Microgravity Vibration Isolation Subsystem (MVIS)
Motion isolation mount (MIM) is a six degree of freedom (6DOF) magnetic levitation (MAGLEV) system that has been designed to isolate experiments from vibratory accelerations (>0.01 Hz) on the Space Shuttle, Mir and International Space Station (ISS). The MIM was operated on the Mir space station between May 1996 and January 1998. A second generation of the MIM, known as the MIM-2, flew on the STS-85 Space Shuttle mission in August 1997. A picture of the MIM-2 is shown in figure 4. Microgravity Vibration Isolation Subsystem (MVIS) is the third generation of the MIM technology.
The Fluid Science Laboratory (FSL) will be part of the Columbus Laboratory which is developed by ESA. The science module Columbus is a huge cylindrical laboratory of 4.5 metres.
The module has room for 10 international standard payload rack (ISPR)s, each hosting an entire laboratory in miniature - complete with power and cooling systems, and video and data links to researchers back on Earth.
Fluid Science Laboratory (FSL)
Figure 4: the FSL (including the FCE and the EU Box).
The FSL, shown in Figure 4, is designed to accommodate experiment specific Experiment Containers (ECs). An EC is installed within the Facility Core Element (FCE). The EC receives power, control and data services from the FSL subsystems.
The FCE includes generic optical diagnostic hardware for viewing a fluid system within the EC. The diagnostic tools include holography, Wollaston/shearing interferometer, Electronic Speckle Pattern Interferometer (ESPI) and thermography infrared camera. Video images from these instruments are recorded by systems installed in the FSL ISPR, outside the FCE.
Facility Core Element (FCE)
The FCE is manufactured out of carbon composite and is designed to maintain the critical tolerances required for the optical diagnostic tools. The optical diagnostic instruments are sensitive to misalignments of the order of microns and are thus also sensitive to vibrations of the ISPR and/or the FCE. The ISPR does contain a AAA fan for air cooling of the FSL subsystems as well as a Moderate Temperature water cooling loop for transferring heat away from the ISPR. Each of these will be significant disturbance sources on the ISPR itself.
The mass of the FCE including the optical diagnostic tools is 135 kg. The EC mass range is 20 kg to 45 kg, giving a total mass for the FCE plus EC combination in the range 155 kg to 180 kg. The FCE and EC are serviced for power, digital video signals, data signals and control signals by up to fourteen electrical umbilical lines and two water lines. The FCE itself does not require water cooling. Water cooling is made available to an EC as necessary. The total stiffness of the umbilical lines are specified to accommodate the MVIS required isolation performance.
The red line, in Figure 6, shows the predicted acceleration for Non-Isolated Rack; vibrational acceleration amplitude is of the order of milli-g. The MVIS performance has been targeted to reduce vibration levels under the specified level, defined by the black doted line, which represents the ISS combined Vibration Acceleration Specification. The blue doted line shows the predicted acceleration level with MVIS. MVIS’ major technology challenge resides with the modeling of and isolating vibrations from up to 22 distributed umbilicals. These umbilicals provide various pathways for the vibration to enter the FCE of the FSL.
The MVIS consists of a distributed set of large gap Lorentz force actuators, with permanent magnets mounted on the FCE and voice coils attached to the ISPR. Three accelerometer assemblies are also mounted on the FCE, housing three accelerometers each. This allows determination of both the linear accelerations and the rotational acceleration of the FCE, and allows for up to three levels of redundancy. There are four Light Emitting Diodes (LEDs) mounted on the FCE with collimated beams directed at four two dimensional Position Sensing Devices (PSDs) mounted on the ISPR. This allows for tracking the position and orientation of the FCE with respect to the ISPR.
The Electronics Unit Box
The Electronics Unit (EU) that controls the MVIS is mounted separately in the ISPR and is shown in Figure 7. The EU box is to be located in the top right section of the FSL in replacement of what was supposed to be a stowage compartment.
