The Black GhostsElectrical Design
Circuit Description
Because the flux through the coils will be different at each position of the coils, the rayís position can be sensed by integrating the current through the coils. This is done using an integrating circuit.
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Figure 2: An integrating circuit.
However we havenít been able to get a precise measure of the coilsí position because of the drift from the op-amp. In fact we tried using several circuits to correct for the drift but none of them allowed us to get totally rid of it. The major problem was that the drift depends on factors such as temperatures. Even when we tuned our circuit to minimize the drift of the op-amp it would come back after running the fins for a couple minutes. We ended up using the following circuit to reduce the drift from the op-amp. (This circuit is not shown in the final circuit diagram).
Figure 3: Trim circuit designed to minimize drift.
In order to drive the ray to the desired position we compare that position with the actual position and use the error between the two as the signal to drive the coils. This is done using a difference amplifier To get enough power we amplify this signal with a push pull amplifier.
In order to run the four fins in a wave motion, we give each of them a phase shifted reference signal. The first fin is run straight off the function generator. We give it a 3.4Hz sine wave with an amplitude of 3.3 V. The second fin runs from a sine wave that is 90† out of phase with respect to fin 1.We use a low pass filter to generate the correct phase shift. The third fin runs from a sine wave that is 180† out of phase with respect to fin 1. The input signal is generated by inverting the signal from the function generator. The fourth fin runs from a sine wave that is 270† out of phase with respect to fin 1. The input signal is generated by inverting the signal from the low pass filter. The three phase shifted signals are then amplified to be scaled back to an amplitude of 3.3 V.
Circuit Diagram
Below is the full circuit diagram for both sensing and controlling one fin ray. The complete circuitry to control all four fins is available for download as an Adobe Acrobat file: Full Circuit Diagram
Figure 4: Closed-loop analog feedback circuit for controlling Fin 1.
Handyboard control
Circuitry
To control the fins with the Handyboard we use the same integrator circuit to sense the position of the coils but we use the Handyboard to generate the error signal. The Handyboard analog pins can read voltages ranging from 0 to 5v so the signal from the integrator needs to be amplified and shifted before being fed to the Handyboard. This is done with the following circuit.
Figure 5: Integrator and signal conditioning for the handyboard.
Because each fin has a slightly different number of coils, the resistance values have to be tuned for each one.