Vibration Isolated Motor Mounts?

[x0054]

While building a vibration isolated electronics plate for my latest H-Quad, something occurred to me. Why am I doing it this way? I mean, why am I putting vibration dampeners on parts I don't want to vibrate, rather then putting vibration dampeners directly on the parts that actually do vibrate, the motors.

Cars, garage door lifters, motorcycles, basically anything I ever worked on that has motors, the vibration dampeners are always installed directly on the motor it's self. So why is that no one (that I have seen) does this in RC Multirotors? I am asking because I wonder if I have missed something, being new to the hobby and all. Have people tried this already and it proved to be a bad idea?

Here are the advantages that I see:

1. Better vibration isolation because the weight of the entire frame is there to resist vibration. In vibration isolation, the bigger the mass of the object you are trying to isolate is, the better chance you have of cutting down the vibration. Here you have the weight of the entire frame + battery + all electronic stuff fighting the vibration.

2. Predefined brake point. In a crash the rubber motor mount can stretch, or even rip, protecting the motor and the arm of the quad.

3. Potentially less weight, and more mounting options for camera and electronics without having to worry about vibration isolation to the frame.

Are there big disadvantages that I have missed? My main problem is that at the moment I can not figure out any good way to make a good vibration isolated motor mount. I mean, it would be super simple to manufacture one. But making one from scratch is problematic. I was considering using something like this:

http://www.ebay.com/itm/30pcs-M3-Ma...140?pt=LH_DefaultDomain_0&hash=item1c4239b7c4

Just wanted to see what other people think.

- Bogdan

 

[Cyberdactyl]

Hmm, I think a graphic is on tap for this one. . .

I can think of two practical reasons why motor dampening is not a 'best practice' in regard to a "floating" multirotor motor vs a solid mounted motor. . .

First you want the FCB's orientation locked with the motor's orientation as tight as possible for crisp control and response. And that would go double for acro flight.

Because of such high torque moments at the motors, it would require them to tilt a fair bit out of the plane the FCB is calculating with, to absorb and lower the vibration frequency. As compared to the very minor tilt offset of a control board with a dampening method. I've exaggerated the graphic below to show the tilt of the prop plane would cause for the FCB.

Second, and a minor point . . the motors are basically spinning independently of each other causing the camera and FCB (assumed to be mounted solid since dampening at the motors is the method) to deal with all sorts of complex vibration harmonics that might get through the dampening mechanism.

 

[RoyBro]

I knew someone would come up with the physics part of this. But also from a practical standpoint, Isolating a plate for the FC & battery, and isolating a camera mount seems less difficult than isolating four, six, or eight motor mounts.

 

[x0054]

Well, the practical part would be trivial if this is something that became popular. A simple rubber motor mount would be all it would take. It would be no more practically difficult then bolting on x number of motors. You would bolt on a motor mount first, and then the motor, or vice versa, depending on the design.

As for the physics of it, it's one of those things that has to be tested out. I ordered a set of those rubber mounts, and I am going to test it out on my tri. I'll report the results. I do agree that angular deflection is a problem. Ideally, you would want to design a rubber mount that resists angular deflection, and instead, converts all vibration into vertical or horizontal motion. Having 4 mounts per motor and spacing them out, like with the commonly included X motor mount should reduce angular deflections. Ideally, you would want to have an upside-down mount, were the motor is sunk in the frame. This way the mounting point of the motor would be close to its top, thus reducing the torsional stress on the mount.

It would be fun to test some of this stuff out. Ahh... if only I had a CNC machine Do you guys know of any services out there that can do custom CNC work for not too much money. I am pretty handy with AutoCAD and would love to try out a few designs.

 

[x0054]

Cyberdactyl, on a completely unrelated note, I love that Flying Monkey avatar. Just noticed it. That would be an amazing Halloween quad!

 

[Cyberdactyl]

Thanks x0054. I have that little guy on my hex for orientation . . .

As to testing a new method, I'm all for it.

I said "best practice", because floating the motors will probably work either great or ok in reducing vibration. The challenge as I see it would be setting up a controlled experiment to see and feel the difference in control response. One way would be to allow the motors a LOT of movement to differentiate the flight characteristics. Another anecdotal method with mounts that are not excessively floating, and may not provide a true result, would be to get someone who can fly extra well in acro, such as FGA to float motors and see if he can sense any control and response difference.

 

[crlock]

I think instead of isolating the motors in the hopes of reducing the vibrations they produce, you could just balance the motors and propellers, that'll do the trick

 

[Craftydan]

In addition to all said above, it still won't remove the last remaining vibe after the motor and prop are pristinely ballanced and the ESC gives clean AC . . . prop thump: the thump of air hitting the boom every time the prop swings over it.

Even if the motor dampening works well, the vibe from the thump would push you toward vibe dampening your control board and camera gear anyway. Not saying it's not worth a try -- experimentaion is a great way to learn things, indivdually and collectivley -- but be prepared to isolate both.

 

[Cyberdactyl]

I think instead of isolating the motors in the hopes of reducing the vibrations they produce, you could just balance the motors and propellers, that'll do the trick

"Perfectly" balanced props and motors go 90-98% of the way to eliminating vibration, but there are still variables that will creep in on any system, most especially if flexible props are used. When a flexible prop bends from off-axis torque and on/off flattening from high RPM changes and/or miniscule different/asymmetric cambers on each blade (but still 'balanced'), etc., may still need some vibration dampening. And as CD said, prop thump.

My brother is flying with 1300KV motors and using those super flexible cheapo plastic props (he's only been flying for a few weeks) and they make a weird sound when you really pour on the throttle. Extremely inefficient when doing so, but a cool throaty roar. They are flattening and fluttering at high RPMs. He's coming down this weekend. I'll try and get some video of it.

 

[crlock]

"Perfectly" balanced props and motors go 90-98% of the way to eliminating vibration, but there are still variables that will creep in on any system, most especially if flexible props are used. When a flexible prop bends from off-axis torque and on/off flattening from high RPM changes and/or miniscule different/asymmetric cambers on each blade (but still 'balanced'), etc., may still need some vibration dampening. And as CD said, prop thump.

My brother is flying with 1300KV motors and using those super flexible cheapo plastic props (he's only been flying for a few weeks) and they make a weird sound when you really pour on the throttle. Extremely inefficient when doing so, but a cool throaty roar. They are flattening and fluttering at high RPMs. He's coming down this weekend. I'll try and get some video of it.

yeah, flexible props do contribute to the vibration issues, stiffer props help, but my main concern with a motor mounted on a soft pad or rubber dampeners, that could contribute even further to increase motor vibrations due to the momentum the prop carry, something similar to a bike wheel, the frame will move, but the motor will drag behind, maybe vibrations will be eliminated, but the gyroscope effect could bring odd behaviours.

or maybe i'm wrong, just somethink to think about

 

[Cyberdactyl]

I think in that case crlock, the gyro effect of the props is there regardless of how they are mounted. That video is to show how substantial the effect can be with a sizable mass on the outer rim in relation to the diameter.

Thankfully props are comparatively low mass for their diameter. But gyroscopic action can almost mimic an anti-gravity system. The effect in the video below is little known, and hard to explain, but if it's the first time you've seen it, it can appear counter-intuitive.

 

[crlock]

I know, maybe it's not enough to affect the behaviour of the craft, and it would depend on the height of the dampeners and the stiffness of the material used. Maybe i'm overthinking it, i just like to overthink
Loved the video, science, you sexy, sexy minx ;D