[ros-users] cmd_vel topic has no attribute linear

Ken Conley kwc at willowgarage.com
Sun Apr 18 19:55:49 UTC 2010


Hi Hitesh,

Please look at the "Writing the Subscriber Node" section here:

http://www.ros.org/wiki/rospy_tutorials/Tutorials/WritingPublisherSubscriber#Writing_the_Subscriber_Node

Your subscriber has no callback. The callback is what receives the
message instance.

Hope this helps,
Ken

On Sun, Apr 18, 2010 at 12:51 PM, hitesh dhiman
<hitesh.dhiman.1988 at gmail.com> wrote:
> Hi Ken,
> Actually i started out by following the tutorials.
> Here's the code I'm using:
>
> velocity = rospy.Subscriber("cmd_vel", Twist)
>
> I checked using rxgraph, my node is subscribing to cmd_vel. I also used
> rostopic echo /cmd_vel, i can see the velocities being published. Now the
> problem is, how to extract the information from the message.
>
> If i try to type :
>
> print velocity.linear.x
>
> I get an error: velocity has no attribute x
>
> Even if i just do velocity.linear[0], or simply velocity.linear, i get the
> error:
>
> velocity has no attribute linear
>
> The cmd_vel topic has a type Twist message, that means two cpp vectors, each
> for linear and angular velocities. I'm not able to get the correct format to
> access these vectors.
>
> On Mon, Apr 19, 2010 at 3:00 AM, Ken Conley <kwc at willowgarage.com> wrote:
>>
>> Hi Hitesh,
>>
>> If you follow the rospy tutorials for publishing and subscribing [1]
>> you will be able to subscribe to any type of ROS topic, including
>> cmd_vel, and extract the fields that you want.
>>
>> [1] http://www.ros.org/wiki/rospy/Tutorials
>>
>>  - Ken
>>
>> On Sun, Apr 18, 2010 at 11:55 AM, hitesh dhiman
>> <hitesh.dhiman.1988 at gmail.com> wrote:
>> > Hi ken,
>> > Well I think you're correct. If I just do a print vel, it says
>> > subscriber
>> > object at 'address'...Basically what I'm doing is getting the cmd_vel
>> > published from navigation stack.
>> > On this note, is it possible to find out the correct way to get the
>> > linear
>> > and angular velocities from cmd_vel ?
>> >
>> > On Apr 19, 2010 2:34 AM, "Ken Conley" <kwc at willowgarage.com> wrote:
>> >
>> > Hi hitesh,
>> >
>> > It would help if you cut and paste the exact error message and include a
>> > sample program. I suspect that the program is using the subscriber
>> > object
>> > instead of the message object, but this is hard to tell without more
>> > information.
>> >
>> > Ken
>> >
>> >> > On Apr 18, 2010 9:43 AM, "hitesh dhiman"
>> >> > <hitesh.dhiman.1988 at gmail.com>
>> >> > wrote: > > Hi all, > I'...
>> >
>> >
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>
>
> --
> Regards,
> Hitesh Dhiman
> Electrical Engineering
> National University of Singapore
>
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