Measuring Strain Gauge without Inst. Amp. - Page 1
Forum section: electronics / beginners

author: Twonius
author_rank: Contributor
timestamp: June 02, 2013, 08:59:41 am
content: Hi,I've been working on a project for which I need to measure 12 strain gauges (3 per module, with four modules communicating to a master via Bluetooth, bluetooth modules connected to the micro via SPI).Needless to say the number of instrumentation amp chips is adding up. The application is a body scale so the precision isn't that high.I've stumbled upon some circuits which use time based measurements in order to measure the resistance of the gauge. Is this implementable with the Atmega 328 and some passive components?Would I be better off just ditching the Atmega and going for an integrated solution like a picostrain PS081 micro? (Remember we're in the beginners forum here)

author: Paul Price
author_rank: Super Contributor
timestamp: June 02, 2013, 09:57:46 am
content: If you are not so concerned about very high accuracy you might be able to substitute cheap op-amps for the instrumentation amps and get the circuit to work well at a reasonable accuracy with software calibration compensating for the small offset errors of the op-amps. Op-amps like the LM324 are very cheap and you get 4 op-amps in a 14-pin package. It is also possible to configure an instrumentation amp from a single op-amp.There are analog multiplexers such as the CD4051/CD4052/CD4053 or the newer ic, for example 74HCT4052 / 74HC4052 that can switch your Arduino inputs into an instrumentation amp to multiplex a single instrumentation amp inputs.What's the part number of the transducer?Why use so many transducers? A commercially built cheap scale would cantilever the weight points to be measured onto a single load point that amounts to no more than the cantilevered weight pushing a ferrite rod into an inductor that is part of a not so very high frequency inductor/capacitor type oscillator in order to make weight turn into frequency for pounds/delta-freq. measurement.Sure, you could buy the one-chip solution picostrain PS081 but how many load cells can each Pico handle for a big fat price?Maybe you can weigh the alternatives to lighten your parts requirements.Where's the fun and learning with the Pico approach?

author: AndyC_772
author_rank: Super Contributor
timestamp: June 02, 2013, 10:10:29 am
content: I fixed a dual channel thermometer the other week, which used a 74HC4052 to switch the output from two different thermocouples to a single ADC. Given the tiny voltages in question I'd never have expected it to work, but it does, and it's pretty accurate too. Moreover, the calibration wasn't significantly affected by replacing the 4052, so I can't believe that it was relying too heavily on software calibration to compensate for the characteristics of any one particular chip.Perhaps you could use a single, good quality instrumentation amplifier and switch between its inputs using an analogue switch?

author: Twonius
author_rank: Contributor
timestamp: June 02, 2013, 10:29:28 am
content: Paul and Andy,After tearing down a few digital scales I saw that almost all of them were using sensors at each corner instead of the cantilever design. I'm not sure if this is a recent change as components have become cheaper but considering I wasn't brave enough to tear down anything that costs more that $10 it seemed like a viable solution which simplified the mechanical design greatly.The multiplexer idea is good but considering each module has 3 sensors this wouldn't really decrease the part count much 3 inst. amps to a multiplexer and 1 inst. amp.Right now I'm just playing around with a load sensor from sparkfun.https://www.sparkfun.com/products/10245I actually just started with AD620s (the way we did it in school) and worked my way up to the instrumentation amplifiers as I learned more about how to make these sensors work but I wanted to make sure I wasn't doing it the way it might have been done 10 years ago considering in my tear-downs I didn't see what appeared to be any separate bridge circuits with amplifiers, just everything running into 1 IC (or at least 1 black glob of goo).The picostrain can handle 4 load cells IIRC. I wouldn't say getting the picostrain to work is going to be easy either since I'd have to write the communication interface to the bluetooth module myself.thanks,Anton

author: SeanB
author_rank: Super Contributor
timestamp: June 02, 2013, 11:35:56 am
content: Using CMOS switches works, provided you switch both inputs to the inamp with the same switch, so you might have a common bus that all the switches in various packages connect to to switch in the assorted gauges. This reduces any thermal offsets which may be affecting the measurement and ensures each switch has a similar on resistance to balance input current of the inamp. As the signal will be noisy anyway you will be low pass filtering in any case, you do not really need a response above 5Hz in any case.

author: Paul Price
author_rank: Super Contributor
timestamp: June 02, 2013, 01:51:49 pm
content: I don't quite understand why you need so many weight load sensors and I would like to see the circuit you are using for just one of them. Please draw a schematic and explain.

