Mechanical Design

Design Considerations:

We went into the design process with the goal of creating a simplistic, low-cost, robust, and manufacturable quadrotor platform. The general design consists of three major components, the motor nodes, the support rods, and the central node. It was essential that the design be reconfigurable, thus a multiple node schematic was followed which allowed for an extent of modularity that enables us to potentially adapt the quadrotor to multiple roles. With this in mind, no part of the quadrotor is glued together (.....yet, as we have not found any significant structural issues, though it is possible that the high frequency vibrations induced by the motor/rotor might reveal a need for gluing). Additionally, we have attempted to utilize electrical connectors over soldering, at a slight weight cost.

Below are photos of our SolidWorks model and of the design process. In the SolidWorks model the rotors are represented as clear cylinders.

SolidWorks-Looking down and Iso views:

As referenced above; A, B and C represent a motor node, the support rods, and the central node respectively. One might note that the central node consists of two layers; this facilitates component placement. Following images will show the configuration of components.




Materials:

With regards to materials, the motor nodes and central node are cut out of 1/8 [in] Acrylonitrile Butadiene Styrene (ABS), while the support rods are 3/8 [in] hollow aluminium. Additionally, a number of 3[mm] steel screws are used for fixing the motor nodes and central nodes to the rods, and for fixing the motors and boards to the nodes. We felt that the aforementioned materials allowed for rapid assembly while maintaining a reasonable overall weight. This was confirmed by our overall frame mass which was about 130 grams. Improvements in the use of materials will be noted in the 'comments' section.

Assembly:

Assembly proved quite rapid and easy. A few iterations for finding the optimal hole sizing for the node->rod interface were necessary, but meticulous modeling ensured that the majority of our parts fit well. It was apparent that the rods would have to overlap in some way, which we solved by crimping the rods at intersection points. Holding the nodes in place was accomplished through the use of screws that pressed down on the rods, providing a surprisingly robust fix. If one watches carefully during the thrust testing video they will see a nut fall off of a screw, while this was not an issue during the test since each screw had an additional redundant nut, we decided to eliminate such uncertainty in our quadrotor by tapping the ABS. The ABS to screw friction combined with the dampening that the ABS provides should keep the screws from rotating during operation.

A motor attached to a motor node:

Close up of the Center Node:

Central Node Configuration:

The slots in the central node house the ESCs for our brushless motors (A2208-14). Wires from the ESCs are routed through the openings at the top of the node through those between the rods out to the motors. The battery is placed on the bottom of the central node, and the 'ping' sensor (our current altimeter, at least until we get our barometer output working properly) will be placed under the node as well. Our Control, IMU, and wireless boards are configured to be placed between the ESCs on the top of the central node.

The Frame with ESCs, Battery, and Motors incorporated:

Central Node with ESCs, Battery, Control Board:

Comments:

We believe that the current frame will prove sufficient for our purposes. It is light, sturdy, easy to construct, and capable of housing the components we require, yet we feel that when we have more time improvements are possible. Wiring up the quadrotor has made us realize that more consideration of wire routing might prove beneficial during the modeling process. Moreover, our nodes seem more robust than necessary and we could probably stay within our design goal of robustness while reducing mass of material used. In the future, use of carbon fiber tubing and sheets, and perhaps even thermoplastics might allow for an equally functional structure that is lighter and more aesthetically pleasing.

Overall though, considering that we designed and assembled the quadrotor over a weekend and accomplished the majority of the wiring in a day (some wiring was on hold while we waited for the appropriate connectors), we are quite happy with our results. And we must admit, this is one cool looking quadrotor.

Initial Thrust Testing

This past weekend we built a test platform in order to do some basic thrust tests with our motors and the two different props we have. This gave us a general understanding of how much current we need per motor in order to keep our aircraft hovering. We were able to more accurately determine this because we have our first body finished! There will be more on the body in following posts. The thrust test platform consisted of a lever made from ABS and a metric scale, so that prop thrust generated a proportional weight on the scale. By scaling this thrust by the ratio of the lever lengths we were able to get an approximate measurement of lift (in grams) versus current for our two props.

Thrust test bed:

Video of tri-blade ramping from idle to 8 Amps (12000 RPM):

Our findings indicate that the two prop configurations are similar, but at lower currents (as in hovering) the 2-blade prop is more efficient than the tri-blade, and at higher currents (above 5 amps) the opposite is true.

Thrust vs. Current plot:

The difference in efficiency at high and low current output is particularly evident when plotting RPM versus current as in the figure below. Note that at higher currents the dual-blade prop has a lower RPM gain per Amp than the tri-blade, and the tri-blade has a higher RPM gain per Amp than the dual-blade at lower amps. The general difference in RPM between the tri-blade and dual-blade is due mostly to the diameter of the props.

RPM vs. Current

To determine the efficiency of our system, we measure the entire weight of our body (adding on some weight to for wires to be conservative) with a particular battery, and determine the hover time based on battery capacity and the current required to hover.

For example, using Blue Lipo lithium polymer batteries with 2200 mAh (at about 185g) we have a total body weight of 615g. In order to hover we need each motor to generate 615/4 or about 154g of "thrust". With the tri-blade this corresponds to about 2.2 Amps per motor (8.8 Amps total). With the dual-blade this corresponds to about 1.8 Amps per motor (7.2 Amps total). With our 2200 mAh battery, this gives a flight time of:

Tri-blade -- 2.2 Amps*Hour * (60 min / Hour) * (1 / 8.8 Amps) = 15 minutes
Dual-blade -- 2.2 Amps*Hour * (60 min / Hour) * (1 / 7.2 Amps) = 18.3 minutes

This indicates that at hovering, the dual blades are (18.3-15) / 15 * 100% = 22% increase with respect to the tri-blade efficiency. This, however, was not enough to convince us to use the two blade props over the three blade props...the tri-blades are much quieter and, let's face it, look way too badass.

With a 1500 mAh Mystery Lipo we found the following flight times using the same metric:

Tri-blade = 12.5 minutes
Dual-blade = 15 minutes

Here's the mess we made in the undergraduate electrical engineering lab:

Initial Motor Testing

So some exciting news: the motors and ESCs from Singapore have finally arrived! We have hooked them up and tested their speed/current characteristic using a 3A power supply set to 11.1V, a 50Hz PWM (Period of 20000 at 1MHz) signal from the MaEvArM, and the tri-blade rotors. Our initial test results and some pictures/video are below!

Dual-blade props with Mystery motor:

Tri-blade prop with Mystery motor:

Tachometer setup to read RPM of tri-blade props:

Temporary test bench with two tri-blade props:

Current and RPM vs. % duty cycle for tri-blade prop:

Tri-blade motor spin test: