Well it is hard to get and set up but all you need is a Gyro in your hand.
I am not the best at detail so please try to understand.
You have to be falling down on top of them more towards the front of them, then you Throw the Gyro down with Z, then instantly catch the Gyro with Z and hold, because you hold Z, it drops it again. This TECHNICALLY Throws the Gyro twice, hitting your opponent two times within a few milliseconds, and you should land as soon as you Z-Catch->Drop the Gyro and as soon as you land you can do MANY things to add to the combo.
Alone, the two drops should hit 10% or 11%. The two things I like doing, and work all the time are:
1.) Instantly F-Smash, this is the 30%.
2.) Instantly run forward and grab them. They will never see it coming and not know what to do, so you will get the grab, I have done it to my brothers mid match, who are better than me, and it works. Trust me. You get the grab, D-Throw and then go for an U-Air. Assuming you do it correctly you should get around 40%. It of course isn't guaranteed if someone of the moves are SDI'd correctly, but then again, you have to realize it is simply a mix-up and R.O.B really doesn't have any TRUE guaranteed combos.
The closest thing R.O.B has to a combo is D-Throwing at a low % and reading where they will go and getting an U-Air. That and his D-Tilt->Grab/etc.
