I. Working Principle of Stepper Motor
First, a stepper motor generates rotation through the interaction of magnetic fields. It consists of two main parts: the stator and the rotor. The stator is composed of coils fixed to the outside of the motor, usually called phases. The rotor, the rotating part of the motor, contains permanent magnets or an iron core, usually called poles. When current flows through the coils of the stator, a magnetic field is generated. This magnetic field interacts with the poles on the rotor, producing a torque that causes the rotor to rotate. The angle of this rotation is determined by the way the current is applied and the way the interaction occurs.
Secondly, stepper motors are controlled by applying current. Stepper motors typically move in fixed steps, meaning that each application of current causes the rotor to rotate by a fixed angle. This step distance is usually determined by the motor's structure and the windings of the coils. Generally, stepper motors can be divided into single-phase and multi-phase types. A single-phase stepper motor requires only one coil to generate the magnetic field, so the rotor rotates only a fixed angle each time current is applied. A multi-phase stepper motor, on the other hand, has multiple coils, and the rotor's rotation is controlled by applying current sequentially. Multi-phase stepper motors typically have higher torque and precision.
Although stepper motors are widely used, they cannot be used like ordinary DC or AC motors under normal conditions. They require a control system consisting of dual-ring pulse signals and power drive circuits to function. Therefore, using stepper motors effectively is not easy; it involves a great deal of expertise in mechanics, electrical engineering, electronics, and computers.
Currently, there are indeed many manufacturers producing stepper motors, but very few possess the professional technical personnel capable of independent development and research. Most manufacturers have only one or two dozen employees and lack even the most basic equipment, merely engaging in blind imitation. This causes numerous problems for users in product selection and use. In view of the above, we have decided to take the widely used inductor-type stepper motor as an example to describe its basic working principle, hoping to provide assistance to users in selection, use, and overall machine improvement.
II. What are the differences between stepper motors and servo motors?
1. Stepper motors and servo motors have different control precision. The step angle of a two-phase hybrid stepper motor is generally 1.8°, while that of a three-phase hybrid stepper motor is 1.2°. Some high-performance stepper motors also have even smaller step angles.
The control accuracy of an AC servo motor is ensured by a rotary encoder at the rear end of the motor shaft. For a servo motor with a standard 2500-line encoder, the pulse equivalent is 360°/10000=0.036° due to the use of quadruple frequency technology inside the driver.
For the vast majority of users, neither mechanical transmission precision nor photoelectric sensor positioning precision can match the physical precision of stepper motors and servo motors. It is unnecessary to unilaterally pursue the highest precision of motors.
2. Stepper motors and servo motors have different torque-frequency characteristics.
The output torque of a stepper motor decreases as the speed increases, and drops sharply at higher speeds. Therefore, its maximum operating speed is generally between 0 and 900 RPM. AC servo motors, on the other hand, provide constant torque output, meaning they can output rated torque within their rated speed range (generally 1000 to 3000 RPM).
3. Stepper motors and servo motors have different overload capacities.
4. Stepper motors and servo motors have different operating performance. Stepper motors are controlled by open-loop control. 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. Therefore, to ensure its control accuracy, the acceleration and deceleration issues should be properly handled.
5. AC servo drive systems are closed-loop control systems. The driver can directly sample the feedback signal from the motor encoder, forming internal position and speed loops. Generally, the stepping motor's skipped steps or overshoot phenomenon is avoided, resulting in more reliable control performance. Servo motors are closed-loop systems; the servo driver can automatically correct lost pulses and provide timely feedback to the controller in case of stall. Stepper motors, on the other hand, are open-loop systems, requiring sufficient torque margin to prevent stalling.
6. Stepper motors and servo motors have different speed response performance.
7. Stepper motors require 100–2000 milliseconds to accelerate from a standstill to their operating speed. AC servo systems have better acceleration performance, accelerating from a standstill to their rated speed of 3000 RPM in as little as a few milliseconds, making them suitable for control applications requiring rapid start and stop.
