Introduction to Drone Systems

Basic UAV Assembly & Components Familiarization

Objective:

• Identify and understand flight controllers (Pixhawk, APM), ESCs, motors, propellers, and battery management.
• Learn about LiPo, Li-ion, and NiMH batteries, their charge cycles, and safety.
• Learn about different types of UAVs (fixed-wing, multirotor, hybrid).
• List the components you would need to make a Quadcopter drone (make sure all the components are compatible with one another) with minimum thrust to weight ratio of 2:1. Make a detailed report on this, along with reasons why the specific component was chosen. Explicitly show proof about each component being compatible with each other.
• Do & show the manual pen and paper calculation of the drone flight time, thrust to weight ratio by reading the datasheet of the components.
• Use E-calc to verify your results.

Learnings:

1. Flight Controller: It can be thought as the brain of the drone, which is responsible for integrating & controlling multiple sensors, aviation systems. There are many sensors which are built inside the flight controller. The scaling technology used here is MEMS (Micro-Electro-Mechanical Systems) technology, here it combines electrical and mechanical parts on a micrometer scale.

   • Noise Elimination:
   • In FCs there can be noise/interference due to unwanted vibrations, this noise ultimately affects the accuracy of the readings shown by these sensors. To tackle this issue modern drones use, "Sensor Fusion Technique", combining the reading of multiple sensors which show the same type of readings, like combining IMU+GPS+Radar can increase the accuracy of the altitude readings of the drone.
   • Protocol used in Pixhawk Orange Cube+ Flight controller:
     • MAVLink [Micro Air Vehicle Link] It's a lightweight communication protocol used to send and receive commands, telemetry, and sensor data. It acts as a link between:
       • Flight controller ↔ Ground control station (e.g., Mission Planner, QGroundControl)
       • Flight controller ↔ Companion computer (e.g., Raspberry Pi, Jetson)
       • Flight controller ↔ Telemetry module or modem
     • The Mavlink can use I²C, CAN, SPI (for specific sensors), USB (Mission Planner), UART (Tx-Rx, Telemetry, GPS etc). Majorly UART is the one which is used, the below image helps us understand how UART is used within another protocol i.e., MAVLink here:
     • MAVLink + LORA:
       • You use LoRa modules to transmit MAVLink messages over long distances.
       • The flight controller sends MAVLink packets via UART, and the LoRa radio transmits those packets. But, for this we would need an external UART-based LoRa modules.

2. What if the drone goes out of the RF range of the RC? Modern drones make use of RTH automation with the help of GPS, tower based tracking technology to make the drone come back to home.

3. Motor & it's nomenclature: In a BLDC motor, N- No. of teeth/coil windings P- No. of magnets/Poles in the BLDC motor So, if a motor packaging tells 12N-14P; It has 12 teeth & 14 magnets as shown in the figure below.

   Thumb rule while selecting the motors should be that the net thrust generated by all the motors should be at least 2x the weight of the drone.

   • For racing drones etc., the thrust to weight ratio is about 8:1 (net thrust generated by the motor is 8x the weight of the drone), for Payload drones it is around 4:1 & for Normal drones it is 2.5:1.

4. Propeller Nomenclature:

Propeller Specs reading: 
There are 2 main factors in the propeller i.e. Diameter & Pitch. Diameter is the measure (in inches) of the propeller from one end to the other  & Pitch is the distance (in inches) travelled by the propeller  in one revolution. It's denoted by 4 digits,
***Note:***
Example: if the numbering on the prop is ABCD R, {ABCD are nos}
AB-> Prop's diameter is AB inches
CD-> Prop's pitch is C.D inches
R-> Clockwise prop
If it has R at the end its a clockwise prop, if there is nothing or L then anticlockwise propeller.
Example: 1045 indicates 10" diameter, 4.5" pitch and its anticlockwise.

