Chapter 3 Structure & Mechanism of ALP Cycle

84 Pages·2014·22.01 MB·English

Chapter 3
Structure & Mechanism of ALP
Cycle
In the development of any small robotic system, the structure of the robot plays a very
important role especially in robots which exhibit inherently unstable behaviours such
as single-wheeled mobile robots. As mentioned in Chapter 1, Subsection 1.4.2, the pre-
liminary work shows excessive structural vibration, structural bending and loosening
of fasteners, which affect the stability and performance of the robotic system. There-
fore, considerable effort and time are allocated in this research to properly design and
construct a robust physical platform for ALP Cycle.
In this chapter, the process of developing the structure of ALP Cycle is presented in
great detail. In Section 3.1, the objective is clearly stated and the requirements that
must be met to fulfill that objective are defined. It is followed by the detailed 3-D
structural design in SolidWorks presented in Section 3.2. Issues such as placement of
components, selection of material, wheel, etc., and designs of the chassis and pendulum
are also discussed in this section. In Section 3.3, the finished structural parts after
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fabrication and the assembled prototype are shown.
3.1 Objective & Requirements of Prototype Devel-
opment
In this research, the main purpose of constructing the prototype is to experimentally
verify the posture-balancing capability of lateral-pendulum mechanism under feedback
control. Theoretical analysis of the proposed autonomous unicycle mechanism is carried
out using the mathematical model of the structure. To achieve experimental success, it
is apparent that the prototype must be constructed in a way such that its dynamics is
as close as possible to the dynamic model derived based on several ideal assumptions.
Therefore, the objective is to develop a prototype which possesses dynamics that is rep-
resented by its theoretical dynamic model while keeping an eye on issues of practical
importance. The relevant practicality issues include ease of experimentation, cost and
appearance.
The following seven design requirements are defined in order to meet the design ob-
jective stated above.
1. Low structural vibration: Excessive structural vibration is undesirable because
ALP Cycle is an inherently unstable system which maintains its balance at only its
upright posture. It acts as a disturbance which perturbs the stability of the system
and also introduces noise into sensor outputs. Furthermore, it may excite the
unmodelled high-frequency dynamics of the system. Definitely, it is not possible
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and feasible to totally eliminate the structural vibration. However, if the structure
is designed properly, its vibration can, at least, be kept at low level, so that it will
not considerably affect the system stability and performance.
2. Low structural bending: For elastic materials, including most metals, structural
failure is normally preceded by structural bending. The static and dynamic char-
acteristics of a structure are also altered by the presence of structural bending.
Therefore, structural bending has to be kept low in order to avoid structural fail-
ure and keep the dynamics of a system close to its theoretical model.
3. Low inertia: The mass and moment of inertia of a robotic structure are important
consideringthefollowingthreeaspects. Firstly,aroboticstructurehavinglowmass
and moment of inertia requires less force and torque to actuate it and, therefore,
can be actuated by smaller, lighter and lower-cost actuator. Secondly, with the
same actuator, a robotic structure with lower mass and moment of inertia has less
reaction time during its motion as compared to that with higher mass and moment
of inertia. Lastly, since we deal with an inherently unstable system, it is easier
and requires less effort to handle a robot having lower mass and moment of inertia
during experiments. Therefore, it is desirable to keep the mass and moment of
inertia of the robotic structure low.
4. Balanced centre-of-gravity placement: Since the theoretical unforced equilibrium
pointofALPCycleisitsuprightposture,theALPCycle’scentreofgravitymustbe
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placed above its contact point with the ground as precisely as possible. Otherwise,
the posture balancing will not be easily achieved or it may even be impossible to
be achieved.
5. High integrity of structural joints: Based on the preliminary work presented in
Chapter 1, Subsection 1.4.2, weak structural joints result in retightening of fasten-
ers after several experiments, which is unnecessarily time consuming. Therefore,
the structural joints of the robot must be highly robust so that it is able to with-
stand multiple experiments.
6. Low cost: To keep the cost of the prototype within the limited budget, we do not
have the luxury of getting the best possible choice for each component. Besides,
trial-and-error approach for performing experiments is to be avoided as failure
is very expensive. Therefore, some trade-off is allowed in selecting components
and more time is invested to design and evaluate the system before performing
experiments.
7. Aesthetic appearance: In the case where the other design requirements above have
been fulfilled, the aesthetic aspect of the design is considered to make the ALP
Cycle more presentable.
The prototype development process presented in this chapter is iterative in nature and
it is summarised in the flow diagram shown in Fig. 3.1. For the brevety of presentation,
only the final result is shown in this chapter and not all stages of iteration. It must be
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notedthat, atnopointinthisthesis, aclaim, thatthedesignoftheprototypeisoptimal,
is made. Instead, the main focus is on developing a prototype which is sufficiently good
to serve the purposes of concept verification and control validation.
Figure 3.1: Prototype Development Process of ALP Cycle
3.2 Computer-Aided Structural Design
3.2.1 Mechanism & Component Placement
The structure of ALP Cycle consists of a wheel, a chassis and a pendulum. The wheel
and the chassis are connected by a rotary joint with a rotary actuator installed rigidly at
the bottom of the chassis providing torque for the wheel. The pendulum is attached to
the chassis through a rotary joint and a second rotary actuator rigidly mounted on top of
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the chassis to provide torque for the pendulum actuation. Two brushless direct-current
(BLDC) motors are used as these actuators. Two motor amplifiers are needed to power
up the two BLDC motors. One microcontroller is needed to function as the robot’s
computer system. Two incremental rotary encoders and two inertial measurement units
(IMUs) are used as sensors in this system. The encoders come together with the BLDC
motors, so they are considered as parts of the motors. The two IMUs are mounted on
two separate printed circuit boards (PCBs). The two PCBs are also used for mounting
the required circuits for power distribution from two Lithium Polymer (Li-Po) batteries
used to power up the entire system. A microcontroller board is used to read the sensor
outputsandcontrolthemotorsaccordingly. WhiledesigningthestructureofALPCycle,
it must be kept in mind that these seven components, i.e. two BLDC motors, two motor
amplifiers, two PCBs and one microcontroller board, are to be securely mounted on the
mechanical structure.
The microcontroller chosen (RSK2+ for SH7216) has the dimensions of 175 mm ×
130 mm × 10 mm. A semi-closed space is allocated on top of the chassis for the mi-
crocontroller. This central arrangement is better for routing the wires to the other
components and for providing a convenient access to the user to give manual input and
to read the LCD output of the microcontroller.
The approximate dimensions of the motor amplifiers (DEC 70/10 4-Q-EC Amplifier)
are 120 mm×103 mm×27 mm. Due to space constraints, motor amplifiers are placed
on two appendage structures attached to two sides of the chassis.
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For the placement of the Li-Po batteries as the portable power source, a hanger is
designed which is hung rigidly below the chassis to securely contain the Li-Po batteries.
The custom-made PCBs are placed on top of the chassis and are exposed to the outside
environment.
The overall arrangement is shown in Fig. 3.2. This design serves the purpose as a
robust platform for this research.
Figure 3.2: Mechanism & Component Placement
3.2.2 Material Selection
The material for the structures of chassis and pendulum is chosen to be 6061 aluminium
alloy. This is due to its following desirable properties.
1. Low mass density: The mass density of 6061 aluminium alloy is 2700 kg/m3 which
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is considerably lower than the mass densities of other common metals such as plain
carbon steel (7800 kg/m3) and titanium Ti-8Mn, annealed (4730 kg/m3) [40].
Therefore, it can make the structure lightweight.
2. High strength: The elastic modulus and tensile strength of 6061 aluminium alloy
is 69 GN/m2 and 124,084,000 N/m2 respectively [40]. Compared to other metals,
these parameters are lower, but they are much higher than other non-metallic
materials.
3. Low cost and wide availability: 6061 aluminium alloy is readily supplied by pro-
fessional mechanical contractors and workshops in Singapore. Therefore, its price
is relatively low.
4. Good manufacturability: Several manufacturing techniques can be applied to form,
shape and join 6061 aluminium alloy. Therefore, there are fewer manufacturing
constraints and the fabrication cost can be kept low.
Other materials such as wood, steel, titanium and acrylic are also given some consid-
eration before choosing this particular alloy. Wood has low mass density, but it has low
strength and durability [17]. Steel has very good strength and durability, but this comes
at the cost of high mass density. Titanium has remarkable strength and moderate mass
density, but its price is extremely expensive. In addition, titanium is rarely supplied
by local mechanical contractors and workshops in Singapore. Acrylic is lightweight, but
its strength and durability are not very good. In conclusion, aluminium alloy provides
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the best trade-off in terms of mass density, strength, cost and manufacturability, and,
therefore, it is the material of choice for the ALP Cycle’s structure.
Inthefabricationprocess,thestructureisblackanodisedtogivethreebenefits. Firstly,
black anodised aluminium alloy has higher corrosion resistance. Secondly, the surface of
black anodised aluminium alloy is non-conductive, so it provides some protection from
accidental short circuit. Lastly, the structure made of black anodised aluminium alloy
hasmoreprofessionalandaestheticappearance, soitismorepresentableforpresentation
and exhibition.
3.2.3 Selection of Wheel
The wheel diameter is determined from the chosen wheel motor set which has a total
length of 173.3 mm. In order to allow ALP Cycle to lean sidewards at maximum of
450 without the wheel motor set touching the ground, the wheel must have a radius of
173.3 mm. Therefore, the wheel diameter is designed to be 346.6 mm. As the wheel
with the specified diameter is unavailable in Singapore, a 300 mm-diameter pneumatic
wheel is chosen instead. With this diameter, the maximum lateral lean angle α before
the wheel actuator touches the ground is calculated as followed.
346.6 mm
max(|α|) = 900 −arc tan( ) = 40.880
300 mm
The maximum lateral lean angle is deemed acceptable. A bicycle wheel consisting of
steel hub and pneumatic rubber tyre is used. It has an axle attached with two ball
bearings, one on each side. The wheel is shown in Fig. 3.3.
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Figure 3.3: Selected Wheel for ALP Cycle
3.2.4 Design of Chassis
The chassis and pendulum are carefully designed in detail in SolidWorks. SolidWorks is
an advanced computer-aided design software which provides a powerful virtual environ-
ment for the design, evaluation and animation of mechanical structure. Therefore, it is
adopted in this research for designing and analysing the structure of ALP Cycle.
The chassis design follows the modular-design concept, in which, it is designed to be
an assembly of several simpler parts which can be easily replaced if some improvement
is to be made. Therefore, should the need arise, future improvement and modification
can be done by redesigning and refabricating a particular part, instead of refabricating
the whole structure.
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