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In this instructable I am going to show build process of a racing drone which is going to be placed in an international drone race. I was asked to build a quadrotor for this specific drone racing event. Drone has to be fast as possible and carry payloads and able to drop different targets on the race track. Whole assembly took 80 hours of work. total cost is about 2000usd.

Step 1: The Competition

This race consists of two missions which are mentioned below. For the mission two payloads should have dimensions 5x5x5cm and minimum weight of 100 grams. Maximum takeoff weight is 4kg.

Mission 1

The quadcopter has to fly over a course for 3 laps. Fastest drone wins.

Mission 2

The quadcopter has to drop 4 cubes at a certain altitude to specified targets on the track. Targets consist of circles that expands 1m radius from the center. Drop in the most inner circle(1m radius) get the full point.


Requirements

There are some safety requirements determined by racing committee. These are failsafe, geofence, kill-switch,fpv equipment. In addition to these requirements, the drone has to pass the technical inpection before the race.

Mission Problem Statements


Mission 1 Problem Statement

In mission 1 uav has to fly through four gates in an order and complete the course in 3 laps. According to scoring rubric, the uav must complete the course in a minimum amount of time. Passing through gates by doing zig-zags requires high speed maneuverability of the uav. For this type of maneuvers, agility, controllability and speed are the key design elements. At the same time pilot has to hold the uavs’ altitude over 5 meters. For this course, estimated flight distance will be approximately 1100 meters. In addition to the extreme flight characteristics, flight time has to be long enough to cover this distance. In this mission our flight type will be video piloting (FPV) in order to have full control of the uav.

Mission 2 Problem Statement

In mission 2 uav has to be loaded with 4 cubes before the mission starts. A release mechanism will drop the cubes to the specified circular targets one by one at a 5 meter altitude. These circular targets will consist of 2 inner circles have increasing 1 meter radius away from the center. There will be 4 targets placed on different coordinates. In order to get get the highest point from this stage, payloads should be dropped with a maximum accuracy to the innermost circles of the targets and drops should be done in minimum amount of time. Main concern of this mission is the release mechanism. The release mechanism has to be designed to drop each cube in the same way. Purpose of this design aspect is to obtain drop precision in order to prevent rotational motion of the cube after it released from the uav that can cause bounce from the target.

Additional weight of the release mechanism and the payload will reduce the flight performance of the uav dramatically. Motors have to be powerful and whole system should be built from lightweight materials in order not to compromise flight characteristics.

Another concern is the shift of the center of gravity. After release of the cubes one by one, center of gravity changes and affects flight capabilities. The release mechanism has to be design to compensate this shift.

As it mentioned before increased weight will reduce the flight time and efficiency. As a result our last concern should be to set enough flight time to accomplish both missions.

NOTE: For the privacy terms, release mechanism plans,drawings will not be given according to the original design.

Step 2: Plannig the Design

In general

For the mission 1 quadcopter will fly without the release mechanism.

For the mission 2, a deatachable release mechanism will be designed and a secondary camera will be mounted under the frame and looking towads the target. From the osd module pinpoint option will be chosen and zeroed according to the 5m altitude.

For the electronics section, we have established a system by selecting a 4S battery which provides 14.8Volts. However devices on the uav such as flight controller, FPV transmitter, OSD, GPS, radio transmitter, cameras and servos operate at different voltages mostly 5V or 12V. In this case we need a voltage regulator for these components. Additionally input voltage of FPV transmitter has to be filtered for the highest image quality. Most of the components mentioned above are going to be connected to the flight controller. Switching between cameras and dropping payloads will be done by aux switches on the radio controller. For the release mechanism electronics, servos are going to be connected parallel and receive same PWM signal and programmed according to 90 degree angle in order to open and close the lid mechanism.

In the conceptual design frame type was selected in six armed configuration for lifting capabilities. However choosing six arm frame brings some disadvantages. Since it requires more motors and esc’s build cost increases dramatically. In addition to the cost, having more arms and components will increase the weight and affect flight characteristics in a bad way. Another concern was the size of a six arm frame. In order to provide required thrust, choosing the sufficient propeller size is important. Angles between six frame arms lesser than a four arm frame. Having less angle between arms, prevents usage of larger propellers those have more lifting capabilities. This simply means even choosing lowest size propeller which cannot provide enough thrust requires to be mounted on a larger six arm frame. Larger frame might fail in mission 1 by means lack of agility and speed. This is the most important trade off by determining the frame type.

In order to obtain the best solution, our starting point will be the maximum takeoff weight which is 4 kilograms. After some research we found a motor that gives thrust equal to 4kg takeoff weight at a four arm configuration. Since it will be four arm frame, the weight of the uav will be lesser than the six arm frame configuration. Less weight means better flight characteristics in general.

