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The Project Report
NAME OF THE TEAM
Istanbul High School TÜBİTAK Research Group
NAME OF THE PROJECT
Improvement of Motion Systems By Using Nickel-Titanium Alloy.
INTRODUCTION
Motion is mostly provided by using electric motors and many different methods.
However these methods generally cause noise pollution and the parts that provide movement are big in
volume and also heavy. Alternatively to these methods, studies about shape memory alloys are done.
Studies done about shape memory alloys are concentrated on medical science and dentistry. In this
project, Nickel-Titanium wire which is one of shape memory alloys is researched. By developing new
movement methods with these wires, it was shown that Nickel-Titanium wires can be used in different
kinds of motions.
GOAL
It is aimed to produce a new and different alternative to motion methods used today. At the same time it
is intended to contribute clean technology, that has a rising importance nowadays. Moreover the
purpose of the project is to bring a new perspective to the studies about robots, to add different methods
to science, to spread the usage and production of shape memory metals by introducing them to the
researchers in our country.
TARGET
The target of this project is to obtain motion of vehicles in small dimensions by using Nickel-Titanium
alloy.
METHOD
Many different methods, generally motors, are used to obtain motion in vehicles.
For example, there is a high ratio of energy loss in electric motors because of friction. Besides, while
working they compose noise pollution, they are heavy and big in volume. They need service after
corrosion by time. These are disadvantages of these systems. Owing to these reasons, energy loss
reaching 40 % can be seen in these motors.
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It was decided to use shape memory alloys as an alternative to these activators. Shape memory alloys
are made of a metallic and crystal structure that can return to original shape determined by heat. And
they have two stable phases, named martensite and austenite. Martensite is low temperature phase,
whereas austenite is high temperature phase. These phases occur because of shape changes in crystal
structure of Nickel-Titanium alloy.”
Shape 1. Crystal form of Shape Memory Alloys
By means of having two phases, shape memory alloys can be used as motion provider in any system.
When they are used in this way, they are more advantageous than other systems. One of these
advantageous is that they operate in silent, clean and non-glittering way and nearly occupy no place.
These wires can work in both alternative and direct current. They are influenced less from magnetic
fields than electric motors. In addition to this, when compared to all movement methods, they have the
highest power/mass (power/volume) effect ratio under low masses (under 100 gr.). Due to this property
and simplicity of the mechanism, they are very charming in micro scaled movements.
Despite these advantageous, Nickel-Titanium wires make things difficult at installation stage and the
wires must be prevented from heating too much.
In the project, it is aimed to make moving vehicles with these shape memory alloys. For this, Nitinol
was chosen as a shape memory alloy. Nitinol is an alloy consisting of 51% Nickel and 49 % Titanium.
It was first found in ‘Naval Ordnance Laboratory’ in 1961. It took its name by adding first letters of
Nickel, Titanium and Naval Ordnance Laboratory.
Nitinol can work till 650°C according to production type. The most important usage areas are medical
science and dentistry.
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Property
Melting temperature (°C)
Density (g/cm3)
Electric resistance (Micro Ohm*cm)
Thermal conductivity (W/cm*°C)
Specific heat (J* kg -1*K-1)
Conversion temperature (°C )
Hysteresis (°C)
One way memory voltage
Value
1300
6,45
Austenite
Martensite
Austenite
Martensite
500
- 100 / +100
5 / 50
3/8
100
70
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8,5
Shape 2. Nitinol's Properties
The wire form of Nickel-Titanium alloy was used. These wires can become 8% short of their length
while changing phase from martensite to austenite. The time of shortening is related with current time.
Current increases with direct ratio in accordance with the thickness of wires. Thick wires are more
powerful than thin wires. They can carry from 20 gr. till 2kg. depending on diameter. By using wires in
different diameters, the wires that give the needed power can be determined. But it must be paid
attention to activation and cooling time.
The energy consumed for wire changes with ambient temperature, length and hardness of wire. Just for
a while much energy can be given, but it must be prevented that the wire is not heated much.
Otherwise, the wire loses it shortening property.
The electric energy used to activate 10 cm. wire changes between 2 V and 4 V with respect to type of
wire. It must be paid attention that current must not exceed the catalogue value. Voltage changes
according to thickness of wire. For example, 2 V is required to activate 0,15mm. diam. Wire, whereas it
is 4 V-4,8 V for 0,1 mm. diameter wire. Furthermore, environmental conditions must be stable to
achieve healthy experiments.
