Doris Knauer, specialist editor, Physik Instrumente (PI)
The resolution of earthbound telescopes can be improved by large primary mirrors, which can only be realised as segmented mirrors. In order for these giant mirrors to maintain their form despite the wind load and the effect of gravity, their individual segments require stiff drives that work with a large travel on the one hand and allow nanometre-precise positioning on the other. So-called hybrid drives that combine a spindle-motor drive with a piezoelectric actuator are the optimum choice for meeting these requirements.
Since 2005, a group of European astronomers, astrophysicists and the industry have been working together with the European Southern Observatory (ESO) to develop an extremely large telescope (ELT) for visible and near infrared light (Figure 1). The revolutionary ELT on the 3,000 m high Cerro Armazones in the Chilean Atacama desert will have a primary reflector with a diameter of 39 m and is therefore the largest optical telescope in the world. The ELT project was approved in 2012 and the green light was given for construction at the end of 2014. It is planned for the telescope to be used for the first time in 2024.
Figure 1
The Extremely Large Telescope (ELT) will have a primary reflector with a diameter of approx. 39 m, which is made up of approx. eight hundred hexagonal mirror elements, and is scheduled to have first light in 2024. (Courtesy of ESO)
Precision positioning of 798 mirror elements
According to Tim de Zeeuw, director general of ESO (Figure 2, left): “The main reflector is a miracle of modern technology.” The main reflector is to be made up of 798 individual hexagonal segments that each have a diameter of 1.4 m (Figure 3). Each mirror element is positioned by three drives. The demand on these is very high.
Figure 2
Prof. Tim de Zeeuw (left), director general of the European Southern Observatory (ESO), and Dr. Spanner, president of Physik Instrumente (PI), during signing of the contract at ESO headquarters in Garching, Germany. PI was commissioned to manufacture the actuators for aligning the 798 segments of the primary reflector of the ELT. (Courtesy of ESO/M. Zamani)
Figure 3
The main reflector is to be made up of 798 individual hexagonal segments that each have a diameter of 1.4 m and weight of approx. 250 kg. Each mirror element is positioned by three drives. (Courtesy of ESO)
Relatively large travel ranges up to 10 mm with a positioning and path accuracy of better than 2 nm are at the limit of technical feasibility. Tracking an object during observation typically requires velocities between a few nanometres per second and +/-0.45 µm/s. If the telescope has to be aimed at a different object, velocities of up to +/-100 µm/s are necessary. Considerable masses must be moved for this; a mirror element weighs approximately 250 kg.
Furthermore, owing to the different alignments of the telescope, an individual drive has to move or hold loads between a pulling force of 463 N and a pushing force of 1,050 N. Altogether, 2,394 actuators are required for equipping all of the 798 mirror elements.
“Realising the technical specifications to the full satisfaction of the customer within a very tight schedule —that is the challenge in this sophisticated and prestigious product and also our strength,” said Oliver Dietzel, project manager at PI (Physik Instrumente).
Hybrid drive combines long travel ranges with nanometre precision
To meet the high technical requirements of the project, PI developed a tailor-made actuator and controller concept. The actuators used for aligning the segments exactly to each other and for fixing the segments to the supporting structure are based on a hybrid drive principle (Figure 4). A motor-spindle drive that is suitable for high loads and large travel ranges is combined with a piezo actuator (Figure 5). All inaccuracies of the motor-spindle drive can be measured with a high-resolution sensor and corrected using the piezo (Figure 6). This ensures an extremely high positioning and path accuracy that cannot be achieved with pure motor-spindles drives.
Figure 4
Positioning accuracy and minimum path deviation: High-stiffness hybrid linear actuator with a diameter of approx. 200 mm with an overall length of approx. 285 mm. (Courtesy of PI)
Figure 5
PICMA piezo actuator with stainless steel housing for hermetic sealing and additional protection against humidity. (Courtesy of PI)
Figure 6
Figure 6: Schematic diagram of the hybrid drive. The common control with one single high-resolution linear encoder allows an extremely constant velocity with high positioning accuracy. (Courtesy of PI)
A dedicated controller controls both drives simultaneously and also controls the high-resolution position measuring system. The servo algorithms consider the motor and the piezo system as a single drive unit and compare the actual motion with a calculated trajectory. This allows ESO to accurately compensate deformations in the structure of the primary mirror.
