Hi everyone,
At first thanks for all information I could get from this forum 
I'm making a project of "smart footbed" which will be able to show what is the overall mass of the person also when standing on 1 foot (limit 80kg)
Does anyone can tell me is there any possibility to connect couple small sensors "in string" to reach my goal? Footbed has to be as thin as it is possible so I hope there is a possibilty to use one of them :
Thank you in advance for any feedback. Have a great day!
Does anyone can tell me is there any possibility to connect couple small sensors "in string" to reach my goal?
No. Although you can connect these Pololu sensors (that what the Google search showed me, hope you got the same) serially to an Arduino you won't be able to measure a person's mass in a way that gives you more information than "person on sensor" or "person not on sensor". Maybe you can distinguish between a child and an adult but you probably won't be able to reach an accuracy of +/-5kg. If you want to weigh people you should look for load cells and not force sensitive resistors.
Ok, at first thanks a lot for such fast and helpfull advice. I have heard about load cells already but comparin them to the FSR sensors they are much thicker which for me is a big problem. As you can imagine my footbed has to be as thin as it possible. Anyway do you think that there is a possibility to connect couple load cells together?
Why don't you contact the folks who make this:
https://www.tekscan.com/products-solutions/medical-sensors/3000e
It is very expensive and I wanted to make something more simple and cheap.
Joshu93:
Anyway do you think that there is a possibility to connect couple load cells together?
Presumably you meant "...connect a number of FSRs together."
Recognize first that FSRs are not precision instruments and that there is likely to be significant error in the "measured" weight, even if you can overcome the following difficulty......
Certainly it is possible to connect a number of FSRs together, but you are not likely to be happy with the result.
Imagine that your footbed consists of four FSRs connected in parallel. The weight carried by each may vary considerably, so for useful results, you would want each of the following scenarios to give you the same total resistance:
a. 100% of the total weight on one sensor
b. 50% of the total weight on each of two sensors
c. 25% of the total weight on each of four sensors
But considering that FSRs have a very nonlinear weight-to-resistance response, the total resistance in each scenario is likely to be different and probably significantly so.
You could demonstrate this to yourself by calculating the total resistance (using the weight-to-resistance data in a datasheet) of four (or "X") FSRs in parallel, for various weight distributions.
A matrix of many small FSR's is also expensive.
Perhaps they can be used to guess the weight, but it won't be accurate. Perhaps 50% inaccurate or so.
The FSR's are more accurate than 50%, but you need to fill the whole area to be able to add all the weights.
There is pressure sensitive fabric by EeonTex. I think they make three different kind of such fabrics (conductive, heating, pressure sensitive). Sparkfun and Adafruit do not sell all three of them at this moment.
Joshu93:
I'm making a project of "smart footbed" which will be able to show what is the overall mass of the person also when standing on 1 foot (limit 80kg)
Any cheap bathroom scale can do exactly this. But 80 kg is a quite low limit for a person's weight.
It is very clear now what you wrote - thanks for all your answers.
In my project I wanted to make some kind of threshold for 10KG,20KG,30KG,40KG and hopefully 50KG
I mean when we set up in program 20KG then, when sensor will measure more than 20KG it will send the signal and the LED start to blink (for example)
What about sensors like this? What do you think ? max weight 100kg?
Again I will be very thankfull for all your support.
You can not fill the complete surface with those sensors, so it will be very inaccurate. You could buy a few and do some tests. Those sensors work when they are attached to a flat surface, I think they can not be used inside a shoe.
A link to AliExpress can be cut off after the .html
This is all that is needed: https://pl.aliexpress.com/item/FSR-robot-touch-insole-flexible-thin-film-pressure-sensor-IMS-C20B-10kg-50kg-100kg-custom-Plantar/32823122790.html.
AliExpress has also a way to make a shortcut to an item, but I don't know how to do that.
Ok, sorry for that I will cut off the link again.
The thing is that I wanted to put 3 sensors (so put them on the flat surface) and then put on it other part of fotbed (3dprinted) with small pins on the other side. While making the step small pin will make a pressure on the sensor - that is my idea.
I understand that it will be inaccurate but it has to give the information if the step is more than 10kg/20kg/30kg/40kg or not.
Regarding that do you think that this sensor will fit?
Joshu93:
I understand that it will be inaccurate but it has to give the information if the step is more than 10kg/20kg/30kg/40kg or not.
How big is that foot bed going to be? What animals are you trying to measure (these numbers are WAY too low for use with humans)? What pattern are you looking for with those three sensors? How big would each sensor have to be?
The sensor should be attached with its back to a flat surface. A plastic pin on top might not work. Even if the pin is flat, the pressure is probably not equally distributed over the sensor. Some kind of rubber or sturdy foam might be better.
I think it is not accurate enough, but that is just a hunch. You have to make it and test it to know if it can be useful.
Beside the questions by wvmarle, can you also make drawing of it ?
It will be a fotbed for human after physical trauma which has started rehabilitation process. I had an knee injury once and during first weeks of rehabilitation I couldn't stand on one feet with all my body mass. That is why I need only a support not a full messege what is the mass. The device only tell me if I put enough mass on the feet or not.
Please correct me if I'm wrong. When I will take this 45kg FSR i wil be able to measure the 10kg or 20kg mass?
Joshu93:
The device only tell me if I put enough mass on the feet or not.
I still don't really understand what you want.
You say you want three sensors under the foot - so I suppose you want to have an idea of where the patient puts (most of) their weight.
But at other times, including here, you say "I just want to know the total", for which you can just take a regular bathroom scale, which does just that, and at much higher accuracy than an FSR can dream of.
Ok, sorry for not being not precise. Lets start from the beginning.
So I had a knee injury once. I had to have a surgery and after that I couldn't walk for the while. Then after 4 weeks walking with orthopaedic supports I could start to put some weight on my leg. Following my therapists instructions in first 3 days I was able to put 10% of body mass (around 10kg) on the leg. I didn't know how much exactly it is so I had to stand on the bathroom scale with 10 kg. Then remember what is the feeling of the 10 kg and walking like this for 3 days. After 3 days - 20 kg... and so on up to 50 kg.
So the idea is that I will create a simple footbed with 3 sensors up to 50kg. The value of the sensors will be this same after standing on the feet (I can stand on your heel or on the toes). If the value of the sensor will exceed 10KG the speaker attached to arduino will make "beep". After 3 days I will change the value for 20 kg on so on. It will be very helpful for people which are in recovery process. In my case in the whole rehabilitation process I was so scared that I will put to much mass on my feet and I will break all the progress.
I hope that now it is clear. What do you think now ?
If you're walking you have the full weight on one leg/foot for about a quarter of the time (not just half of it).
So the limit of 50kg is not sufficient even if ignore that there are many people with weights above 100kg.
