Sensing Force On A PCB? - Page 1
Forum section: electronics / projects

author: hal9001
author_rank: Regular Contributor
timestamp: June 17, 2023, 09:33:05 am
content: Is it possible to sense force on a PCB? I want to sense if a object is placed on the PCB.Searches come up with capacitive sensing but all examples I find are for finger touching a PCB. Can this work with inanimate objects?Cheers!

author: Shonky
author_rank: Frequent Contributor
timestamp: June 17, 2023, 09:38:54 am
content: More info needed. What is the object? Is it metallic? Size/weight? How much force are you expecting to sense?Capacitive sensors don't just work with fingers.What are you trying to do exactly?

author: hal9001
author_rank: Regular Contributor
timestamp: June 17, 2023, 09:45:08 am
content: Quote from: Shonky on June 17, 2023, 09:38:54 amMax weight of 500g anywhere on the surface of the PCB. As low as 100g. Example is if a object like a book is put on the PCB then I want to sense this.I stay away from optical sensing because I want to make sure there is contact with the PCB.

author: Infraviolet
author_rank: Super Contributor
timestamp: June 17, 2023, 04:00:46 pm
content: "anywhere on the surface of the PCB"That might be tricky, I suspect it would be more practical to have a situation where a mass would be resting on a certain subsection of the PCB and measure the deflection between where it is resting and some other area which is fixed. Maybe you could see whether you could build some sort of strain gauge within the PCB layers? Might benefit from using a oard house that could manufacture PCBs in thinner than typical sizes.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: June 17, 2023, 05:24:10 pm
content: These thingsmaybe?Although I think this is more of a mechanical problem and probably cheaper, more robust to solve it by setting up a compliant mechanism that you can measure the deflection / strain of. If the surface of the pcb must contact, e.g. for wireless charging or so then maybe you can integrate it under?The advantage of the linked pressure sensors is that you can use multiple small ones and even build a pressure density map, but I'm not sure if that'd work with the weights you're looking at.HTH

author: dobsonr741
author_rank: Frequent Contributor
timestamp: June 17, 2023, 05:49:34 pm
content: If you are asking can a force sensor be built out from the material of the PCB, just like how capacitive sensor touch areas constructed: the answer is yes.Look at this project how it’s done for a 3D printer hotend:https://www.reddit.com/r/PrintedCircuitBoard/comments/8k3hkr/using_traces_on_a_pcb_as_a_strain_gauge/At the 100…500g sensitivity thin PCB trace zigzag patterns around the 4 mounting points can give you readout. Prepare to use a sigma delta multi channel ADC optimized for strain gauges.

author: babysitter
author_rank: Frequent Contributor
timestamp: June 18, 2023, 12:43:42 am
content: Maybe you would like to use a few of these on the support of the PCB?https://sps.honeywell.com/us/en/products/advanced-sensing-technologies/healthcare-sensing/force-sensors/basic-tbf-series

author: LukeW
author_rank: Frequent Contributor
timestamp: June 18, 2023, 02:54:17 am
content: I remember a project a long time ago, like 30 years ago, in Electronics Australia I think, with a weight scale strain gauge made from a PCB. A long, thin meandered track across a large area, with three active sensing areas in a trefoil shape supported by a bolt in the middle.Force-sensitive resistors might be easier. Or, mount the entire PCB on "real" load cells designed for scale use.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 18, 2023, 04:26:44 am
content: Quote from: Marsupilami on June 17, 2023, 05:24:10 pmI was also thinking of these force sensors. Should work for 100g-500g.Now if only the presence of an object over the PCB is needed and the amount of force is of no importance, probably an optical solution will be more robust.With more details we may be able to come up with a better solution.

author: Terry Bites
author_rank: Super Contributor
timestamp: June 18, 2023, 06:05:56 am
content: You're not going to measure the mass of an item placed at a random location on a PCB using the mechanical properites of the PCB and/or a track pattern.I can see how you might measure the reistance in a cantilever set up as in the cente bolt thing. It wouldn't localise the force in any meaningful way though.PCB materials are (by deisgn) stiff and incompressbile. Eg the elastic modulus for FR4 is 24 GPa! (http://hyperphysics.phy-astr.gsu.edu/hbase/permot3.html)Sensing a position on pcb is relatively simple. Its just good old capacitive touch screen technology. see AR1020-I/SS for instance.

author: inse
author_rank: Super Contributor
timestamp: June 18, 2023, 08:11:25 am
content: What about a straightforward approach: place a number of push buttons with low actuation force on the PCB?

