Electronic scales - Wheatstone vs constant current - Page 1
Forum section: electronics / projects

author: damo
author_rank: Contributor
timestamp: January 27, 2012, 04:46:29 am
content: Hi Guys,I am thinking of building a home made electronic scales using a voltmeter and strain gauges. I was thinking of two solutions to converting the resistance of the strain gauges to voltage for the meter and was wondering about the accuracy, advantages and disadvantages of the two when used for this particular application. The two options are using a Wheatstone bridge or a constant current source.My strain gauges (probably around 4 of these) will be constructed of double sided circuit boards with a long meandering pattern of thin tracks (google electronic strain gauge) such that when they are bent, the tracks on one side will be elongated (increasing the resistance) while the other side will be compressed (lowering the resistance). I will probably include an analogue zeroing circuit through a potentiometer for simplicity (lazy bones doesn't want to work out a tare circuit). If I go with the Wheatstone, would it be better to wire the gauges in series and use a half bridge so I can include the zeroing pot?Other comments on the design are welcome. Thanks guys

author: Rerouter
author_rank: Super Contributor
timestamp: January 27, 2012, 06:17:44 am
content: "The constant current circuit is most effective when dynamic strain is being measured. This is because, if a dynamic force is causing a change in the resistance of the strain gage (Rg), one would measure the time varying component of the output (VOUT), whereas slowly changing effects such as changes in lead resistance due to temperature variations would be rejected. Using this configuration, temperature drifts become nearly negligible."quoted fromhttp://www.omega.com/literature/transactions/volume3/strain2.htmlseems basically that constant current minimises temperature shifts, which would imply a lesser need for a zeroing pot, due to smaller varience, (wires heated at a consistant wattage)however i hope you remeber that strain guages are dynamic if not compressed on the same axis as the tracks

author: damo
author_rank: Contributor
timestamp: January 27, 2012, 12:54:37 pm
content: Thanks Rerouter! Thats a very useful resourceI can tell this is going to be a bit of a learning curve for me as it's been quite a few years since I've played with const current sources. PBC design will be fairly straight forward I think so long as the tracks on either side are well matched to each other. I did realise that they will need to be compressed along the axis of the tracks, but thanks for the infoWhen I said there will be 4 strain gauges, each will be at 90 deg to each other connected to the weighing platform (WP) via some sort of hinge setup. I was thinking of using 8 hinges in pairs to make a sort of movable zig-zag set up. One end attached to the WP, the other to the SG. The other end of the SG attached to the chassis as a rigid body. This way, the WP can move vertically with minimal horizontal or sheer movement.Any other comments?

author: Rufus
author_rank: Super Contributor
timestamp: January 27, 2012, 05:14:03 pm
content: Rerouter got it wrong. Resistance measurement with a current source does not easily provide thermal and drift compensation so it is 'most effective' for measurement of dynamic strain (such as vibration) where you are specifically ignoring low frequency components. A mass and gravity isn't dynamic, you want a self compensating bridge.I don't know what copper is like as a strained resistive element but I suspect FR4 PCB material will be bad for flexures, liable to plastic deformation and compressive and tensile strength very temperature dependent.Commercial strain gauges can be pretty cheap although probably difficult to source and you would have to come up with steel or hard aluminum flexures.For a hobby project you might also consider a force balance approach.

author: harnon
author_rank: Regular Contributor
timestamp: January 27, 2012, 05:27:24 pm
content: A half wheatstone bridge eliminates the impact of temperature but doubles the impact of the bending moment. A full wheatstone bridge compensates for both, but requires four sensors.I'm pretty sure modernstrain indicators (PDF)use bridges for their temperature compensation.

author: SeanB
author_rank: Super Contributor
timestamp: January 29, 2012, 12:45:30 pm
content: Using copper board as a strain gauge will not be good, unless you read the copper temperature and compensate for the effects. As well the flexing of the board will most likely not be linear ( after all it is a laminate that will have voids in it) and will be very limited in it's range. Better to buy some loose foil gauges ( or buy a cheap OHL scale and strip the measurement cell from it) that at least are low TC, or are matched to the substrate they are on.

author: damo
author_rank: Contributor
timestamp: February 06, 2012, 01:30:32 am
content: Thanks for the replies everyone. Been away in Sydney for a week so couldn't get on here.I really wanted to build my own gauges but from what I'm reading this may not be feasible for a long term solution. But since I'm only doing this as a fun project for now, I guess it will do for short term till I get the hang of it and build a more permanent device. Doubt I'll be using it to measure anything over 5kg to start with (kitchen scales). Once I get the design right, I'll look into bathroom and luggage scales using foil gauges (yep, been on a plane recently)Looks like I'm back to the Wheatstone soln. I'll have 8 gauges, 4 in compression mode and 4 in expansion mode. Wire into 4 groups of 2 sensors each (2x2 compression and 2x2 expansion) for a full bridge and read off the centre.

author: amspire
author_rank: Super Contributor
timestamp: February 06, 2012, 01:51:25 am
content: I think it is great making you own sensors. The biggest TC problem will happen if the test current through the bridge is high enough to signifigantly warm the tracks - any difference in track thickness or local cooling differences will result in differing track temperatures and that is when you will start to see errors due to the 0.4%/C coefficient of copper. So the lower the bridge voltage you can live with, the better.