Effect Of Load On Servomotor Dynamics

Effect of load on servomotor dynamics:

 Most important among the characteristics of the servomotor is the maximum acceleration obtainable. For a given available torque, the rotor moment of inertia must be a minimum. Since the servomotor operates under continuously varying conditions, acceleration and deceleration of the rotor occur from time to time.

 The servomotor must be able to absorb mechanical energy as well as to generate it. The performance of the servomotor when used as a brake should be satisfactory.

Let J_m and b_m be, respectively, the moment of inertia and viscous-friction coefficient of the rotor, and let J_L and b_L be, respectively, the moment of inertia and viscous friction coefficient of the load on the output shaft.

Assume that the moment of inertia and viscous-friction coefficient of the gear train are either negligible or included in J_L and b_L , respectively. Then, the equivalent moment of inertia J_eq referred to the motor shaft and equivalent viscous-friction coefficient b_eq referred to the motor shaft can be written as

where n(n < 1) is the gear ratio between the motor and load. If the gear ratio n is small and J_m >> n²J_L, then the moment of inertia of the load referred to the motor shaft is negligible with respect to the rotor moment of inertia. A similar argument applies to the load friction.

In general, when the gear ratio n is small, the transfer function of the electric servomotor may be obtained without taking into account the load moment of inertia and friction. If neither J_m nor n²J_L is negligibly small compared with the other, however, then the equivalent moment of inertia J_eq must be used for evaluating the transfer function of the motor-load combination.