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Sensors and Actuators A *** (****) *** ***

Polymer magnetic scanners for bar code applications

Arda D. Yalcinkaya a,, Olgac Ergeneman b, Hakan Urey b

Bogazici University, Electrical and Electronics Engineering Department, College of Engineering, TR 34342 Bebek, Istanbul, Turkey

b Ko University, Electrical and Electronics Engineering Department, College of Engineering, Rumelifeneri Yolu, TR 34450 Sariyer, Istanbul, Turkey

Received 20 February 2006; received in revised form 2 June 2006; accepted 15 June 2006

Available online 22 August 2006

Abstract

Resonant mode polymer-based electromagnetic scanners are developed for barcode reading applications. The electromagnetic scanner consists

of a mirror suspended by a polymer cantilever beam, a permalloy sheet attached to the mirror, and an external coil to generate the driving magnetic

eld. The simple fabrication process involves polymer molding and electroplating of magnetic material, yielding inexpensive devices suitable for

high volume manufacturing.

Mechanical and magnetic modeling of the device as well as analytical, numerical and experimental results are presented. A barcode reading

system is demonstrated successfully using this inexpensive, easy to manufacture scanner. Fabricated cantilever scanners achieve an optical D

product of 123 mm at 56.5 Hz, consuming an actuation power of 168 mW. 2D actuation of the cantilever scanners is also demonstrated.

Keywords: Electromagnetic actuation; Optical scanners; Polymers; Permalloy; Barcode readers

polymer scanners is especially useful to obtain large mirror rota-

1. Introduction

tion angles to produce wide scan lines at low power levels.

Speci cation on scan line width is normally translated into scan

Performance of a barcode reading system is dependent on the

mirror performance by opt D product, where D is the rotating

properties of the barcode, such as minimum bar width, contrast,

mirror size and opt is the optical scan angle (which is equiv-

code length, code height and barcode quality as well as the scan-

alent to twice the mechanical rotation angle). Typical barcode

ner used in the system. Scanning mirrors, holographic scanners,

reading applications require opt D to exceed 10 mm [1]. An

and rotating prisms are commonly used optical components in

electromagnet and a polymer scanner with magnetic material can

barcode readers to form a scan line on a sequence of dark bars

be utilized to achieve a system with the desired properties. In

on a light background. Regardless of its type, in such a barcode

addition to this, silicon-based MEMS microscanner fabrication

reading system, coding information is contained in the relative

involves expensive process steps such as photo mask fabrication,

widths or spacings of the dark bars and light spaces. Polygon

lithography, etching. Silicon MEMS microactuators for other

scanners have been widely used in the bar code reading applica-

applications are presented in a number of references [3 8] are

tions, but this type of scanners suffer from relatively high power

mostly meant for resonance frequencies that demand expensive

consumption, bulky size and the necessity of a motor to drive

read out electronics. Moreover, relatively large silicon die area

the polygonal scan mirror [1].

adds up to the product cost. Previously reported MEMS-based

The presented device is not limited to barcode reading appli-

electrostatic scanners for barcode readers [14,16] require high dc

cations and can also be used for general scanned imaging appli-

polarization voltages ( 30 V) for the operation, limiting the use

cations. Development of polymer scanners for barcode reading

in the portable systems. In contrast to conventional Si micro-

applications is driven by the need for an inexpensive, compact,

machining, larger material volume used for polymers help to

low resonant frequency scanner suitable for volume manufactur-

lower the production costs and allow utilization of cheap pro-

ing. Combination of electromagnetic actuation technique with

cesses such as mould shaping and UV curing. Furthermore, low

resonant frequency scanners made of polymers allow headroom

for signal processing electronics in terms of speed and power

Corresponding author. Tel.: +90-212-***-****; fax: +90-212-***-****.

consumption. Recently, barcode scanners based on high-speed

E-mail address: ****.**********@****.***.** (A.D. Yalcinkaya).

doi:10.1016/j.sna.2006.06.059

A.D. Yalcinkaya et al. / Sensors and Actuators A 135 (200*-***-***-***

infrared tunable laser integrated with a diffraction grating is pro- Table 1

Parameter de nition and values for cantilever type scanner

posed for 1D barcode reading [13], however this method suffers

from the high cost of the components used in the system as well Parameter Symbol Cantilever

as the limited scan angle. Width of the suspension W1 1 mm

In this paper, electromagnetically actuated polymer-based Width of the mirror W2 8 mm

scanning mirrors are presented. A brief overview of the scan- Length of the suspension L1 15 mm

ner design and the analytical modeling of the mechanical part Length of the mirror L2 8 mm

Structure thickness t1 0.5 mm

are given in Section 2.1. Electromagnetic actuation scheme is

Permalloy thickness t2 30 m

explained in Section 2.2, where nite element analysis (FEA) of Si die thickness t3 0.5 mm

structural mechanics and electromagnetic forces are supplied.

