I was reading the paper "Full-Speed Range Self-Balancing Electric Motorcycles Without the Handlebar" released in March 2016 by Professor Yang and Murakami in Keio University.
I couldn't figure out what the meaning of the following block is :
from the following control system diagram on page 1915 of IEEE Transactions on Industrial Electronics, Vol. 63, No. 3, March 2016.
I would greatly appreciate if someone could explain the meaning of the mentioned block symbol.