SPHERICAL VTOL UAV Summer project14

Project Mentors:Rushikesh ChaudhariKarthik Korada TEAM MEMBERS- Pranab Kumar Prusty Saurabh Suman Akshay Kumar Sanghi Aashi Manglik Jaishri Jain

OBJECTIVEThe objective of this project is to design, build and test fly a Spherical-shaped Vertical Take-Off (VTOL) Unmanned Aerial Vehicle (UAV). This project consists of 5 components, namely Propulsions, Electronics, Control System, Aerodynamics and Structures.This newly shaped UAV allows the propellers to be encased within spherical-shaped struts, which enables the UAV to fly into obstacles without damaging the propeller. This also makes it safe for the pilot, as well as, anyone in the vicinity. Running only on a single propeller-motor configuration, the UAV is able to hover, climb vertically, and transit into translation flight, which is similar to that of a helicopter.

A total of 3 prototypes were constructed where Prototypes 1 and 2 were constructed to investigate and study the aerodynamics and structural components of the UAV. Only after undergoing thorough experimentation and analysis to optimise the various component designs, Prototype 3 was finally constructed. The final product is only made possible by integrating the electronics, control system and propulsion components.

FLIGHT CONTROL THEORY:All the 3 prototypes achieve yaw, pitch and roll motion using 4 control surfaces i.e , four rudders placed symmetrically.

YAW Control:Viewed from the top, the four bottom flaps deflect in the same orientation to give a clockwise or anti-clockwise moment. In the figure on the left, the four flaps are causing an anti-clockwise yaw moment about the CG when the airflow flows into the paper past the flaps. The torque effect from the counter-clockwise motion of the propeller causes the body of the UAV to rotate anti-clockwise (about the Z-axis), causing the UAV to yaw. In order to counter the inherent yaw motion due to the moving propellers, the rudders must be activated at an angle at its default trim position.

PITCH and ROLL control:Pitch and roll describe similar kind of motion as the model is built symmetrically and divided into 4 equal halves. To achieve pitch motion two alternate rudders are moved in the same direction. This causes the UAV to pitch in the same direction in which the rudders are moved. Roll motion can be achieved similarly using the other set of rudders.

PROTOTYPE I:Prototype I was constructed to basically test the electronics. We mainly focused on the structural strength of the model. So stronger and heavier wood was used to build the model. A cuboidal frame for the upper half that would contain the propeller was made. The lower half was hemispherical shaped and it contained the control surfaces. All the electronics including the battery was mounted on top of the cuboidal frame above the motor. This was done to shift the COG of the model upwards. This would increase the distance of COG from the control surfaces thus increasing their efficiency by increasing the torque about COG. Landing gears were also attached at the bottom.The model turned out to be quite heavier. It weighed 995 grams. Even though Four cell battery was used to supply power and avionic motor was used, sufficient lift could not be provided due to huge drag and high weight of structure. Also area of the control surfaces was not enough .Thus, first model was eventually discarded.

PROTOTYPE II:The second UAV mimics a fixed wing aircraft capable of VTOL. Similar to the spherical UAV, it is propelled by a single rotor which is usually placed at the tip of the UAV. It is also capable of hovering as well as transiting into translational flights which looks like a normal fixed wing aircraft.

The all up weight of the structure was about 650 gms. In this model we focused less on the strength and our priority was to reduce the weight of the model. So we used 5mm biofoam instead of the ply used earlier. Avionic 1400 KV motor was used.Use of balsa was limited to the parts which required strengthening such as the legs and bottom of the motor mount. Firstly we kept the CG much above the midpoint which made the control difficult. So we lowered the CG by interchanging the positions of the battery and the KK2 control board. And to our surprise the flight turned out to be so stable that no further tuning was required.After having few successful flights with the KK2 control board we worked on our owm program to control the UAV.

