Robot Arm
Final Report
Cao Yanxin; Cao Zhen; Liu
Xiaowei; Dominggus Yosua
Suitella; Johan van Mijl
Content
Chapter 1: Introduction ................................................................................................. 3
Background ............................................................................................................. 3
Project description ................................................................................................. 3
Chapter 2: Design Phase ................................................................................................ 4
Design of the arm ................................................................................................... 4
Original design ................................................................................................. 5
Improvements design ...................................................................................... 5
Design of the Basement ......................................................................................... 7
Basement Design One...................................................................................... 7
Basement Design two ...................................................................................... 8
Basement Design three ................................................................................... 9
Chapter 3: Calculation ................................................................................................. 11
Chapter 4: Program phase ........................................................................................... 13
Chapter 5: Final Product.............................................................................................. 16
Chapter 6: Conclusion ................................................................................................. 20
Project conclusion ................................................................................................ 20
Technic conclusion................................................................................................ 20
Group conclusion.................................................................................................. 21
Chapter 1: Introduction
Background
As for the Fontys project, the team was allotted a project from Department of Mechatronics to design a
robot arm for domestic applications, for instance picking up a cup or a book, opening a door. And the
robot arm can be mounted on a platform. The team consists of five members, and all from different
backgrounds. A short overview below;
Name of team Member
Specialization
Task
Johan van Meijl
Applied Physics
Team leader
Cao 'Cecillia' Yanxin
Mechatronics
Presentation
Cao 'Tiffany' Zhen
Mechatronics
Presentation
Yosua Suitella
Power Engineering
Administrator
Liu 'Leo' Xiao Wei
Electrical Automation
Keeper of the minutes
A tutor, Antoon Pepping and a consultant, Nick van der Sanden, are assigned to the team to design and
solve the problems.
Project description
The project that the team got is to design and build a robot arm for domestic applications. This robot arm
should meet specifications given by the department concerning lifting capacity and size. To meet the
requirements, the robot has to be able to pick an object at a distance of half a meter, weighing about half
a kilogram. Furthermore it is required to be operated on battery power, should be mountable on a small
platform and have a minimum of three degrees of freedom. In the end, the robot arm has to be controlled
by, for instance, a joystick, using the parallax propeller control board.
The project starts in September of 2010 and should be completed at the beginning of February of 2011.
During this period, the project can be divided into following parts:
General plan of the project
The project goes through around 18 weeks. It starts with making a general plan about what should be
done in these weeks, and the project is carried out by following the plan.
Robot structure research and selection
The first several weeks, the group works on researching the robot arm on the internet to get ideas how to
get start. Sketches, drawings in inventor are made by every member. The group chooses a final design by
combining the advantages of each design.
Searching and ordering components for the design
After determining the final design, the group begins to search for the components, for instance, motors
and pulleys, which should meet the requirements. Aluminum is chosen to make the robot arm, because it
is light and strong.
Building the design
In the last few weeks, robot arm begins to be mounted, which includes making components for robot arm,
cutting aluminum sheets, drilling holes, installing pulley system, and etc. The final design should be
working properly and, of course, meet the demands. The group also needs to implement systems and test
the robot.
Improvement
The group will make some improvements on the robot, for instance applying spring on the robot to
balance the force, if time is available.
The planning chart shows below
Chapter 2: Design Phase
Design of the arm
We did some researches and found the popular use of robot arm is welding and transport. Then we got
the general idea of design a robot arm.
Original design
This design has 6 freedoms as shown in the picture. This is the first idea of the robot arm; the following
improved designs are all follow is general idea.
Axis 1 connect to the base it can rotate the whole up part. Axis 2 and 3 can do up and down part. Axis 4
and 6 can rotate 360°, and Axis 5 can do up and down work at the same time. This design makes sure the
robot arm reach every position around it.
After creating the model in design software, the axis 4 was considered useless and therefore deleted.
