Detecting a non-conductive string above a PCB - Page 1
Forum section: electronics / projects

author: jmaja
author_rank: Frequent Contributor
timestamp: February 19, 2017, 09:54:22 am
content: What kind of ways is there to detect a string (e.g. 2mm diameter wool, non-conducting) at 1-20 mm distance above the PCB? Only flat SMD and low power (preferably <<1mA) components can be used. It would be even better, if the position of the string could be detected in one dimension along the PCB (a few positions or linear scale in 50 mm).The detection method should work also in dark and wet conditions. There will be a water resistant film between the PCB and the string.I would guess some of the proximity sensors should work, but they seem to be mostly geared to detecting a finger.

author: ebastler
author_rank: Super Contributor
timestamp: February 19, 2017, 10:19:49 am
content: You have defined some boundary conditions there which may make this difficult to achieve, and which you have not explained. Why does is have to be a non-conducting string? Also, you have not disclosed all design requirements: E.g. what bandwidth do you need (i.e. how fast can the string position change)? Can the string/PCB be inside a dark enclosure, or operated under controlled ambient light conditions, which might make optical detection easier?Can you please describe the actual problem you are trying to solve? Presenting a partial solution you have thought of may lead the discussion in a wrong direction.

author: jmaja
author_rank: Frequent Contributor
timestamp: February 19, 2017, 12:11:13 pm
content: This is an outdoor solution, no enclosure and there can be nothing on the side or above of the string. So all the detection needs to done from the PCB level. I may try to find out more about possible string materials, but at the moment I'm quite sure all conducting materials are out of the question.Needs to work during night and in the rain. 10 Hz is certainly enough, maybe even 1-2 Hz.The actual problem is how to record the postition of the string with low power. It would be rather easy using a video camera, but that is not possible with the few mW power available. Also it would need quite a lot of image processing to dig out the actual position data, which is all I'm looking for.

author: PointyOintment
author_rank: Frequent Contributor
timestamp: February 19, 2017, 12:17:57 pm
content: Panasonic'sGrid-EYEline of 8×8 pixel thermal cameras might be useful. They say they consume 4.5 mA operating (at 3.3 V or 5 V). Even if the string is the same temperature as the background, its different emissivity might make it thermally visible. If you need more than 8 positions, and you know the string's angle will be consistent, you could tilt the camera slightly and use sub-pixel position measurement.

author: ebastler
author_rank: Super Contributor
timestamp: February 19, 2017, 01:39:07 pm
content: Quote from: jmaja on February 19, 2017, 12:11:13 pmIs the actual thing you want to build a secret?What I was asking for is to take a step back -- what is that string doing? What are its ends connected to? (E.g. if it is some kind of pendulum, could you measure an angle at the string's upper mounting point instead?)Whydo you think it can't be conductive? Could it be a transparent string where you could couple light into it to make it visible? Etc.I feel that it is always more productive to discuss the actual,underlying problemyou want to solve -- rather than the problem with an incompletesolutionyou have in mind.

author: Fraser
author_rank: Super Contributor
timestamp: February 19, 2017, 01:48:53 pm
content: +1When trying to solve a problem, all available data is needed. This includes the objective of the project. Adding incomplete project data to the equation makes it all the more difficult to provide assistance.Not wanting to be mean spirited but why would someone want to spend time assisting when they only get half the story ?Fraser

author: jmaja
author_rank: Frequent Contributor
timestamp: February 19, 2017, 02:20:04 pm
content: Thermal camera is an interesting idea. I just tried that with my Flir E4 at home. Yes you can often "see" the string as clearly hotter or colder than the background. Grid-Eye seems to have about the same NETD as E4 so it should "see" it as well. Hower it doesn't seem straight forward to make a software detect the string and even more so not to detect some temperatute differences in the background as a string. I can imagine that there is a problem with the long range and thus ability to see all kinds of "strings" several meters away. Also the Grid-Eye sensor is a bit higher (4.3 mm above the PCB) than I would like.The string is used to see the air flow direction near the surface. I'm quite familiar with different flow measurement techiques and it's not feasible to use any of those. Also the string is used visually and the aim is just to record that numerically, not to invent a replacement for the string, which could be another project.It's not totally impossible for the string to be conductive, but it needs to be light, flexible, non-sticky (even when wet) and durable. It should work hundreds of hours without replacement. It seems even harder to make an electrical connection from the string to the electronics, but I guess just being conductive would give more options for detection.I understand it would be more interesting with all the background data, but I would think there are many application for detecting a proximity of a small object other than a finger or some active object. So finding a solution for such a detection would be of much more general interest.

