Design and Manufacture of a Small Scale Multi-copter

THE HONG KONG UNIVERSITY OF SCIENCE AND TECHNOLOGY

HKUST President's Cup

Project Report

Design and Manufacture of a small scale Multi-copter

Authors Danish, Mohammed

Asif, Ali Ahmer Hoshi, Nadir Tsoi, Ka Wai ·

Project Supervisors Professor Yang Leng Professor Charles Kwan

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Department of Mechanical and Aerospace Engineering

Introduction

The primary purpose of the project is to design and manufacture a small sized multi-copter. A multi-copter is a rotorcraft comprising of more than two rotors. Multi-copters have become widely-popular since they can be controlled using fixed pitch rotors as opposed to variable pith propellers that are needed for controlling conventional helicopters. Multi-copters have greater control, stability and are smaller in size than conventional aircrafts. Depending on the requirement and user specifications, the multi-copter design can be optimized to further achieve stability while retaining control, speed, payload and size.

Currently, multi-copters are designed to be light and acrobatic. Their usual applications include videography and entertainment. These multi-copters have basically similar designs which have arms containing rotors attached to a central base mounting most of the electronic components. This design results in a light and acrobatic design which can be used for the aforementioned purposes. However, very few multi-copters which are able to lift a certain amount of load are commercially available since there are no designs present which have high structural strength. The current designs also make such a feature difficult to accomplish since the central base exerts a moment while moving with a load. Therefore, it was realized that a new design with a high structural strength and greater stability while moving with a load was required.

The multi-copter designed for this project has the major purpose of carrying a payload along with it providing a means to transport small objects aerially. The multi-copter is remotely controlled and navigated using a camera placed at the front. A remote controlled claw is attached to the bottom of the multi-copter to pick up and release objects. This enables the multi-copter to fly along with the picked up materials. This would allow goods to be transported to areas aerially without human involvement. It would be especially useful for transporting medical supplies to disaster struck areas.

It can also be employed for commercial deliveries of small goods from suppliers to buyers within the area.

Description of the Multi-copter

Payload description

A major function of the quad-copter is to carry and transport some load. For this purpose a mechanical claw is designed and installed in the multi-copter which is able to lift up to lkg of additional load. The claw uses a retracting and contracting mechanism to grasp things and hold them while the quad-copter is airborne.

The payload is simply dependent on the ability of the multi-copter to lift weight. This ability is dependent on mainly three key specifications of the quad-copter that are motor, propeller and total weight ofthe multi-copter. The combination of the motor and propeller determines the thrust that can be produced by each rotor. Subtracting the weight of the multi-copter from the total thrust provides the payload of the multi-copter.

Key issues in the design

1.

Power:

There are a few types of power sources that can be used to run a multi -copter. For electric motors, batteries are the most common choice. There are different types of batteries that are available such

as Li-ion, Li-Po, Ni-Cd, lead acid and etc. most efficient batteries with the best power to weight ratio are Li-Po batteries which are most commonly used in Multi-copters.

2. Stability:

Stability of a Multi-rotor depends on how many rotors are installed. Usually even no. of rotors provides better stability than odd number of rotors. Even number of rotors can be configured such that half of them rotate clockwise while the other half rotates anti clockwise due to which the angular moments are cancelled out and the multi-copter flies stably without rotating at its center. Another factor for the stability of the multi-copter is the position of its center of gravity. It is required that the position of the center of gravity of the multi-copter is at its geometric center. As a result of this, there is no moment acting on the multi-copter and all forces acting on it are balanced. The multi-copter for this project has a circular ring shape structure which ensures that the center of gravity remains at the geometric center at all times. The additional load is also added to the center where the claw is attached keeping the multi-copter stable. As a result, the multi-copter has a high stability which is suitable for lifting loads.

4. Control:

Control and maneuverability of any Multi-rotor is mostly dependent on the design and electronic controllers. The rotors can be arranged in such a way to provide maximum speed in one direction or can be arranged symmetrically to provide easy maneuverability in all directions.

The electronic components such as the control board and remote control are mainly responsible for providing the commands to the motors and other components to produce the desired output from the Multi-rotor. The control board is responsible for receiving information from the controller, which is operated by the user, and converts the commands to useful output signals. Control boards are normally programmable and contain sensors such as gyroscopes, accelerometers, magnetometers to provide a closed loop feedback control of the Multi-rotor and to keep it stable.

