Emerson Exchange 365

Ramp function for SP to a PID in RCAS

Hello Group members,

I am looking for a strategy to implemet ramp function for the SP of a flow controller (PID) in RCAS mode. Any suggestions?

I am aware of the feature SP_RATE_DN/UP in PID block, however they are applicable only in AUTO mode of the PID. 

 

Thanks,Clean Tech

• RAMP
• PID
• mode

CleanTech

• 26 Aug 2015 8:49 PM
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4 Replies

• Youssef.El-Bahtimy

  

  • 27 Aug 2015 4:02 AM

  Do you need to write directly to the RCAS_IN? You can wire a function block into the RCAS_IN, then just write to the input of that block.

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• GeloCortez

  

  • 27 Aug 2015 7:19 AM

  Hi,
  
  You can use RAMP function block and connect that out to PID RCAS_IN

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• Lou Heavner

  

  • 28 Aug 2015 12:52 PM

  I think it is necessary to understand better what you are trying to do. If you simply want to ramp the setpoint and are perhaps using CAS_IN for other purposes then the explanations above can help. If you have something else going on, say logic or APC that is calculating the SP and writing it into RCAS_IN, but you want to limit the rate of change, that might be a bit different kettle of fish. If you want the operator to be able to change the rate of change, you may want to consider how you would achieve that.

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• gamella

  

  • 5 Dec 2015 10:22 AM

  In reply to Youssef.El-Bahtimy:

  This depends on how RCAS mode will be used I mean, continuous logic or secuential logic (from SFC, or PHASE).  If sequential, from SFC in example, a new RCAS_IN value will be written on each SFC scan cycle so ramping vlues may be calculated within sfc. If continuous then RAMP block OUT connected to RCAS_IN may work but is important to properly manage back calculation to let RAMP block know what is happening on PID block. For PID RCAS mode back calculation parameter is RCAS_OUT (it's the same as BKCAL_OUT but for RCAS mode). However, RAMP block does not have BKCAL_IN parameter so it's not possible to directly connect PID/RCAS_OUT to RAMP/BKCAL_IN.

  A solution is to interpose a BiasGain block between RAMP and PID blocks. In this way PID/RCAS_OUT may be connected to BiasGain/BKCAL_IN and RAMP/OUT may be connecto to BiasGain/IN_1.

  The BiasGain must operate in AUTO mode wtih SP fixed to 0. (Another option is to set BiasGain/TRACK_IN_D ALWAYS to True and wire RAMP/OUT to BiasGain/TRACK_VAL).

  If it is desired to allow PID mode changes while ramping it is possible to make RAMP block output track current PID  setpoint. With this, when switched back to RCAS ramping will continue from current PID setpoint.

  To implement this:

  BiasGain/OUT must be connected to RAMP/IN

  RAMP/ENABLED must be set to False when PID is not in RCAS and True when PID is in RCAS

  RAMP/TRACK_IN_D must be set to True when PID is not in RCAS and False when PID is in RCAS

  Final SP value must be written into RAMP/END_VALUE

  Additional logic to write PID/SP_RATE_UP or PID/SP_RATE down into RAMP/RAMP_RATE (depending on current RAMP direction) could be useful if same ramping rates as for PID AUTO mode are desired

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