Laser, DFB module: TEC reaction speed

Google on "cricket game description"

and note that the British national cricket team are now touring Australia, playing a series of five "Test Matces" against the Australian national cricket team. Australia have won the first three matches (and the series) and are now winning the fourth test n Melbourne.

McGrath and Warne are Austraian bowlers of extraordinary effetiveness.

-- Bill Sloman, Nijmegen (but in Melbourne at the moment, ignoring the cricket)

snipped-for-privacy@ieee.org wrote in news: snipped-for-privacy@n51g2000cwc.googlegroups.com:

Steady.. >:) They could, but things aren't so certain now at 84 for 5. That's a worse position that England were in yesterday.

this is a great trick. I picked it up from an IEEE paper a few years ago, tried it and have been hooked ever since. I then found it in one of Astrom & Wittenmarks books. Which I got for US$1, brand new, because the hard-cover had been bound upside down. hurrah for manufacturing screw-ups.

Hi Tim & Joerg,

Tim, have you done much work with so-called Internal Model Control? Its pretty much what you just described. of course the hard part is figuring out the feedforward path. There is a nice description of it in the CRC Press Control Handbook (ed. Levine). I implemented a really nice current controller this way a few years back, and had fabulous results (the control was almost good enough, even with a huge error in my sine(theta) calculation - flipped the sign for 20 degrees or so) at very low sample rates. Almost as good a step-response as dead-beat control (OK, 3 not 2 periods), but very robust to parameter disturbance.

Joerg, thats not much use to you though :(

I hope you guys (and all SED participants) have a safe and joyous festive season. Would you believe it snowed here and in Aussie the day before xmas ?!

Regards,

Terry

Hi Bill,

can you email me copies of those papers please? my snipped-for-privacy@ieee.org

Regards,

Terry_Given

bah humbug! We (NZ) suck so bad at cricket we have to resort to dirty tricks like the stumnping of (IIRC) Muralitharan when we went to congratulate one of his team members.

I saw on the news last night, our captain (and one of the better batsmen) out for 17. Yet we pay these sods around $80k/year each.

Of course our soccer team is far, far worse than the black-craps.

Still, there's always rugby :)

Cheers Terry

-- snip --

-- snip again --

I have a tendency to reinvent methods to solve specific problems, then find out later that I've done something that was invented in the 50's (sometimes it's the 1950s, sometimes it's the 1850's, sometimes...).

I've done this more where you have a nice, repetitive input, which makes adjustments to the feed forward more robust in the face of plant nonlinearities. Making this work well for an arbitrary input would be more difficult, but should be possible using adaptive techniques.

I'm going to have to check out Internal Model Control, though.

Would you like some nice Pacific Northwest (US) rain? We've got lots of it this week.

I am afraid it's going to be tougher than anticipated because we don't know what the plant looks like. Lots of individual delay elements. Then there is the TEC itself which isn't linear at all over +/-10C. Of course, none of that is spec'd anywhere :-(

In the end it might indeed boil down to a uC with lots of PID and feed forward parameters in a lookup table that gets switched through as the DFB module TEC passes through temperature ranges.

If it's any comfort, this stuff has arrived in the Pacific Southwest about two hours ago. With the usual flash flood warnings and all that.

I just checked; it is, indeed, in the Control Handbook.

It came out of chemical process control (he says, quoting near verbatim) in the early 70s. The smartest engineer I've ever met mentioned it once, and as I was stuck with a very low sampling rate along with high accuracy requirements, I looked into it. It turned out to be easy to analyse and design, dead easy to stabilise, and on its own gave me slightly better THD than my BLT+warping discretised PI loops with fancy dead-time compensation (the dead-time compensation used up to the 7th harmonic of the dead-time related voltage error waveform), and a higher CLBW. And the implementation cost me, IIRC, one more MAC per d,q channel than the quadrature PI loops, but removed truckloads of dead-time compensation code.

