PyLabRobot · BioCam · Oct 1, 2026 · Oct 2, 2026
@@ -37,9 +37,9 @@ hand one its configuration before setup and re-running setup does not throw it a
 
 **P3. Every feature has a `.configuration` dataclass, and it holds facts only.** Three kinds, in
 this order: what the device answered about itself, per-machine calibration, and the device facts of
-that generation of drive - encoder resolutions, the ranges each drive accepts, and the firmware's
-own defaults. What the driver chooses to send is not a fact about the device and does not go here
-(P18). `DeviceConfiguration` in `driver/configuration.py` is the same thing for the device.
+that generation of drive - encoder resolutions and the ranges each drive accepts. A default is not a
+fact and does not go here, not even the firmware's own (P18). `DeviceConfiguration` in
+`driver/configuration.py` is the same thing for the device.
 
 **P4. The wire counts in increments; this driver speaks mm, degrees, uL and seconds.** Every
 conversion is a method on the configuration - `x_increments_to_mm` / `x_mm_to_increments` - so the
@@ -130,7 +130,8 @@ that a constant already holds are referenced by name rather than repeated.
 **P18. The driver's defaults are public `default_*` attributes on the feature.** Declared on the
 class with a type, in standard units, round, and slightly below the firmware's own default. A
 caller tunes one device by assigning on the instance, or every device by assigning on the class.
-Named `default_<quantity>[_<condition>]`: `default_y_speed`, `default_z_speed`.
+Named `default_<quantity>[_<condition>]`: `default_y_speed`, `default_z_speed`. Never on the
+configuration.
 
 **P19. A probe names its speeds for what they do.** `search_speed` is the speed it searches at;
 `approach_speed` is the speed to where the search starts, where the command has one. Never a bare
@@ -185,7 +186,7 @@ the Prep's.
    the oldest of the configurations and reads like the document it was transcribed from rather than
    like the rest of them.
 
-8. **The driver's defaults still live in the configurations.** `Head`, `Head384`, `iSWAP`,
-   `Autoload` and `XArm` keep them as `*_default` / `*_default_increments` fields, some in
-   increments, and `Pipettes` keeps a few there. `Head96` and `CoreGrippers` follow P18; the rest
-   move over one feature at a time.
+8. **The driver's defaults still live in the configurations.** `Head`, `Head384`, `iSWAP` and
+   `Autoload` keep them as `*_default` / `*_default_increments` fields, some in increments, and
+   `Pipettes` keeps a few there. `Head96`, `CoreGrippers` and `XArm` follow P18; the rest move over
+   one feature at a time.
@@ -974,8 +974,8 @@ async def request_x_position(self) -> float:
   async def move_to_x_position(
     self,
     x: float,
-    acceleration_level: int = 3,
-    current_limit: int = 7,
+    acceleration_level: Optional[int] = None,
+    current_limit: Optional[int] = None,
     settle_reads: int = 20,
   ):
     """Move channel A1 along X. The whole arm travels, with everything else it carries.
@@ -985,8 +985,8 @@ async def move_to_x_position(
 
     Args:
       x: where to put channel A1, in mm.
-      acceleration_level: how hard to accelerate, 1 to 4.
-      current_limit: the motor current limit, 1 to 7.
+      acceleration_level: how hard to accelerate. Defaults to the X-arm that holds the head.
+      current_limit: the motor current limit. Defaults to the X-arm that holds the head.
       settle_reads: how many reads to take before calling the arm stopped.
 
     Raises:

@@ -1161,8 +1161,8 @@ async def elbow_request_x_position(self) -> float:
   async def elbow_move_to_x_position(
     self,
     x: float,
-    acceleration_level: int = 3,
-    current_limit: int = 7,
+    acceleration_level: Optional[int] = None,
+    current_limit: Optional[int] = None,
     settle_reads: int = 20,
   ):
     """Move the elbow along X. The whole arm travels, with everything else it carries.
@@ -1172,8 +1172,8 @@ async def elbow_move_to_x_position(
 
     Args:
       x: where to put the elbow, in deck mm.
-      acceleration_level: how hard to accelerate, 1 to 4.
-      current_limit: the motor current limit, 1 to 7.
+      acceleration_level: how hard to accelerate. Defaults to the X-arm that holds the iSWAP.
+      current_limit: the motor current limit. Defaults to the X-arm that holds the iSWAP.
       settle_reads: how many reads to take before calling the arm stopped.
 
     Raises:

@@ -1303,16 +1303,16 @@ async def request_x_position(self) -> float:
   async def move_to_x_position(
     self,
     x: float,
-    acceleration_level: int = 3,
-    current_limit: int = 7,
+    acceleration_level: Optional[int] = None,
+    current_limit: Optional[int] = None,
     settle_reads: int = 20,
   ):
     """Move the channels along X. The whole arm travels, with everything else it carries.
 
     Args:
       x: where to go, in mm.
-      acceleration_level: how hard to accelerate, 1 to 4.
-      current_limit: the motor current limit, 1 to 7.
+      acceleration_level: how hard to accelerate. Defaults to the X-arm that holds the pipettes.
+      current_limit: the motor current limit. Defaults to the X-arm that holds the pipettes.
       settle_reads: how many reads to take before calling the arm stopped.
 
