At low frequencies (<0.01 Hertz ) space platforms such as the shuttle, the Mir space station and the International Space Station (ISS) provide a unique near fall-free (10-6g) environment that can be used to conduct material science, fluid physics and crystal growth experiments. However, experience gained on the space shuttle and the Mir has shown that many of these experiments are significantly affected by the vibration levels on these space platforms. Vibrations are driven by on-board activities such as attitude control systems, thermal control systems, air conditioning systems, power generation systems, crew activity and by operation of the experiments themselves. The vibration levels are amplified by the structural dynamics of the spacecraft resulting in vibration environments characterized by mili-g (10-3g) acceleration levels. On the space shuttle, vibration levels above 0.01 Hz can exceed several hundred micro-g (10-4g) RMS, with peaks typically reaching mili-g (10-3g) levels. These acceleration levels are sufficient to cause significant disturbances to many microgravity experiments, especially those with fluid phases which includes many material science experiments. The acceleration environment of the International Space Station will likewise not be as clean as originally hoped for and the ISS will not meet the current vibratory requirements without the use of vibration isolation systems.
The Microgravity Vibration Isolation Mount (MIM) was designed to actively isolate experiments from the high frequency (> 0.01 Hz) vibrations on the space shuttle, Mir and ISS. Since the beginning of the 1990's, Canada has been developing a series of four generations of Microgravity Vibration Isolation Mounts (MIM).
MIM-1 flew onboard the Mir Space Station and the MIM-2 onboard the space shuttle. Now, a third generation, the MIM Base Unit, designed to fly aboard the ISS, is now being developed by Canada. Another generation, called MVIS, is also being developed for the European Space Agency (ESA). MVIS will be mounted in the Fluid Science Laboratory which will be part of the Columbus Laboratory on the ISS.
MIM-1 on Mir
The first MIM unit was launched in the Priroda laboratory module which docked with the Russian Mir Space Station in April 1996.
The system has been operated on Mir beginning May 1996, accumulating more than 3000 hours of operation supporting the following experiments:
MIM-1 installed onboard space station Mir
An upgraded system (MIM-2) was flown on space shuttle mission STS-85 in August 1997 and was operated by Canadian Astronaut Bjarni Tryggvason. The major improvements to the MIM-2 compared to the original MIM are in the design of the electronics and the electromagnet actuators.
MIM-2 has shown the capability to isolated down to 0.3 Hertz
MIM has demonstrated effective isolation from spacecraft vibrations and ability to provide well controlled driven motion to experiments mounted on MIM
The graph below shows an example of the accelerations measured by the accelerometers of the MIM-2 stator (non-isolated) and of the MIM-2 flotor (isolated) during the STS-85 shuttle mission. The accelerations were filtered by a 100 Hz low-pass filter and sampled at 1000 samples per second. The time traces thus have frequency content up to 100 Hz. The MIM-2 controller was set to isolate above a cutoff frequency of 2 Hz for this run.
The graph below shows the flotor accelerations with a scale that is 60 times more sensitive. As can be seen, MIM-2 significantly attenuates the vibration levels. During this run, MIM-2 attenuated 2000 micro-g peak accelerations of the space shuttle to less than 50 micro-g on the MIM flotor.
The graph below shows the Power Spectral Densities (psd) for the accelerations for a run where the MIM-2 controller was set to provide isolation above 0.3 Hz. The attenuation of the vibrations is evident in the separation between the psds of the flotor and the stator accelerations. The black line shows the current ISS specifications for the isolated racks. Note that the acceleration levels on the space shuttle are generally lower than this requirement.
The graph below shows the transfer function between the stator and flotor accelerations.
The overall attenuation up to a frequency of 100 Hz was of the order of 50:1. Note that the
apparent rise in the transfer function above 100 Hz is an artifact of reaching the noise
floor of the electronics since the signals are attenuated above 100 Hz using high order low
pass filters. Around 20 Hz the flotor acceleration levels have been reduced to the noise
floor (0.1 m g2/Hz) of the accelerometers. The model transfer function shows good
agreement with the measured transfer functions up to the frequencies where the accelerometer
noise floor is reached.
Results obtained from the MIM on Mir and the MIM-2 flown on STS-85 have clearly shown the systems ability to attenuate vibrations. The MIM is a unique and valuable tool for providing a better microgravity environment for a wide variety of science experiments such as material science, crystal growth and basic fluid physics.
Based on the STS-85 results the MIM-2 system models have been refined and used to predict the best performance that can be expected on orbit. The figure below shows the predicted isolation transfer function for a 0.02 Hz cut-off frequency.
For more information on the other generations of the MIM technology,
please consult these pages: