EtherCAT (Ethernet for Control Automation Technology) is gaining wide spread popularity in the automation
industry as a real time field bus based on low cost, Ethernet hardware. EtherCAT maximizes use of 100Mbps Ethernet
hardware by using a collision free ring topology, efficient Ethernet frame utilization (> 95%), and data exchange "on the
fly". These characteristics enable EtherCAT to achieve Master to Slave node data exchange rates of > 1000 Hz.
The Green Bank Telescope, commissioned in 2000, utilizes an analog control system for motion control of 8
elevation and 16 azimuth motors. This architecture, while sufficient for observations at frequencies up to 50GHz, has
significant limitations for the current scientific goals of observing at 115GHz. Accordingly, the Green Bank staff has
embarked on a servo upgrade project to develop a digital servo system which accommodates development and
implementation of advanced control algorithms.
This paper describes how the new control system requirements, use of existing infrastructure and budget
constraints led us to define a distributed motion control architecture where EtherCAT real-time Ethernet was selected as
the communication bus.
Finally, design details are provided that describe how NRAO developed a custom EtherCAT-enabled motor
controller interface for the GBT's legacy motor drives in order to provide technical benefits and flexibility not available
in commercial products.