Chapter 4 by Soliman Edris
CHAPTER 4: MODELLING
Introduction
Please refer to Appendix 4.1 for an explanation of the terminology I will be using in this chapter.
My role in the development of the stair-climbing robot was to construct the ‘virtual
robot’ – a three dimensional model of our design using designated Computer Aided Design
(CAD) software. I also produced a short (< 2minutes) video of the robot and its features,
using the CAD programme’s in-built video production feature.
Many CAD programmes allow three-dimensional modelling of any mechanical system, from
constructions as insignificant as a paper-clip, to those as complex as a hover-craft (though the
latter would require powerful CAD software, a very powerful computer, and many hours of
work).
Due to departmental restrictions, I was unable to access the CAD software available on the
Mechanical Engineering departmental computers at UCL, and so I researched into alternative
CAD software available that was available for my use. There are many different varieties of
CAD software available, some to purchase and some for free.
After reviewing many
different versions, and speaking with several members of the Mechanical Engineering
department at UCL [Ref:4.1], I narrowed my selection down to two programmes: Google
SketchUp [4.2] and SolidWorks [4.3].
Google SketchUp is one of the most user-friendly CAD programmes, allowing simple
construction of three dimensional models.
But in its aim to simplify the use of the
programme, it has cut back on necessary features that allow subtler detail to be added to
models. When I viewed models made by users of Google SketchUp, I discovered that the
viewing angles are restrictive, disallowing one to view the underside of the model.
Furthermore, a search for a video production that was made in Google SketchUp proved
fruitless, suggesting that there was no video production feature available in Google SketchUp.
Chapter 4 by Soliman Edris
SolidWorks is professional CAD software that is commonly used to model prototypes of
many commercial items, as it supports many user-friendly features (see The Main Features
Available in SolidWorks section below) that allow easy modelling of complex designs. It
has an ‘intelligent’ three dimensional interface that allows easy viewing from all angles, and
a powerful zoom function to allow a very close, almost microscopic, view of the model, as
well as a distant panned view.
Furthermore, SolidWorks has the ability to produce
professional-looking videos with relative ease. Unfortunately, it is very difficult to obtain a
free edition of SolidWorks but I was fortunate, however, to have a close associate of mine
[4.4] who allowed me free and full access to his copy of SolidWorks.
The SolidWorks Programme
Parts and Assemblies
When the SolidWorks programme is first opened to begin a model, one is offered the
choice of whether to model a ‘Part’ or an ‘Assembly’. One of the powerful features of
SolidWorks is that it requires the modelling of a design to be done component-by-component.
These components are then finally drawn together into an ‘Assembly’. This feature of
SolidWorks becomes more necessary when it comes to adding materials to the components,
as we are unable to assign different materials to different features of the same component; we
must assign a material to a whole component. Thus, a ‘Part’ is a component with uniform
properties that has no internal degrees of freedom. For example, to construct the frame of our
robot, I modelled the length, width and diagonal aluminium bars as separate parts, each with
their own dimensions. I also had to model the bolts that would connect these bars, and the
nuts that would secure the bolts to the bars. These components are then brought together into
an “Assembly”, where they are connected together, and their relative degrees of freedom are
defined (such as allowing the wheel to rotate relative to the frame).
It is possible to create an intermediate assembly that will later serve as a component in a
larger assembly, but in doing so, the intermediate assembly is then considered as a ‘Part’, and
its internal degrees of freedom are restricted as a result. For example, before assembling the
final robot, I assembled the full wheel mechanism, and the frame, before drawing these two
assembled components together into a final assembly.
Chapter 4 by Soliman Edris
Three Dimensional Planes
The figure shows the three
planes that define the three
dimensions of space.
“Front Plane”: x-y plane
“Right Plane”: y-z plane
“Top Plane”: z-x plane
Figure 4.1: The Three Dimensional Planes in SolidWorks
Due to the complexities of modelling three-dimensional objects on a two-dimensional
computer screen, it is of utmost importance that the plane one is considering, and the centre
of coordinates is defined at all times.