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 02, 2013, 02:41:49 pm
content: 3 per module, are you kidding? Each module only needs one opamp, and if you are not aiming for high precision a LM324 will do the job. Did you look at the comments on that sparkfun page? There is a nice video of how to use the module. Anyway, this is how you hook them up, assuming you have a full bridge one. If not, make it into a full bridge with additional resistors:

author: Twonius
author_rank: Contributor
timestamp: June 02, 2013, 03:24:31 pm
content: I hope this helps. Basically it's like 4 separate weigh scales interfaced over bluetooth to a smartphone. Each "scale" has 3 load cells in order to avoid having to design a whole cantilever setup. the INA and BT arrows represent signal conditioning and a bluetooth transceiver in each scale.I've already got the circuit using the INA125 fine but I wanted to see if there was a more up to date way of doing this which could be simper or cheaper than using a bridge circuit considering this did not appear to be the way cheap off the shelf scales were doing it anymore.

author: Paul Price
author_rank: Super Contributor
timestamp: June 02, 2013, 03:35:05 pm
content: PAOPPZHow to Make a cheap instrumentation amp: see attached.The rather large offset voltages of a LM324 may at first seem to be an insurmountable problem, but it really just creates an error that can be corrected by software when no weight is applied to the strain gauge measuring system.If the correct reading is known without a load, then correcting the offset is no worse than adj'ing a scale for tare. And although offset voltage drifts with temperature, it is likely not to be encountered in most application that wide temperature variations occur over a very short interval and it is a very small coefficient of change anyway.(see attached spec)If the situation of using the scale, is that the scale is mostly unloaded, then this error can be continuously calibrated and corrected out of the measuring strategy.If it is known when weight is not applied to the scale, MCU self-correction of the error is quick and easy.The website has a video explaining how to use software and how to bias the strain gauge that uses hardware to continually hardware reverse the voltage applied to bias the strain gauge and subtract the error.The effect is to double the sensitivity of the strain gauge output while canceling offset error.

author: Rufus
author_rank: Super Contributor
timestamp: June 02, 2013, 04:20:29 pm
content: I don't think PA0PBZ made it clear.Multiple bridge or half bridge strain gauges sharing a load can just be connected in parallel to make a single lower resistance bridge or half bridge which measures that load.I think the only advantage of keeping them separate is the ability to adjust and match the gain of each cell to improve accuracy when the load isn't equally shared.

author: Paul Price
author_rank: Super Contributor
timestamp: June 02, 2013, 04:31:28 pm
content: The four edges of a scale are widely separated in Horiz. distance from each other. Since the output of the strain gauge is just a few mV, attempting to wire all strain gauges devices together in parallel would greatly increase noise pickup. It would be best to keep the wires shielded and provide a short electron trip the instrumentation amp input for these reasons.

author: Twonius
author_rank: Contributor
timestamp: June 02, 2013, 04:47:09 pm
content: Well the distance between the three load cells to the instrumentation amplifier would be about 50mm, far shorter than what I've seen in commercial scales, or tried on the bench with inst. amp circuits

author: ejeffrey
author_rank: Super Contributor
timestamp: June 02, 2013, 05:34:57 pm
content: How much are your load cells going to cost? The sparkfun ones cost $10 each. At that point, one cheap instrumentation amp / 3 cells seems totally reasonable and cost effective.My preference would be to use analog multiplexers rather than building several instrumentation amps with cheap opamps. The key factor is CMRR, which requires closely matched resistors. A load cell typically has an output of 0.1% the excitation voltage. That means a CMRR of 100:1 (about what you get if you use 1% resistors) will have a common mode error signal that is 10x bigger than the actual signal. Sure, you can tare that out, but who wants to deal with noise 10x bigger than your signal? How stable is your supply voltage? Do you want to worry about that?A cheap instrumentation amplifier (< $2 each) will have excellent CMRR and be a small fraction of the cost of the load cells. Analog switches will perform very well in this application and they are super cheap.

