Ignore motor soft limits when they are both zero (#1052)

Author: DominicOram

src/ophyd_async/epics/motor.py

@@ -107,12 +107,21 @@ def set_name(self, name: str, *, child_name_separator: str | None = None) -> Non
         self.user_readback.set_name(name)
 
     async def check_motor_limit(self, abs_start_pos: float, abs_end_pos: float):
-        """Check the motor limit with the absolute positions."""
+        """Check the positions are within limits.
+
+        Will raise a MotorLimitsException if the given absolute positions will be
+        outside the motor soft limits.
+        """
         motor_lower_limit, motor_upper_limit, egu = await asyncio.gather(
             self.low_limit_travel.get_value(),
             self.high_limit_travel.get_value(),
             self.motor_egu.get_value(),
         )
+
+        # EPICS motor record treats limits of 0, 0 as no limit
+        if motor_lower_limit == 0 and motor_upper_limit == 0:
+            return
+
         if (
             not motor_upper_limit >= abs_start_pos >= motor_lower_limit
             or not motor_upper_limit >= abs_end_pos >= motor_lower_limit

tests/epics/test_motor.py

@@ -155,7 +155,7 @@ async def test_set_with_zero_velocity(sim_motor: motor.Motor) -> None:
         (10, 9.99, -9.99),  # Goes above upper_limit
     ],
 )
-async def test_set_motor_limits_error(
+async def test_move_outside_motor_limits_causes_error(
     sim_motor: motor.Motor,
     position,
     upper_limit,
@@ -168,6 +168,16 @@ async def test_set_motor_limits_error(
         await sim_motor.set(position)
 
 
+async def test_given_limits_of_0_0_then_move_causes_no_error(
+    sim_motor: motor.Motor,
+):
+    set_mock_value(sim_motor.velocity, 10)
+    set_mock_value(sim_motor.low_limit_travel, 0)
+    set_mock_value(sim_motor.high_limit_travel, 0)
+    await sim_motor.set(100)
+    assert (await sim_motor.user_setpoint.get_value()) == 100
+
+
 @pytest.mark.parametrize(
     "setpoint, velocity, timeout",
     [