It is also possible to define alternative planes and to shift the origin, and this is an essential
feature for more complex designs that require several, carefully positioned planes, as was the
case when I modelled the base attachment for the wheel motors, as is shown below.
Figure 4.2: Creating Multiple Planes
When modelling the base attachment for the motors, it was necessary to define a new
plane tangential to the circular component, in order to construct the plane base.
Views
It is vital to have a good perspective on exactly what the object looks like, and this is
often difficult when trying to represent a 3D image on a 2D screen.
In most CAD
programmes, one is easily allowed to shift the view of and around the component using the
in-built functions as is demonstrated in the figures below.
Chapter 4 by Soliman Edris
The figures show the
available views of a
component, in this case
applied to a solid cube.
Figure 4.3: Views of Model
{Left}: The available
viewing angles of a
component.
{Right}: A helpful set of
axes is always in view in
the corner of the screen
for reference.
Figure 4.4: Viewing Angles of Model
There is also the ability to manually and freely rotate and zoom in on the object using the
mouse buttons alone, which makes the modelling process easier and more efficient, and also
allows viewing from non-regular angles.
Assigning Materials to Components
One of the last features assigned to a component is its material. SolidWorks has a
large database of materials along with in-built material properties. Adding a material to a
component has no structural benefit but is used for display purposes. Once a design is
complete, and has been assigned a material, one is able to ‘render’ it. This makes the model
look realistic, adding the visual properties of the material such as reflectivity for steel, and
transparency for glass. It is this rendered image that is shown in the final video production of
the model.
Chapter 4 by Soliman Edris
Adding Dimensions to Components
SolidWorks’ requirement to have “Fully Defined” components is one of its most
powerful attributes. If a component’s dimensions are not fully defined, it will read “Under
Defined” at the bottom of the screen. This is an indication of the possible power and ability
of the programme as it means that we are able to judge whether a design is physically feasible
by considering just the model; if it is possible in the CAD programme, then it is possible in
the real, physical world. If a program were to allow arbitrary sketches, then one could not be
assured that it is possible in real life.
Figure 4.5: The Impossible Staircase
An example of a design that SolidWorks would not be able to model as it does not
have well-defined dimensions.
source: http://im-possible.info/images/art/computer/raphael-pohl/impossible-stair-white.jpeg
The Main Features Available in SolidWorks
Sketch
The Sketch function is not a tool to draw a design onto the surface of a component,
but rather to define a point, line, or area that a function should be applied to, such as
sketching a circle as the profile to carve out a hole. The sketch lines do not show up on the
final rendered image, but are simply used as a helpful tool for the graphics designer.
Figure 4.6: The Various Sketch Functions
Chapter 4 by Soliman Edris
Extrude
The Extrude function is essentially the sketching function for the third dimension. It
extends a plane 2D sketch into the third dimension, creating a solid body, as is demonstrated
in the figure below. Once the solid body has been created, one is able to begin adding the
finer touches and altering the design.
Figure 4.7: The Extrude Function
Extruded Cut
The Extruded Cut feature carves out a solid volume based on a two dimensional
profile, as is shown in the figure below.
Figure 4.8: The Extruded Cut Function
When one wishes to apply the extruded cut feature, one is able to offset the point from which
it extrudes the cut. So if one wishes to create a hollow cylinder with closed ends (as I did
when I modelled the main wheel body), one is able to offset the extrude start point to ensure
that it leaves the ends closed, but hollows out the centre.
