Tuning basically involves adjustment of proportional. Integral and derivative parameters to achieve good control. The gain, time constants, and dead times around the loop will dictate the settings of various parameters of the controller.
Tuning methods are broadly classified into two :
- Closed Loop Method : e.g. Ultimate Gain Method.
- Open Loop Method : e.g. process Reaction curve.
Ultimate gain method:
The term ultimate gain was attached to this method because its use require the determination of the ultimate gain (sensitivity) and ultimate period. The ultimate sensitivity Ku is the maximum allowable value of gain (for a controller with only Proportional mode) for which the system is stable. The ultimate period is the period of the response with the gain set at its ultimate value.
Process reaction curve:
- To deter mine the process reaction curve, the following steps are recommended. Let the system come to steady state at the normal load level.
- Place the controller on manual.
- Manually set the output of the controller at the value at which it was operating in the automatic mode.
- Allow the system to reach the steady state.
- With controller on manual, impose a step changes in the output of controller, which is an signal to value.
- Record the response of controlled variable.
- Return the controller output to its previous value and return the controller to auto operation.