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Armament Research & Development Establishment (ARDE), DRDO Ministry of Defence, Dr Homi Bhabha Road, Pashan Pune-411021, India Ph. No.+91-20-2588 4795, Fax No.+91-20-2589 3102 E-mail:bpjoshi@ieee.org 2 Department of Electronic Science, University of Pune, Pune-411 007, India Abstract: In addition to sensitivity and bandwidth, the cross-sensitivity is an important design parameter for acceleration/ inertial sensor design. In this paper prediction of cross-axis sensitivity of cantilever type of piezoresistive accelerometer is discussed. The effect of variation in geometrical parameters such as width and thickness of flexure & proof mass (PM) on crosssensitivity are studied. Optimization of cross-sensitivity by varying geometrical parameters has been attempted. This paper deals with simulations of skewed type (Flexure perpendicular to proof mass) and planar type (Flexure in plane with Proof mass) structure for cross-axis sensitivity analysis. The simulation and modeling has been carried using Coventorware MEMSCAD software. Keywords: Inertial sensor, Cross-sensitivity, MEMSCAD, FEM.

1 Introduction

Micromachined accelerometers are widely used in many applications. Large number of scientists all over the world are working on MEMS based acceleration sensors that are mostly either capacitive or of piezoresistive type. A piezoresistive type of acceleration sensor basically consists of a proof-mass attached to a micro-cantilever (Flexure) all made out of silicon. [1-4]. For piezoresistive accelerometer sensitivity S is defined as relative change in resistance per unit of acceleration. Following mathematical equation defines relation between sensor dimensions and its sensitivity [5]. Equation for sensitivity can be written as:

S

=

K .g .L t 2

(In Pa.)

…… Eq. 1

Where, S is the sensitivity [stress level], g is the applied acceleration, t is thickness of flexure in µm, L is length of flexure in µm, K is the constant of proportionality. An accelerometer is expected to have only one sensitive axis. However, cantilever type of accelerometer is also sensitive in other direction. This undesired sensitivity is called as cross axis sensitivity. Cross axis sensitivity is the maximum sensitivity in the plane perpendicular to the sensitive direction relative to the sensitivity in the measuring direction. It is calculated as the geometric sum of the

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sensitivities in two perpendicular directions in this plane [6]. If Z is sensitive axis then cross sensitivity is defined as

……Eq 2 Where suffix (x, y, z) denotes axis in which sensitivity is measured. Effect of cross sensitivity is one of the most important design considerations. Many attempts have been made to reduce cross sensitivity by the accelerometer designers. [7-8]. Since it is a structure deflecting under influence of inertial force, stress is developed in the flexure due to its bending. Therefore it can be stated that if the width of flexure is much greater than its thickness the cross axis sensitivity will be low. Different types of mechanical designs and structures have been tried by designers to reduce cross-sensitivity effect. Efficient use of four-piezoresistors in bridge structure is mostly tried structure [7]. Another way to reduce cross sensitivity is multi flexure accelerometer [8]. However, all these structures have a major drawback, that is, they require more processing steps as well as larger size on chip. In this paper, single cantilever type piezoresistive accelerometer is presented. The crosssensitivity is analysis is carried out by varying width as well as thickness of flexure and proof mass. Paper discusses simulations carried out for skewed and planner structure accelerometer using Coventorware software.

2 Simulations

Cantilever (Flexure) type of piezoresistive accelerometer is...

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