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Load cell problem

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Vikky

New Member
Hi all,

I am in process of developing a force sensor . This force sensor should be work till +/-20 N. So i have choosen load cell to develop this.

I made an instrumentation amplifier circuit using INA326 which would read 600mv at output when no load is applied. At maximum of +20 N (compresion) load it should read 1100 mv and on other side with -20N (tenstile) it should read 100 mv.

I faced while calibrating the sensor. When load cell was given the compresion force by specific weight e.g 200grm the output of amplifier gave me 660 mv and while the same weight was hanged on the load cell (tensitile force) it gave the output of 647 mv. A differene of 13 mv between the compression and tenstile force.

Whether the force sensor has different output on tenstile and compression force or its something with my circuit???
 

ericgibbs

Well-Known Member
Most Helpful Member
Hi all,

I am in process of developing a force sensor . This force sensor should be work till +/-20 N. So i have choosen load cell to develop this.

I made an instrumentation amplifier circuit using INA326 which would read 600mv at output when no load is applied. At maximum of +20 N (compresion) load it should read 1100 mv and on other side with -20N (tenstile) it should read 100 mv.

I faced while calibrating the sensor. When load cell was given the compresion force by specific weight e.g 200grm the output of amplifier gave me 660 mv and while the same weight was hanged on the load cell (tensitile force) it gave the output of 647 mv. A differene of 13 mv between the compression and tenstile force.

Whether the force sensor has different output on tenstile and compression force or its something with my circuit???
hi,
Reference your last thread on this project, did you find the answers of any use.? That thread appeared to have died.:confused:

If its a load cell thats designed for compression AND tensional forces the outputs are linear.

If its a compression load cell it will not be calibrated for tension.

Whats the model number.??
 

Vikky

New Member
hi eric,

Yes, i now understood the difference between the op-amps (the my previous thread).

The load cell i am using is CZS -101 from Joint sensor instruments (szjoint.com) .

I have read its details, their i didn't find any specification whether it is meant for tenstile or compression force. I have even e-mailed them about it application note but still waiting for their reply.
 
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ericgibbs

Well-Known Member
Most Helpful Member
hi eric,

Yes, i now understood the difference between the op-amps (the my previous thread).

The load cell i am using is CZS -101 from Joint sensor instruments (szjoint.com) .

I have read its details, their i didn't find any specification whether it is meant for tenstile or compression force. I have even e-mailed them about it application note but still waiting for their reply.
hi,
The sz site comes up in Chinese.??

Also I have searched the web for CZS-101 with no success.?
 
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ericgibbs

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Yes,

Its a chinese site. I too translated into English through google translator.

For your reference here is detial of the sensor attached.
hi,
Its a brief spec!

It looks like a simple compression cell.

From that spec what do you make the max weight, it cannot be 50,100lbs!!
It must be 50lbs.?? can you confirm that.??

You say you are measuring a force of +/-20N [+/-5lb]
 

Vikky

New Member
You say you are measuring a force of +/-20N [+/-5lb]

Yes,

The force sensor should working till +/- 20N i.e, ~ 2.035 Kg. If i compression the load cell by keeping a weight of 2 kg on one of its arm the reading should give me ~ +20N or ~ 1100 mv.

Same would be for the tenstile force but the reading would be ~ -20 N or ~100mv. Actully we were having this load cell in the stock and i was asked to develop the application using this one.
 

ericgibbs

Well-Known Member
Most Helpful Member
Yes,

The force sensor should working till +/- 20N i.e, ~ 2.035 Kg. If i compression the load cell by keeping a weight of 2 kg on one of its arm the reading should give me ~ +20N or ~ 1100 mv.

Same would be for the tenstile force but the reading would be ~ -20 N or ~100mv. Actully we were having this load cell in the stock and i was asked to develop the application using this one.
hi,
I guess that the 100mV and 1100mV are after your instrument OPA.
The basic cell is 1.6mV/Volt, I assume the excitation is Vmax ie 6Vdc.

Ref your OPA, have you nulled out any standing OPA offset voltages.??

The difference of 13mV could be due to OPA offset, whats the output of the OPA when the cell is unloaded.??

What is the max weight for this cell.?
 
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Vikky

New Member
Ref your OPA, have you nulled out any standing OPA offset voltages.??
Yes i have kept an option for nullfying the effect of the offset voltage. I have made the op-amp to read as 600 mv i.e, 0 Newton when the load cell is unweighted.

Here is my schematic attached.

Tha max weight for the cell is 50 lb, but i would go till 2 kg i.e, 4.5 lb since at weight i will give me 20 N force according to the equation.
 

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ericgibbs

Well-Known Member
Most Helpful Member
Yes i have kept an option for nullfying the effect of the offset voltage. I have made the op-amp to read as 600 mv i.e, 0 Newton when the load cell is unweighted.

Here is my schematic attached.

Tha max weight for the cell is 50 lb, but i would go till 2 kg i.e, 4.5 lb since at weight i will give me 20 N force according to the equation.
hi,
I have been looking thru your diagram.

The 2K offset pot across the bottom two arms of the load cell bridge, which offset is this being used to correct for.?

The offset to pin5 of the INA326 from the pic/dac, is that to give the 600mV point for 0N.?
 

Vikky

New Member
Hi,

The 2K port which is at SS1 and SS2 is for cancelling any of the offset voltage from the bridge (for the factory calibration).


The offset to pin5 of the INA326 from the pic/dac, is that to give the 600mV point for 0N.?
Once it is in the field the user can calibrate the sensor with the help of the switch press. Rest of the operation will be taken by the PIC to set the offset.
 

ericgibbs

Well-Known Member
Most Helpful Member
Hi,

The 2K port which is at SS1 and SS2 is for cancelling any of the offset voltage from the bridge (for the factory calibration).



