Introduction
Whether it is for space exploration, research in toxic environments, or military applications, there
exists a necessity for unmanned autonomous robotic vehicles. Such vehicles are often required to navigate
complex terrain often without any user inputs. Since they are often unreachable by human beings
integrated surface vehicle systems must either be able to generate enough power to complete their tasks,
or must be able to conserve enough energy to do so. Another requirement for these systems is some sort
of sensory input in order both navigate and accomplish mission objectives. Depending on mission criteria
a number of sensors and sensor types may be required.
The purpose of this report is to compare current vehicles of this type in use today and to analyze
their systems in order to design a vehicle that can meet a given set of mission objectives. Analysis will
include construction materials, power systems and battery types, methods of locomotion, sensors, drivetrains, control methods, and on board computer systems.
Current Designs
There are many different existing designs for multi-terrain autonomous vehicles with a wide
range of features designed to fulfill a variety of needs. Some operate on other planets. The pinnacle
examples of these vehicles are the Mars Explorer Rovers (MER). The MER were built by NASA’s Jet
Propulsion Laboratory 2003. They had to be robust since repair was not an option. They also had to be
able to traverse or avoid all obstacles presented by the Martian surface. For this it utilizes a six-wheeldriven, four-wheeled-steered rocker-bogey design. [1]
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Figure 1: MER [1]
Another example is the U-Go robot developed by DIEES Robotic Laboratories. The main goal of
this robot is “to solve problems like transportation, navigations, and inspection in very harsh outdoor
environments.” It was designed to operate autonomously for up to 8 hours, carry up to 200kg, and move
on different types of terrain without creating too much impact for agricultural purposes. For autonomy, it
features an array of sensors including GPS, laser scanner, stereo cameras, bumpers, and ultrasound
sensors. For tackling the demands of motion it features a 650W motor and rubber track drive system. [2]
Figure 2: U-Go robot [2]
There are also a number of legged examples of all-terrain robotic vehicles such as Boston
Dynamics BigDog. The BigDog is said to have about 50 sensors. “Inertial sensors measure the attitude
and acceleration of the body, while joint sensors measure motion and force of the actuators working at the
joints…Other sensors monitor BigDog’s homeostasis: hydraulic pressure, flow and temperature, engine
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speed and temperature, and the like.” Onboard computing allows BigDog to maintain stability on uneven,
and low friction terrains. [3]
Figure 3: BigDog [3]
Locomotion
Since this project is about building an Integrated Surface Vehicle System, a drive system that is
capable of meeting all design goals is required. There are three general branches of drive systems to date
available for the requirements of this project. These include wheeled systems, tracked systems, and
legged systems. Each general branch has its own sub branches. For example under wheeled systems, in
which there are twin-wheels and three-wheel systems. Each of the branches have their own strengths and
weaknesses. It is also important to select a drive system that will carry out the tasks of the vehicle system
with the highest efficiency possible. With these things in consideration, there are four main aspects to
look into when selecting the drive system:
Mobility has to do with both speed and maneuverability. Speed is an important factor because
being able to quickly complete the task contributes to overall efficiency. On the other hand,
maneuverability means the ability to avoid and overcome obstacles with ease in order to allow the system
to complete its task with little or no disturbance.
Stability can be divided into both static and dynamic stability. Static stability is the ability to
maintain equilibrium when the system is stationary. Dynamic stability is the ability to maintain
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equilibrium when the system is moving. If stability is absent in the system, it will not be able to perform
its tasks efficiently.
Power consumption is the tendency to drain power from the system to provide for the operation
of the drive system. The lower the power consumption, the higher the energy efficiency of the system is.
Cost is another important factor for drive system choice. With the concern of limited budget, it is
important to choose a system that fits the budget while still taking the other aspects into consideration.
Wheeled Systems
In terms of mobility, wheeled systems generally have higher speed than the tracked and legged
systems, while the maneuverability depends on the design of different systems. Wheeled systems are best
suited for even terrains. Theoretically, a wheeled vehicle system can have any number of wheel. However,
to achieve both static and dynamic equilibrium, the system needs three or more wheels [4]. Power
consumption of wheeled systems is dependent on the number of motors used and the terrain on which the
system is travelling. Wheeled systems the most popular drive system in use. This is because they are
cheap and simple.
2-Wheeled Systems are significantly harder to achieve static balance in than other wheeled
systems. They need to keep moving to maintain an upright position. To balance itself, the base of the
vehicle system must stay under the center of gravity. The swing-type robot is an example of a 2-wheeled
system [5].
4
Figure 4: Swing-type Robot
3-Wheeled Systems can be broken down into 2 types, two powered wheels and a free rotating
wheel to provide balance (Differentially steered System), and two powered wheels and a powered
steering wheel to provide balance and steering.[6] The center of gravity has to be situated within the
triangular area formed by the three wheels, and mass has to be carefully spread out in that area to ensure
stability when the system turns.
Figure 5: Differentially Steered 3-Wheeled System
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Tracked System
Tracked systems provide mobility to the vehicle system using one or more pairs of tracks which
rotate simultaneously. Tracked systems are usually slow but able to overcome obstacles like rocks and
potholes with ease. With the right material, tracked system can even travel on slippery surfaces while
maintaining good traction. Tracked systems have high stability in both static and dynamic capacities due
to their large surface contact area. However, to operate the tracked system, a large amount of energy is
needed due the numerous gears that must be powered. Tracked systems are usually more expensive than
wheeled or legged systems.
Figure 6: Tracked System
Legged Systems
Legged systems provide mobility to the vehicle system using the walking or crawling mechanism.
Legged systems could be fast or slow depending on variation. They usually have a very high
maneuverability which can overcome large obstacles easily. The stability of the legged system depends
on the number of legs the system has. Balance is maintained as long as the center of mass is within the
area formed by the legs [4]. This is applied to both static and dynamic stability. The power consumption
of legged system is generally lower than wheeled and tracked systems. The cost of this system is
dependent on number of legs used.
2-Legged Systems are known as bipedal system. Their locomotion resembles the locomotion of
human’s legs. Bipedal systems are constantly being challenged by the stability issues. There is currently
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no general algorithm to solve the dynamic stability issue and they are often programmed based on zero
moment point (ZMP) [4]. Asimo from Honda is an example of a bipedal system.
Figure 7: Asimo (Honda)
Most of the 4-legged systems utilize dynamic stable walking because it needs to have at least 3
legs on the ground to maintain balance which leave them with only one leg be able to lift and move [4].
Figure 8: Left: Model of a 4-legged Robot, Middle: Stable Configuration, Right:
Unstable Configuration.
The six-legged or “hexapod” system has proven successful in the realm of biology. In the realm
of robotics it has become popular for its self-stabilizing nature. The complexity of the hexapod gate, and
the cost are often offset by the dynamic stability that can be achieved by this system. [7]
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Figure 9: 6-legged design adapted from nature. [7]
Drivetrain/ Chassis/ Suspension
All of the successful Mars rover missions have involved vehicles with 6 wheels and
wheel mounted motors. The Pantograph was one of the original suspension systems to
experience success on rover-type vehicles; this system was upgraded by the Rocker-Bogie design,
which is still a popular suspension system for extra-terrestrial unmanned vehicular missions [8].
These suspension provide a high level of mobility for the craft by shifting the vehicles center of
gravity over the four backmost wheels and allowing the front wheels to climb obstacles up to
twice the size of the vehicle while the four rear wheels provided enough traction and torque to
propel the vehicle forward. [8]
Figure 10: Left: Pantograph Right: Rocker-Bogie [8]
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These suspensions with wheel mounted motors are advantageous to other suspension/motor
combinations because they have relatively high ground clearance while also keeping the motors from
taking up space in the body of the vehicle. Six wheeled Rocker-Bogie suspensions with wheel mounted
motors strongest quality for lunar missions are due to their high mobility.
(a) Crabbing
(b) Zero radius
(c) Ackerman
(d) Differential/Skid
Figure 11: Changing direction [8]
The disadvantage to the six wheeled systems used on the Mars rovers are that they
require more power to operate and have more mass than many other unmanned vehicular
propulsion systems because of the high number of actuators used to propel each wheel[8]. In the
case of wheel mounted motors on a six wheeled suspension, the motors are exposed to the
ambient atmosphere. While this has caused problems on Mars with dirt and debris getting cause
in the drive train and disabling wheels; the Martian rovers have not had to traverse liquid water.
For the Boeing competition, points will be awarded for traversing water, which could pose a
problem for wheel mounted motors. The Rocker-Bogie suspension system also only has a high
level of mobility while moving forward. While this system has been utilized on the Martian
surface, the vehicles have essentially been traversing new terrain while moving in reverse, so that
they are able to pull out of any adverse terrain they encounter.
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The Dual Axis Drive is a concept that has a lower mass and requires less power to drive a
vehicle. This system has been effective due to the utilization of a linear actuator that separates
two motions of the drive train [9]. While the six wheeled, Rocker-Bogie system could wheel
walk and steer because of its plethora of actuators; the Dual Axis Drive locks links to prevent
wheel walking while the steering drive is in use. This drive has a switching device that will
alternatively lock the steering drive while the wheel walking drive is in use. This isolation of
driving mechanisms allows for fewer motors and actuators to allow similar degrees of mobility at
fraction of the weight and power consumption when compared to the Rocker-Bogie system [9].
While many unmanned vehicles to date have utilized wheel mounted motors; body
mounted motors are more likely to be protected from the elements. Techniques for transferring
motion from a body mounted motor to the wheels can vary. The techniques shown in Figure 10
can be described as a range from “simple and efficient” to “accurate and sturdy”.
10
Figure 12: Dual Output Drivetrain Methods [8]
As illustrated in Figure 10, the motors are located within the body of the vehicle. Body
mounted systems generally have much less mass and require less power to drive because they
contain fewer motors and actuators. The motors’ location inside the body makes it easier to
protect the motor, but also makes the vehicle less maneuverable. While the Rocker-Bogie
system can crab, move in a zero-radius, and skid-steer; these two motor systems can only skidsteer. The body centered motors will generally have a lower ground clearance than the RockerBogie systems. Four wheeled systems generally cannot traverse obstacles taller than the radius
of a wheel because the rear wheels do not have the torque or the traction to propel the front
wheels over the obstacle [8]
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Material Selection
Selecting the best material for any vehicle is crucial. There are three materials
commonly used to rugged autonomous robotic vehicles. These materials are aluminum, carbon
fiber/graphite, and titanium alloy Ti-6AI-4V.
The primary material used is aluminum. This is because aluminum is light weight, strong,
and can be formed into any shape needed. [10]
Carbon Fiber/graphite is used quite often as well. “Graphite composites have
exceptional mechanical properties which are unequaled by other materials. The material is
strong, stiff, and lightweight.” There are many advantages to using carbon fiber, graphite
composites; they have a high stiffness, strength, with a low coefficient of thermal expansion
(CTE) refer to table [11].
Analysis of Carbon Fiber:
Carbon fiber is most commonly used in applications where strength to weight is
desirable. Carbon fiber has high toughness when combined with other materials and has high
thermal conductivity. However, carbon fiber will fail without warning and catastrophically once
the ultimate strength is exceeded. Also very high skill and specialized machinery are required
to make optimized parts. [12]
Another material commonly used for mars rover or rover like robots is the Titanium
Alloy Ti-6AI-4V. It has some beneficial properties such as being able to withstand large
temperature gradients, has a low coefficient of thermal expansion, and has 3 times the strength
to weight ratio of aluminum. However, the titanium alloy is very expensive [13].
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Graphite
Graphite
Fiberglass
Composite
Composite
Composite
(aerospace grade) (commercial grade)
Aluminum
6061 T-6
Steel,
Mild,
AISI
1045?
Cost $/kg
$45-$550+
$10-$45
$3-$7
$7
$0.66
Strength
(MPa)
620- 1380
345- 620
140 - 240
240
410
Stiffness
(GPa)
69 - 345
55 – 69
6.9 – 10
69
207
Density
(g/cc)
1.38
1.38
1.52
2.77
8.3
Specific
Strength
(GPa)
12.4- 27.6
6.9 – 12.4
2.5 -4.4
2.4
1.4
Specific
Stiffness
(E/ρ)
200 x 106-1,000 x
106
160 x 106-200 x 106
18 x 106-27 x 25.5
106
27
6 x 10-6 – 8 x 13 x 10-6
10-6
7 x 10-6
CTE (in/in- -1 x 10-6 – 1 x 10-6 1 x 10-6 – 2 x 10-6
F)
Table 1: Carbon Fiber compared to metals [10]
Materials
Density
Modulus
of
Elasticity
Tensile
Strength
Cost
Aluminum
2.6989g/cc
68GPa
186.3
MPa
$0.79/lb
Carbon
Fiber/Graphite
1.33g/cc
290GPa
5650 MPa
$10/lb
Titanium Alloy
Ti-6AI-4V
4.43g/cc
104.8 GPa
1000 MPa
$1123.95/lb
Table 2: Properties of Metals Common in Robots
Electronics
Electricity is the fundamental energy behind robotics. The conversion from electrical
potential to mechanical movement is essential. A robot does this energy conversion through
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careful manipulation of motors, servos, and sensors. The electrical subsystem of a robot must be
engineered to complete all tasks the robot is designed to perform. [15]
Figure 13: Typical Servo Motor [15]
There are many sizes and strengths of electric motors. For any electric motor, the power
required is dictated by the torque required, and the angular velocity at which the motor is
traveling. This relationship is defined by the equation
the torque, and “
. Where “P” is the power, “T” is
” is the angular velocity. Therefore power demanded by the motor will be
determined by a number of factors including weight of the vehicle, track friction, and track grade.
On one hand, the too small a motor can result in overload of the motors torque ability,
which in turn leads to overheating, or over-current situations. On the other hand, an oversized
motor will use more power, and add unnecessary weight to the vehicle. After the power rating
has been selected, the gear reduction ratio must also be correct to have maximized either torque
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or speed. Hobby grade motors are different in that they perform at tighter tolerance specs so they
can malfunction and become damaged more easily. The hobby grade motor has a greater power
to weight ratio but the cost is greater. Each system requires its own controller unit that interacts
between the main control system and the motor to carry high current signals to the battery. This
is usually accomplished by a mosfet amplifier circuit.[16]
Servos and or actuators can be used in a robot’s design for the system that will be
manipulating objects. The hobby grade servo is ideal for this task because they are readily
available and come in a very large range of sizes and strengths. The servos will not require an
additional controller however if pneumatic actuators are used. In this case, there is need for an
actuator control.[17]
Batteries
There are many types and configurations of batteries. In robotic design, it is important to
maximize capacity while minimizing both battery mass and volume. The battery must be able to
produce a greater current flow than the total electric draw from all of the robots’ devices.
Lithium batteries provide the best performance in this aspect. The lithium battery comes in two
basic forms the lithium ion and the lithium polymer. [18] Lithium polymer batteries have a much
higher current draw capacity than the lithium ion, but are rarely used in household devices due to
their dangerous nature. The lithium polymer battery is very unstable, and if damaged will erupt
in a ball of fire with very little notice. These batteries are mostly used in the hobby world
because of this instability. Both types of battery require a unique type of charger and come in
many different sizes, voltages, and capacities.
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Figure 14: Battery Properties Chart [19]
The figure above shows the relationship between mass to volume in relation to runtime for
different battery types. As described, the lithium polymer is the highest performing per mass,
and volume.
Object Location
The types of object location used today include radar, infrared, microwave, sonar and
electromagnetic. For the location of metal objects there are two types of detectors,
electromagnetic or induction detectors. The EM detector is essentially an antennae that projects
an electromagnetic field, and receives a reflected field from metal objects. [20] The difficult part
is decoding the information returned by the reflected field in a way that will reveal the object to
be located. Depending on how accurate the decoding process is, the device can predict distance
to the object from the device, as well as size and density of the object.
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Sensors
To create a successful Integrated Surface Vehicle System, the robot will require the use
of sensors. A sensor is an electrical/mechanical/chemical device that maps an environmental
attribute to a quantitative measurement. There are hundreds of sensors made today that can
sense virtually anything the human mind can think of. For most robots, three main types of
sensors will be involved. These types include contact sensors, proximity sensors, and
acceleration sensors [21].
Figure 15: Basic Contact Sensor Structure[1].
Contact sensors are those which require physical contact against other objects to trigger.
They are used to measure the interaction forces and torques which appear while the robot
manipulator conducts operations. Proximity sensors have the function of being able to detect the
presence of a nearby object within a given distance, without any physical contact [22]. These
sensors are used for near-field (object approaching or avoidance) robotic operations. Proximity
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sensors can be further broken down into two main types. One uses the principle of a parallelplate capacitor, and the other uses the principle of fringe capacitance. For the parallel-plate-type
proximity sensor, the sensor forms one plate and the object to be measured forms the other
plate.[23]
Autonomous robots also need to be able to self-locate, plan a path, and avoid obstacles
[24]. Some use infrared sensors to find their path, while others will use the global positioning
system (GPS) [25]. Some will use a combination of both for object detection and avoidance. The
point of object avoidance is to keep the robot a safe distance from the object so that no physical
damage is done to the robot or object [25]. Another sensor that is often important to autonomous
vehicles is the accelerometer. Accelerometers can measure the acceleration, and tilt of a robotic
vehicle. They provide data important to both stability, and positioning.
Sensors: Cross Coupled Controls
Technology exists for advanced autonomy regarding all-terrain rovers. Cross-Coupled
Control (CCC) [26] is a system of sensors that has a higher level of accuracy than traditional
drive trains. In a CCC system, each motor is a control loop and each loop shares information
with the other control loops. This allows for the CCC to maintain a more accurate gauge of how
much each wheel/motor combination travels [26]. Another advantage of a CCC is that it can
sense if a wheel is not spinning or is spinning freely, and the controller can distribute power
away from the wheel does not have proper traction [26]. The system tracks and reduces slippage
in each wheel, allowing it to move more efficiently than other systems with closed loops that do
not communicate with one another. A disadvantage to a CCC system is that they move relatively
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slow when compared with similar systems due to the additional information that must be
processed in order to keep the loops communicating properly [26].
Computer Subsytem
Originally, this type of robotic vehicle was controlled by a basic wired controller. Over time they
have evolved to wirelessly controlled vehicles that are able to prerform specific functions. They are now
evolving into autonomous vehicles that only use a controller as a secondary means of control.
Micro-controllers have always been a major part of the computer system of this class of robotic
vehicle. The main one being used now is the Arduino. This micro-controller uses the C programming
language with a few modifications [27]. There are twenty-two different versions of the Arduino to fit
many different needs. The mega 2560 R3 is the most used Arduino out right now. This is due to the sheer
amount of Input/output (I/O) connections it has and that it uses less than 1 Watt (W). These connections
can be used to control other boards, known as shields [27]. These shields can be anywhere from motor
controllers to Bluetooth transceivers. However, the idea of using a micro-controller is beginning to fade.
A new concept has actually been to use an actual computer on board. This is because computers
are now becoming small and power efficient enough to be used alongside, or even replace the microcontroller. For example, the Raspberry Pi has a power consumption of about 3.5 W [28]. All image
processing and other intensive tasks would be done by the computer, while the basic controls would be
done by the micro-controller.
The controller has evolved over time. Cell phones and tablets are now able to control things via
Bluetooth and Wi-Fi. This has brought several advances to the controller. The first being the ability to use
a controller away from a computer and to see video from a camera at the same time. Now that an actual
computer is being used, autonomous robots are beginning to become more and more popular. That is
robots that can move without any manual control.
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References
[1] “Nasa Facts” (2013 September, 20) Nasa Jet Propulsion Laboratory [Website]. Available:
http://www.jpl.gov/news/fact_sheets/mars03rovers.pdf
[2] P. Aranzulla et al., “An Innovative Autonomous Outdoor Vehicle Based On Microsoft Robotic
Studio,” Emerging Trends in Mobile Robotics Proceedings of the 13th International Conference on
Climbing and Walking Robots and the Support Technologies for Mobile Machines, Nagoya, Japan,
August 31 - September 3 2010, pp. 97 -104.
[3] M. Raibert et al., “BigDog, the Rough-Terrain Quadruped Robot,” Proceedings of the 17th World
Congress The International Federation of Automatic Control, Seoul, Korea, July 6-11, 2008, pp.
10822 - 10825.
[4] S. Böttcher, “Principles of robot locomotion,” Seminar, Human robot interaction.
[5] Robot platform, “Types of Wheeled Robot,”
http://www.robotplatform.com/knowledge/Classification_of_Robots/Types_of_wheeled_robots.html.
[6] G. W. Lucas, “A Tutorial and Elementary Trajectory Model for the Differential Steering System of
Robot Wheel Actuator,”2000.
[7] J. E. Clark, and M. R. Cutkosky, “The Effect of Leg Specialization in a Biomimetic Hex pedal
Running Robot,” in Journal of Dynamic Systems, Measurement, and Control, vol. 128, doi:
10.1115/1.2168477
[8] M. Roman, “Design and Analysis of a Four Wheeled Planetary Rover,” M.S. thesis, University of
Oklahoma, Norman, OK, USA, 2005.
[9] A. Shoucri et al., “Dual-axis drive for a Mars rover,” Transactions of the Canadian Society of
Mechanical Engineering, vol. 31, (4) pp. 547-557, 2007.
[10] Rover Missions, NASA, [online] 2013,
http://www.nasa.gov/missions/highlights/webcasts/elv/mera/ld-rover-qa.html (Accessed: 2
October 2013).
[11] Designing with Carbon Fiber, Performance Composites, [online] 2013,
http://www.performancecomposites.com/about-composites-technical-info/124-designing-with-carbonfiber.html (Accessed: 3 October 2013).
[12]Technical, Dragon Plate, [online] 2013, http://www.dragonplate.com/sections/technology.asp
(Accessed: 1 October, 2013).
[13] A. Purura et al., “Mars Advanced Safe Haven Equipped Rover,” [online] 2004,
http://web.mit.edu/mars/Conference_Archives/MarsWeek04_April/Speaker_Documents/Mars
SafeHavenRover-MASHERTeam.pdf (Accessed: 30 September 2013).
20
[14]Matweb: Material Property Data, Matweb, [online] 2013, http://www.matweb.com (Accessed:
30 September, 2013).
[15] “Introduction To Autonomous Mobile Robots” second edition, published by Massachusetts Institute of Technology
[16}2013) http://www.microchip.com/pagehandler/en-us/technology/motorcontrol/motortypes [online]
[17] (2013) http://www.mikeholt.com/mojonewsarchive/NEC-HTML/HTML/article-430-motors [online]
[18](2010) Experimental Characterization of Lithim Polymer Battery Charging Cycles during Bilateral Energy Exchange in
Power-Autonomous Systems by Pushpak Jha [reserch paper]
[19] (2013)http://www.mpoweruk.com/chemistries.htm[online]
[20](2001) Detection and Remediatipon Technologies for Mines and Minelike Targets vi, published by SPIE The International
Society for Optical Engineering
[21] Abinash C. Dubey, James F.Harvey, J.Thomas Broach [book] Dirk Groger, Hosam Alagi, and Heinz Worn. (2013). Tactile
Proximity Sensors For Robotic Application. [Online].
[22] Zhenhai Chen and Ren C. Luo. (1998). Design and Implementation of Capacitive Proximity Sensor Using
Microelectromechanical Systems Technology. [Online].
[23] HeeSu Kang, Kiwon Rhee, Kyoung-jin You, Hyun-chool Shin. (2011). Intuitive Robot Navigation Using Wireless EMG
and Acceleration Sensors on Human Arm. [Online].
[24] E. Georgiou et al., “A Stereo Vision Model For A Small Form Factor Autonomous Mobile Robot
‘KCLBOT’,” Adaptive Mobile Robotics Mobile Robotics Proceedings of the 15th International
Conference on Climbing and Walking Robots and the Support Technologies for Mobile Machines,
Baltimore, USA, pp. 81.
[25] Boyu Wei. “Navigation and Slope Detection System Design for Autonomous Mobile Robot.” The
Ninth International Conference on Electronic Measurement & Instruments, 2009.
[26] Reina, G. “Cross-Coupled Control for All-Terrain Rovers,” Sensors, [online] 12 pp. 785-800,
[27] Arduino - www.arduino.cc
[28] “Power Supply confirmed as 5V micro-USB,” Oct. 2011 - http://www.raspberrypi.org/archives/260
[29] Nikola Mitrovic. “Physical computing and Android in Robotics.” 2nd Mediterranean Conference on
Embedded Computing, 2013.
[30] Android - http://www.android.com
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