Where to source force sensors for electronic balance? - Page 1
Forum section: electronics / projects

author: FenderBender
author_rank: Super Contributor
timestamp: August 14, 2012, 02:27:24 pm
content: Hey guys,My mom is a dietition. She tells people what is best for them to eat. One tools she's always wanted is a scale that can weigh food and then tell you nutrition facts about the food based on the weight. So there would be a lot of prestored foods and all you would have to do is set it to a certain food, weigh it and it will tell you what you're eating.Problem is, there doesn't seem to be one on the market. There are ones that are similar, but don't quite cut it. She also wishes there was an easy way to add more foods to these scales...a problem she had with one she had before. So I thought, hey I could just write a small bit of PC software that can talk to the scale and add new items. So as a project, it seems pretty feasible.The problem I see is actually the sensing part. Where can I get some good quality force sensors to measure between say 0-10kg? I would like for them to be easily interface-able with a microcontroller like an Arduino. If it changed its resistance based on pressure, I could convert that to a voltage and use an ADC. But I need a sensor first. Also, I want maybe 4 significant digits in measurement, and I want them to be accurate. Provided the sensor itself is accurate, what kind of ADC would I need? Would the onboard ADC on an ATMega be too weak?Thanks.

author: robrenz
author_rank: Super Contributor
timestamp: August 14, 2012, 02:37:20 pm
content: Unless you are desperate for a project buy this.http://www.amazon.com/Kitrics-Digital-Nutrition-Scale-Silver/dp/B000U67EI0I have one and it works great.Just the man hours to duplicate this aspect alone (Includes information for 1999 foods; user can easily create and store up to 99 customizable food code entries )would exceed the purchase price if you charged $0.10 per hour for your time.

author: hans
author_rank: Super Contributor
timestamp: August 14, 2012, 02:54:32 pm
content: I happen to just saw this teardown:It says it's 5lbs(?) so about 2kg. Maybe that's enough, maybe not. But atleast it has got an USB connection which may be hackable.. I don't know anything else about the product I'm afraid, and I neither think it's that cheap (though the electronics is made cheap though).

author: FenderBender
author_rank: Super Contributor
timestamp: August 14, 2012, 04:13:56 pm
content: Yes she's looked at that one. I never got into detail about what the nutrition facts were. Sorry. She works at a facility that does dialysis for people with kidneys that have lost functionality. The scale must be able to read outotherthings like phosphorus and potassium content. Without those, any scale is useless for this application.That one would be great if you were just a normal person trying to loose weight or stay healty or whatever, but yeah this is a specific task.I don't know if it's really all that hard of a project really. Maybe that last 10% would be difficult, but as far as interfacing a sensor and then having a readout, I don't think that's all too hard. Could be wrong though!I'll have to look at this teardown!

author: Short Circuit
author_rank: Frequent Contributor
timestamp: August 14, 2012, 05:01:44 pm
content: Quote from: FenderBender on August 14, 2012, 02:27:24 pmForce sensors typically use strain gauges. The change of resistance is pretty small, so they are connected in a bridge. You need a differential ADC to measure these, and probably with fairly high input gain. I believe AnalogDevices has various ADC's specifically for these kind of sensors.

author: robrenz
author_rank: Super Contributor
timestamp: August 14, 2012, 05:17:27 pm
content: Quote from: FenderBender on August 14, 2012, 04:13:56 pmIMO this is a substantial software project. Take the Kitrix scale or something similar and just hack the software/firmware to add the missing data that your mom is after to each food that she would be interested in. I am sure that out of the 1999 foods probably half are things she would not use and then you could use that freed up memory space to add the additional food data to the remaining foods. If you plan to have a PC hooked to it to use the scale then the display on the scale does not matter as the PC will be your interface and could display all the data at once.

author: FJV
author_rank: Regular Contributor
timestamp: August 14, 2012, 05:19:44 pm
content: You could go from a sliding potmeter combined with a spring all the way to professional sensors.Maybe this is something?https://www.sparkfun.com/products/10245

author: FenderBender
author_rank: Super Contributor
timestamp: August 14, 2012, 05:41:18 pm
content: Quote from: FJV on August 14, 2012, 05:19:44 pmVery helpful. I'll go looking around Sparkfun for some stuff.And as far as being a difficult project, I guess I haven't done enough mcu projects to really gauge the difficulty but perhaps just storing the data in arrays in flash and then just accessing that info based on an input from the user.Those numbers would be per a certain mass, say 1gram, and then the measured mass would just be multiplied by these constants. Is that difficult to implement?

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 14, 2012, 05:41:39 pm
content: Quote from: Short Circuit on August 14, 2012, 05:01:44 pmIt's not that bad, I use 1/4 of a LM324 to drive the internal ADC from a USB PIC.

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 14, 2012, 05:49:51 pm
content: Quote from: FenderBender on August 14, 2012, 05:41:18 pmDid you already make up your mind for this being a standalone project? If you just hook up a scale to a PC maintaining the database with content information and printing/storing the outcome would be much easier.

author: FenderBender
author_rank: Super Contributor
timestamp: August 14, 2012, 05:55:37 pm
content: Yeah I suppose you're right. It would be much easier. Most programming I've done has been on PC anyway...but I think in the end, standalone IS what would be best, but then the whole lookup system and navigation would be a pain in the arse...more time than its worth, unless I really start researching. I'm sure I could do something nice, but hmm perhaps I'll get a PC interface-able scale and see if I can just work with that.

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 14, 2012, 06:21:07 pm
content: Here's an example in the range you are looking for (11 Kg)http://www.amazon.com/DYMO-Pelouze-Digital-Compatible-1772059/dp/B004T5Z25U

author: FenderBender
author_rank: Super Contributor
timestamp: August 14, 2012, 06:27:11 pm
content: Thanks a bunch