DESIGN DEVELOPMENT
1.1
Overview
A detailed design process was followed to develop the Shape Memory Alloy
(SMA) actuated gripper as the novel actuator. The customer requirements were
first obtained from advisor by Dr.Y.W.R. Amarasinghe. Benchmarking on those areas
was completed once the specifications were determined. Brainstorming was then
conducted to generate possible ideas for the SMA robotic hand. Applying a
design selection matrix then reduced the multitude of ideas that were generated
by the brainstorming. With the final design ideas selected, the pertinent
calculations, solid modeling and preparations were completed in order to make
the Critical Function Prototype of the gripper.
1.2
Benchmarking
Journals, books and internet sources were consulted to find information relating
to the structure and strength of the gripper. In addition, robotic grippers’ structures
and SMA wire characteristics were researched. These sources determined what
products were currently available, research that was previously completed,
current SMA wire applications and other pertinent information relating to the
project.
4.3. Brainstorming
In order to develop a wide range of possible ideas concept mind maps were
completed for each major component of the project. This enabled a wide range of
ideas to be explored before selecting on a final design. Mind maps were completed
for the gripper structures, the transmission-actuator methods and the holder.
Several possibilities came to light from the mind maps concerning structures
and shapes of the robotic gripper. The possible gripper shapes included both
open and closed structures.
1.3
Experimental Process
The main purpose of this effort was
to conduct original research and provide previously unavailable temperature
dependent performance data of SMA specimens. With this data, design guidelines
for implementing SMAs as deployment actuators can be developed. A unique
characteristic of SMAs is that “all properties change significantly at the
transformation temperatures Ms, Mf, As and Af”
meaning the transformation temperatures of the material must be known in order
to draw meaningful correlations between the structure of the material and its
fracture when heat is applied.
1.3.1
Heat Treatment Process
First we will try to understand the
behaviour of our purchased nitinol sample; at the very beginning we tried to
observe whether our received sample has any reaction with temperature changes.
So we used hot water to increase its temperature form ambient to 1000C
and then cool down it from ambient to below zero degrees of Celsius valves. In
above cases we didn’t see any shape memory effects. Then we wanted to undergo
in a heat treatment process to achieve our objective. First we used blacksmith
fire and increased its temperature for about 1400C for 15 minutes and then
quenched it. But after this process also it did not show any changes with
temperature. So we used the oven in
mechanical engineering production lab of the university to do our heat
treatment experiment process by varying temperature and heat treatment time
period.
1.3.2
Heat Treatment Methodology
In heat treatment process first we give
heat and increase the temperature of the specimen to a relevant value and then
remain in given temperature for a specific time period. After that we got the
specimen from the oven and quickly quenched the specimen while cut offing the
heat supply.
Temperature
(0C)
|
Time (hrs.)
|
Transient temp. approx. (0C)
|
300
|
1
|
Below the ambient
|
400
|
1
|
Below the ambient
|
500
|
1
|
30-40
|
1.5
|
40-50
|
|
550
|
1
|
45-55
|
1.5
|
> 55
|
|
650
|
1
|
> 60
|
1.5
|
> 60
|
After
the heat treatment experiment we observed transient temperature increase with
heat treatment temp and time. By considering above experimental details we
selected transient temperature around 35-45 C (5000C, 1hour). Because
it is the easiest achievable temperature near to the ambient temperature.
1.3.3
Induce heat for specimen
Our second problem was how to achieve the controlling
capability just above the room temperature (35-45 0C) of specimen to
get the relevant shape memory effect. So we identified several techniques that
can be applicable for our application.
Heating method
|
Consequences
|
Hot water channel
|
bulky, no quick response
|
Resistive heating through
NITINOL
|
selected specimen has low
resistance (4Ω)
|
Heating element
|
Since
we are focusing on applying our shape memory sample for robotics applications, using
of hot water chance can be form many drawback like bulky in size and high
response time for operation. When consider resistant heating technique it also not
applicable due to low resistant of our selected sample. This technique is widely
used for Nitinol wires that are having very narrow cross section (micro meter
level). But our selected sample should have considerable cross-section to
obtain required full force in the application. So we decided using the heating
element is the best option to increase the temperature of our sample. Because
we saw some comparative advantages in it like compact in size, quick response
than other techniques and we can easily control the temperature by changing the
amperage across the heating element. These heating element must have higher
resistant and stable in higher value of temperatures. So heating elements like
Tungsten & Molybdenum would be ideal for the application.
