RFFS-100 dual push-pull actuator

[Dave Robelen]

Hi All,
I recently started looking at installing my RFFS-100 system in a scale model (yet to be built) when I kept noting that with the new Li-Poly batteries getting the CG correct with tail mounted actuators could be a problem. After puzzling over the issue a bit, and sketching some options, I came up with the following solution.
The materials required are some .020 aluminum (roof flashing stock), .016" wire, contact cement and epoxy. If the electrons are behaving, I will post a picture of the complete unit. With both actuators, the weight gain is .4 grams including the plywood base. Another benefit is that in this configuration, the magnets are firmly self centering without any interaction, allowing nice clean hinging of the controls.
Cheers, Dave

[Dave Robelen]

And now a close up of a single unit.

[Dave Robelen]

And finally the bits and pieces before assembly. The wire bearing is sandwiched between the two magnets in a groove I cut with my cut-off wheel in a Dremel tool. I had to go real slow to avoid overheating the magnet.
Dave

[FLB]

Hi Dave,

Unfortunatly, I have no experience with RFFS system. So I can't correctly appreciate (is that english?) your work. But it seems to be a precision work regarding to the tiny scale of the set... Nice experimentation.

I have heard about Bob Selman's actuators. His your transformation similar?

Did you already choose your scale model for that system?


Regards

Frederic

[FLB]

I'm still looking at your pictures and I think I begin to understand how it's working... Clever :eek:

Frederic

[Dave Robelen]

Hello Frederick,
The actuators that I used began as a coil and magnet that mount on the control hingeline. The magnet is attached to the control with a curved arm so that it can rotate inside the coil with the axis on the H/L. With the availability of the wonderfully light Li-Poly batteries, it became a problem having that much mass on the tail.
Essentially, what I did was use the coil and magnet furnished with the RFFS-00 system, and assemble them into a configuration similar to the Rujinsk actuators like Bob Selman sells. The movement is completely proportional, just like a servo. I was pleased that this can be done at essentially no cost, and very modest effort.
I am beginning to lay out a Bristol Brownie with rudder, elevator and throttle control. The construction will be all sheet balsa, with the box shaped fuselage retained. I have a sp400 version of this design, and it is inspiring me to build the small one. The wingspan will be 14", and I anticipate the weight to be 22-24 grams.
Regards, Dave

[FLB]

Hi Dave,

Great. I think I will buy a RFFS system... After Chris3D MicroMoth and MicroPitts and your Bristol, I found that this system offers a lot of experimentation possibilities for a small investment in the plane materials (one sheet of 1/32 high quality balsa...). Even if the electronic isn't so cheap... But the result can be so nice. ;)

Regards

Frederic

[hillbilly6]

Man I cant believe how small that is! I hope you have a good magnifying glass! Just how small can these things be made?
Do you think we will see any 1/72 scale RC planes? Seems to me we are already about 1/43? Truly great work Mr Dave, I am impressed!

David

[HouseFLY]

Here is one of a series of tiny projects I am currenty working on to take advantage of proportional tiny actuators like those produced by Bob Selman and others. The other project is a tiny Helicopter smaller than the DIY House FLY, but having a 5 channel Receiver is a problem because currently there are none available yet. I have spoken to Bob S. and Nick L. to see if they'd be interested in helping me with a receiver for 5 channels.

I wil post more photos as projects develop. Pleasse don't mind the huge tires, they are that way for scale look and aesthetics they are made from CNC foam and painted identical to their models which came from some plastic toys.

Mario I. Arguello
www.micro-flight.com

[CalmAir]

That is very clever Dave. Who ever thought that we would start having problems with batteries being too light? Great time to be around!

Dale

[Chris]

That's great, Dave. I would have never thought of cutting a grove into the magnets, but it looks like it worked out quite nice. I'd say the .4 gram weight penalty is well worth the improved scale appearances and resolution of the CG problems associated with LiPolys. I look forward to seeing your Brownie.

