Where do you do scaling?

JeffKiper · Sep 28, 2020

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.

Steve Bailey · Sep 28, 2020

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.

g.mccormick · Sep 28, 2020

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.

TheWaterboy · Sep 28, 2020

g.mccormick said:
... 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.

I_Automation · Sep 28, 2020

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.

geniusintraining · Sep 28, 2020

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.

TheWaterboy · Sep 28, 2020

I_Automation said:
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?

I_Automation · Sep 28, 2020

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.

TheWaterboy · Sep 28, 2020

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.

g.mccormick · Sep 28, 2020

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.

ndzied1 · Sep 28, 2020

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.

I_Automation · Sep 28, 2020

TheWaterboy said:
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.

theColonel26 · Sep 28, 2020

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.

rupej · Sep 28, 2020

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.

NetNathan · Sep 28, 2020

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

Where do you do scaling?

JeffKiper

Lifetime Supporting Member + Moderator

Steve Bailey

Lifetime Supporting Member + Moderator

g.mccormick

Lifetime Supporting Member

TheWaterboy

Lifetime Supporting Member + Moderator

I_Automation

Lifetime Supporting Member

geniusintraining

Lifetime Supporting Member + Moderator

TheWaterboy

Lifetime Supporting Member + Moderator

I_Automation

Lifetime Supporting Member

TheWaterboy

Lifetime Supporting Member + Moderator

g.mccormick

Lifetime Supporting Member

ndzied1

Lifetime Supporting Member

I_Automation

Lifetime Supporting Member

theColonel26

Lifetime Supporting Member

rupej

Member

NetNathan

Lifetime Supporting Member

Similar Topics