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Old 05-09-2011, 06:41 PM   #271
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The novak Sentry has a data aquistion rate of 10data points per second. This should be sufficient for any 1 cell testing.

McPappy Aluminum Flywheel Inertia Calculations Corrected version III


This flywheel can be thought of as two cylinders with holes (or two anular rings)

I first took some dimensions from the flywheel.
Total mass = 95.8 grams
Large ring = 2.450 inch OD x .375 inch ID x .450 thick
Small ring or armature Support Extension = .375 inch OD x .125 inch ID x 1.00 inch long

I calculated the volume of Aluminum in each ring so I could determine by proportion the mass of each separately.


The total Inertia of the flywheel is 4.53 x 10-5 kg m2

A check of this new value would be appreciated. Calculations with superscripts and subscripts are attached.

An armatures inertia is about 4.13 x 10-7 kg m2 from previous work.




This will be needed for calibration of the flywheel dyno.
Attached Thumbnails
Dyno, Homemade, Using a Novak Sentry Data Logger, Continued, The Experimental Thread.-mcpappy-flywheel-003.jpg   Dyno, Homemade, Using a Novak Sentry Data Logger, Continued, The Experimental Thread.-corrected-inertia-calc-iii.jpg  
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Last edited by John Stranahan; 05-11-2011 at 01:05 PM.
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Old 05-10-2011, 06:17 AM   #272
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Let me try and explain. I don't a lot of experience with these small motors or the RC world in general.

I assume you will read more than RPM with the logger. Maybe voltage would be helpful. Sounds like you use a lot of 5 volt gear. So, lets say you want to measure the voltage with the logger. If the logger has say a 0 - 5 volt input and it has a resolution of say 8 bits. This allows you to resolve to 5 / 2^8 or about 19.5mV. This would be in a perfect system. Now add noise on top of that. If the loggers input were say good for 0 - 20 volts. With 8 - bits we could resolve to 78mV. For 100 amps, .4 amps. Add one bit of quantization error and your looking around an amp.

The concept is the same no matter what your measuring. I.E. pressure, temperature, etc.

For the time calcs, lets assume that the motor can spin at say 20,000 RPM. So about 330 revs / seconds. This means it takes 3ms (0.003 seconds) to make one rotation. If you can record at 10 Hz, this means the rotor will rotate 33 times for every one sample.

Normally, with any sampling system, there will be some sort of filter set to at least nyquist. This filter will cause a phase error in the data being collected.

I am not suggesting that the Novak logger uses 8-bits. They don't appear to provide this level of detail. I am just trying to give you some things to consider when looking at data loggers. Better to be informed and have an idea what your needs are before you spend your money.

If I were testing electric motors, I would be interested in seeing what each winding is doing. How well are they balanced. Does one winding make more torque than another. Do the bearings show any signs of wear or damage, etc. All things that could be looked at with a good dyno.

If you start looking at drivetrain components, like clutch slip, the sample rates again would be very important.

For your flywheel setup, I would not have guessed it had a constant accelleration. Also with the power you can put out that the peak accelleration would far exceed the ability of the logger.

I am not sure what the alloy being used is.
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Old 05-10-2011, 12:38 PM   #273
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(Note I have replaced the input with the image below.) Here is part of the output of the sentry It is the early part of a 10.5 run on 2 cell.
We calculate power by mathematically modelling the derivative of the angular velocity curve. Smooth acceleration data results. We measured voltage and amperage at the battery. The sentry has more inputs. These are the ones we use. Attached is a text file direct from the Sentry. The other choice of logger is the Eagle tree. As the sentry appears to be not available that would be my choice.

There was a discrepancy in the density of the flywheel. I have remeasured. The diameter of the flywheel is 2.450 inch instead of 2.5 inch reported. I shall recalculate the Inertia later today. I also forgot to divide diameters by 2 to get radius. This is done and reposted earlier.
John
Attached Thumbnails
Dyno, Homemade, Using a Novak Sentry Data Logger, Continued, The Experimental Thread.-data-master-10.5-run.jpg  
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File Type: txt 1.txt (35.3 KB, 78 views)
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Old 05-10-2011, 02:27 PM   #274
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The raw data is good. Thanks for posting it. Maybe we could take a little time to try and understand what your seeing.

If I look at your data, the voltage step size appears to be about 94mv. So for an 8-bit system, I would guess this logger's input voltage range is about 24.0 volts (256*.094). From their site, they talk about it being a 20 volt input, so I would guess it's an 8-bit ADC.

Could you try a simple step function? So maybe spin the motor up then start the logger but don't have the speed sensor hooked in. Then plug in the sensor. So the logger see's 0 then max speed in a step.

Guessing the analog may be handled a different than the tach. So maybe try the same thing with the voltage. Just connect after it has started recording.

These tests may provide you with some idea about any filters they have. If there is no filtering, we expect the step to cause a verticle line. I would guess the speed input is digital but maybe not. If digital, they may filter it in software.
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Old 05-10-2011, 02:32 PM   #275
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John,
You came out with the right answer but in the equation the 2.5" (later changed to 2.45") is a diameter and therefore should be divided by 2 before squaring. (Also applies to the .375 OD and .125 ID in the second formula)

Last edited by calvin; 05-10-2011 at 02:46 PM.
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Old 05-10-2011, 02:58 PM   #276
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Calvin-Thanks. I made corrections. Please recheck.

Joeqsmith. I am not sure if data logger theory can help me. The RPM sensor is built into the motor sensor circuit which controls the drive of the motor. I cannot do what you suggest. We are just trying to wrangle as much as we can out of an existing data logger.

