fix(api): Switch plunger backlash to pre-load for aspirations (#13076)

api/src/opentrons/hardware_control/ot3api.py

@@ -1467,10 +1467,13 @@ async def _move_to_plunger_bottom(
             # save time by using max speed
             max_speeds = self.config.motion_settings.default_max_speed
             speed = max_speeds[self.gantry_load][OT3AxisKind.P]
-        if current_pos > target_pos[pip_ax]:
+        # IMPORTANT: Here is our backlash compensation.
+        #            The plunger is pre-loaded in the "aspirate" direction
+        # NOTE: plunger position (mm) decreases up towards homing switch
+        if current_pos < target_pos[pip_ax]:
+            # move down below "bottom", before moving back up to "bottom"
             backlash_pos = target_pos.copy()
-            backlash_pos[pip_ax] -= instrument.backlash_distance
-
+            backlash_pos[pip_ax] += instrument.backlash_distance
             await self._move(
                 backlash_pos,
                 speed=(speed * rate),
@@ -1511,10 +1514,6 @@ async def aspirate(
         if not aspirate_spec:
             return
 
-        checked_mount = OT3Mount.from_mount(mount)
-        instrument = self._pipette_handler.get_pipette(checked_mount)
-        pip_ax = OT3Axis.of_main_tool_actuator(mount)
-
         target_pos = target_position_from_plunger(
             realmount,
             aspirate_spec.plunger_distance,
@@ -1525,13 +1524,6 @@ async def aspirate(
             await self._backend.set_active_current(
                 {OT3Axis.from_axis(aspirate_spec.axis): aspirate_spec.current}
             )
-            backlash_pos = target_pos.copy()
-            backlash_pos[pip_ax] -= instrument.backlash_distance
-            await self._move(
-                backlash_pos,
-                speed=aspirate_spec.speed,
-                home_flagged_axes=False,
-            )
             await self._move(
                 target_pos,
                 speed=aspirate_spec.speed,

api/tests/opentrons/hardware_control/test_ot3_api.py

@@ -1106,13 +1106,16 @@ async def test_move_to_plunger_bottom(
     await ot3_hardware.cache_pipette(mount, instr_data, None)
     pipette = ot3_hardware.hardware_pipettes[mount.to_mount()]
     assert pipette
+    pip_ax = OT3Axis.of_main_tool_actuator(mount)
 
     max_speeds = ot3_hardware.config.motion_settings.default_max_speed
     target_pos = target_position_from_plunger(
         OT3Mount.from_mount(mount),
         pipette.plunger_positions.bottom,
         ot3_hardware._current_position,
     )
+    backlash_pos = target_pos.copy()
+    backlash_pos[pip_ax] += pipette.backlash_distance
 
     # plunger will move at different speeds, depending on if:
     #  - no tip attached (max speed)
@@ -1130,7 +1133,12 @@ async def test_move_to_plunger_bottom(
     await ot3_hardware.home()
     mock_move.reset_mock()
     await ot3_hardware.home_plunger(mount)
-    mock_move.assert_called_once_with(
+    # make sure we've done the backlash compensation
+    mock_move.assert_any_call(
+        backlash_pos, speed=expected_speed_no_tip, acquire_lock=False
+    )
+    # make sure the final move is to our target position
+    mock_move.assert_called_with(
         target_pos, speed=expected_speed_no_tip, acquire_lock=False
     )
 
@@ -1139,25 +1147,21 @@ async def test_move_to_plunger_bottom(
     await ot3_hardware.add_tip(mount, 100)
     mock_move.reset_mock()
     await ot3_hardware.prepare_for_aspirate(mount)
-    # make sure when plunger is going down that only one move is called,
-    # and there's no backlash move queued
-    mock_move.assert_called_once_with(
+    # make sure we've done the backlash compensation
+    mock_move.assert_any_call(
+        backlash_pos, speed=expected_speed_moving_down, acquire_lock=True
+    )
+    # make sure the final move is to our target position
+    mock_move.assert_called_with(
         target_pos, speed=expected_speed_moving_down, acquire_lock=True
     )
 
     # tip attached, moving UP towards "bottom" position
     # NOTE: _move() is mocked, so we need to update the OT3API's
     #       cached coordinates in the test
-    pip_ax = OT3Axis.of_main_tool_actuator(mount)
     ot3_hardware._current_position[pip_ax] = target_pos[pip_ax] + 1
     mock_move.reset_mock()
     await ot3_hardware.prepare_for_aspirate(mount)
-    # make sure we've done the backlash compensation
-    backlash_pos = target_pos.copy()
-    backlash_pos[pip_ax] -= pipette.backlash_distance
-    mock_move.assert_any_call(
-        backlash_pos, speed=expected_speed_moving_up, acquire_lock=True
-    )
     # make sure the final move is to our target position
     mock_move.assert_called_with(
         target_pos, speed=expected_speed_moving_up, acquire_lock=True