The static characteristics of water hydraulic artificial muscles (WHAMs) are related to operating parameters and manufacturing parameters. Operating parameters include working pressure and contraction ratio. Manufacturing parameters include initial braiding angle, fiber sleeve material, and initial rubber tube thickness. These manufacturing parameters fundamentally influence the static characteristics of artificial muscle. Orthogonal experiments were designed with an initial braiding angle of 25 degrees and 32 degrees, fiber sleeve of UHMWPE and aramid 1414, and initial rubber tube thickness of 2mm and 3mm to study the significance level of the effects of these factors and their interactions on the static characteristics of WHAMs. Experiments were carried out at different contractions to study the relationship between contraction force and working pressure, and Analysis of Variance (ANOVA) analyzed the test data. The analysis results showed that the significance level of the initial braid angle on WHAM's static characteristics is the most significant; the significance level of fiber sleeve material and initial rubber tube thickness on the static characteristics of WHAMs depends on the working pressure and contractions. The analysis results help people fabricate different WHAM types according to the working conditions, which help people better control the contraction forces. INDEX TERMS Fiber sleeve material, initial braiding angle, rubber tube thickness, water hydraulic artificial muscles. I.INTRODUCTION
Compared with pneumatic artificial muscles (PAMs), water hydraulic artificial muscles (WHAMs) have the advantages of high force/weight ratio, high stiffness, rapid response speed, large operating pressure range, low working noise, etc. Although the physical models of PAMs have been widely studied, the model of WHAMs still need to be researched for the different structure parameters and work conditions between PAMs and WHAMs. Therefore, the geometry and the material properties need to be considered in models, including the wall thickness of rubber tube, the geometry of ends, the elastic force of rubber tube, the elongation of fibers, and the friction among fiber strands. WHAMs with different wall thickness and fiber materials were manufactured, and static characteristic experiments were performed when the actuator is static and fixed on both ends, which reflects the relationship between contraction force and pressure under the different contraction ratio. The deviations between theoretical values and experimental results were analyzed to investigate the effect of each physical factor on the modified physical model accuracy at different operating pressures. The results show the relative error of the modified physical model was 7.1% and the relative error of the ideal model was 17.4%. When contraction ratio is below 10% and operating pressure is 4 MPa, the wall thickness of rubber tube was the strongest factor on the accuracy of modified model. When the WHAM contraction ratio from 3% to 20%, the relative error between the modified physical model and the experimental data was within ±10%. Considering the various physical factors, the accuracy of the modified physical model of WHAM is improved, which lays a foundation of non-linear control of the high-strength, tightly fiber-braided and thick-walled WHAMs.
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