Issue or Bugs report: Accelerometer output not correct. Wrong gravity and wrong direction

[Yan Dong]

Hi, I'm using Chrono 8.0.0, C++.

I want to output accelerometer attached on a ChBody, but the acc output seems to be the acceleration on world frame not on the box frame. Also, I have find the acc output of gravity is strange.

Here're my findings:
1. About the gravity: I set the gravity to be (0,0,-10), and I release the acc attached on a box to fall freely. The acc output is zero which is desired because no external force on box.
Now I rotate the box, making the z-axis pointing downward, by:

box->SetRot(Q_from_AngAxis(chrono::CH_C_PI, { 1, 0, 0 }));

then the IMU output is now (0,0,-20). Which I think should be 0 becase the free falling has no external force. The real imu is 0 indeed.


2. About the extrinsics: I change the IMU attached extrinsics to the body, and I find the IMU output do not change at all no matter the what the extrinsics is:"

auto ext_zero = chrono::ChFrame<double>({ 0, 0, 0 }, Q_from_AngAxis(chrono::CH_C_PI, { 1, 0, 0 }));
auto acc_ = chrono_types::make_shared<ChAccelerometerSensor>(box, 100, ext_zero, nullptr);

3. then I further find that the acc output seems to be the acceleration of the body in terms of the world frame but not its own frame. I add the an external world's X direction force and I find no matter the box is rotated (to negtive x direction) or the extrinsics is rotated, the Accelerometer output is always on x+ direction.


So my questions are:
a). How you implement Accelerometer calculation? What's the Accelerometer output's reference frame?
b). How is the gravity effect on Accelerometer calculated? I guess is by relative moving frame together with the global gravity settings? But it's not true for a real Accelerometer.


Here's my code. The 3 above findings are tested by codes with comments:

---------------------------------------------------
    ChSystemNSC sys;
    chrono::sensor::ChSensorManager manager_(&sys);
    sys.SetCollisionSystemType(ChCollisionSystem::Type::BULLET);
    sys.Set_G_acc({ 0, 0, -10 });
    sys.SetSolverMaxIterations(20); 

    auto ground_mat = chrono_types::make_shared<ChMaterialSurfaceNSC>();
    double friction_mu = 1.0;
    ground_mat->SetSfriction(friction_mu);
    ground_mat->SetKfriction(friction_mu);
    auto box = chrono_types::make_shared<ChBodyEasyBox>(1, 1, 1, 1.0, true, true, ground_mat);
    box->SetPos(ChVector<>(0, 0, 0.5));

    // 1.  Gravity Issue Rotate or Not.
    // box->SetRot(Q_from_AngAxis(chrono::CH_C_PI / 1, { 1, 0, 0 }));
    box->SetMass(1);
    box->SetInertiaXX(ChVector<>(1, 1, 1));
    box->SetBodyFixed(false);
    box->SetCollide(true); //
    sys.AddBody(box);


    // 2. Extrinsics Issue
    auto ext_zero = chrono::ChFrame<double>({ 0, 0, 0 }, Q_from_AngAxis(0, { 1, 0, 0 }));
    auto acc_ = chrono_types::make_shared<ChAccelerometerSensor>(box, 100, ext_zero, nullptr);
    acc_->SetName("IMU - Accelerometer");
    acc_->SetLag(0.0f);
    acc_->SetCollectionWindow(0.01);
    acc_->PushFilter(chrono_types::make_shared<ChFilterAccelAccess>());

    manager_.AddSensor(acc_);
    int imu_last_count_ = 0;

    // 3. Accelerometer direction issue by external force
    // std::shared_ptr<ChBody> ground;      
    // create_ground(sys, ground);          // I created a ground.
    // auto force1 = chrono_types::make_shared<ChForce>();
    // box->AddForce(force1);
    // force1->SetMode(ChForce::FORCE);
    // force1->SetAlign(ChForce::WORLD_DIR);
    // force1->SetF_x(chrono_types::make_shared<ChFunction_Const>(20));
    // force1->SetF_y(chrono_types::make_shared<ChFunction_Const>(0));
    // force1->SetF_z(chrono_types::make_shared<ChFunction_Const>(0));


    while(1){
        auto bufferAcc = acc_->GetMostRecentBuffer<UserAccelBufferPtr>();
        if (bufferAcc->Buffer  && bufferAcc->LaunchedCount > imu_last_count_) {

            AccelData acc_data = bufferAcc->Buffer[0];
            std::vector<double> acc(3);
            acc[0] = acc_data.X;
            acc[1] = acc_data.Y;
            acc[2] = acc_data.Z;
            cout << acc[0] << ", " << acc[1] << ", " << acc[2] << endl;
            imu_last_count_ = bufferAcc->LaunchedCount;
        }
        manager_.Update();
        sys.DoStepDynamics(0.001);
    }
---------------------------------------------------

[Олег Брылёв]

Hello, I faced the same problem. The reason is that the acceleration calculation only takes into account the offset position (m_offsetPose.GetPos()) without the orientation (m_offsetPose.GetRot()) (chrono_sensor/sensors/ChIMUSensor.cpp):

CH_SENSOR_API void ChAccelerometerSensor::PushKeyFrame() {
    ChVector<double> tran_acc_no_offset = m_parent->PointAccelerationLocalToParent(m_offsetPose.GetPos());
    ChVector<double> tran_acc_offset = -m_parent->GetSystem()->Get_G_acc();
    tran_acc_offset = m_parent->GetRot().Rotate(tran_acc_offset);
    m_keyframes.push_back(tran_acc_no_offset + tran_acc_offset);
}

I made my own implementation of the accelerometer. I'm attaching it.

Maybe when I have time I'll make a pull request with these edits to the main repository.

четверг, 19 декабря 2024 г. в 23:08:29 UTC+7, Yan Dong:

[Dan Negrut]

Oleg and Yan – thanks for the input, we’ll do our best to take a look and get to the bottom of this, that is, fix if it’s a problem, or let you know if the code should have been used differently.

 

Dan

---------------------------------------------

Bernard A. and Frances M. Weideman Professor

NVIDIA CUDA Fellow

Department of Mechanical Engineering

Department of Computer Science

University of Wisconsin - Madison

4150ME, 1513 University Avenue

Madison, WI 53706-1572

608 772 0914

http://sbel.wisc.edu/

http://projectchrono.org/

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