author: C
author_rank: Super Contributor
timestamp: June 02, 2013, 05:53:48 pm
content: So 4 scales each with 3 sensors.best case is equal weight on each sensor.worst case is all the weight on one sensor.You might when putting all weight on one sensor cause the other two sensors to read negative weights.Best case each sensor is read, calibrated and the total weights added together.When wired in parallel some calibration could be done.Note: If you were standing on the scale you would most likely shift your weight to get a steady reading that was the lowest.You may be using the 4 sales to weigh the 4 corners of a load and have the same problems as the sensors for one scale.I think the distance could be handled by using twisted pair shielded cable and a few capacitor which would also lower max read speed.So I think the question is when wired in parallel the error level OK for the expected use.C

author: Twonius
author_rank: Contributor
timestamp: June 02, 2013, 06:42:14 pm
content: C,Negative loads shouldn't be a problem since the cells are between the platform and the floor.ejeffrey,Sparkfun's price is a lot higher than what you'll get something like that though a volume producer. I'm just using it as a prototyping stand-in, otherwise every bathroom scale would have to cost > $150 for manufacturers to make a profit.I think this thread is going a bit away from what I originally intended.- I've already made the circuit work with instrumentation amplifiers and understand how they work.- I just wanted to see if there was an alternative to this approach since what I see out in production doesn't look like that (although perhaps they're making their own SOC solutions with instrumentation amps).- The multiplexing looks interesting but for an initial design I think I'd just stick with 3 instrumentation amps and leave that for later.Thanks for the feedback everyone. I appreciate being able to bounce this off of the community.

author: lgbeno
author_rank: Frequent Contributor
timestamp: June 02, 2013, 07:52:27 pm
content: Like someone said, an analog switch can be put in front of the iamp to multiplex the measurements.

author: Twonius
author_rank: Contributor
timestamp: June 02, 2013, 09:34:55 pm
content: It looks like this teardown is my answerhttp://youtu.be/N63IMoJmVOgInside he finds a 24bit Adc with ferrite beads suppressing high frequency noise

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 03, 2013, 09:51:11 am
content: Quote from: Twonius on June 02, 2013, 09:34:55 pmI don't think you need milligram resolution for a body scale, so 24 bit seems a bit over the top. I'm still wondering what you are trying to do, I took apart a few cheap scales myself because I was looking for a solution to measure X-Y force on an object. The (cheap) body scales I have seen just have a load cell in each foot, and they combine the information to get the weight. I did not look at the circuit that much because I was looking for a load cell I could use, unfortunately it turned out that they were way too big for what I was trying to do. I ended up cutting 4 squares out of a small metal plate so that it ended up being a metal cross in a frame, and glued a straing gage on the X and the Y leg (a whole different story, these things are tiny!). I just amplify the strain gauge signal with a LM324 (1K/1M, so 1000x) and then feed the signal into the 2 AD convertors of a PIC processor and use the USB on the same pic to read the values. The reason that there are full bridge strain gauges is that the remaining legs are used for temperature compensation, and it also makes it easy to hook up to an opamp.Can you try to explain again why you need 12 gauges for a body scale? I still don't get it, or are you building 4 scales?

author: Twonius
author_rank: Contributor
timestamp: June 03, 2013, 09:56:45 am
content: Well the resolution would depend on the design of the load cell. That one has milligram resolution because it has a very small sensor. A sensor with a higher capacity would most likely have less resolution.The idea is to put each of the 4 scales under the 4 corners of an object and find out the total weight.Yes you could use only one load cell per corner but they're very sensitive to the direction of the load, so you want to make sure it's all going straight through the load cell. To that end the easiest configuration is 3 or 4 load cells supporting the corners like you see on a lot of kitchen and bath scales.

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 03, 2013, 10:42:18 am
content: I just found the remains of one of the scales I took apart. It looks like the load cells are parallel somehow, since there is only one LM392 in there (opamp+comparator). I think they are doing their own dual slope,I can also see an interesting cap close by. After that the comparator output goes to the cpu cob blob which also drives the display. There is only one other ic, a C21DC eeprom. The cells are only half bridge (2 resistors).Edit: Here is how they hook up the 4 load cells: