GM torque control: robust sig function (#32847)

tricky
old-commit-hash: b7695c00e87819dd6aac49cfec78e438dfc15d16

selfdrive/car/gm/interface.py

@@ -53,7 +53,12 @@ class CarInterface(CarInterfaceBase):
     friction = get_friction(lateral_accel_error, lateral_accel_deadzone, FRICTION_THRESHOLD, torque_params, friction_compensation)
 
     def sig(val):
-      return 1 / (1 + exp(-val)) - 0.5
+      # https://timvieira.github.io/blog/post/2014/02/11/exp-normalize-trick
+      if val >= 0:
+        return 1 / (1 + exp(-val)) - 0.5
+      else:
+        z = exp(val)
+        return z / (1 + z) - 0.5
 
     # The "lat_accel vs torque" relationship is assumed to be the sum of "sigmoid + linear" curves
     # An important thing to consider is that the slope at 0 should be > 0 (ideally >1)