Many embedded systems use sensors that combine an accelerometer and a gyroscope. Quadcopter is one of the examples of that. Already-built cheap toy-like quadcopters are available on e-bay around at $25~$30, but many hobbyists and avid AVR programmers build their own quadcopters. We thought building our own quadcopter would give us a good opportunity to learn about accelerometers, gyroscopes, wireless communications and motor control. Also, we could have a cool high-tech toy at the end of the project!
Joao Diogo Lisboa de Menezes Falcao
Terry Young Hwa Kim
A quadcopter is a multicopter lifted and propelled by four rotors. The motors are mounted in the same plane, on the helicopter, forming a square. This allows this quadcopter to lift vertically without rotating on itself. There are several advantages to quadcopters over comparably-scaled helicopters. The fact that four motors are involved reduces the minimum size of the propellers on each motor. This means that in case the quadcopter hits anything the damage is significant less than if a similar sized helicopter crashed. The following figure shows how the motors are mounted. The rotation direction is critical to the correct functioning of a quadcopter.
All motors have to rotate at the same speed for a vertical lift; however, motor 1 and 3 have to rotate opposite to motors 2 and 4. This is necessary to compensate the angular acceleration generated by each motor. This way the total angular acceleration should be zero, maintaining the quadcopter stable.
According to Wikipedia, flight dynamics is the science of air vehicle orientation and control in three dimensions. Roll, pitch and yaw as shown in the picture below are the three critical flight dynamics parameters, which are the angles of rotation in three dimensions about the flying object’s center of mass. In our project, we use our IMU chip to measure these values and also employ the PID (proportionalintegralderivative controller) system from lab 4 to fine-tune them. Roll, pitch, yaw and throttle (up and down) movements are controlled by an Xbox joystick.
Accelerometer and gyroscope
An accelerometer is an electromechanical device that measures acceleration forces. There is an excellent tutorial on the web at http://www.instructables.com/id/Accelerometer-Gyro-Tutorial/step3/Combining-the-Accelerometer-and-Gyro/
Let us briefly explain how accelerometers and gyroscopes work (a summary from the website). Let the vector R in the diagram below be the force vector that the accelerometer is measuring. Rx, Ry, and Rz are projections of the R vector on the X, Y, and Z axes.
For more detail: Quadcopter Using Atmega644