A capsule robot (CR) with an onboard active locomotion mechanism, has been developed as a promising alternative to colonoscopy due to its minimally-invasive advantage. Predicting the traction force and locomotion resistance of the CR, which are both the friction force, is significantly important for the CR development and control. However, a comprehensive study concerning the coefficient of friction (COF) in the colon, which is necessary for prediction, is not available in literature.
This paper is dedicated to determining a quantitative COF equation in terms of the contact pressure, hoop strain, and sliding velocity. The COFs of three commonly-used materials of the CR (i.e., PDMS, white and transparent ABS plastic), are measured under 144 different friction cases (6 contact pressures×4 hoop strains×6 sliding velocities). The measurements have indicated that the COF decreases with the contact pressure and hoop strain, and increases with the sliding velocity. In addition, the contact pressure and hoop strain both have significant influence on the COF, and the exact influence of one is related to the value of the other one. While the influence of the sliding velocity on the COF is slight and is basically unrelated to the contact pressure and hoop strain. Based on the above influence law, the COF equation involving eight fitted constants has been determined, and its determination coefficients for the three materials are up to 0.9822, 0.9286 and 0.9696, respectively. The COF equation has been used to calculate the traction force and locomotion resistance of a crawler CR, and the good agreement between the calculated and measured results manifests its usefulness in supplying a correct COF for predicting the friction of a CR in the colon.