[Dave Robelen]

Hi Folks,
Thanks for the kind words, It really is a great time to be involved with the micto models. More neat equipment almost every day. I could have purchased similar actuators fron Bob Selman that would have worked fine, but I really wanted to use my RFFS-100 gear.
Mario, have you condidered the 5 channel RX72-PRO receiver sold by Skyhoks and Rigging? It only weighs a couple of grams, and works like a charm. i have used one for a couple of years. It does not have connectors, but solder pads for the leads instead.
Regards, Dave

[GHMBO]

Good show Dave. I have been put off by the Giizmo aspect of this field and your development is a move in the right direction. I can see a pull-pull product in the future. Kind of like the juncture of the regen/escapment and tone/servo developments. Keep up the experiments - we all benefit.
Jim

[HouseFLY]

Hi Dave,

I've had one of the first Rx72-PRO receivers, when they first came out which incidentally I used on a 10" rotor helicopter called the Fairy FLY. I also used it in other smaller projects with the Westek tiny servos which worked out great.

It would be exciting to have a 5 channel RX with built in dual ESC to work direct with actuators, thouh. Bob S. sells the pulsers for the actuators and I am considering going this way in combo with the Rx72-PRO or a GWS RX stripped as an option, I am not sure how the pulsers are going to work out but I am waiting to receive them and check them out. The other is the RFFS-100 system but that is only 3 channels I have a feeling it willl work out great with my Ultralight as in the photos.

I am getting back to using actuators, because of the new systems that are being developed for such and I've has these super light and tiny models waiting to be fitted with smaller RF components. Servos would be great but until they get manufactured in 1/2 to 1/4 the weight of current smallest ones I guess we are stuck with actuators.

I find the whole thing about actutator as means of conrol fascinating and a science in itself.

Another fascinating controls are motors directly used with screws attached concentric to their shafts and used in a CNC fashion to activate control horns and similar. Has anyone tried using these?

Mario I. Arguello
www.micro-flight.com

[Dave Robelen]

Hello Mario,
There is one thing to consider when comparing the magnetic drivers. The RFFS-100 system applies about 1/2 of the battery voltage to the actuator at the maximum deflection. This is a function of the electronics and does require some consideration when using this system. As I understand it, the Selman drivers apply a much larger portion of the available voltage to the coils. So much to consider and balance. Your micro work is an insparation to me, and I am sure others. That little airplane is cute as can be, and those micro heli's just plain delightful. I am in awe at the skills required to work out the control issues and the ability to test fly such tiny machines without damage. Keep up the good work.
Regards, Dave

[bmutlugil]

Hi Dave,

Your actuators look very neat.. The actuator placement is what I don't like about the RFFS system, and this could solve it easily and efficiently. Besides c.g. problems, the normal placement looks very ugly, too.. If I ever buy an RFFS system, I will convert the actuators immediately..

I was wondering if the RFFS actuators had two magnets as sold, or if you added one more.. I also wonder at what frequencies these actuators oscillate - I heard it was much higher than the 3-4 Hz. of the galloping ghosts - but how high?

Do you know if anybody tried bipolarly varying d.c. voltages on these actuators? This would give operation similar to digital servos, alas without feedback - of course the electronics would be more complex.. In fact, theoretically it could also be possible to damp the oscillations (of the standard system) mechanically or electronically, if the mechanical oscillation frequency is high enough.. I wonder if this could be put to practical use, though.. It would require a spring and some mass :( mechanically, and capacitors, coils electronically..

My mind flies sometimes, instead of the model being painted :D.

Best regards,

Bulent

[Dave Robelen]

Hello Bulent,
I have been told (I don't rember who) that the RFFS-100 operates somewhat along the lines of the early Galloping Ghost systems. The difference is the operating frequency. The RFFS-100 operates at a frequency of several thousand hertz. In the typical installtion the actuators are centered with a hinge-line mounted piece of rubber band, and the resulting movements are very proportional to the stick. The principle difficulty with the rubber band arrangement is that it is either too stiff, or likely to be sloppy.
The in-line mounting of the actuators provides a strong mutual attraction between the magnets that provides a strong positive centering force. This does not seem to detract from the output torque, which remains adequate. And yes, the system is furnished with the two magnets butted together.
Although likely that I misunderstood your reference to drivers, the ones available from Bob Selman function very much like bi-polar ESC's. The high frequency pulsed output varies proportionally with the stick movements, and with proper placement, the actuators are magnetically centered. Of course if they are seperated, then a spring type system would be necessary to provide a centering force.
Regards, Dave

[dhurd]

Bulent,
DWE now offers the RFFS-100 with the Bob Selmann actuators instead of our standard actuators as a combo. www.smallrc.com

Dan

[bmutlugil]

Hi Dave,

This might seem foolish, but another point I don't know about the RFFS system is this :D : are the control surfaces oscillating at a few hundred Hertz, or are they only deflecting to a certain angle? This stems from not seeing the system in operation, although the sound of the pulsed signal can be heard while watching some movies on the web.. My intuition says the system should be oscillating like the galloping ghosts..