The signals from the accelerometers are digitized within their respective housings and sent to the EU in digital form to minimize susceptibility to electromagnetic noise. The signals from the PSDs are amplified and digitized at the PSD and sent tothe EU also in digital form. Both the PSD signals and the accelerometer signals are used in the algorithms that control the FCE. The control rate is 2000 cycles per second. This allows active control up to 50 Hz. Above this frequency the isolation is passive, with the various umbilical lines that service the FCE and the experiment within the EC providing only a weak transmission path.
FCE / MVIS Isolation Performance
The MIM-2 work included development of a very complete system simulator, that has been proven through the shuttle flight experiments. This simulator was set up to a model the MVIS system. The inertial properties used for the FCE for the simulation are shown in Table 1 . Most of the umbilical lines are attached to the rear surface of the FCE, with several also attached to the EC through the front surface of the FCE. The total combined stiffness of these lines is specified to be less than 500 N/m, giving a lowest rigid body mode around 0.3 Hz. The estimate of isolation performance is shown in Figure 3. For this simulation case the stiffness of the umbilical lines has been taken as 25% of the specified maximum allowed value and the controller is set to compensate for 90% of this stiffness. This stiffness value and controller compensation will be set as a goals for the final system. As shown in Figure 3 an isolation cutoff frequency of 0.03 Hz is achieved, and the acceleration levels of the FCE are reduced below the ISS vibratory specification. The ISS specification for the DAC 6 analysis cycle is shown in Figure 3 . It is shown in terms of a power spectral density that is equivalent to the one-third octave band spectra typically used in the DAC analysis.
|FCE Mass||Mf||170 kg|
|FCE Mass Moment
|Ixx||30.0 kg m2|
|Iyy||30.0 kg m2|
|Izz||20.0 kg m2|
|Ixy||0 kg m2|
|Ixz||0 kg m2|
|Iyz||0 kg m2|
FCE/MVIS Rejection of on Board Disturbances
The control algorithm is able to attenuate the FCE response to disturbances that are generated on the FCE itself. As an example, the FCE response to a single event impulse such as the opening and closing of a valve was modeled. Figure 8 shows the results. It shows the FCE response within a background of vibrations of the ISPR. These background levels are typical of what was observed on the Mir Space Station. The ISS levels will be higher. The FCE recovers very quickly from this on board impulsive loading.
FCE/MVIS Response to External Impulsive Load
There is a risk that the FCE could receive an externally applied impulsive load from a crew member. Even though the system will ideally be protected from such disturbances, for example by a net mounted in front of the FCE, the system must be able to recover. Figure 9 shows the system response to an externally applied impulse load of 50N. While this is enough to cause the FCE to contact the ISPR, acceleration levels return to very low levels immediately, and the FCE is moved back to the center over a period of more than 100 seconds with low frequency, low acceleration oscillations.
FCE/MVIS Response to Sustained On-Board Disturbances
The reaction of the FCE/MVIS to a persistent on-board excitation is demonstrated by looking at the response to a white noise disturbance. One source for such a disturbance is the water cooling that is available to the EU. The water running through the lines and cooling channels will undergo several changes in direction causing disturbances that can be spread over a wide spectral range. The results are shown in Figure 10 for an input with peak forces of 0.35N along with the background ISPR vibrations. The FCE response is random with peak accelerations of 40 micro-g. Without the MVIS active the peak response would be 227 micro-g.
A Unique Opportunity for Canada
The project will generate significant improvements to microgravity vibration isolation technology - in which Canada is a world leader – and provide additional opportunities for its exploitation by Canadian industry through exposure to the ISS stakeholders, users and researchers. The inclusion of a Canadian technology in a vital ISS facility such as ESA's Fluid Sciences Laboratory is a tribute to Canada’s leadership in this area. This collaboration will provide Canada with access to 5% of the resulting state-of-the-art FSL usage as stipulated in the ESA/ESA Letter of Agreement.
Through collaboration with ESA in their FSL, the FSL/MVIS Project aims to enhance Canada's ability to operate in and exploit space by:
- improving the opportunities for the exploitation of the microgravity vibration isolation technology developed by industry with CSA support;
- providing a significantly improved environment for the successful conduct of microgravity and life sciences experiments by Canadian researchers; and
- contributing to Canada’s utilization of the ISS.
The operational lifetime of the MVIS is 10 years (around 40,000 hours of exploitation).
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