Soft ramp

MrQ · Nov 4, 2014

Hello,

I am programming RS Logix5000 for compact logix.

Looking for a function to do a soft ramp. I have a input value that I want to ramp but at the begining and end of the ramp I want a soft curve shaped ramp.

Can not find anything in AB standard. Any ideas or AOIs?

Peter Nachtwey · Nov 4, 2014

You need at least a third order equation for this.
You will probably need to make your own or search to see if someone has not already made one that is public domain.
http://en.wikipedia.org/wiki/Cubic_Hermite_spline#Interpolation_on_a_single_interval

Ron Beaufort · Nov 4, 2014

the SLC-500 platform has an RMP (Ramp) instruction ... that doesn't directly help you much working in CompactLogix – but the SLC book has some equations that might be helpful if you need to write your own function ...

see the RMP Instruction starting on Page 4-20 ... the equations are on the next few pages ...

http://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1747-rm001_-en-p.pdf

suggestion: just in case you haven't considered it – it's usually very helpful to "graph" your functions by using the built-in Trend feature while you're programming something like this ...

good luck with your project ...

Peter Nachtwey · Nov 4, 2014

Are the AB ramps second order or third order. To do what MrQ wants a 3rd order polynomial is required.

Ron Beaufort · Nov 4, 2014

dude, don't you EVER sleep? ...

Are the AB ramps second order or third order.

as always, Peter, you're way over my head with the math ... here's an excerpt from the book I linked – showing the "acceleration" curve which I think MIGHT handle what the OP asked for ... I'm sure that there are "smoother" ways to write it – but for some systems this might be "close enough for government work" ...

.

Ron Beaufort · Nov 4, 2014

just for giggles I knocked out a quick demo in a CompactLogix ... the curve isn't as "round" as the book shows (I think they just fudged on their example) ...

I'm sure there's a way to calculate a smoother "take off" - but maybe this will be good enough ...

.

osmanmom · Nov 4, 2014

are this possible to apply ?

(attachment)

Peter Nachtwey · Nov 4, 2014

This dude has been in China for almost 2 weeks. I am in a different time zone.
What MrQ describes is an s-curve. The acceleration should be 0 at the beginning and end of the ramp. That is not what your example shows. In the link the m0 and m1 constants are the accelerations at the beginning and end of the ramp. Usually these are zero for a smooth s-curve.

Ron Beaufort · Nov 4, 2014

I added an extra multiplication by (Current_Time / Overall_Time) and the slightly smoother takeoff shown by the green curve resulted ...

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Ron Beaufort · Nov 4, 2014

I'm still in America - and too excited about voting today to sleep ... I missed the part about the end of the ramp ... the book shows an S-Curve too ... let me piddle around with that and see what happens ...

Tom Jenkins · Nov 4, 2014

Here is an example S-Curve

Ron Beaufort · Nov 4, 2014

and an S-Curve based on the book examples ...

.

Ron Beaufort · Nov 4, 2014

and here's the formula that Tom posted - in blue - for comparison with the original formula from the SLC book that I linked - in yellow ...

this is has been fun - and I'm sure that I'll find a use for these examples somewhere along the way - but I've got to get back to work ...

.

Soft ramp

MrQ

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osmanmom

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Tom Jenkins

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