Fixed sensor.accelerometer gravity sign for consider_gravity = True case

CHANGELOG.md

@@ -60,6 +60,7 @@ Attention: The newest changes should be on top -->
 
 ### Fixed
 
+- BUG: Correct the gravity sign an `Accelerometer` applies when `consider_gravity=True`. The gravitational field was added to the inertial acceleration instead of subtracted from it, so the sensor reported the negative of the proper acceleration along the vertical: one at rest read -g rather than +g. Recorded accelerometer data taken with `consider_gravity=True` changes sign in that term. [#1175](https://github.com/RocketPy-Team/RocketPy/pull/1175)
 - BUG: Sample `StochasticFlight` inputs once per simulation [#1126](https://github.com/RocketPy-Team/RocketPy/pull/1126) [#1090](https://github.com/RocketPy-Team/RocketPy/issues/1090)
 - BUG: Fix spurious `ValueError` from floating-point roundoff at exact tank depletion [#1166](https://github.com/RocketPy-Team/RocketPy/pull/1166)
 - BUG: Draw each declared eccentricity once per simulation [#1168](https://github.com/RocketPy-Team/RocketPy/pull/1168)

rocketpy/sensors/accelerometer.py

@@ -13,7 +13,8 @@ class Accelerometer(InertialSensor):
     Attributes
     ----------
     consider_gravity : bool
-        Whether the sensor considers the effect of gravity on the acceleration.
+        Whether the sensor reports proper acceleration, which includes the
+        reaction to gravity, rather than the coordinate acceleration alone.
     prints : _InertialSensorPrints
         Object that contains the print functions for the sensor.
     sampling_rate : float
@@ -166,8 +167,11 @@ def __init__(
             Skewness of the sensor's axes in percentage. Default is 0, meaning
             no cross-axis sensitivity is applied.
         consider_gravity : bool, optional
-            If True, the sensor will consider the effect of gravity on the
-            acceleration. Default is False.
+            If True, the sensor reports proper acceleration, as a real
+            accelerometer does: the inertial acceleration less the local
+            gravitational field, so one at rest reads g along its up axis
+            rather than zero. If False it reports the coordinate acceleration,
+            which is zero at rest. Default is False.
         name : str, optional
             The name of the sensor. Default is "Accelerometer".
         seed : int, optional
@@ -232,7 +236,7 @@ def measure(self, time, **kwargs):
         gravity = (
             Vector([0, 0, -gravity]) if self.consider_gravity else Vector([0, 0, 0])
         )
-        inertial_acceleration = Vector(u_dot[3:6]) + gravity
+        inertial_acceleration = Vector(u_dot[3:6]) - gravity
 
         # Vector from rocket cdm to sensor in rocket frame
         r = relative_position

tests/unit/sensors/test_sensor.py

@@ -5,6 +5,7 @@
 import pytest
 from pytest import approx
 
+from rocketpy import Accelerometer
 from rocketpy.mathutils.vector_matrix import Matrix, Vector
 from rocketpy.tools import euler313_to_quaternions
 
@@ -272,7 +273,10 @@ def test_noisy_rotated_accelerometer(noisy_rotated_accelerometer, example_plain_
 
     # calculate acceleration at sensor position in inertial frame
     relative_position = Vector([0.4, 0.4, 1])
-    inertial_acceleration = Vector(U_DOT[3:6]) + Vector([0, 0, -GRAVITY])
+    # An accelerometer reports proper acceleration: the inertial acceleration
+    # less the local gravitational field. Gravity points down, so the term it
+    # contributes points up, which is why one sitting still reads +g, not 0.
+    inertial_acceleration = Vector(U_DOT[3:6]) - Vector([0, 0, -GRAVITY])
     omega = Vector(U[10:13])
     omega_dot = Vector(U_DOT[10:13])
     acceleration = (
@@ -317,6 +321,48 @@ def test_noisy_rotated_accelerometer(noisy_rotated_accelerometer, example_plain_
     assert noisy_rotated_accelerometer.measured_data[0][0] == TIME
 
 
+def test_accelerometer_at_rest_reads_gravity_upward(example_plain_env):
+    """An accelerometer standing still reads +g along its up axis, not zero.
+
+    The test above recomputes the expression under test, so a flipped gravity
+    term gets mirrored into agreement there instead of being caught. This one
+    says what the instrument does rather than how it is computed: it senses
+    the support force holding it up, so at rest it reports g upward. Reversing
+    the sign in ``Accelerometer.measure`` fails here.
+
+    Every noise, bias and drift parameter is left at its default, all of which
+    are zero, so the measurement is exact rather than bounded.
+    """
+    at_rest = [0.0] * 13
+    at_rest[6] = 1.0  # identity attitude, so sensor axes are the inertial ones
+    still = [0.0] * 13
+    gravity = example_plain_env.gravity.get_value_opt(0)
+
+    sensing_gravity = Accelerometer(sampling_rate=100, consider_gravity=True)
+    sensing_gravity.measure(
+        time=0,
+        u=at_rest,
+        u_dot=still,
+        relative_position=Vector([0, 0, 0]),
+        environment=example_plain_env,
+    )
+
+    assert sensing_gravity.measurement == approx([0, 0, gravity], abs=1e-12)
+
+    # Without the flag the same sensor reports the coordinate acceleration, so
+    # the whole of what the flag contributes is that one upward g.
+    ignoring_gravity = Accelerometer(sampling_rate=100, consider_gravity=False)
+    ignoring_gravity.measure(
+        time=0,
+        u=at_rest,
+        u_dot=still,
+        relative_position=Vector([0, 0, 0]),
+        environment=example_plain_env,
+    )
+
+    assert ignoring_gravity.measurement == approx([0, 0, 0], abs=1e-12)
+
+
 def test_noisy_rotated_gyroscope(noisy_rotated_gyroscope, example_plain_env):
     """Test the measure method of the Gyroscope class. Checks if saved
     measurement is (wx,wy,wz) and if measured_data is [(t, (wx,wy,wz)), ...]