accelerometer magnetic

7/30/2019 Accelerometer Magnetic

1/4

Procedia Engineering 25 (2011) 535 538

1877-7058 2011 Published by Elsevier Ltd.

doi:10.1016/j.proeng.2011.12.133

Available online at www.sciencedirect.com

Proc. Eurosensors XXV, September 4-7, 2011, Athens, Greece

Magnetic accelerometer from polymer

J. Li, W.K. Schomburg a*

RWTH Aachen University, Konstruktion und Entwicklung von Mikrosystemen (KEmikro), Steinbachstrae 53 B, 52074 Aachen,

Germany

Abstract

An acceleration sensor from polymer has been developed which balances a proof mass by magnetic forces. The sen-

sor is fabricated from a polyimide membrane with conductor paths from gold patterned by photolithography and etch-

ing, a frame manufactured by ultrasonic hot embossing, and permanent magnets fixed to the frame. All components

are assembled on a planar substrate and then the frame is kinked forming the desired three-dimensional structure.

2011 Published by Elsevier Ltd.

Keywords: acceleration sensor; magnetic; ultrasonic hot embossing; polymer

1.IntroductionUsually acceleration sensors are made of silicon. Either the deflection of a seismic mass against the

elastic force of a bending beam is measured [1] or it is held in its idle position by a force balancing the

acceleration [2]. The big advantage of employing silicon technology is that the mechanical element of the

sensor can be integrated with the electronics required for amplification and analysis of the signal.

Nowadays, however, there are emerging technologies allowing integrating electronics into polymer

substrates [3, 4]. Therefore, it is interesting to investigate whether acceleration and other sensors can be

fabricated from polymer as well. This could allow employing cheap polymer fabrication processes for

sensor applications.

Since polymers tend to creep and their properties are a strong function of temperature, a polymer beam

appears to be no good elastic element in a sensor. However, if a seismic mass is balanced against the

acceleration force, the characteristic curve of the sensor is no longer a function of the properties of the

* Corresponding author. Tel.: +49-241-80-28444; fax: +49-241-80-22440.

E-mail address: [email protected].


2/4

536 J. Li and W.K. Schomburg / Procedia Engineering 25 (2011) 535 538

beam supporting the mass. Thus an acceleration sensor from polymer was designed with a seismic mass

balanced by magnetic forces (Fig. 1).

Fig. 1. Magnetic micro acceleration sensor from polymer. Fig. 2. Schematic view of the magnetic micro acceleration sensor.

2.Sensor designFig. 2 shows the sensor schematically. A rigid frame from the polymer polyvinylidenefluoride (PVDF)

carries 8 pairs of permanent magnets. Between the magnet pairs there is a polyimide (PI) foil with

conductor paths from gold. The conductor paths form an electromagnetic coil in the field of the magnets.

On the side of the foil opposite of the coil there is glued an aluminum plate stiffening the foil.

Tongues on the left and the right of the foil are bent down and fixed at the substrate. Near the fixation

point of the tongues there are strain gauges from gold which are combined to a Wheatstone bridge detect-

ing the position of the seismic mass. A base and clampers from polymethylmethacrylate (PMMA) are

used to fix the frame and provide electronic connection.

When the sensor is accelerated, the tongues of the foil are bent. This deformation is detected by the

strain gauges and the output signal from the bridge is controlling a feed back circuit driving the coil on

the foil such that the foil is pulled back to its idle position. The driving voltage of the coil is a measure of

the acceleration. The applied magnetic force is equal to the inertial force, thus this senor is a force balanc-

ed acceleration sensor.

The characteristic curve is not a function of the properties of the movable parts of the sensor such as

Youngs modulus, Poissons ratio, length, width, and thickness of the tongues etc. Therefore, geometrical

changes caused by the creep of polymers do not affect the sensor.

Compared to an open loop acceleration sensor the useful bandwidth of a force balanced sensor is in-

creased by a factor equal to the loop gain, thus the trade-off between sensitivity and speed can be relieved

significantly [5, 6]. Besides this benefit, the deflection of the seismic mass is reduced considerably, which

means that nonlinear effects from squeeze film damping and the mechanical suspension system are reduc-

ed obviously.

3.FabricationThe magnetic micro acceleration sensor is fabricated by surface micromachining and ultrasonic hot

embossing [7]. The core component of the sensor is a polyimide foil with a coil, strain gauges, and a

conductive path, which is shown in Fig. 3. It is made by sputtering of 220 nm gold, multiple-layer photo-

lithography and etching on a PI foil, 50 m in thickness.


3/4

537J. Li and W.K. Schomburg / Procedia Engineering 25 (2011) 535 538

Due to the available in-house equipment it was not possible to generate conductor paths for strain

gauges and coil with a width of less than 100 m. The size of the entire foil is limited by the achievable

line width. I.e., with a reduced line width the sensor could be decreased significantly.