Battery

Battery

Battery

Battery

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Radio

Controller

Sensor

Board

After receiving commands from the radio controller, the control board converts those commands and then sends signals to four electronic speed controllers (ESC), which are connected to four motors, accordingly. Those ESCs then control the speed of motors by adjusting voltages supplied by batteries. The rotor then can perform various movements according to the difference of r.m.s of four motors.

5. Navigation

Two live video transmission systems obtained by two separately positioned cameras are used to navigate the multi-copter and to position it over the object to be picked up. One camera is installed in the front of the rotor facing forward and the other one is installed at the center of the claw facing downward. The front camera is used to provide front view of the flying route to navigate the multi-copter and the center camera is used to help adjust the position of the multi-multi-copter so that it can grab objects precisely.

The whole connection of one video transmission system is shown in the figure below. The camera is connected to a transmitter directly and the transmitter transfers video signal through radio to the receiver, which then shows the live video image on a monitor. Two parallel video feeds are obtained from the cameras

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'Remote

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Controller

6. Safety:

Camera/

Mount

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Signal

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Monitor

It is normally the operator's responsibility to look where the Multi-rotor is headed and to save it from any collisions. In some cases such as when the battery ruins out, the operator loses control. For such cases most small scaled Multi- rotors are equipped with a battery voltage level checker which notifies the user of the battery level so that the operator can safely land the Multi-rotor before the power runs out.

In addition to the electronic safety system, the multi-copter designed for this project has extended landing gears made from a tough material. These prevent the rotors from hitting objects as the landing gears are the first component to come in contact with the foreign body. This ensures a mechanical safety for the quad-copter.

7. Aerodynamics:

Since the multi-copter is designed for flight, it is imperative that its design is quite aerodynamic to reduce air drag and to ensure minimum energy losses during flight. The aerodynamics of any multi-copter is dependent almost completely on its design. The main body that connects and supports all the components is mostly designed as such that minimum surface area is achieved. Also in some designs the bodies are curved to allow for smooth flow of air around the multi-copter.

Analyses of the design of the multi-copter:

This design has been inspired from the design of the space ships and UFOs in an effort to make the multi-copter as unique as possible. It is a sandwich design as two metal rings are screwed together by piece of wood in between. It has claw fixed at bottom face of the multi -copter to pick up the object and drop at the designated location.

A circular design is used for the multi-copter in order to be able to fulfill the major requirement of carrying a payload in the middle. The weight of the components at the outer circular rings balances out the moment induced by the added payload in the middle of the quad-copter. As a result a greater stability is achieved. The components used for the design are majorly made of 6061-T6 Aluminum and Pine Wood which ensures a high structural strength for the required purpose. T

The following are the major components of the design of the multi-copter. A short description of the functions and properties of each of the components is provided.

1

.

Ring

A (Top Ring

)

Isometric view of Top ring is as follows:

Thickness: 3mm

Outer Diameter: 475mm

Inner diameter: 425mm

Weight: 340 g

Surface area: 43751 mm2

This is the ring shaped design with the thickness of 3 mm. On this structure, motors and then

propellers are attached. It has 4 sets of screw holes as can be seen in the picture to assemble motor on it. Other than this, there are eight more holes at four different locations in pairs in circular pattern through which long screws are inserted as through it, top ring, wooden support, base ring and landing gear will be assembled.

During the designing, it was brought into the picture that structure does not need to be a complete

ring structure. Therefore, in order to lessen the weight and improve aerodynamics, ring's cross section was reduced.

Aluminum 6061 is used as material and after acquiring the metal sheet, laser cutting machine was utilized to cut the sheet in the required shape.

2. Ring B (Bottom Ring)

Thickness: 2mm

Outer Diameter: 465mm

Inner diameter: 400mm

Weight: 290 g

It is the ring-like structure on to which all the batteries, ESCs and landing gears are attached on. Ring B has many holes as can be seen from the picture. There are 12 holes of 4 mm in diameter and 8 holes with of 3 mm. Out of the 14 holes, 8 holes are drilled to assemble Ring A through the wooden support in between and then the landing port by the long screws inserted from the top. These 8 holes are in pairs in circular pattern. Remaining 4 holes are in pairs at opposite ends to join aluminum pipes by the bottom surface. Smaller holes are drilled to support battery holders.