     Raises:

@@ -69,8 +69,6 @@ class XArmConfiguration:
   x_mm_per_increment: float = 0.1
   x_range_increments: Tuple[int, int] = (0, 30_000)  # what the move accepts; x_range is narrower
   acceleration_level_range: Tuple[int, int] = (1, 5)  # index into five curves, not a rate
-  acceleration_level_default: int = 4
-  current_limit_default: int = 7
 
   @property
   def current_limit_digits(self) -> int:
@@ -98,8 +96,6 @@ def with_device_facts_of(self, other: "XArmConfiguration") -> "XArmConfiguration
       x_mm_per_increment=other.x_mm_per_increment,
       x_range_increments=other.x_range_increments,
       acceleration_level_range=other.acceleration_level_range,
-      acceleration_level_default=other.acceleration_level_default,
-      current_limit_default=other.current_limit_default,
     )
 
   # -- conversions: the wire counts in steps, the driver speaks mm ---------------------------
@@ -212,6 +208,9 @@ class XArm:
   is, how far it travels, and how far along X what it carries reaches.
   """
 
+  default_acceleration_level: int = 3
+  default_current_limit: int = 7
+
   def __init__(
     self,
     driver: "STARDriver",
@@ -354,15 +353,14 @@ async def initialize(self, current_limit: Optional[int] = None):
     """Initialize this arm's drive. This moves it.
 
     Args:
-      current_limit: the motor current limit. Defaults to
-        `configuration.current_limit_default`.
+      current_limit: the motor current limit. Defaults to `default_current_limit`.
     Raises:
       ValueError: If the current limit is outside what the drive accepts.
     """
     c = self.configuration
     # The parameter is sent, so what the drive does is written here rather than left to the drive's
     # own default, which nothing would record.
-    current_limit = c.current_limit_default if current_limit is None else current_limit
+    current_limit = self.default_current_limit if current_limit is None else current_limit
     low, high = c.current_limit_range
     if not low <= current_limit <= high:
       raise ValueError(f"current_limit must be between {low} and {high}, is {current_limit}")
@@ -478,8 +476,8 @@ async def _record_where_it_stopped(self) -> None:
   async def move_to_x_position(
     self,
     x: float,
-    acceleration_level: int = 3,
-    current_limit: int = 7,
+    acceleration_level: Optional[int] = None,
+    current_limit: Optional[int] = None,
     settle_reads: int = 20,
   ):
     """Move the arm to an absolute X position.
@@ -490,12 +488,12 @@ async def move_to_x_position(
     Args:
       x: target X position in mm, at the arm's reference point. Must lie within the arm's travel
         range (`configuration.x_range`).
-      acceleration_level: which acceleration curve to use. The drive's own default is
-        `configuration.acceleration_level_default`; this is the gentler one legacy sends. The
-        hardest curve leaves the arm oscillating about its target rather than approaching it, so it
-        takes longer to come to rest and further still with a 96-head parked forward - it arrives
-        either way, but the settling read below has more to wait for.
-      current_limit: the motor current limit.
+      acceleration_level: which acceleration curve to use. Defaults to
+        `default_acceleration_level`, gentler than the drive's own 4. The hardest curve leaves the
+        arm oscillating about its target rather than approaching it, so it takes longer to come to
+        rest and further still with a 96-head parked forward - it arrives either way, but the
+        settling read below has more to wait for.
+      current_limit: the motor current limit. Defaults to `default_current_limit`.
       settle_reads: how many reads to spend waiting for the arm to come to rest. Each is a command
         round trip, about 10 ms, against a settle of 27 to 90 ms where this was measured.
     Raises:
@@ -504,6 +502,10 @@ async def move_to_x_position(
     """
     c = self.configuration
     self._check_reachable(x)
+    if acceleration_level is None:
+      acceleration_level = self.default_acceleration_level
+    if current_limit is None:
+      current_limit = self.default_current_limit
     low, high = c.acceleration_level_range
     if not low <= acceleration_level <= high:
       raise ValueError(
@@ -565,8 +567,8 @@ async def _unchecked_fw_move_x_with_attached_components_at_z_safety(self, x: flo
   async def move_x_relative(
     self,
     distance: float,
-    acceleration_level: int = 3,
-    current_limit: int = 7,
+    acceleration_level: Optional[int] = None,
+    current_limit: Optional[int] = None,
   ):
     """Move the arm by a distance from where it is now.
 

