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Position sensor for sideways displacement

  1. Aug 2, 2010 #1
    I'm trying to design a position sensor for a specific application but the last time I thought about physics was a long time ago.

    The situation is *similar* to this :

    I have a block of wood with an irregularly shaped tantalum bead (diameter of about 2mm) embedded about 2-4 mm into the wood. There is a 1cm thick gel-foam pad lying over the block of wood with a point marked approximately over where the tantalum bead is.

    The block of wood moves in jerky movements and the gel-foam wobbles over it.

    I want to determine how much the marked point moves in relation to the tantalum bead and in what direction. What would be the best way to do that?

    Things I've considered are :
    - placing an wire coils on the foam, centered over the marked point to create a metal detector-type device (but probably with 2 or 3 receiver coils so that direction can be determined)
    - hall effect sensor (but I think I'd need to use a wire coil to magnetise the tantalum)
    - eddy current probe
    - capacitive sensor

    The problem with the last three, I *think* is that they only detect movement toward and away from the sensor and not sideways displacement, which is the important displacement in this scenario.

    Any suggestions would be appreciated. Thanks
  2. jcsd
  3. Aug 2, 2010 #2


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    Welcome to the PF.

    Can you use a video camera technique? Mount a camera on the wood block, looking down at the dot on top of the gel. Use a frame counter to synch up with the detected (or driven) motion of the block.

    Is there a reason you need to use the metal bead as part of the detection setup? Is this for a competition or something?
  4. Aug 3, 2010 #3
    Hi berkeman. Thanks for your reply.

    No, it's not for a competition. It's for an application related to my work.

    Visual methods won't work because the gel-foam is opaque. I don't necessarily have to use the metal bead. It was just one solution that I came up with within the constraints of the application.

    Sorry I can't be more specific about what the application is.
  5. Aug 3, 2010 #4


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    Just a thought- you mention the gel is opaque, but at what wavelengths? You might be able to image through the gel in either the IR or UV, especially if you're able to distinguish the bead through some sort of heating and/or illumination with specific wavelengths.

    For example, it might be possible to heat the bead and then image in both visible and long wave IR (thermal). The visible imaging system could watch the mark on the gel, and the thermal system <might> be able to image through the gel and see the bead.
  6. Aug 3, 2010 #5


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    But you also said:

    You want to know how the spot on the gel pack moves away from its initial position, right? The tantalum bead's position is irrelevant, it would seem.

    If you can mount the camera on a frame attached to the moving wood block, then your video will just show the movement of the spot on the gel pack with respect to the wood block and the bead or whatever. Start the camera before the block is moved to get your reference position for the spot, and then go crazy!
  7. Aug 5, 2010 #6
    Thanks for both your responses.
    I can see that we're not going to get very far if I don't give you more information, so here it is.

    I'm trying to work out a way to improve motion capture accuracy and specifically at the knee. This is for medical research to record small movements of the femur on the tibia. The problem with the existing system is that the reflective markers are placed on the skin over certain bony landmarks. This creates error in two ways. First of all, as the knee flexes and extends, the skin (with the reflective marker attached) moves in relation to the bony landmark. Secondly, it a patient with a lot of soft tissue between skin and bone, there is more wobbling of the marker, especially in jumping and pivoting movements, which I'm particularly interested in.

    My idea was to implant tantalum beads into the bone at the anatomical landmarks that are used and then to have the reflective markers attached to a sensor that senses where the marker is with relation to the underlying bony landmark (and tantalum bead) at any time so that this can be "subtracted" from the position of the reflective markers.

    I'm thinking of using tantalum beads because they are already used in other situations so ethical approval will not be as much of an issue. The other options would be stainless steel (?318) and titanium, but I'd rather just stick to titanium.

    So, I don't think visual systems are going to work. Heating up the beads so that they are visible with infra-red would probably not be possible because of likely thermal injury to the surrounding tissue.

