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Final Report

Engineering 8936

Term 8 Design Project

Autonomous Hovercraft

Dr. Andy Fisher

April 5th, 2012

Colin Abbott 200621837

Blair Hunter 200642791

Colin Oldford 200717783

Michael Simmonds 200617777

Winter 2012

Faculty of Engineering and Applied Science

Memorial University of Newfoundland

Abstract

The task for the Term 8 Mechanical Engineering design project was to design and

construct an autonomous hovercraft. The project group completed comprehensive

research about the theory of a hovercraft and studied the equations that govern the

motion. Through these studies, the critical parameters of the hovercraft were

determined and several components such as the skirt, motors, fans, and sensors were

tested.

From the initial testing of the components, the design specifications for the hovercraft

were stated. The parameters that would govern the design of the hovercraft were

diameter, weight, operational speed, turning radius, acceleration, and deceleration.

Prototype construction then commenced which included designing and building the

body, skirt, and mounts.

When we finally have a prototype built and ready for testing, the critical parameters

(yaw drag, dart effect, inertia, and thrust) can be determined. The determination of

these parameters would allow for better control integration of the control board, sensors,

and logic that control the hovercraft. Further testing of the hovercraft can then be

conducted to fine tune the code by calculating gains.

To aid in the completion of the project, a project schedule was followed. The project was

also limited to a budget of $450. Throughout the progression of the project, certain risks

were identified and also uncontrollable problems were encountered. Future

considerations for the project such as integrating Bluetooth, better sensors, a cover,

increasing prototype size, and adding video surveillance were also considered.

Table of Contents1.0 Project Scope.........................................................................................................11.1 Hovercraft Background...........................................................................................21.2 Governing Equations..............................................................................................31.3 Parameter Study.....................................................................................................52.0 Design Specifications..............................................................................................63.0 Design Selection...................................................................................................103.1 Body and Skirt Material.........................................................................................113.2 Lift Fan..................................................................................................................133.3 Torque Elimination................................................................................................153.4 Thrust Fans...........................................................................................................173.5 Motors...................................................................................................................183.6 Mounts..................................................................................................................204.0 Testing and Parameter Determination..................................................................204.1 Dart Effect.............................................................................................................204.2 Friction Factor.......................................................................................................214.3 Yaw Drag..............................................................................................................214.4 Moment of Inertia..................................................................................................214.5 Motor Thrust & Torque..........................................................................................214.6 Summary..............................................................................................................225.0 System Simulation................................................................................................236.0 Hardware..............................................................................................................266.1 Control Board........................................................................................................266.2 Power MOSFET....................................................................................................276.3 Sensors.................................................................................................................286.4 Battery..................................................................................................................317.0 Software................................................................................................................327.1 Microcontroller Code.............................................................................................328.0 Project Impedance................................................................................................348.1 Non-Uniform Conditions from an Uncontrolled Environment................................359.0 Project Management.............................................................................................369.1 Project Schedule...................................................................................................369.2 Project Budget......................................................................................................36

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9.3 Website.................................................................................................................3710.0 Risks.....................................................................................................................3710.1 Safety....................................................................................................................3810.2 Sustainability.........................................................................................................3810.3 Ethics....................................................................................................................3810.4 Cost......................................................................................................................3810.5 Schedule...............................................................................................................3911.0 Future Considerations..........................................................................................3911.1 Bluetooth...............................................................................................................3911.2 Sensors.................................................................................................................3911.3 Cover....................................................................................................................4011.4 Prototype Size......................................................................................................4011.5 Video Surveillance................................................................................................4012.0 Conclusion............................................................................................................4013.0 References...........................................................................................................42

List of Figures

Figure 1 - Proposed Hovercraft Path................................................................................1Figure 2 - Hovercraft Free Body Diagram........................................................................3Figure 3 - Root Locus Plot...............................................................................................5Figure 4 - Final Hovercraft Prototype.............................................................................10Figure 5 - Rubber Skirt...................................................................................................12Figure 6 - Foam Body....................................................................................................13Figure 7 - Centrifugal Lift Fan........................................................................................14Figure 8 - Baffles on Underbelly of AHC........................................................................16Figure 9 - Thrust Fans....................................................................................................17Figure 10 - Propulsion Motors........................................................................................18Figure 11 - Centrifugal Motor.........................................................................................19Figure 12 - Weathervane Effect.....................................................................................20Figure 13 – Angular Response......................................................................................25Figure 14 - Translation Response..................................................................................25Figure 15 - DFRobot Romeo Controller.........................................................................26Figure 16 - MOSFET Motor Control Circuit Schematic..................................................28Figure 17 - Sensor Placement.......................................................................................31Figure 18 - Batteries.......................................................................................................32Figure 19 - Code Logic Diagram....................................................................................33Figure 20 - Demonstration Path.....................................................................................35

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List of TablesTable 1 - Hovercraft Parameters......................................................................................3Table 2 - Parameter Study...............................................................................................6Table 3 - Summary of Performance Requirements..........................................................9Table 4 - Material Comparison.......................................................................................11Table 5 - Motor Temperatures.......................................................................................19Table 6 – Motor 1 Thrust................................................................................................22Table 7 - Motor 2 Thrust.................................................................................................22Table 8 - Summary of Determined Parameters..............................................................23Table 9 - Gain Calculations............................................................................................24Table 10 – Control Board Key Features.........................................................................27Table 11 - Sensor Comparison......................................................................................29Table 12 - Project Budget..............................................................................................37