author: Skashkash
author_rank: Regular Contributor
timestamp: February 19, 2017, 02:39:58 pm
content: I'd say you need a camera and an illuminator.If the thread could be IR or UV reflective it would help by having the appropriate LED source and possibly a filter on the camera. You might get away with a ccd line sensor, a cam may be cheaper. And you'll still have to deal with ambient light.But power drain will be high. You'll need a pretty low duty cycle for a battery solution.

author: ebastler
author_rank: Super Contributor
timestamp: February 19, 2017, 03:08:09 pm
content: Quote from: jmaja on February 19, 2017, 02:20:04 pmDo you realize that you still have not told us much at all about that string and the geometry? "String" is a pretty broad term. This could be anything from a hair-thin piece of thread, hardly visible to the naked eye, dangly freely from one end to detect the slightest air movement -- to something much more substantial, which might enable light transmission or electrical charge, maybe in a totally different geometry.Anyway, you are right that this would be more interesting with the background. In fact, it is not interesting enough for mewithoutthe background, and feels quite unproductive, so I'm out of here.

author: Ian.M
author_rank: Super Contributor
timestamp: February 19, 2017, 03:27:59 pm
content: Probablyhttps://en.wikipedia.org/wiki/Yaw_stringor similar. I don't think its applicable to the similar sailing application of telltails, (to monitor air flow attachment near the leading edge of foresails or rotational flow off the trailing edge of mainsails), because there wouldn't be anywhere to mount the PCB.

author: jmaja
author_rank: Frequent Contributor
timestamp: February 19, 2017, 03:30:57 pm
content: Quote from: ebastler on February 19, 2017, 03:08:09 pmI did say in the first post 2 mm diameter wool, which is probably the most common one used. Length is around 10 cm. The string will be rather straight and could be detected at any distance from the point it is fixed. The angle range of interest is probably +-45 degrees and the PCB size should not be bigger than around 50 mm x 20 mm.The string needs to be easily visible to 5-10 m distance with naked eye. So the object is rather big regarding the close distance. Probably the string will never be more than 10 mm above the PCB.

author: ebastler
author_rank: Super Contributor
timestamp: February 19, 2017, 04:29:52 pm
content: Quote from: jmaja on February 19, 2017, 03:30:57 pmSo, a computerized, and slightly miniaturized version of this?https://icons.wxug.com/data/wximagenew/m/MTNesters/0.jpgCan you get away with weaving a single strand of very thin wire into the wool thread, and use capacitive sensing? Send a suitable carrier frequency into the wire, use narrow-band snychronuous demodulation on the receiving end to hopefully make it robust enough against background signals? With multiple electrodes (and amplifiers) on the PCB, you could get some spatial resolution.If the wire makes the "hinge" at the thread's mounting point too stiff, maybe let a segment of wire run along the free-dangling piece of thread only, and use capacitive coupling at the top to couple the signal?Water droplets on the PCB would introduce offsets into the capacitive sensing, I guess. But as long as you only want to detect movements of the thread, i.e. changes over time, that should still be feasible. Although, if I understand the setup correctly -- doesn't wetness totally mess up the calibration anyway, by changing the mechanical properties of the string?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 19, 2017, 04:39:31 pm
content: If you could make the string magnetic, that may open up some options.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: February 19, 2017, 05:02:23 pm
content: This violates your rule about not modifying the string, but a fluorescent dye in combination with appropriate illumination would make the string easily distinguishable from the background. A visible camera in combination with the appropriate dye and filter would do the job.Again, you haven't told us all about your application, but several of us will infer that you are trying to record the position of strings used to visualize flow fields in aircraft, wind tunnels, sailing vessels and other applications. In many of these applications the position really needs to be at least 2D recorded, possibly 3D. Two cameras would solve a 2D requirement, and with some assumptions based on constraints on one end of the string and so on you could get an approximate 3D solution.

