Repost: Making a Mighty Midget-Style Actuator

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Twitcher
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Repost: Making a Mighty Midget-Style Actuator

Post by Twitcher »

I came into this Galloping Ghost caper hoping to avoid the need to make my own actuator but that wasn't to be, because as you GG-ers all know, Rands are as rare as hens' teeth and them as 'as 'em aren't letting go! However, I didn't foresee any difficulty in cooking up my own rotary output, Mighty Midget - style actuator. In a purely mechanical sense, I was right; it was easy, but there were peripheral issues which influenced the final outcome. It often seemed I was taking one step forward and two backward.
All the while, I was in constant email correspondence with Phil, and his support, encouragement and advice was priceless at every stage. Not least, of course, with regard to the all-important recoder. He probably read my successive reports with amazement that such a simple task could be made so difficult!
Having bottomed it out at last, Phil suggested it would be helpful to some others if I posted an account of the ups and downs of the journey, since to the best of his knowledge, no-one had yet used his recoder in conjunction with anything other than a Rand or Adams.
There's no point in re-inventing the wheel, so this may help others who are either forced to take this route, or who just want to try it as a change from whatever they have. In all modesty this simple actuator now works perfectly; it's cheap and it's very easy to make. If you want to alter it that's up to you, but as a fall-back, it will work "as is". So here goes.

I've amassed lots of bits and pieces over the years, including small freeflight 'can' motors, gears, shafts, etc., and I've masses of GRP board, Paxolin, plywood and fixings, so knocking up the first prototype actuator was quick and easy. I'd previously acquired a couple of Phil's L239D recoders, ready for testing. A number of different motors were tried, all rated at 3v or thereabouts and as time went on, several others were purchased in search of the perfect setup. None were expensive but all were lacking. The main thing was the seeming impossibilty of obtaining a decent level of torque and so I went through motors in several sizes, progressively bigger and bigger, without success. It didn't dawn on me for a while that in fact the problem lay elsewhere.
I had been worried by the level of over-heating of the L239D from the start. That's not to knock the unit: I know it works perfectly well alongside a Rand. However, its current capacity is only 600ma, and my actuators were consuming at least this much, and usually more. Adding resistances into the motor circuit helped to bring it down a bit but unfortunately this also reduced the power available. That meant the tension of the spring centering became critical, so there was always a compromise, and an unsatisfactory one at that.
But then, as is mentioned elsewhere in the forum, Phil tested the Pololu DRV8838 driver board, with promising results. Capable of a steady 1.7A output at 5v, as compared to the 600m.a. of the L239D, this was potentially a game-changer.
He sent me one of these new recoders and the huge increase in power, delivered without stress, was immediately obvious. It was brilliant; in fact, the actuator motor was being over-driven, with the poor little 3v motor going like the clappers! A ten ohm 10W resistor helped but this wasn't efficient and it was obvious I needed a 6v motor. Phil suggested a few that might do the job, so I checked them out and bought a couple of Pololu jobbies (that name again) with the same physical dimensions as the smaller 3v motors I'd been using. At just a quid apiece, this was still a cheap option.
Initial impressions were great. Loads of torque from the 7.5:1 single-stage gearing set the birdcage at the back rattling for England! Although rated at 6v, the 5v delivered by the 8838 was clearly more than enough for the motor. A bonus was that from 600ma plus, the current drawn went down to around 350ma, so this was evidently a far more efficient motor. With that I thought I'd cracked it. Job done.
But there was still another 'Gotcha' lurking. It soon become apparent that the motor was generating interference - the dreaded "glitches". Remember those? These were frequent, randomly spaced blanks, where the motor would kick hard over to one side momentarily. Obviously not something I could put up with in a model.
Before this, I'd already soldered 100pf ceramic capacitors to the motor brushes, earthing them on the casing. That worked quite well when the recoder and receiver were well away from the motor. However, space restrictions in the model brought them within about 30mm of the motor, at which point the glitches returned with a vengeance. By now I was getting desperate and emailed Phil to seek advice. Strangely enough, just writing this email dredged up a bit of folk wisdom from years ago, relating to the old Speed 300/400/600 motors we had to use before brushless motors came along.
But Phil thought I might have to go with a separate battery to power the actuator, as being probably the only way to stop the interference from reaching the receiver or PIC, whichever one of them it was that was picking up the rubbish. I really didn't fancy that, because of the extra space it would need and the weight it would add.
I needn't have worried. The final solution was two-fold: 1. I substituted a 9" length of tightly twisted twin wires (heated with a heat gun to soften the plastic. When cool, the twist stays put) from the recoder output to the motor. That was better, but not quite perfect. 2. I then tightly threaded half a dozen loops of this twisted wire through a ferrite ring. This was a suppression trick commonly employed in the days before brushless motors. And, bingo! Problem solved.
I'll post some piccies of the actuator i.d.c., and hopefully also a video of it in action, provided I can work out how to do it! I'll also list the sources of the various bits I've used but I feel I should point out that sometimes it will be necessary to purchase them in multiples, as for instance the gears and shafts. They're not expensive though.
With the high level of torque available from the DRV8838/6v motor combination, a reduced gearing of say 6:1 would probably work just as well as the 7:5 : 1 ratio I've used. It's my intention to try this and I'll report again with the results.

