@ericgibbs I am also looking for solution to combine multiple loadcell without using an junction box.
I see u are in the chat with this related topic and sending the PDF to everyone.
My question is that if I want to combine the loadcell with parallel, than trimming is neccessary right? which mean an extra resistor is needed in each loadcell

 

Adding sensors wouldn't it mean adding their own errors?

 

Hi jim.
What are the details on the L/C's and how many in parallel?
E

 

Hi jim.
What are the details on the L/C's and how many in parallel?
E

I am using the 4 wire load cell combing in parallel just like the PDF display but "without" the junction box.
Not sure will this work or not.

 

What do you hope to achieve by connecting load cells in parallel?

 

What do you hope to achieve by connecting load cells in parallel?

I am using a metal plate as the measuing surface and place the four load cell at each corner.
I hope I can directly get the sum value of these four loadcell

 

I am using a metal plate as the measuing surface and place the four load cell at each corner.
I hope I can directly get the sum value of these four loadcell

Hi Jim,
That is the way I construct and connect my commercial load cells
Ensure that the signal senses are correctly polarised.
What is the metal gauge of your base platform?

E
BTW: if you need to trim each load cell, use a suitable variable resistor in the positive supply to each LC.
Diagram in that PDF.

 

At least one manufacturer of higher quality load cells suggests that similar load cells can simply be connected in parallel to accurately sum the combined load. That does make sense to me.

 

At least one manufacturer of higher quality load cells suggests that similar load cells can simply be connected in parallel to accurately sum the combined load. That does make sense to me.

That would be like connecting multiple voltage sources in parallel. I suppose that since there is some internal resistance in each voltage source, they will form some kind of average voltage. Make sure that all sensors are identical.

 

Hi MrC,
As you can appreciate, the weighing baseplate could be a 'biggish' are say >1mtr sqr.

The L/C's are mounted/bolted on a steel underneath frame, one on each corner area, with the baseplate, which carries the load to be weighed mounted on top of the L/C's.
The L/C's are wired in parallel so that if the applied load weight is off-centre on the baseplate, then the average L/C mV output is input to a suitable INA amp.
To ensure that the L/C's are calibrated,it is usual to add a low value trim pot in series with individual L/C's, and adjust the span mVolt by varying the LC excitation voltage, taking the lowest sensitivity L/C as the reference.

Each L/C should have the same mV/V and be rated to carry the maximum rated load of the weighing system.

E

 

Hi MrC,
As you can appreciate, the weighing baseplate could be a 'biggish' are say >1mtr sqr.

The L/C's are mounted/bolted on a steel underneath frame, one on each corner area, with the baseplate, which carries the load to be weighed mounted on top of the L/C's.
The L/C's are wired in parallel so that if the applied load weight is off-centre on the baseplate, then the average L/C mV output is input to a suitable INA amp.
To ensure that the L/C's are calibrated,it is usual to add a low value trim pot in series with individual L/C's, and adjust the span mVolt by varying the LC excitation voltage, taking the lowest sensitivity L/C as the reference.

Each L/C should have the same mV/V and be rated to carry the maximum rated load of the weighing system.

E

Hi Jim,
That is the way I construct and connect my commercial load cells
Ensure that the signal senses are correctly polarised.
What is the metal gauge of your base platform?

E
BTW: if you need to trim each load cell, use a suitable variable resistor in the positive supply to each LC.
Diagram in that PDF.

Right now I am very confused on why would this work. I understand the 4 wire loadcell is a Wheastone bridge circuit. The circuit should sum up all of the cross voltage of each Wheastone bridge, but by the parrallel combination the strain gauge (which is the resistor in each Wheastone bridge) in different loadcell are all tangle up

 

Hi,
This simulation image shows the effect of two bridges of the same type and specification in parallel.
The left side shows one bridge and the right side two bridges in parallel.

Note: the simulated weight on the right pair has been shared by the two bridges, so the output voltage is half that of a single bridge,
It is the average signal voltages of the bridges connected in parallel.
If 4 bridges were connected in parallel, the voltage out would be Vout/4

That is the limitation of connecting load cells in parallel.

The simulated weights are Vx and Vy.

Did you read and understand the PDF I posted.?

E

BTW: my original question.
What are the details on the L/C's and how many in parallel?

 

@ericgibbs Why did you make Vx and Vy different?
Two sensors in parallel is equivalent to replacing all four resistors with half their values.
I did the calculation by hand and got the same results for the two situations.

 

Of course the load cells must be linear as well as "identical. If you do the math exercise and put the four bridges in parallel the results come out the same as for summing the outputs individually. If done correctly. Of course, all four load cells need to be the same temperature also.
The math proof is indeed a bit tedious.

 

Hi C,
Consider that a single L/C is rated at 10mVout at say 1kg and you place a 1kg weight on the LC it should give 10mVout

By using a baseplate to spread the 1 kg over two L/C's of the above specification, you have effectively created an L/C weighing unit rated a 5mVout for a 1 kg load.

So the resistance change in the bridge arms resistance will be halved as the bridge arms are connected in parallel, that is why Vy is halve of the Vx value.

Look at it another way.
Consider that you had 3 identical coiled springs.
At the end of one spring, you hung a weight of 1 kg and measured the vertical displacement of the end of the spring when weighted and not weighted.

Now hang the 1 kg weight on two parallel vertical springs, the vertical displacement will be half of that of the single spring

Does that answer your query.

Eric

 

Of course the load cells must be linear as well as "identical. If you do the math exercise and put the four

Hi Bill,
The 4 L/C's should ideally be identical, but in almost every case they are not, so the method is to find the L/C which has the lowest mVout at the rated max weight, then add a very low value individual trim pot on the other 3 L/C's and trim the Vext so the mVout matches the least.

The overall mVout when connected in parallel will be the average of the 4 L/C's

E

 

At this point I suggest investigating the applications section of a reputable load cell maker's website, or application literature. This is not a new issue. Others have made it work before.

 

i am not sure i understand why would one choose to parallel them. i mean sure it can work out but ... a lots has changed since 1843. what about when things do not work out, like when there is a short or wire break... paralleling them adds complexity and with single indictor one would not necessarily know if and what is wrong. even if everything is working, one would only see sum and not know of load imbalance.

 

Multiple load cells in parallel is a much less complex scheme than four individual strain gage amplifiers and some summing scheme. And as for "what if something came apart?"
My response is that when done correctly to the applicable assembly standards that is not likely. My guess was that the connections would be done by an individual familiar with the correct assembly standards.