List of Equations

Equation 1 - Equation of Motion in x-direction..................................................................4Equation 2 - Equation of Motion in y-direction..................................................................4Equation 3 - Equation of Angular Motion..........................................................................4Equation 4 - Drive Equation.............................................................................................4Equation 5 - Yaw Control.................................................................................................4Equation 7 - Pressure Equation.....................................................................................15Equation 8 - Lift Fan Pressure.......................................................................................15

List of Appendices

Appendix A – Root Locus & Simulink Plots

Appendix B – Simulink Block Diagram

Appendix C – Existing inventory

Appendix D – Existing Hovercraft Pictures and PIC Code

Appendix E – Drawing Package

Appendix F – Parameter Calculations

Appendix G – Control Board Schematic

Appendix H – IRL-520 Power MOSFET Data Sheet

Appendix I – Sensor Data Sheets

Appendix J – Detailed Microcontroller Code

Appendix K – Project Schedule

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Engineering 8936 Term 8 Design Final Report

1.0 Project Scope

The objective of this project was to design and construct a fully functioning autonomous

hovercraft. An autonomous hovercraft could be used in a variety of environments and

situations. It could be used to locate and investigate something suspicious, perform

inspections or be used to locate missing persons in environments that are not safe for

humans.

For our design project, we aimed to design a hovercraft that can autonomously navigate

a previously defined path and return to the launch site. We initially chose to define a

path in the bottom floor of Memorial University’s Engineering building. Figure 1 contains

the proposed path for our hovercraft.

Figure 1 - Proposed Hovercraft Path

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Autonomy for the purposes of this design project was defined as the ability to navigate a

preset path on its own with no input from an operator. The hovercraft is capable of

avoiding walls, identifying corners, navigating 900 turns, and stopping at the end its

mission. The predefined path is a controlled environment selected by the project team,

which is free of disturbances such as obstacles, slopes or wind.

1.1 Hovercraft Background

A hovercraft is a vehicle that floats or hovers on a cushion of pressurized air. This

pressurized air is achieved by the use of a lift fan. The lift fan pushes a large volume of

air under the hovercraft which is prevented from escaping by the skirt. The pressure

under the hovercraft increases until it exceeds the weight of the hovercraft. Eventually,

the hovercraft raises high enough to lift off the ground. Air begins to escape through the

gap between the ground and the skirt called the hover gap. The skirt is a flexible barrier

attached to the perimeter of the hull that functions to contain the cushion of pressurized

air under the hovercraft and to increase the hover height of the hovercraft.

The hovercraft reaches an equilibrium point at which the amount of air being forced into

the cushion cannot exceed the amount of air escaping through the hover gap. At this

point, the hovercraft is hovering at its maximum hover height. The hover height and

hover gap can be adjusted through the design process.

Once the craft is hovering, thrust fans can be used to provide the propulsion for the

hovercraft. They are typically mounted on the rear of the craft. The hull of the hovercraft

is the main body on which all hardware is stored. Care must be taken to have proper

weight distribution of the hovercraft.

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1.2 Governing Equations

In order to start an analysis of the hovercraft, the project team first defined the

governing equations of the hovercraft. A free body diagram of a typical hovercraft is

shown in Figure 2. The parameters are described beneath in Table 1.

Figure 2 - Hovercraft Free Body Diagram

Table 1 - Hovercraft Parameters

Symbol Parameterα Angle HeadingβAngle o Velocity DirectionE ThrustF FrictionT TorqueM MassI Moment of InertiaJ Yaw Drag

K Weather Vane Effect Coefficient

X Prop CoefficientY Prop Coefficient

Z Coefficient of Friction

The following equations govern the motion of the hovercraft.

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Equation 1 - Equation of Motion in x-direction

Equation 2 - Equation of Motion in y-direction

Equation 3 - Equation of Angular Motion

Q is the control signal for yaw control. The following equations describe the drive of the

hover craft and its controls.

Equation 4 - Drive Equation

Equation 5 - Yaw Control

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By manipulating these governing equations and using Laplace transforms, simulations

can be used to predict the autonomous hovercraft’s (AHC) motion for a given command.

1.3 Parameter Study

To begin the design process, simulations of the hovercraft based on its equations of

motion were used. By doing so, the influence of different parameters on the hovercrafts

motion could be determined. This information was used in the design to either minimize

or maximize certain parameters based on whether they provide positive or negative

characteristics such as: level of damping, speed of response and stability. In particular,

the motion of the hovercraft performing a 90o turn was studied as the goal of our control

system is to accurately turn the hovercraft.

Two methods were used for the parameter study: Root Locus Plots and Simulink

Simulation. The Root Locus plots were used to assess the affect of individual

parameters while a Simulink Simulation was used to verify the behavior as well as to

show the motion of the hovercraft. Interpretation of the root locus plots is shown in

Figure 3.

Figure 3 - Root Locus Plot

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The full set of Root Locus plots and Simulink output can be found in Appendix A. The

derivation of the block diagrams and the completed Simulink simulation can be found in

Appendix B. The summary of the findings of our parameter study are shown in Table 2

Table 2 - Parameter Study

Variable Effect Design ConsiderationsMass: M Faster response more

damping for smaller valuesLightweight

Moment of Inertia: I Faster response for smaller values, damping effect

negligible

Mass located to reduce moment of inertia

Yaw Damping: J Response speed decreases with increasing values.

