Stepper motor as a kind of open loop control system, and modern digital control technology has the nature of the contact.In the current domestic digital control system, stepping motor is widely used.With the emergence of digital ac servo system, ac servo motors are increasingly used in the digital control system.In order to adapt to the development trend of digital control, motion control systems mostly adopt stepper motor or all-digital ac servo motor as the executive motor.Although both similar on control mode (signal pulse sequence and direction), but in the use of performance and applications where there is a big difference.A comparison to be made regarding the use performance of both.
   First, different control precision
   Two-phase hybrid stepper motor step Angle is 1.8 ° and 0.9 °, five steps away from the Angle of the hybrid stepping motor is usually 0.72° and 0.36 °.There are also some high-performance step motor by subdividing hind leg from the Angle of smaller.However the (SANYO DENKI) production of two-phase hybrid stepper motor where the step Angle can dial the code switch is set to 1.8 ° and 0.9 ° and 0.72 ° and 0.36 ° and 0.18 ° and 0.09 ° and 0.072 ° and 0.036 °, combine the two and five phase hybrid stepper motor step Angle.
   Control precision of the ac servo motor by the motor shaft after the rotary encoder.To mount the all-digital ac servo motor as an example, for the line with a standard 2000 encoder of the motor, drive due to the internal used four frequency multiplication technology, the pulse equivalent to 360 ° / 8000 = 0.045 °.For with 17 encoder of the motor, drive round each receive 131072 pulse motor, i.e. the pulse equivalent of 360 ° / 131072 = 0.0027466 °, is a step from the Angle of 1.8 ° of the stepper motor of 1/655 of the pulse equivalent.
   Second, the low frequency characteristics are different
   Stepper motor at low speed is easy to appear low frequency vibration phenomenon.Vibration frequency is associated with load and drive performance, it is generally believed half frequency vibration frequency for the motor no-load takeoff.This determined by the working principle of stepper motor in low frequency vibration phenomenon is very bad for the normal operation of the machine.When the stepper motor work in low speed, general damping technology should be adopted to overcome the low frequency vibration phenomenon, such as add damper on motor, or drive the segmentation technology, etc.
   Ac servo motor running smoothly, even in the low speed also won't appear when the vibration phenomenon.Ac servo system has a resonance suppression function, can cover the mechanical rigidity, and internal system has the function of frequency resolution (FFT), can detect the mechanical resonance point, is advantageous for the system adjustment.
   Third, torque frequency characteristics are different
   Stepper motor's output torque increases with speed decreases, and at a high speed when they fell sharply, so its maximum working speed is in commonly 300 ~ 600 RPM.Ac servo motor for constant torque output, that is, at its rated speed (2000 RPM and 3000 RPM) of less than, can be rated output torque and constant power output above the rated speed for.
   Four different, overload ability
   Stepper motor is generally not have overload ability.Ac servo motor has a strong ability of overload.To mount the ac servo system, for example, it has the speed overload and torque overload capacity.The maximum torque for two to three times the rated torque, can be used to overcome inertia load at startup moments of inertia moment.Stepper motor because there is no this kind of overload capacity, in order to overcome the inertia moment when selection, often need to select a larger torque of the motor, and the machine during the normal work and don't need that big torque, there is a moment of waste phenomenon.
   Five, the performance is different
   Stepper motor control for the open loop control, start frequency is too high or too much load step easy appear lost or blocked, stops frequently occur high speed overshoot phenomenon, so as to ensure the control accuracy, should handle the ascending and descending speed problem.Ac servo drive system for closed-loop control, the actuator can be directly to the motor encoder feedback signal sampling, internal position loop and speed loop, general won't appear step motor lose step or overshoot phenomenon, more reliable control performance.
   Six, different speed response performance
Stepper motor accelerate from 0 to work speed (generally for a few hundred revolutions per minute) 200 ~ 200 milliseconds.Better acceleration performance of the ac servo system, the 400 w ac servo motor to mountain as an example, from static accelerated to the rated speed 3000 RPM it takes only a few milliseconds, can be used for quick start-stop control occasions.
   To sum up, the ac servo system in many performance is better than the stepping motor.But in some occasion not high demand also often use stepper motor to do perform motor.So, in the process of the design of the control system to a comprehensive, with consideration of many factors such as cost control requirements, choose the proper control motors.