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Robot Inverse Kinematics

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We've seen the forward kinematics problem. The inverse kinematics problem is much more interesting and its solution is more useful. At the position level, the problem is stated as, "Given the desired position of the robot's hand, what must be the angles at all of the robots joints?"

Humans solve this problem all the time without even thinking about it. When you are eating your cereal in the morning you just reach out and grab your spoon. You don't think, "my shoulder needs to do this, my elbow needs to do that, etc." Below we will look at how most robots have to solve the problem. We will start with a very simple example.

The figure above is a schematic of a simple robot lying in the X-Y plane. The robot has one link of length l and one joint with angle .  The position of the robot's hand is Xhand. The inverse kinematics problem (at the position level) for this robot is as follows: Given Xhand what is the joint angle ? We'll start the solution to this problem by writing down the forward position equation, and then solve for .

Xhand = lcos      (forward position solution)
cos = Xhand/l
= cos-1(Xhand/l)

To finish the solution let's say that this robot's link has a length of 1 foot and we want the robot's hand to be at X = .7071 feet. That gives:

= cos-1(.7071) = +/- 45 degrees

Even for this simple example, there are two solutions to the inverse kinematics problem: one at plus 45 degrees and one at minus 45 degrees! The existence of multiple solutions adds to the challenge of the inverse kinematics problem. Typically we will need to know which of the solutions is correct. All programming languages that I know of supply a trigonometric function called ATan2 that will find the proper quadrant when given both the X and Y arguments: = ATan2(Y/X). Now we have the tools we need to look at a more interesting inverse kinematics problem:

You may have to use your imagination a bit, but the schematic above is the planar part of the SCARA robot we discuss in the industrial robots section. Here's the statement of the inverse kinematics problem at the position level for this robot:

Given: Xhand, Yhand, hand
Find: 1, 2 and  3

To aid in solving this problem, I am going to define an imaginary straight line that extends from the robot's first joint to its last joint as follows:

B: length of imaginary line
q1: angle between X-axis and imaginary line
q2: interior angle between imaginary line and link l1
Then we have:

B2 = Xhand2 + Yhand2                    (by the Pythagorean theorem)

q1 = ATan2(Yhand/Xhand)
q2 = acos[(l12 - l22 + B2)/2l1B]             (by the law of cosines)
1 = q1 + q2                                     (I know you can handle addition)
2 = acos[(l12 + l22 - B2)/2l1l2]             (by the law of cosines)
3 = hand - 1 - 2
That completes the solution for 1, 2 and  3 given Xhand, Yhand, hand. Most inverse kinematics solutions at the position level proceed in a similar fashion. You will use your knowledge of trigonometry and geometry coupled with your creativity to devise a solution. The solution given above works as is for SCARA robots. If you can imagine turning the SCARA robot on its side, then you will see that the solution above also works for the positioning components of most six degree of freedom industrial robots too. The inverse kinematics solution for Cartesian robots is trivial as all axes are perpendicular by definition and thus there is no coupling of the motions.

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