Small, Simple, Self-Centering Servo Circuit

Kingsparks

Joined May 17, 2011
118
Excellent article and just about the way you described things, just not a detailed. With that in mind and reflecting on my previous post, You could still get forward or reverse flight, just apply the blade movement at the correct point and angle.

Didn't the Russians have a coaxial helicopter? I seem to remember someone having one but if I remember correct it was double trouble as far as breakdown and maintenance.

I just looked it up, check out Wikipedia.

http://en.wikipedia.org/wiki/Coaxial_rotors

Not a lot about it but interesting.
 

SgtWookie

Joined Jul 17, 2007
22,221
Look at the amount of vertical separation between the rotors! Quite a bit. I have no idea how far our OP's blades are separated/are going to be separated.
 

Kingsparks

Joined May 17, 2011
118
Yeah, I saw that and was going to comment but forgot to. I still think I remember hearing they were a nightmare for maintenance.

Oh yeah1 I'll believe they can take a hit from a 20mm cannon and survive when I see it.
 
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Thread Starter

dafremen

Joined Aug 18, 2011
14
Bbak soon to address the points raised and thank everyone properly. Just wanted to mention that the blades should avoid collision or "blade strikes." Eventually, I'll also add a swashplate to the upper rotor and convert it to a Bell-Hiller Setup which means the blades and flybar will stay parallel to one another at all times, eliminating blade strikes.

daf
 

Kingsparks

Joined May 17, 2011
118
It will be real interesting to see what you come up with. If you could, would you post a picture of your helicopter or what you have to date. As I mentioned before, I am a part time RCer and so naturally have an interest. Thanks
 

SgtWookie

Joined Jul 17, 2007
22,221
Well, the term "coaxial rotors" indicates two rotors on concentric shafts, but rotating in opposite directions, and offset vertically from one another.
 

Kingsparks

Joined May 17, 2011
118
Well, the term "coaxial rotors" indicates two rotors on concentric shafts, but rotating in opposite directions, and offset vertically from one another.
Yesss, re-reading the OPs last post it looks like he is still going with the coaxial set up but with only the top rotor using the Bell-Hiller setup..? I really do want to see his helicopter. I can only imagine the linkage but you only have to look at some of the RC helicopter linkage to see how involved they have become.
 

Thread Starter

dafremen

Joined Aug 18, 2011
14
First off wookie, thanks for the differential idea. That'd be perfect for centering, but how to isolate it from the control input?



This is a coaxial heli with variable pitch lower rotor. It was a coaxial heli with fixed pitch lower rotor..but the manufacturer made it as easy as cutting two tabs of plastic, to make the rotor tilt-able.

On a fixed pitch coaxial like this one, the upper rotor/flybar combination tends toward horizontal and so is a stabilizer, providing mostly downward thrust. The bottom rotor is the one that remains tilted when the tail rotor is active, providing forward or backward thrust.

The idea here is to tilt the rotor instead of the helicopter..sending the tail rotor inputs to a servo motor instead of the tail motor.

Here is Fubar II:

FUBAR_II.jpg

Here is a closeup of the swashplate/linkage arrangement. (Notice the swashplate link point is approx 90 degrees behind the rotor link point.)

FUBAR_SWASHPLATE.jpg
View attachment 33503

Finally, here is me providing downward force on the swashplate control input, causing the front of the rotor to pitch upward and the rear to pitch downward. (ie. the rotor is tilted backward.) This would send the helicopter backward.

FUBAR_ROTOR_TILT.jpg

The servo will be strapped to the side. I would like to put a VERY small board under it..just enough for the wipers, the servo and a few components.

Since I have no desire at this time to know the ACTUAL position of the servo on the wipers, simple whether it's centered or not..I thought using the wipers as switches instead of pots might save on circuitry. Apparently not.

So what does a working circuit for this challenge look like gentlemen?

daf
 
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Kingsparks

Joined May 17, 2011
118
Hey Dafremen.

I have to admit to a bit of confusion now. In the original configuration did the tail rotor rotate in one direction to lower the tail and in the other to raise it?

If so then to hover it was stopped, was it not?

You disconnected the tail rotor as you said so what or how is it operating now?

If it is as I am thinking then I am even more confused as to your need to "center" the tail rotor motor. Could you explain please?
 

Thread Starter

dafremen

Joined Aug 18, 2011
14
Ok first off..thanks again for your patience. Let me state again that I'm doing this purely for the fun and challenge of it...most ly because I saw those two tabs that could be cut and thought..what the heck, let's try. (On a side note..the RC shop guy said he's never seen this type of conversion done before in his 30 years in the hobby. He asked me to bring it in when it's done so he can take pictures.)

This type of heli rotates by slowing down and speeding up the main motors. It moves forward by rotating the tail rotor one way, then moves backward by rotating it the other way. The battery is rated 3.7 VDC @ 450maH. The centered remote control stick simply turns the tail motor off.

Here is a close approximation of the Tail Motor circuit:
TailMotorCircuit.jpg

What can we hope to achieve by accomplishing this? Well since the tail rotor has to lift the tailboom weight to get to position, there is a lag between control input and response that can be removed by simply tilting the bottom rotor itself..instead of tilting the entire helicopter in order to tilt the bottom rotor. This should SIGNIFICANTLY improve the speed and handling of the helicopter.

As for my "instead of wiper pots..wiper switches" idea: Here is another idea along those lines. Helicopters have notoriously short flight times. I would like to reduce power consumption as much as possible. Weight too..for similar reasons.
CircuitIdea3.jpg

(The black parallel lines are wiper traces..not capacitors. The gap in the middle is center and would leave a floating output?)


daf
 
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Kingsparks

Joined May 17, 2011
118
Okay, I think I see what you are saying now.

Not to worry about your reason for doing this, as I have said many times, I will spend an hour or more making something on the lathe that I could run to town and buy for a few bucks. It' fun, a challenge and who knows, a learning experience also. I think everyone at AAC understands that or they wouldn't be here.

To make sure I am on the same page as you, you are now trying to use the DC current that operated the tail rotor to tilt the lower main rotor. Is this right?

Another question. What type control or receiver are you using? Is it a regular receiver with discrete servos or an escapement?

Have you actually connected the servo to tilt the rotor?

I have some ideas but just that and it will depend on exactly what you are using before they come together. I guess mainly is the question, are you wanting to use the DC voltage for the tail rotor to tilt your lower main rotor?

That would explain the missing prop on the tail.

There may be a way to use the contacts you showed, an idea is forming if this old brain doesn't just quit on me.
 

Thread Starter

dafremen

Joined Aug 18, 2011
14
YES! =)

I want to tilt the lower rotor backward or forwward using the tailmotor current..and have it re-center when there is no tail motor current.

A breakthrough =D
 

Kingsparks

Joined May 17, 2011
118
Okay, now we are on the same page.

I have never dissected my helicopters different sections and am not sure of the actual operation. Does the swapping of the ground and power occur in the control escapement, reversing the ground and power to the tail rotor?

If that is the case we may be onto something.

I am going out for a while. So if I don't answer until late tonight, don't think I have given up.
 

Thread Starter

dafremen

Joined Aug 18, 2011
14
I posted a schematic for the tailmotor circuit off of a SYMA S107..similar helicopter (same voltage, smaller current requirement.)

It's here:



Here's the servo with the "brush block" and the drawn in "wiper switches" (configure them any way you think might work :) )

 
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Kingsparks

Joined May 17, 2011
118
Okay, the reverse occurs in the controller.

So the idea I have will work if we can put it all together but not at the moment, gota run but that will give this old brain some time to work on the thing.

If my idea works out those contact strips you showed in the last drawing will be needed. Later, hold the fort down.
 

Kingsparks

Joined May 17, 2011
118
Okay Daf.

I'm back home, I think I ate to much. Anyhow, my idea has sort of jelled but I'm going to have to put some lines on paper, well on my computer screen anyhow before I can say much more.

One thing, your "servo"; I took a close look at the pictures and want to make sure I understand what you want.

First, it is no longer really a servo. Sure it has a motor and a place to connect a push rod but no control circuitry. So it will just run as long as it has current or until it hits a limit strip. (More on the limit strip later.) That means your tilt is going to be all the way, one way or the other, you won't have much if any control on that .

The signal or voltage for the tail rotor is low to high, running the motor slow, fast or somewhere in between. You will have to test it but I don't think you will be able to control the tilt position very well, it will be more or less all the way or centered.

With that in mind I have figured out how you can have the motor return to neutral when you release the control but as I said above, it will be all the way then back to neutral, either forward or reverse.

If this will work for you I will see if once I draw it up it still makes sense to me, and you of course. Get back to me and we'll go from there.

Roland
 
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