Publish servo messages on the serial for control of rc car with a arduino

edit

I am trying to subscribe to messages which is being published on a topic (/ecu ) which has the steering and the servo input. I am getting an error [WARN] [WallTime: 1469219387.121113] Serial Port read returned short (expected 8 bytes, received 4 instead). [WARN] [WallTime: 1469219387.121657] Serial Port read failure: my code for publishing to the topic /ecu is :

import rospy
from barc.msg import ECU

def main_auto():
    # initialize ROS node
    rospy.init_node('auto_mode', anonymous=True)
    nh = rospy.Publisher('ecu', ECU, queue_size = 10)
    # set node rate
    rateHz  = 50
    dt      = 1.0 / rateHz
    rate    = rospy.Rate(rateHz)
    t_i     = 0


    # main loop
    while not rospy.is_shutdown():
        # get steering wheel command
        servoCMD = 96.0
        motorCMD = 92.0

    # send command signal
        ecu_cmd = ECU(motorCMD, servoCMD)
        nh.publish(ecu_cmd)

        # wait
        t_i += 1
        rate.sleep()

#############################################################
if __name__ == '__main__':
    try:

        main_auto()
    except rospy.ROSInterruptException:
        pass

the arduino code i have used it from the open source BARC-project

/* ---------------------------------------------------------------------------
# Licensing Information: You are free to use or extend these projects for
# education or reserach purposes provided that (1) you retain this notice
# and (2) you provide clear attribution to UC Berkeley, including a link
# to http://barc-project.com
#
# Attibution Information: The barc project ROS code-base was developed
# at UC Berkeley in the Model Predictive Control (MPC) lab by Jon Gonzales
# (jon.gonzales@berkeley.edu)  Development of the web-server app Dator was
# based on an open source project by Bruce Wootton, with contributions from
# Kiet Lam (kiet.lam@berkeley.edu). The RC Input code was based on sample code
# from http://rcarduino.blogspot.com/2012/04/how-to-read-multiple-rc-channels-draft.html
# --------------------------------------------------------------------------- */


/* ---------------------------------------------------------------------------
WARNING:
* Be sure to have all ultrasound sensors plugged in, otherwise the pins may get stuck in
  some float voltage
# --------------------------------------------------------------------------- */

// include libraries
#include <ros.h>
#include <barc/Ultrasound.h>
#include <barc/Encoder.h>
#include <barc/ECU.h>
#include <Servo.h>
#include "Maxbotix.h"
#include <EnableInterrupt.h>

/**************************************************************************
CAR CLASS DEFINITION (would like to refactor into car.cpp and car.h but can't figure out arduino build process so far)
**************************************************************************/
class Car {
  public:
    void initEncoders();
    void initRCInput();
    void initActuators();
    void armActuators();
    void writeToActuators(const barc::ECU& ecu);
    // Used for copying variables shared with interrupts to avoid read/write
    // conflicts later
    void readAndCopyInputs();
    // Getters
    uint8_t getRCThrottle();
    uint8_t getRCSteering();
    int getEncoderFL();
    int getEncoderFR();
    int getEncoderBL();
    int getEncoderBR();
    // Interrupt service routines
    void incFR();
    void incFL();
    void incBR();
    void incBL();
    void calcThrottle();
    void calcSteering();
  private:
    // Pin assignments
    const int ENC_FL_PIN = 2;
    const int ENC_FR_PIN = 3;
    const int ENC_BL_PIN = 5;
    const int ENC_BR_PIN = 6;
    const int THROTTLE_PIN = 7;
    const int STEERING_PIN = 8;
    const int MOTOR_PIN = 10;
    const int SERVO_PIN= 11;

    // Car properties
    // unclear what this is for
    const int noAction = 0;

    // Motor limits
    // TODO are these the real limits?
    const int MOTOR_MAX = 120;
    const int MOTOR_MIN = 40;
    const int MOTOR_NEUTRAL = 90;
    // Optional: smaller values for testing safety
    /* const int MOTOR_MAX = 100; */
    /* const int MOTOR_MIN = 75; */

    // Steering limits
    // TODO seems to me that the permissible steering range is really about [58,
    // 120] judging from the sound of the servo pushing beyond a mechanical limit
    // outside that range. The ...