After this, it is decided that wires under 50 micron diam. are not suitable for experimental use.
Diam eter (μm )
37
50
100
150
250
Resistance (Ohm / m ) Optim um Current (m A) Total Force (N) Maxim um Load (kg)
860
30
1.7
0.17
510
50
3.43
0.35
150
180
14.7
1.5
50
400
32.3
3.3
20
1000
93
9.3
Power per m eter (W/m ) Activation Tim e (s)
Relaxation Tim e (s)
0.4
0.1
1
0.1
5
0.1
8
0.1
20
0.1
1.1
1.2
1.7
2.9
6.6
Maxim um Num ber of Contraction / m inute
52
46
33
20
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Shape 3. Catalogue values of Nitinol wires
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At the end of examination on catalogues, owing to the difficulty of making models with wires under
100 micron diameter and the flow of excess current in wires over 100 micron diameter, 100 micron
wires were decided to be used.
In the first experiment, without passing current through 10cm. Nitinol wire, different weights were
hung. The regular points of the stretching were identified.
Shape 4. Experiment-1
Shape 5. Mass- Stretching Percentage Graph
As a result of this experiment, the range on which the stretching is regular was determined and it was
decided to hang 200gr weight. In the following experiment, 200gr weight was hanged up to NickelTitanium wire and the percentage of shortening was measured by passing various current degrees
through the wire.
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Shape 6. Experiment-2
Shape 7. Power-Shortening Percentage Graph
By the side of this experiment, at least 0,22 watt (120mA) is required to change Nitinol’s phase to
austenite (shortening) and shortening percentage of wire reaches the maximum value on 0,75 watt
(230mA).
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Shape 8. Resistance-Current Graph
According to this graph, as current is passed through the Nitinol wire, the resistance of wire decreases
until a specific value. The reason for this is the shortening of wire’s length.
In the next experiment masses were changed at 190mA and the shortening percentage was measured.
The reason for using 190mA is that sudden changes occur in the resistance below this value. This
change makes it difficult to fasten the current.
Shape 9. Mass-Shortening Percentage Graph
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Shape 10. Mass-Power Graph
In accordance with this experiment, the power needed to lift up the weight increases with the mass. For
all that shortening percentage decreases.
APPLICATIONS
The goal of the project is to set up motion systems by using wires’ superior properties in the best way.
These properties are lightness and occupying less place than electric motors.
For this reason, it was aimed to set systems that occupy less place. However, installation problems were
met as systems were small. Due to shortening percentage could not move the system, levers were
decided to be used.
1. Six Leg Spider
Shape 11. “Six Leg Spider”
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“Six Leg Spider” was made to learn working principle of Nitinol and to start practising with a model.
Spider was started to be formed by defining movement and structural properties. Legs were fit to front,
back and middle of body in a moving way. In this way, the system was made up of single part each
having one right and one left leg, instead of six leg working independent from each other. Thus, Nitinol
could move ahead as much as possible. Nitinol was bound to front and back of body by passing over
the middle body. Thereby, friction was decreased as much as possible.
Shape 12. Working Algorithm of “Six Leg Spider”
A circuit was set up by using a microcontroller to check Nitinol wires. PIC16F84 was used as the
microcontroller. It was produced by the company “Microchip”, could work at 20 MHz and had 1 KB
flash programme memory. The reasons of why this microcontroller was chosen are that it is one of the
simplest microcontrollers, cheap and widespread. In “Six Leg Spider” microcontroller was used at 4
MHz, because the spider does not need fast operation and the timing of PIC16F84 can be controlled
easier at 4 MHz( 1 operation = 1 second ). “PIC Assembly” language was used to programme
microcontroller. By connecting a LED to PIC16F84- PORTA.0 pin at the circuit, some information
about how the programme works was obtained.
Shape 13. The Control Circuit of “Six Leg Spider”
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At the most 20mA current can be passed through PIC16F84’s pins to move spider automatically. But
Nitinol wires can work at least 120mA. For this reason, ULN2803 integration was added to control
circuit. The function of this integration is to strengthen the circuit with transistor. After connected as in
the circuit, current is directed to the defined entrance. It pulls the output to the ground.
