When faced with challenging alignment applica- tions, designers often have to live with an unsat- isfactory compromise between accuracy, force,and dynamics. Hybrid drives combine two different driveconcepts into a high-performance and precision positioning system—taking advantage of both drives. These types ofsolutions are always in demand when one drive alone is notcapable of meeting all of the requirements of an application.
An example of this can be shown by nanometer-precisionpositioning of heavy loads over long travel ranges. Nanopositioning systems that combine piezo drives with classical drivescrews offer a practical solution while enabling the possibilityof more than one drive concept. Application areas includeprecision movement of telescopes, measuring technology,surface inspection, semiconductor manufacturing, microscopy, and laser technology.
Hybrid Concept with Electric Motor
and Voice Coil
Various concepts have already been developed and implemented for hybrid positioning systems. One solution, forexample, is a combination of linear motors and voice coil actuators (Figure 1).
Linear motors offer long travel ranges with high position-
Hybrid Concept with Piezo Walking Drive
ing accuracy and repeatability but do have the disadvantage
of high friction in the linear guides used for taking up the
load. Nevertheless, voice coil actuators do provide precision
positioning as well as good force control and smooth motion.
However, their travel range is limited to 100 millimeters and
they can only hold loads at rest when they are powered on.
and Piezo Actuator
Physik Instrumente (PI) has already successfully developed and established a further hybrid drive concept for otherapplications, such as those for inspecting semiconductors.
A PiezoWalk drive that offers long travel ranges withhigh stiffness is combined with a PICMA actuator for highdynamics applications (Figure 2). The advantage to this concept—depending on the design of the Piezo Walk drive—isthe possibility of realizing high holding forces. In this case,the high dynamics of the PICMA drive are important for thefast focusing of the object being scanned.
Hybrid Concept with Electric Motor and
PI is working on a project together with engineers and astronomers from the European Southern Observatory (ESO)which will result in the world’s largest terrestrial telescopeon the 3,000-meter-high Cerro Amazones in the Atacama Desert in Chile (Figure 3 - pictured on page 8).
The telescope’s revolutionary design consists of a main
mirror (M1) with a diameter of 39 meters, which is divid-
ed into 798 independent mirror segments. Each mirror
segment has a diameter of 1. 4 meters and is positioned
by three independent hybrid drives with a nominal travel
range of +/- 5 millimeters (Figure 4).
Considerable masses must be moved for this–each mirror segment including its support weighs approximately 551pounds. Due to the different alignments of the telescope, atotal of 2,394 actuators need to be able to move and holdloads with push/pull forces between 463 and 1,050 newtons(Figure 5).
One of the most important tasks of the telescope will be tosupply the sharpest possible images of the universe, whichresearchers can use to search for Exoplanets, i.e., planetsthat exist beyond the solar system. Therefore, the greatesttechnical challenge will be to move the mirror segmentsover the entire range with a maximum position deviationof 2 nanometers.
Figure 1 – Hybrid concept with electric motor and voice coil actuator. Credit: PI
Figure 2 – Hybrid concept with piezo walking drive and piezo actuator. Credit: PI
Figure 4 – Depiction of a main mirror segment from ELT; the three hybrid drives areshown here in blue. Credit: ESO
Figure 5 – Conceptual design of the ELTM1 Segment Subunit. Credit: ESO
Hybrid drives combine two different drive concepts into a
high-performance and precision positioning system—taking
advantage of both drives. These types of solutions are always in
demand when one drive alone is not capable of meeting all of
the requirements of an application.