A stepper motor is a brushless DC motor that rotates in discrete angular steps, converting digital pulses into mechanical movements. These motors offer precise control without needing a position sensor for feedback. Instead of continuous rotation like traditional motors, a stepper motor’s shaft turns in fixed increments in response to a series of input pulses.
Basic Working Principle
Stepper motors work on the principle of electromagnetism. A stepper motor system needs these key components:
- Indexer (Controller): Generates step pulses and direction signals for the driver.
- Driver : Converts the indexer’s command signals into the power needed to energize the motor windings.
Holry Stepper Driver - Stepper Motor: Converts digital pulses into mechanical shaft rotation.
The motor’s operation relies on sequentially energizing the stator windings, which creates a rotating magnetic field. The rotor, which can be a permanent magnet or a toothed iron core, rotates in discrete steps as it aligns with the changing magnetic field. The sequence of energizing the stator windings is controlled by an external driver and controller. By adjusting the pulse frequency and sequence, the motor’s speed and direction can be controlled.
Detailed Mechanism
1. Electromagnets and Rotor: Stepper motors have multiple “toothed” electromagnets arranged around a central rotor. The rotor is a gear-shaped piece of iron or a permanent magnet. These electromagnets are divided into groups called phases.
2. Energizing the Electromagnets: To make the motor shaft turn, one electromagnet is energized, which magnetically attracts the rotor’s teeth. When the teeth align with the first electromagnet, they are slightly offset from the next electromagnet.
3. Sequential Rotation: As the next electromagnet is turned on and the first is turned off, the gear rotates slightly to align with the next one. This process is repeated, with each partial rotation called a “step”.
4. Phases: The circular arrangement of electromagnets is divided into groups, each group called a phase. Electromagnets within the same group are all energized together. Stepper motors with more phases typically have more wires to control the motor.
Driving Modes
There are several different ways of driving the stepper motor:
Wave Drive (Single-Coil Excitation): Only one coil is active at a time.
Full Step Drive Mode: Two coils are always active, providing higher torque output.
Half Step Drive Mode: This mode is a combination of the previous two modes. One active coil is followed by two active coils, increasing the resolution.
Microstepping: Variable controlled current is provided to the coils in the form of a sine wave, providing smooth motion, decreasing stress, and increasing accuracy.
Constant Current Drive System
The constant current drive is the most commonly used drive method because it offers excellent torque performance at high speeds. The stepper motor rotates through the sequential switching of current flowing through the windings. Chopping a DC voltage that is far higher than the motor’s rated voltage ensures the rated current reaches the motor, even at higher speeds.
Advantages of Stepper Motors
- Low cost for control achieved
- High torque at startup and low speeds
- Ruggedness
- Simplicity of construction
- Can operate in an open-loop control system
- Low maintenance and high reliability
- Precise positioning and repeatability of movement
- Excellent response to starting, stopping, and reversing
- Full torque at standstill (if the windings are energized)
- The rotation angle of the motor is proportional to the input pulse.
Disadvantages of Stepper Motors
- Resonance effect often exhibited at low speeds
- Decreasing torque with increasing speed