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PID Autotuning VI and Limitations

Primary Software: LabVIEW Toolkits>>PID and Fuzzy Logic Toolkit
Primary Software Version: 8.6
Primary Software Fixed Version: N/A
Secondary Software: N/A

Problem:
When using the Autotuning Wizard in the PID Control Toolset, the PID gains output from the tuning procedure do not work for my system. What is wrong?

Solution:
The PID Autotuning VI uses the Ziegler-Nichols tuning method. This is a heuristic method that will iterate the PID values until the system has reached a tuned state. The process the PID Autotuning VI uses is modeled by the diagram below.


The autotuning procedure is excited by the setpoint relay which steps the setpoint up or down. This will allow the controller to respond to the change and tune the system for an acceptable response. For a more information about how this VI works, see the Product Manuals: LabVIEW PID Control Toolkit User Manual.

In order to use the PID Autotuning VI you will first need to provide it with the proper information. First set up the autotuning parameters. These are the parameters the VI will use to tune your system.
  • Controller Type — Select what type of controller you are trying to tune. Your options are P, PI, or PID. It is defaulted for PI.
  • Relay Cycles — This tells the setpoint relay experiment how many cycles to use. The more cycles you select, the more precise your results will be, however they will require more time to execute. This will be more pronounced in slower systems.
  • Relay Amplitude — Specifies the step change of the current setpoint up or down. This number will ideally be set so that the each oscillation period takes at least twenty control cycles.
  • Control Specification — This specifies how the systems will respond. Keep in mind that faster respond will result in higher overshoot which may not be desirable. This is defaulted at slow.
  • PV Noise Level — Estimate the noise level that will be in the process variable.
You will now need to provide the normal PID variables, output limits, set point, and process variables. When the PID Autotuning VI runs it will excite your controller forcing it to adapt to the changes in your process variable. Once the autotuning is complete, it will output better PID values if the algorithm was able to find any.

Before you begin autotuning, you must establish a stable open-loop system even if you cannot tune the system on your own. If the system model is too complex or nonlinear the autotune may not work. The algorithm assumes that the controlled system is first-order with a dead-time.

Another problem can be that the control output saturates at the output high and output low limits during autotuning. This adds a nonlinear element to the system and causes incorrect results. To avoid this problem, reduce the relay amplitude parameter and/or reduce the gain Kc of the PID controller before starting the autotuning procedure.

Related Links:
Product Manuals: LabVIEW PID Control Toolkit User Manual
KnowledgeBase 4ETCALGH: LabVIEW PID Toolkit 8.5 Readme
Developer Zone Tutorial: PID Control
Developer Zone Tutorial: PID Theory Explained
Developer Zone Community: PID, Fuzzy Logic and Real-Time Control Seminar

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Report Date: 03/25/2009
Last Updated: 05/29/2009
Document ID: 4VO7GUNC

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