Weight lifting with picometer precision

The controller of the Fabry-Perot Etalon for the Visible Tunable Filter instrument (VTF) has reached the demanding requirements for the controller precision and speed. The distance of the 20 kg Etalon plates can now be adjusted within a few milliseconds to the precision of a medium size atom. This is an important milestone towards the realization of the VTF. The multi-level controller was developed at KIS and has been verified on the basis of a full-size model of the Etalon.

The plots show details from the controller behavior for the case that the distance of both etalon plates is changed by 100nm. Such a step size corresponds to a change of the transmission of the optical filter by 1,1Å (or 0,11nm) in wavelength (at 600nm). Such a step size is a typical use case for the VTF instrument during observations. The upper right plot shows in black the nominal value for the plate distance in nm over time. In blue are the unfiltered and in green the filtered plate distance values shown. The bottom right plot shows details while adjusting to the new plate position (transient period). After about 0.04s the nominal position has been reached to the allowed residual error. The left plot sows the residuals of the controller for all three actuator positions of the Etalon – both unfiltered and filtered. The 30Hz filtered values are within the required +/- 150pm range – about the radial size of the Lithium atom.

The pictures show the so-called „Etalon-Dummy”, that was used to develop and test the etalon controller. The Etalon-Dummy is a full-size model. The etalon plates are made from aluminum and have the size and weight of the final high precision optical plates. On the left is an overview. The close-up image on the right shows one of the three sensor-pairs. The bluish stripe is one of the linear scales that is mounted to the upper plate. The scale on the lower plate is not visible. Also visible is one of the three piezo actors.

The plots of the upper figure show details from the controller behavior for the case that the distance of both etalon plates is changed by 100nm. Such a step size corresponds to a change of the transmission of the optical filter by 1,1Å (or 0,11nm) in wavelength (at 600nm). Such a step size is a typical use case for the VTF instrument during observations.

The upper right plot shows in black the nominal value for the plate distance in nm over time. In blue are the unfiltered and in green the filtered plate distance values shown. The bottom right plot shows details while adjusting to the new plate position (transient period). After about 0.04s the nominal position has been reached to the allowed residual error.

The left plot sows the residuals of the controller for all three actuator positions of the Etalon – both unfiltered and filtered. The 30Hz filtered values are within the required +/- 150pm range – about the radial size of the Lithium atom.

Context information

The "Visible Tunable Filter“ (VTF) is a spectropolarimeter, that is currently being built at KIS and that is planned to start operations end of 2020 at the US  Daniel K. Inouye Solar Telescope (DKIST). The core of the instrument are two Fabry-Perot interferometer (FPI) with a free aperture of 250mm, the largest ever built FPI’s for optical applications. The filter is based on interference of the sunlight between two reflecting optical plates that are kept at a distance of a few 1/10 of a mm. By changing the plate distance, the transmission wavelength can be “tuned”. Beside the demanding requirements on the optical quality of the plates, the control system to adjust and maintain the air gap size has to fulfill challenging performance. The controller needs to make sure that the air gap between the plates with a weight of 20kg is tuned within 40 milliseconds to a precision of 150pm. For comparison: 150pm corresponds approximately to the radius of a Lithium atom.

While the first optical Etalon is still under construction, the Etalon controller was developed for more than a year with the help of a full-size Etalon-model. Now for the first time the control system reached the requirements.

To tune the air gap size three piezo actuators are used that had been developed by the company Physik Instrumente (PI), Karlsruhe, especially for the VTF. To measure the actual plate position three linear scale systems at each plate are used. The system is combined by lithographic phase gratings that are directly glued to the Etalon plates and externally mounted laser sensors to read out the position. The commercial system from Heidenhain is available for a precision of 500nm (0,5 µm). But due to the used method the system can reach an interpolated resolution of about 10pm. KIS has performed an extended qualification program with support from Heidenhain to optimize the system for VTF.

The control of the Etalon air-gap is done in combination of the metrology system from the company Dr. Johannes Heidenhain, Traunreut, and the 3 piezo actuators. The position-values from the linear scale system are read from specific signal converters. The piezos are driven from a high voltage amplifier.

The controller itself is running on a Beckhoff software-PLC. The Etalon hardware is connected to the Beckhoff-PLC via an EtherCAT bus system. The position data from the Heidenhain linear system is acquired digitally every 100µs. The controller is calculating from these position values the actuating values for all three actuators that are then given in the following cycle via analog outputs to the high voltage amplifier.

On the picometer level smallest effects have to be considered and the controller needs to handle various mechanical resonances that in “normal life” are not even recognized.  For example, in this case a falling pencil in a neighboring room acts like an earthquake. A classical PID controller can not fulfill the requirements. Therefore, here a state regulator is used. Part of the resonances are considered in a mathematical model that allows the controller to adjust in advance the needed corrections.

The mathematical model has been created by using the Matlab System Identification Toolbox based on measurements of the frequency response. The determination of the state regulator is also done in Matlab. A specific LabView program was developed to transfer the needed parameters by using an OPC UA interface to the PLC.