Strain gauge Joystick - Could something like this work? - Page 1
Forum section: electronics / projects

author: PeterFW
author_rank: Frequent Contributor
timestamp: January 22, 2020, 12:39:03 pm
content: I would like a 2 axis input device, i have a few wonky design restrictions i set myself, one is no "moving" parts and so i came up with the attachment as a rough idea.The middle hole is just for attaching the stick of the joy-stick part.The deflection gets measured with the PCB strain gauges, the cutout makes it more conforming.I have seen designs like this, so it should work, but what do you think?

author: amyk
author_rank: Super Contributor
timestamp: January 22, 2020, 12:55:18 pm
content: The whole device becomes the (slightly) moving part!More seriously,https://commons.wikimedia.org/wiki/File:Pointing_stick_of_a_Lenovo_ThinkPad_keyboard-4488.jpghttps://commons.wikimedia.org/wiki/File:Pointing_stick_of_a_Lenovo_ThinkPad_keyboard-1047.jpg

author: george.b
author_rank: Frequent Contributor
timestamp: January 22, 2020, 01:33:27 pm
content: Aha, I see you've been studying the IBM TrackpointThat looks like it could work, yes. There has to be a small deflection for it to work, so maybe it's something to keep in mind when it comes to materials/mechanical layout - how much deflection as a result of how much force, and how sensitive a strain gauge you'd have to use.

author: PeterFW
author_rank: Frequent Contributor
timestamp: January 22, 2020, 05:43:37 pm
content: Quote from: amyk on January 22, 2020, 12:55:18 pmThanks for the reply!You are correct, that is why i put the moving in quotation marksI have read the patent and a few more resources concerning this design, i was just hoping someone maybe has tried it yet and got some practical knowledge.Sadly the Trackpoint devices are, as far as i can make out, based on regular FSR.I mean, technically a stain gauge is a FSR too but you know what i mean

author: PeterFW
author_rank: Frequent Contributor
timestamp: January 22, 2020, 05:52:08 pm
content: Thank you for the reply!Quote from: george.b on January 22, 2020, 01:33:27 pmYea, while i know how strain gauges work and what you have to do to get data from them, i never worked with them.On FR4 they should experience a deflection which these usually do not see.Maybe i should just design a couple of different arrangements and try them out but i got a problem with resistance.Using Copper on FR4 i do not get anywhere near >1Ohm.

author: fcb
author_rank: Super Contributor
timestamp: January 22, 2020, 06:05:00 pm
content: Could work. You might want to put gauges on both sides of the PCB. Zero calibration will be a challenge which everway you do it if you go for bridge arrangements.

author: Prehistoricman
author_rank: Regular Contributor
timestamp: January 22, 2020, 06:05:57 pm
content: Quote from: PeterFW on January 22, 2020, 05:52:08 pmYour deflection will be in milliohms anyway so the low total resistance doesn't matter too much.

author: PeterFW
author_rank: Frequent Contributor
timestamp: January 22, 2020, 06:20:25 pm
content: Quote from: Prehistoricman on January 22, 2020, 06:05:57 pmBut in a wheatstone bridge arrangement, i need to match the resistance of all elements to the resistance of the strain gauge.If my gauge is below 1 Ohm, i practically create a short between the supply rails.Or do i have this part wrong?

author: fcb
author_rank: Super Contributor
timestamp: January 22, 2020, 11:58:35 pm
content: Yeah. It’s going to be super low impedance. Commercial strain gauges are typically 120/350/1Kohms, very thin metal printed/etched onto polyamide.Principle is the same though - you’ll just have to be creative with your circuitry to cope with the low Z.

author: skylar
author_rank: Contributor
timestamp: January 23, 2020, 04:04:16 pm
content: Quote from: fcb on January 22, 2020, 11:58:35 pmNot to be annoying, but would not it be easier to glue commercial strain gauges to the PCB cutout originally shown? Then you could use the standard circuit to measure it, no moving parts though there is more assembly than the project seems to aim for.

author: richard.cs
author_rank: Super Contributor
timestamp: January 23, 2020, 04:27:46 pm
content: I have tried printed strain gauges on PCBs before, they weren't great. In my case I was comparing tracks on one side with others at 90 degrees on the other side of the PCB and my main problem was that the two identical geometries ended up 30% different in resistance giving large offsets. I suspect this was mostly plating rather than etching. On mine the major killer was the resistance delta meant that the self-heating didn't cancel so it wasn't as simple as removing a fixed offset.Your sketch suggests you're comparing two areas on the same side of the board which should be bent in opposing directions, they will probably be better matched than mine. You may still find the low impedance problematic though.

