Ikfast

This page describes how to use the ikfast inverse kinematics module from openrave system.

From the openrave web site, "OpenRAVE? provides an environment for testing, developing, and deploying motion planning algorithms in real-world robotics applications." For linuxcnc, we are using the ikfast module to create the IK component, and the qtcoin display application to show the robot model.

Source

A modified version of the openrave source is available here, https://github.com/ftkalcevic/openrave, in the "linuxcnc_gen_c" branch. See the openrave web site for details of the build dependencies, and how to build and install the package. (http://www.openrave.org/docs/latest_stable/coreapihtml/installation_linux.html)

The modified version adds a c code generator that works with linuxcnc. Use the --outputlang=c to create the c code.

The ikfast updates to linuxcnc can be found here, https://github.com/ftkalcevic/LinuxCNC, in the "ikfast" branch. There is a new src/emc/kinematics/ikfastkins.c module that uses the ikfast generated code. There is also some example configurations in configs/ikfast-demo directory.

Building a robot model

openrave supports multiple file formats to describe the robot, http://www.openrave.org/docs/latest_stable/robots_overview/. Openrave xml is a simple format that allows models to be generated quickly. The COLLADA format allows you to generate more realistic looking robots.

Simple robot defined using openrave xml


The same simple robot, but defined using collada file

The robot model is used in all openrave computations, including the generation of the ikfast source. Therefore it is important that it is created accurately.

The creator of openrave recommends opening an interactive openrave session and continually reloading the model...

$> openrave.py -i
In [1]: env.Reset(); env.Load('simple.robot.detailed.xml')

Generating and testing the Inverse Kinematics

The recommended way of generating the ik code is to use the openrave.py program, but advanced users can use ikfast.py directly. It is best to generate the cpp component first and use that for automatic testing. Automatic testing doesn't work with .c output.

For example, to generate the 3d translation ik...

$> openrave.py --database inversekinematics --robot=../simple.robot.xml --iktyp
e=Translation3D --freejoint=Wrist --freejoint=WristR --numiktests=1000

getIndicesFromJointNames [3, 4] ['Wrist', 'WristR']
openravepy.databases.inversekinematics: generate, Generating inverse kinematics 
for manip arm: Translation3D [0, 1, 2] (this might take up to 10 min)
openravepy.databases.inversekinematics: generate, creating ik file /home/frankt/
.openrave/kinematics.587828202130adcfd9bfdcd543fb17e2/ikfast62.Translation3D.0_1
_2_f3_4.cpp

... Lots more output ....

openravepy.databases.inversekinematics: save, inversekinematics generation is do
ne, compiled shared object: /home/frankt/.openrave/kinematics.587828202130adcfd9
bfdcd543fb17e2/ikfast62.Translation3D.i686.0_1_2_f3_4.so
openravepy.databases.inversekinematics: RunFromParser, testing the success rate 
of robot ../simple.robot.xml 
[ikfastmodule.cpp:946] FindIKSolution: No ik solution found, i = 0
Joint Val: 0 0 0 0 0 
Transform: 855638019 0 0 0.4799 
raw ik command: 0 0 0 0 0 0 0 0 0 0 0 0.4799 0.2606354555368424 0.40057338399291
04 

[ikfastmodule.cpp:1001] FindIKSolutions: No ik solution found for, i = 0
Joint Val: 0 0 0 0 0 
Transform: 855638019 0 0 0.4799 

openravepy.databases.inversekinematics: testik, success rate: 0.999000, wrong so
lutions: 0.000000, no solutions: 0.001000, missing solution: 0.000000

Specifying --numiktests runs a series of tests with the ik module to confirm it works correctly. I suspect the output shown above is a bug, as the first test coordinate looks dodgey. Running the tests is important because many ik solutions don't provide a very high success rate. If this happens you'll need to look at changing the iktype, or possibly selecting different free joints.

For more details on how to use openrave, visit the openrave website, www.openrave.org

openrave.py can also do performance testing of the ik solution. Specify the --perftiming= option.

$> openrave.py --database inversekinematics --robot=../simple.robot.xml --ikty
pe=Translation3D --freejoint=Wrist --freejoint=WristR --perftiming=1000

... Lots of output ...

openravepy.databases.inversekinematics: RunFromParser, running time mean: 0.000
005s, median: 0.000005s, min: 0.000005s, max: 0.000015s

Showing a mean of about 5us to calculate the ik. This is good. It means ikfast produced a fast closed form solution.

Changing iktype to TranslationZAxisAngle4D? gives....

$> openrave.py --database inversekinematics --robot=../simple.robot.xml --ikty
pe=TranslationZAxisAngle4D

... Lots of output...

openravepy.databases.inversekinematics: RunFromParser, running time mean: 0.0027
76s, median: 0.002728s, min: 0.001594s, max: 0.005023s

These are terrible times - a mean of 2.7ms and max of 5ms means we can't run with a 1kHz servo loop. Slowing the servo loop down to 200Hz will produce poor motion.

The slow times are because ikfast had to use a numerical inverse kinematics solver. Numerical solvers are a slow iterative process. This limitation is because of the geometry of the robot and the selection of joints.

To generate the ikfast code for linuxcnc, just add the --outputlang=c option, eg...

$> openrave.py --database inversekinematics --robot=../simple.robot.xml --iktyp
e=Translation3D --freejoint=Wrist --freejoint=WristR --outputlang=c 

getIndicesFromJointNames [3, 4] ['Wrist', 'WristR']
openravepy.databases.inversekinematics: generate, Generating inverse kinematics 
for manip arm: Translation3D [0, 1, 2] (this might take up to 10 min)
openravepy.databases.inversekinematics: generate, creating ik file /home/frankt/
.openrave/kinematics.587828202130adcfd9bfdcd543fb17e2/ikfast62.Translation3D.0_1
_2_f3_4.c

... Lots more output ...

openravepy.ikfast: writeIkSolver, generating c code...
openravepy.databases.inversekinematics: generate, compiling ik file to /home/fra
nkt/.openrave/kinematics.587828202130adcfd9bfdcd543fb17e2/ikfast62.Translation3D
.i686.0_1_2_f3_4.so
openravepy.databases.inversekinematics: generate, cannot continue further if out
putlang c is not cpp
openravepy.databases.inversekinematics: save, inversekinematics generation is do
ne, compiled shared object: /home/frankt/.openrave/kinematics.587828202130adcfd9
bfdcd543fb17e2/ikfast62.Translation3D.i686.0_1_2_f3_4.so

Note the name of the output file in the program logs. The directory name and file name are dependant on the robot model. In this case it is...

/home/frankt/.openrave/kinematics.587828202130adcfd9bfdcd543fb17e2/ikfast62.Translation3D.0_1_2_f3_4.c

To use this file in linuxcnc, copy the file to the linuxcnc source tree...

src/emc/kinematics

and rename it from .c to .inc. Also copy the file ikfastkins_c.h.

Edit ikfastkins.c in this directory to include the .inc file. There should already be a line like...

#include "ikfast62.Translation3D.0_1_2_f3_4.inc"

Just change that one.

Building

Simulator

Just do a standard linuxcnc build.

Barret Wam 7DoF? robot