PLC Recommendations for Horizontal Fuel Tank Doser Project
 

Greetings from all the way down here in New Zealand:-)
I have a challenge for you all.....

I have a need for a PLC to run a Doser pump to pump Additive into a Diesel tank.

The tricky bit is that the Main tank/cylinder is horizontal, so the maths isnt nice and straightforward like if it was a vertical tank.

The other tricky part is that tank filling happens at random times by a third party, and is a random quantity. Unfortunately there is no key lock, or hatch cover, or button press or any other easy digital triggers to alert the PLC that the tank is being filled.

We have an Analog pressure/level sensor which needs to understand/realise when the tank is topped up with diesel and that will trigger the Doser Pump routine.

We need to send alerts/updates via SMS/Txt.

So, I was looking at the Siemens Logo with CMR2020 cellular modem.
The modem integrates nicely with the Logo and it will let me send and receive txts/status etc.
Unfortunately I dont think the Logo will be able to do the complicated maths though.
PlanB is to use a calibration Table for the tank contents (more accurate), however the Logo cant handle tables, so was thinking of Move/Compare type blocks, but again I think the Logo would be limited by its available Block memory?

PlanC is to look at a S7-1200 which can do the complicated maths (Calculate). It doesnt do Tables though, so was thinking of Move/Compare type blocks as the S7-1200 has the block memory capacity?
It looks like the CP1242-7 modem works with it and will do what I require.

PlanD(!) - Looked at Eaton Easy4 - as it does Tables, but doesnt do complicated maths. The Easy4s cant receive SMS/txts, only MODBUS TCP.

There are a few other requirements ie. need 2 Analog inputs, 1 High speed digital input (200Hz) for Flowmeter, 2 Digital inputs for buttons, and 1 Digital out for Doser pump.

So, Long story short, I am looking for Hardware suggestions/recommendations please.

Many thanks.

Cheers,
Steve

If the PLC does not have trig functions but does have floating point math, they are, in principle, straightforward to approximate (cf. https://ntrs.nasa.gov/api/citations/...9700026648.pdf), six digits of accuracy is only two terms, but the kabuki dance needed to get to those two terms is even longer, so the only question is whether the logo PLC can provide entough instructions.

Off topic, but I love when you or Peter say something is simple (which it probably realistically is) and then link to a PDF that literally looks like rocket science, which in this case may be true, as it's a NASA technote :lolis:

Well, to be fair, I did not write "simple, but "straightforward," as in "Your destination is straight ahead, a mere thirty-eight miles down this bicycle path" ;). It's all in the bookkeeping.

Whoops, I think this may be quicker: https://stackoverflow.com/a/394512/1992064

Until I see a better platform, my recommendation for PLC platform remains the same - Beckhoff with TwinCAT-3 software.
Basic arithmetic and trig functions
Extended mathematical functions (paid for library)

If those aren't enough, you can always add the free OSCAT user-made functions and function blocks library, loaded with all the math functions you could ever realistically ask for, plus many, many others.
OSCAT Library documentation
OSCAT (adjusted for TwinCAT) download

Matrix Math (TcMatrix) library for TwinCAT

TF6350 - enables the transmission of SMS messages or emails using PLC function blocks
SendSMS Function Block

FYI the S7-1200 would allow you to implement a calibration table if desired.

The way we have been doing this is by using the flow meter pulses and adding what ever per 10,100 or 1000. For gas we were adding as the trucks were filling at the rack. Same gas different additive for each brand.

Thank you very much for your response.
The Logo doesnt do floating point math, so I guess that its not an option.
My head hurts looking at that document!

Very interesting thanks. I had no idea that Beckhoff made such a product.
I have spent the afternoon looking around their site, and reading the manuals etc and have learnt a lot!

Unfortunately, in my requirements, I failed to say that I also wanted to contact/interrogate the device via SMS/txt.
The TF6350 SMTP/SMS Solution appears to be only Outbound?

Thanks for this info!
I cant find any doc or reference to this in the manual.
Conceptually - how would this be done?

Cheers,
Steve

Yes this would be ideal and so much simpler.
However, we have no control over the filling process at all and cant capture flow rate during the fill :banghead:

A flow meter would be the way to go. Probably less expensive, more accurate and very simple. You can use tank gauges. I did a job with omntec and was able to get a 0/10 output to use for other equipment + around here you need a approved high tank alarms.
https://morbros.com/products/gauges-...tronic-gauging
https://www.omntec.com

What type of math does Logo have available? Can you point to a manual? I am looking at this and there isn't even fixed point math available.

I find it hard to believe that hardware could handle SMS.


What sort of accuracy are you after?

You are thinking of something like this?

The program can be in one of several states:

• Idle
  • Dosing not needed
  • Level is stable i.e. not changing, beyond some measurement deadband, over some time, say half an hour
• Filling or draining: level is not stable
• New stable level:
  • If new level is lower than last stable level
    • Save new level as last stable level
    • Return to Idle state
  • If new level is higher than last stable level, then
    • calculate dosing needed
    • execute calculated dosing
    • Save new level as last stable level
    • return to Idle state

Depending on accuracy required, the dosing calculation probably could get away without trig functions; a naive numerical integration scheme is possible with integer multiplies, adds, and compares, although overflow will have to be dealt with, depending on the raw range of the pressure sensor.

But the extra effort to develop such a scheme would pay for a more capable PLC, many times over.

S7-1200 can definitely do the job.
I dont know if there is library function to do a calibration table, but even if there is I would prefer to write my own in SCL (equivalent to Structured Text).
I wouldnt use the Calculate block.
There should be plenty memory available.

I think that any modern PLC can do the job easily. Especially if it supports Structured Text.
A 'smart relay' such as a Logo will probably be hard pressed to do the job. Not impossible, using integer math and various tricks, but the saving on hardware will be lost in time spent to get it working.