PCB Trace Strain Gauge - Page 1
Forum section: electronics / projects

author: Mehran
author_rank: Contributor
timestamp: April 22, 2021, 06:16:09 pm
content: As part of a uni project, I need to develop some modules to attach to a drone and achieve some learning outcome. The idea being the drone is a STEM Platform for students to learn about electronics and programming. One idea for a module I could design is to measure the upthrust of the drone. Tie the drone down and calculate the upward force the drone can produce. In order to measure the force, I plan to make a strain gauge using the FR4 PCB material and fine traces. I have seen one other project online that sort of does this.What I do know:FR4 has different young's modulus depending on how the fibres or orientated i.e. bends more one-way than the other if you know what I mean.Trace width and thickness will vary all over the PCB, and from PCB to PCB, it's not an exact manufacturing process.It's not going to be accurate; there is a reason they don't make strain gauges this way.What I don't know:Is how to calculate the change of resistance as the PCB deflects. I can calculate the resistance of that trace when the PCB is flat. I just don't know how much it will change with the deflection. Will It be a suck it and see sort of thing.MY question is.How bad of an idea is it?Should I use a half Wheatstone bridge? Traces on the top and bottom of the PCB and two precision resistors . or a full bridge and have traces of equal length someplace else on the PCB for temperature compensation...or is that overkill for what will turn out to be a rough/bad strain gauge?Anything else I am missing?The idea of the project is for first/second undergraduates to learn EEE and MECH eng. My vision was for the student to see a Wheatstone bridge in action and visually see the traces on the PCB... also measure the upthrust of the drone.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: April 22, 2021, 06:44:10 pm
content: Sounds like a terrible idea. What's wrong with Hooke's Law?

author: Mehran
author_rank: Contributor
timestamp: April 22, 2021, 06:55:50 pm
content: yes, it is a terrible idea but it's the one I got. was hoping to push the Wheatstone bridge angle to my supervisor. any better idea of a module you can stick on a drone and get some time of learning outcome that will help a first-year EEE student.

author: ogden
author_rank: Super Contributor
timestamp: April 22, 2021, 07:06:36 pm
content: Quote from: Mehran on April 22, 2021, 06:55:50 pmYes, do it. Just don't try to (and fail) building sensor out of PCB. Buy it instead. First search hit:https://www.robotshop.com/en/micro-load-cell-5-kg.htmlOther option - buy digital hand scale and use sensor out of it. Also you don't want to put thrust measurement system on the drone because it is dead weight.

author: phil from seattle
author_rank: Super Contributor
timestamp: April 22, 2021, 07:09:47 pm
content: Your idea (strain gauge connected to a drone to measure lift) is reasonable. Using PCB material as strain gauge is not so much.You could find an inexpensive strain gauge to mount in such a way to accomplish your project. You could find a more predictable carrier for the strain wires. There are lots of ways to do what you want. Part of engineering is sorting though ideas and using the best that meets your needs.

author: tmadness
author_rank: Regular Contributor
timestamp: April 22, 2021, 07:27:36 pm
content: I love your project. I don't know if it will work reliably but I like the spirit.Anyway, my 2¢, what about capacitance. You can get pcbs with good known dielectric. Maybe measure deformation as a function of capacitance ?Ie two parallel know copper geometrys with fr4 in-between. Should be easy to prototype. Get a function gen and a oscilloscope, send a single sine and measure amplitude/phase difference.

author: Mehran
author_rank: Contributor
timestamp: April 22, 2021, 09:06:30 pm
content: Quote from: ogden on April 22, 2021, 07:06:36 pmThe drone is modular, so it will an addon to the drone while it tethered down but you are right, no need for it to be on the drone other than it one extra module I can add.

author: thm_w
author_rank: Super Contributor
timestamp: April 22, 2021, 09:15:32 pm
content: Looks like some sensors use a phenolic material but none are FR4:https://www.elecrow.com/download/Coding%20System%20of%20Strain%20Gauges-AGS-TECH%20Version.pdfAlso its not copper traces. Karma or constantan.You can buy these for <$1. Its more of a mechanical design thing though.

author: tmadness
author_rank: Regular Contributor
timestamp: April 22, 2021, 10:22:48 pm
content: One point on pcb resistors. IRC The tolerance and temp co for pcb resistors is pretty bad. So, the variance between pcbs might drown out your measurements. Also do you really need a whetstone bridge, a good bench dmm might suffice for a proof of consept.There's a discussion on stack exchange on this:https://electronics.stackexchange.com/questions/378731/current-sense-resistor-using-pcb-tracksI highly recommend that you build a minimal proof of consept device. This is something I would tell my younger self when I was building an ambitious university design project.

