{{cite book |title=Introduction to cybernetics |chapter=Chapter 12: The error-controlled regulator |author=W. Ross Ashby |url=http://pcp.vub.ac.be/books/IntroCyb.pdf |pages=219–243 |year=1957 |publisher=Chapman & Hall Ltd.; Internet (1999)}}
</ref>
<ref name=Bequette>
{{cite book |author=B. Wayne Bequette | title = Process Control: Modeling, Design, and Simulation | publisher =Prentice Hall Professional | year= 2003 | page = 5 |url=https://books.google.com/books?id=PdjHYm5e9d4C&pg=PA5 |isbn = 9780133536409 }}