digital scale for weighing -- Alternatives to strain gauges and load cells? - Page 1
Forum section: electronics / projects

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: February 28, 2014, 09:48:39 pm
content: What other ways can weight be measured? Could an accelerometer(s) be used? Or a gyro? Let's say you wanted to make a shipping scale that can weigh from 0.1 oz. up to 100 lbs. Are there any ways to get repeatable, accurate measurements with out using a load cell or strain gauge?

author: IanB
author_rank: Super Contributor
timestamp: February 28, 2014, 10:28:09 pm
content: There are two ways to measure weight. The first way is to see how much something distorts or deforms under the action of the applied weight. The second is to use a balance, where a measured force is applied to balance the weight until the two forces are equal.Balances are OK for hand scales where you can add and remove balancing weights, but I don't know to construct an electronic version.So for automatic electronic scales, I imagine every scale out there probably uses the first option.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: March 01, 2014, 12:29:10 am
content: You can drive a coil (just like a loudspeaker) to lift the load. See what current it takes to get it to some reference point, maybe determined with a photo interruptor or similar. You can lift it pneumatically and measure the pressure.

author: notsob
author_rank: Frequent Contributor
timestamp: March 01, 2014, 12:35:26 am
content: An accelerometer can be used, the measurement is taken at right angle to the gravitational field, and as such you are actually measuring mass not weight [f = m * a]. [ and of course we all know that mass is constant and weight varies according to the gravitational field the object is in ]

author: Alphatronique
author_rank: Regular Contributor
timestamp: March 01, 2014, 01:15:36 am
content: Hivibrating cord , then use sound/frequency for find weightsmall deflection on metal beam will change it natural frequencyhttp://www.digisens.ch/fileadmin/customer/S18E_07_KL66.pdfthat one sensor that use this ..sensor i have see use stench wire under scale surface whit one coil for exiting and one for pick-upthen transform frequency to weight ,that permit to measure dynamic load (scale while moving)i have see it on garbage truck that measure from 0.1kG to 1000Kg while bin was moving ..

author: dannyf
author_rank: Super Contributor
timestamp: March 01, 2014, 01:31:10 am
content: QuoteHard to understand that.

author: Rufus
author_rank: Super Contributor
timestamp: March 01, 2014, 03:35:31 am
content: Quote from: IanB on February 28, 2014, 10:28:09 pmI the most accurate scales use electromagnetic force restoration where the force from a coil and magnet is used to hold the weighing platform in a fixed position. The current in the coil is proportional to the weight on the platform.

author: SeanB
author_rank: Super Contributor
timestamp: March 01, 2014, 04:45:00 am
content: Force restoration balances typically top out at around 15kg, but can measure down to the picogram level provided you can keep it still. This means typically a floating foundation for the massmeter cut through the floor and resting on it's own poured base. For most other application s a variant of the strain gauge is used.Typically you buy either a half cell or a complete 4 wire ( or a temp compensated with a sensor for more accuracy) strain gauge unit and use an instrument amplifier to get the required voltage out of it when fed with a regulated supply, and do some zero and scaling on it then feed it to a display, or digitise it at 12 bits or so and use a MCU to do the zero offset and the scaling. Typically the bridge will be fed with 5V or so, and will have an output at 2.5V with a full scale of around 10mV, so needs a lot of low noise gain, and some quite heavy low pass filtering to keep the noise and hum down.

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: March 01, 2014, 05:45:45 am
content: Quote from: Rufus on March 01, 2014, 03:35:31 amYou're saying the weighing platform is mounted to the axial end of a solenoid. That's what's in my mind's eye anyway. This makes sense.

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: March 01, 2014, 05:48:04 am
content: Quote from: Alphatronique on March 01, 2014, 01:15:36 amThis sounds like a strain gauge, right?

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: March 01, 2014, 05:51:57 am
content: Quote from: Conrad Hoffman on March 01, 2014, 12:29:10 amThis is similar to the other post, but the reference point would be the tricky part. Measuring the current seems simplest.QuoteI guess air could be used so no messy leakage.

