Hi,
we have the problem that we want to use servos on our balloon, but we didn't find servos for temperatures up to -70°C jet. We have already tested the AM Racing 1151 Digital Servo which is waterproof and has a metall gear and one of those standart cheap 9g servos. Both failed at temperatures under -50°C. Both of them also moved slower at lower temperatures. We tested both servos with and without lubricants, (without seemed to work slightly better) but all of our tests failed. Because of our weight limitation, we can't use insultation or heat the servos.
Do you have any experiences with servos or other motors at those temperatures and could help us?
Thanks a lot in advance!
How do you power the servo and the arduino ?
The servo might be mechanically fine (esp. if removing lubricant as grease freezing in the gearbox could also be a real issue) but simply starved of voltage/current as typical batteries would be quite challenged at that temperature (➜ which would perfectly explain the "moves slower then fails" symptom they described)
We have used an external power source which delivered constantly 5V and enough current. That can't be the solution, we measured both during attempted servo movements. The current mounted when the servos tryed to move. It was more like if the servos were jamming...
I suspect it is the grease on the axle inside the electromotor inside the servo that is contributing to the jamming. Even when you rinsed off the grease in the gearbox the bronze sleeve bearing of the motor axle may have grease left, that will freeze solid. You could test that by removing the gears in the servo, so it will be just the motor and then see if it still runs at -70 C
OK then this points at the factory grease in the gearbox solidifying at -70°C as the likely culprit for the jam and the current spike you saw is the motor stalling against it.
Completely flushing the gearbox with isopropyl alcohol to strip all factory grease and then running dry (bad for the equipment) or with a cryogenic grease like Krytox GPL 105 which is rated to -75°C could be something to try.
Also make sure the gears are metal all the way through the gearbox and not just the final stages, since nylon gears get very brittle at those temperatures and can bind under load.
Thinking a bit out of the box, can you clarify what exactly the servo is doing on the balloon ?
Some features could be driven by a shape memory alloy actuator: it is a piece of metal wire or strip that physically changes shape when you run current through it and heat it up, then returns to its original shape when it cools down. That change in shape is what produces the movement, so there are no gears, no motor, no grearbox, nothing to freeze or jam. They are very lightweight which sounds like it fits your constraints too, but the downside is that the amount of movement is fairly small, typically a few percent of the wire length, and the force depends on the wire diameter.
➜ may be if you clarify what you want the servo to do, we could come up with alternative ideas?
Thank you, i will try that out...
Well, there ist exactly one nylon gear, which might be the problem... We want to drop balast (Ethanol) by using the servo and release gas from the balloon with another. I don't think, that we can use a shae memory alloy actuator because the force needet is not that small...
Minus 70-degrees C is a big ask for all sorts of things.
A lot of materials are going to get stiffer and/or brittle.
Motor bearings can shrink, condensation will freeze.
Can't you use some insulation? Polystyrene is very lightweight. Aerogel insulation is very efficient, though expensive.
Weather balloons going to great heights, admittedly no moving parts, but sensors etc., use polystyrene housings.
There are all the other problems as the others mentioned.
Battery output will drop, lubricants can go solid.
Given a choice of gear materials, PTFE might be better. Self lubricating.
That's cold..
Looks like volz servos offers a low temp modification have to contact them for it..
They go down to -70c..
good luck.. ~q
I’m with Oandu the team, don’t wonder…
No, we will fly several days, so insulation will not work so long and heating needs too much energy/weight.
We use the LSH 14 batterie from Saft, that should work just fine.
We will ask them, thank you a lot!
The subject of balloons has cropped up before.
It might be worth checking out any restrictions with aviation authorities.
Most balloons at that height and for that length of time need radar reflectors as well.
Space Shuttle uses Braycote 600
I'm with @hmeijdam. Investigate first. It could be the impregnated oil in the sintered bronze sleeve bearings of the motor. Not sure how to get that out without damaging the motor.
Leo..
We already have the permission for the flight, thanks.
Good idea, maybe we could try to put the whole servo in ethanol to remove any oil of the gears and the motor...
Could you clarify what exactly the servos are supposed to do in your application?
Are they required to move to defined positions (specific angles/positions), or are they effectively just being used as a simple on/off actuator between two end states?
OP said
OK, understood, but I would like to know exactly what the servos are supposed to do in this setup, because I’m thinking more in terms of solenoid valves … or did I misunderstand something?
Sorry for not having explained that earlier, my bad. You are right that we could also try to use solenoid valves, but it seemed to be a lot harder to find such valves that would work at those temperatures and that would not be too heavy. Especially for the venting system it will be impossible to find a valve, that has an diameter of 30mm which we would need to be able to vent gas as fast as possible (the pressure in the balloon is really low in comparison to the enviroment), which is under 25g which we woud probably reach by using a construction with a servo. If you know valves that work at those temperatures and are lightweight, then this would probably also work...
Is the Arduino still running at -70C ?