ARDUINO PROGRAMMINGWe initially started working with Arduino UNO Board as our programmer . But we realized that it has few number of PWM pins. Hence, we switched to Arduino Mega 2560 Board . ARDUINO MEGA 2560IMU

IMU(Inertial Measurement Unit) is an electronic device that measures the vehicles velocity, orientation and gravitational forces, using a combination of acclerometers and gyroscopes, sometimes also magnetometers . We chose MPU6050 as our IMU sensor which has one accelerometer and gyroscope each . Thus , it is a 6DOF(Degrees of freedom) IMU reporting angular velocities and accelerations of aerial vehicle along the three axes .MPU-6050

Firstly , we worked on the code on how to read sensor values . i2cdevlib-master library downloaded from github.com helped us in doing the same. The MPU-6050 features three 16-bit analog-to-digital converters(ADCs) for digitizing the gyroscope outputs and three 16-bit ADCs for digitizing the accelerometer outputs . After succeeding in reading the gyrorates(angular velocities) in radians/sec and accelerations in terms of g (acceleration due to gravity) along each axis , we moved on to writing the code for reading the RC(Radio Control) receiver . RC receivers output pulse width modulated (PWM) signals on each channel. We read PWM signals using hardware interrupts . A hardware interrupt is a signal that is generated by the hardware that literally interrupts the processor. With arduino, hardware interrupts can be generated by a pin changing value, going LOW or HIGH. Arduino has the function attachInterrupt() , which allows to supply an interrupt handler.

Then we started working on the code for stabilizing the UAV during flight.We used PID algorithm in our code to achieve this. A PID controller calculates an error value as the difference between a measured process variable and a desired setpoint. The controller attempts to minimize the error by adjusting the process through use of a manipulated variable.The PID controller algorithm involves three separate constant parameters, and is accordingly sometimes called three-term control: the proportional, the integral and derivative values, denoted P, I, and D. These values can be interpreted in terms of time: P depends on the present error, I on the accumulation of past errors, and D is a prediction of future errors, based on current rate of change. The weighted sum of these three actions is used to adjust the process via a control element. the final form of the PID algorithm is:

whereMV : Manipulated VariableKp: Proportional gain, a tuning parameterKi: Integral gain, a tuning parameterKd: Derivative gain, a tuning parametere: Error = SP - PV t: Time or instantaneous time (the present)T: Variable of integration; takes on values from time 0 to the present t.The current orientation of UAV is the process variable. The desired orientation of UAV is the setpoint.Thus,error is the difference in the current position and desired position. The Manipulated Variable in this case is the PWM signals sent to the servos for controlling roll,pitch and yaw. After measuring the current Position (PV), and then calculating the error, the controller decides what signal is to be sent to the servos (MV). After measuring the temperature (PV), and then calculating the error, the controller decides how to set the tap position (MV). In proportional control the MV is set in proportion to the current error. A more complex control may include derivative action. This also considers the rate of change of error. Finally integral action uses the average error in the past set the MV proportional to the past errors. An alternative formulation of integral action is to change the MV in steps proportional to the current error. Over time the steps add up (which is the discrete time equivalent to integration) the past errors. The current orientation is calculated from the gyro rates. The final orientation is the input that is the input from the receiver.the error is calculated and the desired output of the servos is calculated by using some equations so as to achieve the setpoint. The proportional, integral, and derivative terms are summed to calculate the output of the PID controller. The proportional term produces an output value that is proportional to the current error value. The proportional response can be adjusted by multiplying the error by a constant Kp, called the proportional gain constant. As Kp increases, the speed of response increases. An overly large value of Kp will cause the system to oscillate. It will cause the aircraft to be very sensitive. The integral term is proportional to both the magnitude of the error and the duration of the error. The integral in a PID controller is the sum of the instantaneous error over time and gives the accumulated offset that should have been corrected previously. The accumulated error is then multiplied by the integral gain (Ki) and added to the controller output. The integral term accelerates the movement of the process towards setpoint and eliminates the residual steady-state error that occurs with a pure proportional controller. As I gain increases, the steady state error of sensing the neutral orientation decrease and the UAV maintains its new orientation after the controls are release. An overly large value of I will cause windup in the system, causing the UAV to accumulate a large error in orientation and overshoot continuously as it tries to correct itself. The derivative of the process error is calculated by determining the slope of the error over time and multiplying this rate of change by the derivative gain Kd. Derivative action predicts system behavior and thus improves settling time and stability of the system.

PROTOTYPE III:The third and the final prototype was constructed after the successful testing of the electronic components