Repost: Making a Mighty Midget-Style Actuator
Posted: 28 Feb 2018, 19:03
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
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
) this GG lark & am keen to try & build a few actuators, so welcome any more info if at all possible☺
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.....
