The object is traveling in a circular motion at a constant angular
velocity, 1, with respect to the global z coordinate, gravity pointing
downward z. Therefore, link force in xy plane is a centrifugal force, F =
mw^2R, pointing towards origin, z component balance out gravitational
force. With zero angular acceleration, z-component of link torque is zero,
y-component balance out the torque from gravity, since rotation around y
and z axis are constrained.
Thank you,
Luning
On Friday, March 10, 2023 at 12:10:53 AM UTC-6 Simon wrote:
> 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
>>
>>
--
You received this message because you are subscribed to the Google Groups
"ProjectChrono" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
To view this discussion on the web visit
https://groups.google.com/d/msgid/projectchrono/b3b384ca-cac9-49bc-8fe5-f16b15ad2aeen%40googlegroups.com.