1.3.4
Pull Force Calculation Practical
Our
next target was calculating the pull force which can be delivered from various
cross sectioned strips. So various cross sectioned Nitinol strips where cut
from EDM wire cutting having length of 40mm and cross section of 0.5mm × 1mm,
1mm×1mm, 2mm×1mm and 4mm×1mm. from that we prepared four strips form each sizes
for work. Those pull force values were taken into consideration when deciding
the required size of Nitinol strip for the gripper assembly.
1.3.5
Insulation requirement
As our selection of method for the heating is the application of the
heating element around the Nitinol strip wire. Since, Nitinol has very low
thermal resistivity around 0.76x10-6 Ωm, it
is needed to insulate the Nitinol strip to avoid the current passing through
it. Our primary requirement is to facilitate the heating element for the smooth
current passing along its only.
Typical
insulators are not suitable in this application as the most of the insulators
are electrical and thermal insulators. Though we want to insulate the Nitinol
strip from the electrical flow conducting through the heating element, we want
to absorb the heating conducting through heating element. Following insulators
are identified as electrical insulators but not thermal insulators that are
being used for heating element applications.
Insulator
|
Suitability
|
Disadvantage
|
Mica Insulator
|
Thermal conductivity of 0.528 W m-1 0C-1
|
Mica surfaces can be cracked in the long run
as the reduced fatigue life.
|
Tungsten Insulated Wire
|
Have high thermal conductivity of 1.7 W m-1
0C-1 and lengthy fatigue life.
|
Too much expensive.
|
Rubber Pad
|
Excellent thermal performance and rework capabilities.
|
Bit hard to use in a user defined
application.
|
1.4
Design Specifications
A robotic gripper is being constructed in order to showcase an
application of a Shape Memory Alloy (SMA) actuation system. These SMA’s will be
placed into a groove to create a muscle array. This SMA is actuated by using
the heating element in order to flow the heating and cooling around the SMA
wire. Once heated, the SMA wire undergoes a phase change causing the wire to
contract. The degrees of freedom of the SMA wire is one, as the SMA wire is
contracted then extracted as the temperature around the SMA wire is varied with
the time. The SMA wire’s initial force is greater at the beginning of its
contraction then at the end, technically referred to as reverse biasing.
Therefore, the robotic gripper will incorporate a mechanical device to
compensate for the reverse biasing of the SMA wire.
In order for the force to be transmitted from the SMA wire to the gripper,
mechanisms that will function similar to that of a human hand will be employed.
These mechanisms are similar to the animal behavior seen in the environment and
such mimicking of the behavior is used in the existing SMA based robotic
actuators as we have seen in the literature review too.
1.4.1 Primary approach of 3D modelling
This was our first design and it was developed in order to get an idea
about how a robotic gripper can be actuated through a Nitinol strip. But in
this attempt what we have learned was the heating element would generate high
heat in the Perspex assembly. Therefore, as a secondary attempt we have decided
to install a Nitinol strip outside the Perspex assembly. That would eliminate
the contact of Perspex and heating element so heat generation in the heating
element would not affect the Perspex assembly.
1.4.2
Secondary Approach
In this design we have avoided the defection of the primary design which
was the contact between heating element and the Perspex. The contraction of the
gripper tip is generated by the actuation of the Nitinol strip with application
of current. The extraction of the gripper tip is caused by the spring return
mechanism supported at the both sides of the handle.
As we have eliminated the defection in the primary design, in the
secondary approach we can analyze the variation of the force generated at the
gripper tip according to the variation of the temperature.
1.4.3
Controlling Circuit
Both voltage and amperage can be altered to achieve the differencing of
the temperature of the heating element. As manipulating the voltage is easier
normally, we have decided to build a controlling circuit based on voltage
differencing.