Experimental work and characterization results of the devices

are outlined in Section 3. Section 4 describes the fabrication of desired size with a thickness of t3 is attached to the front side

the devices. Application to barcode reading and also 2D image (Table 1).

scanning are presented in Section 5 and nally, conclusions are Cantilever type scanner is designated to have its fundamental

supplied in Section 6. resonance mode in the out-of-plane direction. Excitation of this

mode is performed by creating an alternating force component

normal to the surface of the scan mirror. Light beam falling onto

2. Actuator design

the scan mirror is de ected by the scanner to form a scan line

on a desired target. The rst resonance mode frequency ( 0 ) of

2.1. Structure of the scanners

the cantilever type scanner is calculated as:

Schematics of scanners studied in this paper is shown in

Fig. 1(a). This cantilever type scanner is based on a mirror with 0 = (1)

Jeff

a size of W2 L2 attached to a L1 long W1 wide suspension

anchored at the end to a larger frame. The uniform thickness of

where ks is the stiffness of the suspension calculated through

the polymer is t1 and selective deposition of t2 thick nickel iron

the Young s modulus of the polymer and the geometry of the

permalloy is performed beneath the mirror. In order to have a

spring. Jeff is the effective mass moment inertia. Essentially,

at mirror with surface deformations less than one-tenth of the

one can tailor the resonance frequency by changing the size of

wavelength used in the system, an Al coated Si mirror diced into

the scanner. Finite element modeling is used in simulation of the

mode frequencies and shapes for validating the analytical calcu-

lations. Fig. 2 shows the rst four mode shapes of the polymer

cantilever structure, where the mirror and the magnetic plating

is not included to the FEA model. The fundamental mode is

used for 1D scan operation for barcode readers, where the scan-

ning mirror is moving out-of-plane at 79 Hz. Note that this FEM

analysis is performed using only the polymer structure, as will

be explained in Section 3, the actual resonance frequency will

be smaller that 79 Hz due to the extra weight coming from the

silicon mirror die and the magnetic material. The second mode

is an in-plane bending mode, which occurs at 387 Hz. The third

mode is the torsion about the suspension and is utilized to form a

scan line orthogonal to the one created by the fundamental mode

for 2D scanning. Finally, the fourth mode is the second order

bending mode of the cantilever where most of the deformation

happens in the suspension at 746 Hz. It is worth noticing that the

scanner works as a coupled multi band-pass lter, where modes

are well separated due to the moderately high quality factor of

each mode.

2.2. Electromagnetic actuation force

An electromagnet is placed under the magnetic material to

generate the driving magnetic eld, HA, as shown in Fig. 1(b).

The de ection of the scanner is modeled using the balance

between the magnetic torque Tmag and the mechanical restor-

ing torque Tmech . For thin lm soft magnetic materials the easy

axis remains in the lm-plane due to strong shape anisotropy.

Fig. 1. Scanner mechanical schematics and dimensions.

238 A.D. Yalcinkaya et al. / Sensors and Actuators A 135 (2007) 236 243

Fig. 2. FEM structural analysis results showing the mode shapes of the cantilever type scanner: (1) out-of-plane bending mode; (2) in-plane bending mode; (3)

torsion mode about the spring; (4) structural rocking mode. In this simulation only polymer-based structural material is analyzed.

The magnetostatic torque can be expressed as [2,4,8]: of magnetic ux lines for this speci c coil is performed and

the results are shown in Fig. 3(b). Both radial and out-of-plane

Tmag = VMp HA sin (2) components of the magnetic eld created by the coil is simulated

as a function of the distance to the coil. The radial component

where V = W2 L2 t2 is the magnet volume, Mp the magnetization

of the permalloy induced by the external eld HA, the scan

angle, and is the angle between HA and Mp Note that, HA and Mp changes with time as the scanner moves. Eq.

(2) assumes and HA are not varying with position across the

scanner. The de ection angle of the scanner for small de ections

is given by [9]:

Tmech

= (3)

The stiffness of the mirror piece is much greater than the

stiffness of the suspension beam, therefore the bending of

the mirror is negligible and is not considered in this paper.

