Hello Simon,
Thank you for your test and feedback. I didn't realize there is a same bug
as in ChLinkMateGeneric().
When we use quaternion to parameterize the finite rotation and derive the
constraint equation (C) and its Jacobian matrix (Cq), we will have a
projection matrix P to convert the Lagrange multiplier (only for rotational
DOFs) to the reaction torque. This projection matrix P =
0.5(s*I33+\tilde(v)) where \pho_{F1_F2}=(s,v) is the quaternion of the
slave frame F1 to master frame F2. When F1, F2 are coincident, P=0.5*I33,
thus we can find a coefficient 0.5 in the code to convert Lagrange
multipler to reaction torque. But in case there is a relative rotation
between F1 and F2, the projection matrix P changes, and the Lagrange
multiplier is expressed in an obscure ghost frame FX (let's denote it as FX
here). The rotation of FX is a half of the relative rotation in
\pho_{F1_F2}. Besides this point, it also includes a component which scales
the Lagrange multiplier by the scalar part 's'. This is why you obtain the
above result in you test.
We fixed this bug in below commit:
Commit: fb7d16caa653d837c9220cf96db5aebeb2df8231 [fb7d16c], on 27th April.
Try to pull it and test again.
I would like to remind here, the tangent stiffness matrix (Kc) of
ChLinkLockLock() is not implemented, although the analytical expression
should be same as ChLinkMateGeneric(), or at least quite similar. Thus you
cannot perform the static and eigenvalue analysis for a pendulum using
ChLinkLockLock().
Best regards,
Chao PENG.
在2023年4月26日星期三 UTC+2 15:32:03<Simon> 写道：
> Hi,
>
> thank you for the updated implementation. I re-ran my test case with
> chrono at commit 841781d6e0e4b09c68cd431e1f7381b402f17ad1.
>
> The reaction torque vector does now rotate as expected with
> ChLinkMotorRotationSpeed. As shown in my example below there is now a
> constant angle difference of 90° between force and torque in the x-y plane.
>
> However, with the ChLinkLockLock implementation I noticed that the angle
> of the torque vector seems to change only at half the speed of the body
> rotation. This leads to a constantly changing difference between force and
> torque direction in the x-y plane.
> So perhaps there is also something wrong with the ChLinkLockLock's
> reaction torque direction (see example with output below).
>
> Sorry for being naggy, your support is highly appreciated!
>
> Best regards,
> 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);
>
> const auto body_position{rotating_body->GetPos()};
> const double body_angle{std::atan2(body_position[1],
> body_position[0])};
>
> const auto reaction_force{rotation_link->Get_react_force()};
> const double force_angle{std::atan2(reaction_force[1],
> reaction_force[0])};
>
> const auto reaction_torque{rotation_link->Get_react_torque()};
> const double torque_angle{std::atan2(reaction_torque[1],
> reaction_torque[0])};
>
>
> std::cout << std::fixed << std::setprecision(3) << "Time: " <<
> system.GetChTime()
> << " *** Body Angle: " << body_angle << " *** Force Angle: "
> << force_angle
> << " *** Torque Angle: " << torque_angle
> << " *** Angle Difference: " << force_angle - torque_angle
> << "\n";
> }
>
> *Output using ChLinkMotorRotationSpeed:*
> ...
> Time: 0.950 *** Body Angle: 0.950 *** Force Angle: 0.940 *** Torque Angle:
> -0.631 *** Angle Difference: 1.571
> Time: 0.960 *** Body Angle: 0.960 *** Force Angle: 0.950 *** Torque Angle:
> -0.621 *** Angle Difference: 1.571
> Time: 0.970 *** Body Angle: 0.970 *** Force Angle: 0.960 *** Torque Angle:
> -0.611 *** Angle Difference: 1.571
> Time: 0.980 *** Body Angle: 0.980 *** Force Angle: 0.970 *** Torque Angle:
> -0.601 *** Angle Difference: 1.571
> Time: 0.990 *** Body Angle: 0.990 *** Force Angle: 0.980 *** Torque Angle:
> -0.591 *** Angle Difference: 1.571
>
>
> *Output using ChLinkLockLock:*
> ...
> Time: 0.950 *** Body Angle: 0.950 *** Force Angle: 0.940 *** Torque Angle:
> -1.096 *** Angle Difference: 2.036
> Time: 0.960 *** Body Angle: 0.960 *** Force Angle: 0.950 *** Torque Angle:
> -1.091 *** Angle Difference: 2.041
> Time: 0.970 *** Body Angle: 0.970 *** Force Angle: 0.960 *** Torque Angle:
> -1.086 *** Angle Difference: 2.046
> Time: 0.980 *** Body Angle: 0.980 *** Force Angle: 0.970 *** Torque Angle:
> -1.081 *** Angle Difference: 2.051
> Time: 0.990 *** Body Angle: 0.990 *** Force Angle: 0.980 *** Torque Angle:
> -1.076 *** Angle Difference: 2.056
> On Monday, April 17, 2023 at 4:48:39 PM UTC+2 [email protected] wrote:
>
>> Dear everyone,
>>
>> We corrected the bugs reported here.
>>
>> Please refer to the commit [d7193ef] on 17th April.
>>
>> Welcome to feedback your test result.
>>
>> 在2023年3月13日星期一 UTC+1 09:14:49<Simon> 写道：
>>
>>> 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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