The Giant Magellan Telescope will be a 25.4-m visible and infrared telescope at Las Campanas Observatory. The optical design consists of 7 8.4-m primary mirror segments that reflect light to 7 secondary mirror segments in a doubly segmented direct Gregorian configuration. GMT is developing a Telescope Metrology System (TMS) to decrease the complexity of alignment and increase observatory efficiency. The TMS has been developed to Preliminary Design Review level. A prototyping, modelling, and analysis effort has been completed. All components of the system were matured, and the edge-sensing strategy was significantly revised. This paper describes the current TMS design.

We present the final design of the Giant Magellan Telescope (GMT) Adaptive Secondary Mirrors System (ASMS), which comprises seven 1m class deformable mirrors segments plus seven hexapod positioners. Each deformable mirror is based on the well established contactless technology developed by AdOptica and already successfully deployed in several 8m class telescopes. The challenge for GMT is that the seven deformable mirrors will function as a single mirror. A subscale prototype made of 72 actuators has been produced to secure system final design: test setup and preliminary results are presented.

Extremely large telescopes are characterized by high degree of freedom control systems used to coordinate multiple segments and mirrors. The dynamics can interact so that single loop requirements do not provide sufficient stability and performance robustness. This paper reviews the relevant multivariable robustness and performance methods, and presents examples from Giant Magellan Telescope (GMT) motion control systems.Singular value bounds of multivariable frequency responses are well developed computational tools that provide a methodology that can be used for telescope analysis. The singular value bounds are relevant because they give the maximum sensitivity for coupled, multivariable systems. Singular values are recommended for analysis, and can be considered for requirements. With sufficient numbers of sensors, these multivariable bounds are measurable and hence can be validated. There is a practical reason for using multivariable tools, to combine many, perhaps thousands of transfer functions and/or measurements that can be compared against singular value bounds. The first example is the AZ/EL mount control. Coupling tends to be small, hence single-input analysis tools suffice, nevertheless the mount control system provides a good introduction to multivariable methodology. The maximum singular value of both the sensitivity and complementary sensitivity functions provide a good bound for crossover robustness near the position control bandwidth, typically +6 dB near 1 Hz. The high frequency region of the complementary sensitivity function provides a good bound on robustness with respect to unmodeled structural dynamics, typically–40dB above the maximum frequency of the finite element modes. Similar multivariable stability robustness bounds can be applied to position control of the M2 assembly, for both the macrocell relative to the top end assembly, and each mirror subassembly relative to the macrocell. The latter includes control of the Fast Steering Mirror, where 21 PZT actuators control the tip and tilt of seven secondary mirrors. The risk is the 21 PZT control loops meet good classical phase and gain margin robustness metrics when measured as individual, single-input-single-output systems, but the multivariable bound exceeds either the +6 dB or – 40 dB bound. This can occur due to interaction in the macrocell, the structure used to support the individual segments. Whether or not this interaction occurs depends on the bandwidth of the control system relative to the structural modes of the macrocell. This tradeoff is important, and the maximum singular value is a good tool to test for this sensitivity.

The Giant Magellan Telescope (GMT) is an advanced Extremely Large Telescope and is being designed for broad wavelength coverage and maximum sensitivity in pursuit of several high priority cosmology science objectives. The GMT will use large primary mirror segments to provide well corrected wavefronts and a very fast optical system with a 20-arcminute diameter field of view. It will have a direct Gregorian optical prescription and deformable secondary mirrors to implement adaptive optics. The project timeline leads to initial commissioning with early instruments by 2023. The Systems Engineering group, along with the chief engineer, is developing and flowing down science and operational requirements and is the guardian of the performance budget. An integrated performance model of the telescope is being developed to support requirements derivation, performance assessment and trade studies that drive telescope architecture decisions. This model has four key elements; a structural plant model which exhibits the complex dynamics of the telescope; a controls model that models both controller dynamics and noise; an optics model to accurately demonstrate performance and optical feedback; and loads models to characterize various environmental loads. This paper will introduce the GMT integrated modeling architecture, but will focus on the specific tools and process developed to build the critical structural model. An efficient process was required to support the variety of required analyses, update frequency of underlying finite element models, and various project critical trade studies. The end-to-end process, from subsystem finite element models to a system level plant and Simulink block diagram has been achieved through development of a new MATLAB class with many useful functions. Functions are covered that combine the subsystem models, manipulate those models, and help them run efficiently in discrete and continuous time. Verification and version control are also discussed. Case study of wind is presented with preliminary performance results.

