Totalizer in Logix5000 vs in field instrument.

I did some searching on totalizing in a PLC versus getting a total from a device, and found a few links, but figured I'd start a new thread specifically on that topic to dig a bit deeper.

My main question is which is recommended and why? Which is more accurate and why?

From the searching I did, it seems like the recommendation is to use the value out of the instrument if possible. But I'm curious why that is?

One issue with doing it in the PLC that I can see would be rounding errors accumulating while doing the totalizing math, but I would figure with modern PLC's and floating point calcs, those should be minimal. Also, wouldn't a transmitter have the same issues? I figure most of those are digital internally these days as well, and I can't imagine they're doing 64 bit math in there or using special float representations internally, but maybe I'm wrong.

I did do some experimenting with Logix5000 on an emulator and got some strange results when using the TOT FBD instruction. It seems to consistently overshoot by a small amount (~0.1) and the amount doesn't seem to change much even if I change the incoming rate, scan times of the totalizer, length of totalizing time, etc. I'm curious if anyone can see what's going on in the code to produce that offset.

I've attached my simple Logix program if anyone wants to run it themselves and see what I'm seeing.

Thanks,

Greg

Id say if you want the most consistent and accurate total you should use the device. I would only use the plc if it was my only option or it wasnt economical, or there was enough tolerance in the total that makes it acceptable to have a slight loss/gain.

You may be able to analyze the value you are totalizing once per second in a plc, but the end device may be sampling 100 times.

The device is the most accurate. The only way to get a truly accurate total in the PLC, is to have a totalizer pulse output from the device wired to a digital input in PLC. For example, the device can be configured to send a pulse every "X" gallons. You can then compute the total in the PLC. The pulse output option is available on almost any flow meter.

Greg what are you totalizing? Are you doing a small batch or a weeks worth of flow from a meter? Things like this make a HUGE difference.
From my experience analog is a joke to do totalization with. A small error in the loop design, a little noise, cheap AtoD converters, scantime updates, card updates, etc.,etc. All reasons to avoid analog signaling when doing long-term tracking. If you need an instantaneous flow then use analog for a quick snapshot.

I prefer to use digital PPx (Pulse Per x whatever ) as long as the card update and scantime isn't to long then you are in good shape.

Give us a few details about doing you are doing, hardware, platform. I have had scan time issues with small programs and quick pulses. I followed behind a guy that setup a meter for PPoz. He was totalizing thousand gallons. He was missing pulses. I changed to PPgal and some logic. System was on the money from then on.

Total is for small batching

Thanks for the feedback.

My application is for a small batching system...the customer is mixing 2 components and needs to control the final concentrations as precisely as possible...we have a tolerance of +/- 0.12 kg for a 20kg charge and the flows will be around 4kg/min. We've decided to go with a Profibus connection to the flowmeter so we can read the total directly out of the device.

It's my first time doing an application like this, so if that solution sounds like it might give us some issues when we implement, any feedback is appreciated.

Regards,

Greg

The Profibus connection will work as well. You'll be reading the exact same total as the flow meter, doesn't get any better than that.

I have seen quite a few people using Hart enabled Analog Input PLC modules
(AB ControlLogix 1756-IF8H for example) to accomplish the same thing.

In general, if your looking for truly accurate totals, don't totalize in the PLC.

wouldn't the HART data be way too slow for control

That's why we went Profibus

Our customer was asking about HART, but I agree, even with a Tri-Loop it probably would have been too slow. That's why we went with Profibus.
With Profi, we can run the bus fairly fast (it's a short physical segment) so I think we won't have any issues with scan times. Although we're using CompactLogix, which uses the ProSoft Profibus card, I've never used it before, so I'm hoping we don't come upon any scan time issues as we implement the project.
If anyone has used the ProSoft and has feedback on it, I'm always interested in what others have found.

Regards,

Greg

Don't think scantime will be a isue, more about valve closing time, and so on.

Banker