The difference between servo motor and stepper motor

The rotor inside the servo motor is a permanent magnet. The U/V/W three-phase electricity controlled by the drive forms an electromagnetic field, and the rotor rotates under the action of this field, while the encoder that comes with the motor feeds back signals to the drive, which adjusts the angle of rotor rotation according to the feedback value compared with the target value. The accuracy of the servo motor is determined by the accuracy of the encoder (number of lines).

What is a servo motor? How many types are there? What are the working characteristics?

A: Servo motor, also called actuator motor, is used as an actuator in automatic control system to convert the received electric signal into angular displacement or angular velocity output on the motor shaft. It is divided into two categories: DC and AC servo motors. Its main feature is that there is no self rotation when the signal voltage is zero, and the speed decreases uniformly with the increase of torque.

What is the difference in function between AC servo motor and brushless DC servo motor?

A: AC servo is better, because it is a sine wave control ball screw, torque pulsation is small. DC servo is trapezoidal wave. But DC servo is simpler and cheaper. Permanent magnet AC servo motor Since the 1980s, with the development of integrated circuits, power electronics and AC variable speed drive technology, permanent magnet AC servo drive technology has developed prominently, and famous electrical manufacturers in various countries have launched their own AC servo motor and servo drive series products and continue to improve and update. AC servo system has become the main development direction of the contemporary high-performance servo system, so that the original DC servo facing the crisis of being eliminated. 90s later, the world has commercialized the AC servo system is the use of fully digital control of the sine wave motor servo drive. The development of AC servo drive in the field of transmission is changing day by day.

Compared with DC servo motor, the main advantages of permanent magnet AC servo motor are: (1) No brushes and commutator, so it works reliably and requires low maintenance and care. (2) The heat dissipation of stator winding is more convenient. (3) Small inertia, easy to improve the rapidity of the system bellows coupling. (4) Adapt to high speed and high torque working condition. (5) With the same power has a smaller volume and weight.

Servo and stepper motor

Servo mainly relies on pulse to position, basically can be understood in this way, the servo motor receives 1 pulse, it will rotate 1 pulse corresponding to the angle, so as to achieve displacement, because, the servo motor itself has the function of sending out pulses, so the servo motor will send out a corresponding number of pulses for each rotation angle, so that, and the servo motor receives pulses to form an echo, or closed loop, in this way The system will know how many pulses were sent to the servo motor and how many pulses were received back at the same time, so that it can control the rotation of the motor very precisely and thus achieve precise positioning, which can reach 0.001mm.

Stepper motor is a discrete motion device, which has an essential connection with modern digital control technology. In the current domestic digital control system, stepper motors are widely used. With the emergence of fully digital AC servo systems, AC servo motors are also increasingly used in digital control systems. In order to meet the development trend of digital control, most of the motion control systems use stepper motors or fully digital AC servo motors as the execution motor. Although the two are similar in the control mode (pulse string and direction signal) flexible coupling, but there are major differences in the use of performance and application occasions. Now the use of the two performance for a comparison.

First, the control accuracy is different

Two-phase hybrid stepper motor step angle is generally 3.6 °, 1.8 °, five-phase hybrid stepper motor step angle is generally 0.72 °, 0.36 °. There are also some high-performance stepper motor step angle is even smaller. For example, a stepping motor for slow-walking machine tools produced by SCT, its step angle is 0.09 °; Germany BERGER LAHR company (BERGER LAHR) produced a three-phase hybrid stepping motor whose step angle can be set to 1.8 °, 0.9 °, 0.72 °, 0.36 °, 0.18 °, 0.09 °, 0.072 °, 0.036 °, compatible with the step angle of two-phase and five-phase hybrid stepper motors.

The control accuracy of the AC servo motor is guaranteed by the rotary encoder at the rear of the motor shaft. For a Panasonic fully digital AC servo motor, for example, the pulse equivalent is 360°/10000=0.036° for a motor with a standard 2500-line encoder due to the quadruple frequency technology used inside the drive. For a motor with a 17-bit encoder, the drive receives 217=131072 pulses per motor revolution, i.e., its pulse equivalent is 360°/131072=9.89 seconds. Is the step angle of 1.8 ° of the stepper motor pulse equivalent 1/655.

Second, the low frequency characteristics are different

Stepper motor at low speeds are prone to low-frequency vibration phenomenon. Vibration frequency and load conditions and drive performance, it is generally believed that the vibration frequency is half of the motor no-load starting frequency. This low-frequency vibration phenomenon determined by the working principle of the stepper motor is very unfavorable to the normal operation of the machine. When stepper motors work at low speeds, damping technology should generally be used to overcome the low-frequency vibration phenomenon, such as adding dampers on the motor, or subdivision technology on the drive, etc.

AC servo motor operation is very smooth diaphragm coupling, even at low speeds, there is no vibration phenomenon. AC servo system has resonance suppression function, which can cover the lack of mechanical rigidity, and the system has internal frequency resolution function (FFT), which can detect the resonance point of the machinery and facilitate system adjustment.

Third, the moment-frequency characteristics are different

The output torque of stepper motor decreases with the increase of speed, and will drop sharply at higher speed, so its maximum working speed is generally 300-600RPM. AC servo motor is constant torque output, that is, within its rated speed (generally 2000RPM or 3000RPM), it can output the rated torque, and above the rated speed is constant power output.

Fourth, the overload capacity is different

Stepper motors generally do not have overload capability. AC servo motor has a strong overload capacity. Take Panasonic AC servo system as an example, it has speed overload and torque overload capability. Its maximum torque is three times the rated torque, which can be used to overcome the moment of inertia of inertial loads at the moment of start-up. Stepper motor because there is no such overload capacity, in order to overcome this moment of inertia in the selection, often need to choose a larger torque motor, and the machine does not need so much torque during normal operation, there will be a waste of torque phenomenon.

Fifth, the operating performance is different

Stepper motor control for open-loop control, too high a start frequency or too large a load prone to step loss or blocking the phenomenon, stopping the speed is too high prone to overshoot the phenomenon, so to ensure its control accuracy, should deal with the problem of speed up and down. AC servo drive system for closed-loop control, the driver can directly sample the motor encoder feedback signal, the internal position loop and speed loop, generally will not appear in the stepper motor step loss or overshoot phenomenon, the control performance is more reliable.

Six, the speed response performance is different

Stepper motor from standstill acceleration to working speed (generally a few hundred revolutions per minute) takes 200 to 400 milliseconds. AC servo system acceleration performance is better, to Panasonic MSMA 400W AC servo motor, for example, from standstill acceleration to its rated speed of 3000RPM only takes a few milliseconds, can be used for the control of fast start-stop occasions.

In summary, AC servo systems are superior to stepper motors in many performance aspects. However, stepper motors are often used as actuating motors in some less demanding situations. Therefore, in the design process of the control system to consider the control requirements, cost and other factors to choose the appropriate control motor.