Wrong answer on robotics forwardDynamics

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barak ostraich
barak ostraich on 24 Jun 2018
Answered: barak ostraich on 24 Jun 2018
i have a 2 rods robot with L1=0.3;L2=0.4 ; cm1=0.2;cm2=0.2; m1=1.8; m2=1.5 with rotational joints. when i load the first joint in forwardDynamics, i get right results. when i'm loading the second joint i get wrong answer. (The analytic answer for torque of [0 1] is accelerations of -9.4737 32.6315)
if true
L1=0.3; L2=0.4;m1=1.8;m2=1.5;cm1=0.2;cm2=0.2;
robot = robotics.RigidBodyTree;
robot.DataFormat = 'row';
body1 = robotics.RigidBody('body1');
jnt1 = robotics.Joint('jnt1','revolute');
setFixedTransform(jnt1,trvec2tform([0, 0, 0]));
jnt1.JointAxis=[0 0 1];
body1.Joint = jnt1;
body1.Inertia=[1 1 0.013+cm1*cm1*m1 0 0 0];
body1.Mass=m1;
body1.CenterOfMass=[ cm1 0 0 ];
addBody(robot,body1,'base');
body2 = robotics.RigidBody('body2');
jnt2 = robotics.Joint('jnt2','revolute');
setFixedTransform(jnt2,trvec2tform([L1, 0, 0]));
jnt2.JointAxis=[0 0 1];
body2.Joint = jnt2;
body2.Inertia=[1 1 0.02+cm2^2*m2 0 0 0];
body2.Mass=m2;
body2.CenterOfMass=[ cm2 0 0];
addBody(robot,body2,'body1');
M=[0 1]
forwardDynamics(robot,[0 0],[0 0] ,M)
%analytic results
% for [1,0] 8.4211 -17.8947; for [0,1] -9.4737 32.6315
%for [1,1] -1.0526 14.7368 ; for [2,2] -2.1053 29.4737
end

Answers (1)

barak ostraich
barak ostraich on 24 Jun 2018
the torque that the matlab is loading is external torque. it means that you need to add the torque at the next joint to the previus to disable the reaction torque. for example to load internal torques of [3 5] on joints 1 and 2 you will put [8 5]

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