@@ -103,6 +103,16 @@ async def test_left_drive(self):
       ["X0XIlw7", "X0XPla05000lr3lw7", "X0XPla04875lr3lw7", "X0XO"],
     )
 
+  async def test_the_defaults_are_the_arms(self):
+    driver = await _both_arms()
+    arm = cast(XArm, driver.left_x_arm)
+    sent = record(arm)
+    arm.default_acceleration_level = 2
+    arm.default_current_limit = 5
+    await XArm.initialize(arm)
+    await XArm.move_to_x_position(arm, 500.0)
+    self.assertEqual(sent, ["X0XIlw5", "X0XPla05000lr2lw5"])
+
   async def test_a_firmware_5_drive_takes_a_two_digit_current_limit(self):
     """From X0 firmware 5.0 the limiter is `lw##`, 00..15."""
     left = dataclasses.replace(
@@ -326,23 +336,23 @@ class TestConfiguringAnArm(unittest.IsolatedAsyncioTestCase):
   async def test_what_is_written_is_where_the_device_holds_it(self):
     driver = await _both_arms()
     arm = cast(XArm, driver.left_x_arm)
-    corrected = dataclasses.replace(arm.configuration, current_limit_default=3)
+    corrected = dataclasses.replace(arm.configuration, acceleration_level_range=(1, 3))
 
     arm.configuration = corrected
 
-    self.assertEqual(arm.configuration.current_limit_default, 3)
+    self.assertEqual(arm.configuration.acceleration_level_range, (1, 3))
     self.assertIs(cast(DeviceConfiguration, driver.configuration).left_arm, corrected)
 
   async def test_the_constructor_takes_one_too(self):
     """The same write, done where every other feature takes its configuration."""
     driver = await _both_arms()
     corrected = dataclasses.replace(
-      cast(XArm, driver.left_x_arm).configuration, current_limit_default=3
+      cast(XArm, driver.left_x_arm).configuration, acceleration_level_range=(1, 3)
     )
 
     arm = XArm(driver, side="left", configuration=corrected)
 
-    self.assertEqual(arm.configuration.current_limit_default, 3)
+    self.assertEqual(arm.configuration.acceleration_level_range, (1, 3))
     self.assertIs(cast(DeviceConfiguration, driver.configuration).left_arm, corrected)
 
   async def test_it_refuses_before_the_device_has_been_read(self):
@@ -357,27 +367,27 @@ async def test_a_simulated_device_then_answers_what_was_written(self):
     arm's device facts across a re-read as a physical device's does - so what was written stays."""
     driver = await _both_arms()
     arm = cast(XArm, driver.left_x_arm)
-    arm.configuration = dataclasses.replace(arm.configuration, current_limit_default=3)
+    arm.configuration = dataclasses.replace(arm.configuration, acceleration_level_range=(1, 3))
 
     await driver.discover()
 
-    self.assertEqual(arm.configuration.current_limit_default, 3)
+    self.assertEqual(arm.configuration.acceleration_level_range, (1, 3))
 
   def test_device_facts_carry_over_and_readings_do_not(self):
     """What a physical device's discovery does with a configured arm. It rebuilds one from the
     reply, then takes the device facts off the arm as it was configured: those are what no device
     answers, so a re-read must not put them back to what this generation documents."""
-    configured = dataclasses.replace(BARE_X_ARM, current_limit_default=15)
+    configured = dataclasses.replace(BARE_X_ARM, acceleration_level_range=(1, 3))
     answered = dataclasses.replace(BARE_X_ARM, width=354.0, x_range=(95.0, 1340.2))
 
     kept = answered.with_device_facts_of(configured)
 
-    self.assertEqual(kept.current_limit_default, 15)
+    self.assertEqual(kept.acceleration_level_range, (1, 3))
     self.assertEqual(kept.width, 354.0)
     self.assertEqual(kept.x_range, (95.0, 1340.2))
     # Neither of the two it was worked out from is changed.
     self.assertEqual(configured.width, BARE_X_ARM.width)
-    self.assertEqual(answered.current_limit_default, BARE_X_ARM.current_limit_default)
+    self.assertEqual(answered.acceleration_level_range, BARE_X_ARM.acceleration_level_range)
 
 
 class TestGeometryDoesNotFollowTheReportedWidth(unittest.TestCase):

@@ -65,9 +65,7 @@
       "acceleration_level_range": [
         1,
         5
-      ],
-      "acceleration_level_default": 4,
-      "current_limit_default": 7
+      ]
     },
     "right_arm": null,
     "min_iswap_collision_free_position": 350.0,

@@ -65,9 +65,7 @@
       "acceleration_level_range": [
         1,
         5
-      ],
-      "acceleration_level_default": 4,
-      "current_limit_default": 7
+      ]
     },
     "right_arm": null,
     "min_iswap_collision_free_position": 350.0,

@@ -68,9 +68,7 @@
       "acceleration_level_range": [
         1,
         5
-      ],
-      "acceleration_level_default": 4,
-      "current_limit_default": 7
+      ]
     },
     "right_arm": null,
     "min_iswap_collision_free_position": 350.0,

@@ -68,9 +68,7 @@
       "acceleration_level_range": [
         1,
         5
-      ],
-      "acceleration_level_default": 4,
-      "current_limit_default": 7
+      ]
     },
     "right_arm": null,
     "min_iswap_collision_free_position": 350.0,

@@ -68,9 +68,7 @@
       "acceleration_level_range": [
         1,
         5
-      ],
-      "acceleration_level_default": 4,
-      "current_limit_default": 7
+      ]
     },
     "right_arm": null,
     "min_iswap_collision_free_position": 350.0,