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Hi, when i fly multirotors one of my main worries is to avoid it from crashing. Mechanically I've practically solved it because I usually fly an octocopter (motor, prop and ESC redundancy). My main concern right now is how to power the flight controller safely.
Initially I've connect all my 8 linear becs (in parallel) to my flight controller because I thought that if one fails, the rest would supply the required power. Later on I found out that a linear regulator may fail and may supply voltages higher than 5v and that would burn the flight controller.
A fuse is the easiest approach to solve this but I've read that sometimes it doesn't work well (fast enough). So i found the Crowbar Circuit.
So, i would use 2 linear BECs for redundancy and the crowbar circuit after each of the becs so in case it fails it will not burn the flight controller:
Another option:
I had the idea to use a switching BEC but i don't know if it's the best alternative. I've read that you cannot put 2 switching BECs in parallel because the voltage is generated in waves that may interference each other. So to solve this, one switching bec would be in use and if it fails, the backup switching BEC would take place.
The diagram for this is shown below:
If the SBEC number 1 fails, the Schottky diode would alternate to the SBEC number 2. I think that using 2 diodes would be better for the redundancy (i don't know if it's ok to do that). Could I use a relay instead of this diode?
What do you guys think? Will it work? Any idea is very welcome! ;D
Does anybody have the part list to do a crowbar circuit for the APM? I think it should supply 5v and 500mA, is that right?
Best regards
Comments
Second thoughts : this crowbar circuit can only work if you have a 1Volt differential between the source and the trigger voltage of the diode. As we know APM accepts a voltage range between 4.8 and 5.25, we just have a 0.45V difference margin to trigger the circuit which is unsufficient.
I'm afraid this would not be the best solution, except maybe by using semiconductor voltage sensing circuits ? like here : http://www.onsemi.com/PowerSolutions/product.do?id=MC3423
Yes indeed. And your power supply should supply between 5 and 5,25 maximum (be careful all of the UBEC, BEC and other SBECs do not necessarily provide exactly 5V. You need to measure to be sure. Plus the measurement without charge is different than when connected to APM)
Explanation from wiki:
This crowbar circuit has an 8V power supply, and triggers at 9.1V. To change the power supply rating, the zener diode, ZD1, needs to be changed to reflect the new trigger voltage. It should be about 1V higher than the nominal supply voltage.
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eduardo, vou responder em ingles para os outros entenderem tambem ;)
You said the crowbar circuit worked fine but would it be possible to list the components that are needed for the circuit?
I agree that are many factors that cause the multirotor to crash but i've been solving the others problems so far:
That's why that I think that I should focus right now on electronics safety. I'm planning to use a parachute too but i haven't enough time to continue the experiments.
Matthew, what do you mean? That the fuse from the flight controller would blow too? Would you have a circuit schematic to share with us?
Hughes, thanks for the information! ;) If I want to change this circuit to supply power to the APM, I'd have to put a zener diode that opens at 5.4 and change the F1 fuse from 0.25A to 0.5A?
Cheers!
i found this schematic on wikipedia that gives more details about how a crow bar circuit works. The example cuts off at 9.1 V (for APM it would be necessary to have a zener diode that opens at 5.4 or 5.5 volts)
One potential problem. If one of the triacs fires then it will short until all fuses are blown, not just the closest one.
You may wish to put an ORing circuit after the fuse to prevent reverse current from blowing the other fuse. That could be as simple as a diode but if you wish to avoid the extra voltage drop you will need an active circuit.
Bernardo, a probabilidade de voce tem problema nos motores, escs e bateria é muito maior que ter um bom regulador bem dimensionado dando problema.
Como eu te disse, a maioria do pessoal liga um monte de conectores de ESC em paralelo e isso causa instabilidade nos reguladores devido a deriva termica.
Vai saber se esses octos que caem por ai do nada nao tem problemas na alimentação dos 5v ?
Ta cheio de pessoas montando multirotor por ai que nem sabem oque é um diodo e ja estão começando a ocorrer quedas frequentes destes equipamentos.
VOu te dizer uam coisa séria, nao pense que porque tem um octo que se falhar um motor ele nao vai cair.
Ele cai sim.
Eu voo multirotores a 4 anos. Tenho varios hexa, uns 3 octos e uns 3 quads.
Ja vi todos eles caindo por problemas de motor ou esc.
Uma vez meu hexa estava voando a uns 700m de longe e ele começou a voar meio BAMBO.
Dei o comando para ele vou para home position e quando ele aterrizou vi que faltava uma helice.
Outra vez aconteceu a mesma coisa mais foi um fio do motor que quebrou.
Isso aconteceu com um hexa.
Outra vez meu octo caiu porque uma helice quebrou e a vibração foi tão forte que o IMU ficou louco e ele acabou caindo .
Como voce pode ver ... nao é só redundancia que resolve problemas.
As vezes coisas simples evitam grandes problemas.
probably i will use an similar circuit on my MUGIN.
I will look at the SBEC i use to see what kind of circuit it use.
Some time the crowbar protection is no needed.
But as i tell you, for cheap equipment i think the protection is not needed.
This is the crowbar circuit i ever used in expensive projects.
It workfine.
I used this on my old mp3car project and this save the mother board 2 or 3 times.
Greg, thanks for the tip, but i'd like something cheaper or homemade / DIY
eduardo, i've never this kind of problem with BECs either but i'd like to turn the multirotor the most reliable possible. I've already had a 7805 that was supplying more than 5v (it was brand new).
no mugin, vai fazer igual ao esquema que postei acima? Um crowbar depois de cada SBEC e depois um diodo schottky?
Por acaso tem como compartilhar o circuito do crowbar?
Abssssssssss!
Bernardo, voce esta enganado se acha que pode ligar varios 7805 (usados na maioria dos escs) em paralelo.
Se voce fizer isso note que um ou mais reguladores irão esquentar muito.
Nos antigos datasheet do 7805 falava que nao se deve fazer isso devido a deriva termica.
Para fazer isso voce tem que colocar resistores equalizadores na saida do 7805.
Tome cuidado com isso. A maioria do pessoal que usa multirotores nem sabe que deriva terminca existe pois a maioria usa somente como hobby e nao entendem nada de eletronica.
O unico equipamento que tenho que vou usar proteção crowbar é no meu MUGIN 3m.
Nesse eu vou usar 2 SBEC com diodo para chavear entre eles e tambem vou usar proteção crowbar porque tenho equipamentos caros dentro do aviao.
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