In this work we present the results of the application, in a novel way, of an automatic proportional-internal-derivative controller to the measurements of the irreversibility line in high temperature superconductors. The system operates by applying to the sample a controlled dc field that maximizes the imaginary (out-of-phase) component of the ac susceptibility, commonly used as a criterion for the determination of such dependencies. Through this technique, for the first time, direct and continous determination of irreversibility lines are possible, allowing the measurement of a whole curve in only one temperature sweep, such in a typical resistivity versus temperature curve. ©1999 American Institute of Physics.