ICP Reference Sensors in Accelerometer Calibration

Exhibiting proven performance in extreme environments, ICP® operation of piezoelectric accelerometers provides excellent reference and stability over time.  The ICP mode of operation excels in operator diverse, low signal level, long cable run, and uncontrolled environments. When one considers the myriad of ICP operation benefits for general vibration measurement use, it becomes evident that these benefits minimize uncertainties in the critical competence of vibration calibration.  The following article outlines the major features that have propelled ICP vibration references to the heart of most modern vibration calibration systems. 

First and foremost since the introduction of the ICP concept in the late 1960’s, the simplified low impedance method of operation provides for a robust signal path impervious to the common contributors of noise, thus improving uncertainty.  This has the effect of minimizing and maintaining one key component of the equipment electrical uncertainties.  Specifically, containing the high impedance signal path within the ruggedized and hermetically sealed sensor housing to reduce potential environmental errors, such as insulation resistance through dirt/oil/chips creating a path to ground or introduction of triboelectric noise through cable whip. 

An additional benefit of low impedance ICP operation is the potential to use ultra-thin unshielded cable to reduce the mass compared to bulky, stiff low noise cable which can greatly influence connector resonances at high frequencies, increase the transverse rocking motion of the shaker armature at mid-frequencies and cause cable induced case strains at low frequencies.

Another process related benefit of simplified operation can be seen in the allowance of more time for the calibration system operator to focus on the few, but critical mounting steps important to any type of accelerometer calibration.  The operator can concentrate to ensure that mating mounting surfaces are clean/flat, the interface is lightly lubricated with silicone grease, consistent mounting torque is applied and that proper cable strain relief is employed.  Proper attention and consistency applied to these key steps further reduces a random error component.  The simplified operation offers a more rapid learning curve and an easier to use process.  This results in fewer gross errors and less time loss due to manual settings and selections.

Ultimately, these benefits combine to form simplicity, ease of use, faster cycle times and fewer errors, thereby lowering the lifetime cost of ownership.  ICP reference accelerometers lower the base cost of a vibration calibration system by eliminating a costly charge amplifier on the reference channel.  Additionally, at the component level, cost of ownership is further reduced by lower cost signal cabling that avoids more expensive low noise cable required for charge mode references.

A final benefit in health and identification can be found in ICP operation. A simple bias monitoring check (either by meter or LED) for channel/cable integrity checks the signal path further eliminating gross errors of shorts/open circuits and cuts troubleshooting time during setup of sensor calibration. With the standardization of TEDS, ICP accelerometers can now provide self-identification, store calibration and further reduce human errors.

In the end, the de-facto standardization on ICP has covered most of the world’s keys laboratories.  While charge mode accelerometer reference sensors served the metrology community accurately with proper training and consideration, ICP reference accelerometers have now been vetted for nearly a half century and have proven a legion of benefits in simplicity and reliability.  ICP references are now the preferred dynamic reference standard sensors for vibration and shock calibration systems.