loops may use conventional AC or DC motors and drive systems with position or speed feedback. As dynamic response requirements increase, more specialized motor designs such as coreless motors are used. AC motors' superior power density and acceleration characteristics tends to favor permanent magnet synchronous, BLDC, induction, and SRM drive approaches. A servo system differs from some stepper motor applications in that position feedback is continuous while the motor is running. A stepper system inherently operates open-loop—relying on the motor not to "miss steps" for short term accuracy—with any feedback such as a "home" switch or position encoder external to the motor system. Stepper motor Stepper motors are typically used to provide precise rotations. An internal rotor containing permanent magnets or a magnetically soft rotor with salient poles is controlled by a set of electronically switched external magnets. A stepper motor may also be thought of as a cross between a DC electric motor and a rotary solenoid. As each coil is energized in turn, the rotor aligns itself with the magnetic field produced by the energized field winding. Unlike a synchronous motor, the stepper motor may not rotate continuously; instead, it moves in steps—starting and then stopping—advancing from one position to the next as field windings are energized and de-energized in sequence. Depending on the sequence, the rotor may turn forwards or backwards, and it may change direction, stop, speed up or slow down at any time. Simple stepper motor drivers entirely energize or entirely de-energize the field windings, leading the rotor to "cog" to a limited number of positions. Microstepping drivers can proportionally control the power to the field windings, allowing the rotors to position between cog points and rotate smoothly. Computer-controlled stepper motors are one of the most versatile positioning systems, particularly as part of a digital servo-controlled system. Stepper motors can be rotated to a specific angle in discrete steps with ease, and hence stepper motors are used for read/write head positioning in early disk drives, where the precision and speed they offered could correctly position the read/write head. As drive density increased, precision and speed limitations made them obsolete for hard drives—the precision limitation made them unusable, and the speed limitation made them uncompetitive—thus newer hard disk drives use voice coil-based head actuator systems. (The term "voice coil" in this connection is historic; it refers to the structure in a cone-type loudspeaker.) Stepper motors are