NEW HE OFFSET
We moved all 7 HE sensors closer to the plate. The AC sensors were burnt out and we replaced them with the DC ones, which we had removed yesterday because they were of no use.
The sensors in their current position feel different magnetic field and their offset is also different (around 100mV, compared to 2.4V before). The smaller value of the voltage offset sounds counterintuitive--because the sensors are now closer and the field around them larger--but it only indicates the direction of the field. The HE sensors read 2.5 at the presence of no field; far away their offset was 2.4V, so they only produced 100mV (2.5V - 2.4V) signal. Now that they are closer, their offset is huge (roughly 2.5V). We are worried they might actually saturate, once the plate starts moving.
For future reference, here are the values of the resistors we use:
AC1: R1= 110K, R2=2.2K
AC2: R1=130K, R2=2.2K
AC3: R1=62K, R2=1.2K
S1: R1=7.5K, R2=2.2K
S2: R1=20K, R2=7.5K
N: R1=20K, R2=6.2K
E: R1=4.3K, R2=1.5K
Noise reduction and debated Cross-Croupling
The new gain for all sensors conditioning board is now 11. The noise would therefore be amplified less (by a factor of 9). Indeed, the improve noise effects were apparent, since signal fluctuated only a couple of mV (see Figure below).
The cross coupling between sensors would now also be smaller (200mV/V for feedback through AC coils would now become almost 20mV; 20mV/V for feedback through DC coils would now be about 2mV/V).
We will measure cross coupling and try to levitate tomorrow for all six degrees of freedom