Mbot ranger doens't move straight forward


Hi world,

i have a mbot ranger und it works generally. I use the latest mblock5 software und programm the mbot with scratch. So I am happy with my mbot. But htere is one thing that drives me crazy.

When I tell the mbot to drive straight forward it drives to the left slightly. If I set the right wheel to 96% and the left one to 100% it drives straight forward. It seems that the optical encoder motor doesn’t work synchronously. I have bought a second pair but it is very similar. It is not possible to drive staright forward. There is alway a slight left turn.

The encoders are connected to the auriga board.

When I build the ranger the I saw that the rubber chain grinded at the motor a bit. Therefore I also build the raptor. With the Raptor the wheels rotate without grinding. But it is the same.

Have someone an idea what could the reason. Thank you in advance.

Background: I want that the mbot drives straight forward until the ultra sonic sensor detect an obstacle. Then the color sensor should detect the color. If red then turn right, blue turn left, green stop (finish). The programmes is working fine. Only that the mbot doesn’t drive straight forward .


It will depend upon which blocks you use to tell it to drive forward.

Some block give a drive at power , this means it sends the amount of power to the motor and the motor will spin at what ever speed happens to be produced by this amount of power. 2 motors are never identical and will always spin at different speeds for the same power.

If you use a block that doesn’t drive at a set power but rather r/min (revolutions per min) the block will regulate the power through a feed back loop to provide and exact speed and your robot will drive straight because both motors are spinning at same speed :slight_smile:


Thank you OutoftheBOTS for your answer, I will test it the next days.


Sorry for the late reply.

Today I have ckecked driving with revolution per min block. I set both block to 90r/min. But the bot still have a slight left turn. It is very simily to the drive at power blocks. It doesn’t drive straight ahead.

Any idea?
Thank you in advance.



try a slower RPM first


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