Method and arrangement for automatic tuning of a controller

An arrangement for determining a relay parameter usable during acquiring of data for relay-based controller tuning of a controller to control a system, comprising:

• a processor configured to
• start acquiring of controller output data and system output data in response to detecting a steady state of system output while controller output is fed to the system as system input;
• and
• determine, based on controller output constraint and on the controller output data acquired and the system output data acquired, a relay parameter representing an amplitude of a relay signal to be added to steady state controller output to obtain a sum signal to be fed to the system as the system input during the acquiring of the data for relay-based controller tuning,
• wherein a first step relay amplitude (RAMP1) is set as follows:
• RAMP1=f1*ABS(Y_WIN),
• wherein f1 is between 0.08 and 0.12, Y_WIN is a controller output averaged over a moving window having a fixed width, and the relay parameter representing an amplitude of the relay signal is based on the first step relay amplitude (RAMP1),
• wherein a second step relay amplitude (RAMP2) is set as follows:
• RAMP2=MAX(RAMP1,ABS(f2*X_MU*G−1)),
• wherein X is a system output, X_MU is a system output averaged over increasing number of samples, G−1 is an inverse of a gain of the controller, f2 is between 0.08 and 0.12, and the relay parameter representing an amplitude of the relay signal is based on the second step relay amplitude (RAMP2).