Re: Lahman, how ya doing?
- From: glhansen@xxxxxxxxxxxxxxxxxxxxx (Gregory L. Hansen)
- Date: Mon, 23 May 2005 17:30:55 +0000 (UTC)
In article <C95ke.15476$_f7.3129@trndny01>,
H. S. Lahman <hsl@xxxxxxxxxxxxxxxxx> wrote:
>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.
Oh, well, it's a traditional PID controller, except instead of outputting
a PV + I\int V dt + D dV/dt, it outputs a sqrt(PV + I\int V dt + D dV/dt).
The final calculation at the bottom of the routine is a square root,
before it's sent to the DAC.
The temperature measurement is just a straight voltage input from the
lock-in. If the lock-in measures 2 nV, then the proportional term is
P*(2e-9), the integral is increased by I*(2e-9)*dt, etc.
>
>>>>>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.
You'd think so. If they can cram a billion transistors onto a chip, you'd
think they could cram 32 comparators on an ADC.
>
>>>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.]
....
>Alas, they never send one to Paris to look at rocks. Between the
Figures they wouldn't. And Paris has many interesting rocks to look at,
too.
--
"Experiments are the only means of knowledge at our disposal. The rest is
poetry, imagination." -- Max Planck
.
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