I have tried to make the quadrant magnetic levitation work but unfortunately I did not succeed. During the experiment, I have made
the following changes to the circuit and setup:
(1) I added small resistors (6K) in parallel to R11, R23, R35 and R47 (indicated in the following schematics) to increase
the control bandwidth from 20 Hz to 80 Hz.
(2) I changed RLED1, RLED2, RLED3, RLED4 from 2.2K to 1.5K in the LED driver such that the current of the
LED increases and in turn the displacement sensitivity increases.
(3) I changed R51 and R51 (in the differencing block for PD1 and PD2) from 5K to 1 K. Correspondingly,
I increased R52 and R54 from 5K to 50K. This changes increase the gain in the differential control by a
factor of 50, which compensates the gain loss after increasing the control bandwidth.
(4) The trim pots in the coil drive block have the following values: 100K for pot1 and pot4, 1K fro pot 2 and pot3.
To increase the gain, I replaced R17, R30, R31, R41 by 102 Om resistors (we run out of 100 Om chip resistors.)
(5) I wrapped the OSEM flags by plastic tubes to block the light from the LED more efficiently. This also removed
the changes of the photocurrent in the photodetector when the levitated plate moves horizontally.
Possible issues that cause the setup not working at the moment:
(1) The feedback gain could be probably still not enough. During the experiment, I can't feel any force changes when the
flags crossing the zero point. The error signals and control signal has the right sign.
(2) The levitated weight may be not enough and the working point is below the extremum of the DC attracting force.
This could give rise to a large negative spring which requires unreasonable feedback gain to compensate.
(3) The DC attracting force between the magnets are differing each other too much (mismatch) and can't
be compensated by the coil driving force.
(4) The control bandwidth may be still not large enough. Initially my design was 100 Hz, but I forgot to divide
the angular frequency by 2pi and the resulting circuit has a bandwidth of 20 Hz. Later I increase it up to 80 Hz
by changing the resistors as mentioned before.
(5) The polarization of the coil could have a wrong sign. I have checked with Gauss meter, but still the absence
of zero-point crossing force change makes me worry about this.
For continuation of this work, I will finish writing my document and summarize all the results and outline what
needs to be done in the future. If everything goes well, I will be back in June and can spend more time on this
experiment. I can also work with the summer students together on this project.