Scaling, Ranging & Normalisation FFb

[Therazh]

I return to the problem of scaling & ranging as no satisfactory reply was forthcoming earlier, by now rephrasing the query.
In this application, I would like to know how an Arithmetic block in FFb can be configured as a divider for a compression ratio calculation for a surge line application, with AI1 (discharge pressure) = 0-1000 psig and AI2 (suction pressure)=500 psig. The ratio output is 1.0-2.5 on 0-100% scales.

As per conventional “normalizing” equation method, a fundamental equation of the form
OUTs = [Km * (Kx * X + Bx) / (Kz * Z + Bz)] + Bo may be simplified further as discharge pressure X and suction pressure Z operate over their 100% span can lead to Kx = 1 and Kz = 1. So the equation becomes: OUTs = [Km * (X + Bx) / (Z + Bz)] + Bo

Conventional methods lead to gain & bias values: Km = 1.33, Bx = 0.0147, Bz = 0.0294 and Bo = - 0.67.

How would I go about in FFb systems in configuring the system? How is scaling, ranging, normalization & computation effected – Is it in EU or % that the calculation performed and at what point does any unit conversion need to take place. Do I need auxiliary blocks?"

[rezabejd]

So the equation becomes: OUTs = [Km * (X + Bx) / (Z + Bz)] + Bo

Conventional methods lead to gain & bias values: Km = 1.33, Bx = 0.0147, Bz = 0.0294 and Bo = - 0.67.

How would I go about in FFb systems in configuring the system? How is scaling, ranging, normalization & computation effected – Is it in EU or % that the calculation performed and at what point does any unit conversion need to take place. Do I need auxiliary blocks?"


I'm not sure how "standard" various vendor's implementation of the ARTHM block are; This block is tested and check-marked when passing the ITK5 & up. See if this thread (http://forums.fieldbus.org/showthread.php?t=1164)is helpful at all.

I have only used the ARTHM block in Rosemount devices and I think your equation could be implemented. There is an OUT_SCALE parameter where you set the low and high engineering units of the computed output. You have to take care of unit conversion when you formulate the expression - the block is not smart enough to know whether your input units are Actual or Normal, gauge or absolute, Celsius or Kelvin, etc. The Rosemount has a limited repertoire of "forms" if you will - e.g., you select the algorithm from a 7-8 item menu that includes flow compensation square root, flow comp. linear, traditional summer, etc. - it can only do these forms.