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: March 01, 2014, 06:04:37 am
content: Quote from: notsob on March 01, 2014, 12:35:26 amI read another old post on this forum and gravity differs geographically. So an accelerometer and a gravity lookup table and we might get a reliable measurement. I've been looking at MEMS and it's possible that multiple devices could be used.I thought a gyro might work too, measuring time and deflection. Not sure but this may require more precision.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 01, 2014, 10:06:08 am
content: Quantum tunnelling composite.I noticed today that the internal oscillator frequency of a PIC varies slightly if you put pressure on the top of the package....

author: lapm
author_rank: Frequent Contributor
timestamp: March 01, 2014, 10:51:50 am
content: Quote from: mikeselectricstuff on March 01, 2014, 10:06:08 amNow im curious, how much is slightly? And im wondering if that's capacitance of sticking finger on top of it or pressure...

author: codeboy2k
author_rank: Super Contributor
timestamp: March 01, 2014, 11:22:50 am
content: Paul Grohe weighs an eyebrow hair using an analog meter movement as a balance beam...

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 01, 2014, 11:36:27 am
content: Quote from: lapm on March 01, 2014, 10:51:50 amI thought it may be capacitance but tried it with an insulated stick.A really hard press with the thumb makes it about 1% faster. This is an SO8 package - presumably straining the leadframe. I feel a video coming on...

author: PeterL
author_rank: Regular Contributor
timestamp: March 01, 2014, 12:17:42 pm
content: Quote from: GiskardReventlov on March 01, 2014, 05:48:04 amAFAIK most strain gauges, like the one's used in loadcell's are basically just resistors wich vary in value when a force is applied to it.

author: SeanB
author_rank: Super Contributor
timestamp: March 01, 2014, 12:26:12 pm
content: Simple way to see a vibrating chord it to take a guitar and pluck the string, then tighten it or loosen it and see how the note changes. for those without an ear for music take a standard hacksaw and pluck the blade and listen, then tighten it up a little and try again. This at least will have some overload capability and shock resistance.I was looking in a catalogue at the Mettler Toledo massmeters, some have a note saying they are sensitive to IR radiation from you coming close in to them causing extra force on the platform. If you ask for the price you want to cry, they are a lot more costly than almost any scope.

author: PA0PBZ
author_rank: Super Contributor
timestamp: March 01, 2014, 01:23:32 pm
content: I took apart an old kitchen scale and to my surprise it did not use a strain gauge but a capacitor.The cap is part of an oscillator circuit, the frequency is translated into weight.You can see the plates of the cap sitting in the middle of the 'load cell' with the 2 points where the wires to the oscillator were soldered.

author: saturation
author_rank: Super Contributor
timestamp: March 01, 2014, 03:05:03 pm
content: The methods that can be used depend on[accuracy needed]/costMany methods have been tried and the most bang for buck at oz precision is a load cell.See appendix:Fundamentals of Weighing Technology-1The 3 top methods are:http://www.fisica.uh.cu/bibvirtual/vida%20y%20tierra/balanzas/Weighing_Principles.pdfQuote from: GiskardReventlov on February 28, 2014, 09:48:39 pm

author: peter-h
author_rank: Super Contributor
timestamp: March 01, 2014, 04:34:59 pm
content: QuoteAbout 0.2% change from the equator to the poles, due to the earth's rotation.There is a similar order effect due to the earth not being spherical.So if you want to get below the 0.5% accuracy level you need a GPS in there, and do some corrections.

author: codeboy2k
author_rank: Super Contributor
timestamp: March 01, 2014, 06:22:59 pm
content: Quote from: PA0PBZ on March 01, 2014, 01:23:32 pmI see it. Does one plate remain fixed and the other moves away from the first one when the weight is placed on the top? or does it compress or flex the capacitor? It's not clear but I am thinking the first one, not the latter.

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: March 01, 2014, 06:33:37 pm
content: Quote from: SeanB on March 01, 2014, 04:45:00 amHow much does my soul weigh? Strain gauge seems to be the most widely used as you and others have said.In another thread on uC there was mention of MEMS devices. I found a 2001 paper that mentions using MEMS for weight measurements. Seems that it was nascent tech then but should be mature by now.

author: codeboy2k
author_rank: Super Contributor
timestamp: March 01, 2014, 06:37:40 pm
content: Quote from: saturation on March 01, 2014, 03:05:03 pmThat's a Scribd link, and I'm really starting to hate Scribd these days.. for 3 (5?) years they encouraged people to upload and share PDFs, it was awesome. Now they locked it all up, taking the treasure trove with them, and charging us to download it..Here's a link to that same document that no one can read from Scribd without paying for it...http://forum.arduino.cc/index.php?action=dlattach;topic=156869.0;attach=40465

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: March 01, 2014, 06:39:40 pm
content: Quote from: SeanB on March 01, 2014, 12:26:12 pm"tighten/loosen" and I think strain gauge, just more apparent.What industry uses those mettler/toledo scales?

