How to include joint offsets in 5axiskins kinematics module?

Hi there

I am trying to wrap my head around the IK code in 5axiskins (the module used in the sim 5axis XYZBC machine) so that I can introduce different joint offsets. As an experiment I tried just adding in an x-offset for the final joint in the vismach script for the 5axis XYZBC machine, and of course I now see the toolpath describe a circle when jogging axis C rather than rotating about a point. I would like to know how to include that offset (and later, multiple offsets for multiple joints in our own machine) in the 5axiskins module.

Forgive me as I am used to seeing long trigonometric monstrosities when looking at IK maths! The section that seems to handle the IK is this:

I understand that PmCartesian is a struct that holds an x, y and z position. A colleague of mine suggested that line 5 appears to transform the cartesian coords in r via a spherical coordinate transformation and lines 9, 10 and 11 subtract those new coords from the x / y / z position.

1. Is the code for introducing built-in joint offsets contained elsewhere, and is this distinct from pivot offset (I will confess to not being able to understand the explanation of pivot offset in the Du Preez paper)? For example with the simulated XYZBC I would like to add my 5cm offset to the final joint to prove the principle to myself. Later on I need to add say x=5cm, y=7cm and z=15cm offsets between joint 0 and 1 in our machine, the x and y axes, and still others later. Can 5axiskins handle this?


2. I noticed there is another kinematics module called xyzbc-trt-kins.c which looks more like the standard matrices I am used to seeing, with what appear to be offsets. Is the maths equivalent to 5axiskins? Further, is 5axiskins also derived from the paper by Rudy Du Preez, or is it just the xyzac-trt and xyzbc-trt modules?

Thanks again for your support!

ik code is generated by the interpreter to the joint values
this is then ofseted by the kinetics so no need to do this

there are all nessesary ofsets in the kinetics modules available
as special as you use standard xyzbc mashine

Thanks for your reply!

So do you mean the kinematics somehow knows the denavit hartenberg link offsets etc? (such as in denavit hartenberg parameters that would be r and d). From one machine to the other they should vary. Imagine mounting your z axis with a large mounting plate for example, or offsetting your final C rotation axis by some margin so that its axis is no longer coincident with the B axis.

As a test, the latter is what I have done by modifying the vismach script for the 5axis XYZBC machine with an offset, and as a result the final joint now describes a circle rather than rotating around a point as with the stock setup, which indicates the kinematics have not adjusted to it.

This is separate to the "pivot length" variable I believe. This is using 5axiskins, where I do not see anywhere to add link lengths / offsets. Would it be better to use one of the Du Preez modules?

Hi Andy

The machine design as given to me is not completely orthogonal in all axes. For example, Joint 0 and Joint 1 look like the attached picture with an offset in the x, y and z axis. This means that the matrices for our machine will feature additional link offsets - the maths will look different to the maths in say the xyzbc-trt file because applying the Denavit Hartenberg convention results in additional offset parameters not found in the standard kinematics modules.

Perhaps I am misunderstanding - is it not usually the case that linuxcnc users build a 'standard' machine with orthogonal axes which match a given kinematics module, meaning that a standard module should work for those users?

At present I am calculating the transformation equations for our machine and intend to edit the xyzbc-trt file with the updated maths.

I cannot show the full machine unfortunately as this is a commercial application.

Hi Andy

Certainly the way I was taught it, DH parameters are meant as a way to simplify creating the equations, and are taught for simple machines as well as complex ones. They make it easier when generating the algebra, but those equations should ultimately be the same as if you found it by other means.

I think the main issue here is whether our offsets will cause the stock kinematics to fail, and so need modifiying? I have no control over the mechanical design unfortunately.

Our machine features two rotary axes as well as the first three cartesian ones also, again with an offset. Imagine a regular 5 axis XYZBC gantry mill but the part holding the tool is offset to the side rather than being in line with the z axis rotation.

The reasons for the quirks is that we are not actually building a mill per se!

That should be fine, genserkins was never an option. I have been looking at using xyzbc-trt-kins.c, which judging from the paper Randy du Preez published on it seems to be the best fit for our machine. That's really where this all started, trying to understand if Randy's file will work without modification, given our offsets.

Thanks for your ongoing support, it is appreciated!