September, 2019
www.us-
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Page 65
Smart Hybrid Actuators Combine Precision, Force and Travel Range with High Dynamics
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will result in the world’s largest ter- restrial telescope on the 1.9 mi (3 km) high Cerro Amazones in the Atacama Desert in Chile. The telescope’s revolutionary
design consists of a main mirror (M1) with a diameter of 128 ft (39m), which is divided into 798 independ- ent mirror segments. Each mirror segment has a diameter of 4.6 ft (1.4m) and is positioned by three independent hybrid drives with a nominal travel range of ±0.2 in. (±5 mm).
Considerable masses must be
moved for this — each mirror seg- ment including its support weighs around 550 lb (249.5 kg). 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 and 1,050 N. One of the most important tasks
of the telescope will be to supply the sharpest possible images of the uni- verse, which researchers can use to search for Exoplanets — planets that exist beyond the solar system. Therefore, the greatest technical chal- lenge will be to move the mirror seg- ments over the entire range with a maximum position deviation of 2 nm. In order to achieve this high
path accuracy, PI developed a hybrid concept where a motor drive screw, that is suitable for heavy loads and long travel ranges, is combined with a piezo actuator. Serial combination of both very
different drives results in a powerful and high-precision positioning sys- tem.
Subnanometer Accuracy Precision motion that results
when an electrical voltage is applied to a piezoelectric material is of par- ticular importance for nanoposition- ing. The electrical power is converted into mechanical energy directly inside the crystalline solid state, which means there are no rotating or frictional parts. The piezo actuators not only
work with high precision, but are also maintenance and wear-free. They can move large loads with weights up to several tons. Electrically, they act as capacitive loads and need virtually no power in static operation. The behavior in the power cir-
cuit is very much like an electrical capacitor. Similar to capacitors, they do not generate any heat in a static condition. The lifetime of piezo actu- ators is also excellent. In the case of PICMA multilayer actuators, the active layers consist of thin ceramic films and are surrounded by an all- ceramic insulating layer that pro- tects the actuators against air humidity and failure from leakage current. The monolithic piezoceramic block of such an actuator is very reli- able even under extreme ambient conditions and high temperature dif- ferences. The piezo actuators used in the
hybrid drive for the telescope seg- ments are also encapsulated in sealed metal bellows filled with nitrogen in order to reach the 30-year lifetime necessary in the adverse ambient conditions of the Atacama Desert.
SMTA International
September 22 – 26, 2019 Booth #1039
Rosemont, IL, USA
Strong performance with large components.
High-Res Sensor for Both Drive Systems
Another feature of the hybrid
drives is the common high-resolution sensor, which helps to control both drives simultaneously and continu- ously. This is the only way to imple- ment the high resolution of the piezo actuators over the entire travel range. The high-resolution sensor is an
incremental optical encoder that is placed near to the drive axis. It oper- ates at a resolution of 0.1 nm and is also not sensitive to the changing
environmental conditions prevailing at the telescope’s location in the desert. The motor drive screw is suit-
able for heavy loads and long travel ranges starting at a few millimeters and going up to one meter. The piezo actuator provides a nominal dis- placement of approximately 0.1 to 0.15 percent of the actuator length, but nevertheless, achieves a position- ing accuracy in the subnanometer range with one high-resolution sen- sor, and can therefore compensate for the inaccuracies of the motor
drive screw. The drive screw is driven by a
brushless, high-torque motor using a high-ratio reduction gearhead. The gearhead ensures zero-play opera- tion, and guarantees a constant transmission ratio. The motor can, therefore, be very small even though large masses have to be moved. The high transmission also supports self- locking of the motor when at rest. A dedicated controller controls
both drives simultaneously and also controls the high-resolution position
Continued on page 68
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