Hi Luning,
thanks for your reply. I agree on the cause of the force and torque
components, but to my understanding the constant torque vector [0.000,
-10.000, -0.000] would be correct only in the rotating frame, not in the
link frame (which is fixed here).
I modified my example, now using ChLinkLockLock to create the same rotating
body setup (see below). The torque vector seems to rotate correctly when
using ChLinkLockLock, in contrast to the ChLinkMotorRotationSpeed case.
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});
// Option 1: ChLinkMotorRotationSpeed
// 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));
// Option 2: ChLinkLockLock
auto rotation_link{chrono_types::make_shared<ChLinkLockLock>()};
rotation_link->Initialize(rotating_body, fixed_body, {});
rotation_link->Set_angleset(chrono::AngleSet::ANGLE_AXIS);
rotation_link->SetMotion_axis({0.0, 0.0, 1.0});
rotation_link->SetMotion_ang(chrono_types::make_shared<ChFunction_Ramp>(0.0,
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 using ChLinkMotorRotationSpeed:*
...
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
*Output using ChLinkLockLock:*
...
Time: 0.970 *** Force: 0.574 0.819 10.000 *** Torque: 4.662 -8.847
-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: 4.707 -8.824
-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: 4.751 -8.800
-0.000 *** Link Absolute Coords: 0.000 0.000 0.000 1.000 0.000 0.000
0.000
On Friday, March 10, 2023 at 6:27:12 PM UTC+1 [email protected] wrote:
> 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
>>>
>>>
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