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Mechanics model:
1. Guidelines
2. Material choices
3. First design
4. Second design

1. Limited dimensions
We wanted to keep the dimensions of the robot reasonable, essentially for aesthetic  

2. Mobility
The whole assembly should be easily transportable.

3. Sturdiness
The device must resist to a normal use.

4. Good hiding of the electronics
Parts such as wires, PCBs, etc. should be concealed and invisible to the user once the robot  
is assembled.

5. Ease of assembly
The assembly had to be kept in mind during the design of all the parts.

6. Reasonable budget
We wished to avoid too much expensive parts.
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In order to prepare the design of our robot, the first step was to establish a list of the  
specifications that we wished:
Material choices

These requirements being settled, a first design was conceived. Before presenting it, let’s  
review the different material choices that were made:
External case
The material chosen for the case is wood.  Firstly because of its opaque nature, allowing us  
to hide the electronic components inside the case.  Since laser cutting was planned to obtain  
the parts, we narrowed the research to two possible materials: multiplex or MDF.

Positioning system
As explained in the homepage of this website, the principle of our robot is to write the time  
on a surface and erase it continuously. To do so, it has to be able to move the writing  
device in two directions.  This implied a review of the different 2D-positioning systems that  
could be applicable. In other words, we had to find a way to transform the rotation  
movement of our motors into a linear motion. Three systems were retained:
The first system considered is composed  
of a conventional spur gear and a rack.  
It has the advantage of being quite  
compact and its parts are easy to find.   
It could even be manufactured by  
ourselves (through 3D printing).
1. The gear and rack drive
2. Belt drive
3. Leadscrews
This system has good accuracy but was  
quickly abandoned because of its size  
and more importantly the cost of its  
We also thought of using pulleys and  
belts to achieve the positioning.   
Although the slip problem is less  
important than for the gear and rack  
system, they need more room because  
a tensioning system must be added to  
ensure its good operation.
Finally, for the first design, we chose the gear and rack system.  Indeed, we appreciated  
the fact that it was easy to obtain and to place on the structure.

Writing head
After the problem of positioning had been dealt with, we had to find a way to achieve the  
up-and-down movement of the writing head on the surface (determining when the tip is  
writing or not).  Two systems were considered:
In the end, our choice was to use the solenoid.  Its control is simple and its dimensions are  
1. DC motor/servo/stepper with screw
System consisting in a small motor  
connected to a screw shaft.  The  
writing tip should then be connected to  
a threaded part to achieve linear  
The other possibility was to use a  
solenoid.  This device generates a  
magnetic field that is able to push or  
pull the metallic shaft.  It is more  
compact than the previous system.
2. Solenoid
At first, we considered using a piece of a conventional  
whiteboard as a support for writing the time.   
However, this choice was abandoned to the profit of  
“whiteboard film”, a flexible writing surface  
presenting the same characteristics as a whiteboard.
Writing surface
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First design

Once the choice of the different components had been made, we could begin the  
development of a model. This was done using Autodesk Inventor:
As seen on the picture, the positioning system lies on top of the global assembly.  Both the  
x-axis and y-axis movements are provided by motors and gear-and-rack transmission  
systems.  The two motors stand on a mobile wooden part that is guided by “rails” on two  
locations. One of the motors is attached by screws directly to the wooden part and the other  
is unified to a sliding plastic piece (3D-printed). To hold the solenoid in place, a second   3D-
printed support is considered. To erase the time, a standard whiteboard eraser would be  
placed on the stepper support.
Although this design helped us to get information on the different components available to  
realise the robot, it posed four important problems:
Second design
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1. Friction
As most of the wood used for laser-cutting have a rough surface, it is  
probable that the mobile wooden part would have difficulties sliding on its  
rails. A potential solution would be to use another material such as  

2. Only one side of the slider is driven
This could cause the wooden slider to rotate and then to block.

3. Two motors on the slider
This might cause overload on the motors due to the important mass.

4. Assembly problems
For the first design, a screw assembly was considered.  This could have  
caused issues at some points where many parts must be assembled (e.g.  
the bottom part, the cover and the rail).
All in all, these issues (mainly the friction issue) caused us to drop this design and restart  
from scratch.
The second concept was designed to avoid the previous problems:
It contains numerous improvements over the first concept:
1. More compact
The first thing to notice is that it has been made more compact since now  
all the components are in the same space.

2. Positioning system
Movement along the vertical axis is now ensured by two belts.  On these  
two belts, a support formed of 3 plexiglas layers is design to hold the  
solenoid in place.  To adjust the tension of the belts, we imagined a new  
system. It is composed of three identical 3D-printed pieces, three bolts and  
an axle. The pulleys are mounted on the shaft and it is then possible to set  
up their position by screwing the bolts, giving result to more or less tension  
on the belts. For the horizontal movement, two shafts support PVC tubes  
(using bearings). Around these PVC tubes, the whiteboard film is rolled. To  
achieve the rotation, the stepper motor is connected to the right PVC tube  
thanks to a belt.  The tensioning system is similar to the one described  
above. Note that this time, to erase what is written on the whiteboard  
film, an eraser is placed under it.

3. External case and supports
The material for the case was kept the same as  
before (MDF).  However, to assemble the  
different panels, a “finger joint” assembly was  
chosen.  This allowed us to avoid using screws  
and to give a nice look to the exterior of the  
robot.  Of course, to make the case sufficiently  
strong, the use of wood glue was considered. On  
top of this modification, three panels were  
added to house the different mechanical  
components and ensure global stiffness of the  
robot. The supports for the vertical axis stepper  
were designed to be laser-cut in the same  
material as the case.
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RoboCLOCK - Mechatronics Project

Technical Details - Mechanics model
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