Electric Pre-Rotator

wolfy

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What RRPM are you looking for Wolfy? What size blades do you have? How many prerotations do you need on a charge? is it important to be able to spin the blades up from a dead stop or can you give them a flip in the right direction? How do you want to control the ramp up, directly with a knob or slider or via buttons?
Gday John, 28' rotors hoping for 220+ if going electric. 5+ prerotations would be good. It is no problem to give the blades a flick before getting in. Something like the way you have controlled yours would be nice, my original thoughts were three buttons off/reset-slow ramp to 100rpm (for taxiing over rough ground)-full power, all within reach of the throttle so that once the roll has started the prerotator could be stopped at any time during the roll. For fun flying on board charging probably not necessary, but flying for work would require on board charging.
Probably asking a lot.
I just bought a new start battery a Lithiumax 300CCA 13 volt that only weighs 800 grams, could possibly be used in series but charged in parallel.
I am watching a few brushless setups at the moment and waiting on testing.
Or I will go again engine driven but with a torque limiting slip clutch, as to be able to go WOT with prespinner still engaged during the roll.

wolfy
 

jm-urbani

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it is a petty that they did not reply me when I have asked them the continuous max discharge rate
the details are mentionning a pulse max discharge of 400 Amp for 3 seconds (5200W) per battery but only for 3 seconds ( the time it takes for starting an engine)
I find this solution really expensive compared to RC command lipos of 60 C
 
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wolfy

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it is a petty that they did not reply me when I have asked them the continuous max discharge rate
the details are mentionning a pulse max discharge of 400 Amp for 3 seconds (5200W) per battery but only for 3 seconds ( the time it takes for starting an engine)
I find this solution really expensive compared to RC command lipos of 60 C
Yeah that's a shame JM I thought they might have been more helpful.
Yes I agree it's probably not the best solution, but the attraction for me was the fact that on board charging is built in to the battery.
But probably still better going RC with the BMS you mentioned.

Cheers
 

jm-urbani

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all I have to say is everyone needs to do the maths ... applying simple electrical laws ... that's it
 

jm-urbani

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a couple of news to share ( I doubt people get excited about this but anyway I try to share ...)

I have used an arduino microcontroler to drive the acceleration of my brushless, it works and it is super easy using the servo.h arduino library and the numerous examples of code available on the internet in which folks show how they are controling a RC servo using an arduino microcontroller

RC brushless motor controllers throttle can be controlled like a servo .

Then, I will be able to program an automatic acceleration, very smooth at the beginning of my tests checking the Amps figures during the process ( with a CC sensor probe that one can find for a few bucks on the internet)

I have found out that cheap hand CC sensor probes were now available, and it is a good news for us (or at least for me) , it will make possible to test our systems to be sure that the intensity will not exceed the controller and battery limits ..

then I will speed up the process according to the Amp figures resulting from my firsts tests in order to find a good balance btw mechanical/electrical limitations and pre-rotation duration ( that must be as short as possible not to eat the battery in excess)

still no tests to show the community ... really frustrating ... one day will come in the next months, years maybe ...
 
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Jean Claude

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Then, I will be able to program an automatic acceleration, very smooth at the beginning of my tests checking the Amps figures during the process ( with a CC sensor probe that one can find for a few bucks on the internet)
To obtain a constant current throughout the pre-launch , the programmed rpm must be according to a law depending on the inertia and dimensions of the blades.
To avoid this it is easier, by programming, to enslave the rpm control with the CC sensor you have.

The curve below shows a typical rotor acceleration under constant torque (i.e. with a constant current in the pre-rotator).
Sans titre.png
 
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jm-urbani

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Hi Jean Claude,


good to hear from you, difficult to know what to do without technical background


thanks for your suggestion, jean fourcade just told me rougly the same thing yesterday


my question is: provided that I have the amperage data and RPM data available for the arduino, how can I enslave the current to the PRM?


the only way to limit the current is to limit the acceleration, the RPM is only a consequence of the acceleration


I mean I can ask the microcontroller to stop pushing the accelerator for X milliseconds, if the current exceeds a certain Amp figure (for example 130 Amp 5900 W at 46v) and then continue to push the accelerator when the current is below a certain amp figure (I would need to write an anti bouncing code a bit like a shmitt trigger but is is possible)


the current would actually be constant

but there is a flaw somewhere in my reasoning as per usual ...

also I think that there is a hazard in controlling the acceleration via the Amp ... imagine that the sensor fails and give less Amp then the actual amp in the circuit ... the acceleration would skyrocket breaking everything and burning everything ..

this system is ok provided one have a good limitng current bms on top of the battery or a current limitation in the esc ...

cordially


jeanmi
 
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Jean Claude

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I piloted gliders C800, Bijave, C 310, airplanes Piper J3 , PA 28, Jodel D117, DR 220, Cessna 150, C
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The DC motor controller needs a speed setpoint that will be delivered by your Arduino
This setpoint must be initialized at 0 rpm at each power-up.
Then have it increment this set point by 0.5 rpm, each 50 milliseconds (for example) as long as the intensity obtained is less than the value entered (130 Amp for example) ), and do not increment it if this is not the case.
Thus, the acceleration obtained will be just that which will keep the current constant, better as if you were gradually turning the speed potentiometer while observing the amperemeter.
 

jm-urbani

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thank you very much, this is way of doing seems perfect
but it is too complicated to me for the time being when I have to focus on my crow hopes and other flying tests ...
I'm not going to break my head I will optimize afterwards to arrive at a "perfect" pre-launcher
and after having added a descent BMS to my lipo pack in order to prevent any over intensity in case the arduino fails to slow down acceleration during the process
the big problem is that the RPM sensors are not perfect at low rpm, as for the current sensors I don't know about their reliability
all of this needs to be tested before giving the system the capacity of accelerating for me
for now i will do like this
I will program a linear throttle increment over a minute and a half (for example), by putting timers between each increment
and then I will test using a current clamp to see what it gives in amps on the pre-launch
if the test is successful i will reduce the timers to optimize
I will maybe vary the timings between the beginning and middle of the pre-launch
I will lose in pre-launch time and in terms of battery consumption but hey no matter I am sure that I would still have enough juice to make at least 6 pre-lacings, I still have almost 400 W / h of reserve up to 30% of the battery
I don't need to pre-spinn in 40 seconds.
 
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Jean Claude

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I piloted gliders C800, Bijave, C 310, airplanes Piper J3 , PA 28, Jodel D117, DR 220, Cessna 150, C
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About 500 h (FW + ultra light)
I get it.
Below is my rough comparison between the amperage controlled method and the constant acceleration method.
The latter introduces a final rpm loss, despite a longer duration.
Of course, a programmed acceleration with several slopes can improve efficiency.

Sans titre.png
 
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jm-urbani

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Thanks again really,

In fact I have power margins, my brushless motor is a 8000W one and the lipos I have bought are high discharge 60C ones, so I think that even if it takes longer to reach my 220 rpm I will be able to get there ... I am not interrested in pre spinning at higher then this, on my monoseater I can get to 240 rpm before the brakes are no longer able to stop me , but I never exceed 220 rpm stick ahead,

if this electric system works at 220 stick back it will be the graal ... and enough for me

as for the constant current process I think I will do it as soon as I have "mastered" how to implement the chineses BMS I have bought

this system your make it possible to optimize the process and to save energy

it will be the next step ( rome ne s'est pas fait en un jour)

and of course the ultimate goal, with a bms would be to recharge the batt pack in flight ... (pérette et le pot au lait)
 
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