Test & measurement


High-frequency jitter can be measured with piezoelectric force sensors, charge amplifiers and low-noise accelerometers and dynamometers


Micro-vibration and jitter testing S


atellite jitter, or blurring of the images originating from micro- vibrations, is a severe deviation source that affects the geometric accuracy of high-resolution imagery. Recent years have seen major advances in terrestrial observation – accompanied by a dramatic increase in the need to measure the earth's surface and atmosphere with ever greater precision. The quality of these images would have been unimaginable only a few


years ago. Critical steps in achieving this progress include the reduction of micro-vibrations on board satellites. Every satellite requires numerous drives, position controls, reaction wheels, actuators and cryo-coolers: the list could go on. These devices comprise mechanical components that cause vibrations when they operate. Micro-vibrations consist of extremely small accelerations of very low intensity. Measuring them is a challenging task. High-frequency jitter can be measured with piezoelectric force sensors, charge amplifiers and low-noise accelerometers and dynamometers. Recent innovative designs using ceramic top plate dynamometers


allow higher sensitivities, higher frequency ranges and the possibility of water cooling optimised for reaction wheel jitter and cryo-cooler micro-vibration applications.


Important technologies for the application High resolution dynamometers: Piezoelectric force sensors and dynamometers combined with high sensitivity charge amplifiers are ideally suited as they offer very high resolutions of up to 100,000. This makes it possible to measure dynamic force changes down to 0.01 N (0.002 lbf) and


Instrumentation Monthly August 2019


Amplitude, phase response and natural frequency in the Fz direction for a ceramic top plate dynamometer Type Z21492


moments down to 0.08 10-3 Nm (0.7 mlbf-in), even if the object to be measured weighs more than 10kg. High frequency response dynamometers: Optimised micro- vibration dynamometers featuring high rigidity allow for very high natural frequencies of more than 1,500 Hz, which enable measurements of up to 500Hz. Low crosstalk dynamometers: The resulting forces and moments are calculated through the three signals provided by each of the four triaxial force sensors constituting the dynamometers. The lower the crosstalk, the higher the force measurement and moment calculation accuracy. Lightweight and low-noise accelerometer solutions: Lightweight acceleration sensors generating lowest possible noise are generally preferred in cases where the micro-vibration levels are still high enough. These properties are the key to prevent mass loading effects while still detecting micro-vibration levels.


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