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 40m Log Not logged in Message ID: 3431     Entry time: Tue Aug 17 23:59:46 2010
 Author: Katharine Type: Update Category: elog Subject: Maglev update

Katharine, Sharmila

Update:
We haven't been posting in the elog regularly, for which we are very sorry.  We have been taking notes in our log books, but we ought to have posted here as well.  We apologize and now present an overview of what we've been up to.

Some time ago, we created a Simulink model to predict the response of our system, but for the model to be useful we needed to include approximately correct gains for each block in the diagram, including the magnet force and coil force gradients and OSEM "gain." We also needed to better quantify the 1x1 levitation.

The circuit which converts current to voltage in the Quadrant maglev control has a variable resistor. This is useful as it gives us a way to zero the current when the levitated object is in the equilibrium position. It was done as follows. The output voltage from the circuit converting current to voltage is fed into the oscilloscope. The voltage values for zero and complete blockage of the LED is noted(say 2V). We adjust the resistor to make the voltage output to be V when the flag completey bolcks the LED. This gives a zero current when the flag is in the equilibrium position.

OSEM Calibration:

The OSEM works by blocking the light that goes into the photodetector from the LED by a flag. To simulate the model we had on simulink, we needed to find what the gain of the OSEM was. The gain of the OSEM is the current it gives per unit displaccement of the flag. To determine this we attached a micrometer to the OSEM flag. The micrometer was long eneough to push the flag to completey block the OSEM. We connected the output of the PD test point (which was the voltage after the photodiode current was converted into voltage) to the oscilloscope. We noted down the voltage difference in the oscilloscope with a fixed reference for different positons of the flag. From the oscilloscope output, we were able to get the PD current. We then selected a linear region of the plot of PD current vs flag position(which is usually in the middle) to fit the graph with a straight line. The slope gives the OSEM gain.