Likewise, the moment of inertia of the cantilever is small

compared to the one of silicon mirror and therefore it is

neglected. Both of these assumptions are veri ed with FEM

simulations.

Large static de ection of permalloy actuators has been stud-

ied in a number of papers [4,6]. This work focuses on the

permalloy actuator s small angle rotations and its dynamic actu-

ation characteristics. The magnetization of the permalloy sheet

is assumed to remain along the easy axis which is parallel to

the surface caused by the high magnetic shape-anisotropy [4].

For thin lms, when an external eld is applied, the magnetiza-

tion vector remains parallel to the lm plane and a mechanical

torque is produced to align the magnetic lm with the external

magnetic eld lines.

The magnetic eld simulations for small excitation signals

are performed using nite element modeling software (FEM-

LAB) [10]. In the simulation test bench, a large coil with 68 mm

Fig. 3. (a) Coil geometry used in this work; (b) cross section of the coil with

outer diameter and 60 mm length incorporating a 16 mm diam- the magnetic core. Magnetic eld and magnetic ux density generated by the

eter magnetic core at its center is de ned. Coil geometry used described coil. The arrows show the magnetic ux density and the streamline

in this work is sketched in Fig. 3(a). Finite element analysis shows the magnetic eld.

A.D. Yalcinkaya et al. / Sensors and Actuators A 135 (200*-***-***-***

Fig. 5. Peak-to-peak de ection of the scanner vs. the distance between the coil

and the permalloy sheet measured for small drive currents at r = 0 (i.e., the

scanner is centered on top of the coil).

3. Experimental results

The dynamic de ection of the scanner was measured using

a laser doppler vibrometer (LDV) by using the setup illustrated

in Fig. 6. This setup allows characterization of electromechan-

ical transfer function of the device, yielding gain phase plots.

Fig. 7(a) shows the peak-to-peak de ection of the cantilever tip

as a function of frequency when drive currents varying between

2 mA and 16 mA are applied to the electro coil. As the drive cur-

rent, thus the magnetic eld, is increased, a slight reduction in

the resonance frequency is observed, due to the spring softening

effect at large displacements. The minimum opt D of 10 mm,

adequate for bar code scanning application, is obtained at a

Fig. 4. FEM simulations of: (a) the in-plane, Hr, and out-of-plane, Hz component power level of 2 mW. Experiments have shown that the peak-

to-peak optical opt D product of 123 mm (15.8 of optical scan

of the magnetic eld HA ; (b) the product of Hr and Hz plotted vs. distance

between the coil and permalloy sheet taken at different radial distances from the

angle using a 8 mm mirror) can be obtained with a power con-

central axis.

sumption 168 mW. Fig. 7(b) shows the peak scan angles of the

tip displacements for different drive currents. The mechanical

magnetizes the sample and the vertical component generates the transfer fuction showing the out-of-plane bending and the tor-

torque on this magnetization. sional modes of the device is given Fig. 7(c). In order to obtain

Fig. 4(a) shows the in-plane (Hr ) and out-of-plane (Hz ) com- this experimental data, the LDV spot is shined onto the corner of

ponents of the magnetic eld versus the distance between the the scanner mirror (as shown in the inset of Fig. 7(c)), where both

coil and the permalloy sheet. As can be seen from this plot, the

maximum radial eld is obtained at about 3.5 mm away from

the electro coil. For small drive currents, the magnetic material

is not saturated and Mp is proportional to Hr . Thus, Mp Hr

product, illustrated in Fig. 4(b), can be taken as proportional to

the magnetic torque.

According to Fig. 4(b), the maximum actuation torque can

be achieved by placing the permalloy sheet about 3.5 mm from

the the coil de ned in Fig. 3(a). In order to verify FEA simu-

lations, the experimental peak-to-peak de ection of the scanner

is extracted by placing the device at the center of the coil. The

device is actuated by a small sinusoidal current waveform with

an offset at the scanner s resonant frequency. Scanner tip de ec-

tion as a function of the distance between the coil and permalloy

is given in Fig. 5, which clearly demonstrates that the displace-

ment of the cantilever tip, thus the magnetic actuation force is

maximum around 3.5 mm. The product of Hr Hz increases as

we depart from the central axis. Fig. 6. The setup used for electromechanical transfer function characterization.

