Suggestions on summing/averaging multiple half-bridge load cells. - Page 1
Forum section: electronics / projects

author: CaveMannDave
author_rank: Regular Contributor
timestamp: January 03, 2016, 03:00:32 am
content: Howdy, all. Noob here, be gentle, please.From ebay, somewhere in Asia, I got several small half-bridge load cells, each only capable of about 1/3 my weight, and thought I could make an analogue output platform scale using 4 of these to use for tracking my diet more accurately than my antiquated bathroom scales.I have the specs for them SOMEWHERE (and can locate, if required), but I believe that I only need general guidance on the methodology front.First off, these devices have a definite limit on the applied load, so I need a method to divide the platform loading, however asymmetrical, roughly equally, between 4 or more load cells (I'm assuming linkages / knife edges, but I'm not welded to that approach).The second (more electronically oriented) question, would be that these independent, probably poorly matched, half bridges could have their outputs linearised and summed in the analogue domain, even utilising multiple trims, to achieve repeatable results.I am not TOTALLY committed to analogue methods, but would prefer to avoid micro-controllers (multiple A/D channels, numeric manipulation, and possible D/A conversion for output), if only for the sheer kicks of forcing it to workThis may be a Don Quixote impossible dream, but I would like to see what the EEVBlog Brain Trust has to say about this concept, positive or negative.I will supply any further information or specs required, but I think this is enough for a general solution.Thank you for your attention, and Happy New Year!Cheers,Dave

author: joeqsmith
author_rank: Super Contributor
timestamp: January 03, 2016, 03:39:05 am
content: Sounds like a fun project. Can you post a part number or link to the datasheet for the parts you have?The last time I used a load cell was helping a friend with a home made engine dyno. 24-bit ADC, chopper amps, etc. We have some nice calibration standards the we load onto the cell and I do a fifth order polynomial fit on that data.Personally, I would stay in the digital world as much as possible.

author: ajb
author_rank: Super Contributor
timestamp: January 03, 2016, 04:11:01 am
content: If the load cells have sufficient linearity, then adjusting gain and offset is dead easy in the analog domain. Likewise summing the outputs requires, obviously enough, a summing amplifier. If the outputs are not sufficiently linear, then correcting them is a bit trickier, and the correction method will depend on the type of nonlinearity. You'd need to characterize the load cells before you could design an analog correction circuit.Really, if you want to be able to do arbitrary corrections and particularly when you don't need to do it very fast, then doing it digitally is going to be simpler.

author: CaveMannDave
author_rank: Regular Contributor
timestamp: January 03, 2016, 04:22:57 am
content: Thank you, ajb,As I said, I am at my limit (physiologically), tonight:I will fill in as many gaps as I can, after I have rested; please be patient, I shall return, after a few hours.Cheers,Dave

author: Ian.M
author_rank: Super Contributor
timestamp: January 03, 2016, 04:42:45 am
content: Lever mechanism to apply 1/10 of the force to a single load cell. That gives you enough margin for the live load when stepping on not to exceed the load cell rating. Calibrate with a water load - smooth sided plastic dustbin, meter stick and a hosepipe. Measure the first 5l with a measuring jug so you don't have to deal with the irregularities iof the bin bottom. Correct for water's change in density with temperature:http://www.engineeringtoolbox.com/water-thermal-properties-d_162.htmlOnce you've got the calibration data in a spreadsheet, you can decide if its practical to linearise it in the analog domain, but implementing an arbitrary transfer function in a stable and repeatable manner using purely analog techniquies is a royal P.I.T.A, and a total waste of time and effort compared to implementing a lookup table with interpolation digitally. It should be noted that if you use the digital system to generate the correction term only and sum it with the load cell output in the analog domain, you don't actually need a very good ADC or DAC.

