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PT1 filter function


  • PT1 filter function

    PT1 filter function
    Hi RMS community,

    I would like to introduce the PT1 function of the RMS.

    Within clean rooms there differential pressure between inside and outside of the clean room area is monitored to keep the polution risk on an acceptable low level. The differential pressure measurement at ranges lower than 50Pa for such applications is very sensitive and shows noise and short time spikes. To keep the control loop of the differential pressure control in a stable process, it is common to apply a PT1 filter on the differential pressure as a process input. The PT1 filter is a low pass filter that dams such noise and spikes. A simple average calculation over the last x measurements is considered as a too strong manipulation of the measurement.

    Measurement values:
    As described it is used for differential pressure but not limited to that measurement value. In the meaning it can be applied to any measurement value.

    Applied for RMS:
    Please click Tools -> Setup -> Measurement Points -> New -> Arithmetic calculration
    Please choose:
    -> Name ...
    -> Type PT1
    -> Measuring pionts
    -> Duration (T - see below)
    -> Measuring point

    Mathematic model behind:
    The PT1 filter function is based on the System Step Response - an answer of a system (system output) to an Heaviside Step Function on a system input (e. g. a 0 -> 1 step function). The mathematic model describes a System Step Response that follows an e-function. Output level(t) = 1-e^(-t/T).

    That means that after a 0 -> 1 step (input),
    the output level is
    • 0.63 after 1 delta t
    • 0.86 after 2 delta t
    • 0.95 after 3 delta t
    • ...

    Formula for RMS:Click image for larger version  Name:	Formula.png Views:	0 Size:	5.8 KB ID:	462
    • MVn -> measurement value at interval n
    • MVn+1 -> measurement value at interval n+1
    • MV_PT1n -> filtered value at interval n
    • MV_PT1n+1 -> filtered value at interval n+1 (next output filtered value)
    • t_interval -> interval time of the based measurement (e. g. raw differential pressure value)
    • T_Duration -> Tau or duration of the virtual PT1 measurement point.
    As higher the T_Duration is set, as stronger the filter function is - that means the outcome value reacts slower.
    After 9 system interval (if T_Duration = t_intervall), the ouput value is approximated to the raw input value. That means the difference is not visible anymore if the displayed value is shown with two digits after the decimal point.

    The next two graphs shows the principle:
    Click image for larger version  Name:	PT1_ideal.png Views:	0 Size:	42.0 KB ID:	466

    Click image for larger version  Name:	PT1_real.png Views:	0 Size:	55.1 KB ID:	464

    • James
      James commented
      Editing a comment
      Do any ISO regulations state that this PT1 calculation can/should be used? ISO 14644-2 states that a delay function can be used when monitoring differential pressure...

    • Clemens.Duzy
      Clemens.Duzy commented
      Editing a comment
      Hi James, the PT1 filter function was a customer request for the RMS SW. The customer application (pharma) behind was differential pressure indeed. The statement of the customer was, that a simple average is not allowed / wished because the real measured value to too strong influenced by an average function. An allowed alternative is a PT1 filter typically used for differential pressure.
      But I cannot say if a ISO standard is based on this requirement.
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