Servo Motor vs Stepper Motor: The Differences Explained

Servo Motor vs Stepper Motor

Stepper Motor

Stepper Motor

Working principle of stepping motor

Stepper motor, as a special motor for control, is the actuator of converting electric pulse into angular displacement.

When the stepping driver receives a pulse signal, it drives the stepping motor to rotate at a fixed angle in the set direction (called “step angle”).

It rotates step by step at a fixed step angle.

The angular displacement can be controlled by controlling the number of pulses, so as to achieve accurate positioning

At the same time, the speed and acceleration of motor rotation can be controlled by controlling pulse frequency, so as to achieve the purpose of speed regulation.

The motor will be reversed if the winding electrifying sequence is changed.

Working principle of stepping motor

The working principle of stepper motor driver

The stepper motor needs to be driven by a special stepper motor driver, which is composed of impulse control unit, power drive unit and protection unit.

The power drive unit amplifies the pulse generated by the pulse generation control unit and is directly coupled with the stepping motor, which belongs to the power interface between the stepping motor and the microcontroller.

Working principle of stepping motor

Control instruction unit receives the pulse and direction signals, the corresponding pulse generating control unit generates a set of the corresponding pulse and then sent to the stepper motor through the power drive unit.

The stepping motor turns a step angle in the corresponding direction.

The stepper motor has some important technical data, such as maximum static torque, starting frequency and operating frequency.

Generally speaking, the smaller the step distance angle, the greater the maximum static torque of the motor, the higher the starting frequency and operating frequency.

Therefore, the operation mode emphasizes the subdivision drive technology.

This method improves the rotation moment and resolution of the stepping motor and completely eliminates the low-frequency oscillation of the motor.

So subdivision drive performance is better than other types of drives.

The rotor inside the servo motor is a permanent magnet.

The driver controls U/V/W three-phase electricity to form an electromagnetic field. The rotor rotates under the action of this magnetic field, and the motor’s own encoder feedback signal to the driver.

The driver adjusts the rotation angle of the rotor according to the feedback value and target value.

Servo Motor

Servo Motor

The working principle of the servo motor

Servo motor, also known as the executive motor.

In an automatic control system, the servo motor is used as an actuator to convert the received electrical signal into angular displacement or angular velocity on the motor shaft and then output.

Servo motors are divided into DC and AC servo motors.

When the servo motor receives one pulse, it will rotate the angle corresponding to one pulse to realize the displacement, because:

The servo motor itself has the function of sending out pulses, so each rotation angle of the servo motor will send out a corresponding number of pulses, which forms a closed loop together with the pulse received by the servo motor.

The system will know how many pulses it sends to the servo motor, and how many pulses it receives.

In this way, the motor can be precisely controlled to achieve accurate positioning.

AC servo motor is superior to DC servo motor in performance comparison.

Ac servo motor adopts sine wave control, with small torque ripple and large capacity.

DC servo motor adopts trapezoidal wave control, which is relatively poor.

Brushless servo motor in DC servo motor has better performance than brush servo motor.

The working principle of servo motor driver

The rotor inside the servo motor is a permanent magnet.

The actuator controls U/V/W three-phase electricity to form an electromagnetic field in which the rotor rotates.

At the same time, the motor’s own encoder feedback signal to the driver.

The driver adjusts the rotation angle of the rotor according to the feedback value and target value.

Brush DC servo motor drive:

The working principle of the motor is the same as that of ordinary DC motor.

The actuator is a three-loop structure, which is the current loop, speed loop and position loop from inside to outside.

The output of the current loop controls the armature voltage of the motor.

The input of the current loop is the output of the speed loop PID, the input of the speed loop is the PID output of the position loop, and the input of the position loop is the given input.

The control diagram is shown above.

Brushless DC servo motor drive:

The power supply is DC, which is converted into U/V/W ac power through the internal three-phase inverter.

The driver also adopts the three-loop control structure (current loop, speed loop, position loop), and the driving control principle is the same as above.

AC servo motor drive:

It can be divided into two modules: the power panel and the control panel with independent functions.

Control panel output PWM signals through the corresponding algorithm, as a drive circuit of the drive signal, to change the output power of inverter, in order to achieve the purpose of the three-phase permanent magnet synchronous AC servo motor control.

The power drive unit first rectifies the input three-phase electricity or municipal electricity through the three-phase full-bridge rectifier circuit and obtains the corresponding direct current electricity.

The three-phase permanent magnet synchronous AC servo motor is driven by the commutator of the three-phase sinusoidal PWM voltage-type inverter after a good three-phase or municipal rectifier.

It is simply an AC-DC-AC converter process.

The control unit is the core of the whole AC servo system, realizing the system position control, speed control, torque and current control.

Performance comparison of the servo motor and stepper motor

Control Accuracy

The more phase and beat the stepping motor has, the more accurate it will be.

The servo motor takes blocks from its own encoder. The more scale the encoder has, the higher the precision.

Low-Frequency Characteristic

The low-frequency vibration of the stepping motor is easy to occur at low speed.

When it works at low speed, it usually uses damping technology or subdivision technology to overcome low-frequency vibration.

The servo motor runs very smoothly, and there will be no vibration even at low speed.

Torque-frequency Characteristic

The output torque of the stepping motor decreases with the increase of speed and decreases sharply at high speed.

The servo motor is constant torque output in rated speed and constant power output in rated speed.

Motor Torque Curve

Overload Capacity

The stepper motor does not have overload capacity, and the servo motor has a strong overload capacity.

Operation Performance

The stepper motor is in open-loop control. If the starting frequency is too high or the load is too large, it is easy to lose the step or stop rotating. If the speed is too high, it is easy to overshoot.

AC servo drive system is closed-loop control. The driver of the servo motor can directly be sampling the feedback signal of the motor encoder. The internal position ring and speed ring are formed. Generally, the stepping motor will not lose step or overshoot, so the control performance is more reliable.

Speed Response Performance

It takes hundreds of milliseconds for the stepper motor to accelerate from static to operating speed.

AC servo system has good acceleration performance, generally only a few milliseconds, which can be used for control occasions requiring quick start and stop.

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