author: jmaja
author_rank: Frequent Contributor
timestamp: February 19, 2017, 09:01:59 pm
content: Quote from: ebastler on February 19, 2017, 04:29:52 pmI could probably put a few very fine copper strands into the wool string, but I don't think they will last at the hinge point. I don't know, if they will even last in the rest of the string with all the flexing it does.The wool string is very light and doesn't absorb that much water in the rain. The idea is that it will always be parallel with the flow, so not like the picture you linked.Will the capacitive touch proximity systems of MCU's with PCB printed touch patterns be able to detect a single fine floating copper strand at 10 mm distance? I don't have much experience about those. The only time I have tried capacitive detection is to measure liquid level through a plastic wall. That seemed to work with rather simple system using MCU ADC for detecting pF range capasitance. But it had some problems with calibration and I didn't do much testing with moisture etc.

author: jmaja
author_rank: Frequent Contributor
timestamp: February 19, 2017, 10:41:39 pm
content: Quote from: mikeselectricstuff on February 19, 2017, 04:39:31 pmHow to make it magnetic? Put a small rare earth magnet inside? I suppose 1 mm cube would be detectable to 10 mm distance? About 1 Gauss or 0.1 mT.It is about as heavy as you can use. Already weighs about the same as 5 cm of the wool string. Then it needs to be coated in order to avoid rust and somehow fixed to the string.Would it work with two or three 1D magnetic sensors? Then you could approximate the position based on the field at those points. The direction of the field could make it difficult. It won't be possible to keep the magnet at the same attitude. It could happen that the field is perpendicular to detection at the nearest sensor and parallel at the other sensor.

author: artag
author_rank: Super Contributor
timestamp: February 19, 2017, 10:48:53 pm
content: Could you use a VL6180 laser TOF sensor ? Or a grid of them, or one scanning ..

author: jmaja
author_rank: Frequent Contributor
timestamp: February 19, 2017, 10:59:32 pm
content: Quote from: evb149 on February 19, 2017, 10:10:33 pmThanks for a lot of different ideas. I have to go through them during the following days. There is no need to measure the 3D position of the string. Just the angle along the PCB surface, not the distance from the PCB.I haven't yet decided about the powering options, but it may be that also peak power is quite limited.

author: Brumby
author_rank: Unknown
timestamp: February 20, 2017, 12:33:13 am
content: The REAL reason for the string......(For those in the appropriate areas of the world, the last entry can be changed to "Tornado")

author: m98
author_rank: Frequent Contributor
timestamp: February 20, 2017, 12:44:39 am
content: You could also try using a bunch of phototransistors in a row to detect the shadow of the string.

author: jmaja
author_rank: Frequent Contributor
timestamp: February 20, 2017, 10:02:41 am
content: Quote from: m98 on February 20, 2017, 12:44:39 amThat would work with a single light source like the sun, but how about cloudy or rainy days when light comes reflected from all the directions. A 2 mm string 10 mm above the surface doesn't give much of a local shadow then. Also it would be difficult in the dark. Would the string reflect enough to be detected?

author: jmaja
author_rank: Frequent Contributor
timestamp: February 20, 2017, 12:39:53 pm
content: Quote from: CatalinaWOW on February 19, 2017, 05:02:23 pmIs there a low power solution for the requires camera (or two) + image processing to find the string? After quick search I could only find ~100 mW sensors. That's way too much and the camera needs to be running most of the time.

author: Fraser
author_rank: Super Contributor
timestamp: February 20, 2017, 01:54:48 pm
content: Impregnate the string with Thorium as done with older gas mantlesUse a cheap Russian or Chinese Geiger tube to detect the low level radiation and distance/proximity. Geiger counters can be very low current as they are all about applying a few hundred volts to the GM tube with very little current.Common cheap GM tubesSi19 BGMSTS-5DOB-50Cheap GM counter kits are available on eBay.About Thorium impregnated gas mantles and Thorium.....https://en.m.wikipedia.org/wiki/Gas_mantle#ThoriumFraser

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 20, 2017, 03:58:29 pm
content: I would probably go for an optical detection with a pulsed light source and a chain of photo sensors using a simple lock-in technique. With that approach I was able to detect a hand movement at ~50cm distance with an ambient natural or fluorescent light present. As long as the string is somewhat reflective you should be able to sense it's position, the power consumption may be quite low, plus both light source and sensors can be SMD.CheersAlex

author: CatalinaWOW
author_rank: Super Contributor
timestamp: February 20, 2017, 04:40:55 pm
content: Sorry, I read low power, and didn't internalize what you meant by low power. At less than 1 mA I don't think you can do any camera approach.Do you have to read the string, or just give the same information as the string? The small handheld anemometers are very low power (they run off of a coin cell). Mount two at right angles, record the results and do the vector sum off line. Biggest problem would probably be slow time response compared to the string. Even simpler would be two vanes with a "zero" fiction optical readout. Same thing, record both vanes and post process the data. Poor mans AOA meter. Possibly you could get by with one vane.