It also lends itself well to adaptive control, inasmuch as its easy to modify the parameters of the reference plant, and its still easy to stabilise. its really just a variant of MRAC anyway. Oh yeah, It also dealt with integrator windup too :)

I will be using it shortly, for some motor control stuff I'm doing, so I'll be digging up my old notes and having a read. If I come across any paper references, I'll post them.

I tested repetitive control for dealing with rectifier/capacitor loads, and it wasnt as good as IMC

We have some right now, although not really NW its the pacific (which I'm on the other side of a mountain range from).

Cheers Terry

Australia seem to have recovered to 372 for 7, against English total of

Between the better bowlers and the better batsmen, the Australians do seem to be difficult to beat.

-- Bill Sloman, Nijmegen (but in Melbourne at the moment, listening to my wife enthusing about the cricket ...)

I thought your wife is Dutch. That's where soccer is all the rage. Very few people on the European continent know what cricket is. To be honest, I don't either.

My wife is Australian. But some of the Dutch do play cricket and there are cricket teams and cricket competitions in the Netherlands. Soccer may be a rage n the Netherlands, but field hockey is also popular and both the mens and womens hockey teams do well in international competitions - but then again, so do the Australian field-hockey teams.

I'm still playing "trimhockey" myself, which is a Dutch invention - field hockey played by the relatively elderly to keep fit, with the players being careful not to damage one another.

-- Bill Sloman, Nijmegen (but in Melbourne at the moment, where field hockey can be very rough - ask my goalee nephew).

Sorry Terry - my attempt to reconstruct your e-mail address failed. Send me an e-mail - my e-mail address isn't munged - and I'll send you the two .pdf's.

Oh, the handy underlining completely obscures the underscore. oops, sorry. will do.

Cheers Terry

It never caught on in Germany. American football did though. Hockey is (or at least used to be) big there, occupying quite some TV time.

That's good not to get hurt. The worst injuries and long term damage I've seen with sports folks were hockey players. Soccer as well but those were mostly wear injuries like permanently shot knees.

Advanced control algorithms presuppose that the actual "plant" state can be accurately measured as the thing to feed back against. It's not obvious to me that you have an accurate realtime wavelength sensor, either in the production system or even for initial laser/tec characterization.

Hell of a storm here last night. There's debris all over, lots of palm tree bits on Dolores Street, and some traffic lights are out. Jim and the boys should get it next.

John

Yep. It's cloudy here... 50°F. Depressing ;-)

...Jim Thompson

Checking ot the mens and womens field hockey results for the last few Olympic games, Germany seems to do as well as the Netherlands and Australia.

My understanding of TEC coolers is that they cool at a rate proportional to KP*I - R*I^2 -- and the 'R' term really is a resistance. If this is truly the case (and if you can characterize it) then it should be easy to calculate a desired rate of heating, then calculate the necessary current.

Of course, that doesn't mean that your laser's response to temperature changes is at all linear.

Eww. I don't like lookup tables, unless there's some linear interpolation going on -- it always seems there's something odd happening at the transitions.

Phil Hobbs's suggestion of using a heater with the TEC going full blast sounds good to me -- I suppose it's too late to slip a heater in there, isn't it?

There is a really nice optical spectrum analyzer. Sweet. Works like a champ. Long story short I got the PID thing going today but it's too slow for my taste, around 3secs per 100pm for TEC control. The difficulty in this case is that we must scan until we hit a resonance and then lock. The location of that resonance isn't known a priori. Pretty much like an etalon where it's all flat and then there is a valley of just a few pm into which we have to lock. Ok, we also have the diode current as a lasso but it still feels like barreling down Lombard Street in a tanker truck and suddenly having to stop on a dime.

Same here east of Sacramento. It even tore the pool sweep quick-disconnect off which never happened before. Now a cold front rolls in. But when I came home from the client my wife had already stoked the wood stove really good. Nice! Time for an Irish coffee.