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Abstract—An electrical bridge system based on differential structure is designed to improve the sensitivity of compressive pressure sensor based on the piezoresistivity of conductive polymer composite. As the sensor works in the condition where all of the sub sensing elements have to bear the same pressure, the classical “excitation-induced differential structure” (i.e. the properties of the sub sensing elements are the same, and they are arranged on the special positions to bear the reversed excitations.) cannot be applied. To solve this problem, a “property-induced differential structure” (i.e. the sub sensing elements are endowed with reversed properties by adjusting the mass ratio of conductive filler to polymer in the composite, and the responses of them are opposite when the pressure exerted on them are the same) is designed. If the mass ratio is lower/higher than the critical mass ratio, the destruction/formation effect of the conductive network of the composite is dominant under compression, inducing that the changing tendency of the resistance of the element is consistent/opposite to that of the pressure. By using the sub sensing elements with reversed properties as the arms of a bridge, the pressure is converted to output voltage. The experimental results verify the feasibility of using the bridge system based on the “property-induced differential structure” to improve the sensitivity.

Index Terms—Compressive pressure sensor, Conductive polymer composite, Piezoresistivity.

I. INTRODUCTION

ESSURE measurement between curved surfaces of industrial equipment is the key to ensure the safety of the system [1]. As the interlayer space is small and the shape of the contacted surface is complicated, the traditional rigid sensor cannot be applied. Therefore, the pressure sensor is needed to possess flexibility. The development of flexible pressure sensor is an important branch of fabricating soft electronics which attracts more and more interests [2-8]. One approach to realize the soft electronics is to utilize new flexible material [9-13].

Conductive polymer composite is piezoresistive and flexible [14-20]. Therefore, this composite can be used to fabricate flexible pressure sensor [21-26]. However, the changing extent in the electrical resistance of the composite under the low

Manuscript received ***. This work was supported in part by the Program for Liaoning Province Excellent Talents in University under Grant

LJQ2014029, and in part by the Fundamental Research Funds for the Central

Universities under Grant N130204001.

Luheng Wang is with the College of Information Science and Engineering,

Northeastern University, China (e-mail: wangluheng@ise.neu.edu.cn). compressive pressure is small. Therefore, it is urgently needed to improve the sensitivity of the sensor based on the composite under low compressive pressure.

Differential structure is an effective method to improve the sensitivity of sensor [27-30]. In the classical differential structure, all of the properties of the sub sensing elements are the same. The sub sensing elements are arranged on the special positions of the measured workpiece to achieve a differential structure (i.e. when the excitation is applied on the workpiece, the sub excitations on the sub sensing elements are opposite, inducing that the output of one sub sensing element increases and that of the other decreases). This kind of classical differential structure is defined as “excitation-induced differential structure”. However, the aforementioned classical differential structure cannot be used in some modern industrial applications where there is no condition for arranging the sub sensing elements on the special positions to construct a differential structure. For example, in the aforementioned engineering application of measuring the interlayer pressure between the curved surfaces of industrial equipment, the pressures applied on the sub sensing elements are the same.

Therefore, the sub sensing elements can only bear the same excitations, leading to the consequence that the “excitation-induced differential structure” cannot be applied.

To solve this problem, a “property-induced differential structure” is designed in this paper. Different from the classical “excitation-induced differential structure”, the differential responses of the sub sensing elements in the “property-induced differential structure” are realized by the reversed properties of the sub sensing elements instead of the opposite excitations applied on the sub sensing elements. The differential responses of the sub sensing elements in the “excitation-induced differential structure” are caused by exerting the opposite sub excitations applied on the sub sensing elements with the same sensing properties, and the opposite sub excitations are realized by arranging the sub sensing elements on the special positions.

The differential responses of the sub sensing elements in the “property-induced differential structure” are caused by using the sub sensing elements with the opposite sensing properties under the same sub excitations, and the opposite sensing properties are realized by fabricating the sub sensing elements with different mass ratio of conductive filler to polymer.

Based on the previous research, the mass ratio of conductive filler to polymer has great effects on the piezoresistivity of the composite [31-32]. There exists a critical mass ratio, at which a transition from insufficient to sufficient percolative network regime takes place. If the mass ratio of conductive filler to polymer is lower than the critical mass ratio, the changing tendency of the resistance is consistent to that of