Construction and controlling of the gripper for medical devices driven by shape memory alloy actuators
Ireneusz Dominik, Renata Dwornicka Construction and controlling of the gripper for medical devices driven by shape memory alloy actuators
Ekonomiczne Problemy Usług nr
ZESZYTY NAUKOWE UNIWERSYTETU SZCZECIŃSKIEGO
In modern medicine the new materials are continuously introduced,
as the needs for new features and parameters are in the high demand. The
medical mechanical devices must be safe in use both for doctors and patients,
they must be also precise, light and energy efficient. These requirements are
fulfilled by Shape Memory Alloy (SMA) materials, which are more and more
commonly used in advanced technics. They are characterized by a relatively
large strain at high values of the generated stress1.
In the case of medical robotics one of the most important issues is
simplicity of the service by the user e.g. a surgeon. Moreover, the construction should
try to minimalize the errors of the user like a hand shaking. For rehabilitation
devices in turn the construction should be light, small in size and energy
saving. That is why all mechanical elements responsible for movement which
1 I. Dominik, advanced controlling of the prototype of s Ma linear actuator, “Diffusion
and Defect Data – Solid State Data. Pt. B, Solid State Phenomena” 2011, Vol. 177.
consume significant amount of total energy should be considered particularly
a pplications of Sma in medicine
The SMA materials are characterized by ability for different type of
actions: superelasticity, one-way effect and two-way effect. In the one-way
shape memory effect initial geometry appears during the heating, in contrast
to the cooling where there is no shape change. The metal “remembers” only
high temperature shape of the parent phase. In the two-way shape memory
effect the alloys’ shape behaves as if it remembers both the high temperature
shape of the parent phase and the low temperature shape of the martensite
phase. This two-way shape memory effect is connected with a cyclic process,
which creates reversible changing of the sample, although the strain is lower
than in one-way effect. In the research which is here presented one-way effect
of the shape memory actuator was involved.
Nowadays a lot of researchers are looking for new industrial
applications based on shape memory alloys. From the view of applicability of shape
memory alloys one can distinguish several important groups such as: fixed
electrical and mechanical connections (e.g.: in aerospace), temperature sensors
(temperature protection), control systems (e.g.: heater), dumped vibration and
noise systems and implants in medicine. Applying SMA wires, called also
artificial muscles2, give the possibility of creating advanced mobile robots,
artificial limps3, as well as other devices for medical purposes4.
In medicine an interesting feature which open a wide area of application
for SMA is constant temperature of a human body. It is possible to create
surgeon construction which are activated by contact with a body e.g. stents or
Grippers which are a hand-shaped end actuators designed for seizing and
holding are becoming more and more important in medicine, especially with
the quick development of a technique called robotic surgery. The main idea
is that a surgeon performs surgery using a computer which remotely controls
very small instruments attached to a robot’s gripper. One of the often used
robotic surgical system is the da Vinci Surgical System which is designed to
facilitate complex surgery using a minimally invasive approach. Up to date
about 2,585 da Vinci Systems are installed in over 2,025 hospitals worldwide5.
In the standard gripper construction either pneumatic or hydraulic
actuators to achieve the gripping function are mostly used6. These actuators
work reliably, but require a fluid such as air or oil. This fluid requirement
makes these types of actuators difficult to adapt for use in medical applications
because of surgical/hospital environment (high vacuum or high cleanliness
applications). Additionally, pneumatic and hydraulic grippers are relatively
expensive. Apart from the hydraulic/pneumatic grippers electro-mechanical
6 G. J. Monkman, H. Schunk, r obot g rippers, Technology & Engineering 2007.
grippers are also used. In the case of electrically driven grippers lubricants
or greases are required which is often problematic also because of required
The alternative, developed by many research centers, are gripping devices
for gripping or grasping small medical elements of the surgery within a set of
jaws utilizing a shape memory alloy material. Most of the constructions based
on the SMA materials are made in micro and mini scale in which small SMA
wires or springs are used e.g. at the Micro Robotics Lab at the University of
Maryland7 or the research center of the Memry Corporation8.
However, only few gripper constructions based on SMA materials which
can be apply for medical purposes are developed. The main disadvantages are
the requirement of additional cooling system e.g. two finger gripper with extra
cooling fans9 or lengthening the SMA wire by its bending (no. 5 in fig. 3) to
increase jaw opening which shorten the lifetime of a device10.
