I have l293d motor sheld. Arduino uno,Mpu6050, and 2 DC motor.

I am making a self Balancing robot, i already have codes but didn't work it has some error because my robot after few seconds of trying to balance itself it start moving forward or backward and didn't stop. I don't understand what is happening i alter kp,ki,kd in my codes but still this happens what is problem? If anyone answer or help me to correct this error that will be very help ful for me thank you. Codes are Bellow..... 

#include <SoftwareSerial.h>   //Include SoftwareSerial for bluetooth connection

#include <AFMotor.h>    //You will need to download and install the adafruit motor shield library. The information you need will be on adafruit's website

#include <Wire.h>   //Include Wire for connection with the mpu6050

AF_DCMotor motor1(1,MOTOR12_8KHZ);    //Initialize motor variables for the two motors connected to the shield 
AF_DCMotor motor2(2,MOTOR12_8KHZ);




int gyro_address = 0x68;    //This is the mpu6050's address needed to access the module                                   
int acc_calibration_value = -6928;    //This is the balancing robot's balance point, use the other program to find it.                        

//Various settings
float pid_p_gain = 12.8;    //Pid gain values  THESE CHANGE FOR EACH ROBOT!                           
float pid_i_gain = .028;                                     
float pid_d_gain = 65;  
                                   
float turning_speed = 30;   //These are constants determining the driving and turing speed                                 
float max_target_speed = 100;                                


byte start;   //Tells the robot to start balancing

int receive_counter;    //This is used for calibrating the gyro

int gyro_pitch_data_raw, gyro_yaw_data_raw, accelerometer_data_raw;   //Gyro data variables

long gyro_yaw_calibration_value, gyro_pitch_calibration_value;    //Gyro calibration data variables

unsigned long loop_timer;   //Times the loop

float angle_gyro, angle_acc, angle, self_balance_pid_setpoint;    //More Gyro data variables
float pid_error_temp, pid_i_mem, pid_setpoint, gyro_input, pid_output, pid_last_d_error;    //Pid variables
float pid_output_left, pid_output_right;    //Pid output for each motor


  
  Wire.beginTransmission(gyro_address);                                     //Start communication with the address found during search.
  Wire.write(0x6B);                                                         //We want to write to the PWR_MGMT_1 register (6B hex)
  Wire.write(0x00);                                                         //Set the register bits as 00000000 to activate the gyro
  Wire.endTransmission();                                                   //End the transmission with the gyro.
  //Set the full scale of the gyro to +/- 250 degrees per second
  Wire.beginTransmission(gyro_address);                                     //Start communication with the address found during search.
  Wire.write(0x1B);                                                         //We want to write to the GYRO_CONFIG register (1B hex)
  Wire.write(0x00);                                                         //Set the register bits as 00000000 (250dps full scale)
  Wire.endTransmission();                                                   //End the transmission with the gyro
  //Set the full scale of the accelerometer to +/- 4g.
  Wire.beginTransmission(gyro_address);                                     //Start communication with the address found during search.
  Wire.write(0x1C);                                                         //We want to write to the ACCEL_CONFIG register (1A hex)
  Wire.write(0x08);                                                         //Set the register bits as 00001000 (+/- 4g full scale range)
  Wire.endTransmission();                                                   //End the transmission with the gyro
  //Set some filtering to improve the raw data.
  Wire.beginTransmission(gyro_address);                                     //Start communication with the address found during search
  Wire.write(0x1A);                                                         //We want to write to the CONFIG register (1A hex)
  Wire.write(0x03);                                                         //Set the register bits as 00000011 (Set Digital Low Pass Filter to ~43Hz)
  Wire.endTransmission();                                                   //End the transmission with the gyro 

  pinMode(13, OUTPUT);                                                      //Set onboard led to output
 
