Modification of a Toy Helicopter into a Highly Cost Effective Unmanned Aerial Vehicle
Abstract — In this paper we discuss the design and development of highly cost effective, semi-autonomous reconnaissance UAV for safe flights in close environments, with real time video feedback. The design and development is based on the modifications/up gradations, (predicated on the results of several small experiments), of a very low cost, small sized - toy helicopter, and a simple non-linear control system designed for the intelligent UAV features. The developed UAV helicopter has been successfully test-flown at higher altitudes, with heavier payload, for longer period as compared to similar helicopters developed at much higher costs.
Keywords —Helicopter, Pay load, Cost effective UAV, BLDC, WCR
In the world of today Unmanned Aerial Vehicles have become a necessary part of a country’s strategic technology. It can serve as a platform for many applications and for pure academic research1, 2. “A huge market is currently emerging from the potential applications and services offered by unmanned aircrafts. If we pay attention to civil applications, a wide range of scenarios appear. For instance; remote environmental research, fire-fighting management, security; e.g. border monitoring, agricultural applications, oceanography, communication relays for wide-band applications. In general, all of these applications can be divided into four large groups: environmental applications, emergency-security applications, communication applications, and monitoring applications”3.
“Diverse methods such as approximated linearization4, neural network5,6 and learning control7, have been used to design flight control laws for the UAV helicopters to improve performances of automatic landing, hovering and automatic flights8.
A typical UAV vehicle should consist of the following essential components:
1) A physical aircraft with engines or motors to perform some basic flight functions;
2) A simple avionic system to implement flight control systems5. Such a system should include:
a) An airborne computer system to collect data, to execute flight control laws, to drive actuators and to communicate with a ground supporting system b) Necessary sensors to measure signals and actuators used to drive the control surfaces c) A communications system to provide wireless communication, which contains two duplex transceivers, one is airborne and the other is on the ground d) An airborne power supply system e) An automatic flight control system; 3) A ground supporting system, which includes:
a) A full duplex transceiver to provide wireless communication with the aircraft b) A computer system to pre-schedule flight courses and collects in-flight data.”1
It is noted that the Military UAVs use specific control designs specially tailored to the particular surveillance mission that they will Implement3. However, a civil UAV should be able to implement a large variety of missions with little reconfiguration time and overhead, if it must be economically viable9.
II. DEVELOPMENT OF THE DESIGN
First of all we used a 2KD design (horizontal Tail Rotor) in the project because it is inherently stable during flight and hovering and provides a good basis for research. The design has Double Rotor Blade configuration that ensures small size and good thrust as shown in Fig. 1. More importantly, it is a very cheap design (see table IV), which makes it easier for us to upgrade the RC helicopter into a UAV helicopter system. It is ideal for serving as the basic aircraft in a UAV helicopter. The size of the helicopter is listed in Table I.
SIZE OF HELICOPTER
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A. Design Experimentation
A number of experiments based on effects like Pendulum effect, Fulcrum effect, Centre of Gravity and Tail Rotor Positioning were designed and performed on the above described initial design, in order to find the best possible modifications for optimum results.
1) Tail Rotor Positioning:
In this experiment we tried to reduce the size of helicopter. We removed the tail rotor along with tail boom and placed the tail rotor under the main rotors. In this way the size was reduced but forward and backward motion was drastically reduced. We then replaced the tail motor with a BLDC motor, due to its very large rpm and small size, and the results were very encouraging. It was a very small size helicopter without a tail rotor but still moving just like an ordinary helicopter.
2) Pendulum Effect:
In this experiment we removed the tail rotor assembly completely, so that the helicopter could move only upwards or downwards. We then designed a special battery compartment to replace the Li-po battery without having to open the front hood all the time. Now placing the battery more to the front or back of the compartment and flying the helicopter; it performed a pendulum like to and fro motion while moving in the direction where battery had been placed. In this way another small size design was created.
3) Centre of Gravity and Fulcrum Effect:
This modification was made while balancing the weight of added modules on the helicopter. The centre of gravity of a helicopter is right under the main rotors. We used fulcrum effect while placing the camera and control module on the helicopter i.e. camera being light weight was placed at the front end and its battery being a bit heavy was placed near the main rotors. A light weight far from C.G with a little heavy weight near the C.G balanced the load.
B. Design Modifications
Based on the findings of the above experiments we decided to introduce the following modifications in the toy helicopter to achieve the desired functionalities.
1) Modified Assembly
Following are the components and systems we added in our design. A carbon fibre rod and specially designed small mechanical parts to incorporate several modules on the helicopter’s body, Special heat sinks for motors of the main rotors, IR Range Finders that serve as the basic sensing units, BLDC Motors to implement the Wrong Command Rejection and Optimization (WCRO) Algorithm, An AVR based Isolated modular System that serves as the main controlling unit, Payload capability enhancing module, Wireless video camera.
