Abstract: In order to balance the hydraulic pressure acted on the multi-axis force sensor of underwater robot, the long-shell theory is selected to research the distortion of bladder used in the sensor and amendment of this theory is carried out based on the mechanics analysis of the long and the short thin shells. An iteration algorithm used to calculate the pressure difference between in and out of the sensor is put forward, which applies the pressure balance method of bladder. Formulas of pressure difference, additional axial force, etc, in the pressure balance method of bladder are educed. The relative factors especially the pressure difference which affects the extra axial force acting on the sensor are analyzed and the characteristics of the pressure balance method of bladder, namely being competent for high pressure balance and possessing a wonderful ability to balance the slight pressure under high pressure. The criterions in designing the structure of the underwater force sensor of bladder are brought forward, namely, compact structure, small length-diameter ratio, large pressure balance diaphragm, minimal wall thickness and little oil inside the structure.

Key words: Multi axis force sensor, Pressure balance, Underwater robot