Strictly speaking, stepper motors are also a type of servo motor . Servo motors specifically refer to motors that can be precisely controlled (meaning that speed, angle, stroke, etc. are controllable), including DC servo motors, AC servo motors, and stepper motors. However, in a less strict sense, they mostly refer to AC/DC servo motors.
Compared to AC/DC servo motors , the biggest advantage of stepper motors is that both rotation angle and speed can be easily and precisely controlled. The control system is simple, using sequential pulse drive. Different pulse current sequences are applied between the stator and stator in sequence, causing the difference in magnetic force between the teeth of the stepper motor to pull the rotor to rotate. The number of control pulses directly corresponds to the number of tooth steps of the rotor. Therefore, when the requirements are not strict, the position sensor can be omitted. Moreover, it has a self-locking capability after stopping. It is much easier to control than AC/DC motors, so it is the most commonly used servo motor, especially in small-power, small-size electrical control machinery where it dominates.
However, the biggest drawback of stepper motors is that they have relatively low torque and power (the maximum is only in the KW level), and their rotation smoothness is not very good. They are generally used in small electromechanical systems.
The main advantages of AC /DC servo motors are high power (up to hundreds of kW), high torque, and extremely high speed range (they can be extremely slow or extremely fast). They also have smooth torque and low jitter and are generally used in large, high-performance CNC systems. However, the control of AC/DC servo systems is very complex. They all require precise angle or position sensors for closed-loop control, which involves complex algorithms, high cost, and large size.
DC servo motors are generally controlled by voltage, but a few can be controlled by current. There is a certain functional relationship between voltage or current and motor speed. The control system controls the motor voltage based on the signal fed back by the angle sensor, and finally controls the motor speed or angle.
Stepper motor principle
A stepper motor, a special type of motor used for control, is an actuator that converts electrical pulses into angular displacement. When a stepper driver receives a pulse signal, it drives the stepper motor to rotate a fixed angle (called the "step angle") in a set direction. Its rotation occurs step by step at fixed step angles. The amount of angular displacement can be controlled by controlling the number of pulses, thus achieving accurate positioning. Simultaneously, the speed and acceleration of the motor can be controlled by controlling the pulse frequency, thus achieving speed regulation. Changing the energizing sequence of the windings will reverse the motor's direction.
Driver principle
Stepper motors require a dedicated stepper motor driver, which consists of a pulse generation and control unit, a power drive unit, and a protection unit. The power drive unit amplifies the pulses generated by the pulse generation and control unit and couples directly with the stepper motor, serving as the power interface between the stepper motor and the microcontroller.
The control command unit receives pulse and direction signals. The corresponding pulse generation control unit generates a set of pulses with the appropriate number of phases, which are then sent to the stepper motor via the power drive unit. The stepper motor rotates one step angle in the corresponding direction. The pulse setting method of the driver determines the stepper motor's operating mode, as follows:
(1) m-phase single-m-beat operation
(2) m-phase double m-beat operation
(3) m-phase single and double m-beat operation
(4) Microstepping requires the driver to provide drive signals of different amplitudes.
Stepper motors have several important technical specifications, such as maximum static torque, starting frequency, and operating frequency. Generally, the smaller the step angle, the greater the maximum static torque, and consequently, the higher the starting and operating frequencies. Therefore, microstepping technology is emphasized in its operation. This technology improves the stepper motor's torque and resolution, completely eliminating low-frequency oscillations. Thus, microstepping drivers offer superior performance compared to other types of drivers.
The rotor inside the servo motor is a permanent magnet. The U/V/W three-phase electricity controlled by the driver forms an electromagnetic field. The rotor rotates under the action of this magnetic field. At the same time, the encoder built into the motor feeds back a signal to the driver. The driver compares the feedback value with the target value and adjusts the rotation angle of the rotor.
