The servos of hind legs seems has a counter direction. The picture is that when I type ‘d’. How can I change the direction of the servos.

I saw on another post that you might not be including InstinctNybble.h inside OpenCat.h around line 76. ( so instead your cat is trying to do a Byttle dog move which has those servos reversed ). But I'm a noob. Hope this helps :) ( this post may have been where I read that )

Did you set "no line ending" for the serial monitor?

You can also download the newest code from GitHub. Remember to set the instinct to InstinctNybble in OpenCat.h.

I just noticed that the servo has a semi-transparent case. When did you get the kit? Is the servo MG90D?

When did you download the code? You can try to download our newest code and it may help solve the problem.