Control characteristics of the ALMA Nutator

The control of a 75 centimeter diameter Atacama Large Millimeter/submillimeter Array (ALMA) nutating subreflector, or Nutator, to meet the specifications of 10 millisecond (ms) settle time and 1.3 arc-second pointing error involves tremendous challenges. The ALMA Nutator has been under development since 2003, and we have improved its dynamic characteristics through robust PID control algorithm, increasing the open loop bandwidth to beyond 200Hz and shortening the settle time to 20 ms. A mode switching associate with model following control scheme is also presented, and expected to improve the settle time and pointing error.