Hi,
[...] the problem is how to synchronous their speed?
1 moving faster and another one slower.
Mean the both motor RPM is different?
Assuming you use wheels of the same diameter on each side, yes.
But even a small difference in RPM would mean that it would be going in (large) circles.
How much is the deviation from a straight line on eg. 10m?
Anyone can give me a solution, how to synchronous both motor speed?
Is it with using feedback encoder? if so, may i know any schematic and how the theory work?
Yes, you need encoders on both wheels.
If you get more pulses from say the left encoder in a given time, you can slow down the left motor an amount like this:
Left motor speed * [Number of pulses right] / [number of pulses left]
Since motor speed won't be a completely linear function of neither voltage, nor PWM duty cycle, you have to do this repetedly (
how often will depend on the number of sgments on your encoder), but the error will be less with each iteration (unless you introduce a step function).
I don't think it would be necessary to involve a PID loop, but that's the next step towards a higher precision.