author: tggzzz
author_rank: Super Contributor
timestamp: April 23, 2021, 11:26:12 am
content: This is a university student project. The quality of the end sensor is relatively unimportant.The important points are that youpredictwhat you don't know and how you will find out, that you plan and implement your design, you assess the quality/defects in the result, and can state what you would do betternexttime.One example of that is that you know you don't know how you can predict the resistance change with deflection. So, whatcouldyou do, theoretically and practically to get an idea? Then how would you use that guess to improve your first sensor? Once you've built your sensor, what can you do tobeaccurate? Then what are the limits of the accuracy?If you can get those things over to an interviewer, they will see that you can tackle stretch projects with unknowns, and make progress - and could do the same ontheirprojects.

author: David Hess
author_rank: Super Contributor
timestamp: April 24, 2021, 02:39:34 am
content: Quote from: Mehran on April 22, 2021, 06:16:09 pmDo not use a half bridge. Besides the mechanical considerations which will lower performance, copper has a high temperature coefficient of resistance and using a full bridge with two gauges on the bottom and two gauges on the top will cancel the temperature coefficient of the copper without an undue increase in complexity.Real loads cells implement compensation of the change of modulus of elasticity of the material, and I suspect this will be so horrible with FR4 that it will dominate over other sources of error. Hysteresis and zero shift with temperature will be horrible as well.I am a dubious however that you can get acceptable signal levels without a lot of copper length because of its low resistance. These are going to be big gauges, and each strut should have a complete full bridge as described above to cancel out some of errors. One trick we used with some load cells which would otherwise have had low sensitivity was to wire multiple gauges in series and bridges in parallel, and I might consider that here with a full bridge on each strut.I seem to recall that someone printed gauges on FR4 to measure force but instead of copper, they used a carbon film like you sometimes find for printed switches. The much higher resistance of carbon film would give a significant boost to sensitivity.

author: branadic
author_rank: Super Contributor
timestamp: April 24, 2021, 06:10:46 am
content: So the task is to make a strain gauge using printed circuit board? Are you limited to FR4?What if you use a single sided aluminum board, with the readout circuit on one side of it and commercial strain gauges elements attached to the second side? This way you creatively combined both, a proper sensor element with low temperature coefificient and proper resistance with a requirement/limitation "circuit board" given by whomever.Full bridge vs. half bridge is just a question of what circuit you are using to digitize the signals, classical differential ADCs / differential to single ended converter plus ADC or modern circuits such as PicoStrain familiy.-branadic-

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 24, 2021, 06:59:33 am
content: Perhaps even worse, PCBs also have hysteresis. That is, as you go around a cycle on the stress-strain curve, you don't end up where you started. It takes a set. There's also some creep (time-dependent strain response), which we're more familiar with as making bolted PCB connections unreliable.A more mundane solution might be preferred -- for example a spring with a deflection meter. Which could perhaps be as simple as a mirror, LED and photosensor -- it won't be linear or anything, but given a series of calibration loads, you'll get an idea of what you're measuring. (Some flex resistors might be okay, but it's actually not great to use turn or slide pots -- they have a lot of stiction, causing blacklash in the measurement.)Or a magnetic solution, using a solenoid or electromagnet of some sort to maintain the counterforce. Now, this can be a bit tricky: normally, the magnetic force varies something crazy like the inverse cube of displacement. That makes measuring a displacement just about impossible, but if a displacement sensor is combined with it, servoed to maintain it at some balance point -- now only the gain of the sensor matters, not its linearity (aside from locally), nor the magnet's. Held at a constant position this way, the magnet applies a force proportional to current (up to a limit determined by size and construction -- magnetic saturation), so can be sensed very accurately.Bonus, such a rig can be turned upside down to levitate objects hanging down from the magnet.Alternately... get one of those levitator thingies and try to beef it up?But yeah at this point just buying a strain gauge is looking pretty nice, I'll admit.Tim

author: richard.cs
author_rank: Super Contributor
timestamp: April 24, 2021, 03:16:50 pm
content: I have tried PCB strain gauges and they were dismally unsuccessful. Along with the inconveniently low resistance the etching and plating tolerances for the small, closely spaced tracks you will need become problematic. I seem to remember we had a 30-40% difference in resistance between the same structure on the two sides of the PCB and even within a single side it wasn't great. Combined with the mechanical properties of FR4, it was unusable for our application.