2. Wheeler
Shape 14. “Wheeler” (Old version to new version)
The next goal is to make a circular motion. For one turn, Nitinol wire has to pull the wheel from
different points. However after the wheel turns 360º, all wires will entangle. Because of this, a rotating
ball was put on the wheel and Nitinol wires were connected to this ball. So wires were prevented from
entangling while turning. While designing, it was determined that 5% shortening of Nitinol wire would
not be enough to turn the wheel. As few wires as possible were decided to be used for this micro scaled
vehicle. Therefore levers were used in the system. In this way covered distance was increased.
The vehicle was designed and four levers were decided to be used. The locations of the levers on the
vehicle were defined. The angles between levers, distance between lever and the wheel, the distance
from the ball to centre of the wheel were calculated. The levers were connected to the ball with ropes
and these ropes were supported with steel wires.
According to calculations to provide the pulling, the angle between iron sticks and the lever must be 90
°C only if the lever is tangent to the wheel.
Lastly, 3 dimensional was drawn to concretize ideas and drawings and to understand the model better.
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Shape 15. Working Algorithm of “Wheeler”
A sheave system was used in this vehicle as planned. By this way, the levers were set more regularly
around the wheel and a stable turn was obtained. But as wheels and cylinders could not be found in the
planned size, the application was bigger and heavier than designed.
Shape 16. The Control Circuit of “Wheeler”
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3. Two Leg Robot
After “Wheeler”, two leg robot was decided to be done. Target is to make two leg robot that keeps its
balance by using Nitinol.
Before making robot, two leg robots were searched. And some information about how they move, how
they keep balance was learned. Movement diagram of robot was drawn. In the diagram, it was defined
when wires shorten, when they loosen. At the end of this study, below diagram appeared.
Shape 17. Working Algorithm of “Two Leg Robot”
Robot was drawn in three dimensions to make things easy during formation.
Shape 18. “Two Leg Robot”
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Springs were decided to be put to joints to hold robot up. Because a flexible object was needed that
could enforce Nitinol, at the same time support legs and the body and balance our robot upright. To set
up a balancing system, the body was composed of two parts. These parts were combined to each other
with a spring from the middle. And corners of body were joined together with Nitinol.
The body would move to different directions, when these four wires that were combined to the corners,
shorten and loosen. In this way the corrupted balance would be improved. To operate the wires in
intended way, firstly it is required to state toward which direction the stability is lost. An acceleration
sensor was used to determine this. The data from acceleration sensor was checked with a
microcontroller and it was aimed that our robot would react suitably.
The body’s reaction time to the data from acceleration sensor must be as short as possible.
Otherwise the robot will fall down losing stability. A fast operation power is needed to control data
from acceleration sensor because acceleration sensor works with ADC.
A production of Philips , LPC2138 was used instead of PIC16F84 as microcontroller to bend the body
as fast as possible. This microcontroller was much faster than PIC16F84. This microcontroller can
work at 60 MHz, has got 512 kb programme memory and 16pins can be used for ADC. All these
properties satisfied our needs extremely.
The circuit to control “Two Leg Robot” was made. In this circuit, the power that would be given to
Nickel –Titanium wire was arranged with voltage regulators. ULN2803 integration was used to control
the current.
Shape 19. The Control Circuit of “Two Leg Robot”
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Some electronic parts used for this project and their properties:
Microchip PIC16F84:
- 1kb flash programme memory
- 68 byte RAM
- 64 byte EEPROM
- Maximum 20 MHz core frequency
- Working temperature : -55°C / +125°C
- Storage temperature: -65°C / +150°C
- Working voltage: 4.5 V – 5.5 V
Philips LPC2138:
- 512 flash programme memory
- 32kb RAM
- Maximum 60 MHz core frequency
- Working voltage : 3 V – 3.6 V (3.3 V ±%10)
- Working temperature: -40°C / +120°C
- 47 inlet and outlet pins for general usage
- 16x10-Bit ADC pin
- 10 bit DAC pin
Motorola MMA7260Q
- 3 pivot sensitivity
- Adjustable sensitivity (1.5g/2g/4g/6g)
- Low current requirement : 500µA
- Low voltage: 2.2V – 3.6V
- Working temperature: -20°C /+85°C
LM317:
- Adjustable output voltage
- Maximum current : 1.5A
- Voltage output : 1.2v - 37 V
- Working temperature : -40°C /+125°C
- Storage temperature : -65°C / +160°C
LM7805
- 5 V constant output voltage
- Working temperature : -30°C /+150°C
- Storage temperature : -55°C / +150°C
- 7V – 25V input voltage
ULN2803
- Maximum 50 V input voltage
- Working temperature : -20°C /+85°C
- Storage temperature : -55°C / +150°C
- Maximum 500mA current for all pins (continuously)
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CONCLUSION AND DISCUSSION
Some mistakes have been seen in the experiments. The first experiments were done with NickelTitanium wires which were brought from America and 8% shortening was seen. After that, differences
were seen on reaction time and shortening percentage in the wires that were brought from Germany
(German wires can shorten maximum 5 % of their length, whereas American wires 8%). Therefore,
there are discrepancy and mistakes in results of the experiments. Besides, measurement devices and
mechanism of experiment do not work with desired accuracy. For this reason measurement errors occur
in datas.
