3. Final BIRD Prototype
Our first Arduino code involved the same setup code as our final, but to find the best PWM for our motor we had a for loop cycle through different values of “j” to determine what the sweet spots were for our motor. After some testing we found that our motor has an arming value of j=1000, as seen in the first for loop, and a low speed at j=1200. From there we could thus crank up the value all the way to a maximum speed of 1700. This final code is fairly conservative due to the fact that our battery was not able to supply the necessary mA-h to the motor. (1000mA-h) Our final code works by running the final for loop but delaying it for a very long time. This allowed our motor to remain at a constant speed.
The motor used in this prototype is a 250 RPM 2200Kv brushless DC motor. The key reason for us choosing this motor was the fact that it weighs in at only 14 g. The controller that we used was the E-Flight 10 Amp Pro Brushless ESC. The ESC has matching inputs for the motor and 3 inputs for the microcontroller. (One voltage input wire, one ground wire, and one signal wire. The battery that we went with was the 7.4V 800mA-h Lithium Polymer Battery by Parkzone. Again most of our descision for picking this battery came for weight reasons, but we later found out that the 200mA-h difference between the battery and the needed voltage for he esc caused the motor to lose a substantial amount of torque once the ESC could not draw in enough amperage from the battery. We used the Arduino microcontroller to control our esc. We had the ground wire attached to the Arduino ground and the signal wire attached to the pin 9 PWM that our code calls upon.
We used a gear train of 4 gears in our drive train attached to the motor. The overall ratio between the output and input torque is around 25. This ratio allowed us to achieve the desired torque that we found for our second prototype from this fairly weak motor. Our gears are arranged in a simple compound gear train. The input gear, which is very small, drives a bigger gear mounted rigidly to another shaft. This shaft also has a smaller gear rigidly attached to it which thus drives our final gear, which is fairly large. This gear then is rigidly attached to a drive shaft which is the input to our four bar mechanism that is involved with flapping the wings.
The four bar mechanism that we used in our final prototype is slightly different than the one that we used in our second prototype. It is still a four bar crank rocker but we changed the orientation of the input torque by 90 degrees. We wanted to condense most of the weight inward towards the center of the body to alleviate vibrations from side to side and make the body more streamline.
We also added some stabilizer rods from the back of the body so a midpoint in the skeleton of the wing. This was also done to form the wing in the second dimension. This is key to our new design as our flight is achieved from the aero elastic effect of the wind on the wing. This rigid triangle allows for a thrust force to be more easily developed against the wing fabric that we mount on top of the wing skeleton. In other words, with the wing fabric attached to this triangular skeleton we achieve an airfoil naturally as the mechanism flaps. This is more along the lines of what smaller birds do when they fly.
Future Plans
Moving forward with this design we would wish to make a lot of changes from what we were capable of doing in the short time that we had. First of all we would want to machine better parts so that we can align our gear train and four bar a lot better. Secondly, we would like to exclude wood from our design. With the changes in humidity and the constant stresses seen on the wooden components, we saw a lot of warping, which further through off the alignment of our gear train.
In the distant future we would like to move back to the complete control of the wing and not rely on the aero elastic effect to drive the thrust of our bird. We would like to create another four bar mechanism in the perpendicular plane to achieve an elliptical motion from the tip of the wing mechanically. Also, we would need wireless controllers to finally get this mechanism in the air. Lastly, once the mechanism is in the air, we would need to attach 2 more four bars to the tail end of the prototype and attach them to servos to that will function as rudders for flight control. We would probably also see a switch to a slightly bigger motor that can generate more torque. This was the biggest sore spot for us in hindsight. Lastly, we would probably revert to our original idea which involved a six bar mechanism to create cupping motion with the wing. This cupping would also reduce the drag force on the wing in the upstroke.
We may also want to do more research on the feathers on the wings of birds. We would like to find a way to achieve the compression and expansion that feathers accomplish when birds are flying. From what we know right now it functions almost as a one way valve that allows for minimal resistance on the upstroke and maximum resistance on the down stroke.
Final BIRD prototype
Final BIRD Arduino Code
Final BIRD Gear Train Assembly (Solid Works)
BIRD test
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