Error implementing ros timer

edit

My goal was to make a Time Stamp on each message published from the commlink node and subscribe them to my pd_controller. I need the Time Stamps to apply numerical methods. In this case is a simple differentiation. But as I go further into the book there are lots of cool methods.

The problem is that my code compiles, but it does not run. I probably did something wrong, and I could not find a tutorial doing something similar with ROS in order to understand what I made.

So, how do I write my own controller? What are the best practices? How do I identify and fix my code?

This my pd_controller node

   #include <ros/ros.h>
#include <geometry_msgs/Twist.h>
#include <std_msgs/Float32.h>
#include <std_msgs/Float64.h>
#include <std_msgs/Int32.h>
#include "surp/Int32Stamped.h"
#include <math.h>

surp::Int32Stamped encoder;

ros::Subscriber encoder_sub;
ros::Publisher vel_pub;
geometry_msgs::Twist vel;       

// subscriber from comm encoder
void encoderCallback(const surp::Int32Stamped::ConstPtr& tk){

//Modify this equation to be the Controller Function
     double timelapse = double(encoder.header.stamp) - double(encoder.header.stamp);
   double kd = (double(encoder.data) - double(encoder.data))/timelapse;
   vel.linear.x = float(((encoder.data*234)*5886) + kd);
   encoder.data = tk->data;
   encoder.header.stamp = tk->header; 
   vel_pub.publish(vel);
}


//make publisher for cmd_vel

//create a rotary encoder to send cmd_vel

int main(int argc, char** argv){
   ros::init(argc, argv, "pd_controller");
   ros::NodeHandle nh;
   vel_pub = nh.advertise<geometry_msgs::Twist>("cmd_vel", 1);
   encoder_sub = nh.subscribe<std_msgs::Int32> ("comm_encoder", 10, &encoderCallback); 


   ros::spin();

   return 0;
}

I get this error:

/home/rmb/gopigo_ws/src/pd_controller/src/pd_controller.cpp: In function ‘void encoderCallback(const ConstPtr&)’:
/home/rmb/gopigo_ws/src/pd_controller/src/pd_controller.cpp:21:61: error: no match for ‘operator/’ (operand types are ‘double’ and ‘ros::Duration’)
    double kd = (double(encoder.data) - double(encoder.data))/timelapse;
                                                             ^
/home/rmb/gopigo_ws/src/pd_controller/src/pd_controller.cpp:24:25: error: no match for ‘operator=’ (operand types are ‘std_msgs::Header_<std::allocator<void> >::_stamp_type {aka ros::Time}’ and ‘const _header_type {aka const std_msgs::Header_<std::allocator<void> >}’)
    encoder.header.stamp = tk->header; 
                         ^
In file included from /opt/ros/kinetic/include/ros/ros.h:38:0,
                 from /home/rmb/gopigo_ws/src/pd_controller/src/pd_controller.cpp:1:
/opt/ros/kinetic/include/ros/time.h:176:22: note: candidate: ros::Time& ros::Time::operator=(const ros::Time&)
   class ROSTIME_DECL Time : public TimeBase<Time, Duration>
                      ^
/opt/ros/kinetic/include/ros/time.h:176:22: note:   no known conversion for argument 1 from ‘const _header_type {aka const std_msgs::Header_<std::allocator<void> >}’ to ‘const ros::Time&’
pd_controller/CMakeFiles/pd_controller.dir/build.make:62: recipe for target 'pd_controller/CMakeFiles/pd_controller.dir/src/pd_controller.cpp.o' failed
make[2]: *** [pd_controller/CMakeFiles/pd_controller.dir/src/pd_controller.cpp.o] Error 1
CMakeFiles/Makefile2:784: recipe for target 'pd_controller/CMakeFiles/pd_controller.dir/all' failed
make[1]: *** [pd_controller/CMakeFiles/pd_controller.dir/all] Error 2
Makefile:138: recipe for target 'all' failed
make: *** [all] Error 2
Invoking "make -j4 -l4" failed

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