Load cell accuracy

Hello,

I am looking for a load cell I could take an analog signal from into my PLC to build a check weighing system for our products. My weight range would be roughly 50 grams to 250 grams and everything I am finding has a =/- .5 gram or more. Does anybody know of anything with a finer tolerance like =/- .1 grams or =/- .2 grams?

Thanks,

-Dave

http://www.futek.com/product.aspx?t=load&m=lsb200

I see a 1lb loadcell there = 453g
That is pretty small.

You're not going to get high accuracy using an analog signal. When interfacing a weigh scale to a PLC, it is more common to use serial communication. I recently did a project with weigh scales over ethernet/IP on a ControlLogix, they worked quite well. In the past I have seen weigh scales interfaced using BCD output from the scale where 4 24vdc outputs represented each digit of the reading.

The basic problem with the analog signal is it's accuracy is in terms of % of full scale. The weigh scale will be rated +/- a specific weight. The two do not interact well when high accuracy is needed.

dploof23 said:

I am looking for a load cell I could take an analog signal from into my PLC to build a check weighing system for our products.

Click to expand...

Is the PLC already picked?

Is this for a scale that must be certified "legal for trade"?

I modified a high speed case check weigher recently because the OEM controls didn't offer the logging, throughput or versatility we required. The results were excellent. I used a Compactlogix brick, and a Hardy Instruments load cell interface card wired directly to the "IT (Intelligent Technician) summing board for the four load cells.

The H.I. 1769-WS card, once past the learning curve, has many outstanding features, high speed and great resolution. With the IT summing board, I can test and adjust each load cell individually for set up right from the HMI without having to open any j-boxes. The real reason for the success, though, was the fact that I used an existing mechanical design which was already proven to work...(I rebuilt an old worn out Metler Toledo CheckMate).

All I had to do was make it faster, smarter and more "report friendly". Mine is not required to be "legal for trade" certified however, which may have been a deal breaker if it were. I didn't dig into what it might take to get my scale certified..maybe it's not that hard, but it should be one of the first questions to ask.

mellis said:

You're not going to get high accuracy using an analog signal. When interfacing a weigh scale to a PLC, it is more common to use serial communication. I recently did a project with weigh scales over ethernet/IP on a ControlLogix, they worked quite well. In the past I have seen weigh scales interfaced using BCD output from the scale where 4 24vdc outputs represented each digit of the reading.

The basic problem with the analog signal is it's accuracy is in terms of % of full scale. The weigh scale will be rated +/- a specific weight. The two do not interact well when high accuracy is needed.

Click to expand...

Agreed, you want accuracy ? Use a communication method to get data from a scale so that the plc sees the same value on the scale.
Whether it is RIO, CNET, DNET, ENET, TCP/IP , Profibus, whatever, way better than analog out from a scale to an analog in to a plc.

Good analogy of using analog out to an analog in, is like taking a photocopy of a photocopy. Reduces accuracy of resolution.

I am having a hard time understanding why you cannot get high accuracy with an analog signal. Accuracy should be
proportional to the max amount of weight being measured.
This is my understanding of the subject and if its not correct then let me know.
Example
An Allen-Bradley 1762-IF4 analog input module which has 15bits (0-32767) bipolar resolution Which means I can detect changes
down to .00305% =(1/32767) or (.0000305 * whatever the weight is).
Using Jac's recommended Load Cell which has a 453g capacity.
We could detect down to.0138165g.I fudged the typical 4-20mA signal and used 0-21mA instead but still


Is this correct?

It's not that you can't be accurate, it's just two unnecessary conversions that pose a risk to that accuracy that makes it less than ideal when a digital option is available.

So the scale already has the load cell conversion stuff, and a scale head, and all you want to do is capture that for use by the PLC? If you can't get a comm channel between the two devices, then sure, go analog. if you can connect them digitally, you will not have to worry about scaling the analog precisely and twice.

Thanks for all the replies, It makes sense to me to use some type of serial communication if possible.

The scale does not have to be certified, just fairly accurate/repeatable.

I don't necessarily have the PLC type picked out yet per say...however I am partial to Mitsubishi.