The Thirty Meter Telescope (TMT) needs to maintain precise positioning of the optical elements to deliver unprecedented image quality. Any vibration from observatory sources must therefore be minimized; model-based analysis leads to maximum allowable forces from any individual source typically of order a Newton or less in the most sensitive frequency band. Careful attention throughout the design process is needed to ensure that these challenging requirements are satisfied. We focus here in particular on cogging forces from the azimuth motor drive. Each motor yields periodic tangential and vertical forces of order 100's of Newtons, with higher harmonics of the waveform potentially exciting telescope structural resonances. There are 56 drive motors, and appropriate phasing between them can ideally cancel most of the net torque or net vertical force. However, the moment created between non-collocated forces still produces image motion even if the net force cancels, and further, small errors in forcer positioning result in imperfect cancellation. We provide a general methodology for estimating total cogging forces and then present an example of the expected TMT performance impact from motor cogging placed in context with the larger challenge of demanding vibration requirement.

This paper describes the development and analysis of a partial authority Receding Horizon Optimal (RHO) controller to improve rotorcraft handling qualities in Degraded Visual Environments. Following guidelines embodied in the military rotorcraft specifications manual ADS-33E-PRF, an attitude-command attitude-hold (ACAH) response type controller is implemented in the pitch and roll axes, rate-command direction-hold (RCDH) in the yaw axis, and rate-command height-hold (RCHH) in the vertical axis. To demonstrate the controller's performance, simulation studies using an EH-60L rotorcraft are conducted for a number of maneuvers and Mission Task Elements. The maneuvers are performed with nominal and slung-load vehicle configurations to exemplify the robustness of the controller. A mathematical pilot model is included in the simulation framework which allows for preliminary pilot workload assessment and pilot-induced-oscillations analysis. Pilot workload and input power are evaluated with a novel technique using scalograms, a wavelet-based metric. The advantages of increasing the control authority setting are also considered. The baseline configuration is a 10% partial authority automatic flight control system. With this setting, investigations are performed to stress the system to assess when undesirable handling qualities occur. Metrics such as bandwidth, quickness, and agility as defined in ADS-33E-PRF are used with moderate to large amplitude inputs. The objective of this analysis is to quantify when performance and handling qualities break down for a 10% partial authority system. The same metrics and tests are then applied to a 20% partial authority controller for comparison. The results of the new RHO controller are benchmarked with the MCLAWS controller. The results show the efficacy of the RHO formulation and validate its candidacy as a desirable rotorcraft control architecture.

A recent flight test program was undertaken to evaluate the Smart Adaptive Flight Effective Cue (SAFE-Cue) pilot-vehicle system loss of control mitigation concept. Three experienced test pilots participated in the flight test evaluations that were conducted using the Calspan Learjet In-Flight Simulator. While the focus of the program was to assess the SAFE-Cue system performance in the presence of failure/damage scenarios, the pilots were also exposed to the baseline aircraft configuration several times during their evaluations. The baseline configuration was designed to have Level 1 handling qualities for the sum-ofsines tracking and bank angle capture and hold tasks that were evaluated as part of the “blind” configurations presented to each pilot. Because of a mechanical issue, only two of the three pilots conducted the roll axis evaluations. From analysis of the test data it was clearly evident that the pilots employed disparate control strategies with one using an unexpected “bang-bang” technique that resulted in a sustained pilot-induced oscillation (PIO). Based on the resulting pilot comments and ratings, the “bang-bang” pilot did not recognize that he was in a sustained PIO. In contrast, the other pilot was able to achieve desired performance with no tendency for PIO. Using a variety of analysis approaches, the two baseline configuration runs were explored in detail to assess the observed differences. Analysis results revealed that both pilots provided lead compensation, however, there were significant differences in pilot input power and effective time delay. Over the course of one baseline configuration evaluation run, the “bang-bang” pilot fixated his input power at a single frequency with a significantly higher gain and an additional 100 msec of effective delay. A new time-varying PIO metric introduced herein clearly illustrates the observed differences in pilot behavior.

Current and planned large optical telescopes use a segmented primary mirror, with the out-of-plane degrees of freedom of each segment actively controlled. The primary mirror of the ThirtyMeter Telescope (TMT) considered here is composed of 492 segments, with 1476 actuators and 2772 sensors. In addition to many more actuators and sensors than at existing telescopes, higher bandwidths are desired to partially compensate for wind-turbulence loads on the segments. Control-structure interaction (CSI) limits the achievable bandwidth of the control system. Robustness can be further limited by uncertainty in the interaction matrix that relates sensor response to segment motion. The control system robustness is analyzed here for the TMT design, but the concepts are applicable to any segmented-mirror design. The key insight is to analyze the structural interaction in a Zernike basis; rapid convergence with additional basis functions is obtained because the dynamic coupling is much stronger at low spatial-frequency than at high. This analysis approach is both computational efficient, and provides guidance for structural optimization to minimize CSI.

