||Friction in control valves is one of the main sources of variability in control
loops. This paper analyzes four non-intrusive methods for friction quantification applied
to two real control valves, one with low and the other with high friction. It was used three
different tunings in the PI controller and the experiments were performed with two
distinct set-points, in order to analyze how they affect the friction parameter estimation.
The obtained results were compared to friction coefficients extracted from an intrusive
method. The four methods presented good results in some cases but, on the other hand,
some problems were detected in their application.