A digital signal processing algorithm for laser particle size and velocity measurement

Size and velocity measurements of particles in a flow field are often required in industrial applications. The signal produced by a nonimaging laser optical measurement system contains the information to obtain particle size and velocity. However, the signal contains noise generated primarily by the photomultiplier tube in the measurement system. A method for processing the digitized electrical signal to reduce the noise in the signal and to extract particle size and velocity is developed. The effectiveness of this processing algorithm is determined with simulated signals with pulses of known width and height. The signals correspond to three particle sizes ranging from the smallest to the largest size of interest. For each size, 100 simulated waveforms are generated using a Monte Carlo simulation of the photodetector. With simple averaging, the processed waveform contains too much noise to reliably yield pulse data without bias or errors. Therefore, a ninth-order Finite Impulse Response filter with specified frequency characteristics is implemented to smooth the raw data. This gives good pulse height measurements and unbiased but imprecise pulse width measurements. Individual baseline calculations and linear interpolation of the pulse endpoints are added to the algorithm to improve the precision of the pulse width measurements. A parabolic interpolation of the pulse minimum did not improve the pulse height precision or accuracy appreciably.