Servo motor principle
Servo motors, also known as actuator motors, are used as actuators in automatic control systems to convert received electrical signals into angular displacement or angular velocity output on the motor shaft. They are divided into two main categories: DC and AC servo motors.
When a servo motor receives one pulse, it will rotate by the angle corresponding to that pulse, thereby achieving displacement. Because the servo motor itself has the function of emitting pulses, it will emit a corresponding number of pulses for each angle it rotates. This forms a closed loop with the pulses received by the servo motor. The system will know how many pulses were sent to the servo motor and how many pulses were received back. In this way, the rotation of the motor can be controlled very precisely, thereby achieving accurate positioning.
In terms of performance, AC servo motors are superior to DC servo motors. AC servo motors use sinusoidal wave control, resulting in lower torque ripple and allowing for larger capacity. DC servo motors use trapezoidal wave control, which is relatively inferior. Among DC servo motors, brushless servo motors outperform brushed servo motors.
Servo motor driver
The rotor inside the servo motor is a permanent magnet. The U/V/W three-phase electricity controlled by the driver forms an electromagnetic field. The rotor rotates under the action of this magnetic field. At the same time, the encoder built into the motor feeds back a signal to the driver. The driver compares the feedback value with the target value and adjusts the rotation angle of the rotor.
Brushed DC servo motor driver: The working principle of the motor is exactly the same as that of a regular DC motor. The driver has a three-loop closed-loop structure, consisting of a current loop, a speed loop, and a position loop from the inside out. 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 controller. The input of the speed loop is the output of the position loop PID controller. The input of the position loop is the setpoint. The control principle diagram is shown above.
Brushless DC servo motor driver: The power supply is DC, which is converted into AC power (U/V/W) by an internal three-phase inverter and then supplied to the motor. The driver also adopts a three-closed-loop control structure (current loop, speed loop, and position loop), and the drive control principle is the same as above.
AC servo motor driver: It can be roughly divided into two modules with relatively independent functions: power board and control board. The control board outputs PWM signal through corresponding algorithm as the drive signal of the drive circuit to change the output power of the inverter, so as to control the three-phase permanent magnet synchronous AC servo motor.
The power drive unit first rectifies the input three-phase power or mains power through a three-phase full-bridge rectifier circuit to obtain the corresponding DC power. The rectified three-phase power or mains power is then converted by a three-phase sinusoidal PWM voltage-source inverter to drive the three-phase permanent magnet synchronous AC servo motor. Simply put, it is an AC-DC-AC conversion process.
The control unit is the core of the entire AC servo system, realizing system position control, speed control, torque and current control.
Performance comparison between servo motors and stepper motors
Control precision: The more phases and steps a stepper motor has, the higher its precision. Servo motors use their own encoders, and the more scales the encoder has, the higher its precision.
Low-frequency characteristics: Stepper motors are prone to low-frequency vibration at low speeds. When they are working at low speeds, damping or microstepping technology is generally used to overcome low-frequency vibration. Servo motors run very smoothly and do not vibrate even at low speeds.
Torque-frequency characteristics: The output torque of a stepper motor decreases as the speed increases, and drops sharply at high speeds. A servo motor provides constant torque output within its rated speed range and constant power output at its rated speed.
Overload capacity: Stepper motors do not have overload capacity, while servo motors have strong overload capacity.
Operating performance: Stepper motors are controlled in an open-loop manner. If the starting frequency is too high or the load is too large, step loss or stalling may occur. If the speed is too high when stopping, overshoot may occur. AC servo drive systems are controlled in a closed-loop manner. The driver can directly sample the feedback signal from the motor encoder and internally form a position loop and a speed loop. Generally, step loss or overshoot will not occur in stepper motors, and the control performance is more reliable.
Speed response performance: Stepper motors take hundreds of milliseconds to accelerate from a standstill to operating speed, while AC servo systems have better acceleration performance, generally requiring only a few milliseconds, and can be used in control applications that require rapid start and stop.
