Petersen spent nearly 20 years at Penn State.
Williams is Penn State's first non-Iowa State transfer addition.
After rising to No. 8 during holiday break, a split with RIT drops the Nittany Lions back to No. 9.
Birchmeier played in 26 games over the last two seasons.
Robinson is the fourth former Iowa State wideout to join Matt Campbell in Happy Valley.
Harvey has three years of eligibility remaining.