4 Wire Strain gauge in Wheatstone Configurations

Thread Starter

abc14

Joined Oct 15, 2017
112
Hello Chaps,


I am familiar with using 3 wire load cells in a Wheatstone configuration. But I needed higher weight values hence decided to go with a 4 wire load cell with higher weight capacity. The load cell I am using is below.

https://www.ebay.co.uk/itm/100kg-42mm-x-38mm-x-3mm-Electronic-Scale-Body-Load-Cell-Weighing-Sensor-/362278699862

I need to connect this a amplifier like the one below.

https://www.ebay.co.uk/itm/HX711-weighing-sensor-module-24-bit-A-D-for-Arduino-and-ESP8266-UK-Stock/122366918062?hash=item1c7da2fdae:g:~QYAAOSwx6pYrMBt&_sacat=0&_nkw=hx711&_from=R40&rt=nc&_trksid=p2047675.m570.l1313.TR1.TRC0.A0.H0.Xhx711.TRS0


I am not exactly sure how would I connect all four at a time. I am successfully connecting one to the amplifier. But all 4 at the same time I am not sure.






I tried connecting by joining all four red, white, black, and green wires together and there output to relevant connectors to the board, but I was not getting any meaningful readings.

Any thoughts are welcome.
 

ericgibbs

Joined Jan 29, 2010
9,350
hi,
Thats the method, I assume all 4 cells will be loaded by the same Base plate.?, so they share the Load equally.?
As you know your 1k impedance cells are rated at 1mV/V , with a 5V recommended excitation.

With all 4 cells connected what excitation voltage do you measure.??

E
 

Thread Starter

abc14

Joined Oct 15, 2017
112
hi,
Thats the method, I assume all 4 cells will be loaded by the same Base plate.?, so they share the Load equally.?
As you know your 1k impedance cells are rated at 1mV/V , with a 5V recommended excitation.

With all 4 cells connected what excitation voltage do you measure.??

E
I am measuring 3.3v excitation voltage on a demo setup. But I could easily supply 5Vs. Now it won't be a base plate. Idea is to create a smart bin. Where I intend to put load cells under four tyres of the bin like the one below.

upload_2018-8-4_16-49-46.png


And run the long wires to MCU on top of bin, or if length of wire is an issue, then maybe do the amplification below the bin and set the extracted signal to the top either wired or maybe using an rf module.

Now couple of things I am not sure.

1. Obviously would it work ?
2. Second the weight of these bins is around 50 KG empty. Would the load cell be able to withstand constant weight over longer periods of time. Unlike bathroom scale where strain would only apply when someone would step on it. In this case at minimum 50KG would always be there and perhaps alot more.
 

ericgibbs

Joined Jan 29, 2010
9,350
hi,
They are specified as 100Kg, so in theory the gross weight could 400Kg.
Many load platforms/bases will apply a constant load on a cell and creep will occur.
So in service the platforms are Zeroed before weighing and also calibrated and the Span adjusted.

The spec sheet does quote a creep figure of 0.05% FSD over 30 minutes.

E
 

Thread Starter

abc14

Joined Oct 15, 2017
112
hi,
They are specified as 100Kg, so in theory the gross weight could 400Kg.
Many load platforms/bases will apply a constant load on a cell and creep will occur.
So in service the platforms are Zeroed before weighing and also calibrated and the Span adjusted.

The spec sheet does quote a creep figure of 0.05% FSD over 30 minutes.

E
Ok I will have to factor in the creep in service. In theory I could eliminate the 50KG bin weight by Calibrating to zero. Hence my weight readings would swing between 0 -440kG rather then 50- 400KG ?
 

ericgibbs

Joined Jan 29, 2010
9,350
hi,
You would Zero the scale by Nulling out the Bin weight then you would need to Calibrate scale using known weights and adjust the Span to calibrate to the correct weight reading.
 

