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How can I implement a smooth "manual move" with Ros-industrial

asked 2017-10-10 06:58:15 -0500

matEhickey gravatar image

I'm searching how to implement a regular "manual" move with ros-industrial, and probably moveit.
By manual move, I mean that when a user press a button, the robot end-effector start moving in a single axis, and when he release it, the robot stop. (The movement on every industrial robot UI I guess (I only know UR and KUKA))

I currently did that, but in a "not smooth" way, like: when he press a button, and until he release it, I publish once each ms on a topic that I want it to go x cm more on that axis. but dues to accelerations, the robot don't do it smoothly.

I also tried like this: on the press event, I call a service so the robot start to go to a far point on this axis, and until I release the button, I publish each ms on a topic to say that I want it to continue, if it didn't receive this topic in time, the robot stop. Problem here was that I can't really define the "far" point, because it's related to the current position, and on the robot articulations (I think I can't, not sure)

Is there something I'm missing ?
Hope you can help, thank you!

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answered 2017-10-10 08:34:53 -0500

gvdhoorn gravatar image

tl;dr: this is probably not something you should try to use (the standard interfaces of) MoveIt for, as it does much, much more than simple interpolation and that takes time. In addition, MoveIt (in its default configuration) was not designed for this kind of use, which is why you run into the issues you describe.

Typical approach: use any of the kinematics plugins available (KDL, Trac IK, Bio IK) to solve IK at a suitable rate and send those directly to your controller. Alternative: use the velocity control interface (if available on your controller and ROS driver) and use the Jacobian.

First: I think this isn't really a "ROS-Industrial" thing other than industrial robots being involved. It's more a motion planning and robot interfacing issue (I do understand why you tagged it with ros-industrial though).

As for an answer and possible approach: this is certainly possible, but 1) is not something that MoveIt was designed for, 2) will need a little manual work and 3) will require a good interface to the motion controller of the robot you're trying to do this with.

re: not design for: MoveIt has a predominantly pipeline based architecture, which receives a motion request (go from A to B), asks a planning plugin to come up with a joint space path (not a trajectory), checks it for collisions, converts it to a trajectory and then returns the result. There is no feedback here, and no concept of a Cartesian space (at least in how it interacts with OMPL and in the default cases, other than the start and end pose). Additionally, the (perhaps implicit) assumption is that trajectories start at 0-velocities (and accelerations, etc) and end with them. These characteristics make the default interfaces you have to MoveIt rather unsuitable for a jogging (ie: continuous motion in Cartesian space).

(there is something called trajectory replacement, but that is not a MoveIt concept per se, and will not change the way MoveIt deals with planning requests)

re: good interface to your robot: many industrial robots don't actually have an external motion interface that will allow you to do anything like this. The two brands you mention (KUKA (RSI: 250 Hz joint space position control) and UR (125 Hz joint space and Cartesian control)) are some exceptions to this, so provided you can come up with the appropriate input to those external motion interfaces, ROS controlled jogging is possible to implement.

re: implementing something like this: there are quite some older questions (and answers) about this: a good one with some background is probably #q74776. Another example is #q245390. And finally, #q69757 (note the response by one of the original authors of MoveIt: "if you already know where you want to go, and you only want to move in a straight line, then the motion planning that MoveIt does is not what you want"). This post on moveit-users discusses a similar question as well.

The typical conclusion of these questions is that ... (more)

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Thanks a lot for this precise answer, indeed I was wrong on my need to use moveit, I only wanted the IK-solver part. Your answer will sure help me! Thank you again !

matEhickey gravatar image matEhickey  ( 2017-10-10 08:48:16 -0500 )edit

Well, I wouldn't conclude you were 'wrong'. It is possible, but not with some manual 'massaging' of MoveIt into a shape where it can do something like this. But it wouldn't be 'standard MoveIt' with the motion-plan request interface as you'd typically use it.

gvdhoorn gravatar image gvdhoorn  ( 2017-10-10 08:51:02 -0500 )edit

I prefer to point new-comers to the RobotState class instead of telling them to use the low-level kinematics_base plugins directly. This provides a better abstraction in my opinion.

v4hn gravatar image v4hn  ( 2017-10-10 08:57:08 -0500 )edit

Also, I would very much welcome a general jogging module as a move_group capability, but (sadly) we don't have something like that at this moment.

v4hn gravatar image v4hn  ( 2017-10-10 08:57:38 -0500 )edit

@v4hn: certainly. Good suggestion.

gvdhoorn gravatar image gvdhoorn  ( 2017-10-10 08:57:52 -0500 )edit

@v4hn: I have it on my list of student-projects. But so far no interest :(

gvdhoorn gravatar image gvdhoorn  ( 2017-10-10 09:02:36 -0500 )edit

same here :-)

v4hn gravatar image v4hn  ( 2017-10-10 10:26:59 -0500 )edit

answered 2018-03-09 14:22:48 -0500

AndyZe gravatar image

updated 2018-03-16 16:06:58 -0500

The jog_arm package does this. It's a Jacobian jogger that can send commands at >125 Hz. Perfect for teleoperation and we're also using it for impedance -- in other words, situations where you need to adjust the trajectory in real time.

I'm not sure the #1 option that @gvdhoorn gave (solve IK) is suitable because the solution could fork at any time, no?

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the solution could fork at any time, no?

I'm not sure I follow? What do you mean by 'fork'?

gvdhoorn gravatar image gvdhoorn  ( 2018-03-09 14:55:54 -0500 )edit

Jump from one solution to a different solution.

AndyZe gravatar image AndyZe  ( 2018-03-09 14:57:20 -0500 )edit

You mean 'configuration'? Different solutions I would expect.

That would depend on your ik solver. Some solvers accept a seed, others can give you 'all' solutions and / or the closest one. That could mitigate that problem significantly. It won't be perfect all the time though.

gvdhoorn gravatar image gvdhoorn  ( 2018-03-09 14:59:08 -0500 )edit

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Asked: 2017-10-10 06:58:15 -0500

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Last updated: Mar 16 '18