When the base ring is assembled with the top ring in the SolidWorks software, it was seen as this ring's top surface area may interfere with the performance of high-speed rotation of the propellers. So in order to tackle that problem and to reduce weight, cutting of some meddling part was done. Aluminum 6061 was chosen for material and similar to ring A, laser cutting machine was used to cut the sheet in the required shape.

3. Wooden support:

Length: 50mm

Width: 25mm

Height: 60mm

Weight: 40 g

This is the supporting part which joins both rings together as this is placed between the plates. The wooden piece has been chosen as this damps the mechanical vibrations caused by the motion of propellers and hence, smoother flight is expected.

The design has two straight holes of 3 mm diameter through which long screws are inserted to join both rings together with the help of nuts on the other end of screws. In the whole design, 4 of these are used to support the structure.

4. Landing gear assembly:

The landing gear assembly includes the following components assembled together: • Landing gear supporting rod (material: Aluminum 6061)

• L-linkages (material: Aluminum 6061)

• Landing gear (material: high-density polyethylene)

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The supporting rod is a hollow square cross sectioned rod. Two are made on each end

but on the adjacent faces to connect to the L-linkages. The L-linkages at top attach the landing gear assembly to the side plates through the two vertical holes in the plates mentioned before. Screws pass through the L-linkages and the supporting rod and connect the landing gear assembly to the side plates. Another set ofL-linkages are connected to the supporting rod through the screws passing through the through holes. They are connected to the landing gear through screws again with the screw heads below the landing gear.

The design has been chosen on criteria of high strength. Other than this, the design oflanding port will prevent any sort of damage happening to the main frame as landing ports are widely spread outwards.

5 .Battery holder:

The battery holder serves the purpose of securing the battery during the time of flight. It is made of

Length: 120mm

Width: 40mm

Height: 17mm

Weight: 10 g

3 mm thick acrylic, has enough width and length to properly secure a battery and has one hole on each end which will be used fasten the battery holder to the main frame using spacer bolts and

6. Attachment Pipes:

Length: 460mm

Diameter: 16mm

Weight: 58 g

Pipe structure has been chosen to join structure to the claw base. Two ofthese pipes are in the design.

There are 4 through holes in the middle of the pipe, two of 5 mm and rest of 3 mm. Through these holes, claw mount will be attached by bolts and nuts. Near both ends of the pipe, two more holes (4mm diameter) are also

7. Claw Assembly:

The claw assembly, as shown in the illustration below, is attached to the bottom of the attachment pipes. The claw arms are made of pine wood since this material provides the required strength and light weight for the design. Each arm is attached to a servo-motor mechanism which moves the arms and contracts them again for picking up and holding objects. A camera would be attached to the bottom of the camera base hanging from the claw base. This camera would be used to position the object to be picked up within the jaws of the claw by using feedback from the video and manually positioning the multi-copter using the remote control.

Discussion and Future Plans

The initial objective of the project was to design and manufacture an autonomous multi-copter. It could navigate on a set path using GPS signals and it could prevent collision with an obstruction using proximity sensors which would have been placed at the outer edges ofthe multi-copter. However, this idea alone was not highly applicable and thus the objectives of the project were revised towards a design which would be able to bear loads. This design proposed an application towards the transportation of goods in disaster struck areas. The claw mechanism was designed and added to the multi-copter producing a product which is able to transport loads along with it. This is combined with the input from the two cameras which are seperately used to navigate the multi-copter and position the object between the jaws of the claw. This transition from the first to the second objective led to changes in focus ofthe design from light and acrobatic to strong and stable. The material selection was also affected as stronger and tougher materials were chosen for the design. Eventually, a stable and strong structure was designed and manufactured to serve the purpose of lifting and transporting goods while mainting structural integrity and strength.

It is realised that the automation system can be combined with the current design in the future with a control board having a greater number of inputs. This would allow the quad-copter to carry loads and move them to a designated spot by itself. This would eliminate human input in the whole process as the coordinates of the desired pickup and drop locations would be set using a GPS system. However, a major issue in combining this system with the current design is that the accurate positioning of the quad-copter by itself over the object for pickup is not technologically possible as it depends on a highly accurate GPS system. This could be overcome by using two separate proximity sensors which are equidistant from the point where the object is to be picked up. These would be used to align the multi-copter relative to the object so that it could be picked up by the claw.

References