A stepper motor , also known as a pulse motor, is based on the fundamental principles of electromagnetism. It is a freely rotating electromagnet that operates by generating electromagnetic torque through changes in the air gap's magnetic permeability. Its original model dates back to the 1950s and 1960s. Around 1960, attempts were made for control purposes, applying it to the electrode delivery mechanism of hydrogen arc lamps. This is considered the earliest stepper motor. In the early 20th century, stepper motors were widely used in automatic telephone exchanges. Due to the Western capitalist powers' struggle for colonies, stepper motors were widely used in independent systems such as ships and aircraft lacking AC power. The invention of the transistor in the late 1950s was gradually applied to stepper motors, making digital control easier. By the 1980s, the emergence of inexpensive, multifunctional microcomputers made stepper motor control methods even more flexible and diverse.
Stepper Motor Holding Torque and Positioning Torque Concepts Explained
The biggest difference between a stepper motor and other motors used for control applications is that it receives digital control signal pulses and converts them into corresponding angular or linear displacement. It is itself an actuator that performs digital mode conversion. Furthermore, it allows for open-loop position control; inputting a single pulse signal yields a specified position increment. This incremental position control system significantly reduces costs compared to traditional DC control systems, requiring almost no system adjustments. The angular displacement of a stepper motor is strictly proportional to the number of input pulses and synchronized with them in time. Therefore, by controlling the number, frequency, and phase sequence of the pulses and the motor windings, the desired angle, speed, and direction can be obtained.
my country's stepper motor industry began in the early 1970s, and the development of finished products took place from the mid-1970s to the mid-1980s, with the continuous development of new varieties and high-performance motors. Currently, with the development of science and technology, especially permanent magnet materials, semiconductor technology and computer technology, stepper motors have been widely used in many fields.
Basic principle of stepper motor
As a special type of motor used for control, stepper motors cannot be directly connected to DC or AC power supplies and must be driven by dedicated stepper motor drivers. Before the development of microelectronics technology, especially computer technology, the controller pulse signal generator was entirely implemented in hardware. The control system used individual components or integrated circuits to form the control loop. This not only made debugging and installation complex and consumed a large number of components, but also meant that once the design was finalized, any changes to the control scheme required a complete circuit redesign. This necessitated the development of different drivers for different motors, resulting in high development difficulty and cost, and significant control challenges, thus limiting the widespread adoption of stepper motors.
Because a stepper motor is a device that converts electrical pulses into discrete mechanical motion, it possesses excellent data control characteristics. Therefore, computers have become an ideal driver for stepper motors. With the development of microelectronics and computer technology, a hardware-software integrated control method has become mainstream, where control pulses are generated by a program to drive the hardware circuit. Microcontrollers control stepper motors through software, better unlocking the motor's potential. Therefore, using microcontrollers to control stepper motors has become an inevitable trend and aligns with the digital age.
Holding torque and positioning torque of a stepper motor
Holding torque refers to the maximum torque that a motor can output when all phase windings are carrying rated current and are in a statically locked state. It is one of the most important parameters when selecting a motor.
Positioning torque refers to the torque generated when the windings of a hybrid stepper motor are not energized and are in an open circuit state, due to the magnetic field produced by the permanent magnet material on the rotor. Generally, positioning torque is much smaller than holding torque. The presence or absence of positioning torque is an important characteristic distinguishing hybrid stepper motors from reactive stepper motors.
Stepper motors, as actuators, are one of the key products in mechatronics and are widely used in various automated control systems. With the development of microelectronics and computer technology, the demand for stepper motors is increasing daily, and they are used in various sectors of the national economy.
A stepper motor 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 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. Stepper motors can be used as special motors for control applications, and due to their characteristic of having no accumulated error (100% accuracy), they are widely used in various open-loop control systems.
Commonly used stepper motors include reactive stepper motors (VR), permanent magnet stepper motors (PM), hybrid stepper motors (HB), and single-phase stepper motors.
