Re: [foxboro] "GDEV" or "VLV,MOVLV,MTR" or "CALC" - Discrete control engineering advice ?

  • From: "Jones, Charles R. \(Chuck\)" <Chuck.Jones@xxxxxxxxxxxxxxx>
  • To: <foxboro@xxxxxxxxxxxxx>
  • Date: Wed, 1 Aug 2007 17:01:16 -0400

As previously stated, the GDEV block is very versatile and useful.  We
use GDEV blocks at every opportunity for all of the reasons mentioned in
earlier posts.  However, I remember a glaring weakness in an otherwise
ideal block.  Unlike a hardwired motor (or ladder logic), the GDEV will
not remove the power request to the motor if the motor loses its
feedback (auxiliary contacts). =20

I have worked around this by tying the MMAIND (Mismatch Alarm Indicator)
into the INTLCK parameter, thus forcing the GDEV into its Interlock mode
(and over-riding either Manual or Auto modes) to drive the GDEV off.  I
tie the INTDSR (Interlock Desired State) back onto itself (so that its
state cannot be changed) and give it the default value of OFF.  Once the
GDEV output is OFF, the desired state matches the feedback state so the
GDEV is no longer in Mismatch Alarm and the GDEV passes out of the
Interlock mode and back into the Manual mode in the OFF state.

The additional connections are as follows:
INTLCK =3D :Blockname.MMAIND
INTDSR =3D :Blockname.INTDSR.0

This solution assumes a few things, such as the assumption that the
INTLCK parameter was otherwise not going to be used.  The same
assumption is made for the desired state of the Interlock, INTDSR.  The
most difficult consequence for us to live with is that the GDEV is left
in the Manual mode.  Most of the time, it has not been a problem.

As always, "your mileage may vary".  I am going from memory here, so
will wish to verify my statements, test the results for yourself, etc.

Chuck Jones
Automation Technologist
Tate & Lyle -- Lafayette Plant
Office 765.477.5324 | Cell 765.586.5290




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