Damping only significant for J = 0.

Low yaw damping desirable

Dart Effect: K Unstable for negative values, slower response for

increasing values, negligible damping

Balance mass as well as possible, front heavy is

unstable, back heavy is stable but reduces response time

Prop Torque: Y Poor damping at low values, negligible response speed until a critical value where

response speed decreases

Ensure sufficient torque available for fast response,

control system design to compensate for poorer

dampingFriction: Z Faster response, more

damping at higher valuesHigh friction desirable

2.0 Design Specifications

There were several considerations that had to be taken into account during the design

phase of our hovercraft. These specifications set goals for the design of the AHC, and

were based on the results of the parameter study. The following are the design

specifications that were outlined by the design team:

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• Sizing Restrictions

Previous hovercraft designs include having two skirts as well as having a

rounded rectangular shape. The body should be no larger than 20 inches in any

direction to limit the moment of inertia. The final hovercraft size is a circle with a

diameter of 15.5 inches.

• Weight Restrictions

Lighter hovercraft designs are preferred to minimize power requirements and to

ease transport and improve responsiveness. The design team used lightweight

materials while using the minimum amount of material required for sensor and

battery mounts as well as the main body. The weight of the hovercraft was limited

to a maximum of 5 kg. The final weight of the hover craft was 1.5 kg.

• Weight Distribution

The weight is to be distributed equally and as close to center as possible. Equal

distribution ensures air escapes uniformly around the skirts circumference as well

as reduces the dart effect and the moment of inertia.

• Endurance

Since this is a proof-of-concept design, the prototype will require a maximum

endurance for testing purposes. Therefore, a minimum endurance of 1 hour is

required for testing purposes. This is determined by the size of battery we

choose to use based on the motor current draw.

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• Control

The hovercraft is required to be autonomous. A DFRobot Romeo Controller

which uses the Arduino open source language is used to interpret sensor

readings and control the hovercraft.

• Hover Height

The hover height of the platform determines the height of the obstacles that the

platform can traverse, hence higher hover heights are advantageous to the

application as it expands the range of operating environments that the hovercraft

can be applied to. The trade off of using a higher hover height is the reduction of

friction which causes the hovercraft to become more unstable and susceptible to

small disturbances. A balance between hover height capable of moving over

obstacles without reducing the friction too substantially was desired.

• Power Requirements

Batteries are used to power the motors, board, and sensors. Separate batteries

are used for the control board and motors due to the difference in required

voltage. The power source should be limited to a 9 volt battery to power the

board and an 11.1 volt rechargeable battery to power the motors. These voltages

were chosen because of the availability of batteries for those voltages.

• Test Environment

The test environment represents the ideal operating environment for the

hovercraft which is a static environment away from inputs that will degrade the

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performance of the craft in operations. Such an environment will provide minimal

drag, and as such will be inside, on a flat, smooth tile surface, away from wind

and other weather effects.

• Maneuverability Requirements

The hovercraft was required to lift off the ground to the appropriate hover height

and also capable in maintaining a stationary position while hovering. The

platform was required to accelerate to its operational speed and continue forward

while maintaining a straight course. The operating speed was defined as walking

speed that is 4-6 km/hr. The platform should accelerate to this speed within 10

seconds. The hovercraft is to be capable of pivoting in yaw resulting in giving a

maximum turning radius of 2 times the diameter of the hovercraft body. A braking

maneuver is required to bring the platform to rest from operational speed and into

the stationary position. Deceleration to rest should be within 10 seconds.

Table 3 summarizes the performance specifications for the design project and the final

design.

Table 3 - Summary of Performance Requirements

Parameter Specification Final ParameterDiameter < 20 inches 15.5 inches

Total Weight < 5 kg 1.5 kgNominal operational Speed ≈ 5 km/hr 4.5 km/hr

Turning Radius < 2x Diameter < 15.5 inchesAcceleration > 0.14 m/s^2 0.2 m/s^2Deceleration > 0.14 m/s^2 0.2 m/s^2

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3.0 Design Selection

Figure 4 contains the picture of final prototype of our hovercraft. Using the specifications

and existing inventory, the project team was able to build this prototype. Appendix C

contains an inventory of the existing materials the design group was given access to.

Figure 4 - Final Hovercraft Prototype

The project team had access to previous prototypes that were built for this design

project. The team decided to take the design approach of studying these hovercrafts

and developing the team’s unique hovercraft design from these studies. Pictures and

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sample PIC code for the previous hovercrafts are contained in Appendix D. Web

searches turned up no useful information that could be applied to the design project.

The design considerations for the hovercraft include the body and skirt material, lift fan,

thrust fan, and motors. Appendix E contains the completed drawing package showing

the prototype construction.

3.1 Body and Skirt Material

Due to the critical nature of the skirt, its design played an important role in the future

success of the AHC. The main considerations for the skirt included coefficient of friction

between the material and the surface (the AHC was designed for tile floor), weight of

the material and the flexibility of the material (i.e. Young’s modulus). Three materials

were selected to be tested for the skirt of the AHC; inflatable rubber inner tube, high

density foam, and fabric. Table 4 shows the key properties of the materials considered

for the skirt of the hovercraft.

Table 4 - Material Comparison

Weight (g)Relative

coefficient of friction

Young's Modulus (GPa) Characteristics

Foam 16.474 Highest 3.5 Poor Seal due to high material stiffness

Fabric 9.290 Low N/A Difficult to control due to low friction

Rubber 34.292 High 0.1Well maintained seal, friction

provides good damping for ease of control

The rubber skirt was considered due to the ability to quickly change the air pressure in

the tube and thus the coefficient of friction to fine tune the handling characteristics. An

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increase in friction would increase the damping during a turn but will decrease the top

speed as well as the speed of response to commands. The rubber skirt is also capable

of creating a consistent seal and lift. It also offered a stable platform for the hovercraft

even when the lift fan was not engaged. One negative of the rubber skirt were that is

was heaviest of the 3 skirts tested. Figure 5 contains a picture of the rubber skirt.

Figure 5 - Rubber Skirt

The selection of the body material and shape were then based on the selection of the

rubber skirt. The shape of the body had to be round to accommodate the rubber inner

tube, otherwise pinch points would be created by deforming the inner tube, and the seal

with the ground would be lost. The material that was selected for the body was rigid

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polystyrene foam due to its easy formability, light weight and rigidity. Figure 6 shows a

picture of the foam body.

Figure 6 - Foam Body

3.2 Lift Fan

The next design consideration that the design group focused on was the lift fan. The lift

fan had to be able to create enough pressure under the body and skirt to allow for the

AHC to hover. The project team had access to numerous pre-manufactured centrifugal

fans which were used to decrease project costs. A picture of the selected centrifugal fan

is shown in Figure 7.

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Figure 7 - Centrifugal Lift Fan

Appendix E contains an isometric drawing of the centrifugal lift fan (DRAWING

NUMBER). These fans increase the speed of air stream with the rotating impellers.

Centrifugal fans accelerate air radially, changing the direction (typically by 90°) of the

airflow. They are sturdy, quiet, reliable, and capable of operating over a wide range of

conditions. In comparison to axial flow fans, the centrifugal fan produces more pressure

per flow rate and is not susceptible to stalling. Equation 7 gives the total pressure

needed to lift the hovercraft. The area (A) of the hovercraft is 0.1217 m 2. The total

weight force of the hovercraft is 14.715 N.

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Equation 6 - Pressure Equation

The total pressure needed to be created by the centrifugal lift fan is 120.9 Pa. One can

estimate the required tip speed of the centrifugal fans by using Equation 8.

Equation 7 - Lift Fan Pressure

Using the density of air (ƿ = 1.2 Pa) and the pressure of the hovercraft, the required tip

speed of 10 meters/second.

Once the tip speed was known of our lift fan, tests of the existing centrifugal fans on the

available motors were conducted which measured the rotational speed of each fan at a

max voltage of 11.1 V. Based on the achieved rotational speed it was determined which

centrifugal fan had a large enough radius to achieve the required tip speed. One of the

available lift fans proved to be capable of providing sufficient pressure. This was

advantageous because the project team wanted to use as much of the existing

inventory as possible. Appendix E contains an isometric drawing of the centrifugal lift

fan (DRAWING NUMBER).

3.3 Torque Elimination

The rotation of the lift fan creates a counter torque which is transmitted to the body of

the hovercraft, causing it to rotate on its own. This created difficulty in controlling the

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hovercraft to both move in a straight heading and while cornering. Two options were

considered to cancel out the torque:

Offsetting the motors to cancel the torque

Installing baffles to extract momentum from the airflow to create a counter torque

The number of baffles, as well as different baffle shapes was tested. As the baffles

remove momentum, the amount of pressure generated diminishes. The air pressure

was measured using a pressure gauge and the effects of different baffle shapes and

number of baffles on the reduction of pressure was studied this way. Using two curved

shape baffles were found to eliminate most of the torque without noticeably changing

the amount of lift. The remaining torque was eliminated by offsetting the motor voltage.

Figure 8 contains a picture of the baffles.

Figure 8 - Baffles on Underbelly of AHC

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3.4 Thrust Fans

The project team was also able to use existing propellers from the inventory. Figure 9

shows a picture of the thrust fans.

Figure 9 - Thrust Fans

These fans are placed on the rear of the hovercraft to allow for forward propulsion. In

order to turn and avoid walls, the rotational speed is reduced in one and kept the same

in the other to create torque which causes the yaw motion.

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3.5 Motors

There are three motors that are incorporated into our AHC which were selected to

match the voltage supplied by our battery. The motors selected are rated for a

maximum of 12V. Figure 10 contains a picture of the propulsion motors.

Figure 10 - Propulsion Motors

The lift fan motor has the additional requirement to run the centrifugal lift fan at a

constant RPM for an extended period of time without overheating. It also had to have

the required clearance to not remain completely inside the AHC skirt. Three motors

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were tested with temperature readings taken from each after running at peak RPM for

10 minutes at 11.1 Volts. The temperature readings for all three motors tested can been

seen in Table 5, these temperature readings are taken on the outer case of the motor

utilizing an infrared temperature gun. The motor temperature must be kept below 150°C

to ensure proper operation of the motors. Figure 10 contains a picture of the motor that

drives the centrifugal fan.

Table 5 - Motor Temperatures

Temperature (oC)Motor 1 104Motor 2 83Motor 3 61

Figure 11 - Centrifugal Motor

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3.6 Mounts

Components are mounted using Velcro, adhesive, screws, bolts and mounts made from

sheet metal. Aluminum sheet metal is attached to the foam base to allow for the mount

of components. Appendix E contains the isometric drawings for the mounts.

4.0 Testing and Parameter Determination

In order to perform an accurate simulation of the constructed hovercraft, the project

team had to undertake rigorous testing to determine the parameters of the governing

equations. The parameters that we needed to determine were friction, dart effect, yaw

drag, and moment of inertia. Appendix F contains details of calculations.

4.1 Dart Effect

The dart effect is the phenomena of a hovercraft to have a moment created about a

natural pivot point in a crosswind, such as one created by turning. The moment is

created by an imbalance of air drag acting about the pivot point. This is shown in Figure

12.

Figure 12 - Weathervane Effect

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For the AHC, a pivot point towards the front would create a moment which would rotate

the hovercraft in the turning direction. By balancing the weight of the hovercraft, the dart

effect can be made to be zero by forcing the pivot point to the exact center, causing the

drag forces to cancel out. Having a dart effect coefficient of zero causes friction to have

no effect on cornering. We can make the centre of mass of the hovercraft in the centre

of the base by carefully balancing out all of the hardware.

4.2 Friction Factor

Due to the dart effect coefficient being zero, the friction factor did not need to be

determined. A friction factor of 0.5 was assumed for the Simulink simulation.

4.3 Yaw Drag

To determine the yaw drag, a tachometer was used to measure the speed that the

hovercraft rotated in one spot from a constant known input torque created by running a

single drive motor.

4.4 Moment of Inertia

The moment of inertia was calculated by approximating the hovercraft as a cylinder and

components as point masses.

4.5 Motor Thrust & Torque

Our thrust versus voltage characteristics from the motors was determined using a

simple weight scale. Based on the collected data, a linear fit was calculated. Using the

measured thrust, the amount of torque produced was found by multiplying the thrust by

the measured distance to the center of the AHC. Tables 6 and 7 show the measured

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motor thrust. The difference in thrust between the motors was utilized to aid in the

cancelation of the lift fan torque.

Table 6 – Motor 1 Thrust

Motor 1 Test 1 Test 2 Average AverageV Thrust (g) Thrust (g) Thrust (g) Thrust (N)

11.1 51 51 51 0.500

10 42 41 41.5 0.4079 34 34 34 0.3348 27 26 26.5 0.2607 20 19 19.5 0.1916 13 13 13 0.1285 9 9 9 0.0884 5 5 5 0.049

Table 7 - Motor 2 Thrust

Motor 2 Test 1 Test 2 Average AverageV Thrust (g) Thrust (g) Thrust (g) Thrust (N)

11.1 45 45 45 0.441

10 38 38 38 0.3739 30 30 30 0.2948 23 23 23 0.2267 18 18 18 0.1776 12 12 12 0.1185 8 8 8 0.0784 0 0 0 0.000

4.6 Summary

Table 8 contains a summary of the parameters that were determined.

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Table 8 - Summary of Determined Parameters

Parameter Tested ValueFriction Factor (Z) 0.5 (assumed)

Dart Effect (K) 0 Nm/radian

Yaw Drag (J)0.002

Nm/radian/sMoment of Inertia (I) 0.0018 kgm^2

Mass 1.5 kg

5.0 System Simulation

Using the governing equations for the hovercraft’s motion and the AHC tested

parameters we were able to study its motion. MATLAB Simulink was utilized to model

the AHC translation and angular movements by creating a block diagram derived from

the governing equations. We were able to use our Simulink simulation template to

determine the optimal proportional, integral and derivative (PID) gains for our system to

turn 90o. This also allowed us to further study the angular motion and translation motion

of the hovercraft.

The Zeigler Nichols method was used to determine PID gains. This method is based on

finding the proportional gain Kp and period Tp, that causes the system to become

borderline stable and enter oscillations. Using various sets of rules, different PID gains

can be determined each of which give different performance. The borderline gain was

found using a trial and error method in the Simulink simulation. It is possible to

experimentally determine the borderline gain and measure the periods of the

oscillations. This was not feasible for our hovercraft as the variations in compass

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readings due to the buildings magnetic field would interfere with our ability to have the

hovercraft enter stable oscillations.

Various PID gains were tested in Simulink, it was found that the Pessen Integral Rule

gave the fastest response with the least amount of overshoot. A modification of this by

removing the integral term improved the response further. While removing the integral

term improves the response, our system will not be able to overcome disturbance

torques, such as counter torques created by the lift fan. However we are able to control

and eliminate disturbance torques therefore, the integral term is not required. It was also

determined that reducing the proportional term eliminated overshoots and improved the

speed of response. Changing the derivative term gave poorer response and was left as

calculated. Table 9 below summarizes the PID rules that were tested with the selected

PD gains shown as well.

Table 9 - Gain Calculations

Gain RulesType Kp Ki KdPID 0.6*Kp 2*Kp/Tp Kp*Tp/8

Pessen Integral Rule 0.7*Kp 2.5*Kp/Tp 0.15*Kp*TpSome Overshoot 0.33*Kp Kp/Tp Kp*Tp/3

No Overshoot 0.2*Kp 2*Kp/Tp Kp*Tp/3Calculated Gains Kp=7 Tp=4.7

PID 4.2 1.8 2.5Pessen Integral Rule 4.9 2.6 3.5

Some Overshoot 2.3 0.5 0.8No Overshoot 1.4 0.6 2.2Selected Gains 1.5 0 3.5

The angular response for the selected PD gains is shown in Figure 13 below.

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Figure 13 – Angular Response

Translation response is shown in Figure 14. This graph predicts that the turning radius

of the AHC will exceed the specified radius. Control over the lift fan was utilized to

reduce the turning radius, which is discussed further in the next section.

Figure 14 - Translation Response

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6.0 Hardware

6.1 Control Board

The final control board selected for this project is the DFRobot Romeo controller. This

board uses an Atmega328 microcontroller with an integrated L298 motor driver. It uses

the Arduino open source programming language. A schematic can be found in

Appendix F. Figure 15 contains a picture of the control board.

Figure 15 - DFRobot Romeo Controller

Two other control boards, a PIC based board and an Arduino, were also assessed.

Advantages of the Romeo controller over the other two is summarized below:

Arduino programming language is easier to use in comparison to C used

by the PIC board

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The Romeo is readily available at low cost in comparison to designing,

fabricating a new PIC based board

The Romeo board has a integrated motor driver, the Arduino board

requires a separate motor driver to be purchased

Romeo has 3 pin connections for I/O pins (5V, GND, Signal), which allows

for easier interfacing and power management for sensors.

Large online community and resources to aid in interfacing with sensors

The Romeo board has greater flexibility for future additions with ports

available for Bluetooth, GPS and servo motors

A summary of key features of the control board is shown in Table 10.

Table 10 – Control Board Key Features

Microcontroller Atmel Atmega 328Microcontroller Operating Voltage 5VDigital I/O Pins 14 (6 PWM)Analog Input Pins 8Communication UART TTL, I2C, SPICPU 8-Bit AVIAnalog to Digital Conversion 10-BitInput Voltage 7-14V (Regulated for microcontroller)Motor Driver L298Motor Control 2 Motors, Direction & SpeedMotor Amperage Max 2 Amps

6.2 Power MOSFET

To allow for more flexibility in controlling the hovercraft, an IRL-520 Power MOSFET,

controlled by a pulse with modulation (PWM) pin, was used to control the lift fan speed.

For our purposes, the lift fan was stopped prior to turning, this reduced the turning

27


radius by eliminating the hovercraft’s forward momentum it had entering the turn.

Reducing the amount of lift could also be used to increase the amount of yaw damping

while turning for better control, however this was not beneficial for our hovercraft. A

schematic of the MOSFET motor control circuit is shown in Figure 16.

Figure 16 - MOSFET Motor Control Circuit Schematic

The MOSFET acts as a switch. When a voltage from the control board is applied to the

G pin, the circuit is completed. When no voltage is applied, the circuit is broken. By

using a PWM, the voltage supplied to the motor can be varied. Appendix H contains the

data sheet for MOSFET.

6.3 Sensors

There are two categories of sensors that were tested during this project, distance and

angle sensors. Distance sensors were utilized to detect walls to determine when a

corner was ahead, and to move away from walls parallel to the hovercraft’s direction of

motion. These included Infrared (IR) sensors and sonar sensors. Angle detecting

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sensors were used to corner and maintain a straight heading, this category included

digital compasses and a gyroscope. A number of sensors were available in the existing

inventory, while additional sensors were ordered on a trial basis to see if they provided

better performance over the sensors already available. Advantages and disadvantages

of the various sensors based off experience are shown in Table 11. Appendix I contains

the data sheets for the various sensors explored during this project.

Table 11 - Sensor Comparison

Category Name Specs Source Advantages Disadvantages

Distance

Short Range IR Sensor

10-80cm Sensing Range

Prof. Hinchey Consistent Performance

Non-Linear Output

Characteristic

Long Range IR Sensor

20-150 cm Sensing Range

Prof. Hinchey Consistent Performance

Non-Linear Output

Characteristic

EZ2-Sonar Range Finder

15-645 cm Sensing Range

Robotshop.ca Long Range Poor Consistency

Angular

CMPS03 Compass

0.1o

Resolution, ±4o Accuracy

Prof Hinchey

Built in Tilt Compensation,

Easier Integration

Affected by Buildings

Magnetic Field

HMC6352 Compass

0.5o

Resolution±2.5o

Accuracy

Robotshop.ca Good Accuracy No Tilt Compensation

IXZ-500 Gyroscope

Dual Axis, ±110o/s Robotshop.ca N/A

Subject to Drift, Poor Accuracy & Consistency for

Angle Calculation

The final sensor selection was:

Long Range IR Sensor mounted on front for wall detection to signal an upcoming corner

Two side mounted Short Range IR Sensors for wall detection and avoiding

CMPS03 Compass for cornering

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The long range IR sensor replaced the sonar module as the sonar module was shown

to have poor consistency during field trials. This would lead to false wall detections

causing the hovercraft to turn before reaching a corner. The sonar sensor was initially

selected due to its long range. This was required to detect upcoming walls at range far

enough to decelerate to a full stop as specified. Adding the capability to control lift fan

allowed the AHC to stop within the sensing range of the long range IR sensor. IR

sensors were shown to be far more consistent for wall detections and do not give false

readings. The non-linear characteristic of the IR sensors is not an issue as these

readings are not fed into a control loop (like the compass).

