Forum: Virtual Classroom - Experimenting With Servo Motors

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Experimenting With Servo Motors

For those interested in experimenting with Arduino, here is an interesting video and discussion from Mechaduino gallery.

which explains how to improve the control of servo motors.Bobbie 25th Apr 2016
Thanks Bobbie, I have not seen Mechaduino before, looks good! On a similar topic, for Thumbpointer, Wade has been working hard on alternative gearing so that the antenna cannot easily be back-driven. This is a major issue, considering ThumbNet locations, potential wind loading, and the fact that I would like them to be fully autonomous. Mechaduino can solve the problem of re-pointing a back-driven antenna but we'd still have the issue of potential damage when wind drives the antenna quickly against end-stops.

Wade is finding the gearing to be something of a challenge, and I've been contemplating whether we could simply use off-the-shelf servos with a wide range (like winch servos). They're expensive if bought individually, but perhaps we could get a good discount on bulk orders, as we did with the NEMA 17s.
Shaun 26th Apr 2016
I don't know how far you have gotten with the planetary gear system, but it is a very compact and strong system.

Maybe a workable way to reduce the stress against the end-stop is to reduce the speed of rotation a few clicks as the rotor reaches the end-stop.. ie; slowly stop the rotor near the end-stop. The last few degrees of rotation could be reduced by 50 percent or more without detrimental signal loss, as the satellite is moving at its slowest rate at AOS and LOS.

And, why have an end-stop since you are using stepper motors. Place the emergency-stops a few degrees beyond the stopping point of the rotor and use the mechanical stop for emergencies like a broken gear or run-a-way controller.,

A tried and true way to reduce the impact against the end-stop is to use a force absorption device such as a coil spring or other impact absorption device (dashpot).

A good explanation is found here:

One of the most steady gear systems is the worm-drive, but is probably the worse type unless made of high-strength steel. the pinion gear has very low level 'teeth' and is easily stripped or slipped with soft materials.

The winch servo is a good idea. Most use a ratchet lock operated by a solenoid to stop reverse rotation. It is a pinion (pin) almost exactly as used in a standard ratchet hand tool wrench. If used in soft material gears, it probably will break the gear.

I think that if you could find a cheap source for high strength steel gears, then most of your problem will be solved. Then the pinion lock will work fine. All medium and heavy duty winches that I am aware use extreme high strength steel gears as they are easily broken. Many machine shops can make the gears for you, but I think that you can find standard steel gears already in the market place that will work...

Another weak point in your rotor is using plastic like material for bearings. Flange mount ball bearing with a cam-lock to lock it to the shaft is rather inexpensive. The sealed bearings will last for many years as they are packed with grease from the factory and are triple sealed to keep dirt out and grease inside.

One last point of interest... using a soft material for the main housing allows a lot of warping and twisting in windy and icy conditions. This allows the gears to move causing friction and maybe slipping. Also allows the housing to bind on the mast causing a lot of friction on the bushings (more gear problems and maybe missed pulses to the servo causing antenna alignment errors)... the friction of the bushings could be removed by using flange ball bearings, as they will not twist..

I do not mean for this to be a diatribe. Hopefully the group will not take it that way... just my 2c worth after a sleepless night and I had nothing else to do...

Bobbie 26th Apr 2016
Diatribe is way too big a word for me, Bobbie, I can't imagine how you managed to come up with it after a sleepless night ;-)

Thanks for your thoughts. I'm not talking about end stops for positioning purposes, I'm just talking about whatever the workings eventually bash up against if the pointer is thrashed by the wind. This will be something, whether it is an end stop that is deliberately included for the purpose, or just the cables tightening. The sudden stop will inevitably not be good for the mechanism, so the best approach is to not let it move in the first place, and that's why I want to use gears that can't be backdriven.

I agree that more sturdy materials are better. As I've said before, we have two approaches to this - one that everyone can build easily, using 3D printing etc, and one that is intended for the more "professional" ThumbNet stations that we will provide to 250 volunteer groups for free. For the latter, we are looking into buying gears, housings etc in bulk to keep them low cost. This means that we will be able to provide non-ThumbNet builders with cheap components too.

PS, Wade tells me that he has made good progress with a worm gear system, I've tried to avoid this for various reasons, engineering and otherwise, but we'll go with whatever works :-) We should know more by the end of the week.
Shaun 26th Apr 2016
Thanks Shaun

Dashpots can be very small and powerful. they work great for absorbing shock such as we are discussing. I imagine something the size of a pencil eraser with about 2 cm of action. The impact forces that it can absorb can be logarithmic or linear or whatever you need. But, your design may not need a dashpot... just a thought..

As for using worm gears, the type of stepper motor used in the 3-D printers may work for you.. such as this one:

It has metal gears in the planetary and has a pinion gear for the worm assembly... I think this one has harden aluminum gears, but there is another listed that has steel gears, but is not as powerful as this one. The threaded shaft is probably too long for your needs, but can be cut to length easily... probably would need a support bearing at the end of the shaft to take some torque from the planetary gear assembly... again, just a thought..

I use a small electric winch on my Honda 4 wheeler which has bronze worm drive. A good freebie with a worm drive is that it will not allow movement of the driven gear if it is designed correctly unless the assembly breaks. This is the only braking mechanism in the winch that I have.. larger winches have a positive lock mechanism to increase braking for safety reasons.

I have owned large wenches, approximately 50 ton pull without snatch-blocks. It used a ratchet system to lock the drum, which was rather large.

Thanks for the hard work by the team, and for working with two platforms parallel so that everyone can take part..

Bobbie 27th Apr 2016
Thank *you* Bobbie! I know that Wade will take a good look at your comments later in the week, so we appreciate your experience. 50 ton is a decent sized winch!
Shaun 27th Apr 2016