Using the new PCB I designed to generate the servo pulse widths, I was able to design and iterate a few wing mechanisms. I studied a variety of mechanisms and ultimately settled on one which was robust, easy to 3D print, and simple to assemble. This mechanism is going to be the starting point for the smaller mechanism I will build in the future. With this step achieved, I’ll return to electronics and dive into the world of surface mount design. You can see this mechanism in motion on my YouTube channel https://youtube.com/shorts/8yZDN7Y712c?feature=share
Render of the high wing stroke amplitude mechanism I 3D printed as part of my insectothopter project.
There are a variety of existing ornithopter designs available to study. Many of these employ a geared down motor and vary the speed of flapping to control the aircraft. There are also designs which use servos and directly drive the wings from the servo horns. Both of these can work and do fly. I tried the direct servo method in my previous butterfly robots (which do not fly). Most servos limit the angle of rotation to 90 degrees, so this was not acceptable to my design parameters.
In order to achieve a natural look I need the wing stroke amplitude to be nearly 180° and the hind pair of wings to be slightly out of phase with the front pair. The sinusoidal motion and phasing was already achieved in my previous project. I built an entirely analog sine wave generator, phase shift, amplitude and center point adjustment controls for driving servos. To achieve the high wing stroke amplitude, the mechanism I used sacrifices half the servo torque in order to double the maximum angle of rotation.
So what are my design parameters? I’ve been watching mechanical engineers on YouTube and they often list the ‘must haves’ and ‘nice to haves’ for their designs. I figured I should do something similar to help keep this project focused and moving forward.
Must Haves
High wing stroke amplitude (nearly 180°)
Analog pattern generator electronics
Compliant materials/shapes
Physically separated motor and axis of rotation (to enclose in a thorax)
Nice to Haves
Analog control electronics
or
Wireless control electronics
Biomaterials or reclaimed materials
Entirely embedded electronics and power supply
Biomimetic design
Viewport render of my preferred wing mechanism showing the boolean cuts for M3 nuts, bolts, and securing the servo.
I’ve learned the hard way that creating mechanisms from 3D prints is most successful when you design parts to be assembled later. There are limitations to the quality and strength of 3D printed parts, so to optimize for 3D printing, it is best to keep parts a simple geometry, optimize for strength following the layer lines of the print, and incorporate holes for embedded nuts. This was my 4th design in this phase of the project. In the past I have explored other designs that do not use motors or servos, but those never achieved good results. This mechansim is reliable, strong, and withstands off axis forces well. An advantage to 3D printing is that using herringbone gears is trivial, and they offer a lot of benefits like being stronger, quieter, and they look cool.
Adding compliance to the wing hinge. Instead of all veins flexing from a single point, they can flex in groups around the center.
I updated the design of my wing veins as well. In previous designs, I simply traced and extruded scans of butterfly wing veins so I could 3D print the veins and sandwich them onto paper. This works okay, but insect wing veins are a complex arrangement of compliant mechanisms. Most of the action happens near the thorax at the base of the wing, where all the different sclerites interact with the thoracic muscles. I took a stab at compliance myself by allowing each wing vein to terminate at a point away from the source of power. Freshly printed, they felt compliant in the right way, but after gluing onto paper to form a complete wing, they get much stiffer. I recorded slow motion video of the flapping motion and I see very little distortion within the wing. This isn’t very biomimetic, but it is also kind of good news because I can always take material away to make the wings lighter and more flexible.
This is not my final design, but it is enough for me to pause here and start another track of work. I can check off many of my essential design requirements, but I really want to miniaturize the electronics and embed them. Having now proved the circuit and mechanical designs will work, it is time to convert my circuit design to surface mount components, re-design the PCB, and develop an onboard power supply. I have almost no experience with SMD parts, so I plan to build a learning kit first. This should let me know what size of parts I can actually solder by hand, and if I am missing any critical tools.
There is one piece of the circuit design that I need to fix and I am having a lot of trouble figuring out how to do it. In order to adjust the amplitude of the sine wave, I employed an overly part heavy method of using a dot/bar display chip to control a stack of pnp transistors that would add the sine signal to itself from 0 to 10 times. This works in terms of giving me an output that goes from zero amplitude to max, but it does it in steps instead off a smooth transition. I wish I could use something like an O.T.A. to create a V.C.A. which would bring my overall part number down. I could use easy to find modern chips too. However, this is going to be a battery operated device, and those OTA and multiplier chips depend on dual rail power supplies. I’d rather not do that as I am already in a 5V DC system for everything else. Also, technically a single quadrant multiplier still wont work because ‘off’ or ‘quiet’ is 2.5V and max is a 0-5V. Multipliers would only let me increase from 0V.
I do have a plan to get around this problem. I’ll try using quad comparators and quad analog switch ICs. These are readily available and come in various SMD packages. Part number and stepped amplitude transitions may still be an issue, but the overall board size will still be smaller. If anyone that happens to be reading this has ideas on a much simpler solution, I would like to hear it!
Overall, I am pleased with these results. It feels like I am finally achieving what I really want with this project. It only took me a year of weekends to build it, but it also feels like it took 10 years to know enough about electronics, mechanisms, and 3D printing. If I can keep this momentum up, I may be able to create the smaller embedded version in a few months. Back to work!