The CMPS03 compass module replaced the HMC6352 compass as while the

HMC6352 has slightly better accuracy and performance inside the building’s magnetic

field, this was negated by its lack of tilt compensation. The coding required to read the

CMPS03 is also simpler and with the advantage of tilt-compensation it was decided to

use this compass module. The gyroscope was eliminated completely for two reasons.

The gyroscope did not provide the accuracy or consistency required to maintain a

proper heading down a hallway. In addition, the short range IR sensors actually

performed exceptionally well in keeping the hovercraft centered in the hallway. As a

result, it would be redundant to include the gyroscope and add unnecessary complexity.

Figure 17 contains the final placement of sensors on the AHC.

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Figure 17 - Sensor Placement

6.4 Battery

A power supply was required that was reliable and portable such as batteries. We had

different options for the batteries such as Lithium Polymer (LiPo) or Nickel Metal hydride

(NiMh). We have access to these batteries in various sizes as well but will be using

2200mAmh, 11.1V LiPo because they supply sufficient power as well being light and

easy to use. We are also utilizing low voltage detectors as well as fuses to protect our

power system. A standard 9V battery is used to power the Romeo control board to

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prevent the onboard voltage regulator from being overpowered. Figure 18 contains a

picture of the batteries used.

Figure 18 - Batteries

7.0 Software

7.1 Microcontroller Code

The purpose of the microcontroller code is to read and interpret sensor readings and

make decisions based on those to navigate our selected path. We have broken down

our selected path into three basic commands, left or right turn and stop, which are

numbered. A command variable will be used to determine the order in which commands

are followed. After the completion of a command the variable has its value increased by

one. For example, when the command variable is a five or a six, the hovercraft will turn

left, as shown in Figure 1. A simplified logic diagram outlining the code is shown in

Figure 19. Appendix J contains the detailed microcontroller code.

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Figure 19 - Code Logic Diagram

The code was developed in increments, with each step increasing its complexity and

adjustments made to improve the logic and consistency. Significant use was made of an

LCD screen and the serial monitor built into the Arduino software to view sensor

readings and debug code. The development was conducted in the following steps:

1. Testing and verification of sensor readings: Determined consistency, accuracy,

effects of close proximity to motors

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2. Basic wall avoidance: Comparison of using while loop versus an if statement. It

was determined using while loops to maintain a distance from a wall made large

adjustments and caused an uncontrollable zig-zag motion. The if statement

makes numerous small corrections, leading to more gentle, gradual adjustments.

3. Cornering & command logic: Tested upcoming corner identification and control

signal calculations, ability to perform right or left turns and stop in the proper

sequence.

4. Integration of wall avoidance, cornering & command logic: Rigorous testing of the

AHC was completed. To eliminate walls parallel to the AHC motion being

detected by the long range IR sensor, the two consecutive readings requirement

was introduced, with the readings being reset when adjustments towards the

hallways center are made.

5. Operation Modifications: Threshold values for wall detection, Kp and Kd gains

and amount of torque used for avoidance were optimized to give the best

performance. The need to stop the lift fan to improve control and reduce the

turning radius was also identified and implemented.

8.0 Project Impedance

The project team encountered many problems that were uncontrollable. These

problems were non-uniform conditions from an uncontrolled environment and sensors.

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8.1 Non-Uniform Conditions from an Uncontrolled Environment

In the design specifications, the test environment was described to be the ideal

operating environment for the hovercraft which is a static environment away from inputs

that will degrade the performance of the craft in operation. These inputs are such things

as uneven floors and wind drag.

The actual test environment contained uneven floors, slopes, dips, and high spots which

would eliminate the pressure cushion and cause the hovercraft to stop. To try and

overcome this problem, bigger batteries were used and also increasing the power

supplied to the motor. These fixes were inconsistent and would sometimes work and

other times not. This proved to be a big frustration for the group and numerous test

areas were tried. We finally found a hallway that was relatively smooth, however it is not

perfect. Figure 20 contains the picture of the path taken during the demonstration.

There are still some imperfections that tend to cause problems.

Figure 20 - Demonstration Path

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9.0 Project Management

9.1 Project Schedule

In order to complete this project on time, the project group developed a schedule to

follow by using a Gantt chart. The updated Gantt chart is contained in Appendix K. The

project team adhered very closely to the schedule and made updates weekly and even

daily. The project team met daily to discuss the project progress and deal with any

issues that occurred. The first item on the agenda for any team meeting was the review

of the project schedule. Each team member was assigned tasks that correspond to

items in the project schedule. Each project member was able to choose the tasks that

matched their personal skills best. The project team met weekly with our supervisor to

keep him updated with the project progression and to ask any questions that any project

team member may have. We also communicated any concerns or questions amongst

the team on a daily basis to keep the time frames outlined in the project schedule.

9.2 Project Budget

The total budge allotted for this project consisted of $250 from the Engineering Faculty

with an additional $200 from group member contributions. To reduce the amount of

money spent parts from previous attempts at making a hovercraft were used which

diminished the amount of new parts that were purchased. Table 12 contains a

breakdown of the final budget, contingency funds and purchases. The total project cost

was $191.40.

