| Popis: |
For nearly 50 years, P/E accelerometers have been used for acquiring pyroshock data with mixed results. For longer distances between the explosive source and the transducer location (e.g., two feet or more), valid data of lesser shock magnitude were usually obtained. However, for shorter distances, a variety of problems were often encountered, causing erroneous results. It was subsequentially determined that most problems were caused by measurement system nonlinearities, i.e., the nonlinear resonant response of the accelerometer, or exceeding the linear amplitude range of the signal conditioner and recorder. In the earlier years, it was erroneously assumed that subsequent low pass filtering of the signal would remove the nonlinearities, hopefully leading to valid data. This only masked the invalid results. Eventually, improved P/E accelerometers were developed with higher natural frequencies and larger amplitude limits that caused substantially fewer problems and allowed measurements closer to the explosive sources. Shortly thereafter, the high frequency noncontact laser doppler vibrometer became available which circumvented the accelerometer resonance problem. However, this velocity transducer is almost always limited to laboratory tests in order to constrain the motion of the laser head by a very rigid and massive support foundation compared to the flexible structure which is attached to the laser target. Other LDV measurement problems have been encountered that must be avoided to achieve valid data. Conventional strain gages have been successfully used to measure pyroshock strain. However, due to the short wavelength of direct and bending pyroshock waves at high frequencies, small strain gages are usually required to avoid spatial averaging over the length of the gage. |
