IK help for the 3DOF phoenix leg

[pyrrhicpk]

Hi all,

I have a coordinate system attached at the tip of a 3DOF leg. I am trying to determine the angles by which the coordinate system would rotate when the leg moves in a certain direction. For instance, I use the IK of Phoenix robot and give the following input motions using the Zenta’s PEP.

1- Vertical down = 5cm.
2- Forward = 7cm.

The PEP sheet shows the following angles of Right Middle Leg:
coxa= -33.7 degree
femur= 35.4 degree
tibia= -16 degree

Now, I assume there is a custom joint at the tip of this leg. This joint is composed of three independent revolute joints which can rotate in x, y & z directions. My intention is to determine these three angles by which the joint would rotate when the leg would move in space.

In this regard, I constructed a 3DOF leg in CAD mechanism and joint it with a custom 3DOF joint at its foot with the ground. When I give the above mentioned motions to the hexapod, the angles shown by the joint at the foot tip are as follow:
About X-axis = 5.926
About Y-axis = 33.69
About Z-axis = -19.4

These are the angles by which the joint at the foot tip rotated when the robot was given 5 cm down motion and 7 cm forward translation.

My problem is with the understanding of these angles.
The angle Y = 33.69 is no doubt the coxa angle, while the angle Z = -19.4 is ( femur + tibia ) or the wrist angle, but how to determine this X = 5.926 angle? I tried summing and subtracting coxa, femur, tibia angles with different combination but could find a way to determine an angle about X axis which would equal 5.926.

My intention is to determine these three angles from the IK or FK of phoenix leg.

Any thoughts?
Thanks,
Umar

[pyrrhicpk]

Can anyone provide me some assistance in this regard please?

Thanks

[zenta]

@pyrrhicpk

My problem is with the understanding of these angles.
The angle Y = 33.69 is no doubt the coxa angle, while the angle Z = -19.4 is ( femur + tibia ) or the wrist angle, but how to determine this X = 5.926 angle? I tried summing and subtracting coxa, femur, tibia angles with different combination but could find a way to determine an angle about X axis which would equal 5.926.

Hi,
I’m not sure if I completely understand your question.

There are no real simple math between the coordinates and the servo angles. Thats the whole point of the IK math. Solve the IK and you’ve solved your “problem of understanding these angles”.

In PEP, try to study the IK Phoenix and the Body&Coxa sheet. Or study Xan’s Phoenix code (the IK sub).

[pyrrhicpk]

Zenta Thanks for the response.

Can you please translate me the relevant portions of the PEP sheet I have uploaded as a screen capture. I believe there are some angles, I might use them to serve my purpose.

My main query is actually to find out the rotation angles at the TAR when phoenix leg is translated/rotated to any position in three dimensional space. For instance, at the mean position a coordinate system at TAR reports 0, 0 , 0 angles in xyz. Now if the femur, tibia and coxa are fed with certain angles, the coordinate system at the TAR would rotate in relation to the leg new position. I need to determine those angles.

I hope I elaborated the issue to some extent this time.

Thanks