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'Square root (was: Pressure to MPH) [OT]'
2000\05\11@115642 by Jim Ham

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Yah, the root routine is brute force. But in its defense I have to say it
works. It usually converges in 6 or 7 iterations. Maybe someone on the list
will extend Scott's algorithm to 24 and 32 bits.

What I ment to post, and where my interest is, is the conversion from A/D
pitot pressure counts to velocity. Somewhere I'm getting this 20% error. It
looks like pure physics to me - the calculation is trivial once you've
looked up all the constants. So where is the error comming from?

Andy can avoid the square root if he doesn't have to display airspeed. Just
square the target speed and do the speed loop comparisons using the squared
numbers.


Regards,

Jim Ham

At 09:10 PM 5/10/2000 -0500, you wrote:
{Quote hidden}

Jim Ham, Porcine Associates
(650)326-2669 fax(650)326-1071
"http://www.porcine.com"

2000\05\11@122810 by Andrew Kunz

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I need to display the numbers, but what I'm planning to do is simply store the
ADC values in the data logger, then when the PC reviews the data work it all
together to come up with actual speed.

Efficiency of a PIC sqrt is of no concern to me, but I did check out Scott's
page and found it very interesting.

Andy










Jim Ham <spam_OUTjimhamTakeThisOuTspamPORCINE.COM> on 05/11/2000 11:50:42 AM

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Subject: Square root (was: Pressure to MPH) [OT]








Yah, the root routine is brute force. But in its defense I have to say it
works. It usually converges in 6 or 7 iterations. Maybe someone on the list
will extend Scott's algorithm to 24 and 32 bits.

What I ment to post, and where my interest is, is the conversion from A/D
pitot pressure counts to velocity. Somewhere I'm getting this 20% error. It
looks like pure physics to me - the calculation is trivial once you've
looked up all the constants. So where is the error comming from?

Andy can avoid the square root if he doesn't have to display airspeed. Just
square the target speed and do the speed loop comparisons using the squared
numbers.


Regards,

Jim Ham

At 09:10 PM 5/10/2000 -0500, you wrote:
{Quote hidden}

Jim Ham, Porcine Associates
(650)326-2669 fax(650)326-1071
"http://www.porcine.com"

2000\05\11@140758 by Spehro Pefhany

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At 08:50 AM 5/11/00 -0700, you wrote:
>
>Andy can avoid the square root if he doesn't have to display airspeed. Just
>square the target speed and do the speed loop comparisons using the squared
>numbers.

Careful with this one, it results in nonlinear controller gain, which can
affect control stability if there is much range in the setpoint.

Best regards,
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2000\05\11@141419 by Andrew Kunz

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Yes that could be a problem in a controller.

Since this is a monitoring project it doesn't matter.

Good point!

Andy










Spehro Pefhany <EraseMEspeffspam_OUTspamTakeThisOuTINTERLOG.COM> on 05/11/2000 02:07:29 PM

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Subject: Re: Square root (was: Pressure to MPH) [OT]








At 08:50 AM 5/11/00 -0700, you wrote:
>
>Andy can avoid the square root if he doesn't have to display airspeed. Just
>square the target speed and do the speed loop comparisons using the squared
>numbers.

Careful with this one, it results in nonlinear controller gain, which can
affect control stability if there is much range in the setpoint.

