429 Testing and Measurement of Mechanism -Induced Disturbances  

Laoucet Ayari*, Michael Kubitschek *, Gunnar Ashton* , Eric Marquardt * and Steve Johnston * 

Abstract  

Many space mechanisms  with moving parts such as gimbals , cryocoolers, shutter mechanisms, filter  wheels, fast steering mirrors, and reaction wheels  produce dynamic forces and torques that have the  potential to interfere with the function of other onboard instruments.  Jitter sources include direct reaction  forces and torques  from linear and rotary actuators , inertial imbalances, misalignments and internal loads  operating against asymmetric inertia and or stiffness  to name a few . The drive to address disturbances  from m echanisms in space hardware is prompted by the need to meet certain mission requirements, e.g. ,  signal quality from a science- producing instrument , line of sight stability, or optical jitter . The minimization  of jitter c an be approached through integrated analytical procedures and experimental test s capable of  identifying the source, nature and magnitude of  the driving forces and torques.  Even if a mechanism is  designed to eliminate jitter , e.g. , having reaction cancellation features, small loads are always  present  due to manufacturing tolerances, slight imbalances, and coupling between deg rees of freedom . In this  paper, we discuss advances made in the measurement verification process of jitter loads at Ball  Aerospace & Technologies Corporation (BATC) over the past few years. In particular, the strategy used  to deal with a wide range of loads and applications prompted the development of a capability to custom build measurement platform s with the needed performance using modular  hardware.  Examples of jitter  mitigation and cancellations are discussed to illustrate some of the work carried over the past decade.  

Introduction  

Force and moment platforms (FMP s), also known as dynamometers , are now commonly used to  measure imparted loads in the six degrees of freedom from either operating mechanical system s with  moving parts placed on top of the device or non- operating systems excited using shakers and stingers.  Such devices can be used in many applications , including JPL’s force limiting  during vibration testing of   space hardware [1], static and dynamic balancing of rotating systems [2], measurement of inertial  properties , and hybrid testing where a subset of the unit under test is accounted for through the  application of equivalent loads obtain from real -time simulations [ 4]. The other use of such devices is in  jitter measurement  [3] and is the subject of the current contribution.   

FMPs  use three or more judiciously encapsulated tri -axial force sensors between two sufficiently rigid  plates. They are designed t o be statically determinate, so that all torques are transmitted mainly as forces  through the load cells . Loads from the tri -axial force sensors are related to the forces imparted on the top  interface plate through a system of equilibrium equations . The unit is then calibrated as a system to  measur e resultant dynamic  loads  on top of its surface.    Depending on the geometry of the FMP , the number and type of force sensors  and the characteristics of  the unit under test, there are limitations on the capability  of the FMP. These l imitations are on the  magnitude and accuracy of the imparted forces and torques, their frequency range and the sensitivity  of  the system as an instrument . In addition, depending on the test location  and whether a low frequency  table is used, there is a noise floor that needs to be characterized prior to each test .    Because of the wide range of requirements from one program to another,  BATC  elected to implement a  strategy that builds custom FMPs tailored for each program using modular hardware.  FMPs are built from                                                         * Ball Aerospace & Technologies Corp., Boulder, CO  

Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center , May 4 -6, 2016