User Manual

9 Tuning (optimization)

9.1 Autotuning (self-optimization)

Oscillation

method

Autotuning (self-optimization, SO) establishes the optimum controller parameters for a

PID or PI controller.

Depending on the controller type, the following controller parameters can be defined:

Reset time (rt), derivative time (dt), proportional band (Pb), cycle time (Cy), filter time

constant (dF)

The controller selects one of two procedures (a or b), depending on the size of the

control deviation:

Step response

method

This type of optimization involves determining the control parameters through an

output step that is applied to the process. First a standby output is produced until the

process value is “steady” (constant). Afterwards, an output step (step size), which can

be defined by the user, is automatically applied to the process. The resulting response

of the process value is used to calculate the control parameters.

Autotuning establishes the optimum control parameters for a PID or PI controller,

according to the selected control structure.

Depending on the controller type, the following control parameters can be determined:

Reset time (rt), derivative time (dt), proportional band (Pb), cycle time (Cy), filter time

constant (dF)

Autotuning can be started from any system status, and can be repeated as often as is

required.

The controller outputs (continuous, relay, solid-state), the controller standby output

and the step size (min. 10%) have to be defined.

Principal applications of the step response method

- Autotuning instantly after “power on”, during the approach phase

Considerable time savings, setting: controller standby output = 0 %.

- The process does not readily permit oscillations (e.g. highly insulated furnaces with

small losses, long oscillation period)

- Process value must not exceed setpoint

If the output (with stabilized setpoint) is known, overshoot can be avoided through

the following adjustment:

standby output + step size <= output in stabilized condition

a) SO in the approach phase

b) SO at setpoint

Start of SO