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Topic: Stretch Sensor to Measure Weight? (Read 510 times) previous topic - next topic

boomshikie

Hi Everyone,

This is my first post, and I did some searching on this topic but with no luck.  I hope the gurus can help shed some light and provide some direction on the question below.  Thanks in advance.


I am working on a project to measure the weight of a messenger bag that is hung over a shoulder.  The goal is to use the should strap as the measuring device (potentially a stretch sensor) that connects to the bag.  Then as the content of the bag changes, the weight change will be reflected in the stretch sensor.

I would like to measure the change in weight in ounces, so precision of weight is important.

question:

1.  Can a stretch sensor measure this weight change?
2.  Am I looking at the wrong sensor type and should be using a different type of sensor?

Something similar to what i am trying to achieve is a luggage weight scale, but I am trying to integrate it into the messenger bag strap.

Thanks for any advice and help the community can provide.

patrick

Delta_G

I think I would use a load cell in the strap instead.

The issue you'll have is that it will read differently depending on how the bag is held.  If the bag is hanging straight down then the strap takes the full weight of the bag.  But if is is slung over the shoulder and resting on a hip or against the side where the strap is at an angle then the force on the strap may not be the full weight of the bag. 
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Please do not PM with technical questions or comments.  Keep Arduino stuff out on the boards where it belongs.

boomshikie

thanks for the reply! :D

that makes a lot of sense with the potential inaccurate readings.  however if the testing was set up to be in the same position every time, can the stretch sensor produce a "weight" reading?  The main goal is to understand the initial stretch reading, and then once the content has been changed, to measure again and see the "difference" from the initial reading.  Do you think this is possible?

With the other suggestion of the "load cell", is the suggestion to have this sensor on the top of the shoulder?  assuming I am measuring the pressure produced from the contact of the strap to the shoulder?

thanks,
patrick

Delta_G

With the other suggestion of the "load cell", is the suggestion to have this sensor on the top of the shoulder?  assuming I am measuring the pressure produced from the contact of the strap to the shoulder?

No, I was assuming that it could be mounted in the strap or possibly where the straps connect to the bag. 

however if the testing was set up to be in the same position every time,
In the same position every time sounds like you're not planning to do this while someone is wearing the bag.  In that case it might be better to have the weighing device not be part of the bag itself.  Hang the bag on a hook to weigh and then go. 
|| | ||| | || | ||  ~Woodstock

Please do not PM with technical questions or comments.  Keep Arduino stuff out on the boards where it belongs.

Wawa

Search for "fish scale" or "luggage scale" on ebay.
Leo..

jremington

#5
Jun 24, 2018, 03:37 am Last Edit: Jun 24, 2018, 03:37 am by jremington
Can you explain why the bag can't simply be put on a scale, or hung from a hook with a scale, for accurate weighing?

boomshikie

Good question jremington!  This needs to be a wearable device. I am using the messenger bag as an example of the product.  Let's say this bag is carried by a patient who has an Urinary Catheter attached. The sensor will measure the initial weight of the empty bag. As the day goes on the bag is filled with urine. Once the bag gets to a certain "weight", it needs to be changed. I want to measure this difference of before with no weight and then what a full bag weight will be. But has to be a wearable device measurement and can't just be put on a standard scale.   

Hope that makes sense. And thanks for the inquiry and interest in the question.

Patrick

Delta_G

Are those bags usually carried in messenger bags? 

Lets be specific about what we're doing here.  You're wasting our time if you're sending us off thinking about how to do this with a messenger bag when what you really want to do it with is a colostomy bag.  There's a pretty big difference there. 
|| | ||| | || | ||  ~Woodstock

Please do not PM with technical questions or comments.  Keep Arduino stuff out on the boards where it belongs.

jremington

#8
Jun 24, 2018, 05:08 am Last Edit: Jun 24, 2018, 05:09 am by jremington
Your post suggests an ambulatory patient, but why would such a patient not be aware of the need to change a waste bag?

Delta_G

Search for "fish scale" or "luggage scale" on ebay.
Leo..
I still think this sort of design is your best bet.  A spring and a load sensor. 
|| | ||| | || | ||  ~Woodstock

Please do not PM with technical questions or comments.  Keep Arduino stuff out on the boards where it belongs.

wvmarle

To accurately weigh an object it must be stationery (to the extent that precision scales are placed on huge chunks of stone and sheltered from any air movements including the operator's breath), and the full weight of the object must be on the scale.

When the person is walking, the object is subject to movement: that disturbs your measurements (weight is basically an acceleration - that of gravity!) a lot. Having the object placed against the person's body disturbs your measurement as part of the weight (and a highly variable part) is carried by the body directly and not the scale. A courier resting their arm on the bag changes your measurement. The person carrying the bag not standing straight up affects your measurement.

So whatever it is you want to weigh (a courier's bag, a colostomy bag, whatever) can not be weighed accurately while attached to the person's body. You have to find another way to do whatever it is you want to do.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

MorganS

I don't think the project is doomed. The accuracy required is not high. It also doesn't have to be fast.

Your bathroom scale has to respond and give an "accurate" result within a second or two. You don't want to stand there for 5 minutes.

The 'messenger bag scale' does not and should not respond to short impulses. If you rest your hand on it, should the alarm go off? It should weigh the bag over a long period, such as minutes, and create an average. Even jumping up and down for a whole minute, what is the average 'weight' of the bag measured by the strap? It should be the same weight as if you weren't moving.
"The problem is in the code you didn't post."

boomshikie

sorry guys, was away all day and didn't get to respond to this post.

Are those bags usually carried in messenger bags? 

Lets be specific about what we're doing here.  You're wasting our time if you're sending us off thinking about how to do this with a messenger bag when what you really want to do it with is a colostomy bag.  There's a pretty big difference there. 
Delta_G, my apologizes on the confusion.  I am not allowed to discuss the actual project but am doing the best I can to explain the intent of the product.  It is not a colostomy bag.

-It is a shoulder strap bag that will carry liquid.

-The bags will be filled with liquid that can't be visually inspected for measurement.  the only way to calculate the weight of the liquid is the measure when before the bag is filled and after the bag is filled to find the difference.
 
-Understanding the variables that come into play (swining, against the body, arm leaning on it), the measurement will ONLY be taken at a stationary state, but needs to be taken while still on the shoulder because of ease in use for the bag.

again thanks for the feed back. 

Delta_G and company, do you still suggest the best sensor is the spring and load sensor? 

thanks

MorganS

You need a "load cell". Here's a good one at Sparkfun It looks like you could just put some eye bolts in the ends (maybe M12 thread, I can't tell from the datasheet) and insert that into the strap system.

You need a load cell amplifier to get the micro signal up to a level that the Arduino can detect. Obviously Sparkfun sells one, with some good sample code too.
"The problem is in the code you didn't post."

Delta_G

I am not allowed to discuss the actual project but am doing the best I can to explain the intent of the product.
Why not?  Is this some proprietary product that you have to keep secret for trade reasons?  And you think that rather than hire a competent engineer to work on it for you, you'll just go to the open source community and abuse them for free help to make your money? 

That's a bit like showing up at the homeless soup kitchen to get soup to serve at your restaurant because it is cheaper than cooking your own and you'll make more profit that way.  It's just despicable. 
|| | ||| | || | ||  ~Woodstock

Please do not PM with technical questions or comments.  Keep Arduino stuff out on the boards where it belongs.

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