Improvements design
Arm part:
Follow the principle of not too complex physical construction and low weight of the whole construction,
we improve the arm part as follow
:
This design effect decrease the arm part's own weight, then the current motors can be used.
Front part:
The idea of front part is there is a up and down part and rotate part to make sure this robot arm can open
the door , pick a cup of coffee and also a place to fix motor, so the design is improved as follow:
The position to put motors:
Where to put the motors is a difficult decision. One suggestion was to put all motors in the arm, but that
would increase the weight of the parts to lift, resulting in the use of stronger motors. This idea was
discarded.
Then the better idea came out, using two towers on the foundation to fix two heave motor. This way we
just need fix one heave motor on the robot arm, and one light one on the front part.
The exactly position refer to the final design.
Design of the Basement
Brief introduction:
For the basement design, there are three concepts totally.
For the final concept, there are three options of motors.
Basement Design One
Rotation part
Slot under
rotation part
Basement
Slot on
basement
Inclined plane
The place where
bearings put on
Tips:
The upper rotation part will be connected with the shaft by using four screws. The screw holes are visible
on the rotation part. There is a slot in the basement and a slot under the rotation part. Six or eight ball
bearings should be mounted on the fixed certain point to reduce the friction between basement and
upper rotation part.
The design of inclined plane is for ball bearings on the underside of the shaft, which work for the decrease
of friction between shaft and basement.
Motors and chips are going to be mounted in the basement. Enough space is left in the basement to
mount all components.
Why the concept changed:
During the design phase of the base, innovative design passes the review. This results in some special
designs, like the one shown below. This complex design requires a lot of materials, like ball bearings which
are too expensive.
Basement Design two
Tips:
It's improved by decreasing the number of ball bearings.
Only one big deep groove ball bearing will be used to connect the upper rotation part and the basement.
The use of coupling will deal with the problem of the connection between motor’s shaft and rotation part.
Why the concept changed:
The price of the big deep groove ball bearing is far beyond the budget. And it’s really hard to find suitable
one. In other words, it’s very hard to find a deep groove ball bearing with large diameter and small balls.
Basement Design three
It’s the initial design for the design three of the basement, which is also the final concept for our basement
design. There are three options of motor for this design to drive the rotational movement of basement.
（The datasheet is shown in the appendix）
Small motor
First of all this motor was chosen according to the first idea and design of our basement. That is motor
connected to the shaft that drives the upper part (arm). The connection between shaft and motor is by
using 2 gears. In order to keep the shaft steady, flanged bearings are used attached to the shell of the
basement. The motor was mounted to the basement with custom bracket as the former design.
But this motor is not chosen since the torque is insufficient. Furthermore a lot of effort is required to
mount the motor, which would require a gearing step.
Big motor
This motor came out after we had discussion with our mentor. The motor is strong enough, so it could
directly drive the upper part without any shaft. But the disadvantages of this motor are big and too fast.
Furthermore, the motor work in lowest speed is insufficient to drive the robot.
Car Power window motor
A car window motor is a good option on the robot. It is a powerful power-window motor that usually used
to drive the power window in the car. It has stable speed and high torque. Looked from its design it
already has 3 screw holes, so can be easily mounted to the shaft. Even thought so, the design needs
improvement by using kind of “T-shape” facility to attached plate to the motor. The motor is nearly
fulfilling the criteria that nearly fulfilling the required criteria, which is slow but strong.
Finally, the basement design was chosen according to the chosen motors and worked out. The results are
the schematically drawings, which will be shown in the appendix. These were used later on during the
production process.
Chapter 3: Calculation
Calculations of torque and moment of inertia
Theory
Torque is the representation of the force required to rotate a mass around an axis. The torque
in formula by:
is given
= r× F ,
in which r is the distance from the axis and F the force on that point.
The formula is angle dependent, which means that the angle between the distance vector and the force
vector is proportional to the total torque.
Therefore the equation can also be written as:
= rFsin
with
,
the angle between the vectors.
