Measurement and modeling of friction in linear and rotary micromotors supported on microball bearings

We report, for the first time, the measurement and modeling of friction in both linear and rotary micromotors supported on microball bearings using transient-response characterization. For the linear micromotor the friction force of 0.33 mN was measured at the velocity of 7 -10 mm/s and normal force of 20 mN. The average coefficient of friction (mu), defined as the ratio of frictional to normal force, and the minimum dashpot coefficient (B), defined as the ratio of frictional force to rolling velocity, were extracted to be 0.02 and 4 x 10^-2 kg/s , respectively, for the linear device. The average friction torque, mu, and B for the rotary micromotor were measured to be 5.7 muN.m, 0.02, and 2.6 x 10^-4 kg/s, respectively, with the tip velocity of 144 mm/s and normal loading of 2 mN. These results enable understanding of frictional forces and their dependencies on operating velocities and normal loadings in microball-bearing-supported micromachines.