Design and performance of a DSP based neutron chopper phase controller

The timing of the production of neutrons at the Los Alamos Neutron Science Center (LANSCE) is tightly coupled to the instantaneous phase of the local commercial power grid. Neutron choppers require active control in order to follow the phase of the grid. A digital signal processor (DSP) based control system has been developed for active phase control of low speed choppers with large moments of inertia. Two of these systems are presently in use and a third system, now being tested, is able to maintain rotor phase control within 2.6 μs (sigma) of the coupled power grid. The amount of time of flight data that must be discarded due to chopper phase errors is typically less than 1 percent. The current system is in the process of being redesigned as a VME module running under the EPICS control software. The design and performance of the phase controller are discussed