In the calculations the assumed point of calculating is the centre of gravity of each object, with the robot
arm completely stretched. This results in a maximum torque for all joints with the distance and force
perpendicular.
The inertia is only relevant for the parts that rotate in the plane parallel to the axis of the motor. In the
robot arms case the lowest motor for the base and the first motor for the wrist have an inertia calculation.
The moment of inertia is defined as the summation of all mass elements multiplied by the square of their
distance to the rotation axis. In formula form this gives:
I = ∑ mi R2i ，
with m the mass and R the distance from the rotation axis.
Following are the calculations of the arm using previously explained formulas. Afterwards there is an
image explanation of the joints and the locations of the masses.
Distance (m)
Force (N)
Radius (m)
Joint 1
Joint 2
Joint 3
Wrist
Base
0,6
5
0,05
0,5
2
0,05
0,5
2
0,48
3
Distance
Axis
Torque
0,5
0,5
0,25
1,75
0
3
0
0,5
1
0
0,5
1
-0,07
0,68
1,43
Inertia
0,0013
0,18
0,0005
0,05
0,38
3
0,2
3
0,13
3
Total Torque (Nm)
0,38
1,13
-0,15
0,6
0,38
0,43
3,65
8,53
Total inertia (kgm^2)
0,05
0,07
0,04
0,01
0
0,41
Remarks
The calculated torque is higher then the output torque of the motors in the base. These are rated at 3.6
Nm. In this case springs would be required to help the motors.
The other motors should perform with a load of maximum 5 N.
Chapter 4: Program phase
Programming Part
The developed robot arm consist of motors that make the robot can move freely in 4 degree. The motors
consist of 4 DC motors as shoulder, elbow, wrist, and 1 servo motor as the hand itself (grabbing
mechanism). Programing the robot arms means programing the motors sequentially to make it able to do
something as the programmer will, for instance to grab and replace a cup of coffee in a distance.
DC motor will run/rotate based on the amount of voltage induced to the motor. Higher voltage induced to
the motor, it will run faster, higher current flow to the motor, higher torque will be gain. The polarity of the
voltage induced also affect the motor. Positive voltage will make it run clockwise and otherwise it will run
counterclockwise.
PWM (Pulse Width Modulation) is used to control the amount of induced voltage. The average value of
voltage (and current) fed to the load is controlled by turning the switch between supply and load on and
off at a fast pace. The longer the switch is on compared to the off periods, the higher the power supplied
to the load is. The term duty cycle describes the proportion of 'on' time to the regular interval or 'period'
of time; a low duty cycle corresponds to low power, because the power is off for most of the time. Duty
cycle is expressed in percent, 100% being fully on.
Figure example of duty PWM signal
And to control the polarity of the voltage, the H-bridge inverter (embedded in Control Board) is used.
Figure the H-bridge
The H-bridge arrangement is used to reverse the polarity of the motor when switches S1 and S4 on the
motor will rotate to the left. When S2 and S3 are switched on, the motor will rotate to right (other way).
Figure the 2 basics state of H-bridge
The H-bridge itself is embedded inside of the Control Board. In this time, Propeller P8X32A-Q44
microcontroller is used to control the motor. It has 23 output pin for Servo Motor and 2 outputs for DC
motor. The specifications of this board are:
 512KB EEPROM for program and data storage
 3.3 & 5.0 VDC on-board regulators
 Surface mount and through-hole components to populate the board
 Barrel jack for 2.1 mm center-positive 6-9 VDC power supply






4-pin programming header for the Prop Plug
Power requirements: 6 to 9 VDC
Communication: Serial for programming via Prop Plug
Dimensions: 2 x 2 x 0.56 in (50.6 x 50.6 x 14.45 mm)
Operating temp range: -32 to +158 °F (-0 to +70 °C)
8 MHz Crystal Clock frequency
Figure Propeller P8X32A-Q44 board
Spin language is featured programming language used in this Propeller board. And 8 MHz clock signal
from Crystal Oscillator inside the microcontroller is used to control the motor. But the motor run in 60Hz
of clock signal, the signal from crystal oscillator was too fast to be applied in the motor. To achieve certain
clock, interrupt command is used to make the signal became slower. Following are the equations.
