Integration of a microgravity isolation mount (MGIM) within a Columbus single rack

Vo]. 22, pp. 119-127, 1990 Printed in Great Britain

0094-5765/90 $3.00+ 0.0 Pergamon Press pie

Acta Astronautica

INTEGRATION OF A MICROGRAVITY ISOLATION MOUNT (MGIM) W I T H I N A COLUMBUS SINGLE RACK R.G. Owen, D.I. Jones, A.R. Owens School of Electronic Engineering Science University of Wales, Bangor, Gwynedd, U.K. A.A. Robinson ESTEC, Noodwijk, The Netherlands. 2. Mierogravity Experiments The range of microgravity experiments extends from materials processing and fluid science experiments to biochemical and biological experiments. A survey. .°f potential microgravity payloads revealed that the most stringent micro~ravity requirements are den~anded by certain materials processing experiments. TI}.e freguancy sensitivity of these and other types ot expernnents is shown in Fig. 1. Also included are the proposed ESA vibration specifications for payloads aboard the Columbus Laboratories. These curves indicate the maximum acceleration levels permitted at the payload interface. Vibrational disturbances above these limits must therefore be attenuated in order to satisfy the specifications.

Abstract An investigation of the predicted vibration environment aboard the two Columbus Laboratories suggests that the microgravity levels demanded by some payloads can only be satisfied b~ isolating the payload b y means of an active isolation mount. A Feasibility Study has shown that the required microgravity level can be most easily achieved by adopting a non-contact strategy, whereby the payload floats inside its rack, its position being controlled by magnetic actuators. This paper describes the main elements and sub-systems of a Microgravity Isolation Mount (MGIM) based on this non-contact strategy and designed to be accommodated inside a Columbus single rack. 1. Introduction One of the main objectives of the Columbus project is the utilisation of the low microgravity environment of space for scientific purposes. Microgravity experiments will be performed aboard the Columbus Attached Laboratory and the Columbus Free-Flying Laboratory. Experimental payloads aboard both pressurized laboratories will be accommodated within single or double racks. However, payloads aboard such racks may be seriously affected by various vibrational disturbances unless they are isolated by some form of anti-vibration mount.

It is seen that all experiments are highly sensitive to low frequency disturbances in the quasi-static frequency range (<.01 Hz.). The lower bound of the materials processing envelope indicates that the Columbus specification is not sufficiently stringent in this frequency range .t° include all these experiments. However, this sensitivity gradually diminishes at intermediate frequencies between .01 Hz. and 2 Hz., whilst at high frequency (> 2Hz.) most experiments appear to be relatively insensitive to vibrational disturbances. This suggests that the Columbus specification at these higher frequencies may be too conservative for many experiments.

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undertaken for the Columbus Attached 4I~aboratory and the Columbus Free-Hying Laboratory.' A summary of the magnitude and frequency of the most dominant disturbances is presented in Fig. 2. Notice that the induced accelerations exceed the Columbus limit by between one and two orders of magnitude at the intermediate and higher frequencies, 3.3 M i n i m i z a t i o n of Vibration Disturbances

3.1 Quasi-steady state accelerations

The quasi-steady state acceleration level attainable depends mainly upon the effects of aerodynamic drag and gravity gradient. Aerodynamic drag is a function of the cross-sectional area, mass and drag coefficient of the spacecraft, its orbital altitude and the date or solar cycle. Gravity gradient aocelerations occur at points which do not coincide with the orbital path of the centre of mass of the spacecraft. Additional steady state accelerations will arise at these points due to periodic attitude correction manoeuvres. 3,2 Spacecraft-induced accelerations These are accelerations induced due to vibrational disturbances originating from attitude and orbital control systems, motors, reaction wheels, cooling fluid loops etc, as well as disturbances induced by flexible elements and crew motion. A comprehensive analysis of both externally and internally induced disturbances has been

In order to provide effective payload vibration isolation, it can be shown that an extremely comjpliant isolator capable of large displacements is necessary at low frequencies, whilst a stiff isolator capable of very smal~ displacements is required at high frequencies. These isolator characteristics are depicted in Fig. 3. Clearly, it is impossible for one type of isolator to be effective over the whole frequency range. The extremely low stiffnesses and high displacements required in the quasi-steady state will be beyond the capability of any isolator. Aerodynamic drag and gravity gradient accelerations are most effectively reduced by increasing the spacecraft's orbital altitude and locating the payload as near as possible to the spacecraft's centre of mass. However, as the altitude of the spacecraft and the payload location are usually

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fixed by other considerations, there is nothing a payload designer can do to further reduce steady state acceleration levels.

