The characteristics of hybrid drives present a practical solution when a position needs to be detected with high precision and moved repeatedly over long travel ranges, or when a target position needs to be reached with nanometer precision. Application areas include precision movement of telescopes, microscopy, surface inspection, semiconductor manufacturing, and laser technology.

The European Extremely Large Telescope (ELT) will be the largest terrestrial telescope for scientific evaluation of electromagnetic radiation in the visible and near-infrared wavelength range. (Image: ESO)

Hybrid drives combine two different drive concepts into a high-performance and precision positioning system. They take advantage of both drives. These types of solutions are always in demand when one drive alone is not in a position to meet all of the requirements of applications.

An example of this is nanometer-precision positioning of heavy loads over long travel ranges. Nanopositioning systems that combine piezo drives with classical drive screws, therefore, offer a practical solution but other drive concepts are possible. Examples of applications for hybrid drives range from semiconductor manufacturing through quality assurance, and optical inspection to biomedical engineering.

Hybrid Concept with Electric Motor and Voice Coil

Various concepts have been developed and implemented for hybrid positioning systems. One solution is a combination of linear motors and voice coil actuators (Figure 1).

Figure 1. Hybrid concept with electric motor and voice coil actuator. (Image: PI)

Linear motors offer long travel ranges with high positioning 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. Their travel range is limited to 100 mm and they can only hold loads at rest when they are powered on.

Hybrid Concept with Piezo Walking Drive and Piezo Actuator

Figure 2. Hybrid concept with piezo walking drive and piezo actuator. (Image: PI)

Physik Instrumente (PI) has developed and established a further hybrid drive concept for other applications such as those for inspecting semiconductors. A PiezoWalk® drive that offers long travel ranges with high stiffness is combined with a PICMA® actuator for high-dynamics applications (Figure 2). The advantage in this case, depending on the design of the PiezoWalk drive, is the possibility of realizing high holding forces. The high dynamics of the PICMA drive is important for fast focusing of the object to be scanned.

Hybrid Concept with Electric Motor and Piezo Actuator

PI is working on a project together with engineers and astronomers from the European Southern Observatory (ESO) that will result in the world's largest terrestrial telescope on the 3,000-meter-high Cerro Amazones in the Atacama Desert in Chile. The telescope's revolutionary design consists of a main mirror (M1) with a diameter of 39 meters that is divided into 798 independent mirror segments. Each mirror segment has a diameter of 1.4 m and is positioned by three independent hybrid drives with a nominal travel range of ±5 mm (Figure 3).

Figure 3. Depiction of a main mirror segment from ELT. The three hybrid drives are shown in blue. (Image: ESO)

Considerable masses must be moved for this — each mirror segment, including its support, weighs approximately 551 pounds. Due to the different alignments of the telescope, a total of 2,394 actuators need to be able to move and hold loads with push/pull forces between 463 N and 1050 N (Figure 4).

Figure 4. ELT hybrid nanopositioning system. (Image: ESO)

One of the most important tasks of the telescope will be to supply the sharpest possible images of the universe that researchers can use to search for exoplanets, i.e., planets that exist beyond the solar system. Therefore, the greatest technical challenge will be to move the mirror segments over the entire range with a maximum position deviation of 2 nm.