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stack-of-tasks / pinocchio

A fast and flexible implementation of Rigid Body Dynamics algorithms and their analytical derivatives
http://stack-of-tasks.github.io/pinocchio/
BSD 2-Clause "Simplified" License
1.96k stars 396 forks source link

Remove Body #1591

Open JonathanKuelz opened 2 years ago

JonathanKuelz commented 2 years ago

Hey, I am working on modular robotics with pinocchio, which means I build lots of robots composed of the same modules. It would be convenient if it was possible to add and remove bodies and joints for this purpose (as mentioned in #1451 ) instead of building every combination from scratch.

I am only working with the python wrappers, where I was able to add as many joints / bodies as I want. However, removing parts of the kinematic chain fails as I have no access to the n-{bodies, joints, frames, qs, vs} properties of a model. Is it somehow possible to decrement these read-only properties in python? I believe I have the rest of the removing procedure figured out, so this would be a workaround until removing parts from the kinematic chain will be properly supported in pinocchio.

EDIT: Theoretically, I could "remove" joints by building a reduced model and then removing the according frames etc., but this keeps model.nbdodies constant still

jcarpent commented 2 years ago

Indeed, it would be a nice feature to add to Pinocchio The only missing part is the need of rebuilding a Data struct as soon as the model has changed. Do you have in mind typical applications for that?

JonathanKuelz commented 2 years ago

My personal application for that is the evaluation of different modular robots for a given task - like e.g. finding a good combination from a large set of possible modules to reach multiple poses in a work space. (Whiman et al. have done something like it for example, but they only expanded their robot while I would like to change it)

Or suppose you want to calculate the forward kinematics for a set of different grippers you are thinking about attaching to your robot - it would be more intuitive to have robot.removeLast(), robot.addNew(gripper) functionalities than to build a new robot for every gripper from scratch.

Whenever you have multiple alternatives for how to build your robot - be it a humanoid for which you want to find the ideal morphology or a modular industrial robot - I think being able to change the robot instance would be of benefit.