Active damping for high precision application

Active damping development

Increasing damping in high precision applications can help to reduce vibrations, hence improve accuracy, significantly. For this purpose, a low-noise non-contact active damping technology was developed. Typical damping bandwidth is 0 – 500 [Hz] with a noise level of 1.5 [nm/s]. Damping constants of more than 10⁵ [Ns/m] are possible in a volume of 1 l.

Introduction to the need for damping

High precision equipment have seen many design improvements over the years. Design principles teach about eliminating over-constrained connections because these can cause thermal strain and mechanical hysteresis. However, by minimizing the constraints, typically also structural damping is reduced. In addition, as machine throughput performance and speeds are going up, the level of vibrations is expected to rise. Explicitly adding damping to the structure has proven to be a very adequate counter measure.

Figure 1. Visualization of the effect of active damping in a production machine, targeting the 100…120 Hz contribution (red: without active damping; green: with active damping)

Design of a high-performance active damper

Passive dampers, e.g. mechanical dampers as used in cars, not only add damping, but they also introduce stiffness. When relative movements are small and the forces across the damper are less than the mechanical friction in the damper, the damper hardly damps and merely introduces stiffness. This stiffness is likely to cause deformations due to hysteresis and thermal excursions. Non-contact damping (such as passive eddy current damping) does not suffer this disadvantage, but typically, the associated damping constants are low.

To overcome the disadvantages of the existing dampers, a multi-disciplinary optimization of an active damper is started. Break-through improvements are combined from the fields of architecture, electronics, sensor (optimized sensor geometry and electronic sensor feedforward), actuator (optimized actuator geometry), control (analogue controller to minimize phase lag), magnetics (external field cancellation), and mechanics (geometric feedback minimization). At the end of this iterative design optimization, a damper resulted (see figures below) that provides a factor 1000 higher damping levels per volume than a passive non-contact damper, without adding stiffness. The main disadvantage of this active damper with respect to a passive eddy current damper is the dependency on electrical power.

Summary of optimized active damper design

To summarize, our active damper technology has unique advantages over passive eddy current dampers:

• Much higher damping constant per volume (more than 10⁵ [Ns/m] per liter).
• Enables integration of force feedforward.
• Allows machine diagnosis (transfer function measurement, vibration logging)
• Analog controller to minimize phase lag
• Optimized actuator and sensor geometry
• External field cancellation
• Geometric feedback minimization
• Extremely low noise level (determined by the thermal-electric noise over a 6 Ohm resistor)

Additional characteristics of our damper technology are:

• Calculated MTBF 150 years.
• 30 dB amplitude margin (@ 180 degrees phase shift)
• 50 kHz internal bandwidth (@ 0dB crossing)

Figure 2. Custom damper as used in production

Conclusions

Adding damping can help to significantly improve accuracy in dynamic challenging situations. Designing active damping is not trivial, but once the rules are understood, very high damping performance can be combined with high reliability.