Once it is in the field the user can calibrate the sensor with the help of the switch press. Rest of the operation will be taken by the PIC to set the offset.
hi,
I would like to see what the manufactures say about the cell ie: comp/tension usage.

Is it possible to measure the actual cell output at the +/-20N extremes.

Also as the cell limit is 50lb and you are using about 5lbs, the cell is down in the bottom 10% of its range, also its only got 4Vext.

So its only a 640uV change over +/-20N.

What is the actual gain that you are expecting from the INA.??
 

ericgibbs

Well-Known Member
Most Helpful Member
hi Vikky,
Done a few calculations and LTspice simulations and the cause of the non asymmetric result for compression and tension in the load cell is due to the method you are using for nulling the factory offset.

You have in effect unbalanced the symmetry of the bridge elements by adding two fixed resistors [the two 1K's and the 2Kvar] across the lower elements of the bridge.
Considering also the total value across the two lower bridge elements will be different after you have nulled the bridge.

So for a given compression force the bridge output is different for the bridge in tension with the same force.
Its only a few uV, but you have a differential OPA with a gain of around 2000 on the bridge output.
So an imbalance or about 6uV will give a 13mV error at the diff amp output.

Do you follow OK.?
 
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Vikky

New Member
hi eric,

i was not at work for past few days and so i didn't get chance to check what you have instructed in the earlier post.

Today i have downloaded the simulation software for INA326 as instructed by the TI and found some strange results which i didn't expected to be happen in my design.

The offset which i have kept (2 fixed resitor of 1K and a var. resisitor of 2K), is not working at all. I have purposely kept it their so that any error at the output ( at no load 600mv)which remains would be eliminated. If i change the value it have no effect on the output voltage. The only thing effect is the gain and reference voltage.

Here i am attaching the simplified circuit through which would sense the voltage change across the load cell (sensor ). It would change by 28.2 uV per 100 grm of weight on it (as per sensor datasheet) which should result in change of 24.5 mv at the output for my case. But the change in output voltage at both the extreme 575.6uV (for max load) is different. On one end it goes to 1.12 V and on the other side it remains at 106mv.

Any idea so that voltage range can be achieved as 1.1 V and 100 mv at extreme linearly??
 

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ericgibbs

Well-Known Member
Most Helpful Member
hi Vikky,
Read and copied your post, I'll go thru it.

Did you follow may last post regarding the bridge unbalance, also any news yet from the load cell company.?
 

Vikky

New Member
Yes Eric,

On the hardware tested it, the 13 mv difference is because of the bridge unbalane condition. So i am designing a new circuit which i will nullify this effect.

Unfortunately their is no any news from the compnay itself after sending 4 reminder mails. May be they are more interested in creating new customer rather than maintaining the existing one.
 

ericgibbs

Well-Known Member
Most Helpful Member
Yes Eric,

On the hardware tested it, the 13 mv difference is because of the bridge unbalane condition. So i am designing a new circuit which i will nullify this effect.

Unfortunately their is no any news from the compnay itself after sending 4 reminder mails. May be they are more interested in creating new customer rather than maintaining the existing one.

hi,
The circuit that you have posted dosnt make much sense.?

The resistors in the bridge input are all different values.?
Also the Gain set resistor is 8k9, for a single supply OPA configuration the datasheet states, when pin5 [ref] has 2meg to the Vref , its only 2K maximum,.? [about *2000 gain]

The circuit as posted by you dosnt work at all in simulation.??

Do you have the LSpice model for the INA326.??? so that I can compare.

EDIT: just installed Tina from TI, do you have the Tina model and file that you could post.??
 
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Vikky

New Member
hi Eric,

I am extremly sorry for uploading the old schematic. Please discard the earlier one. Here is the new schematic attached.

I also found one strange problem. I created a new simulation file and here instead of keeping a variable resistor of 8.9K, i fixed it up with the same amount of resistor. Also the two resistor connected with the reference supply 1.5M and 0.5M , i connected one resistor of 2M ohm.

Making these changes i found that the voltage at the output of the amplifier differs. For matching the output i have to change the gain resistor value (R1).
In one case the R1 = 8.8K and while in the other case R1/Gain resistor is 4.5K!!!

I am also attaching the screen shot of my simulator. The voltage is measured at the output across 1K resistor.
 

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ericgibbs

Well-Known Member
Most Helpful Member
hi,
Look at this image from the datasheet, the gain resistor is over 4 times the specified maximum value.

The INA326 could give misleading results.
 

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ericgibbs

Well-Known Member
Most Helpful Member
hi Vikky
Relooking at your project, just consider a compression load at first.
The L/C is rated at 1.6mV/V for a 50lbs load.

The Vext= 4Vdc so for 50lbs that would be 6.4mV output from the L/C.
Your working range is 20N, say 5lbs, so the bridge output for +20N would be 640uV.
You need to raise this to 500mV from the INA326, thats a Gain of 781.

The same calculations apply for a tensile load of 20N.
ie: -640uV raised to -500mV, still a Gain of 781.

Without offset the output from the bridge, between -20N and +20N would be 1280uV 'change' * 781= 1000mV

As you are using a single supply for the INA326 the -500mV cannot be achieved so you have an Voffset at +600mV.

This offset shifts the output swing of the INA326 to +100mV thru +1100mV, with a dc centre point of +600mV.

Summary: for a INA326 Gain of approx 780, the RG resistor is 2K and the R2/C2 combination is approx 750K and 150nF.

These calculations show that your trial values of Rg of 4k5 and 8k and the R2 at 2MR are well outside of specification.

OK.:)
 
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