author: jmaja
author_rank: Frequent Contributor
timestamp: February 20, 2017, 05:51:38 pm
content: Quote from: Fraser on February 20, 2017, 01:54:48 pmI don't really like the idea of needing to handle alfa-decaying materials. Quite unhealtly when digested or inhaled. Also the tubes you suggested are too big to fit. Otherwise it sounds like the easiest way to calculate the position.

author: jmaja
author_rank: Frequent Contributor
timestamp: February 20, 2017, 06:33:38 pm
content: Quote from: evb149 on February 20, 2017, 06:08:13 pmI would think it is plausible. I was involved with a flow field and mixing measurement using a radioactive measurement in the 90's. There is a company around here that does those, or at least did. They measured how the injected radioactive fluid concentrantion travelled inside a 10 m3 mixing vessel. They had a few detectors outside and they could locate the radioactive fluid quite accurately. That's about 1 m through water. Now only a few mm through air would be needed. Much lower radiation or less accurate detection can be used.

author: ebastler
author_rank: Super Contributor
timestamp: February 20, 2017, 07:17:50 pm
content: Cool, this is turning into a contest of who comes up with the strangest concept!I understand that the string is currently "tracked" by a human observer, watching it from a distance of 5 to 10 meters, and that you want to automate this observation. Apparently you can't supply power to where the air (?) draft measurement occurs, and hence need something battery-powered.Do you have a power supply at the location of the human observer? Why not monitor the string through a telescope and camera, and use whatever image processing power you need to detect the position? Throw in a spotlight if needed to operate in the dark.

author: jmaja
author_rank: Frequent Contributor
timestamp: February 20, 2017, 09:32:08 pm
content: The geiger tube would detect gamma radiation. Not alfa, which can't penetrate a even the glass tube. Gamma will easily penetrate conformal coatings, water droplets etc. You would need several cm thick obstacles to reduce the radiation by 50%. Gamma radiation is most often present when alfa is.That is what makes alfa radiation so bad when the source is inside your body. It will attact the outer surface of tissue it hits. Outside your body it can't penetrate the outer layer of skin, which is dead anyway.So the intensity of radiation will be quite accurately 1/distance^2 and you get to know the ratio of the two distances. Naturally you need enough radiation to get so much pulses that random changes are not important.So it would work, but it would be too large, maybe it would consume too much due to hight voltage and I don't like the idea of making the string radioactive (nor the high voltage).

author: Fraser
author_rank: Super Contributor
timestamp: February 20, 2017, 10:40:40 pm
content: I am pleased that my little suggestion caused so much discussionIt was just a thought for considerationI have always 'thought outside the box' when problem solving and some 'Blue Sky Thinking' sometimes leads to alternative approaches.It is true that Thorium is a potentially harmful radiation source, especially when ingested. That is why it is no longer used for luminous clock and gauge calibrations. It is also being phased out of gas mantles.If nothing else, it made people think about a very unusual, but still feasible, approach to the problem of position sensing. Size can be a challenge but there are very small GM tubes available these days and my examples were just common surplus stock off ebayFraser

author: Nerull
author_rank: Frequent Contributor
timestamp: February 21, 2017, 02:16:04 am
content: If this is for detecting flow direction, why not just use a vane?

author: jmaja
author_rank: Frequent Contributor
timestamp: February 21, 2017, 08:11:13 am
content: Quote from: artag on February 19, 2017, 10:48:53 pmI don't know. Would it detect a rather small object? The datasheet gives 40-70 mm maximum range for a 80x80 mm object in 10 klux ambient light. Direct sunlight would be 100 klux and the target is much smaller. Also the output and receive cones are rather small. It might work as an array of sensors detecting is there something 5-10 mm above or not. But it would probably not work with two sensors both detecting distance to the string and thus giving the position in 2D.

author: Jeroen3
author_rank: Super Contributor
timestamp: February 21, 2017, 08:58:36 am
content: Can't you do something with the mounting point of the string?You can probably measure the direction the string is blown to using some hall sensors if you allow the mount "stick" to move like a joystick. Or maybe you can get miniature load cells.Or just move the detection away, and use the pcb as reference grid and use a distant camera with some software.edit: can the string hold static charge?