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Servo Replacement Specification Wiki

Table of Contents

1 Introduction

1.1 Purpose of this Document

This specification document will outline the specifications for the replacement of the servo control systems on the Green Bank Telescope (GBT). This effort is being undertaken as part of a larger set of enhancements to the GBT under the Precision Telescope Control System project (PTCS). PTCS project work includes a complete modeling to characterize the existing GBT structure, updating the servo system, improving the GBT control systems, implementing advanced measurement techniques, researching advanced instrumentation, and implementing improvements to the active surface.

The goal of the servo replacement project is to give the GBT a level of performance markedly better than current performance. This will allow both an increase in the environmental operational limits of the telescope yielding increased observation time and an improvement and extension of the telescope’s high frequency capabilities.

As in any project of such wide breadth and complexity it is a challenge to identify a definitive differentiation between requirements and design. The specification team has endeavored to keep the requirements general enough not to dictate a design except when external influences (capabilities, cost, scope) indicated only one design could be implemented.

1.2 System Scope

The Servo Replacement Project (SRP) will create a new overlay on the existing hardware systems of motors, brakes, sensors, and interlocks to improve the tracking, slewing, scanning, and stability of the GBT. The existing drive hardware infrastructure will remain in place and is generally capable of operating at the higher levels of performance or too costly to replace and is described in the constraints section of this document.

The SRP will take as its input, the existing capabilities of the telescope drive infrastructure, current operating policies and procedures, any additional environmental or sensor data from the measurement projects, and the structural characteristics derived during the system modeling project.

Generally the SRP deliverables will include replacement of the software and computers that control the telescope drive/brake system, replacement of any sensors lacking sufficient resolution for the new performance requirements, add sensors where they are needed; the integration of sensor data into the control software, and updated user interfaces. Upon completion of the SRP along with the balance of PTCS, a comprehensive characterization of the GBT performance will be documented and worked into an updated pointing model.

Work will be accomplished in phases to allow for ongoing analysis of the progress toward the PTCS project goals. Phasing also allows for better utilization of resources via parallel work activities, and represents a significant component of the risk mitigation strategy. There is a strong desire by NRAO management and the project team to identify any ‘early deliverables’ that can improve the telescope’s performance for astronomers before the complete SRP and PTCS projects are completed

1.3 Definitions, Acronyms and Abbreviations

1.4 References

1.5 System Overview

The GBT is a 100 by 110 meter telescope. The overall structure of the GBT is a wheel-and-track design that allows the telescope to view the entire sky above 5 degrees elevation. The track, 64 m (210 ft) in diameter, is level to within a few thousandths of an inch in order to provide precise pointing of the structure while bearing 7300 metric tons (16,000,000 pounds) of moving weight. The azimuth drive is accomplished by 16 motors on the drive wheels, or trucks. Elevation movement is via a bull gear driven by 8 motors. It is the precision drive of the azimuth and elevation controllers, brake control, and system interlock control by the servo system that allows the GBT to safely accomplish its observational goals of tracking and scanning.

The GBT is of an unusual design. Unlike conventional telescopes, which have a series of supports in the middle of the surface, the GBT's aperture is unblocked so that incoming radiation meets the surface directly. This increases the useful area of the telescope and eliminates reflection and diffraction that ordinarily complicate a telescope's pattern of response. To accommodate this, an off-axis feed arm cradles the dish, projecting upward at one edge, and the telescope surface is asymmetrical. To adjust the focus of the telescope and compensate for changes in the telescope’s geometry as it moves, secondary optics consisting of a sub-reflector with full X,Y,&Z adjustability is also controlled by the servo system. In addition a prime-focus feed exists that can be deployed, but not concurrently, with the sub-reflector.

The GBT's lack of circular symmetry greatly increases the complexity of its design and construction. The GBT is also unusual in that the 2,004 panels that make up its surface are mounted at their corners on actuators, little motor-driven pistons, which make it easier to adjust the surface shape. Such adjustment is crucial to the high-frequency performance of the GBT in which an accurate surface figure must be maintained. Improvements to the active surface, controlled by advanced instrumentation and measurements, are part of the PTCS project but not part of SRP.

The sheer mass of the GBT requires a sophisticated series of structural protections and interlocks; beyond those required for operator safety. These protections have been augmented through the operational life of the GBT and combined with similar activities on other NRAO Green Bank telescopes are well understood.

The servo electronics and controller interfaces are divided between two locations on the moving structure. The azimuth and elevation servos are housed in a servo room located on the base of the structure while the secondary optics servos are located in the receiver room near the top of the feed arm. There are both computer and manual controls for the servos in the respective servo rooms on the GBT, but the vast majority of the time engineering control and all astronomical control is performed from the GBT Control Room in the Jansky Laboratory, 3 Km away and interconnected via a secure, isolated LAN. The majority of the servo control software will reside in one, or both, of the on-telescope servo rooms and the Control Room will access the applications over the LAN directly or via the astronomical control software. There will be an OCU in the servo room, a remote OCU in the control room, and portable OCUs as necessary.

2 General System Description

3 System Capabilities, Conditions and Constraints

4 System Interfaces

5 User Documentation and Training Requirements

6 Other Requirements

7 Appendices

Section 2

Attachment: sort Action: Size: Date: Who: Comment:
Servospecmap.pdf action 37016 06 Feb 2008 - 17:53 MartyBloss  
servoCabinet.jpg action 202119 28 Jul 2008 - 20:54 ToddHunter photo of servo boards in servo room (June 2006)

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