J.L. Colwell and L. Gannon, Defence Research and Development Canada - Atlantic, Dartmouth NS, Canada T. Gunston, VJ Technology Test Laboratory, Southampton, UK
R.G. Langlois, Department of Mechanical and Aerospace Engineering, Carleton University, Ottawa ON, Canada M.R. Riley, The Columbia Group, Virginia Beach, USA.
T.W. Coats, Naval Surface Warfare Center Carderock Division Detachment Norfolk, Combatant Craft Division, Norfolk, Virginia, USA.
SUMMARY
Military personnel on small high speed craft experience sustained extreme motions and repeated high-g slam impacts.
Existing commercial shock mitigation seats can reduce the negative effects of this severe environment on health and safety; however, the procedures used to specify shock mitigation seat performance requirements and especially methods used to demonstrate compliance with performance requirements are not well understood. DRDC Atlantic is pursuing an R&D initiative to reduce the risk of acute and chronic injury to personnel serving in small high speed military craft. This programme seeks to improve the state of the art for modeling, simulation, testing, and evaluation of shock mitigation seat technologies. In support of this programme, two classification schemes are introduced to identify (i) vessel classes, defined in terms of anticipated operating environment, and (ii) exposure severity, based on the motions and slamming actually experienced. Additionally, the characteristics of passive, adaptive, semi-active, and active seat suspension technologies are described in detail, to serve as a guide for classifying shock mitigation seat suspension configurations.
1. INTRODUCTION
Military personnel on small high speed craft experience sustained extreme motions and repeated high-g slam impacts. Existing commercial shock mitigation seats can reduce the negative effects of this severe environment on health and safety; however, the procedures used to specify shock mitigation seat performance requirements and especially methods used to demonstrate compliance with performance requirements are not well understood.
DRDC Atlantic is pursuing an R&D initiative to reduce the risk of acute and chronic injury to personnel serving in small high speed military craft. This programme seeks to improve the state of the art for modeling, simulation, testing, and evaluation of shock mitigation seat technologies. Preliminary discussions regarding this programme with international partners have led to considerable interest, as demonstrated by the list of authors for this paper.
The DRDC Atlantic shock mitigation seat test and evaluation programme began in late-2010 through mid
2011, with the acquisition of representative, contemporary shock mitigation seats from four commercial suppliers in North America and Europe.
Basic static tests will be performed on these seats through the winter of 2011/2012; however, the recent introduction of computer-controlled semi-active seats to the commercial market suggests that simple tests in existing facilities may not be adequate for emerging seat suspension technologies.
In support of this programme, two classification schemes are introduced to identify (i) vessel class, which is defined in terms of anticipated operating environment, and (ii) exposure severity, based on the motions and slamming actually experienced by vessel occupants.
Additionally, the characteristics of passive, adaptive, semi-active, and active seat suspension configurations are described in detail, to serve as a guide for classifying shock mitigation seat technologies.
The “ABCD” High Speed Craft Human Factors Engineering Design Guide [1]1 provides an informative and detailed description of the human factors issues that are briefly discussed in the next section of this paper.
This design guide also describes and shows examples of many commercial shock mitigation seat products that are discussed later in this paper. This document is available online for downloading at no charge, using the link included in the citation for this document, in the Reference section [1].
2. HUMAN FACTORS ISSUES
The effects of sustained exposure to extreme motions and repeated high-g slam impacts in small high speed military craft are becoming increasingly well known to the military operational community. Acute injury associated with one or a few relatively high magnitude slam impacts or with a relatively short-duration exposure to extreme repeated shocks can have consequences varying from a temporary reduction in the ability to perform at peak efficiency, to serious injury that prevents participation in the planned military action. Chronic injuries developed over long periods of time can lead to gradual degradation of performance effectiveness, and in some cases, serious disability and premature retirement from service.
1 numerals enclosed in [ ] square brackets denote references cited at the end of this document.
