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Thread: What are you using for gps and datalogging

  1. #1
    Join Date
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    Default What are you using for gps and datalogging

    Hi,
    I just would like to hear from fellow members what GPS they use to record their speeds and if they were able to measure it against something , a tachymeter for instance, how accurate it was. Id also like to know what datalogging you are using and hows that working out. I'm not interested in any debates or "they are not the holy grail stuff" who doesn't know that I just want your experiences to figure out what will be the best unit to buy. I know it may not be accurate to the trap but I need some semblance of measure other than rpm and pitch speed which is what I use to estimate speed currently. I do have plenty of chronographs with Tachymeters so I guess I could try to verify with that to see if I'm even in the right area at all with calculation.


    Regards
    Hubert

    If you dont know what a tachymeter is:


    A tachymeter (pronounced /t?ˈkɪmətər/) is a scale sometimes inscribed around the rim of an analog watch with a chronograph. It can be used to conveniently compute the frequency in hours of an event of a known second-defined period, such as speed (distance over hours) based on travel time (seconds over distance), or measure distance based on speed. The spacings between the marks on the tachymeter dial are therefore proportional to ​1t, where t is the elapsed time.
    The function performed by a tachymeter is independent of the unit of distance (e.g. statute miles, nautical miles, kilometres, metres etc.) as long as the same unit of length is used for all calculations. It can also be used to measure the frequency of any regular event in occurrences per hour, such as the units output by an industrial process. A tachymeter is simply a means of converting elapsed time (in seconds per unit) to rate (in units per hour)

  2. #2
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    A SAWs trap is just a fancy chronograph, it times you across the measured distance of it and works out your speed from that. The problem with using a watch's chronograph (besides thae fact that you probably want to be looking at your boat not your watch on a speed run) is that of parallax, you can measure out a course on the banks, but unless you can run the same speed as your boat or can stand an infinite distance from the lake to watch it, you need to be in 2 places at once.

    I have used an old fashioned Garmin Etrex handheld GPS, and a Hobbyking Quanum speed meter which is no longer sold but the SkyRC speed meter is an available equivalent. These all worked well to a point, being reasonably consistent in the vast majority of runs to the slower boats that I have run through the traps, and my cars speedo. They do wig out and produce nonsensical results on occasion, so they need a certain amount of filtering of the results, but if i get 2 similar results on the same boat I trust it to be within a couple of MPH of the results it gave. They do have VERY limited datalogging abilities though, with instantaneous maximum speed being the only useful information being recorded for your perusal on shore. Lots of people use these for bragging rights and taken for what it is, I think they are a pretty accurate method of measuring the highest speed a boat achieved during a run. It may have only achieved that speed for a tenth of a second however, which with faster boats is a very different thing to a two way average through the traps.

    I have also used a friend's Garmin GPS running watch, which was by far the most expensive GPS I have used, and by far the worst, it and I believe all the watch type GPS units update at 1hz to extend battery life with their tiny batteries, and it gave readings all over the place. Whereas the handheld units, the RC specific units and I believe most phones, update at 10hz.

    I have since switched to an RCM V2 GPS datalogger for my SAWs boats, which logs data at 10hz continuously during the run, it can also log battery, voltage, RPM and 3x temp sensors, the software will overlay a trace of the run onto a map, and you can get all that data at any point on the run, so when testing and tuning prior to a SAWs you can see your rate of acceleration, how fast you were going into the imaginary traps, how fast you came out, your average speed over the 100m/110yds, what your voltage dropped to under peak acceleration, etc.

    If all you want is an answer for your mates down the pub who ask how fast it is get the SkyRC unit, it is a neat standalone box with an LCD screen, no wiring or other equipment needed.
    If you want information that will help you tune your boat, go for the RCM. With power and all the sensors it is a bit of a mess of wiring, and you need a phone to display the data, so it is not for everyone, but there is a LOT more data available.

    Eagletree Elogger and SM Unilog are dataloggers with 10hz GPS sensors available to them, and pitot sensors too, (which i don't believe are as accurate as GPS, but can be used to filter out erroneous GPS data (not that this is as much of a problem with a GPS logger as one of the speed readers as you can see the spike and the real data before and after it), I have an eagletree which records current, voltage, rpm, 3x temps, and capacity used, but its current shunt is only rated for 150A (it has been many years since I used it so could be misremembering it but I think it actually stopped reading at 200A), while you can add to the shunt and increase its capability to 300A but I don't have a way to measure the resistance well enough to calibrate it properly so it becomes an estimate used for comparison only, (being an EE I bet you have a great multimeter, and could calibrate it OK), the Unilog does the same things for the most part and has a 400A shunt available for it.

    Prices for the SkyRC and RCM are similar, Eagletree is higher, and Unilog is higher still.

