pick_ik Kinematics Solver
pick_ik is an inverse kinematics (IK) solver compatible with MoveIt 2, developed by PickNik Robotics. It is designed to provide robust and customizable IK solutions, offering a wide range of features.
The solver in
pick_ik is a reimplementation of bio_ik,
integrating a global optimizer and a local optimizer. The global optimizer utilizes evolutionary algorithms to explore
alternative solutions within the solution space and identify global optima. Building upon the results obtained by the global optimizer,
the local optimizer applies gradient descent for iterative refinement of the solution. It takes a potential optimum solution provided
by the global optimizer as input and aims to improve its accuracy and ultimately convergence to the most optimum solution.
Before proceeding, please ensure that you have completed the steps outlined in the Getting Started Guide.
Additionally, it is required to have a MoveIt configuration package specifically tailored to your robot. This package can be created using the MoveIt Setup Assistant.
Make sure your ROS 2 installation is sourced and run the following command
sudo apt install ros-$ROS_DISTRO-pick-ik
Create a colcon workspace.
mkdir -p $COLCON_WS/src
Clone this repository in the src directory of your workspace.
git clone -b main https://github.com/PickNikRobotics/pick_ik.git
Set up colcon mixins.
sudo apt install python3-colcon-common-extensions
sudo apt install python3-colcon-mixin
colcon mixin add default https://raw.githubusercontent.com/colcon/colcon-mixin-repository/master/index.yaml
colcon mixin update default
Build the workspace.
colcon build --mixin release
Using pick_ik as a Kinematics Plugin
You can use MoveIt Setup Assistant to create
the configuration files for your robot to use it with MoveIt, or edit the
kinematics.yaml file in your
robot’s config directory to use pick_ik as the IK solver.
You can launch a preconfigured demo with
pick_ik using the following command:
ros2 launch moveit2_tutorials demo_pick_ik.launch.py
The command starts a demo similar to the one in MoveIt Quickstart in RViz
tutorial. However, this demo specifically utilizes the robot kinematics configuration file
located at the
moveit2_tutorials/doc/pick_ik/config directory here.
For an exhaustive list of parameters, refer to the parameters YAML file.
Some key parameters you may want to start with are:
mode: If you choose
local, this solver will only do local gradient descent; if you choose
global, it will also enable the evolutionary algorithm. Using the global solver will be less performant, but if you’re having trouble getting out of local minima, this could help you. We recommend using
localfor things like relative motion / Cartesian interpolation / endpoint jogging, and
globalif you need to solve for goals with a far-away initial condition.
memetic_<property>: All the properties that only kick in if you use the
globalsolver. The key one is
memetic_num_threads, as we have enabled the evolutionary algorithm to solve on multiple threads.
orientation_threshold: Optimization succeeds only if the pose difference is less than these thresholds in meters and radians respectively. A
position_thresholdof 0.001 would mean a 1 mm accuracy and an
orientation_thresholdof 0.01 would mean a 0.01 radian accuracy.
cost_threshold: This solver works by setting up cost functions based on how far away your pose is, how much your joints move relative to the initial guess, and custom cost functions you can add. Optimization succeeds only if the cost is less than
cost_threshold. Note that if you’re adding custom cost functions, you may want to set this threshold fairly high and rely on
orientation_thresholdto be your deciding factors, whereas this is more of a guideline.
approximate_solution_orientation_threshold: When using approximate IK solutions for applications such as endpoint servoing,
pick_ikmay sometimes return solutions that are significantly far from the goal frame. To prevent issues with such jumps in solutions, these parameters define maximum translational and rotation displacement. We recommend setting this to values around a few centimeters and a few degrees for most applications.
position_scale: If you want rotation-only IK, set this to 0.0. If you want to solve for a custom
IKCostFn(which you provide in your
setFromIK()call), set both
rotation_scaleto 0.0. You can also use any other value to weight the position goal; it’s part of the cost function. Note that any checks using
position_thresholdwill be ignored if you use
position_scale = 0.0.
rotation_scale: If you want position-only IK, set this to 0.0. If you want to treat position and orientation equally, set this to 1.0. You can also use any value in between; it’s part of the cost function. Note that any checks using
orientation_thresholdwill be ignored if you use
rotation_scale = 0.0.
minimal_displacement_weight: This is one of the standard cost functions that checks for the joint angle difference between the initial guess and the solution. If you’re solving for far-away goals, leave it to zero or it will hike up your cost function for no reason. Have this to a small non-zero value (e.g., 0.001) if you’re doing things like Cartesian interpolation along a path or endpoint jogging for servoing.
You can test out this solver live in RViz, as this plugin uses the generate_parameter_library package to respond to parameter changes at every solve. This means that you can change values on the fly using the ROS 2 command-line interface, e.g.,
ros2 param set /rviz2 robot_description_kinematics.panda_arm.mode global
ros2 param set /rviz2 robot_description_kinematics.panda_arm.minimal_displacement_weight 0.001