Major Learning:

The following would be the materials needed to assemble a drone;

1. Chassis: If cost is not a concerning factor then Carbon Fiber would be the best chassis as it has best strength-to-weight ratio with high endurance. But if cost is a major concern then plastic or fiberglass would be the best choice for making a chassis.

2. Electronic Speed Controller (ESC): An electronic circuit connected to the motor of the drone and the FC, which:

   • Controls the speed and direction of the motor.
   • Prevents the motor from excess flow of current.
   • Converts control signals (from the FC) into electrical pulses to run the motors.
     • ESC selection: The selected ESC should have 20% more amp reading than the motor. {Amp reading is the amount of current drawn by the motor at 100% throttle}

3. . Propeller: No. of blades ∝ Drag ∝ Overall power req. ∝ (1/Efficiency) {2 blade propellers are the most stable}

Parameter	Threshold	RPM	Thrust	Use Case
High Diameter	> 8 inches	Low	High	Normal drones, heavy payload, stable
Low Diameter	< 8 inches	High	Low	High speed race drones
High Pitch	> 4.5 inches	Low	Medium	Moving Forward (Pitch motion)
Lower Pitch	< 4.5 inches	High	High	Stable Hovering

• High Diameter: Moves more air because of greater surface area, providing greater lift but spins slower.

• Low Diameter: Spins faster, but produces less lift, because of less surface area.

• High Pitch: Designed for speed, with a focus on forward movement.

• Low Pitch: Best for hovering and stability, generating more lift at lower speeds.

  To summarize:

  • Low Diameter & High Pitch = Speed and Agility {Hard to control}
  • High Diameter & Low Pitch = Stability and Hovering {Easy to control}

1. Motors:

   • Understanding the fundamentals of KV:

     • KV is the maximum theoretical rotation speed of the motor (in RPM) @ No Load, when 1 volt is applied to it. or Generator Analogy

     • If you manually install a shaft @ one end of the motor and rotate it 1450 times per minute (i.e. @1450 RPM), it would give you a 1 volt output. Therefore, for a lower KV motor let's say 750 KV, you just need to rotate 750 times a minute to generate 1 volt output.

     • Relation between Kt & Kv of 2 same motors {Torque constant and the velocity constant of a motor}: Consider two motors of the same size, i.e., 2822 1450 KV and 2822 750 KV. We usually associate KV with the torque of the motor, meaning a lower KV corresponds to higher torque, and a higher KV corresponds to a smaller prop and vice versa. However, this logic doesn't hold when the dimensions of the motors are the same.

       If we use the above generator analogy to understand the concept of KV, the lower KV motor should have more copper windings. This is because more windings are needed to produce more output voltage at lower RPMs. These additional windings create a stronger magnetic field, which compensates for the lower RPMs.

       Now, with more copper windings, the length of the copper wire increases, which increases the resistance of the windings. This increased resistance limits the current flowing through the windings, which reduces the changing magnetic field and thus significantly lowers the torque. In comparison, the motor with a higher KV rating has fewer windings and lower resistance, allowing more current to flow through the windings and increasing the torque. Therefore, we can conclude that the torque of the motor can't be judged by the KV value alone when comparing two motors of the same size.

   • If KV is misleading, how to compare 2 motors with the same size or any 2 motors in general? See the Km value, its the motor constant just like Kv & Kt. Higher the Km value better is the overall torque and velocity efficiency of the motor.
   • What should be the motor Kv rating for your prop? This formula isn't a standard formula but gives a rough estimate about the Kv rating of the motor that should be used for each prop size.

