fix(api): Speed up unnecessarily slow "prep" plunger motions (#16022)

api/src/opentrons/hardware_control/ot3api.py

@@ -1877,12 +1877,10 @@ async def _move_to_plunger_bottom(
 
         With wet tips, the primary concern is leftover droplets inside the tip.
         These droplets ideally only move down and out of the tip, not up into the tip.
-        Therefore, it is preferable to use the "blow-out" speed when moving the
-        plunger down, and the slower "aspirate" speed when moving the plunger up.
+        Therefore, it is preferable to use the slower "aspirate" speed when
+        moving the plunger up after a blow-out.
 
-        Assume all tips are wet, because we do not differentiate between wet/dry tips.
-
-        When no tip is attached, moving at the max speed is preferable, to save time.
+        All other situations, moving at the max speed is preferable, to save time.
         """
         checked_mount = OT3Mount.from_mount(mount)
         instrument = self._pipette_handler.get_pipette(checked_mount)
@@ -1895,21 +1893,22 @@ async def _move_to_plunger_bottom(
             self._current_position,
         )
         pip_ax = Axis.of_main_tool_actuator(checked_mount)
-        # speed depends on if there is a tip, and which direction to move
-        if instrument.has_tip_length:
+        # save time while moving down by using max speed
+        max_speeds = self.config.motion_settings.default_max_speed
+        speed_down = max_speeds[self.gantry_load][OT3AxisKind.P]
+        # upward moves can be max speed, or aspirate speed
+        # use the (slower) aspirate if there is a tip and we're following a blow-out
+        plunger_is_below_bottom_pos = (
+            self._current_position[pip_ax] > instrument.plunger_positions.bottom
+        )
+        if instrument.has_tip_length and plunger_is_below_bottom_pos:
             # using slower aspirate flow-rate, to avoid pulling droplets up
             speed_up = self._pipette_handler.plunger_speed(
                 instrument, instrument.aspirate_flow_rate, "aspirate"
             )
-            # use blow-out flow-rate, so we can push droplets out
-            speed_down = self._pipette_handler.plunger_speed(
-                instrument, instrument.blow_out_flow_rate, "dispense"
-            )
         else:
-            # save time by using max speed
-            max_speeds = self.config.motion_settings.default_max_speed
+            # either no tip, or plunger just homed, so tip is dry
             speed_up = max_speeds[self.gantry_load][OT3AxisKind.P]
-            speed_down = speed_up
         # IMPORTANT: Here is our backlash compensation.
         #            The plunger is pre-loaded in the "aspirate" direction
         backlash_pos = target_pos.copy()

api/tests/opentrons/hardware_control/test_ot3_api.py

@@ -1663,13 +1663,11 @@ async def test_move_to_plunger_bottom(
     backlash_pos[pip_ax] += pipette.backlash_distance
 
     # plunger will move at different speeds, depending on if:
-    #  - no tip attached (max speed)
-    #  - tip attached and moving down (blowout speed)
+    #  - tip not attached (max speed)
+    #  - tip attached and moving down (max speed)
     #  - tip attached and moving up (aspirate speed)
     expected_speed_no_tip = max_speeds[ot3_hardware.gantry_load][OT3AxisKind.P]
-    expected_speed_moving_down = ot3_hardware._pipette_handler.plunger_speed(
-        pipette, pipette.blow_out_flow_rate, "dispense"
-    )
+    expected_speed_moving_down = expected_speed_no_tip
     expected_speed_moving_up = ot3_hardware._pipette_handler.plunger_speed(
         pipette, pipette.aspirate_flow_rate, "aspirate"
     )