CCAT will be a 25-meter telescope for submillimeter wavelength astronomy located at an altitude of 5600 meters on Cerro Chajnantor in northern Chile. This paper presents an overview of the preliminary mount control design. A finite element model of the structure has been developed and is used to determine the dynamics relevant for mount control. Controller strategies are presented that are designed to meet challenging wind rejection and fast scan requirements. Conventional inner loops are used for encoder-based control. Offset requirements are satisfied using innovative command shaping with feedforward and a two-command path structure. The fast scan requirement is satisfied using a new approach based on a de-convolution filter. The de-convolution filter uses an estimate of the closed loop response obtained from test signals. Wind jitter requirements remain a challenge and additional sensors such as accelerometers and wind pressure sensors may be needed.

The Keck I and II telescopes have been operational respectively since 1990 and 1996. Operational improvements are sought to decrease the settling time in response to short moves. The structural response of the open loop system has been re-identified and the mount control design has been re-examined. Changes to the mount control compensators and command shaping architecture have been proposed in order to achieve improved response. Results from these studies are presented, both theoretical and experimental.

The Thirty Meter Telescope primary mirror is composed of 492 segments that are controlled to high precision in the presence of wind and vibration disturbances, despite the interaction with structural dynamics. The higher bandwidth and larger number of segments compared with the Keck telescopes requires greater attention to modeling to ensure success. We focus here on the development and validation of a suite of quasi-static and dynamic modeling tools required to support the design process, including robustness verification, performance estimation, and requirements flowdown. Models are used to predict the dynamic response due to wind and vibration disturbances, estimate achievable bandwidth in the presence of control-structure-interaction (CSI) and uncertainty in the interaction matrix, and simulate and analyze control algorithms and strategies, e.g. for control of focus-mode, and sensor calibration. Representative results illustrate TMT performance scaling with parameters, but the emphasis is on the modeling framework itself.

The primary mirror control system for the Thirty Meter Telescope (TMT) maintains the alignment of the 492 segments in the presence of both quasi-static (gravity and thermal) and dynamic disturbances due to unsteady wind loads. The latter results in a desired control bandwidth of 1 Hz at high spatial frequencies. The achievable bandwidth is limited by robustness to (i) uncertain telescope structural dynamics (control-structure interaction) and (ii) small perturbations in the ill-conditioned influence matrix that relates segment edge sensor response to actuator commands. Both of these effects are considered herein using models of TMT. The former is explored through multivariable sensitivity analysis on a reduced-order Zernike-basis representation of the structural dynamics. The interaction matrix ("A-matrix") uncertainty has been analyzed theoretically elsewhere, and is examined here for realistic amplitude perturbations due to segment and sensor installation errors, and gravity and thermal induced segment motion. The primary influence of A-matrix uncertainty is on the control of "focus-mode"; this is the least observable mode, measurable only through the edge-sensor (gap-dependent) sensitivity to the dihedral angle between segments. Accurately estimating focus-mode will require updating the A-matrix as a function of the measured gap. A-matrix uncertainty also results in a higher gain-margin requirement for focus-mode, and hence the A-matrix and CSI robustness need to be understood simultaneously. Based on the robustness analysis, the desired 1 Hz bandwidth is achievable in the presence of uncertainty for all except the lowest spatial-frequency response patterns of the primary mirror.

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.

Finite element models (FEMs) are being used extensively in the design of the Thirty Meter Telescope (TMT). One such use is in the design and analysis of the Primary Segment Assembly (PSA). Each PSA supports one primary mirror segment on the mirror cell, as well as three actuators, which are used to control three degrees of freedom - tip, tilt, and piston - of the mirror segment. The dynamic response of the PSA is important for two reasons: it affects the response of the mirror to fluctuating wind forces, and high-Q modes limit the bandwidth of the control loops which drive the actuators, and impact vibration transmissivity, thereby degrading image quality. We have completed a series of tests on a prototype PSA, in which the dynamic response was tested. We report on the test methods used to measure the dynamic response of the PSA alone and with candidate actuators installed, and we present comparisons between the measured response and FEM predictions. There is good agreement between FEM predictions and measured response over the frequency range within which the dynamic response is critical to control system design.