One important problem we encountered was that the motors started to get heated up after sometime during the flight. We incorporated our specially designed heat sinks inside the body to overcome this problem. We identified the empty spaces inside the helicopter’s body near the main motors and placed the heat sinks there, so that they connect with the body and whole body becomes a heat sink; effectively increasing the heat dissipation.
The desired final result was to ensure safe flights and crash avoidance. To achieve this purpose we decided to use IR Range Finders and BLDC motors. The challenge was to place them on the helicopter’s body so that they monitor a good effective area around the helicopter. For this purpose a Carbon Fibre rod was passed through the centre of the body on which two IR Range Finders and BLDC Motors were mounted using specially designed fittings. The rod is firmly held at the centre using some specially designed mechanical parts placed on the body.
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Fig. 1 Final Design
One of our main objectives was to increase the payload carrying capability of our design. To achieve this functionality a ‘payload carrying capability enhancing’ module was specially designed that incorporated power MOSFETs and opto-isolators. To avoid overheating of these MOSFETs we decided to attach them to the helicopter’s body using mica sheet. This allows better heat dissipation but ensures electrical insulation from the body.
A wireless camera has also been placed at the front end as shown in Fig.1 to provide video surveillance. Finally we needed a main controlling unit that would help implement the novel designed WCRO algorithm. For this purpose an AVR ATMEGA 88 based isolated embedded system was designed to serve as the main controlling unit and was placed under the front hood near the helicopter’s own receiver module.
After incorporating all the changes; the final design gives a very stable flight and because the BLDC motors are placed out of the wing span of the main rotors a very small time operation of these motors provides sufficient counter torque to bend the helicopter in a direction away from the obstacle. The weight of all the extra parts is properly propagated throughout the system to ensure proper manoeuvring during flight.
2) Functions and Results
a) Payload Carrying Capability Enhancement
This is one of our main achievements. Normally these toy helicopters are designed to carry only their own weight. To make the helicopter capable of carrying the extra load of added modules; we increased the operating battery voltages from 7.8V to 11.1V, 1.8A. The motors have a maximum rating of 14V and we are operating in a safe range. To achieve this task we had to go through a series of experiments with various Power MOSFETs incorporated in place of original FETs i.e. D150 used in the starting model.
The problem we encountered in achieving this feature was that there was not enough voltage for the gate drive available at the receiver circuit’s original FETs. To overcome this problem we conducted a complete study of the receiver circuit, did reverse engineering on it and extracted signals at different points as shown in Fig. 2. A total of 200mW power is being consumed by the transmitter module and is working on the principle of PPM (Pulse Position Modulation). There are two power ports for the two motors of the main rotors being powered through the JFETs as numbered. Then there is a small port for the tail rotor. The receiver-transmitter radio set operates at a frequency of 40 MHz. A piezoelectric gyroscope is used on the tail rotor (pitch) control to counter wind- and torque- reaction-induced tail movement. The gyroscope electronically adjusts the control signal to the tail rotor.
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Fig. 2 Receiver Circuit
To overcome the problem of handling extra current from the higher voltage battery, we needed a high current handling Power MOSFET to replace the original FETs i.e. D150. A Power MOSFET P75N75 appeared as the best choice to handle extra current at increased battery voltage and had sufficient switching speed to handle the incoming signal, but it required more gate drive voltage than available on the receiver board. To overcome this pitfall we decided to use the original FETs of the receiver module to drive an Opto-isolator, which inturn switches and provides the required gate drive voltage for the Power MOSFETs and enhance isolation. In this way the problem of driving motors at increased voltages was solved in the most cost effective manner by the reutilization of the redundant D150 FETs.
The helicopter’s own weight is 550g and there is an extra added weight of 410g of different modules making the final design’s weight being 960g. The helicopter was first tested successfully with all this weight for 9 minutes at a height of 8m above the ground. The helicopter was later also successfully tested with an additional payload of up to 2kg.
b) The Sensing System
The sensing system comprises Sharp GP2Y0A710YK0F Package IR RANGERS. This sensor takes a continuous distance reading and returns a corresponding analogue voltage with a range of 100cm (40") to 550cm (~216"). The output of these sensors is non-linear and to use it properly we linearized it using a straight line approximation of the voltage from 0.8m-5.5m as shown in Fig.3. We took voltage readings for every 1’’ distance variation from the sensor in the specified range. In this way we were able to better visualize the voltage varies in accordance with the range and it was easier to take decisions based on range. Here we have only described the working of these sensors; the interlinkage of these sensors with other modules is explained in Section III.
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- Quote paper
- Muhammad Ishfaq Javed (Author), 2012, Modification of a Toy Helicopter into a Highly Cost Effective Unmanned Aerial Vehicle , Munich, GRIN Verlag, https://www.grin.com/document/190796