Basic principle of stepper motor
Working principle:
Typically, the rotor of a stepper motor is a permanent magnet. When current flows through the stator windings, the stator windings generate a vector magnetic field. This magnetic field causes the rotor to rotate by an angle, aligning the direction of the rotor's magnetic field with that of the stator's magnetic field. When the stator's vector magnetic field rotates by an angle, the rotor also rotates by the same angle. Each input electrical pulse causes the motor to rotate one angle and move one step forward. Its output angular displacement is proportional to the number of input pulses, and its rotational speed is proportional to the pulse frequency. Changing the energizing sequence of the windings reverses the motor's rotation. Therefore, the rotation of a stepper motor can be controlled by adjusting the number and frequency of pulses and the energizing sequence of each phase winding.
Heating principle:
Most motors contain an iron core and winding coils. The windings have resistance, and when current flows through them, losses occur. The magnitude of these losses is proportional to the resistance and the square of the current; this is what we commonly call copper loss. If the current is not standard DC or a sine wave, harmonic losses will also occur. The iron core has hysteresis and eddy current effects, which also generate losses in an alternating magnetic field. The magnitude of these losses depends on the material, current, frequency, and voltage; this is called iron loss. Both copper and iron losses manifest as heat, thus affecting the motor's efficiency. Stepper motors generally prioritize positioning accuracy and torque output, resulting in relatively low efficiency, generally higher current, and higher harmonic content. The frequency of the alternating current also varies with the rotational speed. Therefore, stepper motors commonly experience heat generation, and this is more severe than with general AC motors.
Applications of stepper motors
A stepper motor is a special type of motor used for control. As an actuator, it is one of the key products in mechatronics. With the development of microelectronics and computer technology (leading to improved performance of stepper motor drivers), the demand for stepper motors is increasing daily. The characteristic of stepper motors—that they do not accumulate errors during operation—makes them widely used in various automated control systems, especially open-loop control systems.
Stepper motor classification
1. Classification of stepper motors by structure
Stepper motors, also known as pulse motors, include reactive stepper motors (VR), permanent magnet stepper motors (PM), and hybrid stepper motors (HB).
(1) Reactive stepper motor:
Also known as induction, hysteresis, or reluctance stepper motors. Both the stator and rotor are made of soft magnetic materials. The stator has evenly distributed large magnetic poles with multi-phase excitation windings. Small teeth and slots are evenly distributed around the stator and rotor. Torque is generated by changes in magnetic permeability when energized. They are typically three, four, five, or six-phase; capable of high torque output (high power consumption, current up to 20A, high drive voltage); small step angle (as small as one-sixth of a degree); no positioning torque when power is off; low internal damping; long oscillation time during single-step operation (referring to very low pulse frequency); and high starting and running frequencies.
(2) Permanent magnet stepper motor:
Typically, the rotor of a motor is made of permanent magnet material, while the stator, made of soft magnetic material, has multi-phase excitation windings. Neither the stator nor the rotor has teeth or slots around their perimeters. When energized, torque is generated by the interaction between the permanent magnets and the stator current's magnetic field. It is generally two-phase or four-phase; output torque is low (low power consumption, current typically less than 2A, drive voltage 12V); step angle is large (e.g., 7.5 degrees, 15 degrees, 22.5 degrees, etc.); it has a certain holding torque when power is off; and its starting and running frequencies are low.
(3) Hybrid stepper motor:
Also known as 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 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). The rotor structure is more complex (the rotor contains cylindrical permanent magnets, with soft magnetic material at both ends, and small teeth and slots around the perimeter). It is generally two-phase or four-phase; requires positive and negative pulse signals; has a larger output torque than permanent magnet stepper motors (with relatively lower power consumption); a smaller step angle than permanent magnet stepper motors (generally 1.8 degrees); has no positioning torque when power is off; and has a higher starting and running frequency. It is currently one of the fastest-growing types of stepper motors.
2. Stepper motors are classified according to their working method.
(1) Power type: The output torque is relatively large and can directly drive a large load (generally using reactive or hybrid stepper motors).
(2) Servo type: The output torque is relatively small and can only drive a small load (generally using permanent magnet or hybrid stepper motors).
Six stepper motor driver chips
1.L6258EX
Produced by STMicroelectronics (ST), the chips are relatively old, with a maximum of 16 sub-segments.
Control interface: Parallel port.
Advantages: High drive current.
Disadvantages: The chip is relatively old and may be discontinued; the circuit is slightly complex; the program control is slightly complex.
2.A4988
Manufactured by ALLEGRO, a company quite well-known for its stepper motor driver ICs, primarily in small packages such as QFN/LQFP; suitable for small instruments with low power requirements. The A4988's official datasheet states a drive current of up to 2A, but in actual use with a 24V power supply, even 1A is very hot; reaching 2A would likely require a very good cooling system. The A4988 supports 2, 4, 8, and 16 microstepping.
Control interface: step/direction
Advantages: Simple circuit; easy to control; inexpensive.
Disadvantages: During testing, the chip would inexplicably burn out, and the problem was never found. It was not used in subsequent products. The drive current is low, making it suitable for stepper motors with a current of 42 ohms or less.
3.DRV8825
Made by Texas Instruments (TI), a dominant player in analog devices, their motor series (stepper motors, brushed motors, brushless motors, etc.) chips are quite good. The DRV8825's overall performance is similar to the A4988, supporting microstepping of 2, 4, 8, 16, and 32 microsteps. However, possibly due to wiring or program control issues, it is extremely noisy, and was not used in subsequent products.
Control interface: step/direction
Advantages: Simple circuit and easy program control.
Disadvantages: Low drive current and relatively high noise (possibly related to PCB layout).
4. TMC246/TMC249
TRINAMIC, a German company, specializes in motor driver chips; German products are truly excellent. The 246 has an internal MOSFET, while the 249 has an external MOSFET. The control programs for both are fully compatible, only the circuitry differs significantly. It's worth considering for applications with high requirements, but it's relatively expensive, among the highest-priced in its class. Of course, its performance is also generally top-notch. As for how well it works, you'll have to try it to find out!
Control interface: SPI
Advantages: Simple circuit; simple program control; very high efficiency; 246 built-in MOSFET with low heat generation; 249 external MOSFET can drive high current motors; low noise and smooth motion.
Disadvantages: High price; high wiring requirements, hardware engineers design 4-layer boards;
5. TMC260/TMC2660/TMC262
This product is manufactured by TRINAMIC. Their new series features improved performance in various aspects, such as higher drive current and higher microstepping, supporting up to 256 microsteps (although such high microstepping is not very useful in ordinary applications). If using the TMC series, the TMC2660 is recommended as it is relatively inexpensive; for higher power applications, the TMC262 is recommended, as it uses an external MOSFET and is fully compatible with the control program.
Control interfaces: Step/Direction and SPI. You can use only SPI, and related configuration parameters can only be configured via SPI. My method is to use Step/Direction to control the motor and the SPI bus for parameter configuration.
Advantages: High drive current, smooth motion (but it doesn't feel as good as the TMC246 and 249, maybe it's a wiring issue?), and the price is actually cheaper than the 246 and 249.
Disadvantages: Expensive, wiring is slightly troublesome.
6.L6470
This is a product from STMicroelectronics (ST). I haven't used it myself, but a colleague is testing it. From the datasheet, it seems quite powerful, with high current (peak current 7A), built-in speed curve, and hardware origin, etc. Those interested can test it.
