[ros-users] cmd_vel topic has no attribute linear

[Ken Conley]

For 'def callback(data)', the argument 'data' is just a standard
Python function parameter. You can name it whatever you want. It just
so happens that std_msgs/String has a 'data' field as well.

 - Ken

On Sun, Apr 18, 2010 at 1:08 PM, hitesh dhiman
<hitesh.dhiman.1988 at gmail.com> wrote:
> 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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