author: bjbb
author_rank: Contributor
timestamp: April 24, 2021, 06:58:41 pm
content: "...This is a university student project. The quality of the end sensor is relatively unimportant.The important points are that you predict what you don't know and how you will find out, that you plan and implement your design, you assess the quality/defects in the result, and can state what you would do better next time.One example of that is that you know you don't know how you can predict the resistance change with deflection. So, what could you do, theoretically and practically to get an idea? Then how would you use that guess to improve your first sensor? Once you've built your sensor, what can you do to be accurate? Then what are the limits of the accuracy?"Non sequitur. Contradictory to the principles of a professional educator. Contradictory to the expected behavior of an engineer. I had a Cal Tech grad suggest (and later attempt on his own) this very same thing - because that is the solution that the silly people of his alma mater provided as part of his 'professional' education.I have used (mostly MM) strain gauges for over 30 years. Practical experience in the implementation of an actual industrial strain gauge solution is valuable to the engineering student.https://micro-measurements.com/products

author: tggzzz
author_rank: Super Contributor
timestamp: April 24, 2021, 10:11:18 pm
content: Quote from: bjbb on April 24, 2021, 06:58:41 pmI'm not sure whether your response is a good advert for that (?your?) company.Firstly it helps if you use the standard mechanisms available on this website (i.e. the "quote" button next to my post) rather than change my post to allemphasis.Secondly, I will point you to my initial caveat and the word "relatively". This is not a professional product, it is a learning exercise.Thirdly, there is no non-sequiteur. On the contrary, your attempt to compare a post-graduation professional project with an undergrad student projectisa non-sequiteur.Fourthly, what you expect from a student project and what other people would be looking for are two very different things. It is highly unlikely that a student project will have anydirect technicalrelevance to the work required in their first job. Thus it is more reasonable and useful to expect that it can be used to indicate a student's thought processes - because thosewillbe applicable toanyjob. And such mentality and thought processes are indeed what I/we were looking for in undergrads.Finally,"Practical experience in the implementation of an actual industrial strain gauge solution is valuable to the engineering student"would be more accurate as"Practical experience in the implementation of an actual industrial strain gauge solution is only one of 100000 things that might valuable to the engineering student in their first job"Very little in my university course was of direct relevance to my first job, because on half of it was classified and the other half was in a very new technology that was just emerging into industrial applications.

author: tmadness
author_rank: Regular Contributor
timestamp: April 25, 2021, 04:07:37 am
content: Damn op you got a bunch of engineers riled up about your project. What are you going to do about it?

author: SeanB
author_rank: Super Contributor
timestamp: April 25, 2021, 09:13:56 am
content: You can use a PCB, but you probably want to use the copper to hold down some really thin nichrome wire stretched taut, and then measure resistance change of that. Pins in through holes in the PCB, with the wire stretched around them to make a long taut string, and then with the ends secured using more pins to hold the wire between pin and through hole, because you really will have a hard time soldering nichrome wire. Will make a better sensor, still a poor one, but at least resistance that you can measure, using something like 40 to 60 SWG nichrome wire.Tempco is horrid, will need a compensation table and local temperature sensor, but at least you will be able to use a reasonably standard ADC and gain stage to get a usable reading, and one that is able to be calibrated. Will need a temperature chamber to get the tempco per sensor, and then a set of masses to get a deflection set, but will work.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 25, 2021, 11:53:41 am
content: The low resistance is inconvenient and would cause a low voltage and possible quite some self heating in the sensor. Self heating can be a problem as the TC is high and this mismatch in thermal conditions (e.g air flow) can be a problem. Chances are better if you want an air flow sensor and not a DMS, but one may end up with both combined.The reletive resistance change is relatively similar for non magnetic metals (~ 2 times the strain). So the overall resistance change is rather small and the thermal effect quite large.The mechanical properties of FR4 are not what one wants from a load cell, but maybe OK if one wants to demonstrate the negative effects like humidity effect and relaxation.

author: fcb
author_rank: Super Contributor
timestamp: April 25, 2021, 12:13:59 pm
content: I'd give it a go. Even if it fails miserably you've learnt something, failure is still a result! Pretty cheap to try a few different designs also.

author: tggzzz
author_rank: Super Contributor
timestamp: April 25, 2021, 12:20:10 pm
content: Quote from: fcb on April 25, 2021, 12:13:59 pmPrecisely.Particularly if you can saywhatwas a failure, andwhy- and what you would need to do to improve it.