author: pmbrunelle
author_rank: Regular Contributor
timestamp: January 03, 2016, 06:16:47 am
content: I assume these are button-style compression load cells...Mechanically, I think that a triangular platform-type setup would be the simplest. This would consist of an equilateral triangle supported at its corners (with three load cells). Then, due to the low capacity, divide the force at each corner down with a lever as Ian suggested. Note that load cells have a nominal rating, but also a safe overload rating.With a platform-type setup, the complication is that depending on where you stand, each load cell will hold a different percentage of your weight. If you apply your weight perfectly centered in the platform, then all load cells will share the weight equally. At the other extreme, if you stand perfectly above one corner, above a single load cell, then that load cell will carry 100% of the weight.So maybe you think that with pairs of load cells wired in a complementary fashion (to get the full bridge) you'll get some magical tempco or non-linearity cancellation wizardry; it probably won't do that in a platform configuration, because the loading will be uneven between the cells.In a platform, each load cell needs to have its sensitivity individually calibrated. Only then can the weights be summed together to obtain the total weight. Otherwise, if you sum the voltage changes, and then you apply a scaling factor, what scaling factor do you apply? You don't know the scaling factor, because you don't know the load sharing.Electrically, you may want to turn on the load cells a few minutes before stepping on the scale, for thermal stabilization.Then, immediately before stepping on the scale, I would "zero" the scale as follows:For each half-bridge, use a sample-and-hold capacitor to "remember" the idle voltage output of the load cell. So three load cells, three capacitors.Disconnect the capacitors from the half-bridges.Then, step on the platform.For each half bridge, use an instrumentation amplifier to measure the differential voltage between the cell's loaded voltage output and the voltage stored in the sample-and-hold capacitor. Total of three instrumentation amplifiers.In each instrumentation amplifier, use a 25-turn pot to calibrate the gain of each load cell.Sum the three outputs together.Then, display the summed output to a panel meter. I doubt a panel meter (an old-school analog one) will offer sufficient precision, so the meter might need to be referenced to a voltage divider's taps (by rotary switch) corresponding to 100, 110, 120, 130, 140, 150 lbs and so on, and then the panel meter just gives you the least significant digits.If you need to do anything beyond offset/gain, forget the analog approach. But for a bathroom scale, I don't see the need for crazy linearity. Probably the most important is for the scale to be monotonic and repeatable.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 03, 2016, 09:59:47 am
content: If the load cells are sufficiently similar in gain or load could be equally distributed, one could combine two half bridges to a full bridge, especially if they a mounted close by not to get too much signal from the cables. So no more extra refenrence / dummy bridge needed.Getting the average of analog values is just combining the signals with equal resistors.The triangular platform is rather good to distribute the load - though this needs a point load - so ideally 9 load cells would be needed for a stable platform to stand on. One might get away with using the triangular platform just with one (or 2) load cell and 2 or 1 dummies.

author: CaveMannDave
author_rank: Regular Contributor
timestamp: January 03, 2016, 11:11:59 pm
content: Greetings:I know it's not a "real" data-sheet, but this is about all I could quickly find. It may not be the EXACT type that I bought, but it is extremely close in appearance and electrical specs.---------------------------------------Two photos from the listing:http://i.ebayimg.com/images/g/EHsAAOSweW5VbIvG/s-l64.jpghttp://i.ebayimg.com/images/g/3MgAAOSwrklVbIvT/s-l64.jpg---------------------------------------Features:This is a half-bridge load sensor, which is widely used in weight scales. When the half-bridge is being stretched, itsends signal via the red signal wire. You can use multiple load sensors simultaneously to increase the capacity range.Two sensors full-bridge measurements: 50kgx2=100kgFour sensor full-bridge measurements: 50kgx4=200kgParallel use to add additional capacityCapacity (kg): 50Comprehensive error (%F.S): 0.10Output sensitivity (mv/V): 1.0±0.15%Nonlinearity (%F.S): 0.05Repeatability (%F.S): 0.05Hysteresis (%F.S): 0.05Creep (%F.S/3min): 0.1Zero Drift (%F.S/1min): 0.1Temp. Effect on zero (%F.S/10°C): 0.3Temp. Effect on Output (%F.S/10°C): 0.1Zero Output (mv/V): ±0.3Input Resistance (?): 1000±10Output Resistance (?): 1000±10Insulation Resistance (M?): ?2000 (100VDC)Excitation Voltage (V): 10Operation Temp. Range (°C): -10~+50Overload Capacity (%F.S): 150Method of connecting wire: Red= Sig + ; White= Exc –; Black= Exc +Dimension (mm): 34*34*8Package Contents100% Brand New4 pcs x Body Load Cell Weighing Sensor Resistance strain Half-bridge Total 200kg------------------------------------I was thinking in terms of one lineariser/span /zero circuit per cell, a summing amp w/ master gain/cal then perhaps a cheap DPM as readout.My biggest concern, other than mechanically overloading one or more cells due to unequal force division, is the question of electrically directly "adding" the signals: is simple summing enough, or is there some non-linear effect when separate cells are involved.Another question, is DC excitation best, or would advantage be gained by Sine/Square wave excitation and demodulation back to DC towards the output?All in all, this is more a self satisfaction kind of project, not trying for milligram precision or world-class elegance; just a toy for my own use, that can weigh my body OR a puppy, with reasonable accuracy, and perhaps 0.1 Kg (1/5 Lbf) precision.More info, as needed.Cheers,Dave

author: CaveMannDave
author_rank: Regular Contributor
timestamp: January 04, 2016, 01:26:02 am
content: Quote from: Ian.M on January 03, 2016, 04:42:45 amI like that, thank you.Cheers,Dave

author: CaveMannDave
author_rank: Regular Contributor
timestamp: January 04, 2016, 01:31:59 am
content: Quote from: pmbrunelle on January 03, 2016, 06:16:47 amGood thoughts, i will incorporate them.Cheers,Dave

author: CaveMannDave
author_rank: Regular Contributor
timestamp: January 04, 2016, 01:42:44 am
content: Quote from: Kleinstein on January 03, 2016, 09:59:47 amFood for thought, not quite up to your speed on the 9 cell concept, but I would like to catch up. Enlighten me, please.Also, help me understand why 2 full bridges would be better (other than fewer inamps, would they match like a 4 wire full bridge?

author: mjkuwp
author_rank: Unknown
timestamp: January 04, 2016, 03:42:06 am
content: I think you can do something simple. Wire the 4 half-bridges together to make one bridge. You just need one instrument amp. The offset will not be stable but you can zero it with software before taking the reading. Each quadrant of the bridge will have two resistors in it. I've seen this diagrammed elsewhere on this forum but wasn't able to find the drawing just now.or - you can go to Goodwill and pick up a used scale for a few bucks.

author: CaveMannDave
author_rank: Regular Contributor
timestamp: January 04, 2016, 06:06:59 am
content: Greetings, mjkuwp,Would one pair the cells diagonally, or side-by-side?I would love a link to the other thread.Then, still, there is the (hopefully somewhat) equal force division question.But, as I mentioned, this is not to just needing to have a weighing platform, but to see if the concept can be wrestled into realistic functioning; I THINK it should, but I expect there are traps and gotchas lurking.Cheers,Dave

author: mjkuwp
author_rank: Unknown
timestamp: January 04, 2016, 11:38:25 am
content: Do you have 4 load cells of the same design?First, find the color of wire representing the middle of the half-bridge. If that wire is white, then your final circuit will use white wires for all four termination points. White is Excitation+, another White is Excitation -, another white is signal - and the final white is signal +.The other wires are connected to each other in pairs. Red to Red, Blue to Blue. Red to Red and Blue to Blue. These connections don't reference back to your circuit, they just complete it. It's best to draw this out on paper for yourself of course.The mechanical fixing of the load cells is a little tricky and may be harder than making the circuit.

author: pmbrunelle
author_rank: Regular Contributor
timestamp: January 04, 2016, 05:26:33 pm
content: Quote from: mjkuwp on January 04, 2016, 11:38:25 amTo expand on this, the right circuit depends on the mechanical setup.Only once you choose the mechanical topology, then we can have an intelligent discussion of what the proper circuit will be... though the reverse is also somewhat true.

author: CaveMannDave
author_rank: Regular Contributor
timestamp: January 04, 2016, 11:24:07 pm
content: Quote from: mjkuwp on January 04, 2016, 11:38:25 amVery interesting, I will investigate further!Thanks, all,Dave

author: CaveMannDave
author_rank: Regular Contributor
timestamp: January 08, 2016, 05:37:48 am
content: Quote from: mjkuwp on January 04, 2016, 11:38:25 amI have attempted to follow your suggestions, but cannot come up with anything REMOTELY bridge-like.Unless you were meaning 8 cells, paired to form EACH leg of the bridge, with the "white" wires paired at the N/S/E/W corners of a standard Wheatstone configuration, I don't know what you mean.Paired cells, w/outside ends (R-R/B-B) paired, but otherwise floated, using white (centre of 1/2 bridge) as terminals, arranged in a diagonal quad, like a standard "bridge", white pairs at each corner, might be useful, but unless that is what you meant, I won't bother.Cheers,Dave