I updated the design requirements for the Stewart platform. I weighed the unloaded "dirty" SOS that was sitting on the workbench in the control room; its mass is 11 kg. Steve suggested that the OSEMs (not installed on this model) would add another 0.5 kg. From the specs in the final SOS design document, LIGOT970135, I added 0.25 kg for the optic itself; I am therefore taking the total payload mass to be 11.75 kg. (Now, the upper stage of the Stewart platform itself will likely add a nontrivial amount, but I am not worrying about this yet.)
I have emailed Janeen Romie to obtain the actual center of mass and principal moments of inertia of the platform. I also cooked up a simple scheme to measure both quantities, should this information not be available. It would involve rigidly mounting the dirty SOS to a rigid bar hung from a pivot. By translating the mount point in two dimensions and measuring the period of the pendulum, I ought to be able to find the center of mass and moments of inertia by multilinear regression. However, this elaborate scheme is not necessary to just compute some ballpark figures; it could wait until a later stage in the design. For the time being, I just rescaled the moment of inertia proportional to the increase in mass, such that the torquetoforce ratio is unchanged.
As such, the design requirements are now
 linear travel: 40 microns peak to peak (based on SOS design requirements in LIGOT950011)
 angular travel: 3 mrad peak to peak (based on SOS design requirements in LIGOT950011)
 payload mass: 11.75 kg (measured unloaded mass, plus educated guesses about combined mass of OSEMs and optic)
 payload moment of inertia: 0.0232 kg m^2 (wild guess)
 bandwidth: 500 Hz (suggestion of Rana and Koji: ~kHz)
From these assumptions, the revised actuator requirements and dimensions are:
 peak actuator force: 2.04 kN
 minimum radius of top platform: 15 cm
 minimum radius of bottom platform: 30 cm
 minimum height: 26 cm
See the attached PDF document.
It appears that the actuator that I had originally nominated, PI's model P225.80, would very nearly meet the actuator force requirement. Steve also pointed out the following singleaxis shakers that are already in use in the 40m:
 Brüel & Kjær type 4809
 Brüel & Kjær type 4810
I want to find out if either of these would meet the present need, but I'm waiting on a response from the manufacturer to get access to the data sheets.
