# KDL Screw Theory

Submitted by thoseme on Thu, 2010-07-22 13:11 |

Hello,

I had a look into the KDL Documentation, and at the first glimpse I suppose that calculations are based on Screw Theory. This is because of the uncomplicated manner how to define serial chains, and the data structures for wrenches, twists and so on.

I spent some time in understanding Screw Theory, but I am still not familiar with it, having a lot of questions and being unable to calculate some things I am interested in. In order to get familiar with that, I would like to play around with a mighty tool, like Orocos KDL.

Regarding this, it would be very helpful if there would be some examples. For example, applicating a wrench and looking whether the serial chain will move, or what forces can be seen in which joint.

Did somebody do such calculations, and has done a little bit of documentation?

Best Regards ERik

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## KDL Screw Theory

On Thu, 22 Jul 2010, linuxrechner [..] ... wrote:

> I had a look into the KDL Documentation, and at the first glimpse I

> suppose that calculations are based on Screw Theory.

If you like to call it that way, yes, it's "screw theory". But that

corresponds exactly to what comes out of a classical dynamics approach, as

it should be. The only difference might be that 'screw theory' considers

all 6D entities as the basic primitives of the theory, while classical

dynamics (for historical reasons only) always makes a difference between

translational and rotational 'vectors'.

> This is because of the uncomplicated manner how to define serial chains,

> and the data structures for wrenches, twists and so on.

>

> I spent some time in understanding Screw Theory, but I am still not familiar with it, having a lot of questions and being unable to calculate some things I am interested in. In order to get familiar with that, I would like to play around with a mighty tool, like Orocos KDL.

>

> Regarding this, it would be very helpful if there would be some examples. For example,

> applicating a wrench and looking whether the serial chain will move, or what forces can be seen in which joint.

The first one requires a dynamic model (its the "inverse dynamics"

functionality that you would be testing by seeing how a chain reacts to a

6D force ("wrench") at the end point; the latter can also be done via

kinematics only properties of the chain, being the "Jacobian transpose"

mapping (which reflects the _static_ equilibrium between the end-effector

wrench and the joint torques required to counteract it, without taking

dynamics into account).

> Did somebody do such calculations, and has done a little bit of documentation?

The documentation is in every robotics textbook; KDL did not invent

anything new in this context :-)

Herman

## KDL Screw Theory

Hello,

I had a look into the KDL Documentation, and at the first glimpse I suppose that calculations are based on Screw Theory. This is because of the uncomplicated manner

how to define serial chains, and the data structures for wrenches, twists and so on.

I spent some time in understanding Screw Theory, but I am still not familiar with it, having a lot of questions and being unable to calculate some things I am interested in. In order to get familiar with that, I would like to play around with a mighty tool, like Orocos KDL.

Regarding this, it would be very helpful if there would be some examples. For example,

applicating a wrench and looking whether the serial chain will move, or what forces can be seen in which joint.

Did somebody do such calculations, and has done a little bit of documentation?

Best Regards

ERik