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Friction in Robot Joints is the Reason for Nonlinearity in Impedance Estimation
S. I. Hwang(황성일),H. A. Kang(강현아),S. H. Kang(강상훈) Korean Society for Precision Engineering 2021 한국정밀공학회 학술발표대회 논문집 Vol.2021 No.11월
A stochastic estimation of arm mechanical impedance (AMI) using a low-friction robot (MIT-MANUS) under Proportional-Derivative (PD) control reported low coherence, indicating nonlinearity in AMI in the low-frequency range. It was hypothesized that the nonlinearity was due to the nonlinearity in the arm. Another study using an industrial robot under a robust impedance control (IMBIC) compensating all the nonlinearities (e.g., Friction) of the robot reported no noticeable nonlinearity. We hypothesized that the nonlinearity is not from the human arm, and the nonlinearity is due to the friction in robot joints. Mechanical impedance of the spring array was estimated using a direct-drive robot, almost identical to MITMANUS, under PD control and IMBIC, respectively. All the coherence functions obtained with IMBIC were larger than 0.9 in the frequency range of interest. The coherence functions with PD control were less than 0.8 in the low frequencies (< 5 Hz). These results indicated that the nonlinearity is due to the friction in robot joints, even though the friction was small. It is indirect but strong evidence that the human arm behaves linearly for small perturbation, and even if the robot has a low friction level, the friction should be compensated for reliable estimation.
Quantification of sulfur from organic and inorganic materials for determination of <sup>35</sup>C
이흥래,강상훈,송병철,손세철,지광용,Lee, H.N.,Kang, S.H.,Song, B.C.,Sohn, S.C.,Jee, K.Y. The Korean Society of Analytical Sciences 2009 분석과학 Vol.22 No.2
The oxidation studies of a sulfur to a sulfate ion by various oxyhalide oxidants in organic (thiourea, methionine) and inorganic (sulfate, thiophosphate) compounds were carried out in an acidic solution. The optimized result of the oxidation reaction was obtained when a bromate compound (${BrO_3}^-$) as an oxidant and a 3 M $HNO_3$ solvent were used. The chemical yield for the oxidation of the organic and inorganic sulfur compounds to a sulfate ion was monitored as 80% for thiophosphate, 87% for methionine, and 100% for thiourea and sulfate within 5% RSD. The oxidations of thiourea required at least 1.6 equivalents of the bromate in an acidic solution. In the case of the oxidation of methionine and thiophosphate, the oxidation yields were above 80% if the bromate was used at 20 times higher than that of the substrates. The sulfate ion was quantitatively measured by using a GPC counting of $^{35}S$ followed by precipitates of $BaSO_4$. A quenching correction curve for the $^{35}S$ counting was obtained to use the difference via the precipitate weight result.