Math problem I'm too stupid to solve

[Bass]

You are all wrong I tell you!

The answer is: -

Midnight in a glass bottomed boat times three foot six minus a bottle of orange.

and if you want to be utterly precise about it, the constant of integration is a banana about that colour.


Now, what was the question again?

[The Stranger:Me]

Now, what was the question again?

The question is why are you pissed at 9:34 in the morning?

[jono035]

The question is why are you pissed at 9:34 in the morning?

He's in Papakura. That seems to be a common theme among people I know who live out there

[Bass]

The question is why are you pissed at 9:34 in the morning?

Come on! You know that booze doesn't get the colours this bright or the patterns so well defined

He's in Papakura. That seems to be a common theme among people I know who live out there

See above




Sorry about that. Sixties sense of humour I guess.

[Coldrider]

If the cell ps was turned off, the A to D converter being -150mv to 150mv could read full scale deflection negative.
The scaling algorithm could be coded as to inhibit conversions out of the range of 0 to 30 mv, and even if input voltage is less than say 0.2mv, read 0 to inhibit floating values.

[jono035]

If the cell ps was turned off, the A to D converter being -150mv to 150mv could read full scale deflection negative.
The scaling algorithm could be coded as to inhibit conversions out of the range of 0 to 30 mv, and even if input voltage is less than say 0.2mv, read 0 to inhibit floating values.

Yeah, often with SCADA systems for each value they will have a numerical value and a 'health' status, so that if it goes out of range or there is a comms issue you can tell the difference between an actual zero and a zero due to a failure somewhere along the lines.

Doing that such that the value is ignored if it is outside a specific range and an error flag is raised would be a good idea.

Edit: Depending on what the differential amp stage was, my guess is it would read 0mV input with no power. A strain gauge with no supply is basically a resistance between the differential terminals.

[Coldrider]

Yeah, often with SCADA systems for each value they will have a numerical value and a 'health' status, so that if it goes out of range or there is a comms issue you can tell the difference between an actual zero and a zero due to a failure somewhere along the lines.

Doing that such that the value is ignored if it is outside a specific range and an error flag is raised would be a good idea.

Edit: Depending on what the differential amp stage was, my guess is it would read 0mV input with no power. A strain gauge with no supply is basically a resistance between the differential terminals.

Yes, been caught out on an Allan Bradley 5/15, maths overflow error shut down the processor, reset and all OK, they tick away for years then out of the blue they bite....

[jono035]

Yes, been caught out on an Allan Bradley 5/15, maths overflow error shut down the processor, reset and all OK, they tick away for years then out of the blue they bite....

Yikes... That'd be pretty rough, you'd hope they'd just watchdog and reset in that case?

The only PLCs I've really gotten familiar with are the Siemens S7 series. Too big to be of any use to me though, too expensive really. I'd rather just get a board made and use a microcontroller...

[avgas]

Where's Hitcher?

It's "maths" not "math". We are not in 'merica.

As for the question, I'm a simple freight forwarder, so I dunno.

Actually to correct you a bit there. It is math.
Maths is actually different. let me explain.
What is the answer? - Math
Show your 1-10 times tables - Maths

One is a question/subject, the other is a task (or series of)
e.g. I enjoy to run, but i dislike running. I am not good at Math, which is why no one gives me maths to do.
If you want to get pedantic there is computations, computation and Math.

[avgas]

Yeah, often with SCADA systems for each value they will have a numerical value and a 'health' status, so that if it goes out of range or there is a comms issue you can tell the difference between an actual zero and a zero due to a failure somewhere along the lines.

Not if the RTC stops at the same time......
....funny the Citect manual doesn't state "How to put fire out", Guess i better call them up
I also love the aspect that you can raise a flag, the SCADA system crashes, the RTU pages you to tell you the system has crashed yet is all irrelevant because windows update needs to restart the PC.
If only SCADA was designed by a bunch of people who had a procedure rather than a bunch of super intelligent loopy's who wanted to become famous. They can't even talk the same protocol

[jono035]

Not if the RTC stops at the same time.....

So use a simpler clock (doesn't have to be real time, plain RC oscillator for gods sakes) to run your watchdog. With a properly designed watchdog system then this isn't even a problem I've ever bothered with?

Edit: I'm also not sure what you're on about in terms of the 'SCADA system crashing' due to something about windows update but any of the systems we use have failover controllers to avoid this kind of problem. I do agree with you that most SCADA stuff seems to be woefully lacking in the design department though.

