fan: Add off_below option for fans (#1897)

Below off_below the fan will be turned off. When configured correctly this
can prevent stalling, which may lead to overheating and failed fans. To
simplify calibration and use of this setting, off_below is applied in the
input duty cycle domain, prior to any scaling due to e.g. max_power.

Signed-off-by: Nicholas Seckar <nseckar@gmail.com>

klippy/extras/fan.py

@@ -16,6 +16,8 @@ class PrinterFan:
         self.max_power = config.getfloat('max_power', 1., above=0., maxval=1.)
         self.kick_start_time = config.getfloat('kick_start_time', 0.1,
                                                minval=0.)
+        self.off_below = config.getfloat(
+            'off_below', default=0., minval=0., maxval=1.)
         ppins = printer.lookup_object('pins')
         self.mcu_fan = ppins.setup_pin('pwm', config.get('pin'))
         self.mcu_fan.setup_max_duration(0.)
@@ -29,6 +31,8 @@ class PrinterFan:
     def handle_request_restart(self, print_time):
         self.set_speed(print_time, 0.)
     def set_speed(self, print_time, value):
+        if value < self.off_below:
+            value = 0.
         value = max(0., min(self.max_power, value * self.max_power))
         if value == self.last_fan_value:
             return