Accelerometer Calibration and Engine Vibration Monitoring

So you think your work environment is tough? You have multiple projects, competing priorities, shifting deadlines and drastically limited resources. Well at least you have a padded chair, nearby coffee and an office/lab. Now imagine the “work environment” of an aircraft engine monitoring accelerometer…the environment can be: hot to 1200 deg F (650 deg C), wet to 100% humidity condensing, violent seeing shocks up to 1000 g and precarious often on cantilevered mounting brackets and snaking in a cable around components and through passages to the signal conditioning and/or engine monitoring unit (EMU). Sounds like an environment worse than the biggest earthquake, the hottest desert and the wettest rainforest.

Due to this extreme environment, the design of an engine monitoring accelerometer has a number of compromising trade-offs. Foremost, to withstand the extremely hot environment, in addition to requiring a specialized sensing element piezoelectric crystal material, the basic electrical operation is “charge mode.” In this case, the electric signal generated by the accelerometer sensing element is carried a significant distance (often a meter or more) in its high impedance state. Due to environment, the cabling is extremely heavily armored with braided wire sheathing in most areas and encased in hardened aluminum tubing critical areas of extreme temperature or routing through components. The weight of the cable alone can be as much as 5-10 times the weight of the sensor itself. Along its path to the conditioner, the signal is highly susceptible to degradation due to connector loss of insulation resistance (leaks to ground), additive tribo-electric noise due to cable vibrations and to strain effects of the vibrating heavy cable on the mounted accelerometer.

Fortunately for the engine being monitored (and the passengers on the airplane!) once the accelerometer is designed with proper balance of high temperature sensing crystal, durable packaging and armored hardline cabling, the monitoring performance is robust and reliable. Those readers familiar with the subtleties of calibration will immediately realize that all these “features” create a nightmare for calibration:

• The hardline cable is extremely inflexible creating cable strain forces on the mounted accelerometer
• The long loop of integral hardline cable is prone to excess cable vibration, and in charge systems can generate tribo-electric noise which further adds to signal degradation
• The large mass to connector/cable shifts the center of mass of the accelerometer cable pairing to well away from the center of action of the motion on the calibration exciter which amplifies any transverse modes of the calibration shaker/sensor structure. These amplified transverse motions increase the measured errors and the uncertainty of calibration.
• Lastly, the aircraft engine vibration monitoring accelerometers often have a sensing axis parallel to the mounted surface thus requiring a perpendicular bracket to align the sensitive axis to the axis of motion of the calibration shaker. This additional fixturing adds additional dynamics to the calibration setup and drastically limits the “flat line” response range of calibration setup.

As such, the calibration and recalibration of engine monitoring accelerometers can be quite a challenge. There are a number of specialized considerations made in aircraft engine vibration monitoring accelerometer calibration systems used at aircraft Maintenance, Repair and Overhaul (aircraft MRO) facilities. The primary considerations are almost all exclusively mechanical and dynamic.

First, as with any accelerometer calibration system, the performance and validity of the accelerometer calibration at low frequencies can be drastically affected by improperly attending to the cabling. Best practices for low frequency operation of accelerometers include a service loop of roughly 10 to 20 times the diameter of the cable, if possible, and in all cases cable tie down on the moving mount as near to the accelerometer as possible without straining the connection. These two considerations will minimize any extra cable vibration and also minimize the strain effects of a heavily armored cable on the accelerometers.

To further minimize any cable strains in a calibration setup, heavy accelerometer/cable pairings are usually run on a larger calibration shaker that is capable of better handling the imbalance/sideloads of the more massive and irregularly shaped sensor payload. This typically means departing from a precision air bearing calibration shaker and utilizing a calibration exciter with a mechanical flexure. This compromises the increased level of support for lateral loads by accepting the higher transverse resonance contributions of a mechanical flexure support over an air bearing. To a lesser extent, there is also a trade off made in fixturing the accelerometer slightly off center on the calibration shaker face to better orient the center of mass of the accelerometer/cable pair to the center of action of the motion of the shaker. In practicality, this is done by trial and error to find a location of balance to minimize the transverse shaker motion excited by the imbalance.

Also on the topic of fixturing, many aircraft engine vibration monitoring accelerometers need to be mounted in a rotated position to align the axis of sensitivity. To accommodate this aircraft MRO, accelerometer calibration systems are outfitted with a number of right angle bracket fixtures with mounts specific to the engine vibration monitoring sensor. The mounts attempt to balance both the x/y positioning on the planar face of the calibration shaker along with minimizing the distance z away from the face which helps to minimize rocking or rotational inputs. Because the fixture adds additional dynamics between the calibration reference accelerometer and the sensor under test (SUT), the fixture is designed to be as light (still allowing for maximum payload) and as stiff (providing as much rigid/flat line response) as possible. Additional dynamic study is done by the accelerometer calibration system manufacturer to characterize the loaded dynamics. Effectively the bracket adds additional spring/mass combination to the system and lowers the mounted resonance of the test sensor. Therefore the useful flat line performance range of this type of mount is often under 1000 Hz.

We advise you to check with the sensor manufacturer or calibration system provider for additional information and assistance with this type of characterization. Your provider should be well versed in topics like sensor manufacture, accelerometer characterization and performance, structural dynamics of fixturing, calibration grade excitation and measurement/calibration system uncertainties. If you have any problems or additional questions, we’ll be glad to help you!