CANopen in the Grantecan position sensing

Source: CAN Newsletter June 2008

B. Lefort and M. Pi Puig (Grantecan Astrophysical Institute of Canary Islands)

The Gran Telescopio Canarias (GTC) is the largest ground telescope ever built. The segmented primary mirror has a collecting surface equivalent to one circular mirror of 10,4 m. The control system (GCS) includes the hardware and software to control all the telescope subsystems, from the real-time applications to the astronomer interfaces. Most of the GCS subsystems use CANopen as the main control bus. The main CANopen application is the control of the primary mirror, which is composed of 36 hexagonal segments. The GCS is responsible for making these segments behave like a monolithic mirror. Three positioning units per segment (total 108) will move it in tip/tilt and piston. Two position sensors (total 168) between adjacent segments will measure their relative displacement. This loop controls the positioning units and reads the sensors at 200 Hz over CANopen networks. The figure of each segment is also actively controlled thanks to six actuators (total 216) that act on three whiffletrees attached to the back of a segment. This actuation is made at very low frequency. Finally, the temperature of each segment is measured in six points (total 216). Specific CANopen profiles where developed to control the positioning units and to read the edge sensors. CANopen networks are also used to control and monitor several other subsystems of the GTC: tertiary mirror, dynamic counterweights, optics and filter wheels, temperature and humidity control loops, start up and stop systems, calibration lamps, etc.