The input vibration amplitude, and the microgravity level demanded by the various experiments, determines the MGIM's required transmissibility. These factors, together with the physical dimensions of the Columbus rack, are the primary MGIM design constraints, and have a major mfluance in determining the mechanical design of the MGIM, as well as the design of major sub-system elements such as the thermal cooling system, power transmission, sensors and actuators ate (Fig. 4).

The high stiffnesses and small displacements required at higher .fl'equancies can be provided by means of passive anti-vibration mounts. However, no passive isolator can be effective at interntediate ~equcucles due to the range of stiffuesses and displacements required. Satisfying the Columbus specification in this intermediate range therefore requires the use of an active isolation mount, whereby the required stiffnesses (and damping) can be synthesized by varying the parameters of a controlled feedback loop.

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The MGIM platform and payload will require ower, cooling, data and communications serwces m the rack. However, any electrical or cooling fluid connections between the MGIM and the rack (however compliant) will exert forces on the platform. A Feasibility Study showed that the transmissibility curve shown m Fig. 5 could be most easily satisfied by adopting a completely non-contact strate~, whereby the MGIM platform and payload float instde the rack an~! their position is controlled by magnetic actuators. ~ The only forces exerted upon the payload will thus be magnetic forces due to the actuators. However, such a concept would also necessitate non-contact transfer of all electrical services, and furthermore, would preclude the use of a fluid cooling umbilical between the MGIM and rack. The dissipation of heat from the payload to the rack would then have to be realised by means of thermal radiation.

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The prime factors which determine whether a non-contact strategy is feasible are the size of the actuator gaps and the thermal cooling requirements of the various experiments.

Vibrational disturbances can either be minimized by isolating the disturbance at source, or at the payload interface. Much of the high frequency disturbances could be attenuated by passive isolators at each disturbance source. However, the increased size, weight and complexity of an active isolator means that only one sucn device can be afforded, and therefore it must be placed at the payload interface. I inputamplitudes ]

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The size of the actuator gaps is determined by the amplitude of the input vibration. Power can be transmitted to the MGIM across a small gap in a non-contact manner by means of a transformer with a loosely coupled secondary winding. $imilazly, a non-contact data communication can be affected by means of an infra-red optical link. However, there

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is a limit to the size of the gap whereupon transmission of power and data becomes inefficient. The power requirements of the experiments, and the available volume for MGIM accommodation inside the rack will determine whether excess heat can be dissipated by radiation, or whether a fluid cooling loop must be used. This in turn influences the type of feedback control system used to regulate the MGIM's position inside the rack. In a prototype MGIM test rig built to assess the feasibility of the non-contact strategy, control was based on position information obtained from capacitive-type displacement sensors 7with damping provided by a lead-lag compensator. Measurements recorded on this three degree of freedom test rig revealed that acceleration levels approaching lpg were possible at very low frequencies (<.05 Hz). However, analysis shows that a control system using position feedback alone is incapable of overcoming the disturbances introduced due to the compliance of a cooling umbilical. Improved control loop disturbance rejection results when an accelerometer loop is included. However, this is to the detriment of the resulting microgravity level, and in addition requires increased actuator gaps and higher actuator forces.

