A programmable DSP-based neutron-chopper simulator

Neutron choppers at spallation-neutron facilities are rotating devices that are precisely phased to chop a neutron beam. Depending on the mass, moment-of-inertia, and velocity of rotation, a chopper can have a large amount of stored kinetic energy. For example, the PHAROS T-zero chopper at the LANSCE facility at Los Alamos National Laboratory has a rotating kinetic energy of over 1 MJ. When energy levels are this high and for other logistical reasons, it is often much more practical to develop and test chopper speed and phase controllers using a hardware simulator than an actual chopper. This paper describes the design and performance of a programmable DSP-based chopper simulator developed and used at LANSCE. Chopper models are first developed using Simulink and then coded for uploading to the DSP. Several chopper models have been developed all of which include moment-of-inertia, losses, motor parameters, and rotating speed. The simulator receives, as input, an analog control voltage and outputs a digital top-dead-center (TDC) pulse train