A wide spectrum of other issues is also associated with
operations in high speed craft, including; discomfort, fatigue, post-transit degradation of task performance, and (especially when operating at relatively low speeds) motion sickness. Additionally, since passengers and crew of many small high speed craft are not sheltered from the elements, serious problems related to exposure and noise can be experienced. All of these problems are potentially important, especially as they may affect the successful performance of the military mission.
Military requirements for small high speed craft will continue to exist for some time into the future, and so it is of critical importance to provide the best protection possible to personnel serving in these craft. A variety of approaches are relevant, and all should be pursued, including: design and analysis of hull size, shape, structure and materials; operating tactics; driver training;
and, shock mitigation technologies, including decks, platforms, deck coverings, and seats. The main focus for this document is on the testing and evaluation of shock mitigation seat technologies, but all relevant topics should be kept under consideration.
Methods for defining exposure limits or criteria that relate performance, health, and safety to characteristics of the motions and impacts experienced on high speed craft are an important element for high speed craft design and operation. As briefly discussed later, reliable health and safety criteria are particularly important for developing rational strategies for computer-control of semi-active and active shock mitigation seat suspension systems.
The International Standard Organization ISO 2631 Part 5 [2] provides a method for evaluating the effects of repeated shocks on humans, in terms of a dose parameter, which varies according to the magnitude and duration of exposure to repeated shocks (i.e. slam impacts). This dose parameter subsequently leads to the calculation of an equivalent static compressive stress on the lumbar spine. This quantity, Se, can be related to risk of adverse health effects. A particular version of this quantity which represents a normalized 8-hour daily dose is denoted as Sed(8), which is commonly called SED(8), or SED-8. To date, this methodology and recommended limit values are developed mainly for long-term working conditions, for the civilian population.
ISO 2631 Part 5 is currently under review, and will likely be updated in the near future. Reference 2 provides a detailed description of the existing ISO 2631 Part 5 methodology, and provides recommendations for possible improvements to the standard, for application to small high speed craft operations. Reference 3 describes development of impact injury design rules for high speed craft, which includes a comparison of ISO 2631 Part 5 with a number of alternative approaches, including those based on motion Root Mean Square (RMS) statistics, and a Vibration Dose Value (VDV).
It is generally recognized that the RMS statistic is not appropriate for evaluating the repeated shock exposure encountered in small high speed vessels [3,4]. The European Physical Agents (Vibration) Directive 2002 [5]
defines goals for exposure to whole-body vibration, which quantifies exposure in terms of either VDV or SED-8; however, since the VDV methodology is based ultimately on “discomfort” [3], it may not be appropriate for evaluating health and safety, especially for military personnel, for whom “perceived discomfort” may be very different than for the general civilian population [6].
3. TEST AND EVALUATION PROGRAMME Shock mitigation seats offer protection to the occupants of small high speed craft from severe motions and high-g wave impacts that are experienced when operating at high speed in waves. One practical consequence associated with providing better protection to military personnel, is that they will almost certainly drive their boats at higher speeds. It is possible that human protection can be increased to the point where craft performance is limited by power, controllability, and/or hull structure. From the health and safety perspective, this may be desirable; however, the consequences of encountering a limit on high speed craft controllability or structural failure can be very serious, and must be kept in mind.
DRDC Atlantic is leading an R&D initiative to reduce the risk of acute and chronic injury to military personnel who are serving in small high speed craft, by improving the state of the art for shock mitigation seat technologies.
Additional benefits will include the capability to develop concise performance requirement specifications for future acquisition projects, and establishing new methodologies for modeling, simulation, test, and evaluation of shock mitigation seat technologies. The resulting shock mitigation seat test and evaluation programme has the following five phases.
Phase 1 - benchmark contemporary technologies Phase 2 - develop test capabilities and test protocols Phase 3 - develop math models and simulation codes Phase 4 - validate Phase 3 models using Phase 2 methods Phase 5 - document results and recommendations
Phase 1 of the test and evaluation programme began in late-2010 through mid-2011, with the acquisition of twenty-two contemporary shock mitigation seats from four commercial suppliers in North America and Europe, including; Shockwave (Canada), SHOXS (Canada), STIDD (US), and Ullman Dynamics, Sweden.
