. During a
deceleration event the code finds the maximum upcoming head speed
within a configurable time window. The pressure is then only adjusted
to this found maximum. This can greatly reduce (or even completely
eliminate) pressure changes during cornering.