Choosing the frame with four arms will not be enough for this contest. Frame has to be also capable of flying fast and agile in mission 1 and lift 4kg of takeoff weight in mission 2. In order to accomplish these missions, frame has to be versatile. For the mission 1, arms have to be parallel to ground and the heaviest weight in this case battery has to be placed on top the frame. This placement increases maneuverability of the quadcopter by carrying center of gravity to a higher or equal level with respect to the lifting force (motor levels) of the quadcopter. Since the release mechanism will be designed detachable it will not be carried in mission 1. In the mission 2, arms have to be higher than the quadcopters’ base plate in order to obtain dihedral effect which increases stability during releasing payloads. In this case by attaching release mechanism to the lower frame draws center of gravity to the vehicles mid position

Since limited amount of time and lack of resources and opportunities we could not be able to design and build a custom frame. Selected motors provide 4kg takeoff weight with 12 inch propellers. As a result smallest quadcopter frame was chosen according to this propeller size. In this case S500 quadcopter frame was chosen due to its’ tilted arms, lightweight and durable materials and its’ 500mm size which is distance between opposite motor centers.

By combination of powerful motors and lightweight body and small amount of dihedral effect of arms, this uav can fly very stable at high and low speeds. Additionally, having shortest length of arms will provide better maneuverability. During the build process, the uav is going to be built for the maximum balance in order to decrease drag forces. All components will be placed according to optimum center of gravity.

All missions will be done by FPV equipment. Getting the strong video connection between uav and pilot is essential for a proper flight. In addition to the connection image quality should not be distorted during flights. FPV system will consist of two CCD and CMOS cameras (one looking to the front, one looking down for payload release) one 1.5km range 250mw transmitter and one fpv switch in order to select between two cameras. Also there will be an on screen display system (OSD) in order to assist the pilot by monitoring essential information about the altitude, tilt angle, speed, etc. on the FPV screen. Rest electronics systems will be the same in the conceptual design part.

Release mechanism will be made according to the part at the conceptual design with minor changes. Payloads will be loaded in line. Two servos will be operating for the release action. One camera and a laser pointer will be placed near the lid in order to improve the aim on target easily. Payload shifts will be calculated and necessary electronic aided trims will be done from the flight controller software. Average trim adjustments are going to compensate the payload shift in y-direction.

Step 3: Parts List

  1. s500 quadcopter frame
  2. T-motor U-3 700kv motors
  3. T-AIR 40A ESCs
  4. Pixhawk flight controller kit (gps, telemetry,power module,buzzer,switch)
  5. PPM encoder
  6. 12V 300A master switch
  7. MinimOSD
  8. HK fpv 3ch switch
  9. Fatshark 200mw Transmitter
  10. Fatshark 700TVL FPV camera
  11. PowerHD analog servo (for the release mechanism)
  12. Battery Holder (3d printed)
  13. Battery Turnigy 5000mah
  14. Battery strap
  15. UBEC 5v
  16. Secondary camera
  17. Power filter for fpv
  18. matek 12v PDB
  19. camera holder
  20. 12x4.5 propeller
  21. Fatshark dominator goggle
  22. velcro
  23. ziptie
  24. m3x10 screws
  25. m3x40 screws
  26. double sided tape

Step 4: Battery Holder Design

This battery holder is going to be placed on the upper plate of the frame and hold the master switch. I designed in autocad 123d. Turnigy hard case lipo fits perfectly but zop 4s 5000 lipo fits vertical. Bases of the holder and the battery are covered with a velcro. I preassebled the battery holder to check the measurements were correct.

Step 5: Flight Controller Case

I have noticed pixhawk FC cannot be fitted between the plates of the frame so I decided to take out the case of the FC. I made a protective soft case from packaging foam for the Pixhawk. I also added zener diode for brownout protection.

Step 6: Initital Assembly

I soldered 3.5mm bullets for the esc and measured and cut motor and esc wires according to the arm lenght. then I soldered ESCs to the pdb of the frame. I soldered the ubec to the pdb under the frame then placed the flight controller. I isolated connections with an electrical tape and connected esc wires to the FC. Telemetry module and receiver antenna were attached to the landing gear. Finally I attached landing gear. This stage required 9 pre-assembly stages which can be a painful process.

Step 7: Electrical Connections

I placed the matek 12v PDB under the frame with a double sided tape for the 12v supply of the osd. I took negative lead of the main power line and soldered a crimp then attached to the master switch. Later I connected to main power line to the power module of the FC. By doing this anybody can switch off the uav without cutting the power of the FC and gps in case of emergency.

Step 8: Electronics and FPV

All electronic devices were checked before the assembly. Battery tray is inverted and a camera holder was placed on it. then mini osd fpv switch, camera and transmitter installed on the tray. CH5 was assigned for the camera switch to a 2 way toggle on turnigy 9x transmitter and ch7 is assigned to operate servo to control release mechanism. GPS stud was shortened and connected lefthandside of the upper plate.

Step 9: Final Assembly

Finally all connections are done and quadcopter was ready for calibrations and setttings. Motor directions were checked according to pixhawks' motor diagram. Related setting and PID values will be given after the competition. For futher info you are welcome to contact :)

Step 10: FINAL VIEW

I made the release mechanism from the sheet aluminum. Servo mount was welded to the upper part. Release mechanism looks like a magazine which is spring loaded and weights approx. 500grams. It can carry 4 100g cubes.

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