Synchronizing messages without time headers

asked 2013-04-12 06:38:13 -0500

whiterose
148 ●11 ●15 ●19

updated 2014-01-28 17:16:09 -0500

ngrennan
1 ●1 ●1

Hi all,

I am trying to synchronize subscription from a few topics from custom messages.

armj_cmd_pos_sub = message_filters.Subscriber("/arm_controller/position_command", JointPositions) armj_cmd_vel_sub = message_filters.Subscriber("/arm_controller/velocity_command", JointVelocities) armj_cmd_eff_sub = message_filters.Subscriber("/arm_controller/effort_command", JointTorques) armj_states_sub = message_filters.Subscriber("/joint_states", JointState)

It gave me

AttributeError: 'JointPositions' object has no attribute 'header'

I am suspecting the /arm_controller/position_command topic.

The type of the /arm_controller/position_command topic is brics_actuator/JointPositions and the content is

Poison poisonStamp JointValue[] positions

and the content of JointValue is

time timeStamp #time of the data

string joint_uri

string unit #if empy expects si units, you can use boost::unit

float64 value

my question is how do I get rid of the attribute error? How to synchronize topics when their message don't have headers? Is there a way to add headers without meddling with the message files?

Thanks in advance.

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answered 2013-04-12 07:12:20 -0500

dornhege

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Without extra work, you can't. Simply because the filter needs to know what to synchronize.

The main problem here is that the message you pointed out doesn't use a Header, but a custom stamp. At least there is one, so probably the easiest method is to use the filters manually. I haven't checked the API, but I'd try to not directly use the subscriber as an input, but instead put your own "FixHeaderForMyCustomMsg"-Filter in between that just constructs a data type that contains your message + a header (that you fill from the stamp). Given pythons typing, you should be able to just pass that into the TimeSynchronizer.

Meddling with built-in messages probably won't work as the types are defined using __slots__ (probably for efficiency), so you can't change that.

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Does that mean the following steps: (1) Subscribe to the topic and get the message (2) republish the message with another topic with a message with header (3) And subscribe to the new topic ? Or is there a better way?

whiterose ( 2013-04-14 10:37:08 -0500 )edit

No. There is no need to republish. Basically message_filters are filters, so just take the message and put it in something that has a header. That something you should be able to put into the message filter.

dornhege ( 2013-04-14 23:43:51 -0500 )edit

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