(1) Stepping Motor Stepping Motor is a conversion device that converts electrical pulse signals into mechanical angular displacement, and is an executive element of an open-loop servo system.  The stepping motor does not rotate after the power is turned on. Only when there is pulse input does the rotor rotate through a fixed angle. Its angular displacement is proportional to the number of input pulses and is synchronized with the input pulses in time.  When there is no pulse, the rotor keeps its original position and is in a positioning state.  Stepping motor can be divided into fast stepping motor and power stepping motor according to its output torque.  According to the number of excitation phases, it can be divided into three-phase, four-phase, five-phase, six-phase and even eight-phase stepping motors.  According to its working principle, it can be divided into excitation stepping motor and reaction stepping motor.  Since the angular displacement of the stepping motor is proportional to the number of input pulses and the rotation direction is related to the phase sequence of energization, the required rotation angle, rotation speed and direction can be obtained by controlling the number and frequency of input pulses and the phase sequence of energization of the motor windings.  It is widely used in open-loop servo systems because of its wide speed range, fast response, high sensitivity, simple control system and certain accuracy.  The following figure is a block diagram of the drive circuit of a stepping motor, which consists of a variable frequency signal source, a pulse distributor, a power amplifier, a stepping motor, etc.  The frequency conversion signal source of the stepping motor drive circuit block diagram is a pulse signal generator with continuously adjustable frequency from several hertz to tens of thousands of hertz. It delivers pulses of different frequencies to the pulse distributor according to operation instructions, and controls the stepping motor in various operation states.  The pulse distributor receives input pulses and direction instructions and supplies control signals to the power driver.  The pulse output by the pulse distributor is amplified by the power amplifier to drive the stepping motor to work.  (2) DC servo motor due to stepping motor composed of open loop servo system of some technical performance can not meet the requirements of the use of numerical control machine tools, and its performance is also greatly limited.  Therefore, the small inertia DC servo motor appeared in the early 1960s and the large inertia DC servo motor (also called wide speed regulating motor) appeared in the early 1970s.  At present, many numerical control machine tools use large inertia DC servo motors to form closed-loop or semi-closed-loop servo systems.  1. Small inertia DC servo motor Small inertia DC servo motor is developed to improve the fast response characteristic of servo system. It is evolved from general DC motor, but its rotor is different from that of general DC motor: first, its rotor is long and its diameter is small;  Second, its rotor is a smooth slotless iron core, and the coil is directly adhered to the surface of the iron core with insulating adhesive.  Therefore, this kind of motor has the advantages of small rotational inertia ratio, small mechanical time constant, fast response, good dynamic characteristics, stable operation at low speed, etc.  However, because the inertia of the motor rotor is small, the overload capacity is low.  Moreover, the inertia of the rotor of this motor is smaller than that of the moving parts of the machine tool, and only through gear reduction can the two be well matched, thus increasing the transmission chain error.  This kind of servo motor has been widely used in early numerical control machine tools.  2. Wide-speed DC servo motor The wide-speed DC servo motor improves its characteristics by increasing torque on the premise of maintaining the moment of inertia of the general DC motor. The specific measures are: 1) increasing the number of stator magnetic poles and adopting permanent magnetic materials with strong car stiffness;  2) Increase the number of slots and the cross-sectional area of slots on the rotor under the same rotor outer diameter and armature current.  Therefore, the instantaneous acceleration torque of the motor is improved and its dynamic response capability is improved.  Therefore, the motor has the advantages of good dynamic response, strong overload capacity, large torque, wide speed regulation range, large output torque at low speed, etc., and can be directly connected with the screw rod to improve the feed transmission accuracy of the machine tool.  At present, wide-speed DC servo motor is the most widely used of all kinds of linear servo motors.  However, this kind of motor is expensive, complicated in structure and troublesome in maintenance.  (3) AC servo motor although DC servo motor has excellent speed regulation performance, but because its brushes and commutator are easy to wear, commutation sometimes produces sparks, which limits the maximum rotation speed of the motor, and DC servo motor is complicated in structure and high in cost, thus it is limited in use.  Ac servo motor has no brush, simple structure, convenient maintenance, large output power and wide speed regulation range. it is increasingly widely used in numerical control machine tools and has replaced DC servo motor in some occasions.  AC servo motors can be divided into synchronous AC servo motors and asynchronous AC servo motors.  There are two kinds of currents used for asynchronous AC servo motors: three-phase and single-phase.  The magnetic potential sources of synchronous AC servo motors can be electromagnetic, permanent magnetic and reactive. Permanent magnetic synchronous servo motors are mostly used in the feed servo system of numerical control machine tools.