Til123

Ab plc 5/15 ife

14 posts in this topic

Hi, can anybody help me with an analogue input on a plc 5/15 via a 1771 IFE card. The input has been used with a 0-10vdc input, however there is some nasty compute functions to create a logarithmic curve for the Scada. we now need a linear calculation from 2.2v-8.5v. 2.2v = 0.0005mpa through to 8.5v = 1000mpa. I am familiar with the slc but the commands are somewhat different 

Cheers

Share this post


Link to post
Share on other sites

See text for the scaling formula.

The raw data value range for 0-10v input is 0-4095.

So..

2.2v=901 raw data value

8.5v=3481 raw data value

use these value for the raw input min/max in the scaling formula. You can use a "CPT" compute instruction for this, as the PLC5/15 does not have a "SCP" instruction.

ScalingFormula.txt

Edited by Mickey
1 person likes this

Share this post


Link to post
Share on other sites

Thanks Mickey,

I have done as you say and it worked thanks. C an you help me with the requirements compute function? I want low scale to read 0.0001 and high scale to be 1000. 

Dave 

 

Share this post


Link to post
Share on other sites

Your other post said..

.001 at 2.2v

This post says .0001 at 2.2v

Which is it?

 

Quote

0.001 @ 2.5 v, 0.01 @3.5v5v, 0.1 @ 4.5v, 1.0@5.5v, 10@6.5v, 100@7.5v, 1000@8.5v.

 

Share this post


Link to post
Share on other sites

Hi, sorry trying to write these on my phone which isn't going to well!

It should be 0.001@2.5, 0.0001 anything lower than 2.5v

Thanks 

Share this post


Link to post
Share on other sites

Then construct your compute instruction as following.

(((1000-.001)|(3481-901))*(Raw Input-901))+.001=Scaled Value ( this is for a 2.2v min per your first post)

(((1000-.001)|3481-1024))*Raw Input-1024))+.001=Scaled Value ( this is for 2.5v min per your last post)

Only you know the correct min voltage.

Edited by Mickey

Share this post


Link to post
Share on other sites

Also from your other post..

Quote

 

0.001 @ 2.5 v, 0.01 @3.5v5v, 0.1 @ 4.5v, 1.0@5.5v, 10@6.5v, 100@7.5v, 1000@8.5v.

This is linear starting from2.5v through to 8.5v

 

 Those values are not linear to your voltage range.

 

For example 5.5 v is 50% of your range would have a scaled value of 500 (if linear) ??

Edited by Mickey

Share this post


Link to post
Share on other sites

Ok thanks, the measured min v is 2.5v, the transducer goes under scale at below 2.2v is what I was trying to explain.

 

Thanks for the replies, sorry for the confusion 

Share this post


Link to post
Share on other sites

Here is a handy tool for linear response.

 

Created by John Soltesz

scaling chart.xls

1 person likes this

Share this post


Link to post
Share on other sites

Hi, thanks for that. Sorry for dragging this out but they don't seem to be the values I am expecting. This is off a vacuum welding machine. the datasheet for the transducer (mpa) 8.5v at atmosphere 1000mpa, then 7.5v at 100mpa, 6.5v@10mpa, 5.5v@1mpa, 4.5v@0.1mpa, 3.5v@0.01mpa, 2.5v@0.001mpa, any lower iunder scale for the instrument. As it is a vacuum machine they are particularly interested in the lower values. is it am exponential formula?

Thanks again sorry

Edited by Til123
Edit

Share this post


Link to post
Share on other sites

I am not a math geek. So hopefully those that are will jump in.

It is definitely not the formula I provided above. Which is an equation for a straight line.

Edited by Mickey

Share this post


Link to post
Share on other sites

Note that the pressure points are steps of 10 with voltage in 1V steps.  That is a logarithmic scale (Log-base 10).

Vacuum pressure => measured by sensor => output as analog logarithmic value => analog input into PLC => scaled/converted back into pressure

I do not believe you will be successful in converting the analog value back into a 1:1 vacuum pressure value.

Share this post


Link to post
Share on other sites

As stated above, the relationship is logarithmic.

Mpa = antilog base 10 (input volts - 5.5)

The CPT instruction can't handle that, so your alternative is to build a table of values that you can index into using the analog input. You have limited memory in a 5/15 so you'll have to decide what resolution is practical for your application.

 

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now