Autopilot Implementation ■ System Set-up ■ Experiments
This page provides detailed information on choosing the correct hardware based on the goals of your project as well as installing some of the hardware.
The goals of your project will determine which specific motor, propeller, and ESC combination is appropriate for your system’s design. First, begin with the total weight of your quadrotor including the battery and the weight of any potential sensors you may use on your system. For my platform, this is around 1,400 grams. We want to be able to hover the aircraft at 50% throttle so we double the weight of the vehicle resulting in a weight of 2,800 grams. Next, we add a 20% safety factor and multiply the weight by 1.2 resulting in a total weight of 3,360 grams. Therefore, we want to design a thrust system that is capable of producing at least 3,600 total grams of thrust.
To determine the correct motor, we need to determine the total thrust required for each motor. Since we are operating a standard quadrotor aircraft with 4 motors, we divide the total weight by 4 resulting in a thrust of 840 grams per motor. You can shop around to determine which motor best suits your needs. The data sheet should tell you the total amount of thrust that motor is capable of producing with a given propeller. For example, our 880kv motors are capable of producing 1380 grams of thrust each which is higher than our goal of 840 grams. Also note that this specific motor is called the AC2836. The 28 refers to the motor diameter and the 36 refers to the height in mm. The higher the motor shaft, the more torque to motor will produce and the more weight your system can carry. Also, the kv value, in our case 880, is a relationship between input voltage and motor shaft rpms. A smaller kv value will result in more power and a slower propeller speed.
Looking further at the motor datasheet, you can see the recommended propeller dimensions. In this case, they recommend a 12×45 propeller. This means the propeller is 12 inches long and would move forward 4.5 inches after one full rotation. Basically, the higher pitch propellers draw more current but result in faster top speeds. The lower pitch propellers provide more torque and maneuverability while drawing less power. For this reason, they are typically found on small unmanned aircraft. The brand of propeller, APC (Advanced Precision Composites), is common with these aircraft is well because is has reliable and constant power throughout the entire RPM range of the motor.
The motor data also lists the recommended electrical performance. In this case, it recommends a 2-4 series LiPo battery. Each series has an average voltage of 3.7 volts but can be charged up to 4.2 volts and discharged down to 3.0 volts safely. Batteries are also labelled in milli-Ampere Hours (mAH) as well as a “C” rating which determines the maximum allowable discharge current. For example, a 2000 mAH battery can provide 2 amps over a 1 hour period. If the battery had a “C” rating of 30, the maximum discharge current of the battery would be 60 amps. Finally, you can estimate your expected total flight time. Take the typical total current draw, in our case around 2.2 amps per motor at hover. The flight time will be the AH divided by total current draw. In our example, for a 2000 mAH battery, we can expect a flight time of around 13 minutes.
In addition, the motor data provides a maximum current of 20 amps. Therefore, the ESCs must be able to provide up to 20 amps. Looking at the data for our SimonK ESCs, they are capable of providing 20 amps continuous or bursts up to 25 amps. They also support a 2-4 series LiPo battery input.