What I meant by d.c. activation was a theoretical case of applying a pure direct current to the actuators. In this case the magnet will move to an angle proportional with the d.c. current. The current has to be negative for deflection in the other direction. Of course in this case there will be no current at the neutral point and therefore no centering force, and the drivers have to be analog in nature.. The centering spring would be necessary in this case, and it would take some of the available torque..

As much as I know, what is being done with the pulsed system is this: the lengths the positive and negative pulses vary with the control input we give, while the signal period is constant (Pulse Width Modulation), and the magnet oscillates around (or moves to?) an angle corresponding to the average value of the pulsating voltage, which is a dc value actually. If the magnet is pulsating around the neutral position, the voltage has a zero dc average value, as the swings are equal. I don't know why the rubber spring should be necessary for centering in this case.. If the actuator is not oscillating but only deflecting, we should need the spring like in the case of dc activation..

I hope this is clear :D.


Dan,

I will keep this in mind. I have no intention of going to smaller models for the time being (I have no indoor site), so will stick to HS-50's and SHR RX, or similar. The Sperry I am building has a 35 cm. wingspan, and this is smaller than I like. I will go to a slightly larger wingspan on the next project, so the above setup should be enough..

Best regards,

Bulent

[Dave Robelen]

Hello Bulent,
The actuators are sufficiently damped, and the frequency high enough, there is no wiggling or vibrating, just smooth proportional deflections. This is why a centering force is necessary. As far as the indoor issue, I have no indoor site, but my small models fly fine outdoors.
Regards, Dave

[bmutlugil]

Hi Dave,

I should have known that the system was not oscillating, after learning that there was need for a spring.. All of my previous statements are unnecessary - and there is no need for dc activation. I was assuming that the system was oscillatory..

I am painting the Sperry, but could find no way of masking the paper surfaces as yet.. I tried double stick tape, but it tears the paper on removal. The electrostatic plastic does not work.. I found something called Parafilm, but it is not easy to apply.. At least the fuselage is ready, so I can finish installation of the RC gear..

I know some micro models can be flown outdoors, but this limits the period of activity to three seasons, and calm weather in those - wind is almost always present here, so flight seasons are quite limited.. The outdoor season is ending, so I am rushing to get the model finished..

Any news related with the airframe in which the new actuators are to be installed?

Best regards,

Bulent

[Dave Robelen]

Hello Bulent,
Although the actuators do not oscillate, they have zero voltage at neutral and without a feedback arrangement they still need a centering force. The dual conversion that I have shown with the two actuators mounted with the magnets aligned so that the ends point at each other provides an excellent centering force through the mutual magnetic attraction. This was not my idea, I forst saw it used on some actuators that Gary Jones built. It really does work well.
I believe we are on the threshold of being able to design and fly these micro models in a respectable breeze. With the Li-Poly battery cell, my Brownie is showing a thrust ratio (static) of .77-1 with a motor that is not the most powerful. Given enough thrust, and some research into decent low Reynolds no. airfoils, I would anticipate some pretty peppy peformance.
My little Brownie is coming along. The all wood airframe is complete, and I just put a coat of color on the entire model. I have taken some serious liberties with the color scheme. All of the original Bristol Brownies appear to have been painted silver overall with black lettering. In the interests of visibility and a more colorful model, I substituted yellow for the silver. Of course it is not scale, but I believe the effect will look reasonable. I have not taken pictures for the BB as I built it, mainly due to the fact that I have been shooting hi resolution pictures for publication. Betwwen all this talk, and the work on the airframe, I sure hope the silly thing flies all right;)
Take care, Dave