Silicon Piezoresistive 6-DOF Micro Force-Moment Sensing Chip

and Application to Fluid Dynamics Dzung Viet Dao, Toshiyuki Toriyama, John Wells and Susumu Sugiyama

Center for Promotion of COE Program, Ritsumeikan University 1-1-1 Noji-Higashi, Kusatsu, Shiga, 525-8577, Japan. Tel: 077-566-1111 (ext. 6730), Fax: 077-561-3994

1. Introduction

Multi-axis force-moment sensors are used widely in robotics and other fields. Conventional force-moment sensors are formed by bonding metallic strain gauges on a three-dimensional sensing structure. Consequently, the sensor size is large and the sensitivity is small. In this paper, we present a development of a new micro six-degree of freedom (6-DOF) force-moment sensing chip utilizing piezoresistive effect in silicon. This sensor can measure 3 components of force and 3 components of moment independently. The total number of piezoresistors used in this sensing chip is much less that the previous 6-DOF micro force-moment sensors [1, 2] know to the authors.

2. Design of the sensor

The sensing structure is a silicon crossbeam (Fig.1) with 20 conventional piezoresistors diffused on its upper surface. Based on the structural analysis by classical elasticity theory and finite element method (FEM), the positions of piezoresistors have been decided as shown schematically in Fig. 2. The in-plane principal axes of the piezoresistors were aligned with the crystal directions

>< 011 and >< 211 of silicon (111). Table 1 shows the resistance change of piezoresistors due to application of external forces and moments. In this table, the ‘+’ and ‘–’ signs indicate, respectively, an increase and decrease, ‘0’ means unchanged and ‘=’ means a similar resistance change in both sign and magnitude in piezoresistors of a corresponding bridge. Based on these changes of resistance, piezoresistors are connected to Wheatstone bridges to form measurement circuits. Shaded regions in the table indicate where the response of the corresponding bridge is non-zero.

3. Fabrication of the sensor

The sensing chip has been fabricated based on MEMS processes, including thermal oxidization, boron ion diffusion, aluminum vacuum evaporation, and ICP-RIE. Fig. 3 is a micrograph of a fabricated 6-DOF force-moment sensing chip.

4. Characteristics of the sensor

The sensing chip has been calibrated by using nano indenter (Shimadzu Co., DUH-200) controlled by a computer to generate a precise force for the calibration. The sensitivities of each component are: SFz = 1.32 mV/mN, SFx = 0.12 mV/mN, SFy = 0.107 mV/mN, SMx = 4 mV/Nµm, SMy = 3.88 mV/Nµm, and SMz = 1.9 mV/Nµm. The crosstalk was small. Maximum crosstalk of 4% was measured. The outputs were linear and close to the design values.

5. Application to fluid dynamics

The immediate application of this sensing chip is to measure forces and moments on a near-wall particle in turbulent water flow. The sensor configuration is shown in Fig.4. The 6-DOF force-moment sensing chip is located inside a spherical test particle, whose diameter is 10 mm. Loads from turbulent flow acted on the test particle will be transmitted to the sensing chip through transmission pillar. The experiments were conducted with the model, where the test particle mounted with its center was at the level of the bottom plane of a water channel, thus approximately a surface mounted hemisphere. The sensors are placed 6 m from the entrance of an 8m-long, 0.5m-wide water channel.

Two examples of the latest measurement results are shown in Figs. 5 and 6. These figures respectively show the lift (vertical force) and drag (tangential force) coefficients measured with the flow depth kept constant at 5.0 cm, and mean velocity U varied from 0.05m/s to 0.35m/s with an increment of 0.05m/s. The corresponding non-dimensional diameter d+ is from 24 to 147. These figures also show good agreement between our measurement results and the simulation results by ANSYS and DNS (direct numerical simulation), and with experimental results by Taniguchi [3].

6. Conclusions and research plans

A silicon-piezoresistive 6-DOF force-moment sensing chip has been developed based on MEMS technology. A prototype of the sensor for measurement forces on near-wall particle in turbulent flow of water has been assembled and applied. The experimental results confirmed the reproducibility of the measurements. All six components of force and moment have been measured, and the lift (vertical force Fz), the drag (tangential force Fx) and the moment My have been compared with other experimental data and with simulations. Good agreement was obtained.

The work in the near future will be focused on the applications of the developed 6-DOF sensing chip in robotics, and development of nano mechanical sensors, such as nano multi-axis force sensor, nano accelerometer, and so on.

References [1] K. Okada, “Flat-type six-axial force-sensor”, Tech.

Digest of The 9th Sensor Symposium, pp.245-248, 1990. [2] W.L. Jin, C.D. Mote, “A six-component silicon micro

force sensor”, Sensors and Actuators A 65, pp 109-115, 1998.

[3] S. Taniguchi, “Study on the flow surrounding a three dimensional body on a flat wall”, Doctor Thesis (in Japanese), Hokkaido University, (1983).

5003000

100

700

400

20

Y

X

RMy1 RMy2 RMy3

RFz1 RFz2 RFz3 RFz4

RFx1 RFx2

RMx1

RFy1

RMz2

RMx2

RMx3

RFy2

RMz4

RMx4

RMy4

<110>

<112>

RMz1

RMz3

Fig.1. Structure and dimension of the sensing chip. Fig.2. Arrangement of piezoresistors.

Table 1. Resistance changes of piezoresistors due to applied loads.

+ + + + + + + + + + + + + + + + - + - + 0 0 0 0 0 0 0 0 0 0 0 0 + - + - - - + + 0 0 0 0 + - + - + - + - 0 0 0 0 0 0 0 0 = = = = + - - + + - - + + - - + - - - - 0 0 + - 0 0 0 0 0 0 0 0 + + - - - - + + + - 0 0 + + - - + + - - 0 0 0 0 + - - +RFx1 RFx2 RFy1 RFy2 RFz1 RFz2 RFz3 RFz4 RMy1 RMy2 RMy3 RMy4 RMx1 RMx2 RMx3 RMx4 RMz1 RMz2 RMz3 RMz4

FxFyFz

MzMxMy

Fx-Bridge Fy-Bridge Fz-Bridge Mx-BridgeMy-Bridge Mz-Bridge

Fig. 3. Micrograph of the 6-DOF sensing chip. Fig. 4. Sensor configuration.

10

100

1000

0.0 30.0 60.0 90.0 120.0 150.0Nondimensional diameter d+

Lift

coef

ficie

nt

CL

*

SensorDNSANSYS

10

100

1000

0.0 30.0 60.0 90.0 120.0 150.0

Nondimentional diameter d+

Dra

g co

effic

ient

C

D*

Sensor DNS

ANSYS Taniguchi

Fig. 5. Comparison of average lift coefficient CL

*. Fig. 6. Comparison of average drag coefficients CD*.

Transmission pillar

Gold wires

Test particle

Silicone rubber Lead-out cable

Base pillar

6-DOF force sensing chip

Overload protection base