Now that we have version 2.9 and inertial primary control for the Z axis and soon to have it for X and Y axes as well it is necessary to take vibration dampening and isolation of the flight control board much more seriously.
Primary improvements can certainly be made by balancing the props and motors.
So far it seems that the more rigid the frame the better because frame flex introduces undesirable mechanical delay (hysteresis) in translating motor induced actions to the centrally located flight control board. (Do NOT shock mount the motor Arms).
It may be reasonable to somewhat vibration damp the motor mounts themselves because they are on one end of the mechanism.
However, primary damping gains will be made by vibration isolating and or dampening the flight control board itself.
So far we have undertaken this process simply by trial and error sticking on of Foam or Gel pads or using O-ring suspension of the board to outboard standoffs.
This has achieved (barely) acceptable results, but is certainly by no means optimum.
The crucial fact that we have not properly addressed is that the amount and type of dampening medium needs to be matched to the weight (mass) of the item we are trying to isolate.
In fact we are trying to isolate a flight control board that weighs under an ounce or less than 2 ounces in its case which is a very small mass.
Our current "solutions" are actually designed for much larger masses and are not nearly as effective for the light mass of our flight control board as they ought to be.
I have done some on line research which did fully verify this inadequacy.
Virtually all off the shelf solutions (either pad or stud type) basically require a suspended mass that would weigh at a minimum 5 to 10 times what an APM or PX4 / IO board(s) weigh or more for optimal effectiveness.
This includes all pre-made Sorbothane, Alpha gel, memory foam or other silicone or urethane gel or foam mounts including Lord Micro mounts.
However, Alpha Gel or 30 durometer Sorbothane or Kyosho Zeal Gel double sided tape do appear to be the best possible solutions at this time so long as you use small enough pieces of them.
Simply putting a double sided pad under the entire board as we normally do now is entirely inappropriate for maximum vibration isolation and it is amazing it works at all.
Optimally you would use pads of them smaller than 1/2" square (possibly even 1/4" square) on each corner of the board or APM enclosure box. (smaller for the bare board than the board in the box obviously).
You could also improve isolation somewhat by sandwiching the board / enclosure between pads on both sides in slight compression.
So far we have done a dismal job of approaching this like engineers, but the reality is that with the massive excess quantities of vibration absorbing materials that we are using versus the mass of the APM (or PX4) has produced better results than not using them, but no where near what could be achieved by using the proper weight and size of dampening / isolation material.
The basic solution is to reduce the actual isolation medium to the 4 smallest pads you can get by with on each corner and using the softest commercially available dampening materials you can find.
A further gain can be made by placing the item to be damped in 10 to 20 percent compression between 2 pieces of the dampening material.
Thickness of the dampening material does improve dampening and isolation but is not nearly as important as selecting the right material and the right size of the supports made from it.
I believe that Kyosho Zeal tape is 2/10 of an inch thick and that is probably plenty for our use and the frequency range we are trying to damp.
I would very much like to see 3D Robotics produce a APM (and PX4 / IO board for that matter) case with proper internal shock mounting of the board(s) with dampening data for it.
I actually suspected this result from the start of my investigation and a little thoughtful research has completely confirmed it.
Another significant gain in vibration isolation can be had by using a high flex wire and strain relief approach to all wires connected to the Flight control board (and using the minimum number of wires necessary as well.)
I have used the concept of vibration isolation and dampening somewhat interchangeably in this discussion.
Isolation is simple undamped (spring or rubber band support) which allows the movement of the isolated object largely separate from the containing object.
Dampening is the conversion of vibration into heat energy by a shock absorbing medium (car shock absorber for instance.)
Our ultimate goal here is to provide the most high and medium frequency reduction while still allowing low frequency actual board movement to take place with a minimum of delay.
So realistically our methods embody both Dampening and Isolation.
I have covered a lot of ground here, but this is at least a good start for designing some real world vibration solutions that are bound to work better for us than what we have done so far.
