Binrui Wang, Yinglian Jin, Miao Cheng, and Weiyi Zhou
Pneumatic muscle, shape memory alloy, modeling, position control, stiffness
Pneumatic muscle (PM) is one of the most important actuators for bionic robots. To improve the precision of position control and make stiffness characteristics more flexible, a novel PM with a shape memory alloy (SMA) braided sleeve (SMA-PM) was studied. Based on the equivalent of a variable cross sectional area of cylinder and the first law of thermodynamics, PM models were established for calculating inner pressure and mass flow. An output force model was derived with variable SMA wire length, and contraction ratio and stiffness were analysed. Then a position control system was designed and a dynamics model established based on Newtonian mechanics. An incremental proportional-integral-derivative scheme was adopted for the position servo. Simulations of inflation/deflation responses and stiffness were accomplished using MATLAB. SMA-PM can maintain the dynamic characteristics of a fibre braided PM. Furthermore, the variable SMA braided length makes PM stiffness more flexible. Step and slope position control response experiments were implemented on a self-developed SMA-PM test bed. Results showed that inflation response is quicker than deflation, and SMA contraction can compensate for position errors. Thus, PM position precision is improved and response time is reduced.
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