kwilliby

My Feedback: (13)

Could someone please explain a few things about servos to me? I'm looking for servos for my 16 lb. 28% Velox and am a little confused.

1. What is 'dead band width' and how does it affect speed etc?

2. Whick is better? Coreless non-digital high torque vs. a Digital non-coreless high torque.
example. Hitec 945MG vs. 5645MG.
The 945 is Coreless non-digital with 4 usec dead band width
The 5645 is Digital cored with 8 usec dead band width.
3. If I buy Digitals and don't need to sync two on the same control surface do I need a programmer?

4. Would a Digital cored servo have as good centering as a Coreless non-digital?

I've read that Digital gives almost instant torque and better centering. And that Coreless gives better centering and accuracy. So a Digital analog is somewhat better centering than non-digital with almost instant torque? And a Coreless non-digital is more precise centering than non-coreless etc.

Any help would be greatly appreciated.
Thanks.

dirtybird

My Feedback: (5)

I don't remember where but this same point has been discussed just recently on this forum. I suggest you do a search.

kwilliby

My Feedback: (13)

Thanks, I'll look around.

XJet

Deadband is a measurement of the amount fo change in stick movement or servo position required to activate the servo-motor.

For instance, if the deadband is set to 5uS then a change in the transmitter stick position that produced a 1, 2, 3 or 4uS change in the transmitted pulse-width would result in no movement of the servo. However, when the stick moved enough to change the pulse width by 5uS, the servo would wake up and move to the new position.

One might think therefore, that a small (or zero) deadband is a desirable feature -- but (as always) there are some caveats.

Since the motor, gears, output arms and linkages all have mass (and therefore inertia), using too small a deadband will produce a situation where the servo constaintly hunts back and forth as it "bounces" back and forth from each end of the deadband while trying to find center.

A digital servo with too small a deadband is one of the reasons you'll hear them "sing" even when the transmitter sticks aren't being moved -- the other reason is usually stiff linkages.

Generally speaking, a digital servo (regardless of the motor) will have better holding power and more authoritative movement than a non-digital. The reason for this is that instead of checking and correcting the servo position relative to the stick position just 50 times a second (as with a regular servo), digital servos do it many, many times more frequently.

This allows for the use of smaller deadbands and thus more precise centering/resolution, plus it also means that you get almost the full rated torque of the servo even when only very small stick movements are involved.

The real benefit of *coreless* servos is that the moving part of the motor (which must be acellerated and decelerated each time the servo moves, is much lighter than in a cored motor. This means that coreless servos can have an even lower deadband and thus even better centering and resolution -- without runing into that bouncing effect I mentioned earlier.

[quote3. If I buy Digitals and don't need to sync two on the same control surface do I need a programmer? [/quote]
Maybe, maybe not.

In theory, they should be pretty close out of the box, however all electronic components are slightly difference (the better the quality, the smaller that difference) so odds are that no two servos will move an identical amount for a given control input. Bearing in mind that digitals will be far more precise (and therefore notice any small difference when ganged together on a single surface) balancing them with a programmer is almost always a very good idea.

If it weren't for the bouncing created by a deadband that's too small for a given mass of servo-drive-train, then there'd be no difference in resolution. However, because (at least in theory) a coreless servo can have a narrower deadband, it can have a very slight advantage in terms of centering and resolution.

I think you'll find that a digital (with or without coreless motor) will tend to have better resolution and centering than a non-digital - especially once you start putting some load on it.

The only real advantage of a coreless is in very fast servos where the low-mass of the moving part sof the motor mean it can start and stop moving far more quickly. If you look at the Hitec range, the 925/5925 servos (non-digita/digital) servo is their fastest "full sized" unit and it uses a coreless motor. Compare its speed to that of the 3-pole and 5-pole "high speed" servos and you'll see the effect that a coreless motor can have.

kwilliby

My Feedback: (13)

That was a GREAT explanation! Thanks so much for your time and effort explaining this to me. I think I have a good grasp on what i need now.

Thanks again!
Kent.

dirtybird

My Feedback: (5)

One should add that the coreless motor is constructed inside out and the armeture can only be supported on one end. The support for the magnet on the inside must be made from the other end. Also the armeture contains no metal other than the copper of the winding. This results in lower inertia but also means the copper has no heat sink like the cored motor. This make it very easy to burn out when stalled.
For those that don't expect to compete with Chip Hyde, the cored digital is the best chouce in my opinion.

kwilliby

My Feedback: (13)

dirtybird: Well I'm sure not even close to 'Chip Hyde' level. Not for a few weeks anyway! I think the HS5645's will work just fine for me. Thanks so much to everyone for the valuable info. I've learned a lot!

Regards,
Kent.