Step7 Real Data types

Stuntman · Sep 15, 2012

I am working on an application where I am controlling the ramp of a web and I want to increase the web speed by .1 feet per minute every x seconds. In the attached logic diagram I am just showing the addition of .1 to my web speed.

If web speed actual is less than web speed sp, increase web speed actual by .1

If web speed actual is less than web speed sp, decrease web speed actual by .1

If the web speed actual is equal to the web speed sp, do nothing

The problem lies in the way Siemens handles the real value

.1 Plus 0.7 = 0.8000001 not 0.80000000000000000000

Say my web speed set point is 0.8
My logic will always be speeding up and slowing my web speed because we will never truly be at the web speed set point.

I tried converting to an integer multiply .7 * 10 then adding 1 then converting back to a real but still get the same problem 0.8000001

Another problem is the extra values that are showing up after the decimal would be compounded so my ramp rate would not be steady.

I have done 3 other projects with Siemens but I have never ran into this type of task and I was wondering how a Siemens programmer would solve this problem.

Thanks

FYI In my logic diagram in the add and subtract functions I have the IN 1 and IN 2 backwards IN 1 should be the web speed actual.

PLC · Sep 15, 2012

Theres quite a few samples on the Siemens web site

you can start here

http://support.automation.siemens.com/WW/view/en/1853767

PLC · Sep 15, 2012

Also there is a complete converting library available for Step7 that has Ramp, S Ramp, Integrate, Winder,etc.... blocks
Whos you Siemens rep or distributor?
These are available for free

http://www.industry.usa.siemens.com...ments/IND-converting-toolbox-presentation.pdf

KalleOlsen · Sep 15, 2012

Stuntman said:
I am working on an application where I am controlling the ramp of a web and I want to increase the web speed by .1 feet per minute every x seconds. In the attached logic diagram I am just showing the addition of .1 to my web speed.

If web speed actual is less than web speed sp, increase web speed actual by .1

If web speed actual is less than web speed sp, decrease web speed actual by .1

If the web speed actual is equal to the web speed sp, do nothing

The problem lies in the way Siemens handles the real value

.1 Plus 0.7 = 0.8000001 not 0.80000000000000000000

Say my web speed set point is 0.8
My logic will always be speeding up and slowing my web speed because we will never truly be at the web speed set point.

I tried converting to an integer multiply .7 * 10 then adding 1 then converting back to a real but still get the same problem 0.8000001

Another problem is the extra values that are showing up after the decimal would be compounded so my ramp rate would not be steady.

I have done 3 other projects with Siemens but I have never ran into this type of task and I was wondering how a Siemens programmer would solve this problem.

Thanks

FYI In my logic diagram in the add and subtract functions I have the IN 1 and IN 2 backwards IN 1 should be the web speed actual.

I suggest you make a reusable FC to compare the two values, and set them equal if they are within a dead band limit.

You need 3 real inputs:
rIn1
rIn2
rBand // Dead band

and 3 boolean outputs:
xGT // rIn1 - rIn2 > rBand
xEQ // ABS (rIn1 - rIn2) <= rBand
xLT // rIn2 - rIn1 > rBand

It seem to me you are well capable of filling in the code yourself

.

And in 90% of my coding, I use the wrong comparator, please check this.

Kalle

L D[AR2P#0.0] · Sep 15, 2012

In the example you have posted, 0.7 + 0.1 = 0.8 as verified by examining the hexadecimal representation of the floating point numbers.

Instead of using floating point approximations for numbers, use a double integer suitably scaled.

KalleOlsen · Sep 15, 2012

L D[AR2 said:
In the example you have posted, 0.7 + 0.1 = 0.8 as verified by examining the hexadecimal representation of the floating point numbers.

It is imaginable that the result could be different with the PLCSIM(?) and the S71200(?).

Kalle

Stuntman · Sep 15, 2012

Here is an attached pic showing it in action. I used an "hmi Jog" bool to simulate the addition.

-PLC great stuff,

KalleOlsen · Sep 15, 2012

Which CPU type do you use?

Stuntman · Sep 15, 2012

The pic shown is a S7-1200 (CPU 1214), but I have also tried it in a CPU 1215.

HJTRBO · Sep 15, 2012

Would this not work?
"How can you generate a ramped set point with STEP 7 (TIA Portal) V11?"
https://support.automation.siemens....objaction=csview&extranet=standard&viewreg=WW

PLC · Sep 15, 2012

That's the one I was trying to get him to use

Is this for the winder project?

Stuntman · Sep 15, 2012

The ramp generator appears it would work but what i am trying to solve the math problem i have because i see this could be a problem in the future with other applicatiins

KalleOlsen · Sep 16, 2012

Hi!
What if you put something like this in the preceeding network, and use the negated 'Equal' in your rungs?

HJTRBO · Sep 16, 2012

KalleOlsen said:
Hi!
What if you put something like this in the preceeding network, and use the negated 'Equal' in your rungs?

Yes, that is nice. I rolled my own ramp block ages ago and used this similar method. I couldn't remember how to do it quickily but I had the block saved in my library.

Test this and see if it works for you...

Stuntman · Sep 16, 2012

Well I figured out how to make it work. Pretty simple, sometimes its best to work on something else then when you come back to the problem the answer is right there.
Multiply the real value web speed actual by 10 to get the.x decimal into the whole number side (x.). Add 1 to the whole number which would actually be adding (.1) then divide by 10 to place the decimal in the correct place.

Step7 Real Data types

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