These seats were acquired to support development of computer models and physical test methods for shock mitigation seats. This should provide a better understanding of how existing shock mitigation seats work, of how they might be improved, and of how to specify clear and concise performance requirements for
future acquisition projects. It was important to assure
both government procurement officials and suppliers of these seats that this is not an attempt to “pre-screen”
existing products, but rather it is an attempt to acquire the knowledge necessary to promote the development of more effective shock mitigation seats, and to better inform and advise national defence departments on these technologies.
Work on Phases 2 and 3 has recently begun, and will continue through the next few years. The first part of Phase 2 is a series of simple, high-g, single-impact tests scheduled for the fall and winter of 2011, to be performed at the Canadian Naval Engineering Test Establishment (NETE), which is a government-owned and contractor-operated facility, located in LaSalle Québec.
The development of Phase 3 physics-based mathematical models is underway at DRDC Atlantic, and at Carleton University. These models will be developed for implementation in a distributed simulation environment, where they can be ‘actuated’ in real time by one of at least the three following methods: (i) playback of full scale deck acceleration data recorded during actual operations; (ii) ‘standard’ deck acceleration profiles (not yet developed, as discussed later); and (iii), real time calculation of high speed craft motions and slam impact accelerations (also, not yet developed).
Shock mitigation seats generally fall into one of two types: “driver” seats, typically occupied by the coxswain and navigator who are seated at the control console; and,
“jockey” or “pod” seats, typically occupied by the
“passengers in the back”. The individual seat models purchased from each supplier are not specifically identified here. A variety of driver and jockey seat configurations were purchased from the three suppliers that offer both seat types, while one supplier only offers driver seats.
The twenty-two shock mitigation seats acquired to-date represent eleven different configurations, as two of each configuration were purchased. All of the seat models except for one have “passive” or “adaptive” suspension systems (as defined in the next section). One seat model has computer-controlled, semi-active suspension, which represents new challenges for test and evaluation, and has had a significant effect on the goals and scope of the test and evaluation programme.
The original intent for this programme was to develop a series of ‘classical’ mechanical engineering static and dynamic tests to provide validation data for computer models that would in turn be used to analyze seat performance in the complex dynamics of high speed craft motions and slam impacts. This simple plan was developed before computer-controlled suspension seats were readily available; however, in the past year, at least
two suppliers have started producing computer
controlled semi-active suspension seats.
It is quite likely, if not certain, that the control algorithms used in commercial semi-active and active seats will be proprietary, and so these algorithms will not normally be available for implementation in seat suspension mathematical models and simulation codes. It might be possible to infer how these control algorithms work, by employing static and dynamic tests suitable for passive suspension systems to ‘reverse-engineer’ these systems;
however, this might be counter-productive, as its reliability could not be accurately assessed, especially with respect to suspension behaviour in the highly non
linear motion and slam impact environment.
Figure 1 shows an idealized “acceleration profile” for a typical high speed craft motion cycle, with vertical acceleration on the vertical axis, and time on the horizontal axis.
b
Time
Vertical Acceleration
a c
Figure 1: Example “acceleration profile”, with zones (a) free-fall, (b) slam impact, and (c) hydrodynamic re-entry This figure is annotated to show three regions: (a) free fall, after the vessel transits off the top of a wave; (b) slam impact coincident with contacting the water surface at the beginning of re-entry; and, (c) hydrodynamic re
entry. As further discussed in [7], this particular ideal acceleration profile does not represent all likely slam mechanisms, but it provides a good basis for subsequent discussion in this paper. The typical times for a complete motion/slam cycle (i.e. the wave encounter period) varies from about 0.9 seconds to 7.7 seconds, for head seas operations, with vessel speeds from 20 knots to 40 knots, and in waves with modal periods from 4 seconds to 12 seconds. Much longer periods can be encountered for headings through beam to following seas.
