ICAT 2003
December 3-5, Tokyo, JAPAN

3D Form Display with Shape Memory Alloy
Masashi Nakatani, Hiroyuki Kajimoto, Dairoku Sekiguchi,
Naoki Kawakami, and Susumu Tachi
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan{nakatani, kaji, dairoku, kawakami, tachi}@star.t.u-tokyo.ac.jp

Abstract
We propose a new display for presenting 3D forms using
a pin-rod matrix. Due to a long range of movement, it iscapable of displaying large-scale, dense objects such as
human faces or geographical features. In this work, weuse a coil-form Shape Memory Alloy (SMA) as a pinrod actuator. The prototype has a 4×4 pin-rod matrix,
with a 120[mm] range of motion (which is longer thanprevious works), a 12[mm] pin interval and a 1[mm]interval for height accuracy. The pin-rod matrix conveys
visual depth information and is designed to be touchable,
so our goal is to develop a display that appeals to boththe eyes and hands.
Key words: 3D Display, Shape Memory Alloy,
Touchable Screen, Tactile Display, Haptics

but they have only a short range of movement so theycannot display large-scale objects. Other displays use aservomotor [2][3] to actuate pin-rods, but the actuator
has a relatively large volume so it is difficult to realize adense pin matrix. Because of these challenges, a pin-rod
matrix display with both high density AND long rangeof movement has not yet been realized.
In this work, we propose a 3D shape display with longstroke pin-rods using as an actuator a coil-form ShapeMemory Alloy (SMA), material that can be stretched ordeformed from its original shape but would
spontaneously return to its original shape when heated.
SMAs are able to provide a significant amount of
mechanical work due to phase transformation during
heating, and have been applied in many different fields
to date, such as medicine [4], industry [5], tactile display[2][3][6], daily commodity [7], and so on.

1. Introduction
Displaying features of real objects in virtualenvironments has recently attracted considerable
research interest in the rapidly developing field ofVirtual Reality technology. Many studies have been
performed on various aspects of the process, such as
acquiring digital shape information of an object with arangefinder, transmitting the data over a network, anddisplaying it in a remote location.
By far the most common technique for displaying
acquired 3D-information is representing shapes withgeometrical primitives, and presenting them visually
through computer graphics on a visual display such as amonitor or a projection surface [1].
Another technique that has attracted a great deal ofattention is displaying shape attributes through the senseof touch. One example is a haptic display called
FEELEX developed by Iwata [2], comprised of an arrayof linear actuators and a flexible screen. Although thistype of display is successful at realizing naturalinteraction with the bare hand, it is limited in its
expressiveness because it is very difficult to construct a
pin-rod matrix type shape display with high density anda long pin stroke at the same time.
Some displays use piezoelectric ceramic as an actuator,

Although there have been a great variety of studies, it is
not popular to use SMA as an actuator, possibly because
it is very difficult to control this material completely.
This difficulty stems from its hysteresis: it doesn’t move
instantly after applying heat and needs time to shrinkwith cooling. This leads to a serious problem withfrequency response. Although SMA has apparently beenused as an alternative to existing actuators, this approach
isn’t very actively pursued; there have been few studies
and papers on controlling SMA [4][8][9].
Nevertheless, because of the inherent flexibility andcompactness of SMA, it is quite worthwhile to consider
its characteristics and potential to be used as a smartactuator. In the next section we discuss the basic
characteristics of the shape memory alloy we use, andshow some preliminary results from evaluating thisactuator. After an overview of the structuralcomponents, we report the implementation version of 3D
shape display, and examine the position sensor andconsider a novel way to obtain positional information.
We discuss the basic theory, and show preliminaryresults. The final section discusses future applications
for this type of haptic display that allow us to both see
and touch real objects represented in virtual space.

2. Method
2.1 Actuator
The characteristics of the SMA actuator we use, Bio
Metal Helix 200 (manufactured by Toki Corporation),
are as follows: it can be extended to twice its original
length, and it contracts to its original length by heating.
A single SMA is very thin (0.85[mm] in diameter), so it
is possible to place densely in a pin matrix, while it hasenough power to produce a 30[gf] force. We contract theSMA by simply applying an electric current andproducing Joule heat.
Fig. 1 shows the results of evaluating an SMA actuatorby applying 12[V] under various cooling conditions.
SMA contracts rapidly and extends slowly under the nocooling condition. On the other hand, SMA moves
relatively little under constant cooling from an electricfan, and doesn’t move at all under soaking with coolant(42-50% ethylene glycol, 18.5℃). The fastest response
condition is not cooling when SMA is actuated, andcooling with an electric fan when need be contracted.

