future tech questions

[alan 0]

Not currently flying but still observing, and I have some questions:

 

Little brushless motors, being used as steppers, are now used to control camera gimbals. Is anyone using them to direct drive 'plane control surfaces, or as undercarriage servos?

 

Hobbyking is now selling big "pancake" brushless motors for quads. Is anyone using them for direct drive helicopters?

 

Now that data downlinks are common, how far are we from my prediction of stick feedback - meaning use of a strain gauge say, to measure force on a control surface, and sending that data back to an actuator which applies pressure to a control stick?

Then you could actually feel what the little pilot inside feels!

[Martin 0]

Stepper motors have been around for a long time. They have the advantage of being quicker and more precise for small movements (which is why they're commonly used for CNC machinery). They do have some disadvantages though:

* the risk of losing steps, if the load is too great or the steps are too quick (which would then mean your control surface is permanently out of sync with your TX, probably causing a crash in pretty short order)

* the related problem that if the surface is moved while the power is off, it will be permanently out of sync when the power is back on

* slow speed for large movements (as there is a maximum stepping speed which reduces with load)

* you'd still need a gearbox for a control surface, as the typical number of steps per rotation would give unusably poor resolution, so you still have all that weight

* steppers draw motor current continuously to hold a position (as opposed to servos which only draw motor current when the current position is wrong), so battery life would probably suffer

 

The thing that steppers excel at is small precise slow movements, especially if the load is well constrained (hence their wide usage in CNC). Even when steppers are used, they are quite often used in conjunction with servo-style feedback to effectively produce a more precise servo.

 

I'd also say that stick feedback is still a way off. Right now, although telemetry is increasingly common, there is no information passed back from the servos to the receiver (since the servo protocol is very simple). Even SBus and other newer systems don't seem to pass back information from the servos (which is a pity, as it would be interesting to monitor servo performance to tell how much of a safety margin there is and things like that). Servo motor current would probably be the easiest way to monitor control surface load.

 

I have also been thinking about trying to build a stall warning device for a model aircraft, based on airspeed and acceleration.

[alan 0]

Thanks Martin, for that detailed answer.

I was particularly interested if anyone in the club was experimenting with things like this, but I did find reference to direct drive servos which turned out to be conventional servos in a ply housing to directly drive ailerons on big 3d models. And Graupner (I think) brought out the "Funcopter" a few years ago which had a direct drive rotor.

Yes, you would need servo current and direction signals for feedback to the sticks.

[Bravedan 106]

Yes, Graupner did the "Funcopter", and allegedly it was a pretty dire thing, too..............even when "updated", i.e. fixed as much as possible was how it was described to me when I had the misfortune to come into contact with one and was asked to fly it as the owner could not).