To correct for inevitable array misalignment with the motorized stages, a second calibration phase prompts a user to sequentially align the laser irradiation point with three array corners while receiving visual feedback from the camera. The skewed image resulting from the camera’s angled orientation is corrected with a projective transform, where the magnet array corners are aligned to a rectangular image of the same aspect ratio as the array. A two-phase calibration routine 1) obtains a top-down image aligned with the magnet array’s dimensions and 2) translates desired laser irradiation coordinates in pixels into motor commands. 15 In this study, we utilize this effect to aid in PyG milli-robot detection, although significant improvements in actuation force could theoretically be gained by increasing the absorption of radiation incident on the PyG milli-robot.ĭue to the camera’s angled orientation and lack of precise alignment between the magnet array and motorized stages, software calibration is required for each new array or repositioning of an array. A significant portion of incident power at this wavelength is not absorbed but rather reflected from the PyG surface. Although we have previously irradiated PyG samples with laser powers as high as 2W without observable damage to the samples, a 500mW near infrared laser is used in this study for the convenience of its form factor. Lasers at this wavelength with nominal power as low as 300mW can successfully displace PyG milli-robots in this arrangement, 13 but the system reliability and maximum speed are improved with higher laser powers. Based on infrared imaging, estimated maximum temperature increases of ∼20☌ and temperature gradients of ∼10☌ across a 10mm⌀ PyG milli-robot are expected due to irradiation by a laser of this power and wavelength when the laser is completely incident on the sample, which is the case during manipulation operations. The unfocused laser has an estimated ∼2mm spot size at the ∼3cm distance from the tip of the laser to the PyG milli-robot surface. To maximize actuation power, a 976nm, 500mW continuous wave laser diode is located as close as practicable above the magnet array. The observed effect of milli-robot size to magnet array grid spacing ratio on milli-robot in-plane motion is discussed, concluding that larger ratios result in smoother and faster motion control due to relative decreases in magnetic barrier forces on the pyrolytic graphite milli-robot and minimized separation between minimum free energy positions. Sequential control of multiple milli-robots in close proximity without work surface segmentation is also demonstrated successfully. Results from experiments demonstrating automated position control of pyrolytic graphite milli-robots of various sizes levitating over various permanent magnet array configurations are presented. Hardware requirements and considerations are discussed along with software calibration, image processing, and control methodologies. A user may dictate arbitrary desired milli-robot positions to a closed loop control system, which automatically detects and actuates milli-robots to the desired positions and works to maintain them there. A simple control method is presented that harnesses interactions between complex magnetic fields and small-scale thermomagnetic and optical material properties. In this paper, we present methods for addressable, automated position control of levitating pyrolytic graphite samples acting as milli-robots for small-scale assembly and manipulation applications using optical actuation and machine vision techniques. When levitating above an alternating-pole permanent magnet array, pyrolytic graphite can be displaced by asymmetric diamagnetic forces resulting from optically-induced, localized temperature changes and the thermal dependence of pyrolytic graphite’s magnetic susceptibility.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |