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Load Cell/ Sensor Recommendations for Weight Shift Detection

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Randy222:

--- Quote from: Kleinstein on February 29, 2024, 04:19:57 pm ---A test calibration for a relative change would be relatively simple with weights. For the heel / toe part there is anyway a limited accuracy as feets have different size and the position of the foot would matter.



--- End quote ---
I am not so sure it would be trivial. More than one sensor under the pad? One sensor only, then what if the weight is not dead center? Calibration routine yes, but it can get tricky with certain setups if the goal is to cal each sensor in the system.

I thougt I read somewhere long ago perhaps, some force pads that are like a sheet of sensors that form a matrix, from there software reads all the sensory to determine total weight and force point data. Like what they use in the shoe places where you step on and the display shows your foot in color gradient of force, mapping your pressure points. The thing being stepped on is some sort of mat that has sensory in it.

PCB.Wiz:

--- Quote from: am1 on February 29, 2024, 06:36:11 pm ---My concern/issue with in-house calibration is storing the calibration factors/ corrected value. I can do a simple 2 point calibration, but storing it in a microprocessor or saving all 4 values for each sensor for each production device in the software is my challenge. For easy version control purposes, we try to deploy the same software to every machine, so that is why I prefer to use a pre-calibrated sensor and hit a road-block when trying to consider other non-custom, calibrated sensors. The sensors our device currently uses do not require calibration out of the box, so this is new for me, so if anyone has advice on this I would greatly appreciate it.

--- End quote ---
If you cannot find a good enough 'better sensor',  an admin alternative would be to include an EEPROM, or even simpler to manage, a small MCU with each sensor.
Parts like EFM8BB52 have 12b ADC, 10b DAC in a 3x3mm  package, and enough flash to be able to self calibrate in a bench fixture.

PCB.Wiz:

--- Quote from: am1 on February 29, 2024, 03:31:20 pm ---... The device has elliptical-like footplates, so the sensors will be going on a flat form surface, so I plan to put 2 sensors on each footplate (one near the back of each footplate for heel recognition and one towards the front for toe recognition) with 4 total sensors per machine.

I have made a rudimentary "mock system" using off the shelf Flexiforce A502 sensors. It works relatively well with detecting heel/toe loads, and a relatively reasonable % load readout for each foot. These sensors are potentially produced from the same batch, which is a contributing factor with repeatability, but it makes me consider if a reliable system can be made with these sensors.

--- End quote ---

Is this static, or dynamic use ?
You could do a rudimentary auto-calibrate knowing the total weight, but you would hope the sensors drift the same over time/temperature, and do not diverge randomly.

addit: I presume you have already seen this ?

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9331244/

gives

Linearity             ±3% of full scale            Line drawn from 0 to 50% load
Hysteresis           <4.5% of the full scale   Conditioned sensor, 80% of full force applied
Repeatability   <±2.5%                           Conditioned sensor, 80% of full force applied
Drift               <5% per logarithmic time scale   Constant load of 111 N (25 lb)

matb:

--- Quote from: am1 on February 29, 2024, 03:31:20 pm ---I am also concerned about exceeding rated load (within a reasonable range, say ~100-200lbs), with overloading load cells damage it? I'm sure there is some safety factor built in, but that is another thing for me to consider.

--- End quote ---

That question should be in the datasheet of the sensor. From the ones I use (which are industrial ones) it's typically 150% to 200% of the full scale. You could ask the sellers for that information.

am1:

--- Quote from: Randy222 on February 29, 2024, 07:07:42 pm ---I am not so sure it would be trivial. More than one sensor under the pad? One sensor only, then what if the weight is not dead center? Calibration routine yes, but it can get tricky with certain setups if the goal is to cal each sensor in the system.

I thougt I read somewhere long ago perhaps, some force pads that are like a sheet of sensors that form a matrix, from there software reads all the sensory to determine total weight and force point data. Like what they use in the show places where you step on and the display shows your foot in color gradient of force, mapping your pressure points. The thing being stepped on is some sort of mat that has sensory in it.

--- End quote ---

Yes, I have been looking into matrix FSRs that are commonly used in those types of applications. I don't necessarily need the pressure mapping for our application as a simpler UI would be preferred, but the custom sensors would be a matrix sensor.



--- Quote from: PCB.Wiz on February 29, 2024, 11:05:30 pm ---
Is this static, or dynamic use ?
You could do a rudimentary auto-calibrate knowing the total weight, but you would hope the sensors drift the same over time/temperature, and do not diverge randomly.

addit: I presume you have already seen this ?

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9331244/

gives

Linearity             ±3% of full scale            Line drawn from 0 to 50% load
Hysteresis           <4.5% of the full scale   Conditioned sensor, 80% of full force applied
Repeatability   <±2.5%                           Conditioned sensor, 80% of full force applied
Drift               <5% per logarithmic time scale   Constant load of 111 N (25 lb)


--- End quote ---

It would be a dynamic application. The patients would have their feet strapped down to footplates, which are on a free hinge to allow ankle movement as the machine carries them through a gait-like movement pattern (similar to elliptical).

One of the manufacturers I am speaking to for custom FSRs said they can meet the specs you listen above and said the following regarding drift and life cycle: 

" Drift is affected by the following factors: (1) the amount of pressure; (2) the duration of continuous pressure; (3) changes in temperature environment
This project is to compare the difference in force between two sensors, so even if there is drift, the two sensors change simultaneously, so the function will not be affected too much.
The service life is 1-3 years. "


--- Quote from: PCB.Wiz on February 29, 2024, 11:00:23 pm ---If you cannot find a good enough 'better sensor',  an admin alternative would be to include an EEPROM, or even simpler to manage, a small MCU with each sensor.
Parts like EFM8BB52 have 12b ADC, 10b DAC in a 3x3mm  package, and enough flash to be able to self calibrate in a bench fixture.

--- End quote ---


I personally have not worked on any embedded systems using MCU and PLCs, so planning the circuitry while avoiding custom or in-house PCBs is what I have been trying to consider when thinking about load cells/strain gauge application. Thank you for these recommendations, I will look into them some more.

Any other thoughts or advice on this would be greatly appreciated!

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