Re: Lahman, how ya doing?
- From: "H. S. Lahman" <h.lahman@xxxxxxxxxxx>
- Date: Sun, 22 May 2005 19:34:58 GMT
Responding to Hansen...
So when you say, "Input is temperature, output is a voltage difference" you mean the hardware is observing a temperature and presenting its value as a delta-V, right? Does the feedback control processing deal directly in delta-V values or is a software driver converting the delta-V to a temperature as it is sampled and before it is processed?
It's a raw delta-V. Our thermometers aren't calibrated, so we don't know exactly what the temperature is, anyway. We just need to keep delta-V near zero. Even better is that the thermometer is in an AC bridge with a ratio transformer on the warm end. Switching the transformer changes the setpoint, but it also changes dV/dT.
As if that's not enough, when a control action is calculate, the program takes the square root of the result and outputs it as a voltage. That makes the output linear in power, since P=V^2/R. But the output isn't really a power. It goes through an attenuator which reduces the size of the finite voltage steps of the DAC, through a shunt resistor, through the heater resistor... you can't know what the actual power is unless you run through the chain and work it out. The controller just puts out a number.
One of the problems I am having in understanding this is that I don't know what is hardware and what is software in the real controller and how they interact. So far my guess is (in order of appearance):
Thermometer: HW AC Bridge: HW ratio transformer: HW "program" that compute square root: SW Attenuator: HW DAC: HW shunt resistor: HW heater resistor: HW "controller": SW
Yep. Basically everything is hardware except the controller, getting a delta-V in and directing a V out.
Is the "number" put out (by "controller"?) what you were calling delta-V? Or a pseudo power? Or a pseudo temperature?
delta-V is the temperature, T-T0. In a somewhat abstract way, the number that the controller calculates is a power, except scaled by non-Watt units. Then the square root is taken, converting it to a voltage, but it becomes power again in the heater resistor. The square root linearizes the output from the heater, not the output from the DAC.
The previous message indicated that software was reading the DAC. That suggests the resistors are actually before the DAC in the signal propagation. It also seems odd that "program" would be in the middle of the hardware processing if it is software. Is that really part of what "controller" does after reading the DAC?
The output circuit looks like
r1 shunt heater
Vout ----/\/\/-+--/\/\/----/\/\/--
| |
/ gnd
r2 \
/ |
gnd
r1 and r2 form a resistaive divider, Vout'=Vout*r2/(r1+r2). Since it's digitized (13 bits) there's a minimum voltage step dV, therefore a minimum power step dP=(dV)^2/4R. A 1/20 attenuator increases the power resolution by a factor of 400, at the cost of losing some dynamic range.
The shunt resistor is a precision Vishay resistor, about 20K, the voltage across that gives the current since I=V/R. The controller isn't involved in that, it's just saved to disk for later analysis. And the heater resistor is a metal film resistor, also 20K, epoxied to the target, the voltage across that is measured, and again saved to disk for later analysis. And the power delivered is P=IV.
More than I wanted to know. B-)
I was trying to get a handle on how the temperature gets to the controller. The hangup I had with the list above -- that I extracted from your description of what I /thought/ was the thermometer -- was the order. The square root seemed to be done before the attenuator and the DAC output seemed to be fed to the shunt resistor.
However, the schematic above seems to be the hardware on the other side of the controller and Vout is what is produced by the controller after the feedback loop. So I think we are talking about two different things.
However, if that processing is about eliminating 60-cycle harmonics, it might not be necessary to your simulation. OTOH, if it is deemed necessary, then you have to insert the noise in your simulation of the temperature samples. B-) Also, I suspect a prime number that is not a divisor of 60 would be better than others.
I don't think it really matters since the system time constants are much longer than 7/60 second. There actually are issues with digital versus analog controllers because of the discrete sampling time of the digital controller-- the system can change between sampling times and will be less stable under a digital controller. But when the characteristic times of system changes are orders of magnitude longer than the sampling time, it really doesn't matter much.
In these days of cheap GHz processors I would expect the digitization effect would be pretty rare.
