I'm trying to understand PIDs and there is just one thing in the general equation that I can't figure out, that is what does the Tau in the integration mean? in the pdf equation 10.1 there is ki * integral[ e(tau) dtau ] but it doesn't say that it means by tau
http://www.cds.caltech.edu/~murray/books/AM08/pdf/am06-pid_16Sep06.pdf