How to Make a Drone / UAV - Lesson 4: Flight Controller

• Lesson 1: Terminology
• Lesson 2: The Platform
• Lesson 3: Propulsion
• Lesson 4: Choose a flight controller
• Lesson 5: Assembly
• Lesson 6: Get it all working together
• Lesson 7: FPV & Long-range
• Lesson 8: Aircraft

Now that you have chosen or designed a UAV frame, and chosen the appropriate motors, propellers, ESCs, and battery, you can start looking into choosing a flight controller. A flight controller for a multi-rotor UAV is an integrated circuit normally made up of a microprocessor, sensors, and input/output pins. Out of the box, a flight controller does not magically know your specific UAV type or configuration, so you need to set certain parameters in a software program, and once complete, that configuration is then uploaded to the board. Rather than simply comparing flight controllers which are currently available, the approach we have taken here lists which features serve which functions, as well as aspects to look for.

8051 vs AVR vs PIC vs ARM: These microcontroller families form the basis of most current flight controllers. Arduino is AVR based (ATmel) and the community seems to focus on MultiWii as being the preferred code. Microchip is the primary manufacturer of PIC chips. It is difficult to argue that one is better than the other, and it really comes down to what the software can do. ARM (STM32 for example) uses 16/32-bit architecture, whereas AVR and PIC tens use 8 / 16-bit (described below). As single-board computers become less and less expensive, expect to see a new generation of flight controllers which can run full operating systems such as Linux or Android.

CPU: Normally these are in multiples of 8 (8-bit, 16-bit, 32-bit, 64-bit) and are a reference to the size of the primary registers in a CPU. Microprocessors can only process a set (maximum) number of bits in memory at a time. The more bits a microcontroller can handle, the more accurate (and faster) the processing will be. For example, processing a 16-bit variable on an 8-bit processor is a bit of a choice, whereas on a 32-bit processor, it is very fast. Note that the code also needs to work with the right number of bits, and at the time of this article, very few programs use code optimized for 32 bits.

Microcontroller

Operating frequency: The frequency at which the main processor operates. Frequency is measured in "Hertz" (cycles per second). This is also commonly referred to as the "clock rate". The higher the operating frequency, the faster it can process data.

Program Memory / Flash: Flash memory is essentially where the main code is stored. If the program is complex it may take up quite a bit of space. Obviously the greater the memory, the more information it can store. Memory is also useful when storing in-flight data such as GPS coordinates, flight plans, automated camera movement, etc. The code loaded to the flash memory remains on the chip even if its power is cut.

SRAM: SRAM stands for "Static Random-Access Memory", and is the space on the chip which is used when making calculations. The data stored in RAM is lost when power is cut. The higher the RAM, the more information will be "readily available" for calculations at any given time.

EEPROM: Electrically Erasable Programmable Read-Only Memory (EEPROM) is normally used to store information that does not change in flight, such as settings, unlike data stored in SRAM which can relate to sensor data, etc.

Additional I/O Pins: Most microcontrollers have a lot of digital and analog input and output pins, and on a flight controller, some are used by the sensors, others for communication and some may remain for general input and output. These additional pins can be connected to RC servos, gimbal systems, buzzers, and more.

A/D converter: Should the sensors use onboard output analog voltage (normally 0-3.3V or 0-5V), the analog-to-digital converter needs to translate these readings into digital data. Just like the CPU, the number of bits that can be processed by the A/D determines the maximum accuracy. Related to this is the frequency at which the microprocessor can read the data (number of times per second) to try to ensure no information is lost. It is nevertheless hard not to lose some data during this conversion, so the higher the A/D conversion, the more accurate the readings will be, but it is important that the processor can handle the rate at which the information is being sent.

In terms of hardware, a flight controller is essentially a normal programmable microcontroller but has specific sensors onboard. At a bare minimum, a flight controller will include a three-axis gyroscope, but as such will not be able to auto-level. Not all flight controllers will include all of the sensors below and may include a combination thereof. The sensors

Accelerometer Axes

Accelerometer As the name implies, accelerometers measure linear acceleration in up to three axes (let's call them X, Y, and Z). The units are normally in "gravity" (g) which is 9.81 meters per second per second, or 32 feet per second per second. The output of an accelerometer can be integrated twice to give a position, though because of losses in the output, it is subject to "drift". A very important characteristic of three-axis accelerometers is that they detect gravity, and as such, can know which direction is "down". This plays a major role in allowing multirotor aircraft to stay stable. The accelerometer should be mounted to the flight controller so that the linear axes line up with the main axes of the UAV.

Gyroscope Axes

Gyroscope A gyroscope measures the rate of angular change in up to three angular axes (let's call them alpha, beta, and gamma).  The units are often degrees per second. Note that a gyroscope does not measure absolute angles directly, but you can iterate to get the angle which, just like an accelerometer, is subject to drift. The output of the actual gyroscope tends to be analog or I2C, but in most cases, you do not need to worry about it since this is handled by the flight controller's code. The gyroscope should be mounted so that its rotational axes line up with the axes of the UAV.

