remove carParams.mass from the bicycle model

mass cancels out of the bicycle model since tire stiffness and rotational
inertia both scale with it. store stiffness/inertia per unit mass (divided
by the reference Civic mass) so mass disappears from the vehicle model
entirely instead of substituting a fake mass constant.

carParams.mass is still set, just no longer consumed.

Author: pi

opendbc/car/car.capnp

@@ -484,13 +484,13 @@ struct CarParams {
   autoResumeSng @69 :Bool;               # describes whether car can resume from a stop automatically
 
   # things about the car in the manual
-  mass @17 :Float32;            # [kg] curb weight: all fluids no cargo
   wheelbase @18 :Float32;       # [m] distance from rear axle to front axle
   centerToFront @19 :Float32;   # [m] distance from center of mass to front axle
   steerRatio @20 :Float32;      # [] ratio of steering wheel angle to front wheel angle
   steerRatioRear @21 :Float32;  # [] ratio of steering wheel angle to rear wheel angle (usually 0)
 
-  # things we can derive
+  # vehicle dynamics parameters
+  unitMass @17 :Float32;             # [kg] arbitrary mass; cancels out of the vehicle dynamics
   rotationalInertia @22 :Float32;    # [kg*m2] body rotational inertia
   tireStiffnessFactor @72 :Float32;  # scaling factor used in calculating tireStiffness[Front,Rear]
   tireStiffnessFront @23 :Float32;   # [N/rad] front tire coeff of stiff

opendbc/car/interfaces.py

@@ -8,7 +8,7 @@
 from collections.abc import Callable
 from functools import cache
 
-from opendbc.car import DT_CTRL, apply_hysteresis, gen_empty_fingerprint, scale_rot_inertia, scale_tire_stiffness, STD_CARGO_KG
+from opendbc.car import DT_CTRL, apply_hysteresis, gen_empty_fingerprint, scale_rot_inertia, scale_tire_stiffness
 from opendbc.car import structs
 from opendbc.car.can_definitions import CanData, CanRecvCallable, CanSendCallable
 from opendbc.car.common.basedir import BASEDIR
@@ -133,7 +133,8 @@ def get_params(cls, candidate: str, fingerprint: dict[int, dict[int, int]], car_
     ret = CarInterfaceBase.get_std_params(candidate)
 
     platform = PLATFORMS[candidate]
-    ret.mass = platform.config.specs.mass
+    # Arbitrary mass; it cancels out of the vehicle dynamics but keeps the formulation readable.
+    ret.unitMass = 1.0
     ret.wheelbase = platform.config.specs.wheelbase
     ret.steerRatio = platform.config.specs.steerRatio
     ret.centerToFront = ret.wheelbase * platform.config.specs.centerToFrontRatio
@@ -144,13 +145,9 @@ def get_params(cls, candidate: str, fingerprint: dict[int, dict[int, int]], car_
 
     ret = cls._get_params(ret, candidate, fingerprint, car_fw, alpha_long, is_release, docs)
 
-    # Vehicle mass is published curb weight plus assumed payload such as a human driver; notCars have no assumed payload
-    if not ret.notCar:
-      ret.mass = ret.mass + STD_CARGO_KG
-
     # Set params dependent on values set by the car interface
-    ret.rotationalInertia = scale_rot_inertia(ret.mass, ret.wheelbase)
-    ret.tireStiffnessFront, ret.tireStiffnessRear = scale_tire_stiffness(ret.mass, ret.wheelbase, ret.centerToFront, ret.tireStiffnessFactor)
+    ret.rotationalInertia = scale_rot_inertia(ret.unitMass, ret.wheelbase)
+    ret.tireStiffnessFront, ret.tireStiffnessRear = scale_tire_stiffness(ret.unitMass, ret.wheelbase, ret.centerToFront, ret.tireStiffnessFactor)
 
     return ret
 

opendbc/car/tests/test_car_interfaces.py

@@ -41,7 +41,7 @@ def test(self, fuzzy):
     car_interface = get_fuzzy_car_interface(car_name, fuzzy)
     car_params = car_interface.CP.as_reader()
 
-    assert car_params.mass > 1
+    assert car_params.unitMass == 1
     assert car_params.wheelbase > 0
     # centerToFront is center of gravity to front wheels, assert a reasonable range
     assert car_params.wheelbase * 0.3 < car_params.centerToFront < car_params.wheelbase * 0.7

opendbc/car/tests/test_models.py

@@ -193,7 +193,7 @@ def test_car_params(self):
     if self.CP.dashcamOnly:
       self.skipTest("no need to check carParams for dashcamOnly")
 
-    self.assertGreater(self.CP.mass, 1)
+    self.assertEqual(self.CP.unitMass, 1)
     if self.CP.steerControlType not in (SteerControlType.angle, SteerControlType.curvature):
       tuning = self.CP.lateralTuning.which()
       if tuning == "pid":

opendbc/car/vehicle_model.py

@@ -27,7 +27,7 @@ def __init__(self, CP: CarParams):
       CP: Car Parameters
     """
     # for math readability, convert long names car params into short names
-    self.m: float = CP.mass
+    self.m: float = CP.unitMass
     self.j: float = CP.rotationalInertia
     self.l: float = CP.wheelbase
     self.aF: float = CP.centerToFront