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How to publish integers values from a ROS node

asked 2018-03-22 07:18:26 -0500

Marcus Barnet gravatar image

updated 2018-03-22 08:12:18 -0500

Hi to all,

I have a ROS node which publish data coming from encoders and current sensors (the node outputs velocity data, raw encoders ticks and currents). Currently, my ROS node publish all these data as std_msgs/String, but this gives me lot of problems when I need to handle data.

I would like to publish a structured ROS message which reports the robot status; what should I use for encoders ticks, velocity values and currents?

How should I re-write the code in order to implement a correct structured message? I think the best thing to do is to publish a topic for the velocity, another one for the motor current and another for the encoder ticks.

This is my current code, as you can see I declare everything like std_msgs::String.

Thank you for your help.

while (ros::ok())

    std_msgs::String msg_sys, msg_enc, msg_io, msg_vel;
    std:string temp="", volt="", amotor="", enco="", aio="", dio=""; 
    std::stringstream ss, ss2, ss3, ssvel;          

    if (sys == 1){
      device.GetValue(_T, 0, temp);
      device.GetValue(_V, 0, volt);
      device.GetValue(_A, amotor);

      if (temp != "" && volt != "" && amotor != ""){

        std::replace(temp.begin(), temp.end(), ':', ',');
        std::replace(volt.begin(), volt.end(), ':', ',');
        std::replace(amotor.begin(), amotor.end(), ':', ',');

        ros::Time time = ros::Time::now();
        ss << time << "," << temp << "," << volt << "," << amotor << "," << count; = ss.str();


    //Encoder Topic
    if (enc == 1){
      current_time =ros::Time::now().toSec();
      device.GetValue(_C, 0, enco);
       //enco ="15000:16000";

      if (enco != ""){

        if (velocity == 1){
        std::string vel_enco=enco;
        std::replace(vel_enco.begin(), vel_enco.end(), ':', ' ');

        const char * c_enco = vel_enco.c_str();
        char* pEnd;
        double Encoder_sx, Encoder_dx;
        double Distance_sx, Distance_dx, Distance_prev_sx, Distance_prev_dx, Vel_sx, Vel_dx;
        float Vel_linear, w;

        Encoder_sx = std::strtod (c_enco,&pEnd);
        Encoder_dx = std::strtod (pEnd,&pEnd);

        Distance_sx = (Encoder_sx)*sprocket;
        Distance_dx = (Encoder_dx)*sprocket;

        last_time =ros::Time::now().toSec();
        Vel_sx = ((Distance_prev_sx - Distance_sx) / (last_time - current_time))/1000; // m / sec 
        Vel_dx = ((Distance_prev_dx - Distance_dx) / (last_time - current_time))/1000;  

        Distance_prev_sx = Distance_sx;
        Distance_prev_dx = Distance_dx;

        Vel_linear = (Vel_dx + Vel_sx)/2;
        w = (Vel_dx - Vel_sx)/track;

        ros::Time time2 = ros::Time::now();

        //Publish velocity topic
        ssvel << time2 << "," << Distance_sx << "," << Distance_dx << "," << Vel_sx << "," << Vel_dx << "," << Vel_linear << "," << w; = ssvel.str();

        ros::Time time3 = ros::Time::now();

        std::replace(enco.begin(), enco.end(), ':', ',');

        //Publish enco topic
        ss2 << time3 << "," << enco << "," << count; = ss2.str();
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To take this question a bit broader: do you want to stay at the abstraction level of "ticks" and "encoders", or would it make sense to have your node(s) publish more semantically meaningful messages with fields in SI units?

That could be done by the 'next' node btw, but is something to consider.

gvdhoorn gravatar image gvdhoorn  ( 2018-03-22 10:07:31 -0500 )edit

This would be even better, but also the absolute values are OK for me. For example, having something like:

left_ticks: 1234
right_ticks: 2345

works fine for me; what I would like to obtain is to output the values as integers and floats so I can plot them in rqt_plot or use them for other purposes.

Marcus Barnet gravatar image Marcus Barnet  ( 2018-03-22 10:25:11 -0500 )edit

At the moment, the std_msgs::String format is giving me lots of problems when I have to handle the node output.

Marcus Barnet gravatar image Marcus Barnet  ( 2018-03-22 10:26:39 -0500 )edit

1 Answer

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answered 2018-03-22 14:07:59 -0500

Just create your own message file (as detailed in the Wiki).

In your case, something like

time timestamp
int32 left_ticks
int32 right_ticks
float32 left_rotation_radian
float32 right_rotation_radian

should do the trick (I assumed rotations, change for linear motion). The linked wiki page tells you everything you need to know to create, build and use such custom messages.

I agree with @gvdhoorn that a result in SI units is a better idea. If you change something to the hardware, you should only change the driver (or a configuration parameter used by the driver), not all nodes relying on the driver. If you really want to use ticks, add a radian_per_tick (or m_per_tick) field and use that for calculations in other nodes.

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Asked: 2018-03-22 07:18:26 -0500

Seen: 1,495 times

Last updated: Mar 22 '18