Crystal Instruments Receives US Patent for Kurtosis Control

Tim Hsiao (left) and Crystal Instruments president James Zhuge with plaque for patent no. 8,942,930

The United States Patent and Trademark Office awarded U.S. Patent No. 8,942,930 System And Method For Generation And Control Of Mechanical Vibration to Andrew P. Snyder and Tim Hsiao on January 27, 2015. Andrew worked for Crystal Instruments as a DSP Engineer at the time of filing; he is now a part of Elon Musk’s SpaceX team in Hawthorne, CA. Tim is a Product Manager with Crystal Instruments and the patent is assigned to that Santa Clara, CA firm.

This patent protects an important contribution to CI’s Vibration Control System (VCS) business, its unique means of controlling the kurtosis of a random vibration signal. A conventional (Gaussian) random signal has a peak-to-rms ratio (the crest factor) of about 3. In contrast, a high kurtosis random signal of the same RMS intensity, with identical spectrum shape, can have a significantly higher crest factor. A high kurtosis signal is useful to precipatate DUT failures in minimum test time. It is also necessary to meet accelerated damage profiles on the horizon for random testing professionals.

Controlling the kurtosis actually requires a separate control loop. The kurtosis of the Control acceleration is continuously measured and the kurtosis of the Drive signal is continuously changed to achieve the desired Control kurtosis. This happens in parallel and conjunction with the primary loop that controls the spectral content of the same acceleration. The real trick in kurtosis control is to avoid introducing spectral distortion and associated nonlinearity of the spectrum shaping control loop; this can reduce the controller’s dynamic range. The new Crystal Instruments algorithm allows operating at any kurtosis value between 3 and 7 with no loss of control dynamic range.

According to Dr. James Zhuge, Crystal Instruments President: “Andrew and Tim invented an efficient kurtosis control methodology that integrated very naturally with our existing random controller design. In essence they had only to change how the phase spectrum of the drive signal was computed to achieve their objective. This clever modification to our proven random spectrum controller required very little additional processing resources. It’s a fine addition that leaves our expansion options open.”