Best regards,
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Spehro Pefhany --"it's the network..."            "The Journey is the reward"
KILLspamspeffKILLspamspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

2000\05\11@164532 by Harold M Hallikainen

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On Thu, 11 May 2000 08:50:42 -0700 Jim Ham <RemoveMEjimhamTakeThisOuTspamPORCINE.COM> writes:
> Yah, the root routine is brute force. But in its defense I have to
> say it
> works. It usually converges in 6 or 7 iterations. Maybe someone on
> the list
> will extend Scott's algorithm to 24 and 32 bits.
>
> What I ment to post, and where my interest is, is the conversion
> from A/D
> pitot pressure counts to velocity. Somewhere I'm getting this 20%
> error. It
> looks like pure physics to me - the calculation is trivial once
> you've
> looked up all the constants. So where is the error comming from?
>
>

       I'd expect it to also be "pure physics." With a uniform speed, there
would be no airflow in the line from the opening to the pressure
transducer, so it seems to me that nothing can happen in that line. It
must just be how the wind is hitting the opening. Are there some special
requirements related to shape, size, or whatever? To determine the
windspeed, can I just stick a little pipe (pointed forward) out the top
of a car and run it down to a pressure gauge? (and watch it fill with
rain...).
       I still wonder about just having a short rod (cellular phone antenna?)
sticking out the top of the vehicle and measuring the forces on it to
determine windspeed.

Harold




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2000\05\11@171653 by Don Hyde

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The air (or water) inside the pitot tube and its associated plumbing doesn't
move (relative to the tube and the vehicle it's attached to).  This makes it
relatively insensitive to a few drops of moisture etc., though if you look
at airplanes, the pitot tube usually turns up as it goes into the plane, so
that moisture will not travel inside so easily.

The pressure inside is generated because the air entering the opening (it's
pointed forward) is being accelerated(or decelerated) to the speed of the
tube (and vehicle).  So all it takes is a tube pointed in the right
direction.  It's actually not particularly sensitive to the shape, though it
needs to be located away from other objects that distort the wind field.  As
someone else pointed out, it is the static port that is hard to get right,
since Bernoulli effect will tend to generate a vacuum, and locating it and
shaping it just right to avoid that is pretty tricky.

Since the pressure is proportional to the square root of speed, they tend
not to be very sensitive at low speeds.  In a typical light plane, the
airspeed indicator is pretty accurate and responsive at 100 MPH or more, but
at 50, (which is more important since the stall speed is usually around
there), they are seldom accurate to better than 10% or so.  On  most light
planes, the airspeed indicator stays rock steady on 0 until at least 30 MPH
or so.

The little stick sticking up is not often used because it creates more drag,
and is even worse at low speeds because it's proportional to the cube root
or something like that.  It also tends to sort of average over its length,
much of which is inside the boundary layer and is therefor sampling air
that's being dragged along with the vehicle, which will make it read low.

> {Original Message removed}

2000\05\12@063949 by Alan B Pearce

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>Yes that could be a problem in a controller.

>Since this is a monitoring project it doesn't matter.

No, but exactly the "gotcha" that will strike some years later  when someone
comes along with "that speed measuring thingy of yours, I want to use to control
......"

2000\05\12@081827 by Andrew Kunz

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Not likely.

Andy










Alan B Pearce <spamBeGoneA.B.PearcespamBeGonespamRL.AC.UK> on 05/12/2000 06:38:04 AM

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Subject: Re: Square root (was: Pressure to MPH) [OT]








>Yes that could be a problem in a controller.

>Since this is a monitoring project it doesn't matter.

No, but exactly the "gotcha" that will strike some years later  when someone
comes along with "that speed measuring thingy of yours, I want to use to control
......"

2000\05\13@111915 by Peter L. Peres

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Hi,

I've been looking into hydraulics books and I think I know where your
error comes from. My books mention that 'the Pitot tube has a certain
constant C that depends on the device, that must be applied to the
rho*V^2/2 term to obtain pressure'. It also says that 'with a Prandtl tube
the constant becomes 1 (unity)'. The Prandtl tube is a Pitot that is
'fatter' and has the static ports about halfway between the tip and the 90
degree bend. It also has the tip opening smaller than the total outer
diameter of the tube, and in the center of it. In other words, for the
boat it would be a hole for the tube in a small disc, or, a smaller hole
in the flattened front of the tube. I do not have measures for a Prandtl
tube, just drawings.

hope this helps,

       Peter

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