2394 Drives Help To Discover Exo Planets

2023 Author : Hannah Pearcy | [email protected] . Last modified: 2023-05-24 11:12

With its segmented primary mirror, the ELT receives electromagnetic radiation in the visible and near infrared wavelength range. Schott, a technology group for special glass and glass ceramics, is currently producing the mirror support components for the Extremely Large Telescope (ELT) of the ESO (European Southern Observatory). The mirror has a diameter of 39 m and a light collection area of almost 1000 m² . Schott has already delivered segments for the fourth mirror of the telescope optics and two mirror carriers of the 4 m class for the secondary and tertiary mirrors of the ELT. The 3,046 m high Cerro Amazones in the Chilean Atacama Desert is scheduled to go into operation in 2025.

Picture gallery

Picture gallery with 12 pictures

The main mirror will consist of 798 individual mirror segments, each 1.40 m in diameter, and weigh 250 kg. Three drives each position a mirror element . These position actuators (PACTs) come from the company Physik Instrumente, with which ESO has signed a contract. A total of 2394 actuators are required.

The PACTs fix the respective segments, but will also actively control the position in three directions (lifting and tilting in two axes) . In this way, deformations of the mirror can be corrected, which are caused by changes in the telescope height, temperature and wind force. In addition, vibrations can be compensated.

Together, the segments will collect tens of millions of times as much light as the human eye and use additional optics to guide scientific instruments. In order to compensate for deviations from the optimal beam path and thus avoid imaging errors, the mirror segments must be aligned exactly with one another.

High demands on actuators

The travel ranges of the segments can be up to 10 mm. In order to track an object during the observation, the speeds are typically between a few nanometers per second and +/- 0.45 µm / s. The positional deviation on average must not be more than 2 nm. If the telescope is to be aligned to another object, speeds of up to +/- 100 µm / s are required. Considerable masses have to be moved: A mirror segment weighs about 250 kg.

For example, a single drive must move or hold loads between 463 N tractive force and 1050 N compressive force . The maximum permitted waste heat for the actuator and controller must also be taken into account. PI has “tailor-made” a hybrid drive for this demanding task.

Motor-spindle drive combined with piezo actuator

The principle of the hybrid drive is to combine a motor-spindle drive that is suitable for high loads and long travel distances with a piezo actuator. The spindle is driven by a high-reduction gear by a brushless, high-torque torque motor . The gearbox ensures play-free operation and guarantees a constant gear ratio. This allows the motor to be made small, even though large masses are moved. The high gear ratio also supports the self-locking of the motor when it is not moving.

A high-resolution sensor works with a resolution of 100 picometers and measures all inaccuracies of the motor spindle drive . The piezo actuators correct this inaccuracy. They are encapsulated in a sealed metal bellows filled with nitrogen, so that they are protected against moisture and reach the required service life of the positioning solution of 30 years even under adverse environmental conditions.

A special controller controls both drives simultaneously and controls via the high-resolution position measuring system. The control algorithms consider the motor and piezo system as a drive unit and compare the actual movement with a calculated trajectory.

User meeting mechatronic drive technology

The focus of the user meeting mechatronic drive technology is on the mechanical components of gears, clutches and brakes as well as their design, dimensioning and interaction in the overall mechatronic system.

More info

Short encoder lines avoid signal interference

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 avoid signal interference. A single cable then connects the drive to the second function block, the external electronics, which controls the motor, piezo and encoder.

This main controller has a three-channel structure. This means that only one such controller is required to control all three hybrid drives in a mirror segment. It is possible to specify travel commands for each individual drive as well as the desired position of the mirror segment. The controller then "translates" such a command for its three axes.

The control principle of the hybrid drive

The control principle of the hybrid drive is easy to understand: the motor voltage is derived from the control voltage of the piezo. The greater this voltage, the faster the engine runs . So as the piezo expands, the motor drives the spindle in the same direction. The rough positioning of the spindle is supplemented by the fine positioning of the piezo. At the same time, the spindle automatically moves the piezo close to its zero position. Here he has the greatest opportunity to correct the position in both directions. In this way, the relatively long travels can be combined with extremely high positioning accuracy.

The performance of the hybrid drive has been confirmed at ESO in extensive tests. One also appreciates the flexible controller concept, which makes subsequent extensions easy.