author: RoGeorge
author_rank: Super Contributor
timestamp: June 18, 2023, 08:21:57 am
content: SMD MLCC capacitors might get enough capacitance variation when the PCB on which they are soldered is bent or under mechanical tension, but I didn't test how reliable will be to measure that.If not good enough, solder 2 metal plates perpendicular on the PCB, so to make an air capacitor. When the PCB is bent under weight, the metal plates will move relative to each other, and the distance can be detected as a change in capacitance.

author: hal9001
author_rank: Regular Contributor
timestamp: June 18, 2023, 10:11:33 am
content: Thanks to all the fascinating suggestions!Quote from: inse on June 18, 2023, 08:11:25 amThis was my first idea too but the only place I can think to put the buttons is on every corner of the pcb to keep it stable. Is there a better way to place buttons that I didnt think of?Quote from: Terry Bites on June 18, 2023, 06:05:56 amWill capacitance change if the PCB comes contacts a poor conductor like a book? Position isnt important for my application just will liketo know contact is made with the PCB or not.Quote from: SiliconWizard on June 18, 2023, 04:26:44 amI thought about optical solution but I think it can be triggered falsely without contact if something is close enough tothe pcb. Simple example of application is to detect if a book is placed on the pcb and activate a response.

author: Georgy.Moshkin
author_rank: Frequent Contributor
timestamp: June 18, 2023, 03:28:13 pm
content: If exact force value is not important, one option is to measure spectrogram profile of sound waves going through PCB. Using surface mount sound source and microphone. Although need to filter out audible interference.Microwave method, can detect phase change through some line or frequency detuning of coupled resonator array. Difficult to design and too sensitive to metallic objects.There are some flexible pcb sensors for measuring pressure force, maybe gluing an array of most sensitive ones, put each flex pcb tail through PCB hole and solder on other side. Not very DFM if many sensors.

author: David Hess
author_rank: Super Contributor
timestamp: June 18, 2023, 04:38:49 pm
content: It can be done, but typical printed circuit board materials have terrible physical properties to make a load cell. Besides creep and hysteresis, offset and gain will change significantly with temperature.

author: MisterHeadache
author_rank: Regular Contributor
timestamp: June 19, 2023, 03:55:47 pm
content: Quote from: hal9001 on June 17, 2023, 09:33:05 amTake a further step back - what PROBLEM are you trying to solve? I get that you are trying to determine if an object is placed on the PCB. But how is that an undesirable event? And how do you need to react after that undesirable event happens?

author: inse
author_rank: Super Contributor
timestamp: June 19, 2023, 04:46:10 pm
content: And why does it have to be on the PCB?

author: Terry Bites
author_rank: Super Contributor
timestamp: June 19, 2023, 05:07:17 pm
content: Yes, this is the most pressing question. Why. Look into your soul....why?Piezo dots anyone. Rochelle salts, pass the mayo.

author: Georgy.Moshkin
author_rank: Frequent Contributor
timestamp: June 20, 2023, 05:11:12 am
content: Regardless of original problem, I think that sensing force on PCB is interesting. We all know how SMART technology is used in hard disk drives. Some models even provide realtime monitoring of shock events. At first, I thought that someone tries to measure if PCB is being bent, which may lead to solder joint failures and cracks. If mass measurement is not important, I think that audio approach is the best. Try to touch any working audio speaker surface and instantly notice drastic changes of how it sounds. This is measurable, just need to experiment with optimal frequencies, optimal sound source and microphone placement on PCB. Moreover, it could be simulated in some CAD, including size and shape of some mass, and for different PCB thicknesses.EDIT: With flexible pressure buttons and all kind of mechanical buttons - you will end up re-inventing digital scale. For acoustic method you can use PCB or other board material with Eva adhesive strip. Just my opinion! It would be interesting to know a little more details on what you are trying to achieve.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 20, 2023, 06:50:32 am
content: Quote from: Georgy.Moshkin on June 20, 2023, 05:11:12 amAcceleration (including shock detection) is only done using MEMS accelerometers in hard drives.

author: Georgy.Moshkin
author_rank: Frequent Contributor
timestamp: June 20, 2023, 09:24:56 am
content: Quote from: SiliconWizard on June 20, 2023, 06:50:32 amyes. I would not be surprised if it's not the only method. I am sure that I am not the only one who thought that this topic is about stress measurement. And for measurement of full PCB area for object presence I suggested acoustic method.