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:
Can a stretch sensor measure this weight change?
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.
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.
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?
boomshikie:
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.
boomshikie:
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.
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.
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.
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.
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.
sorry guys, was away all day and didn't get to respond to this post.
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.
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?
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.
boomshikie:
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.
thanks for the recommendation. I'll look into Sparkfun some more.
Delta_G,
the project is in the very early conceptualizing state. As a designer I am trying to understand what is possible as I am thinking. I feel this is what these forum/communities are for, to discuss ideas and help think through problems.
I am new to arduino and want to understand if this is a product that can help with the project. I thought this is what this tinkering community is about.
anyways thanks for your feed back thus far. just trying to learn what is possible with arduino.
boomshikie:
the project is in the very early conceptualizing state. As a designer I am trying to understand what is possible as I am thinking. I feel this is what these forum/communities are for, to discuss ideas and help think through problems.
You're close. This is a community of hobbyists and open source enthusiasts. And if your goal is to learn how to work with Arduino and build something cool then this is the place for you.
But if your goal is to keep your ideas all secret so only you can make the money when you had all of us working on it the whole time then you are a thief. Actually, a little lower than a thief IMHO, but you are definitely taking for free what you should pay for.
So it's a matter of what drives you. If you're about learning then you shouldn't have any problem sharing your idea. If you're about the money then it is totally inappropriate for you to be asking us to design the thing for you. There are plenty of great engineers who need work because businesses have decided to cheat the open source community into being their free development department.
It's just like the soup kitchen idea I quoted earlier. If you're there to get free soup because you are homeless and hungry then those folks are there to help you. But if you are there to get soup to go sell in your restaurant then you are despicable.
Besides, don't worry about copies, if your product is remotely useful it will be copied, NDA or not.
I don't care much if it's commercial or not - either way openness on what it is, what it does, and how it does it is expected, in the whole spirit of open source, so we can learn something new and interesting in the process. The more is known about the whole project the more suggestions you can expect, including sometimes very different but much more suitable solutions to the actual problem at hand.
So for the soup kitchen: now if you come to that kitchen specifically to learn how to operate a restaurant (with which you hope to make heaps of money) and in the process teach them how to make a better soup, I'd have no problem with that.
wvmarle:
So for the soup kitchen: now if you come to that kitchen specifically to learn how to operate a restaurant (with which you hope to make heaps of money) and in the process teach them how to make a better soup, I'd have no problem with that.
Sure, but while you're there and the people ask, "why are you here" you wouldn't turn and say, "oh it's a big secret and I can't tell you." You'd say, "I'm here to learn to run a soup kitchen and to help you make better soup, let me show you how."
I'm all for sharing, even with the corporate folks. But let's be open about it. This is an open source community. If Microsoft showed up here and asked for help with a product I'd help even them as long as they're willing to share with everyone what I've done for them. It's done in an educational spirit and so it should be shared with all. It's only when you want to secret it all away for only yourself that it becomes an issue.
My biggest beef is that they do this out of a profit mindedness that is putting good engineers out of work in lieu of getting advice for free from some volunteer just trying to help kids learn to code.