240 A.D. Yalcinkaya et al. / Sensors and Actuators A 135 (2007) 236 243

Fig. 7. Mechanical characterization of the scanner performed in ambient air: (a) resonance peaks of the scanner for different electro coil currents; (b) cantilever tip

displacement at out-of-plane bending mode resonance for varying actuation currents; (c) schematic showing the point where the LDV measurements are taken; (d)

mechanical transfer function of the device, showing the mirror corner displacement as a function of frequency between 40 Hz and 400 Hz.

out-of-plane bending motion and torsion of the scanner are easily

detected. The modes associated with the out-of-plane bending

and the torsion of the scanner occurs at 56.5 Hz and 340 Hz,

respectively. The mechanical quality factors of these motions,

measured at atmospheric pressure, are 40 (out-of-plane mode)

and 87 (torsion mode). Frequency spectrum of the device in a

window between 40 Hz and 400 Hz is given Fig. 7(d). In addition

to the aforementioned out-of-plane bending mode and torsion

mode, there are two more mode peaks appearing at 170 Hz which

is due to the subharmonic excitation of the torsion mode and

240 Hz due to out-of-plane rocking motion.

An interesting, yet useful property of the present device is

its ability to produce two dimensional scan patterns, as shown

in Fig. 8. This 2D image is created by using only one actuation

coil and driving the scanner with a signal given as:

id = I1 sin 1 t + I2 sin 3 t Fig. 8. 2D Lissajoux pattern created by the polymer scanner obtained by apply-

(4)

ing excitation signals at frequencies of 56.5 Hz and 340 Hz.

where 1 and 3 are the resonance frequencies of the out-of-

plane bending and torsion modes of the scanner, and I1 and I2 4. Fabrication

are the magnitudes of the currents exciting the corresponding

modes, respectively. In effect, by applying a drive current given Fabrication of polymer-based scanners involved polymer

in Eq. (4), the scanner is kept in two-mode coupled resonance moulding and permalloy electroplating. Polymer resin (Ren-

where each resonance mode yields in a scan line orthogonal to Shape SL5195) is molded and cured by ultraviolet light into the

the other one. Moderately high mechanical quality factors of desired scanner shape (Fig. 9(a)). The mould de nes both the

the modes separate the motions in two axes, acting as a band- exure and the mirror dimensions which are designed through

pass lter with multi pass bands. A similar actuation principle FEM simulations. As illustrated in Fig. 1(a), the cantilever beam

was previously applied to Lorentz force electromagnetic MEMS is anchored from the left and the rectangular plate on the right

scanners [12]. hand side supports the mirror and the magnetic material. In order

A.D. Yalcinkaya et al. / Sensors and Actuators A 135 (200*-***-***-***

Fig. 10. Block diagram of a scanning mirror-based barcode reader system.

with a sinusoidal signal at the mechanical resonance of the scan-

ner. Photograph of the prototype barcode reader setup is shown

in Fig. 11(a), where light from a laser diode is incident on the

scanner. The light is focused and scanned over the barcode. The

scattered light is collected with a photo diode (PD) while the

beam is scanned over the barcode. As illustrated in Fig. 11(b),

PD output is a combination of peaks and valleys due to re ection

from white and black stripes of the barcode.

Fig. 9. (a) Fabrication sequence of the polymer scanner: (i) aluminum mould

material; (ii) mould preperation for casting; (iii) polymer casting and UV curing

of RenShape SL5195; (iv) release of structures and integration of Silicon mirror

and NiFe permalloy. (b) SEM picture of the NiFe permalloy.

to minimize the distorting effects related to the deformation of

the polymer scanner, Al coated silicon mirror die is integrated

to the scanner using an insulating wax. In this fashion, silicon

mirror is attached to the front side of the scanner and NiFe

permalloy is deposited by electroplating on the back side, where

the actuation force due to the magnetic eld is exerted. NiFe layer

electroplating is performed using a standard sulfate bath [11],

at room temperature. Optimization of dc electroplating condi-

tions with stress reducing agents resulted in thicknesses as much

as 30 m without running into residual stress problems. Scan-

ning electron microscope photo of the plated NiFe permalloy is

shown in Fig. 9(b).

5. Bar code reader application

Fig. 10 illustrates the block diagram of a barcode reader setup

which utilizes an electromagnetic polymer scanner. Such a sys-

tem can be partitioned into optical, mechanical and electrical

blocks. Operation is based on creating a scan line on the barcode

out of the light coming from the light source, then converting

the re ected light into electrical signal by use of a photodiode

and nally processing the signal using electronics.