As discussed earlier, one could postulate how a control system might be devised to provide control over seat response for this typical acceleration profile; however, real high speed craft motions, and in particular the slam impact regime, are often not very ‘clean’. Full scale data sometimes shows multiple peaks in the slam impact region. This is almost certainly due to local structural resonant response to a single wave impact; however, it’s
effect on control system response could be far different
from that intended for the ideal acceleration profile.
The ultimate conclusion from this brief discussion of the challenges related to test and evaluation of computer
controlled suspension seats is that, in the absence of detailed knowledge of how the control system actually works, the only reliable method currently available for performing repeatable tests is to develop shore-based test devices that can accurately replicate full scale motions and impacts. This will be a significant challenge.
In the mean time, the first series of static impact tests at NETE will be performed on a large, high-energy swinging-hammer test device, which is normally used to evaluate the effects of underwater shock and blast loads on shipboard equipment. This device is capable of producing single, high-g impact acceleration peaks that exceed levels expected for deck accelerations from small high speed craft wave impacts, even in the most extreme conditions. The first phase of this series of tests will investigate the range of impact acceleration peak
“shapes” that can be produced by this device, where
‘shape’ is a qualitative way to express the height, width, and other details of the acceleration peak shown in Region (b) of Figure 1. During testing, each seat will be occupied by an un-instrumented anthropomorphic test device (ATD) (i.e. crash test dummy), at weights representative of military personnel in “full kit”. All but one model of seats to be tested are configured for fully
suspended support of the seated occupant, including hands and feet. For the one model of seat that normally has some or all of the occupant’s weight supported by feet-on-deck (at least, before the slam occurs), it is possible to instrument the test deck to measure ATD foot pressure; however, this is a highly complex task that may not be included in these preliminary tests.
Each seat will be instrumented to measure accelerations at the deck, on the suspended rigid seat frame, and at the top of the seat cushion, using pressure pads inserted into custom pockets on the inside of the seat upholstery.
Each seat will be tested three times at each of three acceleration peak values, from approximately 5 g to 20g.
The actual peak acceleration values will be determined through preliminary tests. Additionally, at least a few tests will be performed for seats inclined at various angles with respect to the deck, to represent simultaneous vertical and lateral accelerations. It is intended to publish general results and conclusions from these tests in future open literature publications; however, it is likely that specific seat models will not be identified, which is consistent with the original intent of this programme.
Note that these simple static tests may be of high value for determining computer-controlled seat performance when the seat is operating in a passive state, which could be representative of various failure modes, when the controlling computer, motion sensor, and/or seat actuator fails in service.
4. SEAT SUSPENSION CONFIGURATIONS It is apparent that some confusion or at least inconsistent use of terminology exists in publications from both industry and in academia, with regards to describing shock mitigation seat suspension configurations and technologies. In particular, the presence or absence of a computer-based controller capability is often associated with semi-active and active suspension systems; where semi-active does not have such control, and active does.
This is not consistent with general control theory and seat suspension technology terminology, and so can lead to considerable confusion.
The following section provides an overview of accepted suspension seating terminology, for the four following suspension seat configurations.
1. Passive 2. Adaptive 3. Semi-active 4. Active
Figure 2 shows these four configurations schematically, with ‘passive’ in the upper-left section, ‘adaptive’ in the upper-right section, ‘semi-active’ in the lower-left section, and ‘active’ in the lower-right section.
Passive Adaptive
Semi-active Active
Figure 2. Schematic representation of suspension seating configurations passive, adaptive, semi-active, and active Figure 2 uses standard symbols to represent springs and dampers. The rectangular element in the active system represents an actuator. The dial symbol used in the adaptive system schematic denotes manual adjustments that can be made to the spring and/or damper. The letter
“u” used in the semi-active and active schematics indicates an effectively continuous control input. For semi-active suspension, this input control can only adjust
damper properties (e.g. throttle fluid flow in a damper).