Filleting
Filleting allows edges and vertices of a model to be rounded off. As inputs, it simply
requires the edge or vertex that is required to be filleted, and the radius of curvature for the
fillet. In the real-world, most mechanical items are filleted in the manufacturing process,
either unintentionally due to the inability to construct perfect right angles, or intentionally for
safety reasons, to protect consumers from the sharp corners. I chose to fillet the components
Chapter 4 by Soliman Edris
of our robot for display purposes; in order to make the design look more realistic and
smoother. I chose small radii of curvatures of a few millimetres for the fillets in order to give
them a subtle rounded look.
Video Production
The aim of the video feature is to show how the robot was modelled: its components
and how they relate to each other. It is useful to demonstrate the internal degrees of freedom
of the robot, such as how the wheels rotate with respect to the other components. Its main
purpose is for presentation; to portray the functional capabilities of a design to possible
investors in the design. One is also able to ‘Explode’ the model – separating all of its
components from each other in order to display how they relate, and connect to each other.
A further extension to producing a video is to create a ‘smart’ simulation, which allows one
to define mass and friction properties of various components, and simulate real-life events.
Due to time constraints, I was unable to create a simulation of the robot ascending steps,
though this was my initial intention.
Modelling ‘The Rhino’
There are usually several ways to model any given design, and it is the job of the
graphics designer to decide upon the most efficient method, often exploiting the symmetries
of the design. Our design contains simple symmetries, and by exploiting these symmetries, I
was able to model relatively few components, repeating them for different positions within
the robot. For example when modelling the frame; by using simple rotations, about a well
chosen axis, it was only necessary to model only four aluminium bars for the twelve bar
structure.
Full Example of Modelling
Below I have given a step-by-step example of the process of modelling the coupling
device that is used to connect the motor to the wheels. I have chosen this component as it is a
simple example that uses most of the functions that I commonly encountered in developing
the stair-climbing robot in SolidWorks.
Chapter 4 by Soliman Edris
Step 1:
Sketch a circle:
Diameter = 1.2cm
Step 3:
Sketch circle on one face:
Diameter = 0.75cm
Step 6:
Sketch circle on top
plane, and define
position and diameter.
Step 2:
Extrude circle from sketch plane: Extrusion length = 3.2cm
Step 4:
Extruded Cut the circle from
sketch plane up to next surface.
Step 5:
Switch view to Hidden Lines
Visible for a clearer view.
Step 7:
Extruded Cut the circle up to edge of cylinder.
Extruded cut length = 0.6cm.
Step 8:
Doing the same for the other side gives the
basic shape of the coupler body.
Step 9:
For finer detail; Fillet to smooth the edges,
making the component look more realistic.
Chapter 4 by Soliman Edris
Step 10:
Finally, set the material, in this case to
copper.
Rendering the image gives a highquality, full graphics preview.
Modelling the Frame
Figure 4.9: The Fully Assembled Frame
For the frame of the robot, I modelled the aluminium bars, and the bolts and nuts that
would be used to connect them together. Due to the symmetry of our robot, it was only
necessary to model one aluminium bar for the length of 70cm, and one for the width, which
was 45cm. These were then repositioned and rotated for the various positions within the
robot. It was, however, necessary for me to model two separate diagonal bars; one for the
right side and one for the left side of the robot. They were of the same length (40.8cm) but
differed in the direction of the end angles, as is shown in Figure 4.10 below. This was to
ensure that the outer faces of the bars would be on the outer sides of the robot, as is shown in
Figure 4.11 below:
Chapter 4 by Soliman Edris
Right diagonal aluminium bar
Left diagonal aluminium bar
Figure 4.10: The Difference between the Right and Left Diagonal Aluminium Bars
The images show how the right and left diagonal bars differ in
order to allow their outer faces to point outwards from the robot.
Outer faces of diagonal aluminium
bars point outwards from robot
Figure 4.11: Rotational Symmetry of the Frame Structure
The images show that the front and rear bars differ only by a 180° rotation about the x-axis.
Modelling the Nuts and Bolts
Figure 4.12: Nuts and Bolts - Before and After
The figure on the left shows my initial sketches of the components, the figure on
the right shows the final, rendered components, modelled using SolidWorks.
I decided to add grooves to the bolts to make them look more realistic, but I did not
add grooves to the nut or any of the bolt inputs, as this would have added a certain degree of
complexity when it came to bringing the components together into a final assembly. It would
have required that I exactly match the bolt’s spiral grooves to the nut’s spiral ridges, and the
Chapter 4 by Soliman Edris
bolt’s ridges to the nut’s grooves. In physicality, this is done in the process of screwing the
bolt into the nut, but on a CAD programme, this becomes a complex process as one has to
precisely measure the depth of the grooves, and the pitch of the spiral, and these must match
together exactly, when they are finally assembled. I felt it was unnecessary to go through this
effort for such trivial detail.
In order to make the grooves, I used the Swept Cut function which carves out a 2D shape
along a defined path. For example, in order to make the grooves on the axle rod, I cut out a
circular groove along a helical path, as demonstrated in the figures below.
Figure 4.13: Creating a Spiral Groove in an Axle Rod
A circular profile is drawn such that its centre runs along the helical lines, and its plane is
perpendicular to these lines. Thus, a spiral groove is formed along the axle’s cylindrical surface.
Assembling the Frame
For the assembly of the frame it was my opinion that a diagonal bar that rests on the
width bar is the most stable structure, and also the neatest in terms of appearance.
I
performed some calculations, which can be found in the Appendix 4.2, that show that our
3cm bolt cannot be used to connect the bars if this stable structure were to be adopted, as is
demonstrated in Figure 4.14 below. Instead, bolts of length 1.65cm, rather than 3.00cm, and
nuts of depth 0.63cm, rather than 1cm, were used for the model.
In the stable
structure, the
diagonal aluminium
bar rests on the
width bar.
In this structure, the
3cm bolt intersects
the diagonal bar.
Figure 4.14: The Stable Structure
It was this incident that led me to the decision that I would model the “ideal” construction, as
this is the main objective of pre-construction modelling [4.5] – to design the ideal prototype
Chapter 4 by Soliman Edris
of the design. It is the objective of the construction team to try to recreate this ideal design as
best as possible, subject to the resources at hand. If, for example, the real-life construction
wasn’t stable enough, and the construction team found it appropriate to add adhesive tape to
the supports to make it more rigid, the modelling team would not be expected to also
introduce this adhesive tape as part of their CAD model.
A post-construction modelling
[4.6] team may decide to model exactly how the design has been physically constructed,
including all of the changes that were made due to issues arising during the construction
process. Since the robot had not been constructed during much of the period that I was
modelling, I followed the pre-construction methodology which was to model the idealistic
situation. With this approach, I decided to model the stable structure, where the diagonal bar
rests on the width bar and the nut and bolts are shortened, though this was not adopted for the
real construction. Instead, the construction team chose to shift the diagonal bar along the
length bar to provide room for the bolts and the motors.
Modelling the Wheels
The wheels constitute all of the components that cause the robot to move. A list of these
components, and a brief description of each, is given below.
 The plastic cable spool is the main body of the wheel that is in contact with the
ground.
 The steel axle connects the motors to the wheels (or the wheels to each other in the case
of the non-motorised top-front wheels) via the associated coupler (see wheel-to-axle
coupler and motor-to-wheel coupler below).
Chapter 4 by Soliman Edris
 The wheel-to-axle coupler is used to allow the axle to connect concentrically to the
wheel, as is shown in the figure below.
Diameter of
central hole
through the
wheel
Diameter of axle
 The motor:
Main body
Plastic casing
The motor’s main body contains all
of the electrical components that
make the rotor move, but these have
not been considered in the model.
Screws to secure motor
Base attachment
Rotor
 The motor-to-axle coupler and screws connect the axle to the motor’s rotor. Copper
was assigned as the material for the main coupler body, and aluminium bronze for the
screws.