That's limited by ADCs rather than GHz processors. I don't know off-hand how fast they are, but the faster ones are pricier. But they're fast enough.
I was imprecise. My point was that if one can put a GHz microprocessor in a PC for a manufacturer's cost < $100, one should be able to find ADC and DAC ICs that are fast enough for a whole lot less.
OTOH, 60 cycle pickup can be substantial. I used to do deep crustal resistivity surveys where one lays out 1/2- to 3-mile grounded dipoles on the roadside. (I was a geophysicist in my misspent youth.) One rainy day we had a line break and I went out to fix it. I forgot my wire strippers so I stripped the wire with my teeth while my wet boots were in mud. Afterwards I measured 60V of 60 cycle on the dipole. It was more than enough the make personal wire stripping an adventure.
That sounds like fun. Except for the part about stripping the wire with your teeth. I've applied for a position as a plant engineer, at a power plant, which promised a lot of climbing, crawling, and working outside in extreme weather, and the plant being in the middle of a company-owned nature preserve. It's just about the best I could hope for.
Field Geophysics is second only to vulcanology in the prerequisite degree of looniness. (Anyone who walks on flowing lava has serious problems dealing with reality.) One often has big honking machines (e.g., seismic "thumpers"), explosives, or KV running around under working conditions that are abysmal. Throw in a duct-tape-and-coat-hanger maintenance pragmatism coupled with a Macho Outdoorsman attitude and one loses a fair number of people. [We were a very small company with only a couple dozen employees and we managed to lose two people. One was a tiger snake bite in Australia. (When you are 300 mi from the nearest habitation there's not a lot one can do.) In the other case the guy was laying out a dipole across a narrow valley. He didn't notice that as he went up the far side the wire from the winder in his back pack formed a catenary behind him until it hit a power line.]
<utterly unrelated stream-of-consciousness recollection>
The party chief for that crew in Australia was a local. His nickname was Blackjack because of his preferred operating mode in bar fights. In the field Blackjack ran a tight, traditional ship. At 3 PM the crew had tea from a silver service at a camp table with a nice white table cloth.
One time his crew was about 300 mi from the nearest water, the radio was out, and the water truck missed its scheduled delivery. A few days later they were down to the last ration. Blackjack insisted on using it for the 3 PM tea. One of the crew lost it over the situation. Blackjack dealt with the crewman in his usual fashion and then turned to the rest of the crew and said, "Stiff upper lip, mates! We're the last outpost of civilization." (Without the 'strine accent it needs some imagination.) So they had tea as usual and, fortunately, just as they were finishing up the water truck showed up.
</utterly unrelated stream-of-consciousness recollection>
Earth Science fields crews tend to be a different world. When I was in the Northwest Territories and needed roustabout labor to fill a crew I would routinely go down to the local RCMP post and bail guys out of jail to fill out the crew. Not the sort of management issues that the B-School deals with.
Alas, they never send one to Paris to look at rocks. Between the military and geophysics I spent most of the '60s lugging stuff through various swamps, jungles, tundras, deserts, and mountains and there was no such thing as a Bad Weather Day. [When a field geologist goes out in the morning he makes a bet with himself about whether he will slip on a rock in a stream and get drenched before or after lunch. (Most the good outcrops are are found in stream cuts, so one walks a lot of streams.) So a little rain is not regarded as a big deal.]
For example, I spent a winter 750 mi due north of Edmonton, Alberta living in a plywood shack. The average temperature was -30 and it hit -70 fairly regularly. Personal hygiene was a real character-builder. We had equipment that used mercury cells so it didn't work well in that sort of weather. The solution was an external battery pack on an umbilical cord that was hung in one's crotch. When the receiver started to go flaky it was a pretty reliable sign that it was time to hit the shack for awhile.
But I digress...
************* There is nothing wrong with me that could not be cured by a capful of Drano.
H. S. Lahman hsl@xxxxxxxxxxxxxxxxx Pathfinder Solutions -- Put MDA to Work http://www.pathfindermda.com blog: http://pathfinderpeople.blogs.com/hslahman (888)OOA-PATH
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