Flapping Soft Fin Deformation Modeling using Planar Laser-Induced Fluorescence Imaging

The ability to measure deformations of soft robotic fins non-intrusively is important because we can better inform fin design and control for underwater vehicles and validate computational models. We can repurpose an existing tool within fluid dynamics, called planer laser induced fluorescence and expand it for solids, which could then be used for simultaneous measurements of solids and fluids. This method is generalizable to soft robotic systems and materials.

We can use this technique, not only to validate fluid structure interactions, but also to study flexible materials for sensors and medical applications. Begin by designing and building a custom 3D printed gloss finished mold of fin shape. Prepare an experimental setup by mounting a pulsed laser system on a rectangular glass water tank to generate a planer light sheet intersecting the tank at its midplane at 30 Hertz.

Install a four megapixel charge coupled device, or CCD camera, with a 35 millimeter lens and a long pass fluorescence filter of 516 nanometers. After installation, perform a calibration of the micrometer to pixel conversion by taking a single image from the CCD camera with a ruler placed in the laser sheet plane. Then select two positions on the camera and divide the distance in micrometers by separating pixels.

Ensure the micrometer to pixel ratio is small enough or in the sub millimeter range for the application. Synchronize the laser pulses and camera images with the flapping fin using trigger outputs from the fin software and signals from a delay generator and associated software. Ensure that all laser safety is per the institutional guidelines.

To set the laser system, turn the laser system on with the power key that runs the chiller to cool the laser heads. The fault lights blink until the system is ready to power the lasers. Set the trigger source to external lamp, external Q switch.

For both laser heads, set the laser energy to 60 to 80%of the full power and press each Q switch button. Then turn the lasers on by pressing the power button. Next, plug in the power cables to the camera and ensure proper connections to the computer before opening the camera setting software and selecting the proper port.

After turning the delay generator on, connect the external gate channel to the fin trigger, channel E to the camera, and channels A to D to the laser. Then, open the delay generator software to select the pulse mode to burst and system resolution to four nanoseconds. Set the period in seconds.

Adjust the external trigger/gate mode to triggered, threshold to 0.20 volts and trigger edge as rising. Also, set channels as described in the script. Align the fin, so the laser sheet passes through one cord wise section of the fin at a span wise position, and then secure the fin platform with the mounting hardware.

Connect the power to the fin control hardware and fin motors to begin fin flapping with the selected kinematics, and turn off all the ambient lights. In the delay generator software, press run to begin the synchronized experiments and acquire images of the intersection of the laser sheet with the fin throughout the stroke cycle. Observe the fin flapping in the tank with laser sheet on and ambient lights off.

When done, press stop before disconnecting the fin from the power source. Move the fin platform, so the laser sheet crosses at a new span wise position. Replace the fin with additional desired fin membranes and perform experiments as demonstrated before to acquire the images for the desired number of measurements.

Analyze the image by extracting all the white objects representing fin cross sections from the BW area, filt dot M binary image and displaying the image with the M show dot M.Then create a trace of the binary image boundary for each image to obtain a 2D shape, by selecting all the fin pixels that touch the black background pixels. Compare the resulting fin shapes with the 3D fluid structure interaction, or FSI models, generated from the center lines, to showcase how the process can be used as high fidelity validation. The programmed fin kinematics yielded a stroke amplitude of 43 degrees and a pitch amplitude of 17 degrees.

The image illustrates the comparisons at two positions in the stroke. One in the middle of the upstroke and one in the middle of the downstroke. Furthermore, the comparisons were made between the shape deformations of the PDMS 10 to one, and PDMS 20 to one fin.

The 3D fin shapes were reconstructed from the planer laser induced fluorescence, FSI, and rigid cases in the mid upstroke, demonstrating the capability of the present technique for providing high fidelity validation for the FSI simulations. In the experiment, the fin cross section was not visible at every step due to the opaque rigid spar. The image shows the result where the fin was not visible.

The most important thing to remember is to test the synchronization of the components before running full experiments. Once the timing is set, perform a test run first.