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); }*
---------------------------------------------------
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