For active suspension, the input control can actually increase system energy (i.e. add energy to the system);
for example, consider an active vibration cancellation system. An active system can also remove energy from the system, as is done by a semi-active system. Note that a gas/fluid system that can both decrease and increase gas pressure (and hence modify ‘spring’ stiffness) would be an active system, as adding higher pressure gas is adding energy to the system.
Generally, both semi-active and active suspension systems have a sensor device (e.g. accelerometer), control computer, and control actuator (e.g. controllable valve). Distinctions between configurations can be made in terms of design complexity (mechanical, electrical, and electronic), power consumption, and potential for shock and vibration mitigation. Within each configuration category, various implementation methods or technologies exist – many of which have been demonstrated in commercially-available systems. The suitability of the various suspension types in terms of complexity, power requirements, and potential performance is closely tied to the intended operational role of the seats. Additional details on the distinctions between passive, adaptive, semi-active, and active suspension systems are provided in the following sections.
4.1 PASSIVE SUSPENSION SEATS
Passive suspension seats use some combination of passive components, including springs, dampers, and friction elements, to isolate the sprung mass of the seat and passenger from the motions of the boat. Use of exclusively passive components makes this design relatively simple and robust as it does not involve electrical or electronic components. It also does not require external power. The main limitation of this suspension design is that it must be tuned for particular operating conditions due to the fixed constitutive relationships of the suspension components. The challenge in the design relates to the need to simultaneously provide isolation while working within the limited rattle space of the suspension, to avoid bottoming the seat against hard stops.
4.2 ADAPTIVE SUSPENSION SEATS
Adaptive suspension seats are similar to passive seats in that they make use of passive suspension elements.
However, these seats include a means by which the constitutive properties of the suspension elements can be adjusted periodically for particular operating conditions.
The adjustments could include varying stiffness or pre
compression of spring elements and adjusting the orifice diameter of viscous damping elements. However, it is important to note that these adjustments are not made continuously but rather periodically, either manually or in some cases by some form of external actuation. The
primary functions of suspension elements do not rely on electrical or electronic components and do not require external power during operation. The limitations for vibration isolation are similar to passive seats but adaptive suspensions offer additional flexibility as individual seat can be set for a variety of operating conditions, and/or weights of occupants.
4.3 SEMI-ACTIVE SUSPENSION SEATS
Semi-active suspension seats generally use a combination of passive spring elements and continuously-variable energy dissipation devices (damping and/or friction elements) to allow the rate of energy dissipation to be continuously controlled. Semi-active seats are not able to add energy to the system but can actively influence the rate at which it is dissipated. A typical implementation might include a servo-actuated damper orifice valve.
Recent developments with magneto rheological (MR) and electro rheological (ER) “smart fluids” offer new control mechanisms. Regardless, semi-active seat suspension systems require modest external power to operate the control system, but this external power does not contribute to mechanical work on the sprung mass.
The complexity is greater than passive and adaptive seats because instrumentation and control systems are required to monitor vessel and/or seat kinematics, and to vary the energy dissipation properties of the seat accordingly.
Semi-active suspension seats offer potential for improved vibration isolation within the constraint that they cannot affect the spring component of the suspension force.
4.4 ACTIVE SUSPENSION SEATS
Active suspension seats introduce an active force
generating element into the suspension, possibly in combination with passive elements, to actively and directly vary the force transmitted through the suspension. These devices use a control system to drive the active element, or actuator, which unlike in the case of semi-active suspensions can do positive mechanical work on the sprung mass (adding energy). The complexity and power requirements of active suspensions are greater than alternative suspension configurations. However, significant potential improvement in performance exists due to the decoupling of the transmitted force from the compression of the suspension (and energy stored in passive spring elements). The control of the transmitted force is based on continuously measured boat and seat parameters.
Active suspensions can emulate more simple suspension configurations and can be programmed to vary characteristics automatically with operating conditions.
5. VESSEL CLASS AND EXPOSURE SEVERITY A recent Marine Guidance Note (MGN) issued by the UK Maritime and Coastguard Agency regarding guidance on mitigation against the effects of shocks and impacts on small boats notes the following [8]: