This article introduces and compares the features of stepper motors, closed-loop stepper motors, and AC servo products to help engineers choose the most suitable product for automation equipment applications.
1. Comparison of stepper, closed-loop, and AC servo products
Table 1: Performance Comparison of Stepper, Closed-Loop, and Servo Products
Figure 1: Schematic diagram of closed-loop stepping and servo position tracking errors
Table 2: Examples of Stepper Motor, Closed-Loop Stepper Motor, and Servo Motor Selection
2. Stepper motor
A stepper motor is an actuator that converts digital pulse signals into angular displacement. When a stepper driver receives a pulse signal, it drives the stepper motor to rotate a fixed angle, or step angle, in a set direction.
Figure 2: Leadshine's latest CM series stepper motor
2.1 Stepper Motor Parameter Introduction
Holding torque: The torque by which the stator locks the rotor when the stepper motor windings are carrying rated current but not rotating. Generally, when the speed is less than 1 r/s, the output torque of the stepper motor is approximately equal to the holding torque.
Torque-frequency curve: A curve that describes the relationship between motor torque and speed.
Figure 3: Torque comparison of Leisai 57-frame open-loop and closed-loop stepper motors with the same torque.
Figure 4: Torque Comparison of Open-Loop and Closed-Loop Stepper Motors with the Same Torque on an 86-Frame Machine
Note: Stepper motor voltage balance equation: U = E + IR, where U is the supply voltage, E is the motor back electromotive force, I is the winding current, and R is the winding resistance. The higher the motor speed, the greater the back electromotive force, the smaller the current flowing into the motor windings, resulting in a smaller motor torque.
Rotor inertia: The rotational inertia of the stepper motor rotor. The maximum load inertia should not exceed 5 times the rotor inertia of the motor.
Step angle: The angle through which the stepper motor rotates after one pulse signal. Generally, the step angle of a two-phase hybrid stepper motor is 1.8°, the step angle of a three-phase hybrid stepper motor is 1.2°, and the step angle of a five-phase hybrid stepper motor is 0.72°.
2.2 Stepper Motor Structure
Figure 5: Internal structure of a stepper motor
2.3 Stepper Motor Wiring Method
Figure 6: Stepper Motor Wiring Method
a) Four-wire motor: Set the output current to be equal to or slightly less than the motor's rated current;
b) High torque mode for six-wire motors: The output current is set to 50% of the rated current for unipolar connection of the motor;
c) High-speed mode for six-wire motor: The output current is set to 100% of the rated current of the motor in unipolar connection.
d) Eight-wire motor parallel connection method: The output current can be set to 140% of the rated current of the motor in unipolar connection;
e) Eight-wire motor series connection: The output current can be set to 70% of the rated current of the motor in unipolar connection.
2.4 Characteristics of Stepper Motors
The stepper motor has high torque at low speeds, but the torque decreases as the speed increases, generally accelerating above 800 rpm. Its accuracy is 3%~5% of the step angle, with no cumulative error over the entire revolution. The accuracy of a two-phase hybrid stepper motor is 0.18°. Stepper motors use open-loop control, resulting in fast system response, no overshoot or settling time, and no micro-vibration of the motor shaft when stopped. Stepper motors exhibit low-frequency resonance, with the first resonance point at approximately 1 rpm. Stepper motors use constant current control, resulting in significant heat generation and noise. They also lack overload capacity; insufficient motor force will cause stalling. Therefore, a safety factor of 1.4-2 times should be considered when selecting a stepper motor. Stepper motors and drivers are easy to use and require no complex debugging.
3. Closed-loop stepping
The main body is a stepper motor, with the addition of a position feedback device (photoelectric encoder or magnetic encoder), forming a closed-loop control system using a control method similar to that of a servo motor.
Figure 7: 3D and physical images of Leadshine's new generation CL series closed-loop stepper motor
3.1 Key parameters of closed-loop stepping
Encoder accuracy: The number of pulses fed back to the driver from the encoder per revolution of the motor affects the closed-loop stepper motor accuracy. Common encoder counts for closed-loop stepper motors are 1000 lines, 2500 lines, and 5000 lines. Taking a 5000-line encoder as an example, its resolution is 360°/(4*5000) = 0.018°, which is higher than the accuracy of an open-loop controlled stepper motor.
Torque-frequency curve: A curve that describes the relationship between motor torque and speed.
Position error: The difference between the commanded position and the encoder feedback position. If the position error is too large, the driver will report an out-of-tolerance alarm.
3.2 Block Diagram of Closed-Loop Stepping Principle
Figure 8: Block diagram of closed-loop stepping principle
3.3 Characteristics of Closed-Loop Stepping
Closed-loop stepper motors automatically adjust the winding current according to the load size, resulting in less heat generation and vibration compared to open-loop stepper motors. They have encoder feedback, leading to higher accuracy than ordinary stepper motors. However, their motor response is slower than open-loop stepper motors, and positional errors occur during operation, which gradually decrease within milliseconds after the command stops. They offer higher high-speed torque than open-loop stepper motors and are commonly used in 0-1500rpm applications. When interpolation is used, insufficient mechanical rigidity (belt structure) and a large load inertia can lead to misalignment due to significant position tracking errors. A small number of closed-loop stepper motors require simple adjustments before use.