The spindle is driven by a brushless high-torque motor via a high-ratio reduction gearhead. The gearhead ensures zero-play operation 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.
The piezo actuators are encapsulated in a closed metal bellows filled with nitrogen so that they are protected against moisture and achieve the lifetime of 30 years required for the positioning solution, even under adverse ambient conditions. The high-resolution sensor is an incremental optical encoder, which is placed as close as possible to the tip of the drive. It operates at a resolution of 100 picometres and is insensitive to changing environmental conditions prevalent in the Atacama Desert.
Electronics design and controller structure
The drive electronics consist of two function blocks. The commutation electronics for the motor, the interpolation and the limit switches are located directly in the drive housing. This allows short encoder lines to prevent signal interference. A single cable connects the drive to the second function block and the external control electronics, which control the motor, piezo and encoder. This controller has three channels (Figure 7). This means that only one control module is required to control all three hybrid drives of a mirror segment.
Figure 7
Figure 7: Schematic diagram of the controller. (Courtesy of PI)
At the same time, it is possible to specify motion commands for each individual drive as well as the desired position of the mirror segment. The controller then translates a command for its three axes.
The controller hardware of the real-time system consists of an ARM+DSP Dual SoC with Linux operating system and a quartz clock in an FPGA for buffering all data sequences. A 16+4-bit D/A converter provides the input for the piezo amplifiers and a PWM signal for the motor. While the ARM processor is responsible for the network communication, all real-time relevant calculations, including the servo algorithm, run on the optimised DSP core. To achieve the required energy efficiency, PI developed the entire electronics itself.
The control principle
The control principle of the hybrid drive is easy to understand (Figure 8). 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 spindle in the same direction. In this way, the coarse positioning of the spindle is supplemented by the fine positioning of the piezo.
Figure 8
Figure 8: The controller structure. (Courtesy of PI)
At the same time, the spindle 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 positioning accuracy. The performance of the hybrid drive was confirmed during extensive testing at ESO. The flexible controller concept is very much appreciated by everyone and this simplifies subsequent enhancement.
“We are proud and very pleased to have received the order for this large project and therefore continue our longstanding and successful cooperation,” explained Dr. Karl Spanner, president of PI (Figure 2, right).
In the meantime, the provider of high-precision positioning solutions is involved in a further project for the ELT. PI is developing a new actuator concept together with the Fraunhofer Institute for Applied Optics and Precision Engineering (IOF). It is intended to use 11,000 PICMA multilayer piezo actuators in extremely accurate adaptive optics (XAO) to make it possible to get a clear and sharp view into space with a pitch of less than 4 mm.
Reliable piezo technology
PICMA multilayer actuators are piezo actuators, the active layers of which consist of thin ceramic tapes. In addition, the active piezo ceramic is surrounded by an all-ceramic insulation layer that protects the actuators against air humidity and failure resulting from increased leakage current.
The monolithic piezoceramic block of a PICMA actuator is very reliable even under extreme ambient conditions and this extends the lifetime considerably. These properties mean that the PICMA multilayer actuator is an ideal component for meeting the high demands on quality made by ESO for the ELT project.
In addition, multilayer piezo actuators are now also available in a variant with stainless steel housing. The case is hermetically welded and filled with an inert gas. This means, for example, that the piezo actuators are well suited for applications with increased room humidity and are even protected against splash water. The external dimensions for a piezo ceramic measuring 10 x 10 x 18 mm is then only 22.5 mm in height and 18.6 mm in diameter. For mounting, the piezos are glued on one side and preloaded and the solder pins protrude out of the housing for electrical contacting.
Physik Instrumente (PI)