cheap load cells / force measurement - Page 1
Forum section: electronics / projects

author: djacobow
author_rank: Super Contributor
timestamp: January 19, 2016, 06:56:31 pm
content: Hope you guys can help me out. I've got a question about force measurement that's a bit outside of my wheelhouse.Potential client wants to build a retail product that would involve recording the weight of an object over time (say 20-50 lb). Accuracy requirement would be on the order of 2.5%, though probably a lot of noise could be tolerated as we'd be averaging over very long periods. OTOH, load would be in place for long periods (months+) so opportunities to 'tare' would be infrequent.I know industrial load cells exist for this purpose, but this would be a retail cost-sensitive product, so I think I can't just flip open a Grainger catalog. Probably the right analog cost-wise would be the bathroom scale. Before I go out and buy some such scales to attempt to reverse engineer, I thought I'd ask the forum, what are these mechanisms like, who makes them, and where would I look for suppliers?Thanks,Dave J

author: richard.cs
author_rank: Super Contributor
timestamp: January 19, 2016, 07:03:04 pm
content: Not sure where to buy the load cells from but bigclive recently reviewed some poundland (dollar store) luggage scales with some impressive little load cells in.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 19, 2016, 07:31:25 pm
content: I haven't done any reviews but have taken apart several of these luggage scales and similar things. They all have a steel spring in some configuration, with a conventionally mounted strain guage, and a set of readout electronics in a glop top configuration. I have a similar low cost application in mind and have done some searching to see if there is a chinese vendor for the cells. So far no luck, and at least initially my plan is to salvage the cells from these dollar store scales. There is certainly no one in the western world that sells at a price point that would allow these luggage scales to sell for under $10 dollars as they do. My biggest question so far is whether to salvage the cell and do all of my own readout electronics, or to hijack the LCD driver signals to utilize all of the A/D and other signal conditioning under the glop top. The quantities for my application are low enough that salvage operations like this can actually make sense.

author: SteveP
author_rank: Contributor
timestamp: January 19, 2016, 07:55:10 pm
content: I don't know if the form factor will be suitable for you, but I've used the 50kg load cells from Phidgets.com:http://www.phidgets.com/products.php?category=34At $7.00 qty 1, the price seems right...--Steve

author: Kleinstein
author_rank: Super Contributor
timestamp: January 19, 2016, 08:24:56 pm
content: If price is really critical and a rather compliant mechanics is possible, force can also be measured by a capacitive displacement measurement. Some of the very cheap scales use this.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 19, 2016, 10:51:50 pm
content: Thanks for the Phidgets link. Those really are attractive load cell prices. The OP needs to note that he will also need some form of readout electronics. Phidgets has a USB interface that is also very competitively priced, but it will push the total price of the assembly to $97, right at the edge of his proposed budget.

author: pmbrunelle
author_rank: Regular Contributor
timestamp: January 19, 2016, 11:18:19 pm
content: I will state the obvious here, but to reduce cost, use only a single load cell/force measurement.You can measure the displacement of a small magnet.Any components that are in the load path, which are involved in the displacement measurement must be steel (or something that doesn't creep) and not plastic. If you can't tare the scale, you need to ensure that it doesn't drift over time due to creep.Do you have enough volume to pay for steel stamping tooling and the like?

author: djacobow
author_rank: Super Contributor
timestamp: January 20, 2016, 12:16:33 am
content: Quote from: pmbrunelle on January 19, 2016, 11:18:19 pmYup, creep is definitely a concern. I think the volume is there for some stamped parts.

author: djacobow
author_rank: Super Contributor
timestamp: January 20, 2016, 12:19:35 am
content: Quote from: CatalinaWOW on January 19, 2016, 10:51:50 pmYup, I am prepared to do a little analog stuff before an ADC. I don't thing USB is in the cards for this kind of app.

author: djacobow
author_rank: Super Contributor
timestamp: January 20, 2016, 12:31:34 am
content: Quote from: CatalinaWOW on January 19, 2016, 07:31:25 pmOne definitely gets the sense that knowing Chinese would be helpful. Like if you made a trip to Shenzen and asked around for an hour, you'd soon find yourself directed to suitable supplier(s).