Fig. 1: Evaluating SMA under various conditions

To investigate improvements to the frequency response,
we applied a 40V pulse with various frequencies undersoaking with coolant, using the experimental system
shown in Fig. 2. Fig. 3 shows the frequency response ofSMA, suggesting that it is possible to apply high voltage
for speeding up the response under appropriate coolingconditions without damaging SMA.

Fig. 2: Experimental System of Frequency Response

Fig. 3: System Response of Pulse (upper) andFrequency Response of SMA (below)
40[V] input under cooling with coolant, 18. 5℃, andSMA is always pulled with the spring(spring constant k = 2.69 [N/m])
according to the expansion from natural length.

The range of motion depends on the SMA length. In thepreliminary experiment for a 1[m] rod (42.1[gf]) withSMA (750[mm]), the pin-rod is actuated at 500[mm],and a longer length is possible for larger device units(Fig. 4).

300[mm]Fig. 4: Range of Motion Experimentrod from the photo reflector is forwarded to the PC,
which feeds back a control signal to the pin-rods. Eachpin-rod is operated by this driving-measurement system,
so it is possible to control an arbitrary number of pinrods, given sufficient processing power.
Plastic round bars made of ABS resin (diameter: 7[mm])were used as pin-rods, arranged in a 4×4 matrix. Themaximum stroke of the pin-rod is 120[mm] in thisprototype. The height of each pin can be controlled to0.5[mm] intervals, granting the capability to display finetextures.
To extend the SMA for each pin-rod, we use a springthat produces a competitive force, and a cooling fan thatcools the SMA at the same time (Fig. 7).

2.2 Position Sensor
Because SMA provides no feedback on its transformed
shape, the actuator component becomes an open loopthat must be closed by detecting the position of the pinrods. Each pin-rod requires its own respective position
sensor, so each sensor must be small and thin. For thisreason, we selected a pair of photo reflectors (SANYO,SPI-315-34, dimensions 4.0×3.4×1.5[mm]) that read
black and white stripes printed on the pin-rod itself (Fig.
5). Background noise can be filtered with a slit (1[mm]window width) covering the photo reflectors.

Fig. 6: System OverviewFig. 7: Mechanical Configuration
Fig. 5: Position Sensor

3. Implementation
3.1 System Overview
Fig. 6 shows the system block diagram we developed inthis study. A PC outputs the PWM signal for driving aFET switch, and adjusts the time-averaged voltage
applied to the SMA. Positional information of the pin-

3.2 System Evaluation
This display can successfully present arbitrary forms(Fig. 8). The response time for rise and descent is 0.8 sec
and 2.0 sec, respectively, under room temperature (22℃)
conditions. By forcefully pushing down on each pin byhand, the pin falls temporarily, but upon release it
quickly recovers its form. This feature underscores themerit of durability for using SMA in a tactile display.

Traditional method: Amount of Information = N × N
Fig. 8: 4×4 pin-rod matrix prototype

4. Discussion
An important issue that must be addressed in large-scale
pin-rod matrix displays is the complexity of the control
loop. In methods to actuate a typical N × N pin-rod
matrix such as those driven by servomotors, each pinrod must be controlled individually, requiring an amountof control information proportional to N2. Such schemes
work perfectly well if the number of pin-rods is small.
However, for a high-resolution display with a large
value of N, the required information load could quickly
hamper the control loop of the system.
To speed up the control of the SMA actuators, we use ascheme that is commonly used in CMOS and LCD
displays. Fig. 9 below shows our adopted method, with
an N channel FET switch positioned orthogonally atboth ends of the pin array. To actuate the SMA for asingle pin, each switch needs to be turned on in thecorresponding horizontal and vertical channels, as aBoolean AND operation. Arbitrary height maps can beimplemented with an active matrix drive algorithm, to bedescribed in a later paper. Because the amount of
information required to drive the system is proportional
to 2N, which will be increasingly important as the size ofthe array increases.

The method we adopt: Amount of Information = 2N
Fig. 9: N × N Controlling method (Upper) and
2N Controlling Method (Below, we adapted)

While the compactness of SMA and the flexibility of thematrix drive scheme provide a decisive advantage insupporting a dense array of pin-rods, the need for
position sensors introduces a new technical challenge.
This prototype has a pair of photo reflectors for eachpin-rod, which was readily constructed for a small
number of rods. Naturally, the number of reflectors will
increase if we want to add more rods, and the capacity toread the output of photo reflectors will become a more
serious bottleneck for larger arrays.
Therefore, we will consider adopting other, morescalable sensor devices for measuring position. The firstalternative is using inductance change of a coil byinserting metal rods. Several sensors using theconductance change are already commercially available[10], but we intend to use the spring for extending SMA
as a coil. Using springs reduces the complexity of thesystem and improves pin-rod capacity because it isunnecessary to add another measuring component.