    What would you suggest now? Thanks.
  8. Aug 5, 2010 #7


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    One possible suggestion- Fluoroscopy. http://en.wikipedia.org/wiki/Fluoroscopy

  9. Aug 5, 2010 #8
    The problem with fluoroscopy is firstly the radiation (which although small, will still put off a lot of people) and secondly the small size of the detector head, which means that the subject has to run and then pivot at a very specific spot in the room or else it won't get recorded. Also, most of the fluoroscopy units have a relative small space between the emitter and detector, again making it difficult to use with highly dynamic movements.
  10. Aug 5, 2010 #9
    The first thought that occurs to me is to use a very small neodymium magnet and then use Hall effect sensors to track the movement.

    You can buy some stuff here to try to do a proof of concept.


    A certain amount of calibration would be required to translate the voltage readings into linear position changes.
    Last edited by a moderator: Apr 25, 2017
  11. Aug 5, 2010 #10


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    What comes to mind initially is to use several ultrasound transducers attached to a knee-brace arrangement. I think you would get very accurate position information of the bones, without the need to do any invasive surgery beforehand to implant any markers. Have you done any experimenting with ultrasonic transducers?
  12. Aug 5, 2010 #11
    I did think of using hall effect sensors but my understanding of them is that they only detect movement toward and away from them and not across the face of them. I thought of creating an array of them so that only the one directly over the magnet would detect anything, but the resolution would be fairly poor. Is that right?

    How would I use the ultrasound data? I can't think of a way of getting the position data out without going through it manually frame by frame and measuring how far the bony landmark has moved with relation to the axis of the ultrasound probe. The other problems would be preventing any movement between the probe and the skin, which would have ultrasound gel on it.
  13. Aug 5, 2010 #12


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    I'm not exactly sure how best to use the ultrasound imaging, but you said that you want to get relative position data for the tibia and femur, as the Pt is moving about. I guess I was thinking of a couple sensors to measure the position of the tibia (x & y), a couple to measure the position of the femur (x & y), and a couple to image the patella and meniscus areas. Along with having all of those sensors mounted on a knee brace setup, it seems like it should be possible to get a pretty accurate measure of the x,y,z position of the knee.

    Although, I guess you are wanting to see what abnormal movements are possible with different Pt's knees, so I guess you'e like the "knee brace" to not really brace anything, and rather to just monitor the position of the sensors...

    Along the lines of other ways to do the measurements, do you use bony landmarks at the other ends of the tibia and femur? Since the tibia and femur are rigid, if you add in the position of the other ends of each bone, that could help to improve your sensing of the overall bone positions...
  14. Aug 6, 2010 #13
    There are actually 3d Hall effect sensors on the market.


    I like the idea of using a hall effect since the magnet could be extremely small. I was thinking of using surgical adhesive to glue it directly to the bone.
    Last edited by a moderator: Apr 25, 2017
  15. Aug 6, 2010 #14
    In theory the ultrasound concept would work well. In practice, though, there are a few problems with it that I can't think of a way around. The ultrasound transducers are too large and heavy, which creates more issues. The transducers will probably move in relation to the skin because of the ultrasound gel. Also, the data is difficult to use -it will probably require manual measurements.

    There are markers at both ends of the bones. But that still doesn't solve the problem of the movement of the soft tissue sleeve over the bones.

    thanks for that. I hadn't come across the 3d ones before. That might work.
    I don't think I'll be able to use permanent magnets though. It would be almost impossible to get ethical approval for the implantation of neodymium, especially since it is for research and not for therapeutic purposes. I think regardless of what material is used, if it is a permanent magnet it probably won't be approved for non-therapeutic purposes.
    I was considering using a wire coil to create a magnetic field in implanted tantalum beads. From an ethical approval point of view, that would be ok. The way I envisage it, there would be wire coil about 3cm in diameter with the hall sensor in the middle of it. I'd need to work out how to account for the magnetic field created by the coil so that only the magnetic field around the target is detected.
  16. Aug 6, 2010 #15


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    Just curious. Could you change the positions of the magnet and say a small chip of iron (instead of the tantalum)? Would this make it ethical?
  17. Aug 7, 2010 #16
    Hi dlgoff,

    Iron wouldn't be approved. It has to be biologically inert. It takes years to get any new materials approved for implanation, which is why I want to us one of the ones that is already in use.
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