•Introduction
This article mainly introduces stepper motor drivers and servo motor drivers, and focuses on a detailed comparison and distinction between them.
Stepper motor driver
A stepper motor driver 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 and positioning.
Classification
Stepper motors can be classified by structure: Stepper motors, also called pulse motors, include reactive stepper motors (VR), permanent magnet stepper motors (PM), and hybrid stepper motors (HB), etc.
(1) Reluctance stepper motor: also called induction, hysteresis, or reluctance stepper motor. Its stator and rotor are both made of soft magnetic material. The stator has large magnetic poles evenly distributed on it with multi-phase excitation windings. Small teeth and slots are evenly distributed around the stator and rotor. When energized, it generates torque by utilizing the change in magnetic permeability. It is generally three, four, five, or six phases; it can achieve high torque output (consumes a large amount of power, with the current reaching up to 20A and the driving voltage being relatively high); the step angle is small (the smallest can be 10'); there is no positioning torque when the power is off; the motor has low internal damping, and the oscillation time of single-step operation (referring to very low pulse frequency) is relatively long; the starting and running frequencies are relatively high.
(2) Permanent magnet stepper motor: The rotor of the motor is usually made of permanent magnet material, and the stator is made of soft magnetic material with multi-phase excitation windings. There are no small teeth and slots around the stator and rotor. After being energized, the torque is generated by the interaction between the permanent magnet and the magnetic field of the stator current. It is generally two-phase or four-phase; the output torque is small (the power consumption is small, the current is generally less than 2A, and the driving voltage is 12V); the step angle is large (e.g., 7.5 degrees, 15 degrees, 22.5 degrees, etc.); it has a certain holding torque when the power is off; and the starting and running frequency is low.
(3) Hybrid stepper motor: also called permanent magnet reactive stepper motor or permanent magnet induction stepper motor, it combines the advantages of permanent magnet and reactive stepper motors. Its stator is no different from that of a four-phase reactive stepper motor (but the two magnetic poles of the same phase are opposite each other, and the N and S polarities generated by the windings on the two magnetic poles must be the same), and the rotor structure is more complex (the rotor has cylindrical permanent magnets inside, with soft magnetic material on both ends, and small teeth and slots around the periphery). It is generally two-phase or four-phase; it requires positive and negative pulse signals; the output torque is larger than that of the permanent magnet type (the power consumption is relatively smaller); the step angle is smaller than that of the permanent magnet type (generally 1.8 degrees); there is no positioning torque when the power is off; the starting and running frequency is relatively high; it is a type of stepper motor that is developing rapidly. [1]
System Control
Stepper motors cannot be directly connected to DC or AC power supplies; a dedicated drive power supply (stepper motor driver) must be used. The controller (pulse signal generator) can control the angular displacement by controlling the number of pulses, thereby achieving accurate positioning; at the same time, it can control the speed and acceleration of the motor by controlling the pulse frequency, thereby achieving speed regulation.
model
F3922, F3722L, F3722, F3722A, F3722M, F368, F3522A, F3522H, F3522, F2611, F268C,
Zhongke F223 Stepper Motor Driver
Zhongke F223 Stepper Motor Driver
F875, F556, F256B, F265, F255, F235B, F245, F223
F3522
1. F indicates a stepper driver.
2 indicates the number of phases; 2 represents two phases, and 3 represents three phases.
3 and 5 indicate a current of 5 amperes.
4.22 indicates a voltage of 220V.
Basic principles
The principle of a stepper motor driver is based on a unipolar DC power supply. By energizing each phase winding of the stepper motor in the appropriate timing sequence, the stepper motor can rotate in steps. Figure 1 is a schematic diagram of the working principle of this four-phase reactive stepper motor.
