Non-standard automation equipment servo/stepper motor application, precision, control mode
2021-06-24 20:45:49

Type of servo motor


Permanent magnet synchronous servo motor

The rotor inside the permanent magnet synchronous servo motor is a permanent magnet, actuation-controlled   The U/V/W three-phase electricity forms an electromagnetic field, and the rotor rotates under the action of this magnetic field. At the same time, the encoder in the motor gives feedback signal to the driver, and the driver adjusts the rotation Angle of the rotor according to the feedback value and the target value. The accuracy of the servo motor depends on the accuracy of the encoder (number of lines).


Features are as follows:

The control speed is very fast, from start to rated speed in milliseconds;   An asynchronous motor in the same situation takes a few seconds. Large starting torque can drive objects with large inertia to move.

Large power density, the same power range compared to the asynchronous motor can make the volume smaller, lighter weight.

High operating efficiency. Can support low speed long

Time runs. Power off without rotation phenomenon, can quickly control the stop action.

The starting torque is three times the rated torque.


Asynchronous servomotor

Asynchronous servo motors are virtually identical to asynchronous motors. However, it introduces the encoder to realize the closed-loop control of the motor, so it can also be regarded as a kind of servo motor. Especially with the rapid development of the current frequency conversion speed regulation technology, the actual control performance of the asynchronous servo motor is also very good, with its support of high power, high speed characteristics.


Features are as follows:

The power can be made very large, the design is mature, the operation reliability is high.

Support high speed (over 10000rpm) for long time operation, compared with the highest permanent magnet synchronous motor can only achieve 6000~8000rpm speed.

High cost performance, can replace the use of permanent magnet motor when the control accuracy is not high.

The starting torque is three times the rated torque.


The so-called brushless DC motor “ DC frequency conversion & RDQUO; In essence, it is AC frequency conversion through inverter. Theoretically, the brushless DC motor is similar to the AC permanent magnet synchronous servo motor, and should be classified as the AC permanent magnet synchronous servo motor. The speed can be adjusted by variable voltage frequency conversion, and the highest speed is up to tens of thousands of revolutions /min.


Features are as follows:

Similar to permanent magnet synchronous servo motor


DC brush servo motor

Small size, quick action and quick reaction, large overload capacity, wide speed range, not too high speed, can reach 10000r/min


The speed regulation mode is the following three:

Armature series resistance

Adjust armature voltage (most commonly used)

Regulating magnetic flux



Stepper motor performance overview


1)Frequency torque characteristics

As the pulse frequency increases, the stator of the motor will generate back electromotive force to weaken the working current and reduce the output torque of the motor.


2)Low frequency vibration

Under normal circumstances, the frequency range of low frequency vibration is between 180pps-250pps.



1. Adder and reducer avoid resonant frequencies.

2. Subdivision driving.

3. Switch to a motor with a small step Angle.

4. Switch to servo motor.


3)Start frequency should not be too high, need to accelerate

No-load starting frequency: that is, the pulse frequency at which the stepping motor can start normally under no-load conditions. If the pulse frequency is higher than this value, the motor cannot start normally, and step loss or rotation blocking may occur.



The starting speed of the stepper motor is generally 10~100RPM, and the starting speed of the servo motor is generally 100~300RPM. Depending on the size of the motor and the load conditions, the larger motor generally corresponds to a lower start speed.


4)Acceleration and deceleration time and stop time requirements

The length of acceleration and deceleration time is the key when choosing, but in addition to that, acceleration must be considered. Torque and acceleration and deceleration constant balance, so can not be easily determined.   Therefore, when starting the calculation, set the positioning time of   25%  Left and right for the standard to calculate.


Acceleration and deceleration time [s] =Positioning time [s]×0.25


Note: Acceleration time shall not be less than 0.1s, preferably between 0.2-0.3s (allowable acceleration time of stepper motor is between 0.1-1s, allowable acceleration time of servo motor is between 0.05s and 0.5s)



Control mode of servo motor


1)Torque control mode

The torque control method is to set the external output torque of the motor shaft by the input of the external analog quantity or the direct address assignment. It is mainly used in the winding and unwinding devices that have strict requirements on the force of the material, such as winding device or pulling optical fiber equipment. The torque setting should be changed at any time according to the change of the winding radius to ensure that the force on the material will not change with the change of the winding radius.

According to the size of the input voltage, to control the motor output torque.



2)Speed control mode

Rotation speed can be controlled by analog input or pulse frequency, and the speed mode can also be positioned when the outer loop of the upper control device is controlled, but the position signal of the motor or the position signal of the direct load must be fed back to the upper for calculation. In this case, the encoder at the motor shaft end only detects the motor speed, and the position signal is provided by the direct detection device at the final load end. This has the advantage of reducing the error in the intermediate transmission process and increasing the positioning accuracy of the whole system.

According to the size of the input voltage, achieve the purpose of controlling the output speed of the motor.



3)Position control mode

In the position control mode, the rotation speed is generally determined by the frequency of the external input pulse, and the rotation Angle is determined by the number of pulses. In addition, some servos can assign the speed and displacement directly through communication. The position mode is commonly used in positioning devices because it allows for strict control of speed and position. Applications such as CNC machine tools, printing machinery and so on.