Thread Starter

abc14

Joined Oct 15, 2017
112
hi,
You would Zero the scale by Nulling out the Bin weight then you would need to Calibrate scale using known weights and adjust the Span to calibrate to the correct weight reading.

Thanks.

looking at below configuration

upload_2018-8-4_17-28-20.png



I would get connections 1,2,3,4 for loads cells on left side, and connections 1,2,3,4 on the rightside of the Wheatstone bridge.
To be able to correctly connect to Hx711, I would join both 1s and connect to E+, ad join both 2s and connect them to E-. Then subsequently join both 3s and 4s and connect them to s+ and s- of the amplifier respectively ?
 

ericgibbs

Joined Jan 29, 2010
9,350
hi,
Your description looks correct.
Note: if you do not use the 3.5v from the HX for the excitation and the you use 5V, you MUST follow the notes in the attached image.

Cables should be screened.

I would suggest you start debugging with just one load cell, then add the other 3 later.

How are you mounting the load cells and what are you weighing in the bin.?
E
AA1 04-Aug-18 17.35.gif
 

Thread Starter

abc14

Joined Oct 15, 2017
112
hi,
Your description looks correct.
Note: if you do not use the 3.5v from the HX for the excitation and the you use 5V, you MUST follow the notes in the attached image.

Cables should be screened.

I would suggest you start debugging with just one load cell, then add the other 3 later.

How are you mounting the load cells and what are you weighing in the bin.?
E
View attachment 157532

I haven't got the test bin yet, we have ordered one. its on the way. until now I am been testing the hX711 with 50kg load cells on a bathroom scale. Today I received the 100KG loads cells. Wasn't sure how to connect all four together. I connected 1 by supply 3.3V from MCU to HX711 and got the readings that way.

I will keep you updated once I get the actual bin and also done some more debugging.
 

ericgibbs

Joined Jan 29, 2010
9,350
hi,
If your wheelie bins are emptied in same way as ours, mechanical lift and thump into the back of a truck, the electronics will have to be robust.
E
 

Thread Starter

abc14

Joined Oct 15, 2017
112
hi,
If your wheelie bins are emptied in same way as ours, mechanical lift and thump into the back of a truck, the electronics will have to be robust.
E

Good point. We thought about it, I will make the prototype robust as possible. Although at this stage its a case study, I will get a bit of favour from guys emptying them to look after our electronics. But I see the merits of placing and protecting the electronics efficiently.
 

Thread Starter

abc14

Joined Oct 15, 2017
112
Good point. We thought about it, I will make the prototype robust as possible. Although at this stage its a case study, I will get a bit of favour from guys emptying them to look after our electronics. But I see the merits of placing and protecting the electronics efficiently.

Hi,


Is this correct configuration for connecting 4.

upload_2018-8-5_9-41-9.png
 

Thread Starter

abc14

Joined Oct 15, 2017
112
hi abc,
Looks correct to me.
4 * 1K bridges, load on Vext = 250R at 5V thats 20mA.
E
Hi Eric,

Yes I am getting 272R between 1 and 2 on the indicator. I am running the Hx711 at 3.3V from MCU and getting voltage of 2.5V between 1 and 2. However my weight readings are all over the place. Although I placed the loads cell on a flat table at the moment for debugging.

I am not even sure which side of the load cells should be placing upwards.


upload_2018-8-5_10-4-17.png
 

ericgibbs

Joined Jan 29, 2010
9,350

Thread Starter

abc14

Joined Oct 15, 2017
112
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: -0.0 lbs
Reading: -0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.1 lbs
Reading: 12.7 lbs
Reading: 59.5 lbs
Reading: 78.6 lbs
Reading: 78.4 lbs
Reading: 73.7 lbs
Reading: 51.0 lbs
Reading: 8.8 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 7.4 lbs
Reading: 49.2 lbs
Reading: 73.7 lbs
Reading: 66.1 lbs
Reading: 26.8 lbs
Reading: 1.6 lbs
Reading: 0.0 lbs
Reading: -0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.3 lbs
Reading: 1.5 lbs
Reading: 0.5 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs
Reading: 0.0 lbs




I get these readings even place my hand on it.
 

ericgibbs

Joined Jan 29, 2010
9,350
hi,
Ideally you need a known weight, just pressing it will not help.
For light weights I use UK £2 coins, at 12.00gms
A litre of water is ~1Kg.
Do you have some working bathroom/kitchen scales that you could use as a weight reference.?

