Inverse Kinematics

[noahjanderson]

I would like to use inverse kinematics to program two (2) lifelike arms for a robot to work with children and will appreciate any help or suggestions anyone may have.

The following has been discussed so far on a different thread so I migrated it using the

block as I did not see this feature (or I missed it) on the forum.

Thanks in advance to all!

@Team80_Giraffes

http://blog.oscarliang.net/inverse-kinematics-and-trigonometry-basics/

It’s fun!!! What kind of arm are you looking to control? The more segments and degrees of freedom, the more weird the math can get.

Are you looking for optimal paths or just “good enough” paths? Are there specific orientations the end piece needs to be in? Are there acceleration or velocity limits you need to concern yourself with?

@noahjanderson

Thank you so much for responding and You crazy! I like that.

I was actually reviewing the following two (2) links “to get my feet wet”:

hubo-kinematics-tech-report.pdf

and

[Robotic_Arm_Tutorial.html

Yes this is crucial because I am looking to make this robot lifelike and interact with Children so safety is crucial.

Yes as the robot cannot seem as if it is overextending its joints (DOFs) more than usual for a flexible human being. Its Configuration Space first needs to be calculated first then I can start playing with the non Linear system of equations (FUN to you apparently but not too a microcontroller → poor microcontroller probably thinking… its going to get hot in here LOL). Apparently the issue is that there is the possibility of infinitely many solutions or no solution and “matrix anything” can get high on the order of Big Oh/Omega → for example matrix multiplication is currently something like n^log3 (base 2) or n^2.7~ A microcontroller may or may not be able to process this quickly but a more robust machine probably will have no issue.

Also this robot has 22 degrees of freedom (DOF), but the head is not as difficult as it naturally would have a more limited Configuration Space, and the Arms will be symmetric, right?

I will open another Thread as this is distracting from this threads purpose.](“How to Build a Robot Tutorials - Society of Robots”)

[noahjanderson]

The robot has a total of 22 degrees of freedom(DOF) but the head is not as important and the arms have about 7 DOF and I am hoping should be symmetric.

[Team80_Giraffes]

22 DOF! Oh snap! :eek:

Yes it is very computationally intensive for the poor little Cortex with limited memory and an aging ARM cortex chip in it.

I doubt you’d have enough sensors to manage the angle of those 22 degrees of freedom. Can you pare it down to a few?

If you are looking at Vex parts, each arm would be it’s own Cortex full of motors and sensors.

DOF list:

1. shoulder joint pitch (along the body)
2. shoulder joint roll (away from the body)
3. elbow joint pitch (I think there’s only 1 DOF there)
4. wrist twist
5. wrist yaw (side to side)
6. wrist tilt (up down)
7. wrist twist (turn)

Now you have run just about out of pot’s and IME’s to hook in to the cortex.

From the shoulder outwards infer the position of the hand via math. Get the velocities and accelerations of each joint. Figure out where the hand is as well as each point.

Then comes the path analysis of where you want to get the “hand” to. Like you said, there are constraints on humans for each of these joints you want to apply.

The pot to an angle is not a 1:1 measurement but you can get a fairly good formula to approximate it. From the angle get the angular velocity of each joint. Lengths of the arm segments gets you velocity vectors relative to its space. Go down the arm and you get the whole arm motion computed for where you are.

Now it’s decision on where you want to be and how much force is needed to get there. I don’t know the path that “normal human motion” would take for a hand to get from point A in orientation A to point B. Is it mainly lower arm based or upper arm based? That then can help you in the movement strategy you want to employ.

[Inspector_Gadget]

I made a robotic dragon a couple of years ago that had 12 DOF abs around 18 motors. I ended up using 2 cortexes. The code between the two is fairly easy if you have a master and a puppet cortex. One could control the arms, and the other could control the rest of the robot and send the arms general positions to go to our general functions to perform.

On the idea of the inverse kinematics, look at this site
http://blog.oscarliang.net/inverse-kinematics-and-trigonometry-basics/
It does a pretty good job at explaining some stuff.

[noahjanderson]

Thanks guys!