How to Program a Servo

Technical C/C++
1

Okay I am not sure how to program the servo, I have a servo plugged into the digital sidecar on PWM #3, and I am not sure how to program it moving so this is the code that we have, so could you tell us our problem?


#include "WPILib.h"
#include "xmath.h"

class RobotDemo : public SimpleRobot
{
   RobotDrive myRobot;
   Joystick leftstick;
   Joystick rightstick;
   Servo servo;
public:
   RobotDemo(void):
      myRobot(1, 2, 3),
      leftstick(1),
      rightstick(2),
      servo(3)
   {
       GetWatchdog().SetExpiration(100);
   }
   void Autonomous(void)
   {
       GetWatchdog().SetEnabled(false);
       //CODE
    }
void OperationControl(void)
{
   Servo(3);
   GetWatchdog().SetEnabled(true);
   while(IsOperatorControl())
   {
      servo.Set(170.0);
      GetWatchdog().Feed();
   }
}
};

So any suggestions?

2

I think your major problem is that you haven’t read any of the documentation on C++ coding: http://first.wpi.edu/Images/CMS/First/FRC1718Docs.zip There’s a nice packet of information there. Here’s how to fix your program:


void OperationControl(void)
{
   GetWatchdog().SetEnabled(true);
   while(IsOperatorControl())
   {
      servo.Set(1.0);
      GetWatchdog().Feed();
   }
}

The Set method takes a value from 0.0 to 1.0 (full left to full right)

Also, make sure you have a jumper set on the sidecar next to the pwm output for the servo.

3

The Set method takes a value from 0.0 to 1.0 (full left to full right)

Not on a servo (last i checked it is 0-170) though I don’t know about the team update 3

4

You’re getting your functions confused.
servo.Set() accepts a range from -1 to 1 for min to max values of the servo - this is here so you can just directly accept another float value from something else (like a joystick) and just set it directly.
servo.SetAngle() accepts an angle that you wish to set your servo at. Try using this with your 170 angle and see if that works.

5

it is 0 to 1 not -1 to 1

6

Oh, right. Most of the other functions are -1 to 1, didn’t look at the docs.
If you need to scale (-1, 1) to (0, 1) you can use
prevVal = joystick(1)->Get(); //-1 to 1 value
ScaledVal = (prevVal + 1)/2; //Scaled to 0 to 1

7