Hi Luning,
thanks for the clarification about the link coordinate system.
In my example I added the link coordinate system output to verify that it
is indeed fixed to the global reference frame (see below).
Unfortunately my question remains: Why does the link force vector rotate
and the torque vector does not ?
Thank you,
Simon
*Code:*
ChSystemNSC system{};
auto fixed_body{chrono_types::make_shared<ChBody>()};
fixed_body->SetBodyFixed(true);
auto rotating_body{chrono_types::make_shared<ChBody>()};
rotating_body->SetPos({1.0, 0.0, 0.0});
auto rotation_link{chrono_types::make_shared<ChLinkMotorRotationSpeed>()};
rotation_link->Initialize(rotating_body, fixed_body, ChFrame{});
rotation_link->SetSpeedFunction(chrono_types::make_shared<ChFunction_Const>(1.0));
system.Add(fixed_body);
system.Add(rotating_body);
system.Add(rotation_link);
system.Set_G_acc({0.0, 0.0, 10.0});
for (double time{}; time < 1.0; time += 0.01)
{
system.DoFrameDynamics(time);
std::cout << std::fixed << std::setprecision(3) << "Time: " <<
system.GetChTime()
<< " *** Force: " << rotation_link->Get_react_force()
<< " *** Torque: " << rotation_link->Get_react_torque()
<< " *** Link Absolute Coords: " <<
rotation_link->GetLinkAbsoluteCoords() << "\n";
}
*Output:*
...
Time: 0.830 *** Force: 0.682 0.731 10.000 *** Torque: 0.000 -10.000
-0.000 *** Link Absolute Coords: 0.000 0.000 0.000 1.000 0.000 0.000
0.000
Time: 0.840 *** Force: 0.675 0.738 10.000 *** Torque: 0.000 -10.000
-0.000 *** Link Absolute Coords: 0.000 0.000 0.000 1.000 0.000 0.000
0.000
Time: 0.850 *** Force: 0.667 0.745 10.000 *** Torque: 0.000 -10.000
-0.000 *** Link Absolute Coords: 0.000 0.000 0.000 1.000 0.000 0.000
0.000
Time: 0.860 *** Force: 0.660 0.751 10.000 *** Torque: 0.000 -10.000
-0.000 *** Link Absolute Coords: 0.000 0.000 0.000 1.000 0.000 0.000
0.000
Time: 0.870 *** Force: 0.652 0.758 10.000 *** Torque: 0.000 -10.000
-0.000 *** Link Absolute Coords: 0.000 0.000 0.000 1.000 0.000 0.000
0.000
Time: 0.880 *** Force: 0.645 0.764 10.000 *** Torque: 0.000 -10.000
-0.000 *** Link Absolute Coords: 0.000 0.000 0.000 1.000 0.000 0.000
0.000
Time: 0.890 *** Force: 0.637 0.771 10.000 *** Torque: 0.000 -10.000
-0.000 *** Link Absolute Coords: 0.000 0.000 0.000 1.000 0.000 0.000
0.000
Time: 0.900 *** Force: 0.629 0.777 10.000 *** Torque: 0.000 -10.000
-0.000 *** Link Absolute Coords: 0.000 0.000 0.000 1.000 0.000 0.000
0.000
Time: 0.910 *** Force: 0.622 0.783 10.000 *** Torque: 0.000 -10.000
-0.000 *** Link Absolute Coords: 0.000 0.000 0.000 1.000 0.000 0.000
0.000
Time: 0.920 *** Force: 0.614 0.790 10.000 *** Torque: 0.000 -10.000
-0.000 *** Link Absolute Coords: 0.000 0.000 0.000 1.000 0.000 0.000
0.000
Time: 0.930 *** Force: 0.606 0.796 10.000 *** Torque: 0.000 -10.000
-0.000 *** Link Absolute Coords: 0.000 0.000 0.000 1.000 0.000 0.000
0.000
Time: 0.940 *** Force: 0.598 0.802 10.000 *** Torque: 0.000 -10.000
-0.000 *** Link Absolute Coords: 0.000 0.000 0.000 1.000 0.000 0.000
0.000
Time: 0.950 *** Force: 0.590 0.808 10.000 *** Torque: 0.000 -10.000
-0.000 *** Link Absolute Coords: 0.000 0.000 0.000 1.000 0.000 0.000
0.000
Time: 0.960 *** Force: 0.582 0.813 10.000 *** Torque: 0.000 -10.000
-0.000 *** Link Absolute Coords: 0.000 0.000 0.000 1.000 0.000 0.000
0.000
Time: 0.970 *** Force: 0.574 0.819 10.000 *** Torque: 0.000 -10.000
-0.000 *** Link Absolute Coords: 0.000 0.000 0.000 1.000 0.000 0.000
0.000
Time: 0.980 *** Force: 0.565 0.825 10.000 *** Torque: 0.000 -10.000
-0.000 *** Link Absolute Coords: 0.000 0.000 0.000 1.000 0.000 0.000
0.000
Time: 0.990 *** Force: 0.557 0.831 10.000 *** Torque: 0.000 -10.000
-0.000 *** Link Absolute Coords: 0.000 0.000 0.000 1.000 0.000 0.000
0.000
On Thursday, March 9, 2023 at 4:59:13 PM UTC+1 [email protected] wrote:
> Hello Simon,
>
> "As I understand, the link coordinate system is the 'master' body system
> (Body2)."