Magnet strength
We need to know about the relative strengths of our magnets (levitated and fixed in the coil) in order to do magnet matching.  We used a Gaussmeter to measure the field from each coil magnet at  ~2 mm away from the center (the probe was fixed to an aluminum block, so that the tip had the same vertical separation for each of the four fixed plate magnets).  We labeled each of the four magnets and calculated the field at this distance to be 0.206 kG, 0.210 kG, 0.212 kG, and 0.206 kG,  respectively, for coils 1-4.  However, each measurement had a rather large uncertainty of 0.01 kG, because the field strength varied a lot with position on the magnet, and the measurements were limited by how well we could align the probe tip with the center.

```Fixed Plate Magnets - magnetic field (kG)            meas't    1        2        3        4 1       0.205    0.213    0.209    0.204 2       0.211    0.219    0.223    0.207 3       0.199    0.205    0.211    0.201 4       0.207    0.203    0.206    0.213 average 0.206    0.210    0.212    0.206 stdev   0.005    0.007    0.007    0.005```

We also planned to take the same measurements for the coil magnets.  We noted that the magnetic field varied a lot depending on the probe's location, but not in the way we would expect.  At the edges of the magnet, the field was much stronger (~2 kG) than at the center (~0.5 kG).  We initially thought this might have to do with how we were holding the probe -- for instance, if we measured the force towards the edge by moving the tip all the way across the center of the magnet, there might be some kind of integration effect which does not accurately represent the field.   However, we measured the field at the edge with the probe across the magnet and also with the probe, so this is clearly not the case.

We also noticed that the cylindrical magnet we used for single-magnet levitation was not attracted to the coil magnets in the way we expected.    Though the cylindrical magnet was oriented so that it was strongly attracted to the coil magnet, it was attracted more to the edges than the center, so that it seemed to be repelled by the centers of the coil magnets.  Though this follows somewhat from the Gaussmeter readings, it is not the behavior we would expect when considering the coil magnets as magnetic dipoles.

Attempting single-magnet levitation for each coil:
We attempted to levitate single magnets using all four OSEM/coil combinations.   We assembled the magnets and OSEMs using Haixing's mount, and, adjusting the height of the OSEM plate, attempted to levitate the single magnet with a flag with which we were previously successful.   This was completely unsuccessful using all of the coil magnets (and when we tried to levitate using the south magnets, we flipped the cylindrical magnet's orientation).
Since we had already achieved this levitation, this seemed particularly wrong.  We disassembled the fixed OSEM plate in Haixing's mount and built a cursory OSEM mount, similar to the one we had used for levitation before, and did not fix it in place.  After a little experimenting with the height and position of the OSEM, we were able to achieve levitation with coils 1 and 4.
We noted the levitation magnet separation (~4.5 mm) and the height of the OSEMs at which levitation was achieved (147 +/- 1 mm for coil 1, 146 +/- 1 mm for coil 4).  Then, we reassembled Haixing's OSEM plate and tried to levitate the cylindrical magnet at coil 1 and coil 4, respectively, adjusting the OSEM plate so that the height of the OSEM of interest was the same as when we achieved single-magnet levitation.  This was unsuccessful, which leads us to believe that there is some alignment issue between the fixed coil magnets and the OSEMs in the mount,  possibly due to the unusual field from the fixed coil magnets.
We also were completely unable to levitate using coils 2 and 3.  Coils 1 and 4 have identical circuit paths, whereas 2 and 3 differ slightly.  With more time, we need to investigate this further.

Force-distance measurements
We also measured the repulsive force between the cylindrical magnet and the coil magnets as a function of separation.  We fixed each of the coil magnets, individually, on a stack of sticky notes on a precision balance (the stack of sticky notes was to prevent the coil magnet from interacting with the digital balance) and zeroed the balance.  We then fixed the cylindrical magnet (oriented so that it would be repelled by the coil magnet) to a teflon rod, and mounted the rod so that we could slide it up and down a long cylindrical post.  Noting the position of the rod and cylindrical magnet, we were able to measure the repulsive force as a function of separation (see Excel graph).
However, because the magnetic field varied so much with position on the coil magnet, there is a lot of uncertainty associated with these measurements.    We tried to keep the cylindrical magnet in the same horizontal position, but it was impossible to keep the exact position while sliding the mounted teflon rod up and down the posts.    In spite of this, we fit the linear region of this graph, near the equilibrium separation of the magnets, for a very approximate measurement of the magnetic field force gradient. The slope gives the force per unit distance of the magnet.

Coil-force measurement
We measured the force by changing the current through the coil of wire, using a very similar setup as described above.  Since we are concerned with the magnetic force as a function of current, not separation, we fixed the teflon rod so that the cylindrical magnet and coil magnet were separated by ~4.5 mm, the approximate levitation separation of the two magnets.   We then completely blocked the OSEM with the flag, creating a maximum PD current, and measured the coil current using an oscilloscope when the LED was fully blocked and the current when we removed the flag.  At the same time, we measured the repulsive force by looking at the precision balance.
Unfortunately, we had difficulty taking further readings, because our circuit starting behaving oddly (described below).  Otherwise, we would repeat this process by blocking the OSEM LED by various amount and measuring the change in coil current, and the corresponding reading on the precision balance.   However, the force should be linearly dependent on coil current, and we ought to know one other point: when there is no current in the coil, there should be no magnetic force from the coil to the magnet.  Using this information, we can determine the slope of magnetic force by coil current, but it's not very reliable as we have only one real data point.
One additional aspect makes this reading questionable.  When we switched on the power supply, the reading on the precision balance changed, before we had blocked the OSEM LED at all. Since no light was blocked, theoretically no photocurrent should be coming from the PD and there should not be a coil current from the feedback, so the force should not be changing. We are not sure why this is.

Recent Circuit Behavior
Some noise in the circuit appears to be hugely amplified when the gains of each coil are high, resulting in a high frequency signal of a few hundred kHz.  When the gains are all sufficiently high, this noise can saturate the coil current so that when PD current changes, there is no visible change in coil current.
On Saturday, we noticed some odd behavior from the circuit.  We hooked up the oscilloscope so we could see both PD current and coil current, and were very surprised that the PD current signal was oscillating and continually changing even when no flag was inside the OSEM.   This was also affecting the coil current as well.  We thought this might be due to some component burning out in our circuit, or RC coupling somewhere, but we did not get a chance to pinpoint the origin of this problem.

Modeling
Initially we had attempted to model the force-distance treating the two cylindrical magnets as dipoles, and finding the attraction/repulsions between the four distinct poles.  However, the resulting equation did not have a maximum, which is what we got in our measured values, so it seems this is not the best approach.  We would like to try the current loop approximation.

 Attachment 1: repulsiveforce.png  3 kB  Uploaded Thu Aug 19 18:42:22 2010 Attachment 2: OSEMcalibration.png  4 kB  Uploaded Thu Aug 19 18:42:43 2010 Attachment 3: OSEMslopes.png  6 kB  Uploaded Thu Aug 19 18:43:07 2010 ELOG V3.1.3-