Overview
When a stepper motor runs at low speed, the rotor tends to oscillate repeatedly around its equilibrium position during phase commutation due to speed impact and inertia. This oscillation degrades running accuracy and generates significant noise.
Two primary approaches address this issue:
- Increase the number of motor phases to reduce the rotor's angular displacement during each step transition.
- Apply microstepping drive control, a technique first proposed in the mid-1970s that has since become widely adopted. Microstepping effectively reduces the motor's step angle, improves operational stability, and increases control flexibility.
What Is Microstepping?
Microstepping is a drive method that divides the motor's inherent full step angle into a number of smaller intermediate steps. This is achieved entirely within the driver by precisely controlling the phase currents of the stepper motor; it is independent of the motor's inherent construction.
How Microstepping Works
In a conventional full-step drive, the current in an energized stator phase rises to its rated value instantaneously, and the current in a de-energized phase drops to zero just as abruptly, producing an approximately square current waveform.
In a microstepping drive, the phase currents are ramped gradually rather than switched abruptly. The winding current waveform becomes an N-level approximate staircase wave instead of a square wave. The resultant magnetic field generated by the stator winding currents then creates N new equilibrium positions for the rotor within each full step, effectively forming N microsteps.
Advantages of Microstepping
Microstepping drive technology has been the subject of extensive research worldwide. Modern high-performance microstepping drivers can achieve microstep levels in the thousands or higher. Advanced current-control algorithms now produce highly uniform microstep angles, which delivers several key benefits:
- Greatly improved step resolution of the stepper motor.
- Reduced or eliminated low-frequency vibration.
- Lower audible noise.
- Smoother torque output with reduced torque fluctuation.
These improvements give stepper motors performance characteristics that more closely resemble servo motors.
Microstepping Accuracy vs. Resolution
It is important to distinguish between step resolution and actual positioning accuracy when using microstepping.
Microstepping is fundamentally an electronic damping technique. Its primary purpose is to reduce or eliminate the low-frequency vibration inherent in stepper motors. Improved positioning accuracy is a secondary benefit of microstepping technology.
While microstepping increases the resolution of each command pulse, whether the motor's actual running accuracy approaches that resolution depends on several factors, most notably the current-control accuracy of the microstepping driver. Driver accuracy varies significantly among manufacturers, and higher microstep levels are inherently more demanding to control with precision.
True Microstepping vs. Current Smoothing
True microstepping imposes rigorous technical and manufacturing requirements on the driver, and accordingly carries a higher cost. Some drivers instead employ current smoothing as a substitute for genuine microstepping of the motor phase currents. This approach is sometimes referred to as pseudo-microstepping.
Key differences include:
| Feature | True Microstepping | Current Smoothing (Pseudo-Microstepping) |
|---|---|---|
| Produces genuine microsteps | Yes | No |
| Effect on motor torque | Does not reduce rated torque; effective usable torque may improve slightly due to reduced vibration and resonance losses | Typically causes a drop in available motor torque |
| Vibration reduction | Significant | Limited |
| Driver complexity and cost | Higher | Lower |
Note that true microstepping does not increase the motor's rated holding torque. However, because it reduces vibration and associated energy losses, the effective usable torque delivered to the load may improve slightly compared to full-step operation.