Payloads are accommodated in payload sub-units which can be removed as Orbital Replacement Units (ORUs) for maintenance, exchange and re-configuration (Fig. 6). The payload sub-unit represents a box-like structure which is standard in width and arranged in multiples of constant pitch, each 5U in height (1U = 44.45 m.m.). Total rack height is 35 U or 1556 m.m. which means that each rack is divided vertically into 7 sub-units. Standard Resource Interfaces are provided at payload sub-unit level thus enabling a single rack to accommodate several autonomous payloads~ 6. Proposed Concept A recent survey of potential microgravity experiments concluded that an optimum szze for a MGIM facility accommodated within a single rack would be a 3 sub-unit payload volume accommodated within a 4 sub-unit MGIM "Liner ~, with the MGIM and Payload c~ntrol facilities each occupying a further 1 sub-unit. This is because :(i) There appears to be several experiments which could be accommodated in a MGIM of this size, and which have a heat dissipation of below 1 kW. E~amples of such experiments are presented in Fig. 7. ~.

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The main disadvantage of the non-contact strategy is that the microgravity quality is obtained at the expense of accommodating less payload, as some mass and volume must be sacrificed in order to accommodate thermal cooling panels. Ultimately a compromise must be made between the required microgravity level and payload mass. 5. Columbus Rack System Experimental payloads on both Columbus pressurized laboratories are accommodated within single or double racks. Each single rack accommodates 19 inch standard equipment units, whilst each double rack accommodates two 19 inch equipment units side by side, with a load carrying mid-frame between each unit. All payload rack assemblies are capable of transporting up to 200 Kg. (single) or 400 Kg.

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(ii) Detailed thermal analysis reveals that reducing the Liner size to below 4 su~units makes liquid ~ooling mandatory for a heat dissipation ot 1 k W . - - W f t h radiation cooling the reduced size Liner would lead to a heat dissipation capability of only a few hundred Watts.

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The MGIM platform/Liner, MGIM Supervisor and Payload Supervisor are each designed as individual ORU units. This enables each ORU to be easily replaced in the event of a malfunction without requiring complete replacement of the whole MGIM facility. Also, in the event of a payload re-configuration, only the MGIM platform and Payload Supervisor will need to be exchanged. The proposed MGIM concept is based on the non-contact strategy described earlier, whereby power and data services to the MGIM are supplied by a non-contact power transformer and an optical data link respectively. Payload heat is dissipated by thermal radiation. 6.1 Design of m a i n elements The main elements are the MGIM, Liner, and Supervisor. MGIM The MGIM is basically a light, rigid framework, 15 U (3 sub-units) in height, enclosing a central platform, with a vertical spine running along its back, and cooling fins attached to its sides. Its functions are as follows :(i) It accommodates the payload, which is attached to the central platform. (ii) It provides a structure onto which the cooling fins can be attached. These cooling fins interleave with fins attached to the inside walls of the Liner. (iii) It provides an attachment point for the actuators and sensors, the optical link, and the secondary coil of the power transformer. Control, data and power signals are distributed to the payload via the spine. A mock-up of the MGIM platform is shown in Fig. 9. It is fabricated from an aluminium profile table, and is enclosed within a framework constructed from extruded aluminium sections. The top and bottom surfaces are covered with a thin aluminium sheet. The spine is constructed from an extruded channel section with a removable cover so that the power and data cables can be easily accessed.

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Liner This is essentially a box which is 19 inches wide and 20 U (4 sub-units) in height. Its functions are as follows :(i) To accommodate the MGIM unit within a sealed box, thereby eliminating disturbances due to air turbulence from either the cabin space or rack air cooling ducts. (ii) To panels.

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The front of the Liner consists of a hinged panel, which opens into the Columbus Module central space and allow access to the MGIM. The panel also incorporates inspection windows to enable visual monitoring of the payload. The Liner is firmly locked to the rack by means of screws attached to the front panels. A Liner mock-up fabricated as a welded framework using extruded aluminium sections is shown in Fig. 10. It is installed inside a standard single rack bymeans slide rails. The Liner walls are designed as a framework for two reasons :(i) to reduce weight, (ii) in order to accommodate cold plates for heat dissipation, thereby maximising MGIM width.

Supervisor This unit holds the control and interfacing electronics and is 1 sub-unit in height. It is located in the rack and locked to it in a similar manner to the Liner. Electrical services are received from the Standard Resource Interfaces at the rear of the rack, and are transmitted to the various sub-systems inside the Liner by means of an electrical interface located in the base of the Supervisor. Fig. 9.