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Old 05-10-2011, 03:31 PM   #277
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John,
I don't have one to measure but your percentages don't add up to 100% which would induce a problem in the mass for each portion. I think you need to look at how you treated the hole in the large disk (I assume the hole goes all the way through). Is your small disk actually two disks each .5" wide or is the measurement 1" including the big disk. If it is the latter the "hole" portion for the large disk would be .375 (the OD of the small disc). If it is the former you just need to take out the hole (.125") for the big disk in your first calculation. Hope this helps.

Last edited by calvin; 05-10-2011 at 04:18 PM.
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Old 05-10-2011, 04:24 PM   #278
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Calvin- Revision II is posted. Percentages now add up. It was a matter of carrying through enough digits. McPappy Flywheel inertia only changed in the 3 rd siginificant figure by 1 unit. For purposes of aluminum volume calculation I considered an imaginary hole in the big disk at .375 inch ID. The smaller shaft support, I treated as one cylinder .375 x 1 inch long with a .125 by inch hole in it.

McPappy flyweel Inertia = 4.53 x 10-5 kg m2

Note you won't be calculating this number to use the dyno on flywheel. You will just enter it on the spreadsheet on the data entry page.
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Last edited by John Stranahan; 05-10-2011 at 11:03 PM.
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Old 05-10-2011, 06:20 PM   #279
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I was just wanting to get some idea as to how much of the shape of the curve was due to the logger and how much was real. I guess you could assume it's all real.


I did not know that your logger is part of the motor controller. I was thinking it was a simple unplug the connector from the logger sort of test.
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Old 05-10-2011, 11:00 PM   #280
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We went through a similar set of inquiries or criticisms of the Sentry on our previous thread. My answer is that at this point in history the measure of accurate time seems to be easy. Digital counting also is common and easy. These two measures are probably used to measure RPM. I see no need to distrust them. They are what we use to calculate power. There should be little error here or little flavor seeped into an RPM vs time curve. The amps and volts are of less concern, but they are used to calculate efficiency. I have tested the amps with a turbo thirty and made a small correction for a different 200 amp sensor. The volt measure seems to agree exactly with my digital volt meter.

We measure RPM with a special harness that is optional for the sentry. It is a y type plug. One end goes to your motor sensors, one end goes to the speed control (hence the control aspect) and the middle yís off to the sentry. It may be possible to do what you suggest, but I donít see how its helpful.
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Old 05-11-2011, 03:12 AM   #281
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John,
My calculations still disagree slightly with yours (don't know if it really matters). The difference seems to be in your first calculation where you use the thichness of the "hole" for the large disk to be 1 in, shouldn't it be .450 in? I've attached a zip file of my spreadsheet in case you're interested.

FYI the Eagle Tree V4 is showing in-stock at Tower for about $60.00
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Old 05-11-2011, 05:51 AM   #282
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Agree, the concept of calculating RPM is simple and should be fairly accurate. The question I have is with the filter. This is a dynamic system.

It not a case of mistrust. Its being data driven. Using a volt meter, tach, etc. to compare against the logger is pointless. These will have their own filters and you will be looking at steady state conditions.

Another simple test would be to just remove the flywheel and repeat your first test. No change to your harness.
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Old 05-11-2011, 12:11 PM   #283
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Quote:
Originally Posted by calvin View Post
FYI the Eagle Tree V4 is showing in-stock at Tower for about $60.00
There are two models of the Eagle Tree V4 loggers. The one you quote is the version that has roughly 14 gauge power leads. This might be fine for 1S but for 2S I believe it wouldn't be enough for anything more than 17.5.
Personally I'd get the 150 amp version that has 12-13 gauge leads. I believe it is $20 more.

Also, I'd suggest the magnetic rpm sensor ( if using the flywheel and one can mill a cavity for the needed magnet ) . I've tried the optical, the magnetic, and also the electric rpm sensor. So far the only thing that gives accurate results is the magnetic sensor. It does require a magnet to be attached to the flywheel. The supplied magnets are 1/8 in diameter. I milled the side of the flywheel so the magnet sits flush using super glue to attach it permanently.

It is possible the electric rpm sensor can be made to work but from my experience it isn't plug and play. You have to buy both separately.

Unfortunately, neither the Novak nor the Eagle Tree are without faults. I'd say the wiring simplicity of the eagle tree is superior and also the sample rate is up to 5 times more, but the Sentry's rpm sensor is far superior to the Eagle Tree. The software for the Novak Sentry is fairly polished but the eagle tree software is still under development so it can evolve. The Sentry, discontinued ! Tough call really !
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Old 05-11-2011, 12:18 PM   #284
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Quote:
Originally Posted by calvin View Post
John,
My calculations still disagree slightly with yours (don't know if it really matters). The difference seems to be in your first calculation where you use the thichness of the "hole" for the large disk to be 1 in, shouldn't it be .450 in? I've attached a zip file of my spreadsheet in case you're interested.
Calvin-Thanks. I see this (hopefully) last mistake. yes it should be .450 instead of 1 inch. I will use the spreadsheet.

Joaqsmith-I will do the last thing you suggested. Testing will begin soon.

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Old 05-11-2011, 08:09 PM   #285
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Sounds good. Looking forward to seeing the data. The text files are fine.

Quote:
It is possible the electric rpm sensor can be made to work but from my experience it isn't plug and play. You have to buy both separately.
What is an electric rpm sensor?

This is what I came up with for my super cheap torque sensor. It's a Honeywell linear pot with a spring. Need to make some weights to calibrate it. 100 grams is about half travel on pot.

Last edited by joeqsmith; 05-11-2011 at 08:43 PM.
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