scope:

Purpose

The purpose of force limited vibration testing is to alleviate overtesting associated with the unrealistically high base reaction forces that occur at the test item resonances in conventional base-drive vibration tests (Blake, 1954 [1]; Salter, 1964 [2]; Murfin, 1968 [3]; Ratz, 1966 [4]; Heinricks, 1967 [5]; Painter, 1967 [6]); Smallwood, 1989 [7]; Scharton, et al., 1989 [8]; Smallwood, 1990 [9]; Scharton, 1995 [10]). The purpose of this Handbook is to provide an approach that may be consistently followed by those desiring to use force limiting, without having to conduct an extensive literature search or research and development effort before conducting the test. This Handbook describes the rationale behind force limiting, the hardware required for implementation, the various methods for deriving force-limit specifications, and three flight experiments in which the base reaction forces were measured.

The disparity in the mechanical impedances of the flight and the vibration test mounting configurations is usually the most important cause of overtesting in vibration tests. However, it may not be the only cause. A second contributor [2] to vibration overtesting is the uniformity of the motion at the various attachments of the test item to the vibration test fixture, which uniformity is often in contrast to the uncorrelated motion of the attachments in the flightmounting configuration. The degree of overtesting associated with the uniformity of the attachment motion in the test is usually greater at the higher frequencies and when the flightmounting configuration involves relatively wide-spread attachments to lightweight flight structure excited with an uncorrelated source, such as acoustic noise. Finally, overtesting can simply be the result of unrealistic test specifications, e.g., caused by the cascading of margins associated with the individual steps in the process used to develop the specifications. Neither of these latter two sources of overtesting is discussed further in this handbook.

Applicability

This Handbook is applicable to all force limited vibration tests of National Aeronautics and Space Administration (NASA) flight and non-flight hardware, including but not limited to built-up spacecraft, spacecraft experiments and components, aircraft and launch vehicle equipment, launch vehicle pads, and Ground Support Equipment (GSE).

This Handbook is approved for use by NASA Headquarters and NASA Centers, including Component Facilities and Technical and Service Support Centers. This Handbook may also apply to the Jet Propulsion Laboratory (JPL) or to other contractors, grant recipients, or parties to agreements only to the extent specified or referenced in their contracts, grants, or agreements.

This Handbook, or portions thereof, may be referenced in contract, program, and other Agency documents for guidance. When this Handbook contains procedural or process requirements, they may be cited in contract, program, and other Agency documents for guidance.

For the purpose of this Handbook, a force limited vibration test is any vibration test in which the reaction force at the base of the test item is measured and limited. In addition, a force limited vibration test almost always involves measuring and controlling the acceleration at the base of the test item and may also involve measuring and limiting the acceleration and/or force responses at other positions on the test item as well. For both analysis and test, it is usually convenient to think of the input acceleration as the control and of the base reaction force as a response, similar to the acceleration responses of the test item. The recommended means of measuring the base reaction force is with three-axis piezoelectric force gages, but other means, e.g., shaker armature current or strain gages, may be useful in special situations. Similarly, limiting the force is preferably implemented in real time by the shaker controller, but iterative, off-line control between runs may also be employed.

Since the purpose of force limiting is to reduce the base reaction force at test item resonances, the technique is most useful for highly resonant configurations, some examples of which are structurelike equipment such as telescopes, antennas, and reflectors; lightly damped hardware such as optics and cold stages; flight and development test model (DTM) spacecraft; and equipment with pronounced fundamental modes such as electronic boxes on flexible mounting. Therefore, force limiting is generally not needed in vibration tests of hard-mounted electronic boxes. Force limited vibration tests of various types of aerospace equipment are described in (Scharton, 1993 [11]; Chang and Scharton, 1996 [12]; Scharton and Chang, 1997 [13]; Scharton, 1998 [14]; Scharton, 2001 [15]; Scharton and Lee, 2003 [16]; and Soucy and Montminy, 2007 [17]). For example, force limited vibration tests of the Hubble Telescope 284-kg (625-lb) Wide Field Planetary Camera II (WFPCII), as well as of the 30-g (1-oz) Articulating Fold Mirror for the WFPCII, were conducted at NASA's JPL in the early 1990s [11]. The first of the three guidelines in section 6 of this Handbook is to use force limiting only for vibration tests of highly resonant systems and not for brick-like, non-resonant systems, such as hard-mounted electronic boxes.