feat(csaxs): invert option for BPM and SignalForwarder, enabled for BPM XBox2

The XBox2 BPM amplifier outputs a negative current, so more beam showed as
more negative blade values. invert: true in the device config negates the
input signals; the BPM position/diagonal ratios are unchanged and the
intensity becomes positive. Enabled for bpm_xbox2_slowrb.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017NrH3poJsYBCTVq2Y9fuHZ

csaxs_bec/device_configs/bl_endstation.yaml

@@ -1163,6 +1163,7 @@ bpm_xbox2_slowrb:
     right_top_ref: galilrioesxbox.analog_in.ch1
     right_bot_ref: galilrioesxbox.analog_in.ch2
     left_bot_ref:  galilrioesxbox.analog_in.ch3
+    invert: true  # amplifier output is negative; positive values = more beam
   enabled: true
   readoutPriority: monitored
   onFailure: retry

csaxs_bec/devices/pseudo_devices/bpm.py

@@ -7,7 +7,12 @@ from ophyd_devices.utils.bec_processed_signal import BECProcessedSignal
 
 
 class BPM(PSIPseudoDeviceBase):
-    """BPM positioner pseudo device."""
+    """BPM positioner pseudo device.
+
+    invert=True negates the four blade signals, e.g. for an amplifier whose
+    output current is negative. pos_x, pos_y and diagonal are ratios and do
+    not change; intensity changes sign.
+    """
 
     # Blade signals, a,b,c,d
     left_top = Cpt(
@@ -76,11 +81,13 @@ class BPM(PSIPseudoDeviceBase):
         right_top_ref: str,
         right_bot_ref: str,
         left_bot_ref: str,
+        invert: bool = False,
         device_manager=None,
         scan_info=None,
         **kwargs,
     ):
         super().__init__(name=name, device_manager=device_manager, scan_info=scan_info, **kwargs)
+        self._sign = -1 if invert else 1
         # Get all blade signal objects from utility method
         signal_t = self.left_top.get_device_object_from_bec(
             object_name=left_top_ref, signal_name=self.name, device_manager=device_manager
@@ -131,7 +138,7 @@ class BPM(PSIPseudoDeviceBase):
         )
 
     def _compute_blade_signal(self, signal: Signal) -> float:
-        return signal.get()
+        return self._sign * signal.get()
 
     def _compute_intensity(
         self, left_top: Signal, right_top: Signal, right_bot: Signal, left_bot: Signal

csaxs_bec/devices/pseudo_devices/signal_forwarder.py

@@ -9,7 +9,7 @@ from ophyd_devices.utils.bec_processed_signal import BECProcessedSignal
 
 
 class SignalForwarder(PSIPseudoDeviceBase):
-    """Forward one signal unchanged."""
+    """Forward one signal unchanged, or negated with invert=True."""
 
     signal = Cpt(
         BECProcessedSignal,
@@ -20,8 +20,17 @@ class SignalForwarder(PSIPseudoDeviceBase):
         doc="Forwarded signal",
     )
 
-    def __init__(self, name, signal_ref: str, device_manager=None, scan_info=None, **kwargs):
+    def __init__(
+        self,
+        name,
+        signal_ref: str,
+        invert: bool = False,
+        device_manager=None,
+        scan_info=None,
+        **kwargs,
+    ):
         super().__init__(name=name, device_manager=device_manager, scan_info=scan_info, **kwargs)
+        self._sign = -1 if invert else 1
 
         src = self.signal.get_device_object_from_bec(
             object_name=signal_ref, signal_name=self.name, device_manager=device_manager
@@ -30,4 +39,4 @@ class SignalForwarder(PSIPseudoDeviceBase):
         self.signal.set_compute_method(self._compute_signal, signal=src)
 
     def _compute_signal(self, signal: Signal) -> float:
-        return signal.get()
+        return self._sign * signal.get()

tests/tests_devices/test_pseudo_devices.py

@@ -141,8 +141,9 @@ def patched_dm_bpm(dm_with_devices):
     return dm
 
 
-@pytest.fixture
-def bpm(patched_dm_bpm):
+@pytest.fixture(params=[False])
+def bpm(request, patched_dm_bpm):
+    invert = request.param
     name = "bpm"
     bpm_config = Device(
         name=name,
@@ -165,6 +166,7 @@ def bpm(patched_dm_bpm):
             right_top_ref="right_top",
             right_bot_ref="right_bot",
             left_bot_ref="left_bot",
+            invert=invert,
             device_manager=patched_dm_bpm,
         )
         patched_dm_bpm.devices._add_device(bpm.name, bpm)
@@ -239,3 +241,22 @@ def test_bpm_positions(bpm):
     assert bpm.pos_x.get() == 0
     assert bpm.pos_y.get() == 0
     assert bpm.diagonal.get() == 1
+
+
+@pytest.mark.parametrize("bpm", [True], indirect=True)
+def test_bpm_invert_negates_blades_keeps_positions(bpm):
+    """invert=True: negative amplifier currents read as positive blade
+    signals and intensity; the ratio-based positions are unchanged."""
+    devices = bpm.device_manager.devices
+    devices["left_top"].put(-1)
+    devices["right_top"].put(-3)
+    devices["right_bot"].put(-2)
+    devices["left_bot"].put(-2)
+
+    assert bpm.left_top.get() == 1
+    assert bpm.right_top.get() == 3
+    assert bpm.intensity.get() == 8
+    # same ratios as the non-inverted signals would give
+    assert bpm.pos_x.get() == 0.25
+    assert bpm.pos_y.get() == 0
+    assert bpm.diagonal.get() == 0.25