  
  
    motor1.setSpeed(0);                                                     //Make sure the motors dont turn until calibration is complete
    motor2.setSpeed(0);    

  for(receive_counter = 0; receive_counter < 500; receive_counter++){       //Create 500 loops
    if(receive_counter % 15 == 0)digitalWrite(13, !digitalRead(13));        //Change the state of the LED every 15 loops to make the LED blink fast
    Wire.beginTransmission(gyro_address);                                   //Start communication with the gyro
    Wire.write(0x43);                                                       //Start reading the Who_am_I register 75h
    Wire.endTransmission();                                                 //End the transmission
    Wire.requestFrom(gyro_address, 4);                                      //Request 2 bytes from the gyro
    gyro_yaw_calibration_value += Wire.read()<<8|Wire.read();               //Combine the two bytes to make one integer
    gyro_pitch_calibration_value += Wire.read()<<8|Wire.read();             //Combine the two bytes to make one integer
    delayMicroseconds(3700);                                                //Wait for 3700 microseconds to simulate the main program loop time
  }
    gyro_pitch_calibration_value /= 500;                                    //Divide the total value by 500 to get the avarage gyro offset
    gyro_yaw_calibration_value /= 500;                                      //Divide the total value by 500 to get the avarage gyro offset

    loop_timer = micros() + 4000;                                           //Set the loop_timer variable at the next end loop time    
     
    Serial.println("GO");                                                   //Just for debugging to see if it is working, you can take this out          
                                                      
  }


  
  ///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
  //Angle calculations
  ///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
  Wire.beginTransmission(gyro_address);                                     //Start communication with the gyro
  Wire.write(0x3F);                                                         //Start reading at register 3F
  Wire.endTransmission();                                                   //End the transmission
  Wire.requestFrom(gyro_address, 2);                                        //Request 2 bytes from the gyro
  accelerometer_data_raw = Wire.read()<<8|Wire.read();                      //Combine the two bytes to make one integer
  accelerometer_data_raw += acc_calibration_value;                          //Add the accelerometer calibration value
  if(accelerometer_data_raw > 8200)accelerometer_data_raw = 8200;           //Prevent division by zero by limiting the acc data to +/-8200;
  if(accelerometer_data_raw < -8200)accelerometer_data_raw = -8200;         //Prevent division by zero by limiting the acc data to +/-8200;

  angle_acc = asin((float)accelerometer_data_raw/8200.0)* 57.296;           //Calculate the current angle according to the accelerometer

  if(start == 0 && angle_acc > -0.5&& angle_acc < 0.5){                     //If the accelerometer angle is almost 0
    angle_gyro = angle_acc;                                                 //Load the accelerometer angle in the angle_gyro variable
    start = 1;                                                              //Set the start variable to start the PID controller
  }
  
  Wire.beginTransmission(gyro_address);                                     //Start communication with the gyro
  Wire.write(0x43);                                                         //Start reading at register 43
  Wire.endTransmission();                                                   //End the transmission
  Wire.requestFrom(gyro_address, 4);                                        //Request 4 bytes from the gyro
  gyro_yaw_data_raw = Wire.read()<<8|Wire.read();                           //Combine the two bytes to make one integer
  gyro_pitch_data_raw = Wire.read()<<8|Wire.read();                         //Combine the two bytes to make one integer
  
  gyro_pitch_data_raw -= gyro_pitch_calibration_value;                      //Add the gyro calibration value
  angle_gyro += gyro_pitch_data_raw * 0.000031;                             //Calculate the traveled during this loop angle and add this to the angle_gyro variable
  
  ///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
  //MPU-6050 offset compensation
  ///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
  //Not every gyro is mounted 100% level with the axis of the robot. This can be cause by misalignments during manufacturing of the breakout board. 
  //As a result the robot will not rotate at the exact same spot and start to make larger and larger circles.
  //To compensate for this behavior a VERY SMALL angle compensation is needed when the robot is rotating.
  //Try 0.0000003 or -0.0000003 first to see if there is any improvement.

