Online Autotuning Techniques

To tune PID parameters of a system, some PID VIs allow you to select the technique to apply to the system. You can choose from two techniques that implement a step response-based measurement and two techniques that implement a frequency response-based measurement.

On the PID Advanced Autotuning VI, the autotuning parameters input allows you to configure the technique to use. With the PID Online Autotuning polymorphic VI, select the polymorphic instance that corresponds to a tuning technique you want to use. Other PID VIs control the tuning technique for you.

Step Response Techniques

The following techniques are based on step response measurement:

• Step Open Loop�Performs an open-loop step test. This technique applies a step to the input and waits until it reaches steady-state, when the process variable does not change. This technique assumes that you can model any process as a first-order lag and a pure deadtime system.
• Step Closed Loop�Performs a closed-loop step test similar to the Step Open Loop technique.

If you do not know the characteristics of the model, such as initial PID gains, you can first perform Step Open Loop tuning on the model to obtain the parameters. Wait until the input reaches steady-state. This process is also known as commissioning. Next, perform Step Closed Loop tuning on the model using the parameters computed by the Step Open Loop tuning process as initial parameters. Both techniques assume a first-order model with delay. If your system is not a first-order model, these techniques try to approximate one during tuning.

Frequency Response Techniques

The following techniques are based on frequency response measurement

• PID Relay�Uses the setpoint relay experiment to determine the information needed to tune the controller. The Autotuning Wizard uses this technique.
• Relay Feedback�Uses an on-off relay in a feedback loop to determine ultimate gain and ultimate frequency. This technique is a variation on a closed-loop step test, but is more efficient for systems with a large time constant. This method also incorporates hysteresis to avoid noise in the system.

Related Information

Step Open Loop Autotuning Technique

Step Closed Loop Autotuning Technique

PID Relay Autotuning Technique

Relay Feedback Autotuning Technique