    I also have a Castle Ice 50 ESC with datalogging built in, this is a smaller, neater and lighter solution for electrical datalogging. It offers 2 less obvious advantages too, the main one to me was the built in temp sensor is MUCH faster responding to the quick changes in temp (produced by very high currents of short duration, when running SAWs), than the external dataloggers can be with their external thermocouples. The other being that only the ESC loggers record ripple voltage, which allows you to tune your cap bank. On the down side I don't believe any of them have a GPS sensor so if you want that data you have to run a separate logger anyway, and ESCs with logging are often more expensive. IIRC my Ice 200s stopped reading at 320A.
    Paul Upton-Taylor, Greased Weasel Racing.

  3. #3
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    The tachymeter in that case just requires another spotter and a hand signal when it crosses. I would expect a nuts on number but ur trying to see if you in the ball park. I got logging on all my drives and I believe eagle tree has in addition to datalogging gps for speed but it also has a tach so you can make some comparisons there as well. Let me digest more what I've read here before I say more.

  4. #4
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    Yes as I mentioned above, the Eagletree has RPM sensors available (both hall effect, and brushless), as does the Unilog (adding an optical option too), but both are both are sold at an additional cost, as are the temp and GPS sensors.

    The RCM comes with GPS, Voltage, brushless RPM sensor, and 3x temp sensors included, whereas the Elogger base has only has volt and current sensors, and the Unilog base has only voltage, current and altitude sensors, when you add those sensors on the Elogger is about double the price, and the Unilog almost 4x as much.

    You do miss out on current with the RCM, but while my Eagletree v3 uses a shunt that can be beefed up, I just looked at their website to confim pricing and the v4 now uses a hall effect sensor to measure current not a shunt, so I have no idea if they can be tweaked to read over 150A. At this point for your use with a SAWs 1:8th scale I would discount the Eagletree and chose between the RCM with no current capability and the much more expensive Unilog.



    Another thing to consider is their means of gathering the data, I consider the RCM a datalogger but really it is telemetry, there's a transmitter in the boat that sends real time data to a base station, that then sends it on to your phone, and their phone app displays and records it. Whereas the Elogger and Unilog need the data downloading to a PC after a run either through a cable or a memory card.

    There are positives and negatives to both, the RCM takes up more space in the boat, but the base station is easier to lug around than a laptop and having the data without un-taping the boat is very convenient if you are doing several tests in a day. The true Loggers are a chink smaller and a little lighter in the boat, and don't require an antenna, and are very convenient for a single run then review the data back home with no equipment other than the logger needed at the lake, but are not nearly as convenient to use multiple times a day, as they do require a PC to view the data, and physically connecting to that PC after every run.
    Paul Upton-Taylor, Greased Weasel Racing.

  5. #5
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    Okay... Thanks for this. Glad I read everything. Let me ask I've been thinking about a drone flight card . It would allow real time logging and telemetry to your radio with the castles available today. The hobby drone racing is pushing the control hardware tech working with the like of the us military and many other top tech companies. If you used the first person google then you wont have to look at the radio for the telemetry it will be a HU display and the flight card would allow alot of other cool things as well. With APD HV PRO and betaflight I can utilize low pass and notch filtering on the drive to attenuate triplen commutation frequencies. Remove noise and modal vibrations and core losses from the stator etc. Does any of that sound worthwhile to you or do you think its too much? I can explain how it does that but it would be lengthy. In short it looks at the electrical revolutions of the motor and based on a majority function implemented in the MCU coding it creates the active filter. If u want a deeper look ill send you a technical brief from Microchip?.


    Edit : I will grab a sky rc as well for the speed measurement. Dont wanna use the phone.

    Regards
    Hubert
    Last edited by eXoNerated; 07-01-2020 at 09:51 PM.

  6. #6
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    Hi Paul.

    They say....
    'NO one believes in GPS anymore" But all the civilized world guides there vehicles with them everyday in 2020.No t to mention the military etc, When they stop believing paul? I know u didnt say this. But anyway.....

    Check this idea out. You dont have to be a child prodigy to grasp it either.....
    If two people had head to head speed run battles the GPS wouldn't need to be accurate to the real speed anyway as long as both individuals had the same logger. So all a non believer has to do is buy himself a sky rc like you or I and see where his designs are. All wed have to do is approve on style gps and thats waht everyone would have to use Simple....meeting silly metrics that still don't change anything about how things work. most people that have used good ones usually report only a few mph difference. Who test and tune only in rowing pools with timing......? That idea is absurd.

    Like you said il take your advice and by the sky rc at least we can not believe together.

    Some Things are Just Silly.
    Hubert

  7. #7
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    I know very little about drones, having only messed with a toy grade one for long enough to know that it is not for me.

    In case you misunderstood, when you use the RCM system your phone stays safely ashore, once you open the app you can put it back in your pocket until you want to see the data.
    Paul Upton-Taylor, Greased Weasel Racing.

  8. #8
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    No, I got that part. I was just saying I didn't want to use the smartphone gps to record speed. I dont wanna put a 1000 dollar phone in the boat and lose it. I tested the phone gps against the car speedo and it was on.


    Thanks,
    Hubert

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