A simple method for real-time DSP-based neutron chopper speed and phase control

Spallation-neutron facilities often use precisely-phased rotating choppers to parse the neutron beams that are used in various experiments. Typically, the reference signal is a digital pulse train of nearly-constant frequency. A chopper provides phase and frequency information to a controller by outputting a digital pulse each time its rotating blade passes a top-dead-center (TDC) location. Early controller designs used analog techniques to measure speed and phase and perform the necessary closed-loop control. More recently, purely digital techniques using digital signal processors (DSPs) and counter-based phase detectors have been implemented. This paper describes a relatively simple and low-cost method of implementing chopper speed and phase control with an off-the-shelf DSP card. In this technique, the reference and chopper-TDC digital-pulse trains are slew-rate limited to form trapezoidal waveforms which are sampled by 16-bit ADCs on the DSP board. The DSP finds the zero crossings, computes the frequency and relative phase of the two input signals and performs the dual-loop feedback control functions. A 16-bit DAC outputs a control voltage to a chopper-motor power driver. The paper also describes the rate-limiting circuitry, the real-time dual-loop control algorithm, and system performance