The triangles used for angle calculations

I am trying to control a Dobot arm. The arm moves with angles whereas I need to work with cartesian coordinates. From inverse kinematics equations and polar coordinates I have implemented x,y and z coordinates working very well on their own. But I can not combine the coordinates in order to work all together. When I add them up it is not going to the desired place. How can I combine these coordinates? I got some help from (https://github.com/maxosprojects/open-dobot) but could not manage to successfully move dobot.

Edit: I've written the codes in Qt and also I've added the triangles used for angle calculations.

//foreArmLength=160mm rearArmLEngth=135mm

float DobotInverseKinematics::gotoX(float x) //func for x-axis
float h=qSqrt(qPow(lengthRearArm,2)-qPow(x,2)); //height from ground
QList<float> zEffect=gotoZ(h); //trying to find the effect of x movement on z-axis
float cosQ=h/lengthRearArm; //desired joint angle
float joint2=qRadiansToDegrees(qAcos(cosQ));    
//move in range control
if(joint2 != joint2)
    qDebug()<< "joint2NAN";}
return joint2;

QList<float> DobotInverseKinematics::gotoY(float y) //func for y-axis
QList<float> result ;
float actualDist=lengthForeArm+distToTool; //distance to the end effector
float x=(qSqrt(qPow(actualDist,2)+qPow(y,2)))-actualDist; //calculating x movement caused by y movement
float joint1=qRadiansToDegrees(qAcos(actualDist/(actualDist+x))); //desired joint angle
float joint2=gotoX(x);  //the angle calculation of y movement on x axis
if(joint1 != joint1)
    qDebug()<< "joint1NAN";}
return result;

QList<float> DobotInverseKinematics::gotoZ(float z) //func. for z-axis
QList<float> result ;
float joint3=qSqrt(qPow(160,2.0)-qPow(z,2.0))/ 160; //desired joint angle
float temp=160-qSqrt(qPow(160,2.0)-qPow(z,2.0));
float joint2=qSqrt(qPow(lengthRearArm,2)-qPow(temp,2.0))/lengthRearArm; //desired joint angle
if(joint3 != joint3)
    qDebug()<< "joint3NAN";}
return result;
  • $\begingroup$ Could you please edit your question to include the work that you've done so far? I took a look at the Dobot documentation and didn't see anything specifically about kinematic lengths or reference datum for individual joints. $\endgroup$
    – Chuck
    Aug 17, 2016 at 13:48
  • $\begingroup$ @Chuck I've edited the question sorry for the previous format I was in a hurry I've forgotten to add the codes $\endgroup$
    – hatoz
    Aug 18, 2016 at 4:46
  • $\begingroup$ For others trying to help, it seems that the Dobot people are trying to do a simplified analytic inverse kinematics by 'converting x and y to polar coordinates to get angle and radius'. @hatoz, There are two steps to the solution: f(x,y)->joint1,radius f(radius,z)->joint2,joint3. $\endgroup$
    – hauptmech
    Aug 18, 2016 at 10:48
  • $\begingroup$ @hauptmech thx for the reply but I am trying to calculate joint1 from x and y by getting simply atan(y/x) but the result does not seem to be correct $\endgroup$
    – hatoz
    Aug 18, 2016 at 12:16
  • $\begingroup$ atan2(y,x) always. You can Google why. $\endgroup$
    – hauptmech
    Aug 18, 2016 at 12:55

1 Answer 1


$a_1, a_2$ : Link lengths

$p_x,p_y$ : Target $x$ and $y$ coorditate


$p_r=\sqrt{p_x^2+p_y^2}$ : Target radial distance

$p_z$ : Target $z$ coorditate


If $\kappa>1$ the position cannot be reached.


There are 2 solutions, elbow up or elbow down. Below use $\theta_3^*=\theta_3$ or $\theta_3^*=-\theta_3$





These are joint angles. To get motor angles, you will have to adjust the angles based on how the motors are connected to the robot. $\theta$ is 0 when horizontal, $\theta_3$ is 0 when straight (in-line) with Link 2.

A detailed explanation can be found in Robot Analysis by Tsai. ISBN 9780471325932 pp 71

  • $\begingroup$ thank you for your answer, I've implemented it but I can't get a result since the cos value is calculated as something bigger than 1, also I've assumed these a2 and a1 lengths are the arm lengths am I right? $\endgroup$
    – hatoz
    Aug 19, 2016 at 7:30
  • $\begingroup$ Sorry, I was missing a parenthesis. I added a note on how to detect when there is no solution and how to get both solutions. Sorry I don't have time to do a diagram. $\endgroup$
    – hauptmech
    Aug 19, 2016 at 11:32
  • $\begingroup$ I've implemented your solution this time run without errors, but the angles are really bigger than the desired values I could not find where the mistake is, there is nearly a 2 cm slip. Do you have any ideas why something like that may happen? Thank you for your effort. @hauptmech $\endgroup$
    – hatoz
    Aug 22, 2016 at 5:54

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