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kin_extruder: Fix numerical stability when using pressure advance 

Avoid using the absolute E position when calculating pressure advance
as that position can grow arbitrarily large, which can result in
"numerical stability" problems.  That instability could eventually
lead to internal errors during step compression.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
This commit is contained in:
Kevin O'Connor 2020-10-23 22:59:20 -04:00
parent 2bcf06a295
commit 1b6b7fc58c
1 changed files with 11 additions and 7 deletions
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18
klippy/chelper/kin_extruder.c
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@ -52,7 +52,8 @@ extruder_integrate_time(double base, double start_v, double half_accel
// Calculate the definitive integral of extruder for a given move // Calculate the definitive integral of extruder for a given move
static double static double
pa_move_integrate(struct move *m, double start, double end, double time_offset) pa_move_integrate(struct move *m, double base, double start, double end,
double time_offset)
{ {
if (start < 0.) if (start < 0.)
start = 0.; start = 0.;
@ -60,7 +61,7 @@ pa_move_integrate(struct move *m, double start, double end, double time_offset)
end = m->move_t; end = m->move_t;
// Calculate base position and velocity with pressure advance // Calculate base position and velocity with pressure advance
double pressure_advance = m->axes_r.y; double pressure_advance = m->axes_r.y;
double base = m->start_pos.x + pressure_advance * m->start_v; base += pressure_advance * m->start_v;
double start_v = m->start_v + pressure_advance * 2. * m->half_accel; double start_v = m->start_v + pressure_advance * 2. * m->half_accel;
// Calculate definitive integral // Calculate definitive integral
double ha = m->half_accel; double ha = m->half_accel;
@ -75,20 +76,23 @@ pa_range_integrate(struct move *m, double move_time, double hst)
{ {
// Calculate integral for the current move // Calculate integral for the current move
double res = 0., start = move_time - hst, end = move_time + hst; double res = 0., start = move_time - hst, end = move_time + hst;
res += pa_move_integrate(m, start, move_time, start); double start_base = m->start_pos.x;
res -= pa_move_integrate(m, move_time, end, end); res += pa_move_integrate(m, 0., start, move_time, start);
res -= pa_move_integrate(m, 0., move_time, end, end);
// Integrate over previous moves // Integrate over previous moves
struct move *prev = m; struct move *prev = m;
while (unlikely(start < 0.)) { while (unlikely(start < 0.)) {
prev = list_prev_entry(prev, node); prev = list_prev_entry(prev, node);
start += prev->move_t; start += prev->move_t;
res += pa_move_integrate(prev, start, prev->move_t, start); double base = prev->start_pos.x - start_base;
res += pa_move_integrate(prev, base, start, prev->move_t, start);
} }
// Integrate over future moves // Integrate over future moves
while (unlikely(end > m->move_t)) { while (unlikely(end > m->move_t)) {
end -= m->move_t; end -= m->move_t;
m = list_next_entry(m, node); m = list_next_entry(m, node);
res -= pa_move_integrate(m, 0., end, end); double base = m->start_pos.x - start_base;
res -= pa_move_integrate(m, base, 0., end, end);
} }
return res; return res;
} }
@ -109,7 +113,7 @@ extruder_calc_position(struct stepper_kinematics *sk, struct move *m
return m->start_pos.x + move_get_distance(m, move_time); return m->start_pos.x + move_get_distance(m, move_time);
// Apply pressure advance and average over smooth_time // Apply pressure advance and average over smooth_time
double area = pa_range_integrate(m, move_time, hst); double area = pa_range_integrate(m, move_time, hst);
return area * es->inv_half_smooth_time2; return m->start_pos.x + area * es->inv_half_smooth_time2;
} }
void __visible void __visible