Mock-up of MGIM platform

A mock-up of a single rack with the Supervisor and Liner units installed is shown in Fig. 11.

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Locking Mechanism The locking mechanism is required to secure and release the MGIM inside the Liner and withstand maximum launch stresses. A sketch of the proposed locking mechanism design is included in Fig. 8. One essential requirement of the locking mechanism is that, when actuated to enter the locked state, it should be capable of capturing the MGIM at any combination of displacements away from the nominal position, within a 10 m.m. limit. This can be achieved by using conically shaped mating surfaces to centralise the MGIM before locking. Each locking mechanism therefore consists of two conically shaped pistons driven in opposite directions on a lea&crew shaft to engage with a pair of conically shaped lugs fixed to the side of the MGIM platform. The MGIM is thus pulled towards its nominal position, without exerting a net force or torque on the MGIM, and contact with the cooling fins is avoided.

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6.2 Sub-systems Design Two of the main design objectives, after having determined the basic MGIM/Liner dimensions were

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Cooling System Heat is removed from the MGIM by radiation, using cooling panels attached to the sides, rear, and lower surfaces of the MGIM and Liner respectively. Each panel consists of numerous fins, each with a thickness of 2 m.m., with cold plates attached to their reverse sides (Fig. 12). Cooling water circulates through all the cold plates which will be interconnected. The cold plates are designed to be integrated into the Liner walls, and therefore do not occupy additional space inside the Liner. A thin aluminium sheet is added to seal the Liner wails. The dimensions of the fins allow the MGIM 10 m.m. movement in any direction.

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The internal width of a single rack (452 m.m.) is small in comparison to its depth (635 m.m.) and height (up to 1556 m.m.). A MGIM's width will be further reduced due to the need to attach cooling fins to its sides, and also dye to the need to allow it approximately 10 m.m. free movement in any direction. Accordingly, all other sub-systems (except the sensors and actuators) are accommodated away from the sides of the MGIM. MGIM Cage The MGIM Cage consists of a strong, rigid central belt supporting and enclosing the MGIM, and is capable of withstanding all launch stresses. Slide rails are fitted on the outside of the Cage and inside the Liner to enable the Cage to be inserted inside the Liner (Fig. 8). The MGIM is thus secured to the Liner at a point as near as possible to its geometric centre. This is desirable as it can be assumed that the payload's centre of mass will be close to the MGIM's geometric centre. The upper and lower sections of the Cage, however, are only required to support the sensor/actuator units, power transformer and optical data link, and can therefore be made much lighter. based on amplitude.

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Power Transformer Electrical power is supplied to the platform by means of a transformer with a loosely coupled secondary coil. The primary winding Is wound tightly on a ferrite core, whilst the secondary has a clearance of 7 m.m. around the core in all directions. A prototype of this transformer has been constructed whereby the primary winding is driven with a square wave derived from a 150 V d.c. suppl7 by the action of a bridge of MOS power transistors. The secondary is connected directly to a bridge rectifier and smoothing capacitor with a resistive load. A po~ver transfer of 1 kW has been successfully achieved.

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Sensors a n d Actuators Capacitive bridge displacement sensors and magnetic attraction actuators are used. Both are designed to be integrated into a compact single sensor/actuator unit. The units are located in the aP between the MGIM platform and Cage. The precise tion of each unit is illustrated in Fig. 13. Sensors located at x . . x measure the relative displacement between the MGIM and Cage along the x axis. Any corrective action is provided by the actuators at xC,x, whose line o f action will be through the geometric centre of the MGIM. Clockwise and anti-clockwise rotation about the z axis will be controlled by actuator pairs at xy x3 and x2, x respectively. Corresponding units at y . . y ~ and z . . z 4 control motion in the yz and xz planes.