The schematic diagram of a four-phase stepper motor shows that at the start, switch SB is connected to the power supply, and SA, SC, and SD are disconnected. The B-phase magnetic pole aligns with the rotor's 0 and 3 teeth. Simultaneously, the rotor's 1 and 4 teeth misalign with the C and D phase winding magnetic poles, and the 2 and 5 teeth misalign with the D and A phase winding magnetic poles. When switch SC is connected to the power supply and SB, SA, and SD are disconnected, the rotor rotates due to the interaction between the magnetic lines of force of the C-phase winding and the magnetic lines of force of the 1 and 4 teeth. The 1 and 4 teeth align with the C-phase winding magnetic poles. Meanwhile, the 0 and 3 teeth misalign with the A and B phase windings, and the 2 and 5 teeth misalign with the A and D phase winding magnetic poles. This process continues, with the A, B, C, and D phase windings being powered alternately, causing the rotor to rotate in the A, B, C, and D directions.
Four-phase stepper motors can be classified into three operating modes based on their energizing sequence: single-four-step, double-four-step, and eight-step. The step angles of single-four-step and double-four-step motors are equal, but the torque of single-four-step motors is smaller. The step angle of the eight-step operating mode is half that of both single-four-step and double-four-step motors; therefore, the eight-step operating mode can maintain higher torque while improving control accuracy.
The difference between stepper motor drivers and servo motor drivers
The power-on sequence and waveforms for single four-phase, double four-phase, and eight-phase operation modes are shown in Figures 2.a, b, and c, respectively. The driver is equivalent to a combination unit of switches. The motor is powered sequentially by pulse signals from the host computer to make it rotate.
Servo motor driver
Servo drives, also known as servo controllers or servo amplifiers, are controllers used to control servo motors. Their function is similar to that of a frequency converter for a regular AC motor. They are part of a servo system and are primarily used in high-precision positioning systems. Generally, they control the servo motor through position, speed, and torque to achieve high-precision transmission system positioning. Currently, they represent a high-end product in transmission technology.
Working principle
Currently, most mainstream servo drives use digital signal processors (DSPs) as their control core.
It can implement relatively complex control algorithms, achieving digitalization, networking, and intelligence. Power devices generally employ drive circuits designed around intelligent power modules (IPMs). The IPM integrates the drive circuit and includes fault detection and protection circuits for overvoltage, overcurrent, overheating, and undervoltage. A soft-start circuit is also added to the main circuit to reduce the impact on the driver during startup. 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 frequency-converted by a three-phase sinusoidal PWM voltage-type inverter to drive a three-phase permanent magnet synchronous AC servo motor. The entire process of the power drive unit can be simply described as an AC-DC-AC process. The main topology of the rectifier unit (AC-DC) is a three-phase full-bridge uncontrolled rectifier circuit.
With the large-scale application of servo systems, the use, debugging, and repair of servo drives are important technical issues in servo drive technology today, and more and more industrial control technology service providers are conducting in-depth research on servo drives.
Servo drives are an important component of modern motion control and are widely used in automated equipment such as industrial robots and CNC machining centers. Servo drives for controlling AC permanent magnet synchronous motors, in particular, have become a research hotspot both domestically and internationally. Current AC servo drive designs commonly employ a three-loop control algorithm based on vector control, encompassing current, speed, and position. The rationality of the speed closed-loop design within this algorithm plays a crucial role in the overall performance of the servo control system, especially in terms of speed control performance.
Basic requirements
Requirements of servo feed system
1. Wide speed range
2. High positioning accuracy
3. It has sufficient transmission rigidity and high speed stability.
4. Fast response, no overshoot
In order to ensure productivity and processing quality, in addition to high positioning accuracy, good fast response characteristics are also required. That is, the response to tracking command signals must be fast, because the CNC system requires sufficient acceleration and deceleration when starting and braking to shorten the transition time of the feed system and reduce contour transition error.
5. High torque at low speeds, strong overload capacity
Generally speaking, servo drives have an overload capacity of more than 1.5 times for several minutes or even half an hour, and can be overloaded by 4 to 6 times in a short period of time without damage.
6. High reliability
The feed drive system of CNC machine tools is required to have high reliability, good working stability, strong adaptability to environmental conditions such as temperature, humidity, and vibration, and strong anti-interference ability.
