In order to achieve this high path accuracy, PI developeda hybrid concept where a motor drive screw, that is suitablefor heavy loads and long travel ranges, is combined with apiezo actuator (Figure 6). A serial combination of both verydifferent drives results in a powerful and high-precision positioning system.
Piezo Actuators Position with
Precision motion that results when an electrical voltage is applied to a piezoelectric material is of particularimportance for nanopositioning. The electrical power isconverted into mechanical energy directly inside the crystalline solid-state, which means that there are no rotatingor frictional parts.
The piezo actuators not only work with high precision butare also maintenance and wear-free. They can move largeloads with weights of up to several tons. Electrically, they actas capacitive loads and need virtually no power in static operation. The behavior in the power circuit is very much like anelectrical capacitor. Similar to capacitors, they do not generate any heat in a static condition.
The lifetime of piezo actuators is also convincing: In thecase of PICMA multilayer actuators (Figure 7), the activelayers consist of thin ceramic films and are surroundedby an all-ceramic insulating layer that protects the actuators against air humidity and failure from leakage current.The monolithic piezoceramic block of such an actuator isvery reliable even under extreme ambient conditions andhigh-temperature differences.
The piezo actuators used in the hybrid drive for the telescope segments are also encapsulated in sealed metal bellowsfilled with nitrogen (Figure 8) in order to reach the 30-yearslifetime necessary in the adverse ambient conditions in theAtacama Desert.
A High-Resolution Sensor for Both Drive Systems
A further feature of the hybrid drives is the commonhigh-resolution sensor, which helps to control both drivessimultaneously and continuously. This is the only way to implement the high resolution of the piezo actuators over theentire travel range.
The high-resolution sensor is an incremental optical encoder that is placed near the drive axis (Figure 9). It operatesat a resolution of 0.1 nanometers and is also not sensitive tothe changing environmental conditions prevailing at the telescope’s location in the desert.
The motor drive screw is suitable for heavy loads and longtravel ranges starting at a few millimeters and going up to onemeter. The piezo actuator provides a nominal displacement ofapproximately 0.1 to 0.15% of the actuator length, but nevertheless, achieves a positioning accuracy in the subnanometerrange with one high-resolution sensor, and can therefore compensate for the inaccuracies of the motor drive screw.
The drive screw is driven by a brushless, high-torque motor via a high-ratio reduction gearhead.
The gearhead ensures zero-play operation and guaranteesa constant transmission ratio. The motor can, therefore, bevery small even though large masses have to be moved. Thehigh transmission also supports the self-locking of the motorwhen at rest.
A dedicated controller controls both drives simultaneouslyand also controls the high-resolution position measuring system. The servo-algorithms consider the motor and the piezosystem as a single drive unit and compare the actual motionwith a calculated trajectory.
The control principle of the hybrid drive is easy to un-
derstand (Figure 10). The motor voltage is derived from the
control voltage of the piezo. The greater this voltage, the
faster the motor runs. When the piezo expands, the motor
drives the drive screw in the same direction. In this way,
the rough positioning of the drive screw is supplemented
by the fine positioning of the piezo. At the same time, the
drive screw always moves the piezo near to its zero position
automatically. This gives it the best chance of correcting
the position in both directions. In this way, relatively long
travel ranges can be combined with an extremely high po-
The characteristics of these types of hybrid drives are notonly useful for telescopes, but are always a practical solutionwhen a position needs to be detected with high precision andmoved repeatedly over long travel ranges, or when a targetposition needs to be reached with nanometer precision. Other typical application areas include measuring technology orsurface inspection, semiconductor manufacturing, microscopy, and laser technology.
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Figure 6 – The electric motor is suitable for heavy loads and long travel ranges. In conjunction with a piezo drive, the hybrid system also provides additional positioning accuracyin the subnanometer range. Credit: PI
Figure 7 – All-ceramic insulated, high-power piezo actuators, durable even under difficultoperating conditions in industry, life science and microscopy, as well as in medical technology and research. Credit: PI
Figure 8 – An encapsulated PICMAlinear actuator of part of the hybriddrive concept. Credit: PI
Figure 9 – Schematic diagram of the hybrid drive. The common control with one singlehigh-resolution linear encoder allows an extremely constant velocity with high positioningaccuracy. Credit: PI
Figure 10 – The controller structure. Credit: PI
The characteristics of
these types of hybrid
drives are not only
useful for telescopes,
but are always a
when a position needs
to be detected with
high precision and
over long travel
ranges, or when a
target position needs
to be reached with