Speed PID loop

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Hello,

So i am trying to program a PID control loop so my motors will reach a certain speed more accurately. How it works is on our shooter, we have two axles with wheels around them that spin and shoot the ball. These axles are spun by 2 motors coupled together on each axle. So I have in total 4 motors that need to be powered, but each pair will be powered the same.

My problem is, is I don't know alot about PID loops or how to make one to control speed. Could someone please explain in depth how to make one and if anyone has any source code they would be willing to share, it would be greatly appreciated.

Thanks
Dimitri