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Control Experiment: Model Helicopter

INTRODUCTION

The AERO 320 course is an introduction to linear time-invariant (LTI) control theory and currently it is limited to just that, theory. Introductory control theory classes are often dreaded by students because the material can be abstract and difficult to understand. The goal of this project is to design, build, and implement a lab experiment for the AERO 320 class. This experiment is intended to be a teaching aid for the students by giving them the opportunity to work hands on with a control system.

The proposed design of this lab experiment is a simplified model helicopter. The helicopter will be constrained to vertical motion, moving up and down a linear steel pole by varying the speed of the brushless DC motors and therefore the lift created by rotor blades. Using MATLAB/Simulink students will command the helicopter up some height of the pole. In real time, by utilizing the plotting capabilities of Simulink, they will be able to see the distance, velocity and acceleration plots versus time.

The students will determine the gains of a proportional, integral, derivative (PID) controller that will filter the error signal produced by comparing the student input height and the fed back current height. Adjusting the gains associated with the different components of the PID controller will demonstrate the effects on the stability and closed-loop system response parameters.

APPARATUS

Figure 1 . Carriage Assembly

Figure 1 . Carriage Assembly

The sleeve separates the linear bearings, which prevents the bearings from binding on the pole when a moment is produced by unbalanced lift forces from the two propellers on either side of the carriage. Each linear bearing is attached to the sleeve with two screws. One set of screws attaches the aluminum strip and the linear bearing to the sleeve. Table 1 lists the components of the carriage assembly along with a picture and its specific purpose. Figure 1 shows the assembled carriage.

PROCEDURE FOR MANUAL CONTROL

The manual control mode is an open loop setup with helicopter motors being controlled by the RC transmitter, the first step before testing the helicopter model closed loop. The purpose is to test whether all the mechanical components work together and can lift the model. The model w as successfully tested with manual control. In the process of testing the model, it was discovered that the ESCs do not start at the same throttle position, or duty cycle.

ANALYSIS

Figure 3 . Block Diagram of Simulink Model

Figure 3 . Block Diagram of Simulink Model

The input is a simple step source block that represents the desired distance for the helicopter to travel. The controller for this system is a PID controller. The classic PID controller has three terms: proportional, derivative, and integral. For this report, it is assumed that the read er has a basic understanding of the use and design of PID controllers.

Figure 4. Closed Loop Step Response with Ideal PID Controller

Figure 4. Closed Loop Step Response with Ideal PID Controller

It is important to understand the effects of changing the controller gains on a system. It is for this reason that all three controller gains were swept through a range of values to see the effects while holding the other values constant. The nominal values for the system were chosen to be the same as in Figure 4. The gains were increased and decrease d twice by increments of two.

FUTURE IMPROVEMENTS

There are several future improvements to the model and supporting components that will increase the performance and robustness of the experiment. The following section will call attention to these areas that need improvement and suggest alternatives to the present state of the model. This section could also serve as a starting point to anyone who will be continuing on with the project.

CONCLUSION

This project is the first iteration of a model helicopter control experiment for the AERO 320 class. The original goal of the project was to design, build, test, and write a complete lab experiment. Because of setbacks in the project, the original goal was not met. This project did successfully fly a first iteration of the physical model. In addition to flying the model, a test stand and carriage were built to support the model. A theoretical Simulink model was developed to predict the effect of PID controller gains on the performance of the system. An ideal PID controller was analyzed, along with a PID controller with real zero and pole locations.

The PID controller with the real zero and pole was determined to not have any additional benefits to the normal PID controller. An Arduino microcontroller was used to implement a Ping Ultrasonic Distance Sensor as a potential feedback sensor, test the control of motor speed in a loop, and develop a PID controller with discrete controller terms. The Arduino will provide the necessary signals to run the motors and control the speed separately through dual PWM outputs which compensate for the offset and gain mismatch in the ESCs and motors. If the goal of the project remains the same, the Future Improvements section of this report outlines some areas to focus on to run the helicopter model closed loop.

Source: California Polytechnic State University
Author: Matthew D. Lattanzi

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