> This is not correct. Your link coordinate system is defined here,
>
> rotation_link->Initialize(rotating_body, fixed_body, ChFrame{});
>
> ChFrame{} initialize a reference frame coincide with the global reference
> frame. Because your Body2 (fixed_body) has the same orientation as the GRF,
> and it is fixed, it just so happens that your link coordinate system is the
> same as the 'master' body system.
>
> You can check the orientation of the link coordinate system using this:
> auto quat = rotation_link->GetLinkAbsoluteCoords().rot
>
> Thank you,
> Luning
>
> On Tuesday, March 7, 2023 at 9:45:38 AM UTC-6 Simon wrote:
>
> Hi,
>
> as per the documentation
> <https://api.projectchrono.org/classchrono_1_1_ch_link_motor_rotation_speed.html>,
>
> ChLinkMotorRotationSpeed's member functions Get_react_force() and
> Get_react_torque() return the respective values in link coordinates.
> As I understand, the link coordinate system is the 'master' body system
> (Body2).
>
> In the example below a body rotates around the absolute z axis using
> ChLinkMotorRotationSpeed (the 'master' body is fixed here). As expected,
> the link reaction force consists of a the constant body weight in z
> direction and the rotating centrifugal load in the x-y plane. However, the
> reaction torque (due to gravity) is constant despite the link's rotation.
> Shouldn't the torque vector also rotate or do I misunderstand something?
>
> Thanks in advance!
> Simon
>
>
> *Code: *
> ChSystemNSC system{};
>
> auto fixed_body{chrono_types::make_shared<ChBody>()};
> fixed_body->SetBodyFixed(true);
> auto rotating_body{chrono_types::make_shared<ChBody>()};
> rotating_body->SetPos({1.0, 0.0, 0.0});
>
> auto rotation_link{chrono_types::make_shared<ChLinkMotorRotationSpeed>()};
> rotation_link->Initialize(rotating_body, fixed_body, ChFrame{});
>
> rotation_link->SetSpeedFunction(chrono_types::make_shared<ChFunction_Const>(1.0));
>
> system.Add(fixed_body);
> system.Add(rotating_body);
> system.Add(rotation_link);
>
> system.Set_G_acc({0.0, 0.0, 10.0});
>
> for (double time{}; time < 1.0; time += 0.01)
> {
> system.DoFrameDynamics(time);
>
> std::cout << std::fixed << std::setprecision(3) << "Time: " <<
> system.GetChTime()
> << " *** Force: " << rotation_link->Get_react_force()
> << " *** Moment: " << rotation_link->Get_react_torque() <<
> "\n";
> }
>
>
> *Output: ...*
> Time: 0.810 *** Force: 0.697 0.717 10.000 *** Moment: 0.000 -10.000
> -0.000
> Time: 0.820 *** Force: 0.690 0.724 10.000 *** Moment: 0.000 -10.000
> -0.000
> Time: 0.830 *** Force: 0.682 0.731 10.000 *** Moment: 0.000 -10.000
> -0.000
> Time: 0.840 *** Force: 0.675 0.738 10.000 *** Moment: 0.000 -10.000
> -0.000
> Time: 0.850 *** Force: 0.667 0.745 10.000 *** Moment: 0.000 -10.000
> -0.000
> Time: 0.860 *** Force: 0.660 0.751 10.000 *** Moment: 0.000 -10.000
> -0.000
> Time: 0.870 *** Force: 0.652 0.758 10.000 *** Moment: 0.000 -10.000
> -0.000
> Time: 0.880 *** Force: 0.645 0.764 10.000 *** Moment: 0.000 -10.000
> -0.000
> Time: 0.890 *** Force: 0.637 0.771 10.000 *** Moment: 0.000 -10.000
> -0.000
> Time: 0.900 *** Force: 0.629 0.777 10.000 *** Moment: 0.000 -10.000
> -0.000
> Time: 0.910 *** Force: 0.622 0.783 10.000 *** Moment: 0.000 -10.000
> -0.000
> Time: 0.920 *** Force: 0.614 0.790 10.000 *** Moment: 0.000 -10.000
> -0.000
> Time: 0.930 *** Force: 0.606 0.796 10.000 *** Moment: 0.000 -10.000
> -0.000
> Time: 0.940 *** Force: 0.598 0.802 10.000 *** Moment: 0.000 -10.000
> -0.000
> Time: 0.950 *** Force: 0.590 0.808 10.000 *** Moment: 0.000 -10.000
> -0.000
> Time: 0.960 *** Force: 0.582 0.813 10.000 *** Moment: 0.000 -10.000
> -0.000
> Time: 0.970 *** Force: 0.574 0.819 10.000 *** Moment: 0.000 -10.000
> -0.000
> Time: 0.980 *** Force: 0.565 0.825 10.000 *** Moment: 0.000 -10.000
> -0.000
> Time: 0.990 *** Force: 0.557 0.831 10.000 *** Moment: 0.000 -10.000
> -0.000
>
>
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