Nanopositioning

Nanopositioning systems are used wherever components need to be positioned with nanometer-scale accuracy. Most nanopositioners are designed for precise linear motion, either along a single axis or across multiple axes, although rotary configurations also exist.

Typical nanopositioning applications include wafer stages, microscope sample stages, research setups, beamlines and metrology equipment. Nanopositioners may also need to operate in demanding environments, including high vacuum and cryogenic conditions.

The performance of a nanopositioner depends on more than positioning resolution alone. Travel, force density, bandwidth, stiffness and thermal stability all influence the accuracy and responsiveness of the complete system.

Reluctance actuators provide nanometer-level positioning accuracy over a millimeter-scale stroke, combined with high force density. In addition, their fast response and low power consumption make them well suited to many of the nanopositioning applications described above.

Nanopositioning Actuators

Most commercially available nanopositioners use either piezoelectric actuators or Lorentz-style actuators such as voice coils to generate motion, with piezo-based-ones being the most common.

Piezo nanopositioners offer excellent positioning resolution, high stiffness and fast response in a compact package. However, piezo actuators typically provide only micrometer-scale travel. This limits the distance over which a piezo stage can position a component without additional mechanisms. Flexure guides are commonly used to maintain accurate, frictionless motion, while larger travel ranges may require mechanical amplification or a separate coarse-positioning stage. These additions can increase system complexity and introduce compromises in stiffness, force, available space and dynamic performance.
Voice coil nanopositioners provide millimeter-scale travel and offer a smooth, predictable force response because their force is directly proportional to the applied current. However, this also means that generating more force requires more current. Their relatively low force density further increases the electrical power required to produce high forces, introducing thermal challenges and making it difficult to maintain a compact nanopositioning system.

Wafer stages are a common nanopositioning application

Reluctance actuators offer another combination of characteristics. They provide millimeter-scale stroke, high force density, fast response and low electrical power requirements, making them an interesting alternative for nanopositioning systems.

These trade-offs create a fundamental challenge for FSM designers: achieving both large tip-tilt range and high control bandwidth while maintaining pointing precision and thermal stability.

This creates an opportunity for a third type of FSM actuator: the reluctance actuator. Reluctance actuators combine large travel, high dynamic performance and low electrical power requirements, offering a different balance of tip-tilt range, control bandwidth and thermal performance.

Reluctance Actuators for Fast Steering Mirrors

Reluctance actuators can expand the performance envelope of fast steering mirrors. The main advantages include:

Wide bandwidth

Reluctance actuators combine large travel with high dynamic performance, enabling a better trade-off between tip-tilt range and control bandwidth than conventional tip-tilt actuator technologies. This allows FSMs to achieve higher control bandwidth at a given tip-tilt range, or larger tip-tilt range at a given control bandwidth.

Research has shown that reluctance actuation can outperform established piezoelectric and Lorentz-force technologies across a significant part of the FSM performance space, particularly at bandwidths above 700 Hz and angular ranges below 120 mrad. Fluxthor's newer reluctance actuator technology is now pushing this performance further.

This is particularly valuable in applications where fast beam steering is critical. For example, in free-space optical communication, higher control bandwidth enables higher data rates, and therefore faster (internet) communication.

Large tip-tilt range

The 1–7° tip-tilt range represents an attractive performance region for fast steering mirrors, offering a strong balance between angular range and control bandwidth. Reluctance actuators offer larger angular range than piezo tip-tilt actuators, and can sustain these larger travel ranges at higher frequencies than voice coil actuators. This gives them the potential to outperform both piezo and voice coil technologies in this performance range.

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Low heat generation

Reluctance actuators can generate the required force with relatively low electrical power, reducing heat generation close to the mirror. This helps minimize thermally induced distortion and angular drift, supporting stable and precise beam steering during continuous operation.

For FSMs, thermal performance also depends on the mirror and optical coating. Minimizing optical absorption is therefore important alongside minimizing actuator heat generation.

High force density

Reluctance actuators have a motor constant 5× higher than voice coil actuators, enabling high dynamic force from a compact actuator with low electrical power requirements. This allows the actuator to be integrated close to the mirror while keeping the overall FSM compact.

High angular resolution

Reluctance actuators provide precise control of actuator position and force, with actuator positioning down to the sub-nanometer range. Combined with suitable mechanical design and position sensing, this enables very high angular resolution for precise and repeatable tip-tilt control in fast steering mirrors.

Conclusion

Reluctance actuators can expand the Pareto front of fast steering mirror performance by combining large tip-tilt range, high control bandwidth and low electrical power requirements. This enables greater steering authority and fast, precise beam control while reducing thermal effects that can compromise optical stability.

Fluxthor reluctance actuators therefore offer a compelling alternative to conventional piezoelectric and voice coil actuators for demanding optical applications.

About Fluxthor

Fluxthor is the world's first provider of commercial reluctance actuators, with extensive experience in developing high-performance actuation solutions for precision optical systems.