Low-level control software for the WEAVE spectrograph

Abstract

WEAVE is a wide-field spectroscopy facility for WHT which includes a multi-object dual-beam spectrograph which will operate in the visible wavelength range. The blue beam will cover the range 360-600 nm and the red arm will cover the 580-960 nm range. In these ranges the spectrograph will offer a mid-resolution (~5000), while in three narrower wavelength intervals, two for the blue arm and one for the red one, the instrument will provide a high (~20000) spectrograph resolution. The spectrograph is currently entering the assembly and integration phase and the first light is foreseen in 2019. The entire WEAVE project is managed by an international consortium led by the University of Oxford. The spectrograph is controlled by a coordination process, the so called High-Level Server, which is part of the Observatory Control System (OCS) software suite, and is the single point of access to the embedded control system, the so called Low-Level Control Software, which is based on PAC technology. This paper describes the design of the embedded software for the control of the spectrograph mechanisms. We first describe the interface between high and low level software, then we present the PAC architecture and discuss the low-level state machine. Finally, we provide details on the principal program routines and describe the engineering interface.

The WEAVE observatory control system

“…• The Spectrograph Control system: this subsystem is divided into two layers: the low-level software in the embedded control system (PAC) 4 , the high-level control and feedback coordination server that resides in the WEAVE OCS.…”

“…All status and control requests are routed through a centralized server on the OCS (the spectrograph high level server) which has the responsibility of coordinating the control of the spectrograph embedded control system 4 , which is based on PAC technology. This is implemented in a manner that is congruent with the techniques that have been utilized with the successful PAC conversion of various WHT facility instruments, using RSLogix 5000 and Allen Bradley ControlLogix processors.…”

“…This control system will be responsible for performing the actions requested under the constraints of reliability and time. It will also be responsible for updating the internal data structures 4 with an accurate snapshot of the instrument state. In practice, it will not be aware of which external component is requesting a demand (in fact, the Engineering Interface and the high-level software will use the same method).…”

“…The Autoguider System for the LIFU will image the sky directly and perform the calculations necessary to provide the guiding corrections to the Telescope Control System. • The Prime Focus Corrector system: this subsystem is embedded in an Allen-Bradley PAC and it is responsible for controlling the WEAVE rotator, telescope focus 4 and the ADC unit. It is interfaced to the TCS using the standard PLCIO library.…”