Lag compensator to improve my plant Phase Margin

[Chong]

Hi,

I'd like to design a lag compensator to improve the Phase Margin of my plant. For example, the Transfer function (in s-Domain) of my lag compensator is (Ts+1)/(aTs+1). Where T is time constant and a affect the plant attenuation.

[Soroush]

Hi,

You need to design a lead compensator to improve the phase margin. Lag compensation results in improving steady state error.

Anyway, the controller structure for both lead and lag compensators are very alike, except DC gain that may be omitted when designing a lag controller.

As you will implement a digital controller you must Specify the sample rate of measurement and control action. The latter one is related to actuator's bandwidth.

Then, find 3 parameters of the lead controller, using analytical methods, (I mean Gain - Pole - Zero)

Implementing a lead-lag controller in Z-Domain with LabVIEW is not difficult. The key is knowing how to relate Z-transfer funstions to dataflow programming style, e.g. when you have a loop with a shift register or feedback node in it, this shift register equals to Z^-1 (means one unit of delay).

Note: This is a nice problem in control theory. try to draw a block diagram in Simulink and LV. The results are different. because Simulink (and LV simulation module) don't think of block diagrams as dataflow problems, but LV depends on Dataflow.

Hope it Helps.

[Barp]

Chong,

You don't need to use Simulation or Simulink if you have Control Design Toolkit (CDT). You can use CDT to create the Lead or Lag Transfer Function in S-domain and convert this in the z-domain (discrete function) and Simulate it with the Implementation VIs. However, notice that you have to have Control Design 2.1 or later to use the implementation VIs.

This VI would allow you change the constants on-the-fly, however, if you don't need this, you could do the calculation outside the loop. This would make the controller more deterministic.

Hope this help!

[Chong]

Hi Soroush,

Thank you very much for your prompt reply. I'd like to confess that I'm new to NI. But I will learn more what you have mentioned in your reply. If you have any good advice, It will be very appreciated. Thanks again.

Best Regards,

Chong

[Chong]

Hi Barp,

Thank you for your prompt reply and other option of the lag compensator design. I'm new in using labview. It will take sometime for me to familiarise. I will learn more about the Control Design Tool Kit. Thanks again.

Best Regards,

Ivan