Hello all;
I spend some time with LDC1000EVK and I make some measurement with various coils.
My requirements:
measurement distance: 6cm
resolution and repeatability: better than 0,5mm
size of coil and steel plate in not critical, and both may be quite big.
Tested coils:
air coil - diameter ~40mm, ~100 turns/~850uH, Ct=1nF, f=180kHz, Cf=220pF, rod located parallel to steel plate (best results in distance range)
ferrite 3F3 coil1 - ~200 turns on ferrite rod, 5cm long, Ct=1nF, f=95kHz, Cf=470pF, rod located parallel to steel plate (best results in distance range)
ferrite 3F3 coil2- ~80 turns on ferrite rod, 5cm long, Ct=1nF, f=198kHz, Cf=470pF
ferrite coil3 - WE coil for QI, but this have very short measurement distance
photo shows my test bench
my test bench allows to set distance with 0,05mm or better repeatability (micrometer screw), this micrometer test bench my company buy at farnell. all others components was designed specially for test LDC1000EVK
PROBLEM DESCRIPTION
Problem is with coils with ferrites.
Example, ferrite coil1:
1) set distance to 5,00cm - L=470,000uH
2) set distance to 0,30cm - L= I'm not remember now, i think this is not important now
3) set distance back to 5,00cm - L=469,970uH
4) if I repeat 5 times steps 1)...3) then every final result in step 3 was smaller. Of course error is big only at first step, all next step has smaller errors, ~0,003uH
It looks like some hysteresis.
In my implementation this problem is critical, it makes measurement error quite big - at about 0,5mm or worst.
I have not tested yet this problem with air coils.
My observation: current which flows via coil looks like half-sine wave. this makes some DC offset/component which may magnetize ferrite. Similar problems are with flyback converter, but in SMPS has little importance.
Does have someone similar experience?
Is it possible to avoid this problem? I think that problem is hard to solve, because avg current depends on distance.
Regards
Wojciech Błędziński