7.1 MGIM Rack Options The design of the proposed MGIM concept has been based on a single Sub-unit Rack. However, since the adoption of this concept the availability of another type of payload rack, ,~nown as a Facility Rack, has been a n n o a n c e d . ' - - A Facility Rack is designed to accommodate a single autonomous payload, and consequently is provided with Standard Resource Interfaces at rack level only. This enables a payload designer to decide the optimum payload configuration within the rack, since there are no fixed interface locations at sub-unit level. An additional benefit is the increased payload volume available due to the absence of the rear interface panel (Fig. 14). A Facility Rack would therefore appear to be a more suitable rack for MGIM accommodation. The main disadvantage of the single rack is the limitations imposed by its narrow width. Another rack option, perhaps more suitable for some experiments, might be a MGIM accommodated inside half a double rack. Estimates of the typical volumes occupied by a MGIM in both single and double racks are presented in Fig. 15.

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7.2 Types of M G I M Platform It is envisaged that several MGIM platform configurations would be available, from which payload designers could choose the most suitable design for their experiments. Three possible contlguratlons are depicted in Fig. 16. It is estimated that a payload mass o f approximately 75-85 Kg. can be accommodated on the mock-up MGIM platform.

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D a t a Link High rate data transmission is achieved by a number of infra-red emitter-receiver pairs shielded from ambient light interference by concentric tubes. 7 7. MGIM User Details The following sections are intended to give a payload designer details of the mass and payload volume of a MGIM, the types of MOIM platform which might .be made available and a possible operational scenario.

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Fig. 15. MGIM Dimensions and Volumes inside Single and Double Racks 7.3 Operational Scenario The MGIM will be assembled within its Cage, and will initially be installed inside the Liner on Earth. It will then be transported as an ORU into orbit. The Cage is attached to a slide system inside the Liner to enable easy withdrawal of the MGIM in-orbit, whilst mechanical stops prevent its complete withdrawal whilst being serviced by a crew member. Re-confignring the MGIM facility to accommodate a new payload would only involve the installation of a new MGIM/Payload/Ca~e unit together with the corresponding Payload Superwsor. 8. Conclusion An outline of the microgravity levels of future payloads aboard the Columbus Laboratories, together with the predicted vibration environment inside the laboratories has been given. A Micrograviry Isolation Mount capable of being integrated reside a Columbus single rack has been described, and a hardware mock-up has been constructed. Design of the various sub-system elements are currently in progress and will be installed inside the mock-up at a later date. It is intended that this mock-up will be used as a 3 degree of freedon~ test rig to confirm earlier experimental results.

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Acknowledgement The work presenetd here was performed as part of ESTEC contract 7637/88.

9. References 1. Owen R.G., Jones D.I. "Columbus Applications Study', Report BTN 001, University of Wales, Bangor, 1988. 2. Langbein D. "Allowable G-Levels for Spacelab, Columbus and EURBCA', ESA Contract No. 6.76/86/F/FL(SC), April 1987. 3. "Gravitational Biology Facility for Columbus" Pre-phase A F i n a l Report GTS/ESA Contract No. 6723/86/F/FL(SC), January 1987. 4. MBB/ERIqO : "PM-4 Induced Disturbance Analysis', ESA Contract Report No. COL-MBER-100-TN-0522-01, September 1987. 5. MBB/ERNO : "MTFF Induced Disturbance Analysis', BSA Contract Report No. COL-MBER-000-TN-0432-02, February 1988. 6. Jones D.I., Owens A.R., Owen R.G. "Microgravity Isolation Mount: Design Report', ESTEC Contract No. 6380/85, by University of Wales, Bangor, (1986). 7. Jones D.I., Owens A.R., Owen R.G., Roberts G. "Microgravity Isolation Mount: Final Report', ESTEC Contract No. 6380/85, by University of Wales, Bangor, (1987). 8. Jones D.I. "Assessment of the feasibility of using an accelerometer loop for umbilical disturbance rejection', Report BTN 002, University of Wales, Bangor, 1988. 9. Owens A.R. "Calculation of MGIM Cooling by Thermal Radiation', Report BTN 004, University of Wales, Bangor, 1989. 10.Fenerbacber B., Hamacher H., Naumann R.J. "Materials Science in Space', Springer-Verlag, 1986. ll.Biolab Phase-A Study, Mid-Term Presentation by Aeritalia Space Systems Group at ESTEC, Noordwijk, June 1989.

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