SMA actuator used in construction
The main aim of the project presented in the article was to create the new
contraction of the gripper for medical devices and medical robots which can grip
objects in a range of centimeters. The used actuators produced by Miga Motors
company are able to multiply the displacement which provides long strokes in small
devices (fig. 4). In contrast to the presented gripper constructions the SMA wires
are not bend (shortening the lifetime of a device) but are activated linearly (fig. 5).
Additionally, the actuator housing allows SMA wires for quick heat dissipation
so there is no need for an additional cooling system. In the gripper two actuators
of the different construction were used: push DM0115PH and pull DM0115PL.
The internal structure of the actuator consist of moveable parts
connected by SMA wires (fig. 5). The small diameter wires typically in the range
0.025÷0.5 mm are made of nickel-titanium. During heating the contraction of
the wires is observed. The value of the contraction depends on the length of
the wire because typical contraction equals 5% wire length. So, the longer the
wire the more contraction can be observed. The contraction of heated wires
is opposite to ordinary thermal expansion and in comparison it is larger by a
hundredfold. The most user-friendly way of applying the heating is by using
electrical current which flows through the wire.
Because of the titanium SMA wires are really powerful, e.g. a wire with
0.2 mm in diameter can pull 0.6 kg. That is why SMA wires are characterized
by one of the highest in technology weight ratio, which describes the ratio of
a maximum external load to their own weight. It allows for building miniature
devices which are extremely efficient. The main advantages of the actuator based
on SMA wires besides the weight ratio are: silence, smooth motion and a really
small size. A safe assumption is that any task requiring physical movement in a
small space with low to moderate cycling speeds is something that most likely
will be better done with actuator wires. Many of the tasks currently being done
with small motors or solenoids can be done better and cheaper with SMA wires.
The actuator DM115 with its own weight 30 grams can create force
equal 20N. In the construction two actuators were used. The first one created
the one-way movement of pushing and the second one of pulling. Working
together as an antagonistic pair they allowed fully control the movement in
both directions: for closing and opening of the gripper. The actuators are
electrically driven only during operating the gripper, which saves the energy.
The reason why the SMA actuators were used in the application is their
exceptionally good cooperation with the medical devices and in surgery
environment. The elimination of the vibrations is one of them. The used actuators
are linear by design and most appropriate for linear motion, which can easy
translate into movement of the gripper jaw. The creating movement based
on the SMA wires placed in the actuators is very smooth and continuous. It
increases the accuracy of the gripper positioning. In opposite to the classical
devices e.g. motors there is no requirement for gearboxes and linkages and as
result no lubricants or greases are used. The construction is entirely out of high
strength engineering thermoplastics and stainless steel which is demanded for
the high cleanliness applications. The last but not the least important feature
of the actuators used in the medical applications is that actuator components
are non-magnetic, and are even compatible with Magnetic Resonance Imaging
(MRI) applications or other highly specialized diagnostic systems e.g. Positron
Emission Tomography (PET).
c onstruction of the gripper
The base of the gripper was designed to ensure stability and possibility
of fixing moveable parts. There are two parts which create a foundation and
side walls. All elements of the gripper were made out of sheet metal 1mm
thick (fig. 6).
The gripper size is depends on the actuators size which is 86 × 22 × 7,5 mm.
The construction was optimized to restrict the size of the gripper. The
diameters are presented in fig. 7.
The whole construction consist of 9 parts and it is symmetrical. The angle
irons with arms were fixed to the one side walls. The linear movement of the
actuators is transferred into rotary movement by a curved pivot jaws.
The construction was designed in Solid Works software. After drawing
single parts the assembly project was built. Next the simulation of the
movement was conducted which allowed any collisions to be located. In that way the
accurate selection of the diameters of the single parts as well as modification
of their shapes and joints location were performed. Finally the 12 mm stroke
of the actuators allowed the gripper jaw to be opened in the range 0÷17 mm
which is completely sufficient to grip and hold any small medical elements of
the surgery set e.g. scalpel, surgical suture or surgeon’s tourniquet.