According to the experiments, Nickel-Titanium wire works most efficiently in 170mA. It starts to
shorten in 120mA. It begins to change the phase from martensite to austenite in 120mA.
As the current passed increases, heating of wire increases too. So shortening percentage goes up.
Related to this, after a certain flow a big difference is seen in wire’s shortening. The reason for this is
the changes in crystal structure of the wire. The diameter of wire increases and the length decreases
with changes in crystal structure. As seen in the experiments, there is a big increase in shortening
percentage in 170mA. Because at 170mA wire completes its phase change in a big ratio. There is not a
big difference in percentage of shortening at higher currents. But the current is fixed at 190mA when
current is stabled in experiments and applications. The reason of why it is fixed at 190mA is that
sudden resistance changes are seen below 190mA. This change complicates to stable the current.
According to data’s obtained from the experiments, the efficiency depending on the mass and electrical
power is calculated.
Efficiency=
η=
Work done
Energyused
ΔE Pot m.g.Δh
=
E El
V.I.t
To find out the electrical energy used up, the elapsed time of shape change in crystal structure of
Nickel-Titanium wire must also be measured. But with existing equipments the elapsed time of current
in and out of the wire cannot be measured. Because of this the reaction time in catalogue is used to
calculate efficiency. As the value in the catalogue is not certain, mistakes occur in efficiency
calculations.
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Shape 20. Power-Efficiency Graph
According to the data, the wire works most stable at 0,47 watt ( 190mA ) when 200 gr. is hung on
Nickel-Titanium wire. In the applications when shortening was not enough, 0.69 watt (230mA) was
given.
Shape 21. Mass-Efficiency Graph
The weight which Nickel-Titanium wire could lift at constant current (190mA) was identified to use
most efficiently in exercises. The experiments showed that Nickel-Titanium wire works most
efficiently when 0.47 watt is given and 300 gr. is hung. The efficiency is 22.5 % with these values.
After finding the force needed to provide movement in the applications, the points to which NickelTitanium wire would be combined to be defined.
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As the applications were done completely with hands, there have been some difficulties in installation
step. This caused the applications not to be as small as planned. Also no devices, as light as needed
could be found. For this reason applications are in model stage at the moment. The ways to produce
lighter and smaller models are being looked for. By this way, it is aimed to move the systems as
planned.
In “Six Leg Spider”, it was aimed at having as high and long steps as possible. In this way, the practice
was provided with moving easily on rough and smooth surfaces. In addition, this practice can be used
in the places where the friction is high. It can climb to plane obstacles thanks to its high steps. This goal
was achieved. The studies are going on in order to let this practice to have a lighter structure.
In “Wheeler”, the improvement was provided with circular movement. This application can move
much faster than the other ones with the help of circular movement. Another feature of this application
is that it works in a silent way while moving in circular motion. With this feature it can be used in
different areas. As weight of vehicle is heavy enough, power requirement is high. Studies are being
done to reduce the weight.
In “Two Leg Robot”, the desired result has not been achieved yet. It was understood that this vehicle
could not move because of the mistake done in planning step. The centre of gravity is in upper part of
the body. This hardens to balance. Although microcontroller works fast, the heating-cooling speed of
Nickel-Titanium wire is not enough to balance the system. However, this design and system is an
example to the robots that can be designed in the future.
Source
-
Erfolgreich Experimenten mit Nitinol Mini-Robotern – Fied Wagenknecht. (2000),
München, Germany
Muscle Wires Project Book – Roger G. Gilbertson (2000), ABD
Prof. Dr. Ayşegül Akdoğan – Şekil Hafızalı Alaşımlar (Shape Memory Alloys)
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