Closed-loop control. The motor carries a feedback device, usually an encoder or resolver, that reports actual shaft position and speed to the drive, which compares that with what was commanded and corrects continuously. Without that loop the same motor is simply a motor. This is why servo performance is a property of the motor, the feedback device and the drive together rather than of the motor alone.

A stepper moves in fixed increments and holds position by construction, without knowing where it is. That is simple and inexpensive, but if it is overloaded it loses steps silently and the machine's position is wrong with nothing to indicate it. A servo knows its position at all times and corrects, so it cannot lose position undetected, delivers more torque at speed, and runs cooler at rest.

A variable frequency drive controls the speed of an induction motor, and does so economically over a wide range, but it does not position. A servo system positions: it moves to a coordinate, settles within a stated tolerance and holds there under load at zero speed. Where the requirement is a controlled speed, a VFD and induction motor are cheaper and entirely adequate; where it is a controlled position, they are not.

Incremental and absolute encoders, both optical and magnetic, and resolvers. Encoders offer high resolution and are the norm. Resolvers are more tolerant of heat, vibration and contamination and remain common in harsh and military applications. Resolution, accuracy and the communication protocol all matter, and the feedback device must be one the intended drive supports, since compatibility is not universal.

An incremental encoder reports change, so the system does not know where it is at power-up and must be homed by moving to a reference. An absolute encoder reports actual position immediately, removing the homing move. Multi-turn absolute devices also remember how many revolutions have been made, which matters on axes that travel more than one turn and cannot safely be homed by moving.

It is the ratio between the load inertia reflected to the motor shaft and the motor's own rotor inertia. If the load inertia is much larger, the system becomes difficult to tune and responds sluggishly or oscillates. Gearing helps disproportionately, because reflected inertia falls with the square of the ratio, which is frequently the real reason a gearhead is fitted rather than a need for more torque.

Continuous torque is what the motor can produce indefinitely without exceeding its thermal limit. Peak torque is a much higher figure available for short periods, typically for acceleration and deceleration. Sizing uses both: peak must cover the acceleration demand, and the root-mean-square torque over the whole duty cycle must sit within the continuous rating, or the motor will overheat even though no single move exceeded its capability.

On any vertical or gravity-loaded axis, where the load would fall if power were lost or the drive disabled. The brake is a fail-safe device, spring-applied and electrically released, and it is intended to hold a stationary load rather than to stop a moving one. Fitting one is a safety decision, not a convenience, and it must be specified with the motor since retrofitting usually is not possible.

Within one manufacturer's system as a rule. The drive must support the motor's feedback device and protocol, match its voltage and current ratings including peak, and hold the motor's parameters for tuning. Servo motors and drives are sold as matched pairs precisely because the loop depends on the combination. Mixing brands is possible in limited cases but rarely worth the commissioning effort.

Very little mechanically, since they are brushless and sealed. Attention concentrates on the cables and connectors, which flex, chafe and are the most common failure point, and on the feedback device, which is sensitive to contamination and shock. Check the holding brake functions where fitted, keep cooling surfaces clear, and confirm shaft seals remain intact where the environment is wet or dusty.