Write Stuff Yourself

I've basically struggled over the past week+ to figure out what the deal is with the optimal filter. The puzzling thing was that simulations from the command line would look good, whitening an otherwise off-white output PSD, but when it came time to Simulink the outputs would look almost identical.

Finally on Monday, after much cursing I figured it out the problem after comparing basically every internal signal in the Simulink model to what it should be using lsim. It turns out that the multi-channel transfer function block I was using in the LTI controller was spitting out garbage. Fixing this involved copying the little fucker from a working diagram I received from my advisor. What was maddening was that both blocks implemented the identical transfer function...at least identical algebraically.

Now, I had to use this custom block because Mathworks, in their infinite, overpriced wisdom, doesn't have a transfer function Simulink block that works with multiple channels to my knowledge. Our lab created this one to use, but unbeknownst to me it assumes the denominator is monic, which wasn't the case in the block I was using. This is what I get for using software without really knowing the details under the hood.

Anyway, shits sort of working now with multiple channels in the experiment, just meeting my little deadline. Saturation's the enemy now, but I think I can fix that with the forthcoming hardware mods. There's still a paper deadline in a couple weeks though that I'm going to try for.

9.21.10 [2]

I've ordered parts for the new and improved experiment. Mainly just a ton of lenses, they should arrive relatively quickly. I'm hoping realignment should be straightforward, but I'm worried since the tolerances on the reimaging telescopes is pretty thin. I might have to resort to CADing everything out, but even that would involve a lot of guesswork.

There's apparently a conference submission deadline at the end of the month, so it turns out that my goal to do multichannel adaptive control by the end of September is a good plan. I'm going to try to get some decent results that can go into a preliminary paper before I tear my experiment asunder and start over. For the time being I'll have to run <10Hz so that the single delay DM model holds.

While the adaptive loop seems to work well without much modification, so far I haven't gotten much out of the optimal IIR filter/Kalman predictor. It works well enough in flattening the PSD when I use the identified disturbance model directly to generate the noise, but in the experiment its effect is negligible. I suspect this is a result of a shitty ID. It might be interesting to look at the prediction error with the actual experiment/simulation, instead of just driving the identified model.

Overall the plan is to spend the remainder of September getting preliminary results for this conference, then worry about experimental modifications in October. Make it so.

9.21.10

Vacation injury report:
- 1.5" cut, left shin
- 2" cut, right shin
- 5" bruise, hip
- multiple bruises, left thigh
- 1" bruise, chest (x2)
- skinned left elbow (despite armor)
- scraped left knee (various)
- slightly chapped lips

hey, what's the point of paying for health insurance if you're not going to use it?

9.12.10

So I'm taking a week of to visit places like this



But on the other hand I have plenty of time to digest what I learned on my little business trip last weekend. The good news is that the DM delay I've experienced was well known, and it turns out not to be due to the DM at all. Instead, the problem is that the WFS returns a frame from a buffer somewhere when requested, and at higher frame rates there's no guarantee how fresh or stale that image might be.

The solution is to use a hardware trigger for the camera so it only returns a recent frame when requested. The good news is that the current driver box I'm using for the 61 actuator DM can be modded to do this. The bad is that I have to solder a connection inside this $5000 box. We'll see when that happens.

Other than that I learned just how short my optics knowledge is when I talked about my experiment. Contrary to what I'm seeing, the beam size and intensity shouldn't change with DM commands as long as the WFS and disturbance DM are on imaging planes. Right now light is just beamed in without regard for human life. Some relatively simple hardware changes should solve this, but that means yet another alignment. Really I shouldn't be tinkering with the experiment at this point, but you can't fix stupid.

All said, I should be able to use a 10x10 lenslet array and fix the DM delay, which would allow me to boost the speed up to 100Hz and beyond. I'm aiming for the end of Oct for this, but who knows.

9.6.10

So I'm headed out to ABQ tomorrow to check out some other AO experiments irl. Although it probably would have been more useful to go there months ago when I was still tinkering with hardware, hopefully I'll get some nice ideas about how to improve the frame rate of my experiment, something my advisor seems to focus on.

I'm more concerned about the control aspects, and I doubt I'll hear anything interesting about that although you never know. I've made some progress computing the Kalman predictor like I described. Identifying the right disturbance model turns out to be a shittier experience than I thought, and I'm still not sure if I have the best procedure nailed down yet. I wasn't even able to get er done in the single channel case. The one IIR filter (for 5 channels) I managed to implement didn't do much to improve the wavefront error in either simulation or experiment. So either something is wrong with the calculation, or the whole idea of using a simple n-step predictor is mistaken somewhere.

That'll have to wait until I get back, or longer since I'm taking next week off to get out of town before the unwashed masses return to campus at the end of the month. Hopefully I can use that time to wrap my head around some simple RLS and (ideally) primitive lattice filter example in between mountain bike runs.

If I work hard like a good grad student, I think I'm still on track to get a multichannel version of the adaptive filter running by the end of the month, although surprisingly the multichannel optimal IIR filter may come first.