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[lise] Informatics
Automation of the gondola’s positioning system

The gondola that holds the beam-combining optics is suspended on a North-South oriented cable stretched between both slopes of the Moutière Valley. Located 100 meters above the ground on the focal sphere of the hypertelescope, this gondola is maneuvered by two winches with which it can be pointed towards the observed object. Its position can be gently corrected during the observation period to follow the East-West apparent motion of the object. The positioning of the gondola also allows orienting the coudé beam towards the telescope on the ground.
Gondola suspended in the sky ©MR
Each winch has one or two step-motors controlled by a Snowball (Linux Micro PC) that communicates on a local TCP/IP network. Batteries charged with a solar panel power these motors.
West Motor ©MR
Interior of the command cabinet ©MR
Solar Pannel ©MR
Installed near the 20 cm telescope – which observes the focal plane of the hypertelescope – the control station communicates via WIFI with the winch motors, located in the East and West points and 300 m away from each other. Software developed specifically for this purpose adjusts each second the driving speed of the East and West winches. This software – named "motor-speed-carlina" – takes into account both the position in the sky of the observed object and the specific geometry of the system, i.e. the mirrors on the ground, the suspended focal gondola, the observation telescope on the South point. It runs on a Windows laptop and communicates by TCP (Transmission Control Protocol) with the Snowball cards at the East and West winches. The operator monitors by eye or with the help from a camera the position of the gondola at the 20 cm telescope located on the South point. The diurnal tracking of the telescope allows properly controlling the motion of the gondola, that is, the gondola position must not change with respect to the field of view of the south telescope. Therefore, the operator can make small corrections using keyboard commands to adjust the gondola motion.
R.P. et P.N. test the Software ©AL
P.N. tests the gondola’s control system ©AL
Local testing of the motor ©PN
The distance between the telescope and the gondola – which must be kept constant to the millimeter – is measured with a distance meter. The adjustment of this distance, manually controlled, will be automated. 

The execution of the successive operation is launched by "command line" with the keyboard. A graphical interface simplifying this procedure is under development.

Technical details:
  • The Snowball electronics were acquired from Calao Systems at Grenoble. Equivalent to a Galaxy Samsung, dual core processor, 1Go Ram, 1 GHz smartphone, they use the Ubuntu Linux operating system. They have a high power CPU (Central Processing Unit) that can be easily programmed in Linux.
  • Developed by LISE team, the motor-speed-carlina software includes a set of modules communicating with each other via TCP messages on a "client-server" mode. These messages consist of ASCII chains, easily used in all programming languages (we programmed them in C++, Common Lisp for testing). They can be executed on Linux or Windows 7. Each message corresponds to one specific function and contains all the information needed for its execution. They are easily testable since these messages are not connected with each other ("stateless").
  • A basic DS in C++ and Common Lisp library allows quickly enhancing the application of new components. If we have a simple component, its inclusion can be done in one day. The structure of the Carlina application in independent components allows a collective development. Several computers are equipped with development tools necessary for the creation of the software components. The synchronization of the "sources" uses the SVN (SubVersion) management software.
  • The East side winch, unlike the West one, is double. Both motors are mounted head to tail and controlled with a single Snowball card. One of the two cables ensures the gondola tracking of the apparent motion of the object. The other cable allows, through a light differential, correcting the inclination of the optical axis of the gondola in the East-West plane as the object moves in the sky. The position correction of the optical axis of the gondola on the North-South plane and the rotation of this axis around the vertical axis are manually controlled at the southern observation point.

    East Point ©MR
    WIFI Antenna - South Point ©MR
    The Snowball card ©MR