The structural details are illustrated in Fig. 10. Thissystem has two modes: an actuating mode and a sensing
mode. In the sensing mode, the Pulse Generator emits apulse wave, and transmits it through the spring and theSMA. The resulting inductance change creates a wave
distortion observed by a voltmeter. The voltmeter isconnected to an analog-digital converter, and the PC
determines the position based on the change.
We currently use stainless steel and iron rods, although
we are still investigating which material is most suitable
for this purpose.

Fig. 11: Concept of 3D Shape Advertisement System

6. Conclusion

Fig. 10: A Novel System Configuration

5. Future WorkWe anticipate applying this device in two ways. One
application is a 1m-square large 3D display system (Fig.
11), which creates a rough texture using a pin matrix,
and projection technology to visually present anillusionary fine texture. To the authors’ knowledge, a 3D
display system with this scale, arbitrary shape and fast
movement has not yet been achieved. We can use thissystem as a dynamic street advertisement.
Another potential application is a novel communicationinterface called “Face Phone”. Fig. 12 demonstrates theconcept of the system. The device displays dynamic
rangefinder data obtained from a human face. Like
conventional videoconference technology, the user canhear the other person’s voice, and also see his/her facemovement. It is one realization of face-to-facetelexistence, where the haptic display will convey aheightened sense of presence to help our daily
communication with others.

We have successfully verified the validity of using a
Shape Memory Alloy as an actuator in a 3D form
display. Due to the long stroke of the pin-rod array that
presents both visual and haptic feedback, it is possible todisplay almost any kind of shape, even for large objects.
Each pin-rod is controlled for height with a height
accuracy of 1[mm], so the device can present finetextures. In the implementation device, we used a pair ofphoto reflectors to obtain positional information of therods in order to control the SMA actuators. However,
due to the imminent engineering challenges for large
arrays, we suspect that a measuring method using theinductance change produced by inserting metal rod
inside of the coil would be a more scalable approach.
As these issues are resolved, and large-scale 3D form
displays with increased density and long pin-rod stroke
become more feasible, we anticipate developing
communication tools such as a dynamic advertisement
system, and a facial expression transmitter. Suchinnovations promise to enhance the human ability toboth see and feel objects represented in virtual space.

Fig. 12: Concept of 3D Face Display

References[1] N. Kawakami, M. Inami, D. Sekiguchi, Y.
Yanagida, T. Maeda, and S. Tachi: “Object-orientedDisplays: A New Type of Display Systems-from
Immersive Display to Object-oriented Displays,”
IEEE SMC’99 Conference Proceedings, Vol. 5, pp.
1066-1069, December, 1999.
[2] H. Iwata, H. Yano, F. Nakaizumi, and R.
Kawamura: “Project FEELEX: Adding Haptic
Surface to Graphics,” SIGGRAPH 2001 Conference
Proceedings, pp. 469-475, 2001.
[3] C. R. Wagner, S. J. Lenderman, R. D. Howe,“Tactile Shape Display Using RC Servomotors,”
10th Symposium on Haptic Interface Environment
and Tele operator, March, 2002.
[4] K. Ikuta, M. Tsukamoto, and S. Hirose: “ShapeMemory Alloy servo actuator system with electric
resistance feedback and application for aendoscope,”
ProceedingsofInternational
Conference on Robotics and Automation, 1988.
[5] C. M. Pemble and B. C. Towe, “A Miniature shapememory alloy pinch valve,” Sensors and Actuators
A: Physical, Vol. 77, Issue 2, pp. 145-148, October,
1999.
[6] P. S. Wellman, W. J. Peine, and R. D. Howe:“Mechanical Design and Control of a HighBandwidth Shape Memory Alloy Tactile Display,”
Proceedings of the International Symposium ofExperimental Robotics, Barcelona, Spain, June,
1997.
[7] J. V. Humbeeck, “Non-medical applications ofshape memory alloys,” Materials Science andEngineering, Vol.273-275, pp. 134-148, 1999.
[8] R. D. Howe, D. A. Kontarinis, and W. J. Peine,“Shape Memory Alloy Actuator Controller Designfor Tactile Displays,” Proceedings of the 34th IEEE
Conference on Decision and Control, Vol. 4, pp.
3540-3544, December, 1995.
[9] D. Grant and V. Hayward, “Constrained ForceControl of Shape Memory Alloy Actuators,”
Proceedings of the IEEE International Conferenceon Robotics & Automation, pp. 1314- 1320, April,
2000.
[10] K. Nagai, R. Fujimoto, S. Miyawaki, N. Oshie, S.
Toda, and S. Sugiyama, “Consideration on theProperties of a Small Position Sensor with Low
Temperature Drift,” IEEJ Proceedings of the 20thSensor Symposium, pp. 205-208, May, 2003.