Re: Making a Mighty Midget-Style Actuator
Postby Twitcher » Sat Feb 13, 2016 8:46 pm


Grabbing the opportunity last week to test my newly-built "Mangled Wot" GG test vehicle, I'm pleased to report that all went well. It was a cold, frosty morning (at 9 am) and completely calm, so the conditions were ideal.
The home-made rotary actuator and birdcage linkage worked perfectly, fully vindicating Phil's efforts in giving the recoder a healthier output. The 6V Pololu motor is clearly well suited in this application - there's absolutely no shortage of grunt. In fact, I'm confident that the actuator would easily fly a bigger model.
For the record, the model's wingspan is 48"; weight 37 ounces; wing loading 13.5 oz/sq. ft. The motor was rated back considerably to 230W on the throttle curve (100 WPP) and by using a smallish prop. I've now changed to a less powerful motor ready for the next round of flight-testing.
I think someone remarked elsewhere that it was slightly odd being able to hear the actuator noise above the motor, but as there was very little sound coming from that at less than half throttle, perhaps that was not so surprising. It does make a din!
Incidentally, my apologies for not having been able to upload any photos: I'm a forum virgin and I just haven't been able to suss it out!
As you can tell, I'm really chuffed with the outcome of all the experimentation I've carried out over recent months and I sincerely thank both Phil and Shaun for their help and encouragement. The upshot is that if you haven't got/can't find/can't afford a Rand actuator, all is not lost. For approx. £6 you can easily make something just as good.

And June 6th at Pontefract is in the diary.

Re: Making a Mighty Midget-Style Actuator
Postby Phil_G » Sat Feb 13, 2016 11:45 pm


David if you send the photos to me by email I will add them into your post.
What would be really fantastic is if you could somehow get some video, maybe a friend could catch your next venture and post it?
I for one know how much effort you've put into this so we'd love to see the results.
Once you've settled on a formula, a "How-to" with a list of suppliers would be great!
Very well done and congratulations on your successful Galloping Ghost flights !
Cheers
Phil

PS incidentally I'm a bit disappointed that you've had no replies, but you have had 87 reads so people are definitely interested,
I think maybe they're just keeping quiet until all the research is done! GG is certainly a hot topic right now, here and in the USA.

PPS
Twitcher wrote:
...And June 6th at Pontefract is in the diary.
I hope not, we'll all have gone home by then!
Its Sunday 5th with open flying on Saturday 4th!
www.singlechannel.co.uk
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Wayne_H
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Re: Repost: Making a Mighty Midget-Style Actuator

Post by Wayne_H »

Thanks for the repost :D
Did any more details, such as pictures or sketches ever surface? I'm enjoying finally (only taken 40+ years :shock: ) this GG lark & am keen to try & build a few actuators, so welcome any more info if at all possible☺
Cheers,

Wayne
Once a Retrobate, always a Retrobate............ ;)
Twitcher
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Re: Repost: Making a Mighty Midget-Style Actuator

Post by Twitcher »

Hi Wayne

I've only just seen your request for details of the re-born actuator, so apologies for the silence. It's dead simple; just a 6v Pololu motor driving a 7.5:1 reduction stage, and from there to the birdcage, an old-fashioned balsa torque rod. I'll sort out a few photos for you.

Some explanation of the birdcage might not go amiss. You could do no better than to go to Outerzone and study the plans of John Ralph's " Wagtail" and Bill Grundy's "PhleetPhoot". Those two were my mentors as a teenaged GG virgin, and you can be sure the set-ups shown on the plans will work. Essentially, you need to aim for twelve degrees of "down" and thirty to thirty five degrees of "up" elevator. Rudder movement needs to be thirty five to forty degrees each way.

The trim set-up is important too. Ideally the model should be VERY slightly tail heavy. That's so that you're normally needing a tiny bit of down elevator in S&L flight which, because it's a slightly faster pulse rate, helps to reduce "galloping".

As I mentioned in the earlier posts, the actuator is very powerful and I've found it beneficial to tame it by reducing the supply voltage to the recoder to around 3v. I used a small voltage reducer taking its supply from a spare receiver channel. Otherwise, driven direct from the recoder, there's bags of oomph to fly a model of up to 60" w/s.

