# how to create a robot from DH parameters

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Ali Souliman on 20 Jan 2021
Commented: Jose on 5 Apr 2022
Hello,
I have the DH matrix of my robot manipulator.
[0.033 -pi/2 0.147 q1;
0.155 0 0 q2-pi/2;
0.135 0 0 q3;
0 pi/2 0 q4+pi/2;
0 0 0.218 q5];
I am trying to create a model of the robot along what's used here:
the forward kinematics function getTransform() doesn't return the right Homogeneous transormation matrix, which means the model isn't correct.
the code I wrote:
dhparams=[0.033 -pi/2 0.147 0;
0.155 0 0 -pi/2;
0.135 0 0 0;
0 pi/2 0 pi/2;
0 0 0.218 0];
robot = rigidBodyTree('DataFormat','row');
body1 = rigidBody('body1');
jnt1 = rigidBodyJoint('jnt1','revolute');
body2 = rigidBody('body2');
jnt2 = rigidBodyJoint('jnt2','revolute');
body3 = rigidBody('body3');
jnt3 = rigidBodyJoint('jnt3','revolute');
body4 = rigidBody('body4');
jnt4 = rigidBodyJoint('jnt4','revolute');
body5 = rigidBody('body5');
jnt5 = rigidBodyJoint('jnt5','revolute');
;
setFixedTransform(jnt1,dhparams(1,:),'dh');
setFixedTransform(jnt2,dhparams(2,:),'dh');
setFixedTransform(jnt3,dhparams(3,:),'dh');
setFixedTransform(jnt4,dhparams(4,:),'dh');
setFixedTransform(jnt5,dhparams(5,:),'dh');
body1.Joint = jnt1;
body2.Joint = jnt2;
body3.Joint = jnt3;
body4.Joint = jnt4;
body5.Joint = jnt5;
transform = getTransform(robot,[0.1 0.1 0.1 0.1 0.1],'body5')

Yiping Liu on 21 Jan 2021
Ali, the getTransform method is working correctly. But from the way you set the DH parameters, I assume you expect "theta offset" on joint angles 2 and 4.
% a alpha d theta
dhparams=[0.033 -pi/2 0.147 0;
0.155 0 0 -pi/2; % theta offset?
0.135 0 0 0;
0 pi/2 0 pi/2; % theta offset?
0 0 0.218 0];
In the current setFixedTransform implementation, the theta offset is ignored for a revolute joint (similary, the d offset is ignored for a prismatic joint), and that is a documented behavior, see here. In the future, we may consider to allow user to specify the offsets to ease some of the practical use cases, but for now, there are two workarounds:
1) manually adding fixed bodies in between to account for these offsets.
2) Modify the config with the offsets. i.e. instead of
getTransform(robot,[0.1 0.1 0.1 0.1 0.1],'body5')
You do
getTransform(robot,[0.1 0.1-pi/2 0.1 0.1+pi/2 0.1],'body5')
Valeria Leto on 13 May 2021
Hi, I have the same problem, but I haven't understood how to change the code. Could you make an example with the second joint?
Jose on 5 Apr 2022
Hi, in version 2022a you can set the home position of a joint after creating the robot object like this:
robot.Bodies{2}.Joint.HomePosition=-pi/2;
robot.Bodies{4}.Joint.HomePosition=pi/2;

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