In my previous post, I explained the saturation issues of the ADC and DAC we faced. To prevent saturation of the DAC, we will implement our gain after the feedback filter -we already introduced a gain of 25 at the coil's conditioning stage-and use a gain of approximately 1/25 for the digital filter. In this way, even if the ADC saturates at 10V, the feedback filter will send maximum 400mV (10V/25) to the DAC. However, the possibility that the ADC saturates still exists.
To make matters worse, we cannot even exploit the 10V saturation range of our ADC. The reason is that the undesired cross-coupling between coils and sensors is almost as large as the signal produced by the plate. Although vector fitting was very successful at cancelling cross-coupling, this method can only be implemented inside the digital control, after the ADC. That been said, if we are to stay within the 10V range, only 5V can come from the plate; the rest 5V will inevitably come from cross-coupling. Practically, this means that the signal from the plate is successfully sensed and transferred to the digital control for very small displacements, beyond which the ADC saturates and feedback control is impossible. We use two ways to tackle this obstacle:
We measured cross-coupling effects before and after the use of mu metal and noticed a drastic reduction in cross-coupling (about a factor of 3). The figures below show the measured transfer function before and after the use of mu-metal.
One of the things to also note is that the bottom strain gauge motors readings are damaged and no longer worth looking at to determine when the plate is near equilibrium. This is probably the result of excessive weight on them from the plate. The top strain gauge motors seem to still be working fine.