You are not registered yet. Please click here to register!


 
 
plc storereviewsdownloads
This board is for PLC Related Q&A ONLY. Please DON'T use it for advertising, etc.
 
Try our online PLC Simulator- FREE.  Click here now to try it.

New Here? Please read this important info!!!


Go Back   PLCS.net - Interactive Q & A > PLCS.net - Interactive Q & A > LIVE PLC Questions And Answers

Reply
 
Thread Tools Display Modes
Old September 28th, 2020, 09:37 AM   #1
JeffKiper
Lifetime Supporting Member + Moderator
United States

JeffKiper is offline
 
JeffKiper's Avatar
 
Join Date: Jun 2006
Location: Indiana
Posts: 2,231
Where do you do scaling?

Where do you prefer to scale instrumentation?
I was at customers shop and they had an instrument that was a 1-5VDC to a signal conditioner to make it a 4-20mA then in the PLC the had a SCP instruction .

They had scaling in the signal conditioner and the PLC. I dont know why they used the signal conditioner for anything other than a V-mA converter.

I like to do all my scaling in the PLC. If I can keep the instrument at factory default I will. A lot of the temperature probes that are out there are 0-100°C. Di the conversion in the PLC and any maintenance guy can change it without configuring anything.
What are your thoughts? Im getting more and more into the process world and trying to make better decisions from the collective experience here.
__________________
There should be a minimum IQ before people can breed.

FS Eng (TÜV Rheinland, #11004/15, Machinery)
  Reply With Quote
Old September 28th, 2020, 09:58 AM   #2
Steve Bailey
Lifetime Supporting Member + Moderator
United States

Steve Bailey is offline
 
Steve Bailey's Avatar
 
Join Date: Apr 2002
Location: The boondocks of Western Massachusetts USA
Posts: 7,275
Like you, I prefer to keep field devices in their out-of-the-box state whenever possible. It makes things lot easier when you need to replace one.
  Reply With Quote
Old September 28th, 2020, 10:37 AM   #3
g.mccormick
Lifetime Supporting Member
United States

g.mccormick is offline
 
Join Date: Jul 2012
Location: IN
Posts: 753
I agree with the scaling in the PLC. In the case of a logix system, do you have the IO module engineering set to signal level or engineering level? For instance:
transmitter -25-150F = 4-20MA. On the input card do you set engineering values to 4000-20000 or -25-150? I have seen both ways, but I always set the card to 4000-20000 and do the scaling with a scaling AOI.
  Reply With Quote
Old September 28th, 2020, 11:07 AM   #4
TheWaterboy
Lifetime Supporting Member + Moderator
United States

TheWaterboy is offline
 
TheWaterboy's Avatar
 
Join Date: May 2006
Location: State of Denial
Posts: 1,146
Quote:
Originally Posted by g.mccormick View Post
... but I always set the card to 4000-20000 and do the scaling with a scaling AOI.
I'm curious why you prefer that solution. I do mine in the module (where possible) but I'm always open to a better ideas.
  Reply With Quote
Old September 28th, 2020, 11:39 AM   #5
I_Automation
Member
United States

I_Automation is offline
 
I_Automation's Avatar
 
Join Date: Jun 2020
Location: Detroit, Michigan USA
Posts: 359
I do scaling in the PLC always.

As for the 4000 to 20000 range it will give better resolution than a range of 175 (-25-150) would give per mA or Volt.

That might not be a problem for a tank temperature between 33 and 200, but a laser distance sensor or analog tank level sensor would give a lot more precise positioning at the higher resolution.
  Reply With Quote
Old September 28th, 2020, 12:20 PM   #6
geniusintraining
Lifetime Supporting Member + Moderator
United States

geniusintraining is offline
 
geniusintraining's Avatar
 
Join Date: Jun 2005
Location: SC
Posts: 7,251
Back in the day... we had an outside company do some of our calibrations (ISO9000) the first and last time we let them in the logic of the PLC, they were only required to calibrate a few of the instruments and this was due to liability, the others were done in house in the PLC when possible, but there were a handful that they calibrated at the device and put their yearly sticker on and to be honest most of them were tweaked on both ends especially when they had a local display.
__________________
www.PLCCable.com PLC Communication Cables, PLC Trainers, MicroLogix, ControlLogix, Siemens, Allen Bradley and more...OEM and aftermarket supplies... Aftermarket 1784-U2DHP Allen Bradley USB to DH+, new USB to 485 modbus
  Reply With Quote
Old September 28th, 2020, 12:20 PM   #7
TheWaterboy
Lifetime Supporting Member + Moderator
United States

TheWaterboy is offline
 
TheWaterboy's Avatar
 
Join Date: May 2006
Location: State of Denial
Posts: 1,146
Quote:
Originally Posted by I_Automation View Post
As for the 4000 to 20000 range it will give better resolution than a range of 175 (-25-150) would give per mA or Volt.
Why would the resolution be any different? I assume the analog module (in CLX anyway) would be doing the same math on the 4-20ma signal as code would, just out of view.