Requirements for motors
1. The motor can run smoothly from the lowest speed to the highest speed with little torque fluctuation. Especially at low speeds such as 0.1 r/min or lower, it still maintains a stable speed without creeping.
2. The motor should have a large and long-term overload capacity to meet the requirements of low speed and high torque. Generally, DC servo motors are required to withstand overloads of 4 to 6 times the rated load for several minutes without damage.
3. In order to meet the requirements of fast response, the motor should have a small moment of inertia and a large stall torque, and have the smallest possible time constant and starting voltage.
4. The motor should be able to withstand frequent starting, braking and reversing.
The difference between stepper motor drivers and servo motor drivers
1. How to choose between stepper and servo motors?
It mainly depends on the specific application. Simply put, you need to determine: the nature of the load (e.g., horizontal or vertical load), torque, inertia, speed, accuracy, acceleration/deceleration requirements, higher-level control requirements (e.g., port interface and communication requirements), and the primary control method (position, torque, or speed control). Also, determine whether the power supply is DC, AC, or battery-powered, and the voltage range. Based on this, determine the model of the motor and the corresponding driver or controller.
2. Should I choose a stepper motor or a servo motor system?
In fact, the choice of motor should be based on the specific application, as each type has its own characteristics.
3. How to use a stepper motor driver?
Choose a driver with a current greater than or equal to that of the motor. For applications requiring low vibration or high precision, a microstepping driver can be used. For high-torque motors, use a high-voltage driver whenever possible to achieve good high-speed performance.
4. What are the differences between 2-phase and 5-phase stepper motors, and how do you choose between them?
Two-phase motors are inexpensive, but they vibrate more at low speeds and their torque drops rapidly at high speeds. Five-phase motors, on the other hand, vibrate less, have better high-speed performance, and are 30-50% faster than two-phase motors, making them a viable alternative to servo motors in some applications.
5. When should a DC servo system be selected, and what are the differences between it and an AC servo system?
DC servo motors are divided into brushed and brushless motors. Brushed motors are low-cost, simple in structure, have high starting torque, wide speed range, and are easy to control. They require maintenance, but maintenance is convenient (replacing carbon brushes). They generate electromagnetic interference and are subject to environmental requirements. Therefore, they are suitable for cost-sensitive general industrial and civilian applications. Brushless motors are small in size, light in weight, have high output, fast response, high speed, low inertia, smooth rotation, and stable torque. Control is complex but easily achieved through intelligent systems. Their electronic commutation is flexible, allowing for square wave or sine wave commutation. These motors are maintenance-free, highly efficient, operate at low temperatures, have minimal electromagnetic radiation, and have a long lifespan, making them suitable for various environments. AC servo motors are also brushless motors, divided into synchronous and asynchronous motors. Currently, synchronous motors are generally used in motion control due to their wide power range, allowing for very high power outputs. They have high inertia, a low maximum speed that decreases rapidly with increasing power, and are therefore suitable for low-speed, stable operation applications.
6. What precautions should be taken when using a motor?
Before powering on, perform the following checks: 1) Ensure the power supply voltage is appropriate (overvoltage may damage the drive module); the DC input polarity must be correct, and the motor model or current setting on the drive controller must be appropriate (do not set it too high initially); 2) Ensure the control signal lines are securely connected; in industrial environments, shielding should be considered (e.g., using twisted-pair cables); 3) Do not connect all necessary wires at the beginning; only connect the most basic system. After it runs smoothly, gradually connect the others. 4) Be sure to understand the grounding method, whether to use a floating ground or no grounding. 5) Closely observe the motor's status for the first half hour of operation, such as whether the movement is normal, the sound, and the temperature rise. If any problems are found, stop the machine immediately for adjustment.
7. When the stepper motor starts running, sometimes it moves briefly and then stops, or it moves back and forth in place. Sometimes it also loses steps during operation. What is the problem?
