Helmet-Mounted Displays: - USAARL - The - U.S. Army

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186 B. Joseph McEntire operational capabilities demand the helmet also serve as a mounting platform for such systems as weapon targeting, night vision or image intensification devices, flight symbology displays, chemical defense masks, and nuclear flash protection. These requirements demand more complex mounting devices on the helmet and, ultimately, result in increased system weights and potentially less than optimal CM location. Ultimately, there is a limit to how much mass can be supported by the aircrew without increasing the fatigue rates and neck injury risk in accidents. Mass and CM The mass of flight helmets has been a concern since "hard shell" helmets first appeared in the 1950s. These helmets were introduced to provide increased head protection during a crash, but at a significant weight increase over the previously worn cloth caps. The total head supported mass increased from 0.5 kg for the leather or cloth cap to 1.5 kg for early hard shell helmets, which included noise-attenuating earcups, earphones, microphone, and integral, adjustable visors. The hard shell helmet, lined with polystyrene foam, provided an order of magnitude improvement in impact protection. In the 1980s, the introduction of various visual enhancement devices further increased the mass to 3 kg for the standard Army SPH-4 flight helmet equipped with the AN/PVS-5 NVG. The increased mass of this helmet system is believed to have a detrimental effect on pilot performance due to neck muscle strain and fatigue and to increase the risk of severe neck injury in crashes. The disadvantages of increased helmet mass, however, are offset by the enhanced visual capability for night flying and increased weapons aiming capability offered by helmet-mounted image intensification devices and other helmet-mounted displays. In order to permit the use of 3-kg helmets without overloading the neck in severe crashes, the U.S. Army's Night Vision Laboratory (currently NVESD), Fort Belvoir, Virginia, developed a spring-loaded, ball-socket mount which permits the latest generation night vision device (ANVIS) to break free during a crash. The 0.6-kg NVG device was designed to break free of the helmet at a goggle deceleration of 10 to 15 times the acceleration of gravity (G) (Military specification, MIL-A-49425 (CR), 1989). Although this approach may offer one solution to the problem of increased head-supported mass in Army aviation, little is known about the dynamic
Biodynamics 187 behavior of this device in a crash or of the physical limitations of the human neck to support these masses. In an initial attempt to define a safe limit on flight helmet mass for the Army, USAARL in 1982 proposed a limit of 1.8 kg (3.96 lb) during the development of the AH-64 Apache IHADSS helmet. The helmet system subsequently developed met this mass limitation while providing the desired platform for the HMD and the required acoustical and impact protection. Nonetheless, the SPH-4 helmet with NVG attached used for night operations in all other Army helicopters continued to exceed the proposed 1.8-kg limit by more than a full kilogram. Although there have been anecdotal reports from aviators complaining of considerable discomfort with this system, particularly after long missions, the effects on pilot performance of bearing this much mass has never been systematically studied. Furthermore, the dynamic consequences of crashing with headborne masses approximating 3 kg remain largely speculative. Historically, helmet mass and CM requirements have been nonexistent or vague. These requirements often were written loosely and based on existing designs. Language in helmet development specifications often resembled “. . . the helmet CM must be located as close to the head CM as possible,” “. . . lighter and CM no worse than current helmet systems,” “. . . provide ease of head movement,” and “. . . (have) reduced bulkiness.” These requirements provided little guidance to the design teams and could not be quantitatively evaluated. Seven parameters are required to define the mass properties of helmet systems. As illustrated in Figure 7.1, these include mass, the center of mass position along three orthogonal axes, and the mass moment of inertia about the three respective axes. The coordinate system used by the Army aviation community is based on the head anatomical coordinate system and is illustrated in Figure 7.2 (Rash et al., 1996). The x-axis is defined by the intersection of the mid sagittal and Frankfurt planes with the positive direction anterior of the tragion notch. The y-axis is defined by the intersection of the Frankfurt and frontal planes with the positive y-axis exiting through the left tragion notch. The z-axis is oriented perpendicular to both, the x- and y-axes following the right hand rule. The rationale for defining aviator helmet mass requirements can be segregated into three areas: Aircrew health, operational effectiveness, and user acceptance. Aircrew health can be affected by both short- and longterm exposures of head and neck loadings. Long term exposures are the result of helmet mass and its mass center location in normal flight conditions (vibration and 1 to 2G flight environment). These effects
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Helmet-Mounted Displays: Design Iss
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The views, opinions, and/or finding
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vi Contents 4. Visual Coupling Clar
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viii Contents References ..........
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x Ben T. Mozo, Research Physicist U
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xii Foreword began its phenomenal e
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xiv Preface Acknowledgments The aut
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Introductory Overview 1 Clarence E.
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Introductory Overview 5 Figure 1.2.
