LDC1000EVM Linear Position Sensing Target Requirement

Hello Huichen,

The LDC1000 is only recommended for metal type identification applications. For all other applications, I recommend using one of our newer devices; In your case, I recommend the LDC1612, which offers many advantages. The high-resolution multi-channel LDC1612 is an L-only devices which offers greatly improved measurement accuracy, better part-to-part variation, lower power consumption, easier design integration, and multiple channels.

Here is an apps note which covers linear position sensing in detail: http://www.ti.com/lit/an/snoa931/snoa931.pdf

Here is more information on the sensor design: http://www.ti.com/lit/an/snoa930/snoa930.pdf 

Hello Huichen,

I don't have access to dropbox; can you please add the picture to the forum thread?

I don't know what your system looks like, but here is a generic example of how a second coil in opposing direction can be used to improve the z-axis dependence.

Hi Ben,

The information you provided is very helpful! I have come up with two design options, could you please take a look at it? The first design is pretty much similar to the example you provided. The second design uses circular coil, placed at two sides of the rotational arm.

And in the mean time, could you advise if any insulation between the aluminum target and aluminum balance arm is required? Do I need to carry out a calibration process as the example you suggested in your post before I conduct the real measurement? Which coil shape do you think is better for my application?

Thank you so much for your help so far!:)

Hello Huichen,

The triangular target (similar to what you've drawn in figure 1) gives better resolution than the stretched coil approach from figure 1. However, the better approach is typically the one with more controlled mechanical tolerances. I would expect that it is easier to control tolerances with the first approach because it does not require a second target.

In both drawings, your target moves closer/further away from the coil as the arm spins, which was not the case in the compensation information that I sent previously. We don't have simulation or measurement data on the suitability of the approach in this case.

The aluminum arm will certainly be sensed by the coil if it is within approximately 2 coil diameters away. If both coils see the arm at equal distance at all time, this will simply result in a fixed offset and a reduction in dynamic range, which may not be a problem. If it is not seen by both coils equally, then it would interfere with the measurement and would likely need to be dealt with. Here's some information on shielding from interfering metal: e2e.ti.com/.../inductive-sensing-how-to-shield-from-metal-interference

Hi Ben,

If I understand correctly, what you meant by "target move closer/further away from the coil" actually refers to the fact that target is not parallel to the coil surface as it will be in a certain angle as arm rotates? It is a small angle rotation though, typically around 1 degree, so I am assuming a quasi linear motion here, and the dz variation maximum should be 1.1mm.

And do you have any way to over come this in my application of sensing end displacement of the balance arm? What if I sense the displacement using axial measurement, the center shift should be only 1.1mm while my measurement range is 30mm. Let me know if you have some ideas. :)

Cheers,
Huichen