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Rotating the Servo Using PID controller and SONAR

nafis

I have to make a bot which has front wheel steering by one servo motor like real cars..and one driving motor to run the robot..by rotating the servo the bot will turn right and left..there is Two SONAR on the left and right side to detect the distance from left and right wall. By taking the distance from both sides of the wall the robot has to be moved continuously forward at the middle position by steering the front wheel with help of the Servo.
How can I rotate the servo with PID for smooth turning? At 118degree the servo is straight and the robot is entirely forward. that means at 118degree of servo the robot will be completely straight.
How can I code the PID algorithm to rotate the servo by taking the distimageance from two SONAR?image

  • I'd start by tackling the PID loop, once you understand the PID loop you can work to implement it.

    There are some great PID tuning and creation videos on youtube!

    What I did in a similar project was 3 if/while commands, one for when the left side is closer,

    one for roughly center +- sensor error, and another for the right side closer.

    Then based on those statements get your difference and use that as your Porpotinal part of your PID loop. You might want to use the Map(int, int, int, int) command to change the distance from the wall to an appropriate steering angle.

    I don't have my code anymore unfortunately otherwise I'd send it over to you! I hope this helped a little bit.

  • What makes you think that PID is the best way to control the servo in this applicaton ?

    PID control doesn't care about offset (that's what the I term is for).

    Any real PID control system needs to be initialised so you could preset the integral term then.

    Alternatively you could add the offset to the output of the PID controller.

    MK

  • in reply to michaelkellett

    i think if i use PID then it it will be more smooth

  • in reply to WumblingBorkeys

    thanks for your reply...can you please give the code?

  • Sounds like a school homework project. The servo is a PID. Tell it angle by the PWM pulse width, it regulates it.

    Not sure what 118 degrees is reference to, is the servo 0-180 degrees, with 90 degrees being your steering zero point ?  If so, the servo horn can be popped off while the controller tells it to be at 90, to set the zero closer to the servo 90 degree mark as possible. 

    Ping left wall & ping right wall - the difference is your error signal into a PID - IF the goal is to track the middle between them. 

    The "D" part of PID is not needed because nothing here is high inertia.   The Integral part  can be thought as "memory" , again, once a steering correction has been made, the wheels go straight and the car stays on the angled path anyway until another steering correction is made.

    So down to the "P" part depends on the steering rod linkage adjustment. Does, for example, the full servo 90 degrees in both directions make the tires turn 90 degrees, 45 degrees, or maybe even 30 degrees ? 

    The desired angle output needs then converted to a PWM width , where 1.5 msec is straight, 1 msec is full left (or right) , 2 msec is full right (left), the proportional gain is a scalar adjusted based on the desired corrections. 

     

  • in reply to nafis

    The Arduino IDE has example code for servos and the ping sensors, also a PID library.