According to the number of input pulse wave, achieve the purpose of controlling motor positioning.



Stepper VS servo motor


1)Different control precision

The step Angle of two-phase stepper motor is 1.8° ; The three-phase hybrid intake motor and driver produced by Bagra, Germany, can be subdivided to achieve a step Angle of 1.8° , 0.9 & deg; , 0.72 & deg; , 0.36 & deg; , 0.18 & deg; , 0.09 & deg; , 0.072 & deg; , 0.036 & deg; , which is compatible with the step Angle of two-phase and five-phase stepper motors. The control precision of the AC servo motor is guaranteed by the encoder at the back end of the motor. For an electric drive with a standard 2500-line encoder, the pulse equivalent is 360&deg with a 4x frequency technology inside the drive. / 10000 = 0.036 & deg; ; For a motor with a 17-bit encoder, the drive receives 217 =131072 pulse motor turns, that is, its pulse equivalent is 360&deg. / 131072 = 0.00274658 & deg; Is the step Angle of 1.8° The pulse equivalent of the stepper motor is 1/655.



2)Low frequency characteristics are different

The two-phase hybrid stepper motor is prone to low frequency vibration when running at low speed. AC servo motor runs very smoothly, even at low speed there will be no low-frequency vibration phenomenon.


3)The torque frequency characteristics are different

The output torque of the stepper motor decreases with the increase of the speed, and will decrease sharply at higher speeds. AC servo motors have constant torque output, that is, they can output rated torque within rated speed (such as 3000RPM).



4)Different overload capacity

Stepping motors generally do not have overload capacity, and AC servo motors have strong overload capacity, the general maximum torque can be 3 times of the rated torque, can be used to overcome the inertia torque of the inertia load at the start moment. Because the stepping motor does not have this overload capacity, in order to overcome this inertia moment, it is often necessary to select the motor with a larger torque, which results in the phenomenon of torque waste.


5)Different operating performance

Stepper motor control for open loop control, starting frequency is too high or the load is too large easy to lose the step or the phenomenon of blocked rotation; When stopping, if the speed is too high, it is easy to overshoot. Therefore, in order to ensure the control accuracy, the problem of rising and falling speed should be handled well. AC servo drive system for closed-loop control, internal composition of position loop and speed loop, generally do not appear to lose step or overshoot phenomenon, the control performance is more reliable.



6)Speed response performance is different

Stepper motors need 200~400ms to accelerate from rest to working speed (usually several hundred RPM). AC servo drive system has good acceleration performance, from static acceleration to working speed (such as 3000RPM), generally only a few milliseconds, can be used for quick start control occasions.


7)Different efficiency indicators

Stepper motor efficiency is relatively low, generally less than 60%. The efficiency of AC servo motor is relatively high, generally more than 80%. Therefore, the temperature rise of the stepper motor is higher than that of the AC servo motor.



Driving mode of stepper motor


Open and closed loop control of servo motor


◎Open LOOP control (OPEN LOOP) by the controller output instruction signal, used to drive the motor according to the instruction value and stop at the specified position.

◎The position or speed detector is installed on the motor shaft to obtain the position and speed return credit number.

◎Full-close LOOP control directly feedback the displacement of the object to the control system at any time by using the optical ruler and other position detectors.



Type and resolution of encoder


An encoder is a device that programs and converts signals or data into a signal form that can be used for communication, transmission and storage.

An encoder can convert an angular or linear displacement into an electrical signal.



According to the working principle, encoders can be divided into


Incremental encoder

Incremental encoder


Absolute type encoder

Each position corresponds to a definite digital code, and its indicated value is only related to the starting and ending positions of the measurement, and has nothing to do with the intermediate process of the measurement.



Discussion on control precision and positioning precision of stepping motor


1)Stepper motor control accuracy and positioning accuracy difference



Control accuracy (step Angle)

Two-phase stepper motor 1.8°

Three-phase stepper motor1.2°

Five-phase stepper motor 0.72°



1. Although the subdivision driver can improve the control precision, the subdivision control precision can not be used as a reference for the control precision if the subdivision level exceeds 4.

2. When considering the control accuracy, the minimum step Angle should be multiplied by 2 to see if the control accuracy is satisfied.


Stepper motor positioning accuracy

The positioning accuracy of stepper motor refers to the difference between the actual positioning Angle and the theoretical positioning Angle, which is completely dependent on the motor itself and has no direct relationship with the step Angle or the subdivision level. Positioning accuracy =3%-5% step Angle.



1. Step Angle accuracy error does not accumulate.

2. The one-step angular error does not decrease with subdivision.

3. Subdivision can improve the control accuracy, but there are also errors in subdivision, and the subdivision error will accumulate with the single step error.


Mechanical error

The mechanical error has nothing to do with the control accuracy, but to some extent, it has something to do with the single-step positioning accuracy.


1)Method of improving positioning accuracy

Add reducer

Switch to servo motor

Add grating ruler or encoder