Supplying heat around the is controlling through a voltage regulation
using a mosphate based drivers with Pulse With Module. PWM signal is generated
through Atmel microcontroller.
1.4.4
Nitinol behaviors analysis with input voltage
Heat transfer through
Tungsten coil is calculated using following equation
dE/dt
= W –H
E = thermal energy stored in wire
W= power generated by Joule heating
H = heat transferred
to surroundings
E = CwT
Cw
= heat capacity of tungsten wire
W
= I2 ×Rw
Rw
= Residence of tungsten coil
Heat transferred to
surroundings is taken as H,
H
= Σ (convection to fluid+ conduction to supports + radiation to surroundings)
For equilibrium
conditions the heat storage is zero:
dE/dt
=O
∴ W=H, and Joule heating W equals the convective heat
transfer H
To
find the temperature in the Nitinol, we have taken in to consider the following
assumptions,
- Radiation losses neglected
- Steady state condition arrived
- Tw uniform all over
the length of the wire
- Fluid temperature and density
constant (Air)
Figure 4‑9 Thermal
conductive pad insulated Nitinol strip wound with heating element - tungsten coil
Resistivity of
tungsten element (~300K), ƥ = 52.8 nΩm
Diameter; D = 0.025 mm
Length; L= 6.6 cm
Resistance
of tungsten element, R;
R = 7.099 Ω
Power generated by
Joule heating, W;
W= (V2)/R = mC(Tw-Ta)
Surface
temperature of Tungsten coil, Tw ;
Tw = V2/(R×m×C)
+ Ta
Tw = V2/{7.099
Ω [
( 0.00252 6.6) cm3 × 19.3 × 10-3 kg/cm3] ()}+280C
= V2 /0.237765074 +28
=
Temperature of tungsten element at given voltage value
Considering
the temperature of tungsten element equal to temperature of thermal conductive
pad;
To =
Outside temperature of thermal conductive pad = Tw
Ti = Inside temperature of thermal
conductive pad
= temperature of Nitinol strip at given voltage
h2= Thermal
conductivity of thermal conductive pad = 5 W/mK (0-1000C)
t2= Thickness of
thermal conductive pad = 0.5 mm
A2 = effective area =
{(0.5 +1) × 2 mm 4cm} = Surface area of Nitinol strip
By
using Fourier’s law
Qx= -k
dT/dx
V2/R = -k
(Ti-Tw)/dx
V2/7.099 Ω
= -5 W/mK (V2 /0.237765074 +28- Ta) K/ 0.5mm
Ti = V2 /0.237765074 +28
K
≈ Tw
Because,
insulation thickness is so smaller though insulation has low thermal
conductivity
Input
Voltage
|
Temperature
of Nitinol strip
|
1
|
34.73
|
1.5
|
43.14
|
2
|
54.92
|
2.5
|
70.06
|
3
|
88.56
|
3.5
|
110.4
|
4
|
135.7
|
4.5
|
164.3
|
5
|
196.2
|
5.5
|
231.5
|
6
|
270.2
|
Since Nitinol temperature vs. deflection can be get through a coupe
field analysis in Ansys, we have model the Nitinol strip in there. Direct
couple field analysis has been taken into account and we have developed a
material model for Nitinol properties. As the temperature varies with the
voltage differencing in the nitinol the apparent deflection at the tip of the
Nitinol simulated in Ansys model.
Temp (C)
|
Deflection(mm)
|
45
|
0.713
|
55
|
1.024
|
65
|
1.335
|
75
|
1.645
|
85
|
1.955
|
95
|
2.265
|
105
|
2.576
|
115
|
2.886
|
125
|
3.197
|
Temperature (C)
|
Young's Modulus (Gpa)
|
45
|
42
|
55
|
50
|
65
|
61
|
75
|
65
|
85
|
65.1
|
95
|
67
|
105
|
67.5
|
115
|
68
|
125
|
68.5
|
The experimentally determined values
of modulus of elasticity are presented as a function of temperature in the
above reference. Those values are taken into account when we are going to do
the force calculation with temperature.
