In precision motion systems, the focus is often on the actuated degrees of freedom (DoF) since these involve actuation, position sensing and motion control. In most/many cases, however, the non-actuated, constrained DoFs are equally important since these are exposed to similar disturbance forces as the actuated DoFs and accuracy requirements are comparably challenging.
Known solutions for constraining degrees of freedom in precision motion systems are roller bearings, flexure bearings, gas bearings and active/passive magnetic bearings, but what technology fits your application best? In order to answer that question one needs to understand the essence, pros/cons and limitations of each of these bearing technologies.
In CD players that were developed in the 1980s, originally, a swing arm supported by roller bearings was used to move the optical pick-up unit from the inner to the outer radius of the disk and to accurately follow a particular track. In later generations, driven by a smaller pitch and tighter accuracy requirements, the swing arm was replaced by a translational guide way based on sliding contacts for coarse positioning, combined with a flexure bearings for fine positioning of the pick-up unit in radial and focus direction.
Typically, when designing precision motion systems, various design parameters have to be balanced and simultaneously optimized, such as range of motion, accuracy and reproducibility, stiffness and damping. Although sliding and also rolling contacts suffer from stick-slip behaviour and (virtual) play, cost and complexity are low and performance might be good enough for coarse positioning. When low friction and hysteresis is required, or strict requirements on particle generation hold, it may be advantageous to fully separate surfaces via full film bearings, either aerostatic or hydrostatic. For large range of motion in vacuum conditions, such as in electron microscopes, E-beam inspection or EUV lithography systems, active magnetic bearing systems (AMBs) are highly advantageous. Despite higher cost, these systems have the additional advantage that frame vibrations can be isolated from the motion stage. When only small translations or rotations are required, flexure bearings can provide excellent reproducibility, particularly if manufactured monolithically. These bearings, however, suffer from parasitic stiffness and have very little damping.
This course will focus on the design, modelling and implementation of bearing elements in precision motion systems. We will discuss the trade-offs between performance on the one hand, and cost and complexity on the other hand. Both passive and active bearing technologies will be extensively discussed. We will focus on different application areas, such as medical equipment, machine tools, scientific instrumentation and semiconductor applications.