stepper: Have caller calculate max jerk velocity

Allow the owner of the stepper object to cacluate the maximum step
jerk velocity.  This is used to ensure there is no communication error
between mcu and host.

Disable checking of jerk velocity for extruder stepper motors.

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

klippy/cartesian.py

@@ -15,6 +15,8 @@ class CartKinematics:
                          for n in steppers]
         self.stepper_pos = [0, 0, 0]
     def build_config(self):
+        for stepper in self.steppers[:2]:
+            stepper.set_max_jerk(0.005 * stepper.max_accel) # XXX
         for stepper in self.steppers:
             stepper.build_config()
     def set_position(self, newpos):

klippy/extruder.py

@@ -14,6 +14,7 @@ class PrinterExtruder:
         self.stepper_pos = 0
     def build_config(self):
         self.heater.build_config()
+        self.stepper.set_max_jerk(9999999.9)
         self.stepper.build_config()
     def get_max_speed(self):
         return self.stepper.max_velocity, self.stepper.max_accel

klippy/stepper.py

@@ -15,6 +15,7 @@ class PrinterStepper:
         self.inv_step_dist = 1. / self.step_dist
         self.max_velocity = config.getfloat('max_velocity')
         self.max_accel = config.getfloat('max_accel')
+        self.max_jerk = 0.
 
         self.homing_speed = config.getfloat('homing_speed', 5.0)
         self.homing_positive_dir = config.getboolean(
@@ -26,6 +27,8 @@ class PrinterStepper:
 
         self.clock_ticks = None
         self.need_motor_enable = True
+    def set_max_jerk(self, max_jerk):
+        self.max_jerk = max_jerk
     def build_config(self):
         self.clock_ticks = self.printer.mcu.get_mcu_freq()
         max_error = self.config.getfloat('max_error', 0.000050)
@@ -33,9 +36,10 @@ class PrinterStepper:
 
         step_pin = self.config.get('step_pin')
         dir_pin = self.config.get('dir_pin')
-        jc = 0.005 # XXX
+        jc = self.max_jerk / self.max_accel
         inv_max_step_accel = self.step_dist / self.max_accel
-        min_stop_interval = int((math.sqrt(inv_max_step_accel + jc**2) - jc)
+        min_stop_interval = int((math.sqrt(3.*inv_max_step_accel + jc**2)
+                                 - math.sqrt(inv_max_step_accel + jc**2))
                                 * self.clock_ticks) - max_error
         min_stop_interval = max(0, min_stop_interval)
         mcu = self.printer.mcu