Emerson Exchange 365

Question regarding Asi_bus card/network

Hi,    we need  make a Pulse Width Modulation PID, I know how to make with DO card, but I don´t know how to make it in Asi_bus card. I will use a  on-off valve in Asi_bus network.

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Irving_Aguilar

• 14 Jul 2014 8:44 PM
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6 Replies

• Bruce Brandt

  

  • 14 Jul 2014 9:55 PM

  The function of Pulse Width Modulation is contained in the logic not the card type so you do the exact same thing you would do with a standard DO card.

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• Andre Dicaire

  

  • 14 Jul 2014 10:23 PM

  The DO card and CHARM Do Can be configured as continuous pulse type allowing for an analog value to set the duty cycle of a discrete output. The ASI devices do not support this.
  
  To do this in a module the resolution of the duty cycle will be the exec time of the module at best since the DO can only change state when commanded.
  
  How long can the period of the cycle be? Assuming a 10 Second period and a 100 ms module you would have 1% resolution.

  Andre Dicaire

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• Andre Dicaire

  

  • 14 Jul 2014 10:33 PM

  To do this. You could use a counter to create a ramp say 0 to 100. Compare the PID/OUT and set the output channel true when counter is below PID output and false when counte is above.
  
  Reset the counter automatically by wiring the out_D to reset. Use limit block to compare and write to Cas_in od DO block.
  
  Downside is you are running a PID at its fastest rate on a process that has to be very slow for this to work.

  Andre Dicaire

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• Irving_Aguilar

  

  • 14 Jul 2014 10:56 PM

  In reply to Andre Dicaire:

  ok, I work in the sugar industry and control of a continuous pan feeding involves feeding molasses to the pan, the input is a density measurement to  pid  and the output is an on-off valve to control the density within the pan. the duty cycle must be possible adjust via a value in the screen of Delta V. all  valves are in a Asi-bus network. had never worked with this type of control before, which is why I'm very concerned

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• Bruce Brandt

  

  • 15 Jul 2014 12:37 PM

  In reply to Andre Dicaire:

  My mistake. I forgot that you had to set the duty cycle of the module. Your ramp idea is probably the best solution.

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• Andre Dicaire

  

  • 16 Jul 2014 4:41 PM

  In reply to Irving_Aguilar:

  As I mentioned, the key issue is the process time constant for your Density.  Since you are manipulating the DO channel from the module, the resolution of your duty cycle is a function of the module exec time and period for the valve cycle.  It will be hard control the loop tightly if the time constant is short.  If you shorten the period, you will loose resolution on the duty cycle as you cannot speed up the module exec time past 100 ms.  That's why this type of control is typically done with a continuous pulse IO Channel.

  You might want to run the DO channel from a 100 ms module and run your PID loop at a more moderate rate.  That way you don't waste CPU on the PID and user interface portion of the logic.  Since this has to be a slow process for this to work, your PID loop could be run at 2 seconds.  Move the Ramp and compare logic to the DO Module.  Embed this module in the PID module and wire the PID Out to the Duty Cycle % value.

  Do you know what the period of the valve cycle is?   (Duty cycle is the percent on time over the period).  If this is <10 seconds, your resolution will be greater than 1% i.e. at 5 seconds, the module exec time of 100 ms represents 2%, and so on.  

  DeltaV AS-I card does not support analog values.  This is a 3.0 AS-I spec.  

  Certainly a control challenge when you don't have the right control signals driving the control element.

  Andre Dicaire

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