author: fcb
author_rank: Super Contributor
timestamp: January 23, 2020, 07:16:45 pm
content: Quote from: skylar on January 23, 2020, 04:04:16 pmSure - but then you have a $20 thing, I think the OP wants to make a low cost thing. Also, a standard strain gauge setup might still need an instrumentation amp with a gain of 100-1000, ADC etc..Handling the low-Z is the first headache, compensating for thermal drift will be a big deal also. But then thats 90% of the fun with electronics.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: January 24, 2020, 07:03:21 pm
content: Typical strain gauge has 1-2microstrains limit before deformation becomes plastic and irreversible.Have you done stress-strain analysis on it?Self-heating is usually prevented by gating the excitation voltage.Leo

author: PeterFW
author_rank: Frequent Contributor
timestamp: January 27, 2020, 11:29:33 pm
content: Thank you everybody for your ideas and feedback, i want it to be a cheap solution because i would like at least... 44 or 88 of them.But after a lot more thinking an research i ended up with a FSR matrix from Sensitronics, at least i ordered one.I think i can get the desired effect if i glue on suitable rubber feet with considerations of the matrix pitch.That should get me where i want.

author: jfitter
author_rank: Contributor
timestamp: January 31, 2020, 07:34:41 am
content: Electrically the design is simple whether you use strain gauges, optical rotation sensors, etc. What really needs to be considered is whether the mechanical design is practical.Real joysticks used for machine control have a mechanical arrangement that provides for a centering force. What this means is that a certain critical force (torque) must be exerted before there is any output from the sensor, after which the output is linear with deflection. Check out radio control joysticks, especially the high end ones, aircraft side-sticks, joysticks for cranes and hoists, coal mining machines, etc.The problem with simple force sensors on fixed sticks is that there is an output for any force. A tiny force yields a tiny output and the practical reality of this is that it is almost impossible to control anything with it.This is why non-gymbal joysticks are NOT used for machine control.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: January 31, 2020, 08:51:31 am
content: I would not sayanything. A lot of critical control systems use force sensitive control as opposed to displacement sensitive controls.E.g. most modern fighter planes.Humans control forces much better than position.LeoQuote from: jfitter on January 31, 2020, 07:34:41 am

author: MarkF
author_rank: Super Contributor
timestamp: January 31, 2020, 02:36:13 pm
content: Quote from: Leo Bodnar on January 31, 2020, 08:51:31 amCorrect.The force sensitive stick in the fighter moves less than +/- 1/4" at the top of a 12" long stick.It works surprisingly well. Very easy to control.

author: jfitter
author_rank: Contributor
timestamp: February 01, 2020, 09:27:14 am
content: Quote from: MarkF on January 31, 2020, 02:36:13 pmThis is by your definition a displacement sensitive stick.In a fighter the stick must be controlled when under a G loading of up to 9G. Elastomer sticks are the only practical solution.The MOST precise joystick operators are high level radio control aerobatic competitors. They require a level of precision which is almost super human. The top radios have precision machined gymbals floating on ceramic bearings with 16 bit hall effect angle sensors on each axis. Force sticks have been tried and found unsuitable.As a past national champ I believe I am well qualified to speak on this matter. You need to actually get out there and do it to understand the practical issues involved - it's called engineering.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: February 01, 2020, 10:53:58 am
content: Quote from: jfitter on February 01, 2020, 09:27:14 amI am chuffed to learn from the best - thank you for this lesson.Leo

author: Kilrah
author_rank: Unknown
timestamp: February 01, 2020, 02:06:12 pm
content: Quote from: jfitter on January 31, 2020, 07:34:41 amThat's why you use the difference as your input, proper thresholds etc. I did play with FSR-based sticks in the past and had good results for the intended purpose.Quote from: jfitter on February 01, 2020, 09:27:14 amNothing in the OP says he requires such precision though. And having a foot in that field, a lot of what you just touted is just clever marketing

author: jfitter
author_rank: Contributor
timestamp: February 01, 2020, 11:50:45 pm
content: Quote from: Kilrah on February 01, 2020, 02:06:12 pmGarbage - do you actually have one of these. I do. Open it up and have a real look. As an embedded design engineer for 30 years I do know what I am looking at.The problem with these forums is the answers from people who clearly have no idea what they are talking about. I reply to almost nothing, and only when I see a question that I have experience in and know a great deal about do I answer and offer advice. It is usually met with responses from the peanut gallery, which is why I don't reply very often.You lot are on your own. Figure it out yourselves.

author: MarkF
author_rank: Super Contributor
timestamp: February 02, 2020, 07:19:31 am
content: Quote from: jfitter on February 01, 2020, 11:50:45 pmAre you all talking about RC Model airplanes or actual aircraft?I was referring to the actual aircraft and we just used the un-certified ground version.Still far too expensive to take apart ($16,000). I don't know the internal mechanism. Just that they are force sensitive.

author: i_am_fubar
author_rank: Regular Contributor
timestamp: February 02, 2020, 10:57:23 am
content: I question the need for 16 bit hall effect. Assuming 50mm of tip travel, that means your controlling down to 800nm of positional accuracy. That is..... Not going to happen. 50um (10 bit), can't believe you would notice a difference?

author: Kilrah
author_rank: Unknown
timestamp: February 02, 2020, 07:53:37 pm
content: Quote from: jfitter on February 01, 2020, 11:50:45 pmI never said it's not what was inside, only that it's a marketing gimmick rather than something actually significantly beneficial. I've seen a few fun blind tests.