fabrik chain implementation

[ahundt]

I'm interested in using the fabrik chain implementation, and the version in devel looks easier to use. Should that work or is it still expected to change substantially?

[wolftype]

hi -- devel branch is pretty active right now with major revisions, but vsr_chain is fairly stable. actually your question led me to find a small bug in the calcJoints() method, I also added an examples/xFabrikChain.cpp for reference on how to use the fabrik() solver. let me know if/how it works for you!

[ahundt]

Thanks! I'll give it a try.

I had to hack things a bit to get stuff to compile. I wanted to see some of the stuff in scratch because it looks like there are some good examples of IK in there. I manually set some of the paths and compiled GLV, but it seems to be a bit of a mess. :-)

[ahundt]

I made a gist with a cmake patch and errors for the versor scratch folder

[ahundt]

I'm also interested in using the new model constraints published for FABRIK with vsr_chain. Any chance those are present or could be added relatively easily?

• Extending FABRIK with model constraints paper
• Overview of IK methods, including some FABRIK model constraints details
• FABRIK IK algorithm creator's site.

In my case I just have 7dof revolute joints and need to set joint limits.

There is also an interesting thread related to constraints, and another on math.stackexchange

[wolftype]

fyi right now what i use to compile projects in scratch is

./run.sh scratch/projects/…../xProjectName.cpp

and optionally add “configure” to the end to force cmake to add it to the targets list if it is a new project

./run.sh scratch/projects/…../xProjectName.cpp configure

note that not all projects in scratch currently compile

On Jul 20, 2015, at 10:19 PM, Andrew Hundt notifications@github.com wrote:

Thanks! I'll give it a try.

I had to hack things a bit to get stuff to compile. I wanted to see some of the stuff in scratch because it looks like there are some good examples of IK in there. I manually set some of the paths and compiled GLV, but it seems to be a bit of a mess. :-)

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[wolftype]

thanks for these newer references, i had not seen them but will look over them . . .

happy to help get something going, if you find a particular algorithm you want to implement.

On Jul 20, 2015, at 11:26 PM, Andrew Hundt notifications@github.com wrote:

I'm also interested in using the new model constraints published for FABRIK with vsr_chain https://github.com/wolftype/versor/blob/master/vsr/vsr_chain.h. Any chance those are present or could be added relatively easily?

Extending FABRIK with model constraints http://onlinelibrary.wiley.com/doi/10.1002/cav.1630/full paper
Overview of IK methods, including some FABRIK model constraints details http://www.andreasaristidou.com/publications/CUEDF-INFENG,%20TR-632.pdf
FABRIK IK algorithm creator's site http://www.andreasaristidou.com/FABRIK.html.
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[ahundt]

The two constraints I care about are very simple: Revolute joints and joint limits. Essentially, each joint on the arm I use is a 7 rotating joint arm, and it only has a range of +/- theta from the center which varies for each joint.

I think the revolute joints are already implemented for devel, do you think the joint limits could be added easily?

[wolftype]

Traveling a bit at the moment (to the AGACSE conference!) and have not had a chance to look at the extended technique,
but I have experimented with constraining points during fabrik solver to circles, rather than spheres, which equates to
constraining the axes of rotation (or to constraining to two distances at once).

Constraining the angles would then be a matter of clamping the thetas of each to a lower and higher bound, I suppose.

Currently the ik fabrik solver in vsr_chain just constrains to spherical distances (so, ball joints), but the Constraint class in vsr_rigid.h shows some
of the ideas behind forming other constraints (between two points for instance).

I haven’t tested below, but i think the forward rotation constraint would look something like this:

1 - get location of point closest to chain[i].cxy();
2 - intersect line through that point with sphere surrounding chain[i].cxy();

Circle circle = chain[i].cxy(); /// circle of rotation
Sphere sphere = chain[i].bound(); /// sphere of rotation

Point pa = chain[i].pos();
Point pb = chain[i+1].pos();

/// Note: I would encapsulate the following into a method which takes a circle and a point p, and returns a point on the circle closest to p.
/// probably put it in Constrain::PointToCircle( p, circle)

auto cen = round::location( tangent::at( circle, p ) ); /// point on same plane as circle
auto line = pa ^ cen ^ Infinity(1); ///line through center of circle and p
auto meet = (sphere^line.dual()).dual(); /// meet of line and sphere
auto res = round::split( meet ,true); ///point on circle closest to p

then it is a matter of taking the result and limiting the rotation of the joint.

I can try this out if I get a moment….

On Jul 22, 2015, at 4:03 AM, Andrew Hundt notifications@github.com wrote:

The two constraints I care about are very simple: Revolute joints and joint limits. Essentially, each joint on the arm I use is a 7 rotating joint arm, and it only has a range of +/- theta from the center which varies for each joint.

I think the revolute joints are already implemented for devel, do you think the joint limits could be added easily?

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[wolftype]

ok there were a few bugs in that of course, but i pushed a working

Constrain::PointToCircle( const Point& p, const Circle& c );

to vsr_rigid.h which calculates the point on circle c closest to p.

examples/xPointToCircle.cpp shows this particular constraint in action.

it is a little easier than i suggested, we can project the point to the plane of c,
and then interesect the line through that projected point and the center
of c with the sphere surround of c. perhaps there is an even simpler way.

this constrains a point to a circle . . . limiting its angle is then the next step
with a dot product. give a bit of time to get that into a working kinematic example…

pc

On Jul 24, 2015, at 1:20 AM, wolftype wolftype@gmail.com wrote:

auto cen = round::location( tangent::at( circle, p ) ); /// point on same plane as circle
auto line = pa ^ cen ^ Infinity(1); ///line through center of circle and p
auto meet = (sphere^line.dual()).dual(); /// meet of line and sphere
auto res = round::split( meet ,true); ///point on circle closest to p

[ahundt]

awesome! Also, enjoy the conference it sounds super interesting.

[ahundt]

Any chance you could explain joint angle limits? I'm eager to try this out.

[wolftype]

I'll try to take a look at this this week - ;)

On Aug 8, 2015, at 2:02 AM, Andrew Hundt notifications@github.com wrote:

Any chance you could explain joint angle limits? I'm eager to try this out.

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[ahundt]

cool thanks!