IMU 6 Axes

Inertia Measurement Unit (IMU) An IMU is essentially a small board that contains both an accelerometer and gyroscope (normally these are multi-axis). Most contain a three-axis accelerometer and a three-axis gyroscope, and others may contain additional sensors such as a three-axis magnetometer, providing a total of 9 axes of measurement.

Compass

Compass / Magnetometer An electronic magnetic compass is able to measure the earth's magnetic field and is used to determine the drone's compass direction (with respect to magnetic north). This sensor is almost always present if the system has GPS input and is available in one to three axes.

Atmospheric Pressure

Pressure / Barometer Since atmospheric pressure changes the farther away you are from sea level, a pressure sensor can be used to give you a pretty accurate reading of the UAV's height. Most flight controllers take input from both the pressure sensor and GPS altitude to calculate a more accurate height above sea level. Note that it is preferable to have the barometer covered with a piece of foam to diminish the effects of wind over the chip.

GPS Satellites

GPS Global Positioning Systems (GPS) use the signals sent by a number of satellites in orbit around the earth in order to determine their specific geographic location. A flight controller can either have onboard GPS or one which is connected to it via a cable. The GPS antenna should not be confused with the GPS chip itself and can look like a small black box or a normal "duck" antenna. In order to get an accurate GPS lock, the GPS chip should receive data from multiple satellites, and the more the better.

Distance

Distance Distance sensors are being used more and more on drones since GPS coordinates and pressure sensors alone cannot tell you how far away from the ground you are (think hill, mountain, or building) or if you will hit an object. A downward-facing distance sensor might be based on ultrasonic, laser or lidar technology (infrared has issues in sunlight). Very few flight controllers include distance sensors as part of the standard package.

Below is a list of the most popular flight modes, though not all will be available on all flight controllers. A "flight mode" is the way the flight controller uses sensors and RC input in order to fly and stabilize the aircraft. If you have a transmitter with five or more channels you may be able to configure the software to allow you to change the flight mode via the 5th channel (aux switch) while in flight. Each mode is defined below.

Manufacturers of flight controllers generally try to provide the most features possible - either included as standard equipment or purchasable separately as options/add-ons. Below are just some of the many additional features which you might want to take a look at when comparing flight controllers.

Damping: Even small vibrations in the frame, normally cause by unbalanced propellers and/or motors, can be picked up by the onboard accelerometer, which will in turn send the appropriate signals to the main processor, which will then take corrective action. These minor corrections are not wanted nor needed for stable flight and it's best to have the flight controller vibrate as little as possible. For this reason, vibration dampeners/absorbers are often used between the flight controller and the frame.

Case: A protective case around the flight controller can help in a number of ways. Aside from being more aesthetically pleasing than a bare PCB, a case often provides some level of protection from the elements, as well as added protection in the event of a crash.

Mounting: There are various different ways in which a flight controller can be mounted to a frame, and not all flight controllers have the same mounting options.

• Four holes spaced 30.5mm or 45mm apart in a square
• Flat bottom for use with a sticker
• Four holes in a rectangle (no set standard)

Community: Since you are creating a custom drone, being part of an online community may help considerably, especially if you encounter issues or want advice. Obtaining advice from a community, or seeing user feedback regarding the quality and ease of use of specific flight controllers also helps.

Accessories: Aside from just the flight controller itself, additional products (accessories or options) may be required in order to make full use of the product. Such accessories may include, but are not limited to GPS module and/or GPS antenna; cables; mounting accessories; screen (LCD / OLED);

So given all of these different comparative features, what information might you be able to obtain about a flight controller, and what might a flight controller include? We have chosen the Quadrino Nano Flight Controller as an example.

Main Processor The ATMel ATMega2560 used onboard is one of the more powerful Arduino-compatible ATMel chips available. Although it has a total of 100 pins, including 16 analog to digital channels and five SPI ports, due to its small size and intended use as a flight controller, only some of these are broken out on the board. AVR vs PIC:  AVR CPU: 8-bit Operating Frequency: 16Mhz Program Memory / Flash: 256Kb SRAM: 8Kb EEPROM: 4Kb Additional I/O Pins: 3x I2C; 1x UART; 2x10 pin GPIO; Servo with 5x outputs; OLED port A/D Converter: 10-bit

ATMega 2560

Sensors   The Quadrino Nano incorporates the MPU9150 IMU chip which includes 3-axis gyroscope, 3-axis accelerometer and 3-axis magnetometer. This helps make the board quite small without sacrificing sensor quality. The MS5611 barometer provides pressure data and is covered by a piece of foam. The integrated Venus 838FLPx GPS with external GPS antenna (included)

Quadrino Nano Sensors

Software   The Quadrino Nano was made specifically to use the latest MultiWii (Arduino-based) software. Rather than having to modify the Arduino code directly, a separate, more graphical software was created.

Quadruino Software

Communication Direct input from a standard RC receiver. Dedicated Spektrum satellite receiver port Serial (SBus and/or Bluetooth or 3DR radio links)

Quadrino Communication

Additional Considerations Case: A protective semi-transparent case is included as standard equipment Mounting: There are two main ways to mount the Quadrino Nano to a drone: screws and nuts or foam sticker Compact design: The controller itself (GPS antenna not connected) measures 53mm x 53mm.

Lynxmotion Quadrino Nano