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2015-12-09 08:25:31 -0600 asked a question Velocity control from posestamp format of a 6 DOF robot

Hello,

I have a question about a problem which we are facing. We have a working 6DOF robot where we can place the end-effector by posestamp X,Y,Z. If we enter the position of the end-effector to position x,y,z this works in RVIZ correctly. I use the simulation of arbotix. Now we are in reality working with coply motors. These are driven by velocity. We have a slider where we can set the velocity of the coply motor. But this gives the velocity to the joint. However, we would like to control the position of the motors in x,y,z. We are using Moveit! which works with the arbotix simulator. For us in the project it is important to give in x,y,z position to the end-effector. However, by some choices we have to deal with cople velocity drivers and not with for example arbotix servo drivers. Can someone of you tell me how we could convert the x,y,z position in a simple way to velocity to control the coply drivers . We have seen that via the arbotix sim that when we enter an x,y,z position the joint are velocity driven. So somewhere in the setup we are have an velocity component. I would likt to know where and possible how. I was looking for Twist or odometry, but I am not sure about this options. So please help me to find the correct way to position my robot via x,y,z postition and velocity.

Sincerely,

Marcel Weusthof

2015-11-19 06:21:48 -0600 asked a question Can moveite cope with mimic joint (parallel kinematic) and/or liniear constraints for planning and trajectory in IK

We have a parallel robot which we want to model and control using ROS. For this we need a Forward Kinematic (FK) model as well as calculating the Inverse Kinematics (IK) on velocity level. The kinematic structure of the robot partly behaves serial and partly parallel (2 DOF’s parallel, 3 DOF’s serial). For more info I can refer for the short movie, which can be send on request, which shows the behavior of each DOF.

Implementing the FK was quit straight forward by introducing mimic joints in the URDF-file. These mimic joints are simply a linear function of other joints. This approach works as expected for the FK, however not for the IK. For the IK problem we tried MoveIt, however the standard algorithm used in MoveIt, does not cope with these mimic joints. Maybe we could calculate the manipulator Jacobian by hand and implement our own IK algorithm, however we prefer to use a package like MoveIt, as this package comes with a lot of other possibilities like collision detection, joint limits etcetera.

So our question is, how should we implement a parallel kinematic structure in ROS?

This question can be divided by the following sub questions: • Is the use of mimic joints the correct choice? • Can MoveIt cope with mimic joints? • Can MoveIt cope with linear constraints (e.g. q3=q2-q1), which is basically coping with the mimic joint? • Is there an alternative for MoveIt which can do the above? (IKFast?, OpenRave) • Is there a package which uses the Jacobian matrix directly? • Can we fool MoveIT to solve this

2015-11-03 06:40:23 -0600 asked a question Error Group " " has mimic joint will not initialize dynamics solver

Hello,

Who can help me with the following problem.

We have a robot which has 2 mimic joints on one arm. When we generate the urdf via Moveit! there is no error. Via the GUI I can control all the joints including the 2 mimic joint. The multiplier is simple -1 and offset is 0. Via RVIZ and planning/execute I can position every joint or effector. No errors.

I am using the KDL library, which should be able to work with mimic joints.

But when I use the roslauch demo.launch or move_group.launch it says every time "Group 'right_arm' has a mimic joint. Will not initialize dynamic solver".

When I try to position the end effector via xyz then I get the following "Fail: No motion plan found. No execution attempted".

I would like to get a solution of the mimic joints. When removing the mimic joint and replace them by default joints, the robot works. However we need to mimic two joint. Maybe one of you has a solution to mimic two joints, without using the mimic joint as described in the urdf file. I think that I made a mistake somewhere in the setup assistent of moveit, but I seem not be able to see the fault. Who can help me to solve this.

I am using ubuntu 14.04 LTS rosdistro Indigo and rosversion 1.11.13 using on a intel core I7 with 8 GB and 64-bits

Marcel