Carbon Fiber Reinforced Polymer (CFRP) Drone Frame Project Update 2

Project Update 2

 

      Carbon Fiber Reinforced Polymer (CFRP) Drone Frame

 

ME514

Hongrui Chen

Zeyuan Ma

Shujing Wei

Department of Mechanical Engineering

University of Wisconsin-Madison

April 19th, 2019

 

 

 

Finalized Part Development

  1. Composite Layup

1) Cutting Carbon Fiber

Figure 1. Indication of each position for carbon fiber binding

 

IDENTIFICATION L (mm) W (mm)
A 112.75 21
B 94.75 16.76
C 127.62 16.76
D 145.62 21
E 85 11.5

Table 1. Dimension for each surface

 

After we received our CLIP-printed arms, the next step for us was to attach carbon fiber onto the surface of arms. We cut small pieces of carbon fiber from a whole pre-preg carbon fiber, which was originally stored in refrigerator. The dimensions we used for cutting and corresponding position for sticking carbon fiber were shown in table 1 and figure 1 respectively.  For part A,B,C,D, we need two of each pieces since there were two arms. To prevent failure of the pieces, we actually prepared 3 pieces for each part. We did not cut the part of E as a single piece. Instead, we cut three relatively large pieces to attach A, E and D together because it could potentially add mechanical strength to the arms by adding more carbon fiber. By doing so one layer on each of the A and D side will be added, which can compensate for the gap between part E and part A and D. Since the carbon fiber is sticky under room temperature, and it has good attachment to the surface of the CLIP-printed surface, we sticked the carbon fiber pieces to the corresponding position directly. The real part is shown in figure 2.

Figure 2. The real CLIP-printed part

2) Waterjet mold

It was desired to have a smooth and shiny carbon fiber finish laying onto the arm. This could be done with some metal covering the carbon fiber during curing.  We used a 0.5mm galvanized steel metal sheet as the mold. In order to manufacture the mold, we utilized the water jet in TEAM lab to cut to the desired shape. The shop staff helped to configure and operate the water jet. Finally, we bend the sheet metal into the shape of the arm. The metal cover after waterjet cutting is shown in figure 3.

Figure 3. Metal cover using waterjet cutting

 

3) Curing the CLIP part

After we placed the carbon fiber onto the arm, we could cure the arm together with the carbon fiber. The curing process would bound the carbon fiber with the arm. After curing, the carbon fiber would permanently bound with clip printed part. This resulted in a very strong part with the strength of carbon fiber and the complex geometry of CLIP printing. However, the curing setup was relative complex. The processes are listed below and the corresponding picture is shown in figure 4.

1) Spray mold release onto the base plate and the sheet metal mold

2) Stick vacuum seal around the build plate

3) Attach the mold onto the part

4) Cover the parts will peel ply, peel ply makes it easy to peel off after curing is completed

5) Cover with a sheet of plastics to prevent resin from leaking

6) Cover with a piece of fabric, the fabric enables air to be suck out freely later

7) Apply the final layer, the vacuum bag onto the part.

8) Stick the vacuum bag with the vacuum seal.

9) Apply vacuum, meanwhile make sure that no air is leaking into the bag

10) Program the oven to cure the part

Figure 4. The curing process

 

  1. Electric Wiring

The drone consists of many electronics. The receiver receives the signal sent by the radio and sends the signal to the flight controller. The flight controller controls and stabilizes the drone. We used a DJI Naza M Lite controller. The controller has an 3-axis accelerometer, a 3-axis gyroscope for stabilization, and a barometer sensor for hovering. The controller calculates the appropriate motor spin speed and sends a PWM signal to the ESC. The ESC is used for converting DC battery voltage input into three phase AC used for powering brushless motor.

The drone also features two RGB LED strips at the front. The LED can be used for illumination and let the drone operator see the heading of the drone. The LED strip, which controlled via a 8 switch junction, is  shown in figure 5.

Figure 5. Switch for RGB

 

Performance Validation

  1. Successful assembly

When designing the drone, it is important to take into consideration that 3D printed part requires larger tolerance. During modeling in Solidworks, an extra 0.5 mm gap was added to mating features. As a result, the 3D printed part mated easily.

  1. Static Test Success

After wiring up all the electronics, we performed a static test with propeller removed. All the motor rotated at the right direction, and the drone responded to the input from the controller. After verifying the reliability of the electronics, we will perform the maiden flight later.

  1. Better Stiffness of CLIP arm

After redesigning the arm structure for CLIP printing, the latest CLIP part showed better stiffness than that of the last print trial. In addition, the surface finish was improved and the strength of each lattice was stronger. Due to its increased stiffness, the new CLIP part could be then post-processed, especially the step applying vacuum discussing above. In addition, the new CLIP part could be prepared for further test on binding of CLIP and carbon fiber.

In conclusion, our drone performed our intended functions successfully.

 

Final Obstacle

  1. Design clip version of the whole drone

While we have printed most of the parts of the drone using FFF successfully, we were only able to try and print two of arms using CLIP technology. We cannot do CLIP printing for the whole part partially due to the design, since most of the parts were not designed using lattice structure. Unfortunately, lattice structure is required for CLIP if we want to have the optimal performance from this technology.

  1. Difficulty in Manufacturing CLIP and Composite part

The post-processing of the CLIP-printed part is arduous because it requires human labor. From cutting and sticking the carbon fiber onto the arms to curing the final CLIP, it took us about 10 hours for preparing only two arms. The human labor would make 3D printing manufacturing inefficient. We would expect that in the future, the post processing could be modified and become “smarter”. For example, we could design a way that can automate the cutting processing or the curing to make the whole process of 3D printing more efficient.

 

Appendix:

  1. Print Time of each Print

 

Part Name Volume (cm^3) Weight (g)/From Cura time(min) Material
Main Cabin 107.88 82 357 PLA
Each Arm (FFF version) (x4) 54.75 29 144 T-PLA
Each Arm (CLIP version) (x4) 29 Currently Unknown Currently Unknown CLIP
Front Cover 34.51 34 276 PLA
Rear Cover 16.48 16 108 PLA
Battery Strut (x2) 8.10 8 47 PLA
Battery Hook 2.36 3 28 PLA
Center Strut 6.25 8 43 PLA
End Strut 5.35 5 24 PLA
Front LED Cover 11.31 14 69 PLA
Rear LED Cover 5.36 7 33 PLA
Front Landing Gear (x2) 70.5 40 170 PLA
Rear Landing Gear 39.45 25 114 PLA
XT60 Bracket 1.4 1 17 PLA