It's the first time a post something here so I hope I do it correctly.
A friend of mine asked to help him designing a system to weight a small silo that contains cereals using load cells. I tried to search a bit online but could not find many resources about it.
We would like to use 6 compression load cells; each load cell should go under a silo's "foot", the silo has 6 feet (it looks like this one).
We would like to connect the load cells to an arduino board, using the HX711 Amplifier.
I have never done anything similar, but I think we should connect the load cells in parallel, following something like this tutorial. The problem is, that most of the tutorials/boards I have found supports up to 4 load cells, how can I do it with 6? I though maybe using more of those cell-combinator boards, but I'd like to have some insights on this.
The meaning of 1mV/V -- at rated load , the load cell will output 1 mV signal when the excitation voltage is 1V. Thus if you apply 5V excitation, the load cell will output 5 mV signal at the rated load.
The load cell is compatiable with the HX711 Module which contains a high gain amplifier and a 24-bit Serial ADC.
You can operate your system in one of the following two ways: 1. Use of the HX711.h Library which determines the calibraton factor in order to determine the unkown weight.
2. Make a 2-point calibrartion of your load cell and then measure the unknown weight.
I should go with the HX711 then, and thanks for the advice on the calibration for the load cell, but how would you put everything together? I don't think I will be able to connect all the load cells to a single HX711 amp, right? I would need either to sum up all the analog output from the cells and than connect it to the HX711 or connecting each cell to a single HX711 (6 HX711 in total), then summing up the digital output and connecting it to the Arduino board, right?
The following diagram (Fig-1), taken from Load Cell's specifications, indicates that the load cell is to be opperated as a single unit. However, you may use 4052 (4-Channel differential) type analog multiplexer and a single HX711 Module to operate 4 Load Cells.
What if we create a large board to put the silo on and use 4 load cells instead of 6?
I'm thinking like creating a big scale. This way, I could use the multiplexer and operate 4 cells. If we are able to create a good platform (that has to be parallel with the ground) it should work since the silo has 6 symmetrical feet, right?
Thank you very much for the insights and the information.
Each HX711 has two analog channels (Fig-1), which could be used to connect two load cells. If using four load cells, then there will be a need of two HX711 Modules and no need of 4052.
I would recommend simply using 6 load cells and 6 HX711 boards. My guess is, compared to the load cell cost everything else is very small.
It looks like your silo is about 12 feet high (rough guess based on the ladder). I would not consider adding a platform just to make the measurements easier.
The really problem I foresee is connecting everything up and surviving:
The OP also needs to consider how to connect the feet to their footings to allow the load cells to function while still restraining the feet against lateral loads from wind (if outdoors) and earthquakes (if those are a factor where the OP lives).
In addition, depending on the proportions of the silo and silo contents, there may be uplift loads at the feet during severe wind or seismic events that need to be resisted (to prevent overturning).
Thank you all for the inputs, I'm starting to get a grip of what is necessary to do in order to complete this project.
I think those problems are not concerning because the silo will be inside a building, earthquakes are not a factor where I live and the contents of the silo will be cereals. The only environment problem should be due to the large amount of dust, but a suitable enclosure for the Arduino, the HX711 and schermature for the cables should be enough, I hope.
As @JohnRob said, the load cells are the main cost here. I think 1000kg single compression load cells should go from 100-500€ (depending on the certifications), so the 6 Load cells connected to the 6 HX711 seems the more straightforward solution to me. I still need to figure out how much electric power is needed, since I won't be able to connect all to a constant source of power and I will need a battery pack, but I think I can find loads of resources on that matter on the web.
Remember you can shut off the sensor power, put the Arduino to sleep, periodically waking up to take a reading then back to sleep.
You will have to worry about ESD and lightning even in a building.
I would imagine ESD is an issue in a building with a lot of dust but still the sensor electronics need to be protected.
Lightning if it strikes in the general vicinity of your silo and electronics will cause a voltage differential as the energy dissipates. It could reach 100's of volts per meter.
Next is to consider how to get the data from the Arduino - Sensor package to where ever you need to monitor it.
You might consider a LoRA Tranceiver. They are really simple to setup and have significant range and are designed exactly for your type of application.
Thanks a lot, I will search a bit about ESD and how to properly protect all the electronics.
As for the sleep mode, yes that was actually the idea and my friend doesn't want IoT functionality, he just wants to read the weight when he needs (so we were thinking about an LCD screen that powers up with a button and display the weight).
Also, I think we will need a way to turn on the Arduino whenever he starts putting cereals in the silo (either automatically or with a button). Then, we will connect a small alarm or a light to the Arduino so that my friend can easily see when a certain weight is reached (e.g 3000kg of cereals). Sorry if I didn't mention all these things in original post.
I haven't actually seen the place where the silo is placed, only seen a similar silo. But my friend said that he would like to use a battery pack, so my guess is that the silo in a storage and that the power outlets available are used for other machinery.
Battery packs are fine if he finds it more convenient. Mains would be less upkeep. Not trying to necessarily add complexity but you might consider adding a solar panel to keep the batteries charged.
ESD is not very difficult to deal with. Mostly it can be squelched with a capacitor and a TVS diode. The real key is physical layout. You have to keep ALL the wires coming in and out of you enclosure in one area and protect all of them in that same area.
Once I will visit my friend's we will deal with the power problem; right now I am thinking about using 9V battery pack (rechargable maybe?). As for solar panel, since all will be inside a close (I assume) building, with no/really few windows, I don't think it's duable but correct me if I am wrong.
As for the ESD protection, a good layout would be to keep all the HX711 and the arduino inside an enclosure with only the cables from the load cells coming in right? Should I keep the battery pack in a different enclosure?
Lastly, any recommended program to try a simulation?
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