View Full Version : 4-20 signal
July 2nd, 2002, 02:37 PM
Hi. I am fairly new to PLC programming and I have a simple question. We use AB SLC 5/04 and 5/05 PLCs. The programmer who originally set up all of our 4-20 signals scaled them from 3.5 AMPS to 20.5 AMPS, and set up the corresponding output range from a negative number below the expected range to a positive number above the expected range. Ie: 0-600 F is scaled –18.75 to 618.75. The engineer that I am working with tells me that this should not be the case. Can anyone tell me if this in necessary and explain why or why not?
July 2nd, 2002, 03:46 PM
It's not necessarily good practice, but it isn't necessarily bad practice either. It depends on why the original programmer did this and what his logic does with it.
One advantage of a 4-20 mA over a 0-20 mA signal is the "live zero". The 20% offset is used to distinguish between a dead transmitter (0 mA) and a zero process measurement (4 mA). Some I/O modules set a bit or use another means to indicate that the signal value has dropped below a threshold (say 3.5 mA) and the transmitter or wiring may be no good. The original programmer may have been using the 3.5 mA bottom range so he can set an alarm in his logic to indicate a failed transmitter.
On the top end, most transmitters will go slightly over 20 mA if the proces variable is beyond the nominal range of the device. The original programmer may have wanted to catch this condition as well.
Or, the origninal programmer may have been compensating for calibration errors in his logic.
Or, he may just have been trying to be cute!
Is it necessary? No. Is it a bad thing? Maybe, maybe not.
July 2nd, 2002, 03:49 PM
With the over/under range setup that you indicated, it will give you some room
to detect a shorted or open device and still have the normal range available
from 0 to 100 percent.
July 2nd, 2002, 04:35 PM
I think he was just being cute.
You don't say what kind of modules you are using, but if it's not a Class 3, then you can't change the scaling of the input module, but you can make it look like you are.
Normally, a 4-20 mA signal comes in 3277-to-16383. Or
0 ºF = 4 mA = 3277 counts
600 ºF = 20 mA = 16383 counts.
If you use an SCP (SCale with Parameters) instruction, you would normally just plug in 3277 and 16383 for the Input Min and Max, and 0 and 600 for the Output Min and Max. But on the same scale:
-18.75 ºF = 3.5 mA = 2867 counts
618.75 ºf = 20.5 mA = 16793 counts
If you do an SCP with 2867, 16793, -18.75 and 618.75 as your values, you really haven't gained anything, you've just confused people. If the transmitter produces 4 mA, the input module will still produce 3277 counts. This will still mean that the transmitter should be seeing 0ºF
As I think on it further, there is no way to "set up all of [the] 4-20 signals [and] scaled them from 3.5 AMPS to 20.5 AMPS". Some cards allow you to change the scaling of the input, so that if you don't like 3277 and 16383, you can get 4000 and 20000, or 0 and 16383 instead.
But the card itself is always going to give some count when the transmitter is supplying 3.5 mA, and another count the transmitter is generating 20.5 mA. Theoretically, most of the SLC analog input cards can handle 0 to 21 mA. Whether the transmitter is capable of sending out a reliable singal within that range is another matter.
July 2nd, 2002, 06:46 PM
I have a truck-load of pressure and temperature transducers that I deal with. These transducers are calibrated once a year. We have a couple of major shut-downs through the year. The transducers are calibrated in groups according to their turn.
Sometimes a transducer goes out of calibration and we can plainly see that this is so. It has an effect on the process.
Most of my transducers have back-ups. Both transducers (the Primary and Secondary for any given part of the process) are running but only the Primary is normally used for control.
If the Primary fails, the process automatically switches to the Secondary. At that time, a visually painful remainder is posted on the screen every cycle until I, or one of my compadres, acknowledges the failure and puts the message into a less aggressive mode. The replacement is installed as soon as practical.
If, on the other hand, through simple observation, it is seen that a Primary transducer has gone out of calibration, the Primary can taken off-line and the Secondary will automatically jump in to control that part of the process. (Of course, this is playing the Odds-Game"; the odds being that the Secondary is OK.)
IF, the Primary transducer has not gone too far out of calibration, I attempt to "calibrate" the transducer by software means. This means adjusting the scaling. Hopefully, the transducer has still maintained a certain amount of linearity. Even if it lost a large part of its linearity (at the extremes), I try to take advantage of what linearity there is. Again, HOPEFULLY, the linear part is across the normal (or critical) operating range.
I have to assume that the Secondary, which is now controlling the process, is reasonably calibrated and produces reasonable results.
So, using the Secondary as my "Yard-Stick" I try to "re-calibrate" the Primary using software. Since both are operating together I can follow and compare the response of each. Sometimes that adjustment takes me outside of the normal areas. The numbers you finally end up with depend on the linearity of the transducer.
I then change the operating status of the Secondary to Primary and the Primary to Seconday. The less reliable transducer becomes the back-up until such time that the scheduled calibrations happen again.
Of course, if the new Primary goes bad, then the both of them are scheduled ASAP!
The point of this whole walk-through-the-park is that sometimes, when necessary, software calibrations are done on transducers. And sometimes those calibrations take the numbers outside of the normal range.
It certainly is not the right way to do it, but it can produce reasonable results - depending...!
July 2nd, 2002, 09:38 PM
I see that I may have misunderstood what the previous engineer had done. I read it as the input modules had been somehow altered from 4-20 to 3.5-20.5, not the tranmitters. I'm still not sure that callibrating a transmitter to generate 3.5 mA at -18.75 ºF buys you anything more than calibrating it to 4 mA at 0 ºF. Without knowing more about the transmitter, I couldn't say if it's even accurate with signal strengths less than 4 mA (or greater than 20).
Nice redundant transmitter scheme.
July 3rd, 2002, 10:48 AM
Thanks for all of the help. I come from an IT background, and all of this is new to me. I am learning on the fly so to speak.
All of our cards are class 3, but I think the original programmer was being cute. He has tended to over complicate some things where a more strait forward approach would have worked just as well.
I really like the redundancy scheme that you outlined Terry. I would like to try to incorporate some of those practices here.
I will be taking some AB programming classes later this year. Hopefully that will clear up a lot of the technical questions that I come across day to day.