SKEDSOFT

Control Systems - 1

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 Jm and bm be, respectively, the moment of inertia and viscous-friction coefficient of the rotor, and let JL and bL 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 JL and bL , respectively. Then, the equivalent moment of inertia Jeq referred to the motor shaft and equivalent viscous-friction coefficient beq 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 Jm >> n2JL, 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 Jm nor n2JL is negligibly small compared with the other, however, then the equivalent moment of inertia Jeq must be used for evaluating the transfer function of the motor-load combination.