The metal chosen for the construction has to be strictly selected because
of the surgeon environment in which should operate. It should be biocompatible
and resistant to corrosion. In the prototype version the stainless steel AISI 303
was chosen. Because of the material stiffness and high precision requirement
the parts of the gripper were cut out by a CNC machine11.
11 R. Łuszczak, c ontrolling of actuators with shape memory alloy used in medical devices,
master thesis, supervisor I. Dominik, AGH – University of Science and Technology 2011.
c ontrolling of the gripper
As it was mentioned to move the gripper an energy must be applied
to the actuators which in the DM series electrical current is used. The first
actuator DM0115PH model creates push movement an DM0115PL model pull
movement – in that way the position of the gripper is controlled.
The internal resistance of the single actuator was 4.4 Ω. The 12V DC
power supplied was chosen for safety reason. 1 A current value was the
threshold value from which the actuator started moving. For the maximum
12 mm stroke of the actuator 1.3 A must be applied. However, the actuator
worked quite slowly. Further current value increasing ended at 1.6 A value
above which there was no faster actuator movement observed. Summarizing
this part of the experiment 12 V DC voltage and 1.5 A current values gave the
maximum speed of the actuator stroke which did not cause any damage. Next
it was decided to use PWM controlling.
The duty factor should be about 60%, so the maximum current value in
the high state was calculating from the formula:
where: – duty factor value, nominal current value, maximal current value.
The calculated maximum current value in the high state was 2,85A.
The value seems to be high, however with 12 V power supply the energy
consumption is still acceptable.
The position of the gripper was controlled directly by DAQ measurement
cards made by National Instruments. The first model NI 9263 was analog
output card working as a generator in the range 0÷10V and second model NI
9215 was analog input card with signals range 0 ÷ +/-10V. The SMA actuators
needed over 2 A current for activation so the additional electronic circuit was
built. It was decided to use NPN transistors for amplifying the output signal
of the DAQ card. The problem was to low value of the gain (maximum 40dB).
Taking into account that maximum current generated by the card was 1 mA
the maximum amplified signal was 100mA, so it was too low. The solution
was to use a Darlington Pair (BDW42) which can amplify signal even a few
thousand times which was sufficient for the task.
Having solved the problem of power supply the next step was to measure
the current value flowing through the actuator. To fulfill the task a special
standard ceramic resistor with 1Ω value was connected in series to the
actuator. The voltage drop value on the resistor is directly the value of the current
in the circuit.
The voltage drop on the actuator can be calculated from the formula:
where: voltage drop on the actuator, voltage between Darlington base and
circuit ground, voltage drop between base and emitter of transistor,
actuator resistance, standard resistor resistance.
During testing the circuit presented in fig. 8 it was observed that
during longer periods of feeding the actuators the Darlington Pair increased its
temperature. To ensure the safety of the circuit additional two temperature
measurement chips LM35 were fixed to the transistor drains made of metal.
The output signal of the chip is calibrated in such a way that 10 mV indicates
1 °C, which means that the value 0.214 V equals 21.4 °C. For protection against
any unstable states in the circuit which may influence the measurements the
4,7 µF capacitors were connected parallel to the power source.
c ontrolling application
The controlling application was created in LabView software made by
National Instrument which is a commonly used in controlling and measu
rements area. From the beginning it was decided to build the multithread
application, where different events have different priorities. The firs thread
was connected with a reliable user interface. The delay time between pressing
the buttons and the device reaction was assumed to be shorter than 10 ms,
which is about ten times faster than reaction time of the user. Moreover the
indicators and control switches of the interface have to be placed logically and
friendly for a user.
The second thread was responsible for sending data to the output card.
The task was to not overload the system resources and avoid losing data.
Similarly the third thread was created to cooperate with the inputs signals.
The buffering operation was required as well as decoding received data. Not
all data gathered by the third thread were presented to the user in the interface
(the first thread), there was no need because with a higher sampling time a user
could not see any difference, the visible plots were just for orientation while
the full data were saved on a disc.
Fig. 9. Data flow between program events
Source: own elaboration.
According to the described above assumptions each of the threads must
have been realized in the separate loop. The first thread responsible for run
ning the interface was chosen for the master loop. The loop was created with
an event structure often called an interruption structure. Depending on the
event which occur in the interface e.g. a button pressing the structure will
run an appropriate minor event. In the case of starting a new event while the
previous one is running the queuing operation is executed. The data flow
between program events are presented in fig. 9.
