The development of high-precision hexapod actuators for the Hobby-Eberly Telescope wide field upgrade

Abstract

Hexapods are finding increased use in telescope applications for positioning large payloads. Engineers from The University of Texas at Austin have been working with engineers from ADS International to develop large, high force, highly precise and controllable hexapod actuators for use on the Wide Field Upgrade (WFU) as part of the Hobby Eberly Telescope Dark Energy Experiment (HETDEX) ‡ . These actuators are installed in a hexapod arrangement, supporting the 3000+ kg instrument payload which includes the Wide Field Corrector (WFC), support structure, and other optical/electronic components. In addition to force capability, the actuators need to meet the tracking speed (pointing) requirements for accuracy and the slewing speed (rewind) requirements, allowing as many observations in one night as possible. The hexapod actuator stroke (retraction and extension) was very closely monitored during the design phase to make sure all of the science requirements could be met, while minimizing the risk of damaging the WFC optical hardware in the unlikely event of a hexapod actuator or controller failure. This paper discusses the design trade-offs between stiffness, safety, back-drivability, accuracy, and leading to selection of the motor, high ratio worm gear, roller screw, coupling, end mounts, and other key components.

Thermal morphing anisogrid smart space structures: Part 1. Introduction, modeling, and performance of the novel smart structural application

“…As the six legs are actuated, the two surfaces’ relative position and angle can be controlled in all six DOF. A recent example of the hexapod technology can be seen in the implementation of the hexapod system in telescopes (Yang et al., 2015), specifically the Hobby–Eberly telescope (Wedeking et al., 2010; Zierer et al., 2010, 2012). The thermal morphing hexapod, discussed elsewhere by the authors (Phoenix and Tarazaga, 2017), introduces rotational constraints in conjunction with thermal strain actuation to improve the minimum morphing capability.…”

An alternative architecture and control strategy for hexapod positioning systems to simplify structural design and improve accuracy

“…Conventional approaches for controlling hexapod systems exploit actuator designs with imbedded/collocated sensors to determine actuator length (for example, see [1]. The sensed length is intended to be close to directly along the line of action of the actuator.…”

“…The hexapod PID controller outputs fully graduated current commands to the servo-amplifiers. Unless the actuators specifically employ stepper motors (uncommon for high force actuators), actuator output is ultimately achieved via current control through the servoamp, which is the equivalent of torque control on the motor and effectively force control on the actuator (for example, see [1]). …”