How to use loadcells?

I am going to upgrade a machine that has 4 loadcells attached to the same frame.
What would be the best way to get the signal from the loadcells into the plc.
Do I buy a transducer with an analog output or is one with a digital ouput (rs232/485) the way to go? Since I'll be using an AD 06 plc I guess that a digital output would be the best option. However, I didn't find a lot of info on the protocol used by these transducers. Is it a simple ASCII based protocol or do you have to be a rocket scientist to communicate with these beasts?
Anyone has a recommendation?

Another question is, how does one wire the 4 load cells into the tranducer?

Thanks for the help....

A few years ago, I set up some load cells, and had the same problem you did.

My initial thought was to use analog 4-20mA, but the boss had some bad experience in the past with that and said no.

Next try was using ASCII communications. This ended up being too slow for the job, and also to complicated for that brand of PLC.

Next was field bus. We hadn't used any field busses on site at that time, but after we tried it, we never looked back.

The brand of "transducer" was Mettler Toledo (Actually a "Panther" weight panel) and the protocol was Modbus plus. Later we ended up using some more elsewhere on Profibus, with about a dozen panels in total.

The panels also came with the option of Device Net and Modbus Ethernet.

I believe this brand has had a few more advances since then, so you could look at their lates offering.

I would also look at your PLC brand, since many also have load cell cards, eliminating the need for any additional devices or wiring.

Hope this helps,

Doug

Doug, I forgot to say that speed is not important. The machine takes a sample of potatoes, about 50 kilograms. Then it fills 1 of 3 bins with these potatoes (potatoes are sorted to size), and then the bins have to be weighted.
Since the DL06 PLC supports ascii communication as standard I would prefer this if it works. I guess you didn't have a bad experience with ascii communication, the only problem was the speed, right?

I'll take a look at the Toledo products...
Thanks

Thierry,

I have been using loadells with the AD line of PLCs for many years now. I have interfaced with several different indicators using the CP-128 in a 205 rack. I recently set up an 06 to do the same thing. Most indicator have ASCII RS-232 out. With the 06, you will need to use the VPRINT and AIN instructions to get the data from the indicator to the 06. AD has several sample files showing how to do this with the 06.

Since you are using 4 loadcells, you will need to get a summing box to bring all 4 signals together and output as one signal to the indicator.

There are a few strain gage modules out there that you can bring in to the PLC using a 0-10 VDC(or 4-20mA) analog input card but realize that the resolution of these analog cards is only 4095 (12 bit cards). If you want a 1:1 with kilograms then the best resolution (for 50kg) you will be able to get is .1 kg. At that resolution you will be able to measure 409.5 kg. You will do all the calibrating (scaling) from within the PLC.

I am replying to this from home tonight (Sunday) but will give you a couple of sources tomorrow when I get in to work.

Bob

Thanks Bob, it would surely be helpfull if you could send me the link of the indicator you use with the 06. That way I can be sure that it will work. I'll have enough new things to do (i need to interface the 06 with a pc through ethernet), so if I use the same equipment as you at least I'll know the weighing stuff will work.

Tx

Hello Thierry,

I've build several installation with weighingcells. Mostly 3 per machine and sometimes just 1. Why did they install 4? But that's besides the question.
There are several suppliers in Belgium, like Doug was telling you have Toledo,
another good one though is Precia-Molen, they have a good technical support coordinator

Luk Van De Look
Tel 03/886.00.84
GSM 0475/62.39.75

Secondly you have

Jtec
Rudi Thys
tel 03/660.52.05
GSM 0475/59.18.61

And as last one you have Ktron, is a company that builds
dosing devices.

Hope this helps.

Thierry,

The other problem I had was the early model Modicon Quantum I was using. At that time, the ASCII card was quite cr@@@@, I mean, it wasn't very good. I ended up using about 300 lines of instruction list to get it to work (including error checking and other features). A PLC with a better ASCII card would have made it work better.

Doug

If you want to use a field bus, the DL-06 supports Modbus RTU. If you load cells have serial communications, the odds are that they will support Modbus and you can use the RX instruction in the DL-06 to get the data.

I have used load cells with 4-20 mA signal conditioners successfully in the past. The signal conditioners are usually 24 VDC powered, and supply the excitation voltage to the strain guages in the load cell and ouput a 4-20 mA signal. If you use a quality product (I used M-Systems but Red Lion and Action Instruments and many others have quality devices) you will not loose much in accuracy, and you can calibrate them to the range and accuracy you need.

Remember, even with a 12 bit analog input, 0-4095 data range, you get resolution of 0.02% (1 part in 4095). This is probably in hte same range of accuracy as the load cell itself. If you select the range of the devices properly, with the probable max weight at about 75% of the range of the transmitter/load cell, you will have sufficient accuracy for your application.