The figure above shows my initial sketches, and the final, rendered coupler and screw.
Chapter 4 by Soliman Edris
The rotor is inserted into one end of the cylinder, and the axle into the other. The
screws are screwed into the holes on the cylinder’s lateral surface in order to secure
the rotor and axle in position. With this set-up, the axle will rotate with the rotor.
 The battery was modelled as a simple rectangular cuboid with filleted edges. All of
its internal electrical components were neglected for the model.
All of the frame and wheel components were then brought together into a final assembly, and
their relative degrees of freedom were defined. Fully rendered images of the complete,
modelled robot are shown at the beginning and end of this chapter.
Advantages of Modelling
The main benefit of pre-construction modelling the robot is the ability to foresee difficulties
that may arise during the physical construction of the robot. This is because a program like
SolidWorks requires precise dimensions so that it can ‘intelligently’ recognise whether
designs are physically plausible. That is to say, one is unable to model appliances that can’t
be physically and tangibly created.
One example of when the modelling aspect could have helped in the physical construction
was during the assembly of the frame, in particular in attaching the bolts and nuts to the
Chapter 4 by Soliman Edris
frame. When I was modelling this, I came across the difficulty that the 3cm bolt intercepted
the diagonal bar, as is explained earlier in the chapter. I found that a 1.65cm (maximum) bolt
and a 0.65cm (maximum) nut should have been used in order to ensure maximum structural
stability. Unfortunately, the frame had already been physically constructed when I was
assembling the final model and so I could not forewarn the construction team, who did indeed
encounter this problem. This serves as an example of how modelling can be very helpful in
making the construction procedure smooth and efficient.
Modelling is of great importance as it allows a design to be analysed without the need for a
physical construction. It also allows us to confront issues that may be problematic during the
process of physical construction.
Due to constraints on resources and time, it is often difficult to physically construct many
different robot designs, but with powerful CAD software available for usage, it would be
possible to model, or ‘virtually construct’, many realistic robot designs without spending
money on materials.
Possible Future of Modelling
I was unable to make a video of our modelled robot ascending steps. This was due to the
added complexity of having to sketch a staircase, and define the interactions and degrees of
freedom between the steps and the robot. This would even include defining that the wheel
cannot penetrate the step. One can now begin to see the amount of parameters and conditions
that would need to be applied in order to realistically simulate a stair-climbing robot. Had it
not been for time constraints, this simulation feature would have been included as part of my
project.
There are also functions in SolidWorks that allow the mass of the components to be input
thereby allowing an extension to running ‘real-world’ simulations that would account for
gravity, as well as other forces, such as friction.
I had initially intended to model other designs for our robot such as structural variations (for
example adding wheels that run up the front diagonal aluminium bars to allow smooth
ascendance onto the first step), and variations to appearance (for presentation purposes).
Furthermore, I would like to have modelled other robot designs such as the 8-Segment
Origami-Inspired Stair-Climbing Robot which was designed and constructed by Ambroise
Chapter 4 by Soliman Edris
Morel [4.7].
It was unfortunate that Ambroise Morel’s robot did not succeed – it’s
mechanism locked up as it began moving. It appeared to be that the actual construction was
too rigid to allow the smooth motion that such a design should have been capable of, and thus
it was more of a structural fault, as opposed to a conceptual fault. It is this fact that would
have made modelling Ambroise Morel’s design on SolidWorks very beneficial as, in doing
so; I could have simulated the motion and pinpointed the source of the problem. This would
have been possible since the robot was available to us, and so it would have been possible to
take accurate measurements freely, but I was unable to do this, however, due to time
constraints.
It becomes clear, therefore, that modelling serves a crucial and beneficial role in the
production of any mechanical design. Not only does it highlight any flaws in a design, but it
also allows the freedom for the graphics designer to be creative and innovative in
manipulating designs before they are finally manufactured.