The primary mirror control system (M1CS) stabilizes the 492 segments of the Thirty Meter Telescope primary mirror in the presence of disturbances. Each Primary Segment Assembly (PSA) has three actuators and position sensors that control the piston, tip, and tilt of the mirror segment. Requirements for the PSA position controller are presented, with the main requirements being 10 Newton per micron stiffness below one Hertz, where wind is the primary disturbance. Bandwidths of the PSA position controller of about twenty Hertz, assuming a soft actuator, are needed to meet this requirement. A finite element model of the PSA was developed and used for a preliminary control design. PSA structural modes at 40, 90, and 120 impact the control design. We have studied control designs with different actuators, sensors, and structural filters in order to assess disturbance rejection properties and interactions with the PSA structural modes. The performance requirements are achieved using voice coil actuators with modal control architecture for piston, tip, and tilt. Force interactions with the underlying mirror cell are important, and we present the status of our studies of the control structure interaction effect (CSIE). A related paper presents further analysis of the CSIE and MICS global position control loop.

The TMT mount control system provides telescope pointing and tracking. Requirements include wind disturbance rejection, offsetting time and accuracy, control system robustness, and the magnitude of response at structural resonances. A finite element model of the complete telescope has been developed and the transfer functions used for the control designs are presented. Wind disturbance, encoder, and wave-front-sensor models are presented that are used for the control design. A performance analysis translates the requirements to a required bandwidth. Achieving this bandwidth is important for reducing telescope image motion due to wind-buffeting. A mount control design is presented that meets the demanding requirements by maximizing low frequency gain and using structural filters to roll-off structural modes. The control system analysis includes an outer guide loop using a wave front sensor. Offsetting time and accuracy requirements are satisfied using feed-forward control architecture.

The Thirty Meter Telescope (TMT) project(double dagger) has revised the reference optical configuration from an Aplanatic Gregorian to a Ritchey-Chretien design. This paper describes the revised telescope structural design and outlines the design methodology for achieving the dynamic performance requirements derived from the image jitter error budget. The usage of transfer function tools which incorporate the telescope structure system dynamic characteristics and the control system properties is described along with the optimization process for the integrated system. Progress on the structural design for seismic considerations is presented. Moreover, mechanical design progress on the mount control system hardware such as the hydrostatic bearings and drive motors, cable wraps and safety system hardware such as brakes and absorbers are also presented.

The largest optical telescopes today use a segmented primary mirror, with the outof-plane position of each segment actively controlled. The segments are supported by a flexible structure that introduces dynamic coupling. This coupling leads to controlstructure-interaction (CSI), which limits the achievable bandwidth of the primary mirror control system. As telescopes increase in size, the number of mirror segments increase, and the potential for CSI increases. The dynamics can be approximated by n identical oscillators coupled through the support structure. These dynamics are explored herein. First, in the special case where the support-structure modes provide an orthonormal basis for the oscillator dynamics, the problem can be transformed into n separate coupled oscillator problems. The application to more realistic support structures is then investigated using the dynamic model of the Thirty Meter Telescope. The achievable bandwidth is estimated by projecting the dynamics onto Zernike basis functions, with higher bandwidth possible for higher spatial frequency.

The primary mirror control system (M1CS) keeps the 492 segments of the Thirty Meter Telescope primary mirror aligned in the presence of disturbances. A global position control loop uses feedback from inter-segment edge sensors to three actuators behind each segment that control segment piston, tip and tilt. If soft force actuators are used (e.g. voice-coil), then in addition to the global position loop there will be a local servo loop to provide stiffness. While the M1 control system at Keck compensates only for slow disturbances such as gravity and thermal variations, the M1CS for TMT will need to provide some compensation for higher frequency wind disturbances in order to meet stringent error budget targets. An analysis of expected high-wavenumber wind forces on M1 suggests that a 1Hz control bandwidth is required for the global feedback of segment edge-sensorbased position information in order to minimize high spatial frequency segment response for both seeing-limited and adaptive optics performance. A much higher bandwidth is required from the local servo loop to provide adequate stiffness to wind or acoustic disturbances. A related paper presents the control designs for the local actuator servo loops. The disturbance rejection requirements would not be difficult to achieve for a single segment, but the structural coupling between segments mounted on a flexible mirror cell results in controlstructure interaction (CSI) that limits the achievable bandwidth. Using a combination of simplified modeling to build intuition and the full telescope finite element model for verification, we present designs and analysis for both the local servo loop and global loop demonstrating sufficient bandwidth and resulting wind-disturbance rejection despite the presence of CSI.