Optical Accelerometer Calibration References

Laser Primary and Encoder Low Frequency

Optical techniques are becoming more and more commonplace in the sensing industry. We have observed that they also provide some extraordinary benefits when used as references for accelerometer calibration. The Modal Shop and PCB have developed two new optical methods/systems: one for extending low frequency calibration capabilities and another for further simplifying and expediting laser primary calibration.

For the very low frequency range (one quarter of a hertz to a few hertz) there is now a patent pending method of low frequency dynamic calibration via a displacement based optical encoder. The encoder base reference provides near laser primary accuracies. It is extremely unusual to have improved accuracies at these very low frequencies because of the extremely low acceleration levels at low frequencies due to stroke limitation of calibration exciters. The inherent benefit of employing an optical displacement reference is that its noise floor is flat with respect to stroke length. Hence, for a given shaker stroke, the optical response is flat to DC rather than exhibiting the 1/f type rising noise floor that is typical of a piezoelectric accelerometer. When referencing the encoder the practical low frequency limitation becomes the function noise floor of the sensor under test rather than stroke of the shaker or the noise floor of a reference accelerometer. Practically, this allows calibration professionals to now use the same high accuracy, low transverse motion, mid stroke length air-bearing calibration exciter to calibrate low frequency high sensitivity seismic accelerometers. This method can eliminate the need, cost and trouble of second long stroke exciter. In many cases one calibration shaker is enough now! Due to the twist that this method provides dual benefits of improved uncertainties at low frequencies AND simplifies/streamlines the calibration process by using the same exciter for both regular and low frequency tests, we anticipate that this innovation will serve the needs of both national laboratories and end users.

A second development we have been sharing with the calibration community is a dual beam laser primary accelerometer calibration system. A laser primary represents the ultimate in accelerometer calibration accuracy and, as a result, often compromises calibration throughput and ease of use for improved accuracies. In this new system, the system includes dual laser heads as standard. Coupled with the beam splitting capability of each head, this system now simultaneously measures up to four points around the sensor under test (SUT) which reduces overall calibration time ½ or ¼ when compared to the conventional means of making separate measurements around the SUT with a single point laser. The technique also conforms to the ISO 16063-11 standard for accelerometer calibration via laser methods.