With an understanding of the fields applied to rectangular cross sections as well as new material samples, we created a second testing platform and started testing simultaneously in San Diego and Pittsfield. Rather than using open-loop or interesting and fun analog feedback circuit to maintain continuous motion, we put together an interface with a dsPIC33F that could provide conditioning for an optical emitter-detector pair used to measure the position of the suspended mass as well as enable closed loop control as a function of position to allow current pulses to be conducted through the beams at a controlled rate. The dsPIC was also connected to the LCD screen shown to provide some immediate feedback to the operator.
I initially started testing non-oriented and grain-oriented 0.25mm thick electrical steel. Much of the work was focused on exploring the setup, duty cycle, and different control loops, and was intended to be exploratory work for another paper. In truth it was very easy to become lost in the details and miss the forest for the trees. While significant numbers of experiments were conducted and we observed differences in the peak stroke obtained as a function of: (i) material, (ii) material composition, (iii) driving current, (iv) pulse width etc., it was difficult to really control each variable independently. It would really be better to return to this work with a better measurement of position and use laboratory control electronics to take control of the experimental setup. That said, the interface we came up with does function as a great showpiece for how simple an apparatus might be to implement the concept in practice (and still sits on my desk ready to be used) – a few dollar emitter / optical detector, low cost microcontroller, and some basic electronics.
Sample data is shown below, in which the shaft length of 63.5mm had 3A current pulses applied at an ~duty cycle of 20%, in which the pulse was ideally applied after the stroke passed through its lowest point and was beginning to rise. After approximately 33 seconds, the feedback was turned off such that the slow decay in oscillation magnitude indicated just how little damping was present in the system. The frequency of the oscillation can be see in the 3rd figure as around 12Hz, which was the expected resonance frequency. The scatter on the FFT was associated with the noise of the position measurement and as it would taken over the full duration of the data being recorded, such that there was likely some change due to the factors such as temperature variation resulting in changes in the shaft length through thermal expansion.
What we learned and demonstrated as part of this is both how simple of an interface can be used to generate the effect, but also how careful one must be when taking data to really characterize one variable at a time. Following this, still in exploratory mode, the effect was turned to a setup that fixed a cantilever at two ends, such that the resonance mode was significantly greater.