My company manufactures plastic bottles and we have a couple of customers in the pharmaceutical industry that have pretty tight tolerances on the weight of the containers.

Thanks again for the help,

-Dave

Okie how does the "digital" option get the 0-30mV from a loadcell to the PLC without converting it? The analog route has only one ADC (the analog card) the same as the digital. I do agreed the amp with communications would be much nicer but in all likely hood not perform a lot better.
Gabsdad is correct in his thinking for the accuracy. That is why you would use a 100lb loadcell in that application.

Cheers

jac; said:

Okie how does the "digital" option get the 0-30mV from a loadcell to the PLC without converting it?

Click to expand...

Yes, I was skipping that obvious part, so that makes three conversions.

jac; said:

The analog route has only one ADC (the analog card) the same as the digital. I do agreed the amp with communications would be much nicer but in all likely hood not perform a lot better.
Gabsdad is correct in his thinking for the accuracy. That is why you would use a 100lb loadcell in that application.

Cheers

Click to expand...

I thought they were talking about communicating between a scale and a PLC, not directly to load cells, although after using the Hardy stuff, that is exactly what I would do, if it fit the application.

Maybe I misunderstood, but I was describing this:
[load cell(s)]<-->[scale head with brains]<-comm cable->[PLC Comm channel]
being better than this:
[load cell(s)]<-->[scale head with brains]<---4-20mA--->[PLC Analog In]

-The original post was asking for a "loadcell" capable of 0.2gram accuracy.

- I'm not really understanding what a Scale Head with Brains is? Typically load cell amp's are brainless and their job in life is to amplify the 0-30mV from the loadcells to something usable such as 4-20mA after setting the span and zero. With the Hardy you are moving the scaling to engineering units from the PLC to the amplifier and probably some diagnostics. Don't get me wrong I would prefer using Hardy if I could justify the cost difference as it is some of the best hardware around. But the same could be accomplished with other loadcells and amp's .

jac said:

With the Hardy you are moving the scaling to engineering units from the PLC to the amplifier and probably some diagnostics. Don't get me wrong I would prefer using Hardy if I could justify the cost difference as it is some of the best hardware around. But the same could be accomplished with other loadcells and amp's .

Click to expand...

Actually, with the HI card, we move the excitation circuit and mV to digital ADC to the PLC card (not sure if there is any amplifier involved). The summing board is just a summing board. The IT summing board has relays and an extra pair of comm wires that let you select individual load cells instead of summing them all.

I am also familiar with the "dumb" load cell amps you are talking about too. it's still 3 conversions if you convey it through a 4-20mA loop.

jac; said:

-The original post was asking for a "loadcell" capable of 0.2gram accuracy

Click to expand...

I suppose I inferred that from his 2nd post, he meant a scale not direct to load cells.

jac; said:

- I'm not really understanding what a Scale Head with Brains is?

Click to expand...

http://cardinal-detecto.centralcarolinascale.com/215_Features.jpg

Sorry for the poor choice of lingo. Are you the same person as dploof23?

FYI
I took a class on Control Logix to Siemens S7 conversions and I believe it was said that siemens Analog Cards can take in mV signals typically generated by load cells. So you could use it for 4-20, 1-10vdc or##mV etc.

You can use any mV input card, but the HI does it with high resolution and 10ms update rates, not to mention the built in functions (like tare and zero) and engineering units.

Okie,

- I understand now what you are using. I can see why you think you would have more conversions the Cardinal unit would have an LD amp to an ADC(Display) then an DAC then to a PLC ADC. You would be correct in this case of an extra conversion but not if you used a “dumb” amplifier to a PLC ADC.
-As far as the “HI card” in the PLC chassis (HI-1746-WS?) with the load cells directly tied to it. I don’t know the electronics of it but I would assume the amp is built into the card then to an ADC with the processor reading it data tables. Same amount of steps less the shielded cable. But this way of doing it though need to have the PLC chassis close to the loadcells where a “Dumb” amp could be placed close and 4-20mA to the Analog In. It is best to not to extend the Loadcell cables. Anyways either way is no more accurate than the other.

Anyways I’ve enjoyed this little chat but I see no purpose of extending it. I look forward to your reply. Until next time.


PS. I have no idea who dploof23 is…..