Why finer microstepping does not always mean quieter motion

Conventional microstepping generates the phase-current waveform from a finite number of current levels. Increasing the number of microsteps brings that waveform closer to the ideal sinusoidal profile, but small errors remain between the generated and ideal currents.

These errors produce periodic variations in motor torque. Couplings, gears, belts, lead screws and supporting structures all have natural resonant frequencies, and torque ripple at or near one of these frequencies can increase vibration in the motor and driven load. The vibration can then pass through the mechanical assembly and be radiated as audible noise.

In a surveillance camera, both effects can become visible at system level. The positioning mechanism has to move accurately without introducing vibration or objectionable motor noise. Current waveform accuracy therefore has a direct bearing on the mechanical and acoustic behaviour of the assembly.

Key takeaways:

• Discrete microstep currents remain an approximation of a sinusoidal waveform.
• Waveform error creates torque ripple that can excite resonances in the motor and driven mechanism.
• Mechanical damping can suppress vibration after it has been generated, but may add components, mass or design constraints.
• Smoother phase-current generation reduces the disturbance before the mechanical system can amplify it.

Torque ripple can excite the mechanics

An ideal pair of sinusoidal phase currents produces a uniform rotating magnetic field and smooth torque. A conventional microstepping driver generates those currents from a finite number of levels.

Small differences between the generated and ideal waveforms create corresponding variations in motor torque.

The frequency content of those variations matters. A camera mechanism contains its own resonant frequencies, determined by the motor, transmission, mounting structure and load. When a torque-ripple component approaches one of those frequencies, the mechanical response can amplify the original disturbance.

Audible motor noise can therefore originate in the electrical drive waveform even when the mechanism itself is functioning correctly.

Mechanical damping can add size and weight

Damping materials, vibration isolation, flexible couplings and structural reinforcement can limit the transmission of vibration through the mechanism. Operating conditions can also be selected to avoid known resonant regions.

Mechanical damping may increase system size, weight or cost, but avoiding resonant regions can restrict the available speed range. Current waveform accuracy provides another design variable: lower torque ripple reduces the vibration available to excite the motor, load and supporting structure.

A smoother current waveform reduces the excitation

Continuous microstepping adjusts phase current continuously rather than constructing the waveform from discrete current levels. The closer sinusoidal current profile produces smoother torque and reduces the energy available to excite mechanical resonances.

Toshiba’s TB67S579FTG is one implementation of this approach. Its continuous microstepping function sits within a two-phase bipolar stepper motor driver intended for precision motion-control applications.

Follow the path from current waveform to acoustic noise

The whitepaper Advanced Microstepping Technology for Precise Stepper Motor Control examines how current waveform approximation becomes torque ripple, how the mechanical system can amplify it, and how continuous microstepping changes that relationship.

It also considers the broader design trade-offs involved in achieving smoother motion without adding excessive mechanical or control complexity.