Permanent magnet stepper motors are generally two-phase, with smaller torque and size, and a step angle of typically 7.5 degrees or 15 degrees. Reluctance stepper motors are generally three-phase, capable of high torque output, with a step angle of typically 1.5 degrees, but they generate significantly more noise and vibration. The rotor windings of a reluctance stepper motor are made of soft magnetic material, and the stator has multi-phase excitation windings, utilizing changes in magnetic permeability to generate torque.
Hybrid stepper motors combine the advantages of permanent magnet and reactive stepper motors. They are further divided into two-phase and five-phase types: two-phase stepper motors typically have a step angle of 1.8 degrees, while five-phase stepper motors typically have a step angle of 0.72 degrees. This type of stepper motor is the most widely used and is the one selected for this stepper motor microstepping drive solution. Some basic parameters of stepper motors are as follows:
Motor's inherent step angle:
It represents the angle the motor rotates for each step pulse signal sent by the control system. The motor is given a step angle value at the factory, such as 0.9°/1.8° for the 86BYG250A model (meaning 0.9° for half-step operation and 1.8° for full-step operation). This step angle can be called the 'motor's inherent step angle,' but it is not necessarily the actual step angle during motor operation. The actual step angle depends on the driver. Number of phases in a stepper motor:
This refers to the number of coil groups inside the stepper motor. Commonly used stepper motors include two-phase, three-phase, four-phase, and five-phase motors. Different numbers of phases result in different step angles. Generally, two-phase motors have step angles of 0.9°/1.8°, three-phase motors 0.75°/1.5°, and five-phase motors 0.36°/0.72°. Without microstepping drivers, users primarily rely on selecting stepper motors with different numbers of phases to meet their step angle requirements. However, with microstepping drivers, the 'number of phases' becomes meaningless; users can simply change the microstepping setting on the driver to change the step angle.
Holding torque:
Holding torque refers to the torque with which the stator holds the rotor in place when the stepper motor is energized but not rotating. It is one of the most important parameters of a stepper motor, and typically, the holding torque at low speeds is close to the holding torque. Because the output torque of a stepper motor decreases with increasing speed, and the output power also varies with speed, holding torque becomes one of the most crucial parameters for evaluating a stepper motor. For example, when people say a 2 N·m stepper motor, unless otherwise specified, they mean a stepper motor with a holding torque of 2 N·m.
Determinator Queue:
This refers to the torque with which the stator locks the rotor in a stepper motor when it is not powered on. The term "DETENTTORQUE" lacks a standardized translation in China, which can lead to misunderstandings. Because the rotor of a reactive stepper motor is not made of permanent magnet material, it does not have a DETENTTORQUE. Some characteristics of stepper motors:
1. The accuracy of a typical stepper motor is 3-5% of the step angle, and this accuracy does not accumulate. 2. The maximum permissible temperature for the surface of a stepper motor.
Overheating of a stepper motor will first cause the magnetic material of the motor to demagnetize, resulting in a decrease in torque or even loss of steps. Therefore, the maximum allowable temperature of the motor surface depends on the demagnetization point of the magnetic material of different motors. Generally speaking, the demagnetization point of magnetic materials is above 130 degrees Celsius, and some are even as high as 200 degrees Celsius or more. Therefore, a stepper motor surface temperature of 80-90 degrees Celsius is perfectly normal.
3. The torque of a stepper motor decreases as the rotational speed increases.
When a stepper motor rotates, the inductance of each phase winding generates a back electromotive force (EMF); the higher the frequency, the greater the back EMF. Under its influence, the phase current of the motor decreases as the frequency (or speed) increases, resulting in a decrease in torque.
4. Stepper motors can operate normally at low speeds, but they cannot start above a certain speed and are accompanied by a whistling sound. A key technical parameter for stepper motors is the no-load starting frequency, which is the pulse frequency at which the stepper motor can start normally under no-load conditions. If the pulse frequency is higher than this value, the motor cannot start normally and may experience missed steps or stalling. Under load, the starting frequency should be even lower. To achieve high-speed rotation, the pulse frequency should have an acceleration process; that is, the starting frequency is low, and then it accelerates to the desired high frequency (motor speed increases from low to high). Stepper motors, with their significant characteristics, play a crucial role in the digital manufacturing era. With the development of various digital technologies and the improvement of stepper motor technology itself, stepper motors will be applied in even more fields.
