# How to find theta1 to theta5 after D-H parameter

Joint   θi    di    ai-1    αi-1
1     θ1-90  -d1     0      180
2      θ2     0      0      -90
3      θ3     0     a2       0
4     θ4-90   0     a3       0
5      θ5     0     0        90


I am confused about the right way to look for my theta1-theta5. Probably from the offset limit of the angles or calculation from x0 to x5 angle rotation or from atan2(x,y).

I am not sure if this answers properly your question, but in this case theta1-theta5 are supposedly your joint variables, i.e. they are the value read from the encoders of your robotic arm. The -90 placed on joint 1 and joint 4 are instead the offset between the encoder readout and the actual joint configuration.

I don't know about the Robot which you have. But for your α1, try using 0 instead of 180. I think, two parallel Z axis confuses us, but taking 0 for parallel rotation axes will make difference in values (in case of COS θ).

• @Alecive, thanks for your comment. I really need to confirm my joint variable from theta1-theta5 before moving on to the next stage.Is there any way i can attach d picture of my robot arm movement. Apr 1 '15 at 14:02
• the reason for the 180 is because my Z0 is facing up while Z1 is facing down. Thanks 4 the reply. Apr 1 '15 at 14:05

In the Denavit-Hartenberg parametrization the theta parameters represent the degrees of freedom of your system. If you are using a robotic manipulator then the theta values correspond to your joint angles.

I would highly recommend that you look into the possibility of using the product-of-exponentials formulation instead. This is an alternative approach to using the Denavit-Hartenberg parametrization. This approach has a number of benefits such as the removal of singularities which are attributed only to your choice of parametrization.

I can imagine that some people would be put off from using it as you require a little more mathematical knowledge to fully understand how it works. However, as with many things you can still use it without knowing where it comes from.

If you do decide to continue with the D-H parametrization, remember that there are many possible values for your parameters which will result in equivalent models of your system.