  gyro_yaw_data_raw -= gyro_yaw_calibration_value;                          //Add the gyro calibration value
  //Uncomment the following line to make the compensation active
  angle_gyro -= gyro_yaw_data_raw * 0.0000003;                            //Compensate the gyro offset when the robot is rotating

  angle_gyro = angle_gyro * 0.9996 + angle_acc * 0.0004;                    //Correct the drift of the gyro angle with the accelerometer angle

  ///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
  //PID controller calculations
  ///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
  //The balancing robot is angle driven. First the difference between the desired angel (setpoint) and actual angle (process value)
  //is calculated. The self_balance_pid_setpoint variable is automatically changed to make sure that the robot stays balanced all the time.
  //The (pid_setpoint - pid_output * 0.015) part functions as a brake function.
  pid_error_temp = angle_gyro - self_balance_pid_setpoint - pid_setpoint;
  if(pid_output > 10 || pid_output < -10)pid_error_temp += pid_output * 0.015 ;

  pid_i_mem += pid_i_gain * pid_error_temp;                                 //Calculate the I-controller value and add it to the pid_i_mem variable
 if(pid_i_mem > 400)pid_i_mem = 400;                                       //Limit the I-controller to the maximum controller output
 else if(pid_i_mem < -400)pid_i_mem = -400;
  //Calculate the PID output value
  pid_output = pid_p_gain * pid_error_temp + pid_i_mem + pid_d_gain * (pid_error_temp - pid_last_d_error);
  if(pid_output > 400)pid_output = 400;                                     //Limit the PI-controller to the maximum controller output
  else if(pid_output < -400)pid_output = -400;

  pid_last_d_error = pid_error_temp;                                        //Store the error for the next loop

  if(pid_output < 5 && pid_output > -5)pid_output = 0;                      //Create a dead-band to stop the motors when the robot is balanced

  if(angle_gyro > 30 || angle_gyro < -30 || start == 0 ){    //If the robot tips over or the start variable is zero or the battery is empty
    pid_output = 0;                                                         //Set the PID controller output to 0 so the motors stop moving
    pid_i_mem = 0;                                                          //Reset the I-controller memory
    start = 0;                                                              //Set the start variable to 0
    self_balance_pid_setpoint = 0;                                          //Reset the self_balance_pid_setpoint variable
  }

  ///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
  //Control calculations
  ///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
 
     pid_output_left = pid_output;                                             //Copy the controller output to the pid_output_left variable for the left motor
  pid_output_right = pid_output; 
                                                                            //Copy the controller output to the pid_output_right variable for the right motor

  
  
  //The self balancing point is adjusted when there is not forward or backwards movement from the transmitter. This way the robot will always find it's balancing point
  if(pid_setpoint == 0){                                                    //If the setpoint is zero degrees
    if(pid_output < 0)self_balance_pid_setpoint += 0.0015;                  //Increase the self_balance_pid_setpoint if the robot is still moving forewards
    if(pid_output > 0)self_balance_pid_setpoint -= 0.0015;                  //Decrease the self_balance_pid_setpoint if the robot is still moving backwards
  }

  


    motor2.setSpeed(abs(pid_output_left));    //Sets the motor speed to the pid value
    motor1.setSpeed(abs(pid_output_right));
    
    if(pid_output_left < 0){                  //If the pid value is negative the motor need to go backwards
       motor2.run(FORWARD);
   
    }
    else if(pid_output_left > 0){             //If the pid value is positive the motor needs to go forward
       motor2.run(BACKWARD);
       
    }
    else if(pid_output_left == 0){            //If the pid value is 0 the motor can stop moving
      motor2.run(RELEASE);
  
    }
     if(pid_output_right < 0){
     
        motor1.run(FORWARD);
    }
    else if(pid_output_right > 0){
     
        motor1.run(BACKWARD);
    }
    else if(pid_output_right == 0){
    
      motor1.run(RELEASE);
    }
}