   Propeller Coefficient values for different sizes/diameters of the props are as follows,

   • For 5" Prop, Pc~ 9600. {This gives 2400 KV, assuming 4S battery}
   • For 6" Prop, Pc~ 7600. {This gives 1900 KV, assuming 4S battery}
   • For 7" Prop, Pc~ 6400. {This gives 1600 KV, assuming 4S battery}
   • Rule of Thumb for selecting a motor w.r.t props: Bigger the size of the prop, bigger should be the size of the motor too.
   • Shaft of the BLDC Motor: The shaft of a BLDC motor is the rotating metal rod at the center of the motor. For an outrunner motor shaft is internally fixed to the spinning outer part therefore, we can't see it. The thicker the shaft the better it is, as it is more strong against damages. But a thick shaft increases the weight of the motor.
   • Motor Hub: Part of the motor which protrudes outwards where we mount the propellers.
   • Changing the rotation direction of the motor: In order to change the rotation direction of the motor, just swap any 2 wires connected to the ESC.
   • Understanding the volume of a motor and how it affects the performance:
     • Bigger motors i.e., motors with large volume are better because more copper windings can be efficiently fit (with least empty spaces) inside it, generating more magnetic field and thus, more torque.
   • What motor to choose for different propeller sizes? The below chart shows the minimum volume of the motor that should be used for different propeller sizes.

Propeller Diameter (Inches)	Minimum Volume of the motor Req. [It's the min value, so you can always choose motors bigger than what's specified here]
5	600 * pi mm^3
6	700 * pi mm^3
7	900 * pi mm^3

Note: If we use propellers larger than what is recommended for a particular motor, then the load on the motor increases which may result in more current draw and cause overheating of the motor.

1. Battery: We use Li-Po batteries in drone because of their low weight & high discharge rate. C Rating/Discharge rating is how quickly the current can be discharged from the battery, basically supplying burst shot of current which usually lasts for a very less time, they are useful when giving throttle, when the throttle is given the discharging rate should be as high as possible.

• Battery terms breakdown:
  • mAh: Tells us about the capacity about the battery, more the mAh, more is the capacity, but as the capacity of the battery increases the weight of the battery also increases.
  • C Rating: Discharge rate of the battery, it gives us the measure of how quickly the battery discharges itself.
    • Ex: If a battery of 3000 mAh is rated at 30C, then 30 times the capacity of battery, is the amount of burst of current it can supply without damaging the battery/drone, i.e. in this case it is 30x3000 mA= 90 A
• Battery selection:
  • Lithium Polymer(LiPo) Vs Nickel Metal Hydride Battery (NiMH):
    • In NiMH batteries, the capacity slowly reduces as we keep using them, but in LiPo batteries, even after discharging, the output voltage stays more constant.
    • LiPo batteries have higher charge density than NiMH batteries, i.e. for the same size, a LiPo battery might give 11V, while a NiMH battery might only give around 8V.
    • A 6-cell NiMH battery can be roughly equal to a 2-cell LiPo battery in performance.
    • LiPo batteries are much more expensive compared to NiMH batteries.
  • General Knowledge:
    • The selected battery should have high capacity and high C rating, with minimal weight.
    • The selected battery should be compatible with the ESC, if the ESC is rated at 3S and the battery used is > 3S, then it can cook the ESC.
  • Technical Knowledge:
    • First calculate the net current drawn by the each of the motor @ 100% throttle, then multiply it with the no. of motors used in the drone, this will give the total current drawn by the motor.
      • Ex: The motor draws 24 A of current peak at 100% throttle for some XYZ propellers, so assuming its a quadcopter, the total current drawn would be 24 * 4=96 A. In order to avoid heating issues, we choose a battery with sufficient spare, which can supply about 105 A of current easily. Now we can select the batteries by two way,
        • First way- Trial & Error: Here a battery pack is given to us, and we calculate the total current it can supply by multiplying the battery capacity and the C rating (As shown in the above example for C Rating). In this case since we need 96 A of current and the supply current of battery is just 90 A. Therefore, we cant use that battery. So, we should either increase the capacity of the battery (mAh) or the discharge rate (C Rating). But If weight is a concern (like in racing drones) then we can't increase the battery capacity (mAh) above a certain threshold, so the only option left is to increase the C