Scaling help

Well this is my first post and want to say this is one of the best forums that i have the oppurtinity to visit!

Been reading a bit and trying to get my hands around a issue with a sensor.

1. Sensor is for measurement of dew point
2. Temp range is -112 to +140 F
3. Output is 4...20 mA
4. Using Analog card 1769-IF4
5. Micrologix 1500

I get alot of flucuation and think it might be noise? also having a tough time scaling it correctly?
This is the rung of info i have tried to tackle the problem.
Rung # 5

SOR BST SCP N9:5 3277 16384 -112 68 N7:5 NXB ADD I:2.1 550 N9:5 BND EOR
SCP N9:5 3277 16384 -112 68 N7:5

Sorry couldnt get a screen shot to paste? all trying to do is get signal in to display to HMI unit.

Any help would be greatly appricated!
BSG

Have you set up all of the configuration bits for the analog input channel that you are trying to use?

I would think your scaling should look something like this:


Maybe calibrate it and tweak the input min and max values...

First, welcome to the forum.

Are you using the actual analog register as your display register?

If your numbers are fluctuating, you may want to consider doing some averaging...and thus displaying the averaged value rather than the raw value.

Also, is your probe grounded or isolated? If grounded is it attached to the machine? Is the machine grounded? We had an experience whereby the customer's analog was fluctuating terribly. We found out that their ground potential was between 250 and 400 mV (acceptible is 50 mV) and their temperature probe was not isolated, and attached to the equipment which was grounded. Thus causing the erratic behavior.

I am not familiar with AB programming, so I am certain that if you needed some assistance with averaging, that there are more than a few capable programmers that could offer you suggestions.

Hope this helps.

God Bless,

Averaging post

Scaling post

BSG,

Those dew point and humidity sensors are very tricky. I tried to use one (unsuccessfully). I found that the temperature must be fairly stable, and not jumping up and down, because the sensor measures both moisture and temperature, then calculates the dew point. If it is in an air stream, the flow must be laminar, and not turbulent. Also, if the sensor is "saturated" (water condenses on the sensing head) then it can stop working, or work erratically. You definitely should average your readings over some time period. If you are getting erroneous readings mixed with a few good readings, you can set per cent deviation min and max limits to throw out the spurious data.

When you're tweaking with an SCP I've seen lots of people trigger a maths overflow error; the SCP doesn't clamp the output min or max so you can end up with numbers that you weren't expecting in your code. I'd suggest putting in a "debug" line on the next rung that unconditionally unlatches the maths overflow bit (S:5/0 I think; sorry, memory like a goldfish at the moment)

Ok seems like a great amount of information! But some of this is items are new to me! Does any one have examples of
1. Average
2. DeBugging

I am fimular with the logic but just green!
also a note this dew point sensor is in a compressed air line, so it is measuring dryness of compressed air.
Thanks for all the great help!

I guess it falls to me to answer point 2!

By "debug" I meant a line of code that is in there merely to catch errors that occur during installation/commisioning; you'd might want to delete it when the commisioning is finished.

BSG said:

Does any one have examples of
1. Average

Click to expand...

Did you visit the links I put earlier in post#4 ?

yes i have read both related post, but as i stated i am a new to PLC logic and trying to understand it fully. Does any one have any sample logic that i can review? I will keep on reading also!