Yesterday, a sequence of force and gain measurement was made to determine the imbalance in the
quadrant, magnetic-levitation prototype. This was the reason why it failed to achieve a stable levitation.
The configuration is shown schematically by the figure below:
Specifically, the following measurements have been made:
(1) DC force measurement among four pairs of magnets at fixed distance with current of the coils on and off
From this measurement, the DC force between pair of magnets is determined and is around 1.6 N at with a
separation of 1 cm. This measurement also lets us know the gain from voltage to force near the working point.
The force between pair "2" is about 13% stronger than other pairs which are nearly identical. The force by the
coil is around 0.017 N per Volt (levitation of 5 g per 3 Volt); therefore, we need around 12 volt DC compensation
of pair "2" in order to counterbalance such an imbalance. Given the resistence of the coil equal to 26 Om, this
requires almost 500 mA DC compensation. Koji suggested that we need a high-current buffer, instead of what
has been used now.
(2) DC force measurement among four pairs of magnets (with current of the coils off) as a function of distance
From this measurement, we can determine the stiffness of the system. In this case, the stiffness or the
effective spring constant is negative, and we need to compensate it by using a feedback control. This is
one of the most important parameters for designing the feedback control. The data is still in processing.
(3) Gain measurement of the OSEM from the displacement to voltage.
This measurement is a little bit tricky due to the difficulty to determine the displacement of the flag.
After several measurements, it gave approximately 2 V/cm.
Plan for the next few days:
From the those measurements, all the parameters for the plant and sensor that need to determine the
feedback control are known. They should be plugged into the simulink model and to see whether the
old design is appropriate or not. Concerning the experimental part, we will first try to levitate the configuration
with 2 pairs of magnets, instead of 4 pairs, as the first step, which is easier to control but still interesting.
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