jeljaik
commented
10 years ago :+1: Nice summary! There are only two things I personally dislike. One is the word 'shank' to describe the part of the lower limb from knee to ankle. I know it's meant to sound more anatomically correct, but then neither 'leg' should be used to describe the entire lower limb, as it is also defined as the part between the knee and ankle (it's a synonym actually). I also don't like 'l' and 'r' for left and right. More proper acronyms such as rt, lft, lt look better IMHO, e.g. lt_foot as opposed to l_foot. It also removes ambiguity between letters like 'l' and 'I' (capital i and l).
traversaro
lornat75
miggia
alecive
iron76
pattacini
In the process of producing software-friendly models for the iCub robot directly from the Creo CAD models in a semi-automatically, we needed a convention for naming the different elements that constitute the robot. I will proposed a convention extending existing naming schemes, and I will be happy to get any feedback. Sharing a common convention in existing software would help to reduce incompatibilities and integration problems.
Joints
1-DOF Joints
Note: it could be useful to distinguish between the two different concepts of degree of freedom and joint. For simplicity we decide to ignore for the time being this difference, that is relevant for multi-DOF joints and closed kinematic structures.
The joints are mechanism that connect two different links (as defined in the following section) of the robot. 1-DOF joints allow motion only along one direction between the two connected links.
For joints an existing convention is introduced in http://eris.liralab.it/wiki/ICub_joints . I do not know if this names are actually used anywhere in the code and/or documentation (@barbalberto perhaps in IAxisInfo implementation?). Unfortunately in that convention the names are not unique across the robot, so for example
kneeis used to refer to both left and right knee. So we propose to use that convention, with an additionall_orr_prefix for joints that come in symmetric pairs. Disregarding for the moment eyes and hands, then we have the following joint names:I am personally ok with this names, but perhaps someone (@luca-fiorio ?) has some comments and may thinks that this names are misleading.
Fixed (0-DOFs) Joints
When dealing with 6-axis FT sensors, it is convenient to model them as joints that do not allow any motion between the two connected links.
In the iCub can be mounted up to 6 6-axis ft sensors.
Links
The links are the physical rigid bodies that constitute the robot. Each link is characterised by a mass (represented in models by the inertial parameters) and a physical shape (represented in models by meshes).
For defining the links we are representing the robot as a directed tree, where the
root_linkis the body that can be attached to the pole, and the meaning of the other links can be deduced by their parent joint, as defined in the previous section.The main idea behind this naming scheme is that "big" links (roughly speaking, the one that can reasonably interact with the user) are named with intuitive names such as
upper_arm,forearm,upper_leg,lower_leg,chest,head,foot,hand. All the little links that instead are part of more complex linkages, take their name from the articulation, such astorso_1,torso_2.Frames
In literature and in robotics software, the links and frames concepts are often confused, because every link is usually associated with a frame rigidly attached to it. However this link frame definition is dependent on the formalism that one uses for describing the robot. For example, if the Denavit Hartenberg convention is used the link frame origin is required to be placed on the axis of the child joint, while if the Modified Denavit Hartenberg convention or the URDF format the link frame origin is required to be place on the axis of the parent joint. To avoid inconsistency, I think it is better to clearly separate frame and link concepts. For the iCub, it could be useful to explicitly state the frame used for defining the kinematic chains used in the iKin library. In particular a useful set of frames could be:
Skin Frames
An interesting set of frame is the frame defined by the iKin convention ( http://eris.liralab.it/wiki/ICubForwardKinematics ). This reference frames are used by the skin system to express contact points, force and torques (in skinDynLib data structures) and taxel positions. The one currently used by the skin system are:
LEFT_FOOTSkinPartRIGHT_FOOTSkinPartSKIN_LEFT_UPPER_ARMSkinPartSKIN_LEFT_FOREARMSkinPartSKIN_LEFT_HANDSkinPartSKIN_RIGHT_UPPER_ARMSkinPartSKIN_RIGHT_FOREARMSkinPartSKIN_RIGHT_HANDSkinPartSkinPart
RIGHT_LEG_UPPER,LEFT_LEG_UPPER,RIGHT_LEG_LOWER,LEFT_LEG_LOWERandSKIN_FRONT_TORSOare not currently correctly processed by the system and they are not assigned any frame. In particularSKIN_FRONT_TORSOis problematic because the Torso link for the DH convention used by iKin for frame placement has three different frames, depending on which chain is considered (if head, right arm or left arm).For all this frames, an alternative naming could be: "linkName_ikin_frame", to have a name that is directly related to the iKin software.
Other frames
Probably some other intermediate frame could also be useful, as something related to the neck.
Additional frames could be defined to improve compliance with REP-120 that is already used for different humanoids URDFs (Nao, Romeo, HRP-2, Coman).
This is an initial proposal that is not discussing several important aspects (eyes, hands, accelerometers, gyroscopes), but I guess it is already ready for getting some feedback.
cc @EnricoMingo @jeljaik @arocchi @hu-yue