how to create a robot from DH parameters

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:

https://www.mathworks.com/help/robotics/ref/rigidbodytree.html#d122e13383

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;

addBody(robot,body1,'base')

addBody(robot,body2,'body1')

addBody(robot,body3,'body2')

addBody(robot,body4,'body3')

addBody(robot,body5,'body4')

transform = getTransform(robot,[0.1 0.1 0.1 0.1 0.1],'body5')

[Umar]

Hi @Ali Souliman,

You mentioned, “I am trying to create a model of the robot along what's used here:

https://www.mathworks.com/help/robotics/ref/rigidbodytree.html#d122e13383

the forward kinematics function getTransform() doesn't return the right Homogeneous transormation matrix, which means the model isn't correct.”

Please see my response to your comments below.

To address your concerns regarding the transformation matrix generated by the getTransform() function, it is crucial to understand how DH parameters work, especially in the context of joint offsets. The DH parameters consist of four values: link length, link twist, link offset, and joint angle. The last parameter theta is particularly important for revolute joints because it defines the angle of rotation about the joint axis. In your current setup, you have defined some theta offsets for joints 2 and 4, but these offsets are ignored by default in MATLAB’s setFixedTransform method for revolute joints. This is why you may not be seeing the expected results when calling getTransform(). So, to compensate for these ignored offsets, you can insert fixed bodies between joints. This approach will allow you to manually account for any necessary transformations due to offsets. As pointed out by Yiping Liu, you can modify the joint configuration directly when calling getTransform(). For example, instead of passing the raw joint angles:

     transform = getTransform(robot,[0.1 0.1 0.1 0.1 0.1],'body5');

You would adjust the angles considering the offsets:

     transform = getTransform(robot,[0.1 0.1-pi/2 0.1 0.1+pi/2 
     0.1],'body5');

Here is how you can implement these changes in your code:

   % Define DH parameters including necessary adjustments
   dhparams = [
       0.033  -pi/2    0.147    0;
       0.155   0       0       -pi/2; % Adjusted theta offset
       0.135   0       0        0;
       0       pi/2    0        pi/2; % Adjusted theta offset
       0       0       0.218    0
   ];

   % Create robot model
   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');

   % Set fixed transforms
   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');

   % Assign joints to bodies
   body1.Joint = jnt1; body2.Joint = jnt2;
   body3.Joint = jnt3; body4.Joint = jnt4; 
   body5.Joint = jnt5;

   % Add bodies to robot
   addBody(robot, body1, 'base')
   addBody(robot, body2, 'body1')
   addBody(robot, body3, 'body2')
   addBody(robot, body4, 'body3')
   addBody(robot, body5, 'body4')

   % Get transformation with adjusted angles for offsets
   transform = getTransform(robot,[0.1 0.1-pi/2 0.1 0.1+pi/2 
   0.1],'body5');

After implementing these changes, verify your robot model by testing various configurations and observing if the transformations align with your expectations.

Please let me know if you have any further questions.

Answer 1

[Yiping Liu]

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]

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]

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;