FAN: Kick start fan when the increase is more than 50% (Klipper3d#2085)

Large increases in fan speed can suffer from acceleration lag, so this
change kick starts the fan for a change of 50% or more in the same way
that starting the fan from 0% does.

Signed-off-by: Justin Schuh <code@justinschuh.com>

config/example.cfg

@@ -253,8 +253,9 @@ pin: ar9
 #   constrained by the implementation and may be significantly
 #   different than the requested cycle_time. The default is False.
 #kick_start_time: 0.100
-#   Time (in seconds) to run the fan at full speed when first enabling
-#   it (helps get the fan spinning). The default is 0.100 seconds.
+#   Time (in seconds) to run the fan at full speed when either first
+#   enabling or increasing it by more than 50% (helps get the fan spinning).
+#   The default is 0.100 seconds.
 #off_below: 0.0
 #   The minimum input speed which will power the fan (expressed as a
 #   value from 0.0 to 1.0). When a speed lower than off_below is

klippy/extras/fan.py

@@ -42,8 +42,8 @@ def set_speed(self, print_time, value):
         if value == self.last_fan_value:
             return
         print_time = max(self.last_fan_time + FAN_MIN_TIME, print_time)
-        if (value and value < self.max_power
-            and not self.last_fan_value and self.kick_start_time):
+        if (value and value < self.max_power and self.kick_start_time
+            and (not self.last_fan_value or value - self.last_fan_value > .5)):
             # Run fan at full speed for specified kick_start_time
             self.mcu_fan.set_pwm(print_time, self.max_power)
             print_time += self.kick_start_time