Abstract: Direct-drive (DD) motors have the characteristics of simple structure and high controllability without reduction gears. However, DD motors are more sensitive to the disturbance and parameter variation than other motors which have reduction gears. Enhancement of the stiffness of DD motors is one of effective methods to reduce these influence on the accuracy of the systems, but very high stiffness will lead to unstable, even become vibratory. Viewed from mechatronics design, a variable-viscous electrorheological (ER) fluid damper whose viscous coefficient depends upon the applicable voltage is designed, and directly attached on the shaft of DD motor to achieve high servo stiffness control. The structure of the variable – viscous ER damper is presented, and the viscous coefficient model of the designed damper is developed. The validation of this mechatronics design is demonstrated with a simulation of the position PD control DD motor system.

Key words: Direct-drive motors, Electrorheological fluid damper, Mechatronice, System stiffness