Fix critical issue with wheel encoder process

- The process approach cannot rely on ValueError since the FSM will
  never see such an exception raised
- Use an additional fault value to indicate when a fault has happened

pod-control/src/components/process_encoder.py

@@ -36,7 +36,10 @@ class WheelEncoder:
     """
 
     def __init__(
-        self, counter_value: Synchronized[int], speed_value: Synchronized[float]
+        self,
+        counter_value: Synchronized[int],
+        speed_value: Synchronized[float],
+        fault_value: Synchronized[bool],
     ) -> None:
         GPIO.setmode(GPIO.BCM)
         GPIO.setup((PIN_ENCODER_A, PIN_ENCODER_B), GPIO.IN)
@@ -45,6 +48,7 @@ def __init__(
 
         self._counter = counter_value
         self._speed = speed_value
+        self._fault = fault_value
         self.reset()
         log.info("Process encoder setup complete.")
 
@@ -64,7 +68,7 @@ def measure(self) -> None:
 
         if inc == 2:
             log.error("WHEEL ENCODER FAULT", state, self._last_state)
-            raise ValueError
+            self._fault.value = True
 
         if inc != 0:
             log.debug("counter: ", self._counter.value, current_time - self._start_time)
@@ -85,9 +89,11 @@ def __del__(self) -> None:
 
 
 def wheel_encoder_process(
-    counter_value: Synchronized[int], speed_value: Synchronized[float]
+    counter_value: Synchronized[int],
+    speed_value: Synchronized[float],
+    fault_value: Synchronized[bool],
 ) -> None:
-    wheel_encoder = WheelEncoder(counter_value, speed_value)
+    wheel_encoder = WheelEncoder(counter_value, speed_value, fault_value)
     while True:
         wheel_encoder.measure()
         time.sleep(0)
@@ -98,6 +104,9 @@ def wheel_encoder_process(
 
     counter_value = Value("i", 0)
     speed_value = Value("d", 0.0)
-    p = Process(target=wheel_encoder_process, args=(counter_value, speed_value))
+    fault_value = Value("b", False)
+    p = Process(
+        target=wheel_encoder_process, args=(counter_value, speed_value, fault_value)
+    )
     p.start()
     p.join()

pod-control/src/fsm.py

@@ -75,6 +75,7 @@ def __init__(self, socket_server: PodSocketServer):
         self._brakes.engage()
         self._wheel_encoder_counter = Value("i", 0)
         self._wheel_encoder_speed = Value("d", 0.0)
+        self._wheel_encoder_fault = Value("b", 0)
         self._pt_downstream = PressureTransducer(ADDRESS_PT_DOWNSTREAM)
         self._motors = Motors()
         self._signal_light = SignalLight()
@@ -83,7 +84,11 @@ async def run(self) -> None:
         """Tick the state machine by loop."""
         p = Process(
             target=wheel_encoder_process,
-            args=(self._wheel_encoder_counter, self._wheel_encoder_speed),
+            args=(
+                self._wheel_encoder_counter,
+                self._wheel_encoder_speed,
+                self._wheel_encoder_fault,
+            ),
         )
         p.start()
 
@@ -137,6 +142,11 @@ def _enter_running(self) -> None:
     def _running_periodic(self) -> State:
         """Perform operations when the pod is running."""
         self._running_tick += 1
+
+        if self._wheel_encoder_fault.value:
+            log.error("Wheel encoder faulted")
+            return State.STOPPED
+
         self._motors.drive(20)
 
         asyncio.create_task(