HCAL Background

Hand-held calibration of accelerometers is a very simple and straightforward method. A primary advantage of hand-held calibration is that it can perform an end-to-end calibration of the transducer and input channel of the data acquisition system. A hand-held calibrator is a small, portable, battery- powered device. The system consists of an electromagnetic exciter driven by a oscillator at a constant frequency, (typically 79.6 or 159.0 Hertz, nominal). An accelerometer is mounted on top of the calibrator and a built-in reference accelerometer controls a servo-feedback loop to maintain a constant reference level (typically 1 g rms or 10 m/s2 rms, nominal) for calibration. Please click on Thumbnail to enlarge.

RCAL Background

Static calibration of the force sensor of an impactor is not satisfactory because the sensitivity is dependent on several factors, including the type of tip and the mass of the impactor. The recommended method of calibrating transducers for impact testing is ratio calibration [Halvorsen and Brown, Sound and Vibration, Nov. 1977]. With this method, the frequency response of a large inertial mass is measured by mounting an accelerometer on one side of it and impacting on the other side. The calibration FRF can be used as a calibration curve or a single, averaged value can be determined for a given frequency range as a calibration value.

When measuring frequency response functions with impact testing, the absolute calibration values of the transducers are not necessary, but it is convenient to calibrate the ratio of the force sensor and accelerometer. In this case, the calibration value of the force sensor is typically set to unity, and a number of accelerometers are calibrated relative to the impactor. To determine the absolute calibration of the impactor, an accelerometer that has been calibrated by some other method serves as the reference transducer.  Please click on Thumbnail to enlarge.

DCAL Background

Drop calibration, which is also known as free-fall calibration, is a very basic and natural method of calibrating accelerometers and force sensors. It is based on a predictable and repeatable motion, that a body in free-fall experiences a constant gravitational acceleration, and by measuring the step response of the transducer to the one-g input, its sensitivity is determined. An actual drop calibration response differs from the ideal free-fall step response in several aspects, and the measured data is curve-fit to determine the sensitivity. Drop calibration is an absolute calibration method and does not require a reference standard because the results are referenced to the local  gravitational constant. Please click on Thumbnail to enlarge.

Roving Response Impact Testing is a modal data acquisition method in which frequency response functions are measured between a fixed impactor and one or more roving response transducers. The basic procedure is that the test system is repeatedly impacted at an input position while the response transducers are roved about the measurement positions. RRIT is a single reference technique, but to collect multiple reference data with RRIT, the procedure is repeated for other input positions. Alternatively, the impacts could be made at all input positions before relocating the response transducers. Since only one input is applied at a time, a multiple-input FRF estimator is not required. The RRIT method is a convenient and practical technique that is very well suited for troubleshooting vibration problems and field testing. Additional information on impact testing procedures are included in the relevant sections of this manual. Please click on Thumbnail to enlarge.

MRIT Background

Multiple Reference Impact Testing (MRIT) does not imply impacting a test structure with more than one hammer. Instead, the outputs are taken as the references, rather than the input. MRIT is a modal data acquisition method in which frequency response functions (FRF) are measured between a single, roving impactor and multiple, fixed response transducers. Possible reference transducers are accelerometers, microphones, and proximity probes. A set of multiple reference FRFs is generated by applying the reciprocity relationships to determine the response at the impact locations. The set of measurements can then be treated as multiple input data and processed with multiple reference modal parameter estimation software.  Please click on Thumbnail to enlarge.