Altimeters - NARAM60 - MicroPeak?

Discussion in 'Rocketry Electronics and Software' started by vcp, Aug 12, 2018.

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  1. Aug 12, 2018 #1

    vcp

    vcp

    vcp

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    So before NARAM60, I had zero experience in dealing with altimeters. After NARAM60 I have mostly bad experience. I'd like to try and figure out what I'm doing wrong. I had four flights with normal, very straight boosts to altitude, but three of those reported altitudes in the 30-50 meter range (using two different MicroPeaks). I don't think I was the only one having this problem, as I counted about 20 flights in the results with qualified flights but similar unlikely low altitudes. There may have been more, as there were some reported as 'TL' or not returned. (But I also don't know how many of those were MicroPeaks.)

    Along the way, I heard a couple of comments in the nature of: "People are having trouble with MicoPeaks."

    I don't have any reason to think that there is a particular failure in the MicroPeak altimeter, but I do think that there is some procedural problem with the way I, and perhaps others, are using it.

    The instructions say to turn it on, wait for it to report the prior altitude, then after a delay of one minute for prep time, it will be ready to detect a launch. The problem that I had with this is that after prepping, checking-in, getting pad assignment, setting up the tower and piston; easily thirty minutes could have elapsed. 'De-prepping' the model to recycle the altimeter before launch would have been difficult and wasn't built into the process. Perhaps it should/must be?

    Vent holes. Per some threads and comments here I'd assumed that vents wouldn't be necessary. But on a second flight, a vent was added with the same low reading result (utterly certain the altimeter was on). Thoughts on vents re this problem?

    Finally, I've heard about a 'light-sensitivity' problem, but no information on the nature of this problem. Does exposure cause low readings/no readings/failure to detect launch/ what? Is light exposure a bad thing before, during, or after a launch?

    Anything else I'm missing or should/shouldn't be doing?

    TIA.
     
  2. Aug 12, 2018 #2

    UhClem

    UhClem

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    All pressure sensors, since they are built from silicon strain guages, are light sensitive. If strong light strikes their sensing port it can result in anomalous readings. So long as that port is not in line of sight of the vent holes, you are safe.

    The Micropeak records data, have you looked at any data for the suspect flights?

    It appears that the only thing recorded is the pressure data and not the internal state. That is important because it uses a Kalman filter. Although with the data and the Kalman gains it should be possible to see what was happening.
     
  3. Aug 12, 2018 #3

    BEC

    BEC

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    I worked returns at NARAM. Most of the “track lost” altimeter flights - at least when I was on duty - were MicroPeaks, but not all of them. I don’t know why this was...but we saw them both having not detected launch (still slow single flashes) and some which seemed to have not detected landing (faster single flashes when presented to be read). I knew when I saw one that was not flashing at all that we were in good shape to get some kind of reading, since the MicroPeak turns itself off when it thinks it is done flying.

    As to why this was....I don’t know. I’m sure that there are going to be conversations amongst the experts and the maker to see what might be going on.

    On static ports: the idea that they aren’t needed is nonsense. All of the altimeters we were using record air pressure and if they can’t sense the air pressure at apogee while the rocket is there, then they are going to give lousy (generally low) readings. Some of them then do the conversion to altitude onboard while others - notably the Adrels - only record pressure and depend on their computer interface program to turn that data into a time vs. altitude graph and data table.

    On light: yes, MicroPeaks (and others which don’t have onboard protection including Adrels and early FireFlys) should be protected from bright light. This generally manifests itself as noise in the data but the effects are not - at least for me - predictable.

    I flew two Adrels and a MicroPeak in my Classic Model entry (Nova Payloader on a C6-5). All of them were protected from sunlight inside that clear payload section. One Adrel and the MicroPeak were in dark fleece sleeves my wife made and one Adrel was wrapped in blue Quest wadding. The data from all three agreed very nicely (within a meter in max altitude). The Adrel that was in the tissue wrapping rather than cloth pouch had more wiggles in the descent data (the payload section swung around under the ‘chute quite a bit) but it wasn’t severe.

    One MicroPeak (and one FireFly) was lost and not returned, though one FireFly did turn up at the lost and found and was there for a couple of days and so far as I know never claimed.

    I flew Adrels in my altitude flights and had generally good luck with them. Some did not, but this was more a function of the mishmash of versions of altimeter versions, computer interface hardware versions and computer software versions that are out there....

    I really like the MicroPeak and hope that whatever is going on can get sorted out.
     
