Electrical – Reading 2 Quadrature encoders using a single Arduino Due

Your while loop logic is incompatible with the conditions inside it.

Your while loop is saying:

as long as BOTH wheel encoders read less than clicks, execute the code inside

and your if statements are saying:

if any of the wheel encoders is greater or equal to clicks, stop the corresponding motor

This is generally false except for the rare condition where one of the encoders happen to increment between the arduino evaluating the while and the if statements. In order words just about never.

So the first thing you need to do is fix your logic. Then you need to investigate more sophisticated means of keeping the wheels in sync, such as computing an error term from the different in ticks, and employing a PID controller algorithm to adjust the speed of both motors via analogWrite.

It looks to me that your control loop is essentially:

if speed is too slow:
    increase PWM duty cycle one unit
if speed is too fast:
    descreate PWM duty cycle one unit

As you have observed, this doesn't work so well. Your control loop cyclically overshoots the target speed.

The canonical solution to this sort of problem is a PID controller. The concept is essentially the same, measure a thing and compare it to a target to calculate an error. Then adjust something (in your case, PWM duty cycle) based on the error.

However, a PID controller has three error terms:

• P: the current error
• I: the integral of the error
• D: the derivative of the error

For each of these terms, the controller has programmed some gain. It then multiplies each term by the respective gain to calculate how much the control input (PWM duty cycle) should be changed. Properly tuned, this allows you to get a feedback loop that at first ramps up the duty cycle, then as your vehicle approaches the target speed, backs off smoothly so you get a minimum of overshoot.