Using 8 bit overflow interrupt with prescale 1the program is executed in
= 1 (prescale) x 2^8 x 0,125 us (period of 8 MHz clock)
= 32 us
60Hz = 0,166 ms
And using repetition command
= 0,166ms
32 us
= 520, 83 or equal to 520 times repetition to get 60 Hz clock signal
After the signal builder is done, the next step is just change the value of duty cycle in % (percentage)
Potentiometer (regulated resistor) is used to gain a feedback from the motor. The potentiometer is
connected to the angle (rotation part), so it will change if the angle also changes, and the changing of
potentiometer is equal with the rotation changing (from the motor). The potentiometer is connected with
Wheatstone bridge where the bridge also connected to the DC supply. So if the value of the resistor is
change due to position change the voltage output across the bridge will also change. It can be an
indication of the position of the motor. But the output from the Wheatstone is an analog signal so it needs
to be converted in digital signal to be used as a feedback in the microcontroller. To get those conversion
ADC chips is used.
Potentiometer (regulated resistor) is used to gain a feedback from the motor. The potentiometer is
connected to the angle (rotation part), so it will change if the angle also changes, and the changing of
potentiometer is equal with the rotation changing (from the motor). The potentiometer is connected with
Inverting Op-amp where the input and the op-amp connected with DC voltage source. Using the following
configuration the changing of the resistance in the circuit can draw between 0-10V output’s voltages. So it
can measure the position precisely.
V out = - R2 x V in
R pot
The output from the Op-amp circuit is an analog signal so it needs to be converted in digital signal to be
used as a feedback in the microcontroller. To get those conversion ADC chips is used.
Chapter 5: Final Product
3D drawings:
Real final product:
Chapter 6: Conclusion
Before finishing this final report, we are going to make a conclusion for this project from three aspects,
which are: project conclusion, technic conclusion and group conclusion. Project conclusion is related to
how we control the progress of the project and what we learn from the project. Technic conclusion is kind
of conclusion related to the knowledge of science and some technical tips for this project. Group
conclusion explains how we deal with the communication with the group during the project.
Project conclusion
Having meeting with teacher once a week is very important.
Holding Regular meeting once a week makes the members always be responsible for concluding progress
before the meeting and planning better after the meeting.
Communicate with tutors as much as possible.
All the projects in school focus on education. Finishing the project is a mission, while learning something
from it is the final goal. Group members always can get some new inspirations from the conversation with
the tutors.
Some deadlines can be postponed, under the situation where group members have contemplated the
reason and make sure can catch up later.
Not everything can be controlled by people. Some progress can be stopped easily by a simple reason. No
matter how simple it is, the group need to know what it is and make sure the project can be caught up
later. What if we can’t, and then change the plan.
Technic conclusion
The servo motor is easy to program but is not as strong as DC motor.
Using the spring on the lamp is good design. Actually the whole arm can bring the idea of the design of
lamp.
There is much more space for improving the position of the motors.
Considering the fact that the shoulder part, elbow part and wrist part should be lighter and lighter in
sequence; the position of the motor can influence the value of moment of inertia:
Group conclusion
Every team member should know the progress of the project clearly, even other team member’s
deadline.
This is a very important factor for the communication within the group. Everyone’s mission is just a small
part of the project.
Should be fair, but there’s no need to be concentrate on being or not being fair.
Focus everyone’s energy on our own mission instead of calculating too much about who works more and
who works less. But all of this should be under one situation where everyone is responsible for their own
part and care about the whole project. One’s mission is according to his/her ability. A good team is that all
the members are working for the team and the final goal, not the personal benefit. Have to mention that,
if a team eventually gets its glow, everyone in the team will get a radiant smile definitely.