The threads responsible for servicing the measurement cards were
located in two other loops. In the structure they are considered as slave loops. The
used environment allows many threads to be executed parallel. Additionally
modern CPUs are constructed as multicore with hyper-treading feature, which
significantly decrease the cycle time of the program. To reach the maximum
program efficiency minimum three core CPU is required. In the case of two
core processor it is recommended to assign the master loop for one core and
the slave loops for the second.
The communication between the master loop and the slaves loops was
realized by synchronic operation “Notifier”. In that way both slave loops
received the same data bunch and after processing it they are waiting for
a next bunch. This structure caused no data loss. The return transmission from
the slave loops to the master one is controlled by “User Event” structure. The
transmission is realized by an event structure together with dynamic events.
Every user event is registered by the event structure. Sending the bunch to the
master loop led to execute the appropriate event in it. The architecture does
not allow slave loops to communicate between each other. The communication
from the one slave loop must be sent to the master loop via user event and
after verification via notifier is send finally to the second slave loop. Thus the
used architecture prevent a bunch of circulating between the slave loops when
an errors occur.
The structure of the implemented in application consist of the main
program (fig. 10) and 22 subprograms e.g. Init, Gen Loop, Acquire Loop or
errors handling (fig. 12).
The interface of the program “Manipulator Control Application” was
created to maximize the simplicity (too many options are often difficult to
work with) with the full functionality (fig. 11).
The three most important switches are “STOP” which closes the ap
plication while “OPEN” and “CLOSE” operate the gripper adequately. The
indicator “Power ON/OFF” shows whether the power supply is connected or
not. At the left bottom side a section with measured signals values is placed.
Two data columns match two actuators, where for each of the actuator the
current value flowing through the actuator, the temperature of the Darlington
Pair, current output of the DAQ card and gain of the Darlington are presented.
The main part of the window presents a graph where four different plots may
be presented. The selection of the plot is available below the graph: Actuator
Current Graph and Transistor Temperature Graph both of them available for
opened and closed jaws action.
Apart from the events controlled by a user from the interface panel the
program operates also with dynamic events. The error arising activates a
subprogram “Error” which displays an error message (fig. 12). In the pop-up
error window short information about the type of the error is shown as well
as its code number.
If the error occurs in any of the slave loops it will close this loop and send
the information to the master loop. Otherwise if the error occurs in the master
loop the three variable responsible for closing all three loops are activated.
In the article the prototype construction of the gripper for medical devices
was presented. The main advantage of the new solution is exchanging the
traditional drive e.g. an electrical motor for SMA actuator. The main advantage of
the SMA actuators is their weight ratio, one of the highest in modern technics.
In the case of used actuator which weights 30 grams force equal 20N can be
created. Other advantages especially important in surgeon environment are:
silence, smooth motion and a small size. What is more actuator components
are non-magnetic, and are compatible with Magnetic Resonance Imaging
(MRI) applications or other highly specialized diagnostic systems e.g. Positron
Emission Tomography (PET).
The gripper controlling was realized by created application which
allowed for simple manual controlling of the jaws as well as observing the crucial
values in the system e.g. current and temperature. Currently the research on
controlling accurately the jaws position and the gripping force are in progress.
The precise distance and force sensors were already installed.
The application for controlling the gripper jaws was built not as a simple
serial executed source code but as the multithread program. It was prepared
for further development where the cycle time of the application is crucial e.g.
force controlling of the gripper holding an object.
The built prototype is the first step in creating the whole series of the
SMA grippers. The size and force of the gripper is determined by used SMA
actuator. Nowadays the actuators are available in wide variety of size from
a few millimeters to decimeters and force up to 70 N (with own 30 grams
Dominik I., a dvanced controlling of the prototype of s Ma linear actuator, “Diffusion
and Defect Data – Solid State Data. Pt. B, Solid State Phenomena” 2011, Vol. 177.
Hadi A., Yousefi-Koma A., Elahinia M., Moghaddam M., Ghazavi A., a shape
memory alloy spring-based actuator with stiffness and position controllability,
“Journal of Systems and Control Engineering” 2011, Vol. 225, No. 7.
