Servo design and analysis for the Thirty Meter Telescope primary mirror actuators

Abstract

The Thirty Meter Telescope has 492 primary mirror segments, each incorporated into a Primary Segment Assembly (PSA), each of which in turn has three actuators that control piston, tip, and tilt, for a total of 1476 actuators. Each actuator has a servo loop that controls small motions (nanometers) and large motions (millimeters). Candidate actuators were designed and tested that fall into the categories of "hard" and "soft," depending on the offload spring stiffness relative to the PSA structural stiffness. Dynamics models for each type of actuator are presented, which respectively use piezo-electric transducers and voice coils. Servo design and analysis are presented that include assessments of stability, performance, robustness, and control structure interaction. The analysis is presented for a single PSA on a rigid base, and then using Zernike approximations the analysis is repeated for 492 mirror segments on a flexible mirror cell. Servo requirements include low-frequency stiffness, needed for wind rejection; reduced control structure interaction, specified by a bound on the sensitivity function; and mid-frequency damping, needed to reduce vibration transmission. The last of these requirements, vibration reduction, was found to be an important distinguishing characteristic for actuator selection. Hard actuators have little inherent damping, which is improved using PZT shunt circuits and force feedback, but still these improvements were found to result in less damping than is provided by the soft actuator. Results of the servo analysis were used for an actuator down-select study.

Robustness of Thirty Meter Telescope primary mirror control

“…TMT has considered both "soft" (voice-coil) and "hard" (e.g. piezoelectric) actuators, 13 and the analysis here focuses primarily on the selected voice-coil actuator design. The unsteady forces on the mirror segments due to wind turbulence within the enclosure lead to requirements both on the M1CS control bandwidth, of roughly 1 Hz, 3,4 and also on the lowfrequency stiffness of the M1CS actuators, and hence on the servo loop gain.…”

Primary mirror dynamic disturbance models for TMT: vibration and wind

“…Various different actuator technologies have been considered, prototyped, and characterized . 9 The analysis below summarizes this process for a soft (voice-coil) actuator with a collocated high-bandwidth servo loop, and for a hard actuator with parameters representative of one of the options considered. A hard actuator is not viable without some added damping, due to the resulting vibration response, however, a collocated force-feedback loop can be used to add damping to a high-impedance position actuator, and this is assumed here.…”

Control system modeling for the Thirty Meter Telescope primary mirror

“…Finally, the actuator model is also fit to experimental data taken from a prototype actuator. 5 Model validation has been conducted by constructing two fully independent models, one interconnecting the component models in Fig. 8 in state-space, and the other in the frequency domain; both yield identical results for CSI predictions.…”

“…The focus here is to summarize these modeling tools. Earlier analyses have been described in [3][4][5] and similar analyses for the European ELT in [6].…”