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: March 01, 2014, 06:48:07 pm
content: Quote from: peter-h on March 01, 2014, 04:34:59 pmWeight a minute! We are measuring gravity already and there's been mention of an accelerometer, maybe throw in a gyro too. Self-calibrating via sensitive MEMS.

author: jmaja
author_rank: Frequent Contributor
timestamp: March 01, 2014, 06:50:47 pm
content: Quote from: peter-h on March 01, 2014, 04:34:59 pmIt's actually about 0.5%. 9.7804 at the equator and 9.8322 at the poles.

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: March 01, 2014, 06:52:15 pm
content: @saturation, @codeboy2k,Will have a look at those links.Never liked scribd and only used it by accident.So they're not charging me to download anything. This is a failed business model.

author: PA0PBZ
author_rank: Super Contributor
timestamp: March 01, 2014, 07:15:39 pm
content: Quote from: codeboy2k on March 01, 2014, 06:22:59 pmIf you have a second look at the picture you can see that only the left side is supported, the right side is hanging in free air. So when you put pressure on the top the whole thing deforms and the plate distance will increase. (the bottom plate is attached to the right side)

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: March 01, 2014, 07:22:52 pm
content: Quote from: codeboy2k on March 01, 2014, 11:22:50 amWhat make is the panel meter?

author: PA0PBZ
author_rank: Super Contributor
timestamp: March 01, 2014, 07:36:36 pm
content: Quote from: GiskardReventlov on March 01, 2014, 07:22:52 pmNot sure about the one he used but these guys have a lot:http://scientech.cwfc.com/products/spokes/08_VersatileProducts.htmNow how many eyebrow hairs do you need to calibrate it?

author: Rerouter
author_rank: Super Contributor
timestamp: March 01, 2014, 09:48:27 pm
content: My old tafe used to have those kind of meters, they are called Edgewise Panel Meters

author: saturation
author_rank: Super Contributor
timestamp: March 02, 2014, 02:57:49 pm
content: Yes, most files I linked are CC on many sites, if not I'd upload it. codeboy2k has a fine alternative link. Enjoy.Quote from: GiskardReventlov on March 01, 2014, 06:52:15 pm

author: G7PSK
author_rank: Super Contributor
timestamp: March 02, 2014, 03:42:53 pm
content: What about using a piezo crystal as the load element.

author: PA0PBZ
author_rank: Super Contributor
timestamp: March 02, 2014, 04:47:27 pm
content: Quote from: G7PSK on March 02, 2014, 03:42:53 pmPiezo only reacts to changing pressure.

author: SeanB
author_rank: Super Contributor
timestamp: March 02, 2014, 05:31:46 pm
content: Piezo works well for doing on the fly mass measuring, like on a dynamic weighbridge. Easy to interface but poor resolution, but a fast response and can determine easily if a HGV is over an axle mass limit on the fly. The ones near me are embedded in the roadbed, and have a simple signal system of a green or red light to alert the RTI if there of the need to flag a vehicle. Here the max allowable axle load varied but is not more than 22.5 tons per axle AFAIK remember. Front axle loads are lower at around 12.5 tons. I see a lot of light commercial vehicles with dual tyres with flat inner tyres, funny how the driver never checks those pressures. A lot of heavy trailers also have the same, but as the driver often has to change the tyre they do tend to check pressure or they will be struggling alone with logs and big hydraulic jacks to do a change on the road. They tend to do so unless they are a fleet rental vehicle with a service breakdown agreement. Then the driver does not care.

author: N2IXK
author_rank: Frequent Contributor
timestamp: March 02, 2014, 05:34:42 pm
content: Piezo elements can be used to measure static pressure, as well. Changes in applied load cause a shift in the resonant point of an oscillating quartz crystal. Paroscientific is one commercial manufacturer of piezoelectric pressure transducers. High precision, high $$$ stuff.http://www.paroscientific.com/transducers.htmAnother good technology for precision measurements of mechanical deflection (which is all a scale really is) would be the Linear Variable Differential Transformer, or LVDT. Commonly used in precision gaging heads, force transducers, etc.Bonded resistance strain gages are really the easiest way to go. All you need to make them work is a regulated DC excitation source, and a high gain differential amp.

author: Willson
author_rank: Newbie
timestamp: April 11, 2014, 07:21:46 am
content: But you canget accurate measurementwith digital scale rather than using analog scale or any instrument that you made for weighing. Because some time you get accurate results for these handy made scale if you are weighing heavy things but i prefer to use a scale to get accurate measurement.