W artykule przedstawiono nowy typ konstrukcji chwytaka medycznego dla
urządzeń i robotów medycznych. Tradycyjny aktuator, tj. silnik elektryczny, został
zastąpiony przez aktuator (siłownik) zbudowany ze stopu z pamięcią kształtu (SMA).
Zastosowany komercyjny typ siłownika wyprodukowany przez firmę Miga Motors
zdolny jest do zwielokrotnienia przemieszczenia elementu SMA, co pozwala na
wydłużenie zasięgu wysuwu siłownika przy pozostawieniu zwartej obudowy. Aktuator
potrzebuje energii tylko podczas operacji zamknięcia i otwarcia szczęk, dzięki czemu
jest energooszczędny. Wysuw siłownika o długości 12 mm pozwala na chwytanie
niedużych elementów chirurgicznych, np. skalpela, nici chirurgicznej lub pensety
anatomicznej. Podczas projektowania konstrukcji wykorzystano oprogramowanie
Solid Works, a samo kontrolowanie położenia chwytaka zrealizowano, stosując
2 A. Hadi , A. Yousefi-Koma , M. Elahinia , M. Moghaddam , A. Ghazavi, a shape memory alloy spring-based actuator with stiffness and position controllability , “Journal of Systems and Control Engineering” 2011 , Vol. 225 , No. 7 , pp. 902 - 917 .
3 Z.W. Zhong , C.K. Yeong , Development of a gripper using s Ma wire , “Sensors and Actuators”, Elsevier 2006 ,Vol. 126 , Iss . 2, pp. 375 - 381 ; Y. Shaoze , X. Feng , L. Xiajie , W. Jinhui, a g ripper a ctuated by a Pair of Differential s Ma s prings , “Journal of Intelligent Material Systems and Structures” May 2007 , 18 , pp. 459 - 466 .
4 J. Kwaśniewski , I. Dominik , The s Ma wires application in the b raille Monitor, “Diffusion and Defect Data Solid State Phenomena” 2010 , Vol. 165 , pp. 290 - 293 .
7 J.E. Rajkowski , A.P. Gerratt , E.W. Schaler , S. Bergbreiter , a multi-material milli-robot prototyping process , “Intelligent Robots and Systems” 2009 , pp. 2777 - 2782 .
9 Y. Shaoze , Y. Tianfu , L. Xiajie , W. Rencheng, Tactile feedback control for a gripper driven by s Ma springs , AIP ADVANCES 2 , 032134 . 2012 .
10 U.S. Patent no. 4 , 900 ,078 Gripping device utilizing a shape memory alloy 1990 , Inventor Bloch ; J. T. Assignee: the Boeing Company. Fig. 3 . Grippers driven by SMA elements Source: Y . Shaoze, Y. Tianfu , L. Xiajie , W. Rencheng, op. cit. (left); U.S. Patent no. 4 , 900 ,078, op. cit. Kwaśniewski J., Dominik I. The s Ma wires application in the b raille Monitor, “Diffusion and Defect Data Solid State Phenomena” 2010 , Vol. 165 . Łuszczak R ., c ontrolling of actuators with shape memory alloy used in medical devices , master thesis, supervisor I. Dominik , AGH - University of Science and Technology 2011 . Monkman G.J., Schunk H. , r obot g rippers, Technology & Engineering 2007 . Rajkowski J.E. , Gerratt A.P. , Schaler E.W. , Bergbreiter S., a multi-material millirobot prototyping process, “Intelligent Robots and Systems” 2009. U.S. Patent no. 4 , 900 ,078 Gripping device utilizing a shape memory alloy 1990 . Inventor Bloch; J.T. Assignee: the Boeing Company. Shaoze Y., Tianfu Y. , Xiajie L. , Rencheng W. , Tactile feedback control for a gripper driven by s Ma springs , AIP ADVANCES 2 , 032134 , 2012 . Shaoze Y., Feng X. , Xiajie L. , Jinhui W., a g ripper a ctuated by a Pair of Differential s Ma s prings , “Journal of Intelligent Material Systems and Structures” May 2007 , 18 . Zhong Z.W. , Yeong C.K. , Development of a gripper using s Ma wire , “Sensors and Actuators”, Elsevier 2006 , Vol. 126 , Iss . 2.