CNC Machine displacement error

Hi, the cnc machine i built does not move exactly the distance it's commanded to move.
It's XYZA, 4 axis cnc machine (milling machine config) , and the issue is the Y axis.
I'm using AC servo motors with 5:1 speed gear, controlled in position mode.
the controller board is LPT-CNC-H parllel board ( AL-ZARD, 2013) , which is very cheap, only 14 US$.
Linuxcnc version is 2.7.12

The puzzle is, under python interface,Y axis moves to the exact location, whereas in gcode auto-run mode,
it does not.

and, straight line movement commands tend to have different error scale with arc movement commands,
also different error scales for forward movement and for backward movement.

to solve the issue, I performed many tests to find the correct offsets for straight line movement and for arc movement respectively,

I wonder if someone could hint me about the underlying reason that's causing the issue,
so that i can solve the issue once and for all.

Also, I'm planning to upgrade my controller board, is there any recommendation for a parellel controller board, 4 axis. ?

the error scale is different for different arc-radius,
i'm guessing acceleration-deceleration is a factor to the error, but I don't know how to cope with this error.

so, for example, relay the signal from the parellel port via a ULN2803APG with a 5V power supply to the motor driver, let me have a try =)

by checking the test data, the error scale is very consistent in each different case,
thus i suppose the error is due to physical intrinsic attributes of the machine, instead of the controller board.
a photo of the machine is attached. the machine has two layers, the bottom layer has a sponge covered upon it.

right now the upper layer and bottom layer share the same motor scale config although the bottom layer has radius larger than the upper layer due to the sponge. During operation, the two layers squeeze together and move coordinatedly.

the layers are not equal in Thickness as the Sponche can squeeze
youmay consider 2 ROT encoders to the moving Band to get a feedback of movement

Do I guess correct that your axis with the green "belt" is the one that gives you an error?
If you do not measure the real surface, I'm confinced that you will never get it precise.
One effect/reason is called trum ( in German... No chance to translate to English as word always replaced by a.....) other are thickness error, slipping, tention changes...

The only way would be some kind of roller, but that will have a slight slop or you go the marking way. Means you identify position on belt by either magnetic or optic strip.

I did a lot in that Metier when industrialized the digitran digital printing process. ( nope, not Kammann; they only bought the rest after we got broke)
Mike

i believe you made a great point,
the motor spin speed should always be the same for both the upper and the bottom layer.

That's probably why I'm having a error when ploting arc/circles using G02, G03 commands. I set the bottom layer Y axis, and the upper layer B axis, probably, G02/G03 only coordinates Y axis with X axis, thus B axis moves independently.

So, if there is a simple way to make G02 commands treat B-axis the same as Y-axis.