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Message ID: 697     Entry time: Tue Aug 6 09:27:43 2013
Author: Giorgos 
Type: DailyProgress 
Category: SUS 
Subject: Negation of Cross Coupling with Feedback 

Negation of Cross Coupling with Feedback

In my previous post, I commented on how it is possible to negate the coupling from the coils' signal. This can adequately happen only if we know the amount of coupling. To measure this quantitatively, Haixing removed the plate from our setup, so that any signal reported by the sensors would be a product of the magnetic field created directly from the coils; this is what we want to subtract. Also, Haixing believes this technique is sufficient enough so that we no longer need to move the coils further from the sensors. That being said, we switched back to the original behavior of the ACs (for feedback) and DCs coils (DC magnetic offset).

In previous measurements, I calculated the correlation between the coils voltage and the sensors voltage for the AC1 coil and sensor (that are next to each other; we ignore rest of cross-coupling between coils-sensors) and found it to be around -11.28 dB for our DC (f about 0Hz) signal. We also measured the transfer function between the AC1 coil and sensor and found it to be around -11.133 at low frequency; the data are in close agreement. Then, we introduced a factor -GCGs in our feedback (Gc was measured to be about 400mV/V and Gs is known from the whitening filter) and measured the transfer function again. The magnitude dropped to -40dB(shown below).

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At low frequencies, we need this value to drop even more, to approximately -70dB since the transfer function of the plate is around 2mV/V or -54dB. Further, we only cancelled the coils' coupling in the low frequency range and we should modify our feedback so that we improve the system's behavior over all frequency range.

Cross Coupling between Coils-Sensors

Here I summarize my findings for the calculated cross-coupling: for AC1 (coil-sensor), I found -11.28dB, for AC2 -10.808dB,and for AC3 -11.258dB.

Simulink Model

For some unknown reason, the Simulink Model for the feedback needs at least 2 filter modules and one subsystem in order to work; otherwise it fails to operate. In order to work, we also need to include a time delay so that the coil's output is not at once fed into the feedback. I worked and finished a generic Simulink model for all six degrees of freedom, however all the coefficients are unknown. Even so, I will post it along with some description of what each components does.

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