Remember, even with a 12 bit analog input, 0-4095 data range, you get resolution of 0.02% (1 part in 4095). This is probably in the same range of accuracy as the load cell itself.

Click to expand...

Tom,

In the size that he will need (probably 75kg/100kg) if he calibrates for full scale his resolution would 18g/24g. The A/D hardware in a typical indicator is at least 16 bit, some as high 20 bit. This allows a higher degree of resolution than the analog input cards that are offered by AD. I wish they made a 16 bit card. I would have been all over it like butter on bread. As I stated before, if thierry decided to use an analog input card, his resolution would have to be in 100g increments if he wants the least significant digit to increment by one.

I've build several installation with weighingcells. Mostly 3 per machine and sometimes just 1. Why did they install 4? But that's besides the question.

Click to expand...

Fritz,

Four loadcells are typically used in large platform scales. There is one loadcell in each corner and they bring all four leads back to a summing box that joins all the signals together and outputs as one main signal to the indicator.

Theirry,

AS far as the 06 is concerned, unfortunately the loadcell I am using is the FIT II from HBM.
http://www.hbm.com
This loadcell has the A/D transducer physically inside the cell's case. HBM does offer a seperate A/D transducer though. There are two: the AD104 and the AED. This company is located in Germany so you should not have any trouble getting a sample. If you decided to go this route, I could help you with the command set. I think it is the same for the FIT II and the AED.

I have had no experience with this device but have seen it used in other scales.

http://www.actionio.com/products/slimpak2/wv448.html

The indicators that I have used in the past (strictly ASCII) are:

From Rice Lake Weighing Systems:
http://www.rlws.com
UMC-222
IQ-520
IQ-700
IQ-710
IQ-800
IQ-810

From Mettler/Toledo:
The Viper

I hope this helps you,

Bob

testsubject said:

Tom,
In the size that he will need (probably 75kg/100kg) if he calibrates for full scale his resolution would 18g/24g. The A/D hardware in a typical indicator is at least 16 bit, some as high 20 bit. This allows a higher degree of resolution than the analog input cards that are offered by AD. I wish they made a 16 bit card. I would have been all over it like butter on bread. As I stated before, if thierry decided to use an analog input card, his resolution would have to be in 100g increments if he wants the least significant digit to increment by one.

Click to expand...

First of all, let's not confuse resolution with accuracy. Allen Bradley PLCs have 16 bit resolution (or 15-1/2 bit or some such nonsense) on a lot of their analog inputs. But my experience in the field is that the accuracy isn't any better, because of drift and zero shift and so on.

Second, if the analog input to the PLC is scaled properly, the accuracy and resolution as a percent of full scale is unchanged. If his strain guage output (the sum of the four signals) is 4-20 mA at 0-100 kg his resolution is .02% or 0.02 kg or 24 grams. Only Thierry can tell us if that is sufficient for his application or not, but he shouldn't worry too much about 0.02% accuracy on the input card if his strain guage is ±0.1% accuracy, for example! It doesn't matter how many significant digits the field bus data contains if the primary sensing element, the strain guage, isn't more accurate than the input card!

The same percentage resolution and accuracy applies if the stran guage was 0-6,000 tons. In that case the resolution would be 2,930 lbs or about 1-1/2 tons. That seems pretty bad, but at 0.02% is still better than most primary sensing elements.

It doesn't matter how many significant digits the field bus data contains if the primary sensing element, the strain guage, isn't more accurate than the input card!

Click to expand...

Tom,

You are right. You assume that the loadcell accuracy is less than the resolution. I went the other way and assumed the loadcell accuracy is greater than the resolution.

I typically deal with smaller loadcells(<50KG) with an accuracy of +-0.03%. Let us not forget that this limit represents the guarantee from the loadcell manufacturer. I have found that it is often much better than that in operation for the loadcells that I use. If I wanted to use a 20kg loadcell (implied accuracy +-6g) to measure a 10kg load with 1 gram increments (10.000kg), I cannot with the analog input card. The resolution is only 4.88g/div. I know it is possible because I have done this with strain gage indicators.

So, assuming that the loadcell is much more accurate than the resolution of the analog card, the analog card is the limiting factor. That is why I do not use them.

Did you try to use F0-04THM it have mV input and should work with loadcell with 16 bit resolution.
I will try to do something with that at soon.
If it will work it will save some monay and will give accurated system.

Arik,

No I have not. Maybe someone else here has.

Bob

Bob,

Yes, you were right. We even got several weighing devices which use 4 weighingcells. We use those for truck weighing.

I was to focused on dosing devices and gauging.

thanks for pointing that out.

Rudi

Thanks guys for the help, to bad my internet connection went down (due to some work in the house).
I will order a conditioner from sensy (as loadcells from them are used on the machine) and try to use ascii communication with the 06 PLC. I'm sure I'll be back for some more help ;-)