Abstract
The output member of an actual servomechanism has always the Coulomb friction. We show the effect of the Coulomb friction on the parameters in a 2nd order servomechanism, which uses a D. C armature-controlled servomotor. First, we assume that the Coulomb friction can be treated as the viscous friction varied with the output speed. Then we can use the well-known transfer function of a servomotor, in which Kv and Tm, are the functions of output speed. Therefore, the damping ratio ξ and the natural frequency of the closed-loop system also vary with the output speed. Because the Coulomb friction increases the steady state error and causes the delay in the response, we propose a method of cancelling out the Coulomb friction and linearizing the system by impressing the compensating voltage to the controller. Furthermore, we present a new servo system, in which the Coulomb friction is utilized to improve the response by stopping the cancellation intermittently according to the combinations of the signs of error and output velocity. Sometimes the limit cycle occurs in the linearized system, but the stable system with very small steady state error is obtained in the latter case.
