Adding fourth axis to delta

Hi all,

I have a delta robot based on linuxcnc. I will use it for pick and place applications. I want to add fourth axis to this delta as a rotary axis. My delta is three axis based but I want to add fourth axis to rotate my objects which objects robot picking and placing.

As I said I want to add this axis for rotate the objects. I want to control this axis 0-360 degrees. How can I add this fourth axis? Or any other suggests?

Thanks in advance...