The frame (cf. Fig. 4) was fabricated by ultrasonic hot embossing [7], because, this way, complex

micro structures from polymer are manufactured in seconds. The inverse of the micro structures requiredfor the frame were milled into the surface of an aluminum plate, 5 mm in thickness, to fabricate a molding

tool. A stack of 5 PVDF foils, each 100 m in thickness was placed on the tool followed by the PI foil

shown in Fig. 3 and another two PVDF foils. Then the sonotrode of an ultrasonic welding machine

pressed the entire stack onto the tool, heated it up by ultrasound, and released the work piece again within

2.35 s.

Then the not embossed rim and center parts of the PVDF were cut off resulting in the work piece

shown in Fig. 4. The frame can be kinked at hinges formed by ultrasonic hot embossing. An aluminum

plate 12 mm, 8 mm, and 300 m in length, width, and thickness, respectively, was glued on the back of

the PI foil as a seismic mass.

Similarly two magnet holders were fabricated of PVDF foils by ultrasonic hot embossing.

The permanent magnets (Q-05-1.5-01-N from Supermagnete, Germany) were glued attractively posi-tioned into the magnet holders. Then, the two holders were glued on upper and bottom surfaces of the

PVDF frame to generate a uniform magnetic field across the coil on the PI foil. The PVDF frame was

kinked at the hinges forming the desired three-dimensional suspending structure and fixed to the base by

two clampers with four screws. At the development stage, base and clampers are made by milling, but in

mass production injection molding would be more efficient. In each clamper there are three copper tapes

which provide the electric contact between gold pads on the PI foil and wires soldered to the tapes.

4.ExperimentsThe sensor was characterized in two ways: It was tilted in the gravitational field and swinging mounted

at the end of a pendulum. The characteristic curve of the sensor was measured by tilting it (Fig. 5). Two

measurements resulted in nearly the same curve.

The pendulum was equipped with an angular position sensor and the acceleration sensor was mounted

parallel to the pendulum, thus measuring tangential acceleration. Fig. 6 shows the results obtained when

the pendulum was lifted up to 45 and then released. The signal from the bridge of the strain gauges is

constant in certain limits because the coil pulls the seismic mass back into its idle position while position

and acceleration are changing.

Fig. 3. Polyimide foil with coil, strain gauges, and conductive path. Fig. 4. PVDF frame with PI foil.


4/4

538 J. Li and W.K. Schomburg / Procedia Engineering 25 (2011) 535 538

Fig. 5. Characteristic curve of the sensor tilted in the gravi-

tational field of the earth.

Fig. 6. Signal from the bridge containing the strain gauges,

position of the pendulum, and voltage supplied to the coil.

5.ConclusionsAn acceleration sensor from polymer was built and tested successfully. This shows that even inertial

sensors can be fabricated from polymers if the design is adapted to the special needs of this material class.

In this case the acceleration was balanced by the magnetic force of a coil in the field of permanent mag-

nets and the electrical current necessary for balancing is a measure of the acceleration.

Ultrasonic hot embossing was employed for fabricating the sensor from a stack of polymer foils

allowing cheap fabrication of the sensing element. It appears to be possible that in future even the control

electronics could be integrated into polymer sensors.

References

[1] L.M. Roylance, J.B. Angell, "A Batch-Fabricated Silicon Accelerometer", IEEE Trans. Electron Devices ED-26 (1979)

1911 - 1917.

[2] L.A. Rocha, E. Cretu, R.F. Wolffenbuttel, Pull-in Dynamics: Analysis and modeling of the transitional regime, Proc.

MEMS04, (2004) 249-252, ISBN 0-7803-8265-X.

[3] F.J. Touwslager, N.P. Willard, D.M. Leeuw, I-line lithography of poly-(3,4-ethylenedioxythiophene) electrodes and

application in all-polymer integrated circuits, Appl. Phys. Let. 81 (2002) 4556 - 4558.

[4] P. Khuntontong, T. Blaser, W.K. Schomburg, Fabrication of Molded Interconnection Devices by Ultrasonic Hot

Embossing on Thin Polymer Films, IEEE Transactions on Electronics Packaging Manufacturing 32 (2009) 152 156.

[5] M. Elwenspoek, R. Wiegerink, Mechanical Microsensors, Springer-Verlag (2001) p. 143, ISBN 3-540-67582-5

[6] S. Beeby, G. Ensell, M. Kraft, N. White, MEMS Mechanical Sensors, Artech House (2004) p. 177, ISBN 1-58053-536-4.

[7] W.K. Schomburg, K. Burlage, C. Gerhardy, Ultrasonic hot embossing, Micromachines 2 (2011) 157 166, ISSN 2072-

666X, doi:10.3390/mi2020157.

accelerometer magnetic

Documents