Scaling Analog Input to float and getting negative number

Gotxreddragonx · Jun 28, 2018

Hello,

I have a Micrologix 1400 PLC and using RSLogix 500. Using a Multi Ranger Ultra Sonic Level Sensor and the input is moved to N7:11, then scaled from 3120 / 15,602 to 0 / 13.5. I also added the less than/equal to 3120 and greater than/equal to 15,602 and moving them into the appropriate spot to scale the input properly, which it does. Somehow it's still showing a negative number in the SCP output, which is saved to a Float.

Gotxreddragonx · Jun 28, 2018

Here is a picture of what I have

GaryS · Jun 28, 2018

For one thing I see your actual input I:2.1 shows 2961
That is less than the scaled min of 3120
Any input value less than the 3120 min value will be a neg value
it could actually be correct for your application

Gotxreddragonx · Jun 28, 2018

Even though I move 3120 to the minimum value if it shows less then 3120?

Aabeck · Jun 28, 2018

Yes, you are moving the minimum value AFTER the scaling.

Move the scaling to the last branch.

As it is the scaling is scaling the lower value from the input, then changing it to the minimum before the scan reports to RSLogix what the value is. As smart as controllers are, they only do one step of a rung at a time and can't comprehend (yet) what is really wanted.

ndzied1 · Jun 28, 2018

The order that things are happening is important.

When the first rung is scanned, 2961 is moved into N7:11

When the SCP instruction is scanned, the Input is still 2961

Then, after the SCP instruction is done, you check for max and min and update N7:11.

If you put the max/min check before the SCP instruction then you will see those limits effect the output of the SCP.

perhaps your ultrasonic needs to be scaled/offset properly.

Gotxreddragonx · Jun 28, 2018

Moving the scale to the end fixed the problem. Thanks for the help!!

GaryS · Jun 28, 2018

I would change thins a little
The scale input should be I:2.1 ( Input directly Into the Scale Function)
To set the limits
if F8:11 > 15602 then F8:11 = 15602
If F8:11 < 0 then F8:11 = 0

Both compares are after the scale is executed
The scale function scales the input then you check the limits of the scaled result

Gotxreddragonx · Jun 28, 2018

Ok I can do that. I just took over programming for our plant and had to debug the program, since the original programmer passed away in the middle of the project. There is no organization at all, so when I get the time I will rewrite it. I just started learning ladder logic a couple months ago. But I'm loving it.

Ken Roach · Jun 28, 2018

Speak no ill of the dead. Okay, maybe just a tiny bit.

The automation business gives me an opportunity to learn something new every day.

At a Sunday dinner last week a 22-year-old son of a neighbor commented in passing that he had been bored at work and was goofing off. I must have paused in an unusual way; he asked "what do you do when you're bored at work ?"

"I, um. I don't know. I haven't been bored at work since I was sixteen."

Now, that doesn't mean I don't sometimes goof off; my 12,000+ posts on this forum prove that.

But it's not because I'm bored !

Gotxreddragonx · Jun 28, 2018

I am never bored, that's for sure. There's always something to learn. I shall get to 12,000+ posts on here someday.

Gotxreddragonx · Jun 28, 2018

Actually, is the scaling for -10 to +10 VDC the reason for the 3,120 - 15,602? It's used for the Ultrasonic level probe and a hydraulic screen. I understand the 4-20 mA and PID scaling. Never used the -10 to +10 VDC.

Firejo · Jun 29, 2018

Ken Roach said:
"I, um. I don't know. I haven't been bored at work since I was sixteen

Yes Ken but you aren't the usual nut

Actually I've been fortunate lately to not having boredom be an issue. Having said that in my previous job it wasn't boredom that was the problem, anger and frustration bad enough to make me want to... well I better not say. Someone is going to do what I wanted to do one of these days and I don't want to be the suspect that confessed online

Aabeck · Jun 29, 2018

Firejo - this?

Gotxreddragonx · Jun 29, 2018

Scaling Analog Input to float and getting negative number

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