Set up your rig for using just one LC connected, use a reference weight and compare the displayed value for each of the 4 LC's in turn.

E
 

Thread Starter

abc14

Joined Oct 15, 2017
112
hi,
Ideally you need a known weight, just pressing it will not help.
For light weights I use UK £2 coins, at 12.00gms
A litre of water is ~1Kg.
Do you have some working bathroom/kitchen scales that you could use as a weight reference.?

Set up your rig for using just one LC connected, use a reference weight and compare the displayed value for each of the 4 LC's in turn.

E
I have connected one load cell. Before I do the weight calibrations I am observer something strange.

Connections

upload_2018-8-5_11-40-57.png


With Red and black like that I got the weight readings in -.




eading: -8.9 lbs
Reading: -9.2 lbs
Reading: -9.4 lbs
Reading: -9.8 lbs
Reading: -10.1 lbs
Reading: -10.2 lbs
Reading: -10.0 lbs
Reading: -10.0 lbs
Reading: -10.0 lbs
Reading: -10.0 lbs
Reading: -9.8 lbs
Reading: -9.5 lbs
Reading: -9.2 lbs
Reading: -8.6 lbs
Reading: -7.7 lbs
Reading: -6.2 lbs
Reading: -3.8 lbs
Reading: -1.2 lbs
Reading: -0.5 lbs
Reading: -0.5 lbs
Reading: -0.5 lbs
Reading: -0.5 lbs
Reading: -0.5 lbs
Reading: -0.5 lbs
Reading: -0.5 lbs
Reading: -0.5 lbs
Reading: -0.5 lbs
Reading: -0.5 lbs
Reading: -0.5 lbs
Reading: -0.5 lbs
Reading: -0.5 lbs
Reading: -0.5 lbs
Reading: -0.5 lbs
Reading: -0.5 lbs
Reading: -0.5 lbs



However swapping red and black wires fixed the readings into + ?


Reading: 0.5 lbs
Reading: 0.5 lbs
Reading: 0.8 lbs
Reading: 1.5 lbs
Reading: 2.7 lbs
Reading: 3.8 lbs
Reading: 5.0 lbs
Reading: 6.1 lbs
Reading: 6.7 lbs
Reading: 7.2 lbs
Reading: 7.7 lbs
Reading: 8.1 lbs
Reading: 8.4 lbs
Reading: 8.5 lbs
Reading: 8.5 lbs
Reading: 8.3 lbs
Reading: 7.8 lbs
Reading: 7.6 lbs
Reading: 7.5 lbs
Reading: 7.4 lbs
Reading: 7.5 lbs
Reading: 7.4 lbs
Reading: 7.1 lbs
Reading: 6.1 lbs
Reading: 3.8 lbs
Reading: 1.4 lbs
Reading: 0.5 lbs
Reading: 0.4 lbs
Reading: 0.4 lbs
Reading: 0.4 lbs
Reading: 0.4 lbs
Reading: 0.4 lbs
Reading: 0.3 lbs
Reading: 0.3 lbs
Reading: 0.3 lbs
Reading: 0.3 lbs
Reading: 0.3 lbs
Reading: 0.3 lbs
Reading: 0.3 lbs
Reading: 0.3 lbs
Reading: 0.3 lbs
Reading: 0.2 lbs
Reading: 0.2 lbs
Reading: 0.2 lbs
Reading: 0.2 lbs
 

Attachments

Top