GBT Active Surface Software

The main input to the system is a command vector of desired actuator lengths for each of the 2209 actuators. The main outputs are the lengths of the actuators. Commands are transmitted to the active surface system via an Ethernet connection, using a Remote Procedure Call (RPC) protocol. For convenience, the command vectors are ordered per the rib and hoop topology of the telescope backup structure, where the actuators are physically located. The Active Surface Master Computer receives the commands and processes them in several ways. Initially the commands undergo a sanity check. Following this, the "ideal" commands are turned into "real" commands by compensating for the temperature, gain, and offset characteristics of the actuators. The commands are then permuted to account for the mapping of telescope ribs and hoops to control system addresses.

Again using an RPC protocol, "real" position commands are transmitted to the Slave Computers, whose main task is to close the position loops. To control the actuators, the Slave Computers communicate with Intelligent I/O Processors (IIOPs), which basically interface the VME computer bus to proprietary serial busses. In turn, the serial busses communicate with two types of control modules, the LVDT Module, and the H-drive Module. An LVDT module functions as a readout for sixteen Linear Variable Differential Transformers, or LVDTs, (the actuator position transducers), while an H-Drive module functions as a motor driver for sixteen DC Brush motors (the actuator motors). Thus, a pair of modules (one LVDT Module and one H-Drive module) contains the control electronics for sixteen actuators. The system requires 139 pairs of modules to control the entire complement of actuators.

The system runs under the multitasking real-time operating system VxWorks (distributed by Wind River Systems) which allows the use of remote consoles. The software includes several diagnostic functions which are callable from the consoles, permitting troubleshooting both at the master and slave levels. Heavily integrated into the telescope's monitor and control system, the software makes use of the Message System to communicate warnings and faults to the operator, and of the logging system, to maintain a record of running time for each actuator. Several monitor tasks are included to measure the health of the system, both at start-up and during operation, and take appropriate action upon finding anomalies. The main monitoring task which runs on the master computer is called the "Master Monitor" and oversees all of the hardware continually checking for fault conditions. A graphical monitoring screen permits rapid evaluation of the status of the system. It shows which actuators are at their requested positions, which are in transition to their requested positions, and which are ignoring commands by using an array of color coded dots on a window. The dots are arranged to mimic the rib/hoop locations of the actuators on the physical structure. Additional capabilities can be gained by clicking on any of the dots. These include a detailed status and an ability to issue an individual position command. The rib/hoop to control-system address is dynamically displayed as the mouse is dragged across the array of dots. The higher level software, which decides which actuators to move and by how much is not part of this system.

*taken from GBT Memo 184 by Richard J. Lacasse