More later.

David
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Shaun
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Re: Repost: Making a Mighty Midget-Style Actuator

Post by Shaun »

The Twitcher Rand is a very powerful unit.. I can confirm that as when I accidentally connected a 5 cell NiMh when testing it nearly shook the tail off my Das Ghosten Flugboat.
The only change I made was to construct the frame from PCB instead of wood as per David's original.

Cheers,

Shaun
To reduce the chance of interference I used a couple of capacitors and RF chokes on the motor.JPG
Stewart
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Re: Repost: Making a Mighty Midget-Style Actuator

Post by Stewart »

I think a reasonable copy of the Controlair actuator could be made using a GWS IPS geared motor, to get the throttle - drill a nylon bolt to be a press fit on the output shaft and assemble it between the bearings with an aluminium throttle lever fitted and filed to mesh with the bolt threads.
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Re: Repost: Making a Mighty Midget-Style Actuator

Post by Twitcher »

I've now got some photos of my GG actuator but don't know how to post them. Any ideas, anyone?

David
Twitcher
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Re: Repost: Making a Mighty Midget-Style Actuator

Post by Twitcher »

Having been shown the error of my ways, I'm hoping there will now be a couple of images below, depicting the parts needed to make a DIY GG actuator and the finished article, installed in my Mangled Wot Mk. 2.

It's not essential to copy this slavishly: a simpler way of mounting the shaft would be to attach a length of 2mm ID brass tube as a bearing to the top of the motor, packing it as necessary to achieve a "fag-paper" clearance between the pinion and gearwheel. That's a bit crude but it would do the job and save the bother of making the larger frame in which to hang the 2mm ID flanged bearings used in mine and Shaun's actuators. Your choice.

As you'll see, this is a modeller's, rather than an engineer's solution. The pinion is an 8T; the main gear is 60T, giving a ratio of 7.5:1, which is the same as the old Mighty Midget set-up. Pinions may be a little tricky but look at www.kpaero.com. They supply cheap pinions for all their motor/gearboxes. You need the one for the KP01, which may be a 9T, giving a ratio of 6.6:1, which should still be okay. It will be compatible with the 60T.

The motor was sourced from www.hobbytronics.co.uk. Another useful site for gears, shafts, bearings etc., is www.technobotsonline.com. The spring is nice but again, not essential - a simple rubber band will do, but will need to be checked regularly. Aim for as near as possible nil tension at rest; it merely gently assists the gear train to return to centre from the extremes of movement. You need 270 degrees of movement to get the deflections required.

Because this actuator is so powerful, I recommend driving Phil's recoder with a reduced voltage from a spare Rx socket. Have a look at www.uv3r.com/FlightChip/index.html and scroll down to voltage reducers. These are very small and light and wonderfully inexpensive - £1 each or 3 for £2-50. You'll need to modify the recoder (a simple job) but it's worth doing for the extra flexibility it confers.

Any problems, PM me. Should you have difficulties, I have spares of most of the parts and could probably help you out.

David

PS: Apologies - I see I've forgotten to mention the source for the 60T gear - www.technobotsonline.com. The shaft is from the same source but you'd need to buy a pack of them - far more more than you'd ever use - better is to find a scrap of 2mm piano wire. This will be a very tight fit in the bearings and will need reducing with emery cloth or wet'n'dry.
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Wayne_H
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Re: Repost: Making a Mighty Midget-Style Actuator

Post by Wayne_H »

David,

Thanks for sharing all the info from your considerable efforts - I shall definitely have a go at building my own! :) I actually have the gears, carbon 2mm shaft + bearings, and from memory, some suitable motors as well, all from my Wayne's Indoor World days in the early E-Zone years (mid - late 90's). Most of it is packed away in storage, so shall have to be "rediscovered" after our (soon) next & final move to Temora, but that is a whole other story.....😕

First step - learn more about bird cages :o :lol:
Cheers,

Wayne
Once a Retrobate, always a Retrobate............ ;)
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Shaun
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Re: Repost: Making a Mighty Midget-Style Actuator

Post by Shaun »

Here you go.. get bending :D

Shaun
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Twitcher
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Re: Repost: Making a Mighty Midget-Style Actuator

Post by Twitcher »

Out flying the o/d Mangled Wot Mik. 2 and the Veron Robot today at Middle Wallop: light wind and tolerable temperature, so a good time was had by all. The MW2 has the homemade actuator and the Robot has a Controllaire "Ghost" actuator (also GG). They attracted quite a bit of attention and probably at least one new recruit to the forum and to Phil and Shaun's technical website.

David
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