Length
of the Nitinol strip = 0.04 m
Cross
section area = 0.5 mm2
Temp (C)
|
Deflection(mm)
|
Young's Modulus (Gpa)
|
Force(N)
|
45
|
0.713
|
42
|
0.37
|
55
|
1.024
|
50
|
0.64
|
65
|
1.335
|
61
|
1.02
|
75
|
1.645
|
65
|
1.34
|
85
|
1.955
|
65.1
|
1.59
|
95
|
2.265
|
67
|
1.9
|
105
|
2.576
|
67.5
|
2.17
|
115
|
2.886
|
68
|
2.45
|
125
|
3.197
|
68.5
|
2.74
|
1.5
Finite Element Analysis approach for the gripper
Typically, engineers develop working design models prior to fabrication
and implementation in order to predict behaviors when the design is subjected
to environmental stresses. Doing so allows the engineer to make necessary
modifications before any chips are cut with a direct correlation to reduced
costs. Designs also typically involve well characterized materials (e.g.
aluminum) with well-defined material properties and predictable behavior.
Evident from the literature, development of an algorithm addressing
large displacement and thermo mechanical behavior of SMAs is in itself an area
of research beyond the scope of this effort. Thus, commercially available computational
analysis tools were used in an attempt to create a Finite Element Model (FEM)
that closely modeled Nitinol response when restrained and strained to failure.
1.5.1
Selection of Multi-physics Modeler – ANSYS
Workbench
ANSYS Workbench Release 15.0 was
chosen as the multi-physics program due to its project schematic view ties
together the entire simulation process, guiding the user through complex
metaphysics analyses with drag-and-drop simplicity. Given the experimentally
derived behavior of the material, it was modeled as nonlinear with large
displacement.
1.5.2
Select Analysis System
ANYSYS offers many different
analysis systems to aid the designer in predicting concept behavior. It is
anticipated that the SMA will be instituted in a pre-strained static condition
with heat being applied while in that static condition. Thus, the Static
Structural analysis system was selected for developing the SMA finite element
model.
1.5.3
Determine Analysis Material Properties
Nitinol is not available in the
martensitic shape memory form as a standard material preloaded in ANSYS and so
was created. Material characteristics required to accurately model nonlinear
material behavior include
1.5.4
Assign Material to Model
The default material assigned to the specimen by ANSYS was generic
structural steel. The material was changed to the created Nitinol for both the
linear and nonlinear analyses.
Nitinol possesses very unique material properties. As such, a new
material model must be created in ANSYS Workbench 15.0 to obtain satisfactory
results. The team understands that Nitinol can have very different properties
depending on the thermal processing it is subjected to after being cut. For
extremely accurate results, a sample of the Nitinol strip must be tested for
the various parameters listed below:
- Density
- Coefficient of thermal expansion at different temperatures
- Reference temperature value
- Stress vs. strain data points
- Hardening parameter
- Elastic limit
- Temperature
scaling parameter
- Maximum
transformation strain
- Martensite
modulus
- Load dependency
parameter
- Tensile yield strength
- Tensile ultimate strength
Nitinol properties of the model has to be determined by carious
experimental tests. However to achieve such details, we have to perform so many
experiments which are mostly out of our touch. Therefore, we assumed a Nitinol
model and carried out our experiment on Nitinol behavior considering following
- Density: 6500 kg/m3
- Coefficient of thermal expansion at different temperatures: 6.6 × 10-6 0C -1
- Reference temperature value: 32 0C
-- Hardening parameter: 6.894× 106 Pa
- Elastic limit:
3.4474 × 105 Pa
- Temperature
scaling parameter: 26062 Pa 0C -1
- Maximum
transformation strain: 0.04 mm-1
- Martensitic
modulus: 3.3095 × 108
Pa
- Load dependency
parameter: 0.05
By using the
above material model we have obtained the figure 4-12; result for the Nitinol strip temperature with
deflection variation.
Hi
ReplyDeleteFrom what sources did you get this information?
I got help to find SMA analysis from your post. But still I face some problems. I get error messages of incorrect property data of nitinol. What I have to do to solve the problem?
ReplyDelete