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Introductory Overview 7 The trend f
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Introductory Overview 9
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Introductory Overview 11 aviator to
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Introductory Overview 13 1970's. Ta
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Introductory Overview 15 Image sour
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Introductory Overview 17
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Table 1.3. HMD performance figures-
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Introductory Overview 21 direct vie
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Introductory Overview 23 Figure 1.9
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Introductory Overview 25 Figure 1.1
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Introductory Overview 27 Ercoline,
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Introductory Overview 29 II, Procee
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Introductory Overview 31 Armstrong
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34 Clarence E. Rash, Melissa H. Led
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40 Liquid crystal Clarence E. Rash,
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Optical Designs 3 William E. McLean
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Optical Designs 57 and plotted in l
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Optical Designs 59 phosphors, (b) u
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Optical Designs 61 optics. Examples
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Optical Designs 63
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prism combiner. Optical Designs 65
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Figure 3.2. (continued) Optical Des
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Optical Designs 69
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Optical Designs 71 Figure 3.4. Comp
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Optical Designs 73 Figure 3.6. Opti
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Optical Designs 75 Figure 3.8. Refl
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Optical Designs 77 Figure 3.9. Ray
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Optical Designs 79 incidence and re
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80 Clarence E. Rash hinted at befor
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82 Clarence E. Rash metal tolerant
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84 Clarence E. Rash the performance
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86 Clarence E. Rash Figure 4.3. Hig
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88 Clarence E. Rash imagery in HMDs
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90 Clarence E. Rash as with ANVIS.
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92 Clarence E. Rash performance, on
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94 Clarence E. Rash effects on visu
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96 Clarence E. Rash concerning roll
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98 Aeromedical Research Laboratory.
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DESIGN ISSUES PART TWO
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102 Clarence E. Rash and William E.
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104 Contrast Clarence E. Rash and W
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Page 153 and 154: 136 Clarence E. Rash and William E.
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Page 187 and 188: Visual Performance 6 William E. McL
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Page 193 and 194: Visual Performance 175 perception.
Page 195 and 196: Visual Performance 177 with monovis
Page 197 and 198: Visual Performance 179 individuals
Page 199 and 200: Visual Performance 181 0 to -5.25 d
Page 201 and 202: Visual Performance 183 Report No. 9
Page 203: Biodynamics 7 B. Joseph McEntire He
Page 207 and 208: Figure 7.2. Head anatomical coordin
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Page 211 and 212: Biodynamics 193 al., 1972), and man
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Page 219 and 220: Biodynamics 201 Ty pic al acc ele r
Page 221 and 222: Biodynamics 203 exceed the mechanic
Page 223 and 224: Biodynamics 205 complexity and comp
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Page 227 and 228: Biodynamics 209 the fitting problem
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Page 231 and 232: Biodynamics 213 down. The helmet ma
Page 233 and 234: Biodynamics 215 Desert Shield/Deser
Page 235 and 236: Biodynamics 217 Donelson, S. M., an
Page 237 and 238: 219 Fort Rucker, AL: U.S. Army Aero
Page 239 and 240: 220 Ben T. Mozo Figure 8.1. Noise l
Page 241 and 242: 222 Ben T. Mozo available in the ne
Page 243 and 244: 224 Ben T. Mozo Protectors”(ANSI,
Page 245 and 246: 226 Ben T. Mozo language. Tests of
Page 247 and 248: 228 Ben T. Mozo Figure 8.6. Speech
Page 249 and 250: 230 Ben T. Mozo the communications
Page 251 and 252: 232 Ben T. Mozo Item Mass (grams) C
Page 253 and 254: Ribera, J. E., and Mozo, B. T. Unda
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236 Joseph R. Licina operational us
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238 Joseph R. Licina Standard MIL-S
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240 Joseph R. Licina On electro-opt
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242 Joseph R. Licina For HMDs with
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244 Joseph R. Licina 1994b) that wi
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246 Joseph R. Licina Table 9.2. Hea
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248 Joseph R. Licina provided a 93.
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250 Joseph R. Licina and buckles, a
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252 Joseph R. Licina authorized to
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254 Joseph R. Licina a nuclear flas
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256 Joseph R. Licina requirements:
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HMD PERFORMANCE ASSESSMENT PART THR
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262 Clarence E. Rash, John C. Mora,
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Glossary 11
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270 making stereoposis possible. Bi
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272 John C. Mora and M elissa H. Le
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NOTES ON CONTRIBUTORS Melissa H. Le
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Notes on Contributors 281 Research
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284 Index Breakaway force (see Fran
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286 Index movement, 55, 80, 84, 90,
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288 Index 198, 204, 205, 208, 210,
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290 Index Microphone, 15, 186, 224,
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292 Index Speech intelligibility (S
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