The following aspects should generally be considered during inspection: 1) Whether the motor torque is large enough to drive the load. Therefore, we generally recommend that users select a motor with a torque 50% to 100% greater than the actual requirement, because stepper motors cannot be overloaded, even momentarily, which will cause loss of steps, and in severe cases, stop or irregularly move repeatedly in place. 2) Whether the current of the input step pulse from the upper controller is large enough (generally >10mA) to ensure stable conduction of the optocoupler. Whether the input frequency is too high, resulting in no reception. If the output circuit of the upper controller is a CMOS circuit, a CMOS input type driver should also be selected. 3) Whether the starting frequency is too high. Whether an acceleration process is set in the starting program. It is best to accelerate from the motor's specified starting frequency to the set frequency, even if the acceleration time is very short, otherwise it may be unstable or even in an idle state. 4) This situation sometimes occurs when the motor is not properly fixed, which is normal. This is because it actually causes strong resonance in the motor, leading to loss of steps. The motor must be properly fixed. 5) For 5-phase motors, if the phases are connected incorrectly, the motor will not work.
A stepper motor is an open-loop control unit that converts electrical pulse signals into angular or linear displacement. Under non-overload conditions, the motor's stopping position depends only on the frequency and number of pulse signals, and is unaffected by load changes. When the stepper driver receives a pulse signal, it drives the stepper motor to rotate a fixed angle in a set direction, called the "step angle." Its rotation occurs step by step at fixed angles. The angular displacement can be controlled by controlling the number of pulses, thus achieving accurate positioning. Simultaneously, the motor's speed and acceleration can be controlled by controlling the pulse frequency, thus achieving high speeds.
A servo motor, also known as an actuator motor, is used as an actuating element in automatic control systems. It converts received electrical signals into angular displacement or angular velocity output on the motor shaft. The rotor inside a servo motor is a permanent magnet. The U/V/W three-phase electricity controlled by the driver creates an electromagnetic field, causing the rotor to rotate under the influence of this magnetic field. Simultaneously, the motor's built-in encoder feeds back signals to the driver. The driver compares the feedback value with the target value and adjusts the rotor's rotation angle accordingly. The accuracy of a servo motor depends on the accuracy (line count) of the encoder. In other words, a servo motor itself has the function of emitting pulses. For each rotation angle, it emits a corresponding number of pulses. Thus, the pulses from the servo driver and the servo motor encoder correspond, making it a closed-loop control system. Stepper motors, on the other hand, are open-loop control systems.
The differences between stepper motors and servo motors are as follows: 1. Control precision: The more phases and steps a stepper motor has, the higher its precision. Servo motors rely on their built-in encoders; the more graduations on the encoder, the higher the precision. 2. Control methods: One is open-loop control, and the other is closed-loop control. 3. Low-frequency characteristics: Stepper motors are prone to low-frequency vibration at low speeds. When operating at low speeds, damping or microstepping techniques are generally used to overcome this vibration. Servo motors operate very smoothly and do not vibrate even at low speeds. AC servo systems have resonance suppression capabilities, which can cover insufficient mechanical rigidity, and the system has a built-in frequency analysis function (FFT) to detect mechanical resonance points for easy system adjustment. 4. Torque-frequency characteristics:
5. Different overload capacities: Stepper motors generally lack overload capacity, while AC motors have strong overload capacity. 6. Different operating performance: Stepper motors use open-loop control, which can easily lead to step loss or stalling if the starting frequency is too high or the load is too large. They can also cause overshoot when stopping if the speed is too high. AC servo drive systems use closed-loop control, where the driver can directly sample the encoder feedback signal, forming internal position and speed loops. This generally avoids the step loss or overshoot issues common in stepper motors, resulting in more reliable control performance. 7. Different speed response performance: Stepper motors require hundreds of milliseconds to accelerate from a standstill to operating speed, while AC servo systems have better acceleration performance, typically requiring only a few milliseconds, making them suitable for applications requiring rapid start and stop.
In summary, AC servo systems outperform stepper motors in many aspects, but their price-performance ratio is different.