  4. Aug 13, 2018 #4

    vcp

    vcp

    vcp

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    I should have mentioned that all of my altimeters were not directly, but essentially in the parachute compartment. I'd heard people talk about 'flying naked', with the altimeter just tied in with the parachute, so I thought this would be ok. By 'essentially' I mean that they were in a slot fitted into the shoulder, but open to the parachute compartment.

    So re light, they weren't exposed except during prep, and after ejection they were in their shoulder slot, which shouldn't have been direct sunlight. Though I wonder if the plastic was opaque with respect to sunlight. But if sunlight just causes noisy data, would I be concerned about that? Can it trigger a false launch detection?

    I guess launch detection is what I'm really wondering about. Did they detect a false launch and record a 'flight', before the flight? Changes in ambient air pressure over time before launch? On venting, they were fully exposed to the pressure at altitude after ejection, I'd have thought it would have recorded apogee then if it hadn't already recorded a 'false' flight.

    So being 'in' the parachute compartment, there was the overpressure of the ejection charge, which would be a 'low altitude' spike - would that terminate the flight recording?

    Don't have the magic cable, so I can't see the recorded data.
     
  5. Aug 13, 2018 #5

    James Duffy

    James Duffy

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    I suspect that this is the problem. Thirty minutes is an awfully long time for an altimeter to bake in the sun, be subjected to gusts, and get jostled about during prep. The flyer's prep procedures should be optimized to minimize the time between altimeter arming and launch, and the NARAM range procedures should be tweaked to help. At a minimum, pad assignment should take place before altimeter issue/check/verification, allowing the flyer an opportunity to get everything at the pad tweaked and ready to go beforehand. That way it would be a simple matter of popping the altimeter into the rocket, sliding the rocket into the tower, engaging the piston, and launching.

    I've just returned from flying Scale Altitude at the World Championships, and this flow was almost exactly what was used. We were typically able to go from altimeter issue to launch in less than three to five minutes (note that the Adrel altimeters we used have a three-minute arming delay). Admittedly, we had a dedicated prep and launch area assigned to the US team, but a similar flow could be achieved by simply making pad assignment the first step in the process.
     
  6. Aug 13, 2018 #6

    BEC

    BEC

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    Gary - Shoot - I had “the magic box” at NARAM and could have helped you take a look at the data to see what we could learn from the data. Often there is a big spike at ejection - sometimes up, sometimes down - between the pressure change (if the altimeter is exposed to the ejection charge as in your case) and sometimes up or down due to all the shaking that happens - or both. The sensors are also affected by very high G loading. The software tries really hard to filter this stuff out, but it doesn’t always succeed.

    As for sunlight and plastic transitions - Apogee has you paint the inside of the transition black in their payload altitude model (the Midge) to mitigate this. I can’t see how sun exposure would trigger a false launch detect (the MicroPeak needs to see a pretty rapid altitude increase to 30m to detect a launch) but I suppose it is possible.

    There was some inconsistency in altimeter checkin at NARAM - I didn’t have to show my altimeter was “ready” for any of my four altitude flights. I don’t think that I was using an Adrel was the reason for this. I certainly agree with James that the checkin flow where altimeters are involved needs to take into account the times that altimeters aren’t “watching” for pressure changes - a period which is generally around one minute (PerfectFlite devices, MicroPeak) and defaults to 3 minutes with the Adrel (I have mine set to two minutes). That’s the time when folks should be stuffing altimeters in and buttoning up the rocket. Unfortunately, no altimeter indicates “ready” until after this “I’m not watching” period is over.

    Also, if you pop open the compartment that contains the altimeter (especially with your hand over the static vents or if you don’t have any vents) outside this period you can certainly cause a false launch detect.
     
  7. Aug 13, 2018 #7

    vcp

    vcp

    vcp

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    Never even occurred to me to ask about a cable or download the data.

    Black paint. Got it.

    That last is probably the killer. There was a lot of fumbling going on in getting the models set in the tower/piston.
     
  8. Aug 14, 2018 #8

    zog139

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    Because the amount of time from check in to launch can be a while. What you want to do is check in, get your pad, then right before your model is ready to fly, turn the micro peak off and back on. Make sure it flashes the last flight, then install in your model and fly. I've had good results with this process

    YMMV
    Jim
     
  9. Aug 14, 2018 #9

    BEC

    BEC

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    Jim is quite right, but of course this process only works if your model is set up so that you can do it that way without too much hassle. Another part of integrating altimeters into competition....
     

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