The scanner explained in this paper is used in a barcode reader

system as depicted in Fig. 10. A smaller coil than that was mod-

eled in Section 2.2 is used to actuate the scanner and generate Fig. 11. (a) Photograph of the barcode scanner system during operation; (b)

high in-plane and out-of-plane magnetic elds. The coil is driven oscilloscope output of the photodiode.

242 A.D. Yalcinkaya et al. / Sensors and Actuators A 135 (2007) 236 243

The output of the photo diode is processed with an electronic rials can be utilized for better performance. The small angle

circuitry to obtain the desired signal. As shown in Fig. 10, the operation of the device ensures low stress and therefore more

current output of the photodiode is ampli ed and converted into reliable operation for the polymer.

voltage by a transimpedance ampli er. Time derivative of this

ampli ed signal is taken to clearly nd whether the scan line is Acknowledgements

crossing over a black or a white stripe on the barcode. Follow-

ing that, analog to digital conversion with 10-bit resolution is Authors wish to thank Aselsan A.S., Ankara, Turkey for their

performed to allow communication with a microcontroller used help in fabrication of some of the prototypes. We are grateful to

for decoding. Low resonance frequency of the scanner allows KUMPEM and Scienti c and Technological Research Coun-

use of low-cost, widely available electronic components for data cil of Turkey (TUBITAK) for funding of the project (grants

processing. Output of the photo diode on the scope is shown in MISAG-280-2004 and 104M161).

Fig 11(b). The scope output is ltered then digitized and pro-

cessed with the microprocessor. Different width black and white

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Olgac Ergeneman was born in Ankara, Turkey in 1981. He completed the high

[21] H. Miyajima, Development of a MEMS electromagnetic optical scanner

school in TED Ankara College in 1999. He received his BS degree from the

for a commercial laser scanning microscope, J. Microlithogr. Microfabric.

Department of Electrical and Electronics Engineering, Middle East Technical

Microsyst. 3 (2) (2004) 348 357.

University, Ankara in 2003. Same year, he joined Optical Microsystems Lab-

oratory (OML) of Koc University, Department of Electrical Engineering as an

Biographies MSc candidate. Olga Ergeneman is a student member of SPIE and IEEE.

Hakan Urey received the BS degree from Middle East Technical University,

Arda D. Yalcinkaya received a BS degree from Istanbul Technical Univer-

Ankara, Turkey in 1992, and MS and PhD degrees from Georgia Institute of

sity, Electronics and Telecommunication Engineering Department, Istanbul,

Technology, Atlanta, Georgia, USA in 1996 and in 1997, all in electrical engi-

Turkey; MSc and PhD degrees from Technical University of Denmark (DTU),

neering. He is currently an assistant professor at Koc University, Istanbul, Turkey.

Mikroelektronik Centret, Kgs. Lynby, Denmark, all in electrical engineering in

He joined Microvision Inc., Seattle in 1998. Since 2002, he has been with Koc

1997, 1999 and 2003, respectively. Between 1999 and 2000 he was a research

University. He has published more than 50 journals and conference papers, 5

and development engineer at Aselsan Microelectronics, Ankara, Turkey. He

edited books, 2 book chapters, and he has 9 issued and several pending patents.

had short stays as a visiting reseacher at Interuniversity Microelectronic Cen-

He is the chair of Photonics West MOEMS Display and Imaging Systems Confer-

ter (IMEC), Leuven, Belgium, Centro Nacionale de Microelectronica (CNM),

ence and Photonics Europe Symposium MEMS, MOEMS, and Micromachining

Barcelona, Spain in 2000 and 2003. Since 2003, he has been a research associate

Conference. His research interests are generally in the area of information optics

at Koc University, Istanbul, where he is engaged with start up of a cleanroom

and microsystems, including microoptics, optical system design, microelectro-

facility. He is also working as a part time ASIC designer for Microvi-sion Inc.,

mechanical systems (MEMS), and display and imaging systems. Dr. Urey is a

Seattle, USA. His research interests include design, fabrication and character-

member of SPIE, OSA, IEEE-LEOS, and vice-president of the Turkey chapter

ization of MEMS, CMOS-MEMS integration, nanotechnology and design of

of IEEE-LEOS.

analog ASICs. He has been a Sabanci Foundation (VAKSA) scholar between

ilever scanners is also demonstrated.

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