[peasea]

He's in Papakura. That seems to be a common theme among people I know who live out there

I think living in Papakura would be a good excuse for being pissed 24/7.

[Winter]

yeah, be careful about that... Is it flagging a warning or actually sending an e-mail to make the order? You might find that if something goes wrong (load cell stops giving output, wire corrodes off, power supply dies etc.) then a rate of change detection will trigger...

Absolute value would be the way that I would do it with it e-mailing you in such a way as to be a reminder rather than an automated system... You can do some basic low pass filtering on the results easily enough too (measure values once per second, sum them every minute and divide by 60 to give you 1 update per minute that attenuates any changes that occur faster than over a 30s or so timeframe).

If your thing can e-mail you or raise a warning or whatever if the output drops below a certain amount too (no input at all) then that could be useful. May require some filtering as well to not trigger on a bottle change (unless you have a bottle cradle heavy enough to be measured).

If the system is switching between bottles when the pressure drops then that would be why it waits until one is pretty much empty... The pressure should be constant with temperature until the bottle is empty (ignoring the outgoing rate of the gas etc.).

I want it fully automated. I'm lazy enough to code something to do it for me. When it reaches the Empty weight (EW) of 35.4kg it will raise a flag. If that flag is still there after 2 days, then its not going to get any smaller. if the weight is within +/- 3kg of what it was when it first hit EW 2 days ago, and as long as it hasn't ordered a bottle in the last 4 days, it will go ahead and order a new one.

The platform on top of the loadcells the bottles sit on are folded from 3mm stainless steel and weigh 5.86923018234529kg. The software knows something is wrong if it falls below that value.

Okay now you got me a bit confused, and without seeming adversarial I have a few thoughts/questions. In terms of the "# of divisions" surely that is just about how many SFs you run the conversion to. In terms of you stating the scale as 300, what does a 0mV output from the loadcell represent (and before anyone jumps in with the obvious joke - it's turned off - it's a serious question).

0mV means 0kg.

Sure thing, ask away!

I'm not really sure what you mean by "how many SF's you run the conversion to". In terms of the ADC itself, the number of divisions comment was meant to say that a 16 bit input ADC will spit out a number between 0 and 65535 which corresponds to what the input voltage is relative to the reference voltage.

If the input range is 0-1V then 0V in will be 0. 1V in will be 65535. Everything else will be scaled accordingly (0.5V being 32768 for instance). To get a number in your program that refers to the actual voltage being measured then you would divide the ADC output by 65535 in this case to get the measurement. 0-1V doesn't mean there are 1001 counts or anything like that, it isn't integer mathematics.

Don't worry about getting anything exactly right, there is no way that those modules will be good for all 16 bits of precision. Noise will eat away a pretty massive chunk of that precision, especially when your reference levels are 150mV. To change the ADC output value by 1, the input would have to be accurate to 4.6uV.

Not entirely sure if that answers your question?

The loadcell is accurate to about 12grams, compensates for temperature within -10/+40°C.
The AD is accurate to +/- 0.1% @ +/-150mV, zero drifts +/-6μV/°C

[jono035]

I want it fully automated. I'm lazy enough to code something to do it for me. When it reaches the Empty weight (EW) of 35.4kg it will raise a flag. If that flag is still there after 2 days, then its not going to get any smaller. if the weight is within +/- 3kg of what it was when it first hit EW 2 days ago, and as long as it hasn't ordered a bottle in the last 4 days, it will go ahead and order a new one.

The platform on top of the loadcells the bottles sit on are folded from 3mm stainless steel and weigh 5.86923018234529kg. The software knows something is wrong if it falls below that value.



0mV means 0kg.


The loadcell is accurate to about 12grams, compensates for temperature within -10/+40°C.
The AD is accurate to +/- 0.1% @ +/-150mV, zero drifts +/-6μV/°C

0.1% is 10 bit precision more or less, which is pretty much exactly what I would have suspected. What you described at the start sounds good.

Your platform weight has a pretty serious excess of precision though given the accuracy numbers you've already stated!

If you're getting numbers out and they seem all good then sweet.

[Winter]

0.1% is 10 bit precision more or less, which is pretty much exactly what I would have suspected. What you described at the start sounds good.

Your platform weight has a pretty serious excess of precision though given the accuracy numbers you've already stated!

If you're getting numbers out and they seem all good then sweet.

they 'seem' all sweet. But I'd love to get my hand on a 50kg set weight to check it all.

Can you get 'accurate weights' from somewhere?