[ros-users] cmd_vel topic has no attribute linear

[hitesh dhiman]

Hi Ken,
Thanks for the help.
I just have a small question regarding this example node. The callback
function takes in a 'data' argument. Does this mean that the message being
published has a 'data' attribute? Is it true for all messages?

On Mon, Apr 19, 2010 at 3:55 AM, Ken Conley <kwc at willowgarage.com> wrote:

> 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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-- 
Regards,
Hitesh Dhiman
Electrical Engineering
National University of Singapore
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