I can understand that other analog modules that only allow the variations of Raw, Proportional or PID would benefit from selecting a larger range and scaling from there. I can get behind that. But does it matter in CLX?
  Reply With Quote
Old September 28th, 2020, 12:43 PM   #8
I_Automation
Member
United States

I_Automation is offline
 
I_Automation's Avatar
 
Join Date: Jun 2020
Location: Detroit, Michigan USA
Posts: 359
Since the value will probably end up in a DINT using the following:

4000 to 20000 = 16000 range = 1000 counts per mA

-25 to 150 = 175 range = about 11 counts per mA

If a change of 1mA is temperature then 11° per mA would be OK for water temperatures.

If the change of 1mA is 2 meters distance change then a range of 175 would be readings that jump one count about every 18cm, where a range of 16000 would change one count every 0.125mm. Trying to position something within 18cm of target would probably be useless unless it was a railcar or ship.
  Reply With Quote
Old September 28th, 2020, 12:54 PM   #9
TheWaterboy
Lifetime Supporting Member + Moderator
United States

TheWaterboy is offline
 
TheWaterboy's Avatar
 
Join Date: May 2006
Location: State of Denial
Posts: 1,146
Sure if you have to use integers. But in a CLX analog module you can scale the 4-20ma input to any engineering unit you desire and output to a float. All the work is done.

In hindsight I suppose though that's not the majority of analog input options in existence, and I do have my small share of those where I do the same thing, so in that case I can see your point.

I was only asking in case the CLX module scaling was inferior in some way to code based.
  Reply With Quote
Old September 28th, 2020, 01:40 PM   #10
g.mccormick
Lifetime Supporting Member
United States

g.mccormick is offline
 
Join Date: Jul 2012
Location: IN
Posts: 753
I guess for me it comes down to a few things:
1. I like to be able to see the signal value (ie 4000-20000 for 4-20MA) in the PLC and feed it into the AOI. This allows me to monitor for faults (ie <4ma or >20ma etc.)

2. The plc module configuration become agnostic to what sensors are connected (as long as they are the corrct signal type). Which just seems better in my mind.
  Reply With Quote
Old September 28th, 2020, 02:09 PM   #11
ndzied1
Lifetime Supporting Member
United States

ndzied1 is offline
 
ndzied1's Avatar
 
Join Date: Aug 2002
Location: Chicago, Illinois
Posts: 2,553
We don't use CLX that often but even if we had the capability to scale in the card, I'd still prefer to do it in the program.

ditto what g.mccormick said.
__________________
nOrM
======================
nOrM=Norman Dziedzic Jr.
I've never been to China but my phone has.
  Reply With Quote
Old September 28th, 2020, 04:27 PM   #12
I_Automation
Member
United States

I_Automation is offline
 
I_Automation's Avatar
 
Join Date: Jun 2020
Location: Detroit, Michigan USA
Posts: 359
Quote:
Originally Posted by TheWaterboy View Post
Sure if you have to use integers. But in a CLX analog module you can scale the 4-20ma input to any engineering unit you desire and output to a float. All the work is done.
Yes, but then the reading can be from 4.0000000 to 20.000000, giving the actual input.

Since the values mentioned are so large it is presumable they are going to a DINT and not a REAL tag.
  Reply With Quote
Old September 28th, 2020, 04:31 PM   #13
theColonel26
Lifetime Supporting Member
United States

theColonel26 is offline
 
theColonel26's Avatar
 
Join Date: Feb 2014
Location: West Michigan
Posts: 500
If the Device is IO-Link then I will scale in the Device and read that digital value from the PLC, over the communications.

If not then yes, I do it in the PLC Logic, and not the module config.
__________________
SkyCad is by far the Best ECAD Schematic Software I have used.


Fact my "Fun Facts" are never fun
  Reply With Quote
Old September 28th, 2020, 09:19 PM   #14
rupej
Member
United States

rupej is offline
 
Join Date: Sep 2014
Location: NC
Posts: 815
OP, I do strictly process control and always prefer to do it the way you suggested. Leave the instrument scaling default if possible, then scale in the PLC.

I prefer 4000-20000 then a scaling to put it in proper units. This makes it easy and consistent to check for loss of input or overrange.
  Reply With Quote
Old September 28th, 2020, 10:28 PM   #15
NetNathan
Lifetime Supporting Member
United States

NetNathan is offline
 
Join Date: Nov 2011
Location: Corona, CA
Posts: 1,791
GE PLC automatically converts 4-20mA input to 0-32000 counts...

Also in regards to where the scaling should be...
I think it makes it way easier just to replace the device and not worry about scaling it or the signal conditioner (if you have to replace it). Then anyone can replace it. IMHO
__________________
****Control Freak****
Net is where I be and Nathan is me.
  Reply With Quote
Reply
Jump to Live PLC Question and Answer Forum

Bookmarks


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Topics
Thread Thread Starter Forum Replies Last Post
PLC5 Scaling Question jrowe4 LIVE PLC Questions And Answers 3 July 18th, 2014 07:27 PM
PT100 Scaling in RSlogix 5000 InstrumentationUK LIVE PLC Questions And Answers 6 February 5th, 2014 09:55 AM
ControlLogix and Devicenet Christoph LIVE PLC Questions And Answers 16 October 5th, 2013 11:17 AM
how to determine low,high,and mean scaling for Omron? kartika LIVE PLC Questions And Answers 5 August 2nd, 2011 03:20 AM
ControlLogix PID Scaling - Help! kdcui LIVE PLC Questions And Answers 4 May 10th, 2011 07:09 PM


All times are GMT -5. The time now is 01:10 AM.


.