4. AC servo
A servo system is an automatic control system that enables the output controlled variables, such as position, orientation, and state, of an object to follow any changes in the input target (or given value). In position mode, the servo motor uses pulses for positioning; when the servo motor receives one pulse, it rotates by the angle corresponding to that pulse. Simultaneously, the servo motor encoder has a feedback function; for each angle the servo motor rotates, the encoder emits a corresponding number of feedback pulses. These feedback pulses and the pulses received by the servo driver form a closed-loop control, allowing the servo driver to precisely control the motor's rotation, thereby achieving accurate positioning.
Figure 9: Actual product image of Leadshine L5 series AC servo products
4.1 Important Parameters of Servo Motors
Rated speed: The speed at which the motor operates at its maximum continuous output torque (rated torque) and rated power.
Rated torque: refers to the amount of torque that the motor can continuously and safely output. When the ambient temperature is 25 °C, the motor winding temperature and the driver power device temperature will not exceed the maximum allowable temperature during continuous operation at this torque, and the motor or driver will not be damaged.
Maximum torque: The maximum torque that the motor can output. Short-term operation at maximum torque will not cause damage to the motor or irreversible performance loss.
Maximum current: The maximum current that the servo can pass through for short-term operation, which is generally 3 times the rated current.
Maximum speed: The highest speed at which the motor can operate for a short period of time. At the maximum speed, the motor torque decreases and the motor generates more heat.
Rotor inertia J: The unit of rotational inertia of the servo motor rotor is kgcm^2. Generally, the maximum load inertia should not exceed 20 times the motor rotor inertia.
Encoder line count: The number of pulses the encoder sends back to the driver per revolution of the motor, affecting the closed-loop stepping accuracy. Common servo encoders have 2500 lines, 5000 lines, 17-bit, and 23-bit counts. A 17-bit encoder has an accuracy of 0.0027°, higher than conventional stepping and closed-loop stepping.
Two important formulas for servo motors: T=Kt*I, P=n (unit converted to radians/second)*T.
4.2 Servo Motor Structure
Figure 10: Internal structure of the servo motor
4.3 Servo Three-Loop Control Principle Block Diagram
Figure 11: Block diagram of servo three-loop control principle
4.4 Characteristics of Servo Motors
AC servos are characterized by constant torque at rated speed. Common 200W and 400W low-to-medium inertia AC servos have a rated speed of 3000rpm and a maximum speed of 5000rpm, offering high speed performance. Torque is proportional to current, allowing operation in torque mode, such as in applications requiring constant torque, like screw tightening or terminal crimping. AC servos operate with minimal noise and vibration, and generate little heat. For the same size, their rotor inertia is small; a 400W servo's inertia is equivalent to that of a 2NM stepper motor with a 57mm base. Servos possess short-term overload capability; the motor overload factor during acceleration and deceleration must be considered during selection. Servos use closed-loop control, and like closed-loop stepper motors, they exhibit position tracking errors. Leadsai servos incorporate trajectory tracking and resonance suppression functions to improve servo rigidity, thereby reducing position errors and positioning time for better interpolation performance. Servos require debugging before use.
5. Comparison and summary of three motor drive solutions:
1) Stepper motors are suitable for low-speed, short-distance, small-angle, rapid start-stop, and low-mechanical-connection rigidity applications, as well as applications with low requirements for vibration, noise, heat generation, and precision. Leadshine stepper motors are easy to debug and economical. They offer advantages over closed-loop and servo motors for rapid interpolation motion due to the lack of positioning time and smaller positional error. When selecting a stepper motor, ensure the torque is 1.4-2 times the theoretically calculated torque, and the load inertia is less than 5 times the rotor inertia.
2) Closed-loop steppers are suitable for medium-speed applications where ordinary steppers cannot reach, for medium-to-long stroke point-to-point movements, and have certain requirements regarding noise, vibration, heat generation, and positioning time. They offer high accuracy, include alarm output to prevent damage to mechanical equipment and waste of valuable raw materials, and are generally easy to operate without debugging and are reasonably priced. When selecting a stepper, no safety factor needs to be reserved for torque, and the load inertia should be less than 5 times the rotor inertia. Rapid interpolation movements require strong mechanical connection rigidity (lead screw or rack and pinion structure) and a low load inertia ratio (less than 3 times the rotor inertia).
3) Servo drives are suitable for high-speed, medium-to-long-stroke, high-precision, low-noise, low-vibration, and low-heat applications. Leadsai servo drives feature multiple custom I/O ports and frequency division outputs, offering comprehensive functionality and strong expandability. However, debugging is relatively complex, and the price is higher. When selecting a servo drive, a safety factor is not required for torque calculations, and the load inertia should be less than 30 times the rotor inertia. Rapid interpolation motions require strong mechanical connection rigidity (lead screw or rack and pinion structure) and a low load inertia ratio (less than 3 times the rotor inertia).
Lei Sai Introduction
Leadsun is a world-renowned brand and industry leader in the field of motion control for intelligent equipment.
Since its establishment in 1997, Leadshine has been committed to "replacing human manual labor" as its corporate mission, focusing on the research, development, production, sales and service of a series of high-quality products such as servo motor drive systems, stepper motor drive systems, motion control cards and motion controllers. Through persistent and meticulous efforts, Leadshine strives to help customers achieve their dreams and realize common growth.
Through two decades of unwavering commitment to product innovation, market expansion, and application services, Leadshine has become a leading global provider of motion control products and solutions in terms of production and sales volume. Due to the dual advantages of stability, reliability, and superior performance, Leadshine products are used by tens of thousands of excellent equipment manufacturers in industries such as electronics, robotics, machine tools, lasers, medical, and textiles, and are exported to more than 60 countries including the United States, Germany, and India.
