Ultrasonic Piezo Motor Enables a Compact 2-Axis Steering Mirror

Inertia-based piezo motors are one established example. These systems often use stick-slip motion to drive a spindle or screw. For a miniature two-axis steering mirror, however, the authors identify several drawbacks, including limited traversing speed, audible-frequency operation, and the space required for multiple actuators and screw mechanisms.

The ultrasonic motor investigated here was developed to address those limitations while preserving two valuable characteristics of piezo motors: wide travel range and self-locking behavior. Once positioned, the mirror can hold its orientation even after drive power is removed.

At the heart of the design is a hollow cylindrical resonator fabricated from PIC 181 hard piezoelectric ceramic. The mirror is mounted within a hemispherical ceramic rotor, while three contact tips spaced at 120° intervals transfer motion from the stator to the rotor. A spring preload maintains the friction force required at these contact points.

The resonator uses a common inner electrode and three outer electrode segments, each spanning 120 degrees around the circumference. Applying a sinusoidal voltage to one segment excites a resonant vibration mode within the ceramic structure.

Like any friction-driven motor, performance depends on maintaining sufficient normal force at the contact interface. In the prototype, the actuator is designed to generate approximately 1 N of force at the contact points between the spherical tips and rotor.

The preload mechanism does more than simply maintain friction. The flat spring used in the design also influences the attainable motion range. As the mirror moves away from its neutral position, the spring's restoring force increases. The farther the mirror tilts, the more of the available actuation force is consumed, overcoming that restoring force.

Eventually, the restoring force becomes equal to the available driving force, defining the practical motion limit of the system.

This creates a motion envelope that is more complex than a simple rectangular or circular operating range. The effect becomes visible in the measured two-dimensional trajectories and ultimately shapes the attainable workspace.

To verify the resonator design, the researchers characterized the stator using a laser vibrometer. An open-loop sinusoidal frequency sweep from 380 kHz to 430 kHz was applied at a mean operating voltage of 15 Vrms while oscillation amplitudes were measured at the three stator tips.

The key result is the close agreement between simulation and measurement. For each electrode segment, the corresponding stator tip reached an oscillation amplitude of approximately 150 nm at resonance, with the measured resonance occurring near 406 kHz.

When non-corresponding electrodes were excited, tip motion remained comparatively small, confirming that the intended vibration mode was being generated.

For a functional prototype, this correlation between finite-element predictions and measured performance is particularly valuable because it demonstrates that the modeled resonator behavior translates directly into the physical device.

Mirror motion was measured using an autocollimator. The evaluation consisted of 55 individual movements across the two-dimensional workspace, with trajectories executed in alternating directions.

The resulting operating area forms a roughly triangular region rather than the circular or square workspaces commonly associated with many positioning systems. According to the authors, this shape is primarily determined by the interaction between motor force and the changing restoring force produced by the preload spring.

To define a practical operating specification, the researchers identified the largest circular region that could be fully enclosed within this measured workspace. Based on that criterion, the prototype demonstrated an angular travel range of ±10 mrad.

Importantly, this value represents a usable two-dimensional operating range rather than a maximum excursion achieved along a single favorable direction.

The work demonstrates that a single ultrasonic piezoelectric resonator can generate controlled motion about two angular axes within an extremely compact package.

The measured resonance frequency closely matches finite-element predictions, validating the resonator design, and the prototype achieves a two-dimensional angular range of ±10 mrad under the range definition used in the study.

Just as importantly, the authors present the device as a functional proof of concept rather than a finished positioning product. Testing was performed open loop, and future development efforts are expected to focus on integrated position sensing and closed-loop control strategies.

From an engineering standpoint, that is precisely what makes the work interesting. Before addressing advanced control algorithms, the team first demonstrated the underlying electromechanical principle, verified the resonator behavior, and established the achievable motion range of the concept. The result is a compact beam-steering architecture that shows how ultrasonic piezo motor technology can be applied to generate two-axis mirror motion using a single actuator structure.