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 in a set direction; this fixed angle is called the "step angle." Stepper motors cannot be directly connected to DC or AC power; a dedicated driver is required. The principle of a stepper driver is to use a unipolar DC power supply. By energizing the windings of the stepper motor in the appropriate timing sequence, the stepper motor can be made to rotate in steps. Stepper motor drivers generally require pulse signals and direction signals, while an enable signal is usually not required.
Microstepping control of a stepper motor driver allows for the adjustment of the motor's speed and acceleration, achieving more precise control. Furthermore, the pulse and direction connections will differ depending on the controller's output type. When selecting a stepper motor driver, factors such as motor type, voltage, current, and microstepping parameters must be considered. Additionally, the relationship between the driver's power supply and the manufacturer's design must be taken into account, as well as the method for modifying the direction internally within the program.
Types of stepper motor drivers and their underlying principles:
Bipolar drive: Bipolar drive is a popular driving method suitable for two-phase bipolar motors with four wires. The stepper motor operates by alternately reversing the current in each phase.
Unipolar drive: A unipolar drive requires a motor with a center tap in each phase (6 wires). Since it does not require twisting the current in each phase, the driver only needs to switch the current from one coil in each phase to another. This drive method simplifies the driver but utilizes only half the copper windings, resulting in a reduction of approximately 30% in torque or force.
L/R Driver: An L/R driver represents the electrical relationship between an inductor (L) and a resistor (R). The relationship between the motor coil impedance and the stepping rate is determined by these parameters. The power supply output voltage should be "matched" to the rated voltage of the motor coil for continuous operation. Most published motor performance curves are based on the full rated voltage applied to the motor leads.
Chopper Driver: A chopper driver is a constant current driver and is almost always bipolar. The name "chopper" comes from the technology of rapidly switching the output power on and off (chopping) to control motor current. For optimal performance, a power supply to motor rated voltage ratio of 8:1 is recommended.
Other step angles: To achieve smaller step angles, more poles are needed on both the rotor and stator. A 7.5° motor has 12 pole pairs on the rotor, with 12 teeth on each pole plate. Each coil has two pole plates, and each motor has two coils; therefore, a 7.5° step motor has 48 poles per step. Multiple steps can be combined to provide greater motion.
Miniature stepper drivers: Many bipolar drivers offer a feature called microstepping, which electronically breaks down a full step into smaller steps. Microstepping effectively reduces the step increment of the motor. However, the accuracy of each microstep has a larger percentage error compared to the accuracy of a full step. As with full stepping, the incremental error of microstepping is non-cumulative.
Full-step drive mode: In full-step drive mode, the stepper motor controller cyclically excites the two coils of the two-phase stepper motor according to the pulse direction command. Each pulse causes the motor to move by a basic step angle, i.e., 1.80 degrees. This drive method has the advantages of simple structure and low cost, but it has greater vibration at low speeds and relatively lower positioning accuracy.
Half-step drive mode: In half-step drive mode, the motor shaft stops at the full-step position during single-phase excitation. When the driver receives the next pulse, if the other phase is excited and maintained in the previous sequential excitation state, the motor shaft will move half a step angle and stop in the middle of the two adjacent full-step positions. This cycle of single-phase and then dual-phase excitation is repeated, and the stepper motor will rotate in half-steps of 0.90 degrees per pulse. Half-step drive mode offers the advantages of twice the accuracy and less vibration at low speeds.
Microstepping drive mode: In microstepping drive mode, the two coils of the motor are precisely current-controlled according to sinusoidal and cosine steps, respectively, so that the distance of one step angle is divided into several microsteps. This drive mode has the advantages of minimal low-speed vibration and high positioning accuracy, and is suitable for applications that require low-speed operation or high positioning accuracy.
When selecting a stepper motor driver, it is necessary to choose the appropriate drive mode based on the actual application requirements. For applications requiring high precision and low speed operation, a microstepping driver can be selected; for applications requiring simple structure and low cost, a full-stepping driver can be selected; and for applications requiring a balance between the two, a half-stepping driver can be selected.
