Method and arrangement for automatic tuning of a controller

1. A method of determining a relay parameter usable during acquiring of data for relay-based controller tuning of a controller, usable to control a system, comprising:

starting 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

determining, 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 =f 1*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( f 2 *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).

2. The method of claim 1 , wherein the determining of the relay parameter is further based on a relay hysteresis (HYS), as given by:

HYS= f 4 *X _STD

wherein

f4 is between 4 and 6,

X_STD=sqrt (Var_X),

Var_X is a recursive mean of VX or low pass filtered mean of VX,

VX=Sqr (X−X_WIN), and

X_WIN is windowed mean of system output (X).

3. The method of claim 2 , wherein, the relay signal switches from low to high or from high to low in response to ABS(X−XR) being larger than the relay hysteresis (HYS), wherein XR is a system output reference value.

4. The method of claim 1 , wherein the controller output constraint comprises an upper limit controller output (PHOUT) and a lower limit controller output (PLOUT), wherein a third step relay amplitude (RAMP3) is set as follows:

RAMP3=MAX(RAMP2 ,P RAMP_MIN),

wherein

P RAMP_MIN=5*HYS*MAX( G −1 ,ABS( Y MAX)/ABS( X _ MU )),

HYS is relay hysteresis,

Y MAX= f 3*MIN( Yss−PL OUT, PH OUT− Yss ),

f3 is between 0.85 and 0.95,

Yss is a steady state controller output, and

wherein the relay parameter representing an amplitude of the relay is based on the third step relay amplitude (RAMP3).

5. The method of claim 4 , wherein the relay parameter representing an amplitude of the relay (RAMP) is obtained according to:

RAMP=MIN(RAMP3,ABS( Y MAX)).

6. The method of claim 1 , wherein

steady state of system output is detected,

if X_WIN is between X_UL and X_LL, setting Index=1,

wherein

X _UL= X _ MU +0.5 *X _STD, and

X _LL= X _ MU −0.5 *X _STD,

wherein X is a system output or process output, X_UL is a threshold upper limit and X_LL is a threshold lower limit and X_WIN is a windowed mean of X.

7. The method of claim 6 , further comprising:

detecting a steady-state offset comprising performing:

If ABS(X_WIN-XR) is greater than 2*X_STD,

then setting BOFS=1,

else setting BOFS=0,

wherein

XR is a system output reference value,

BOFS is an offset flag and

X_STD=sqrt (Var_X), Var_X being a recursive mean of VX or low pass filtered mean of VX.

8. The method of claim 7 , further comprising:

setting a new system output reference value (SETPOINT) by performing:

If (Index=1 and BOFS=1),

then set SetPoint=X_MU;

else set SetPoint=XR.

9. The method of claim 1 , further comprising:

determining an inverse gain (G −1 ) of the controller by:

estimating windowed maximum of controller output, Y_MAX

estimating windowed minimum of controller output, Y_MIN

estimating windowed maximum of process output, X_MAX

estimating windowed minimum of process output, X_MIN

setting INVGtemp=(Y_MAX-Y_MIN)/(X_MAX-X_MIN); and

setting G −1 =Recursive mean of INVGtemp.

10. The method of claim 1 , wherein, for performing relay-based controller tuning, controller output data and system output data are acquired, while the sum signal is fed to the system as system input data using relay amplitude as determined previously.

11. The method of claim 1 , wherein, for performing relay-based controller tuning, controller output data and system output data are acquired over several cycles of the relay signal switching from low to high or from high to low.

12. The method of claim 1 , wherein the acquiring of the controller output data and the system output data is stopped, in response to several cycles being performed a maximum number of times or if the controller output is outside a range bounded by PLOUT, PHOUT, wherein PLOUT is lower limit controller output and PHOUT is an upper limit controller output.

13. The method of claim 1 , wherein f1 is 0.1.

14. The method of claim 2 , wherein f4 is 5.

15. 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 =f 1*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( f 2 *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).