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Table 12 - Project Budget

Starting BudgetFaculty of Engineering $ 250.00

Group Members $ 200.00 Total Budget: $ 450.00

Costs to Date

Sonar Range Finder $ 26.89 Gyroscope $ 41.19

Digital Compass $ 37.11 DF Robot Microcontroller $ 37.11

Propellers $ 6.00 Motors $ 12.00

HST/S&H $ 31.10Total Project Cost $ 191.40 Remaining Funds $ 258.60

9.3 Website

The project team maintained a website that contains all of the technical documentation

pertaining to the project. Our website is www.autonomoushovercraft.yolasite.com. It

contains the meeting minutes from our supervisor meetings, videos and pictures of

progress, project schedule, source code, and results of all engineering analysis and

simulations. All of the work completed discussed in this project report is contained on

the project website. The website was maintained throughout the whole life of the

project.

10.0 Risks

The risks for this project were broken down into 5 sections; safety, sustainability, ethics,

cost and schedule.

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10.1 Safety

Due to the nature of the hovercraft, there were some common areas of concern when it

came to the safe operation of the vehicle. The first safety hazard presented was the

possibility of the main propulsion fans causing bodily harm. To mitigate this risk,

protective covers will be installed to avoid the possibility of damage to the surroundings.

The lift fan was also identified as a possible safety hazard. This risk was mitigated by

the introduction of a protective screen cover of the fan intake port. As we are using

batteries for this project, electrical shock or electrical fire is also a risk. To eliminate this

risk, all electrical sources will be properly grounded and any exposed leads will be

properly isolated prior to the energization of the system.

10.2 Sustainability

There were two main risks presented with respect to environmental concerns. These

were the usage of lithium polymer batteries to run the hovercraft and the overuse of

materials. To mitigate the risk of environmental damage due to the LiPo batteries, a

proper disposal process must be followed. To reduce our risk of overuse of materials,

the hovercraft was designed in Solidworks to eliminate any excess material and as well,

to keep the number of prototypes constructed to a minimum.

10.3 Ethics

There are no ethical risks associated with this project.

10.4 Cost

The risk of cost overrun was a possibility and could have been detrimental to the

completion of the project. To mitigate this risk, a detailed budget was constructed which

38


included all aspects of the vehicle. A contingency fund is also included in the budget to

allow for limited cost overruns.

10.5 Schedule

To mitigate the risk of schedule overruns and delays, milestones were selected for the

project and weekly meetings with our project sponsor determined if we were meeting

our preset milestones. Extra time was allotted within the schedule to account for

shipping delays of components.

11.0 Future Considerations

After the completion of this design project, there are several future considerations that

can be explored if this project was to be continued. These considerations include adding

Bluetooth, better sensors, covering the AHC, making a bigger prototype, and having

video surveillance.

11.1 Bluetooth

In addition to the autonomous code, one can also implement Bluetooth onto the

hovercraft to allow for remote control. This could be useful given the situation it is being

used in. With the use of Bluetooth, different paths can be switched between while it is in

use. It would also give the option to allow for emergency start and stop of the hovercraft.

11.2 Sensors

With more time and budget, better sensors can be implemented onto the AHC to allow

for better handling and also full autonomy. Such sensors that could be implemented are

39


a GPS to allow for better positioning and turning, 3D Laser range scanner for better wall

following, and a better gyroscope for better turning and heading control.

11.3 Cover

Another future consideration for the autonomous hovercraft would be to develop a cover

to encase the board, sensors, and wires. Aerodynamic studies can be undertaken to

minimize the drag on the hovercraft and can also be made waterproof to allow for travel

over water.

11.4 Prototype Size

Increasing the size of the hovercraft can allow for travelling greater distances by

increasing the number of batteries and also to potentially carry payloads. In order

develop a bigger prototype, motors and fans would need to be scaled up.

11.5 Video Surveillance

The final future consideration is to integrate video surveillance into the AHC. This, along

with integrating the Bluetooth, can be a worthwhile venture. A video camera would allow

one to be able to see actually what the hovercraft is seeing. Additionally, integrating the

control of the camera with the control board to record at particular times could be

considered.

12.0 Conclusion

All in all, this final design project has been an invaluable learning experience. It not only

taught us more about developing and engineering design from beginning to end but it

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also gave us the confidence in being able to design something and have it accomplish

what we wanted it to accomplish.

Achieving complete autonomy is a difficult task to accomplish. It was realized that it was

important to set specific goals to achieved based on the limits imposed; in this case,

these included the budget available to purchase sensors and time allotted to produce an

operational prototype. We feel we produced a prototype that met our expectations as

well as showed an improvement over previous attempts at an autonomous hovercraft

within the Faculty of Engineering at Memorial University. We were able to learn from

past mistakes and feel the progress we have made can attribute to the future successes

of other design groups if this project is undertaken again. In the end, this was a good

learning experience for the group because it taught us how to trouble shoot to be able to

figure out problems and also keep a tight schedule. Overall, we all had fun completing

this project and will carry these memories for ever.

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13.0 References

Beardmore, Roy. Root Locus Methods. Ed. Roy Beardmore. N.p., July. Web. 17 Jan.

2012. <http://www.roymech.co.uk/Related/Control/root_locus.html>.

Kalpakjian, Serope, and Steven Schmid. Manufacturing Engineering and Technology.

Fifth ed. Upper Saddle River: Pearson Education, 2006. N. pag. Print.

Rethwisch, David G. Materials Science and Engineering An Introduction. Seventh ed.

New York: John Wiley & Sons, 2007. Print.

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