Please try your own experiments and kick in your own thoughts here, that's how we get better and this is just a launching point.
Here is an excellent link to some definitive research and testing that will help:
The Secrets of Prop Balance, Tip Trace, and Dynamic Balancing.
Balance (mostly x/y vibs): I use two plastic cones on a steel rod between two magnets. then sand off a bit of the heavy side of the prop.
If looking at the prop from the front, release the prop from a horizontal position and let it pendulum and:
- ... stops at 6ish o'clock - sand along the entire end of the downward blade.
- ... stops at 4ish o'clock - sand along the left half of the downward blade
- ... stops at 8ish o'clock - sand along the right half of the downward blade
- ... doesn't move. release from a vertical position. doesn't move, you are done.
Pendulum the prop twice at each check step before you sand. before the second try, rotate the prop on the steel rod by 180 degrees. this will help eliminate center hole bias.
Tip Trace (mostly z vibration): Mount the prop onto the motor. Manually spin the prop. Check the tip trace at two spots 90 degrees from each other. Both blade tips should trace to the same height off of the floor at your check locations. If one traces higher, then the hub of the prop needs to be leveled until both blades trace the same place.
Dynamic Balancing. The next step you can't do, but thought i'd share it anyway (you might figure out a way). I then mount the prop onto the motor in its four possible orientations relative to the screw holes (this only works with pancake mounts). Bring the prop up to hover speed (one at a time on my motor test stand) and check the vibration levels with my meter. Record the best orientation and mount the prop for flight in that position. On average, this last step decreases vibs by an additional 25% over the worst orientation.
What i would pay for a dynamic prop balancer.
Thank's Forrest, I'm going to try like this; I have the idea to check dynamic balance with my phone, not shure how presicion it has, I comment this to a friend and he tells me that it's no difficult for him to make one with a osciloscope, i'm going to disturb him to do it ;)
osciloscope method didn't work :(, What do you think about this?http://www.turbinemuseum.de/Gasturbines/Balancing_Tool/balancing_to... any other idea for dynamic balancer?
Impressive. good find.
Now if someone can figure out how to do that with the prop mounted on our motors, it would sell big time. my solution cost over $1K (an Extech Vibration Meter). If someone could build and manufacture one that could be priced at $200 or less, it would sell.
If I find a diy solution I share it but, if you didn't do it yet.......it's look difficult :( .
When I chek the props side to side (tip trace) the worse has 3mm and I can repair it :) , it has a little excess of balancing glue
- tip trace is moving the props to your measuring tape (measuring tape stays at the same place). Actually the measuring tape has two locations that are 90 degrees from each other just to ensure that you aren't measuring the null point.
- haven't talked about this one too much. this is more of a efficiency issue than a vibration issue (maybe).
- every blade should be level tip to tip to each other
- one way to measure this is to
... set the ship on a flat surface and level the four motors to that surface
... rotate the props so they align with two opposite motor bolts
... measure the height of the tips from the floor; the measurements should be the same.
... rotate the props 90 degrees so they align with the other two opposite motor bolts
... measure again.
If errors, determine if the issue is the
... motor mount not being level (if so then shim or adjust)
... prop hub not being level (if so sand or shim the hub on the motor side)
i used to hate soldering too. found out that the key was the equipment:
- temperature controlled 700 - 900F
- enough wattage for quick heat replacement (LF-1600 works)
- using the right tip (my favorite is a small flat tip)
- tinning the tip (periodically douse the tip in flux and clean it then tin it) so it's shinny
- adding a wee solder to the tip (critical for heat transfer) to the wire or connection
- etc etc
Thank's for the tips Forrest, you are an open book :O , i'm using a 42W (Good) solder, I don't use flux, I'm going to ask for it.
In the UK there are restrictions on lead in solder and I found difficulty using the normal solder that they sell in the shops. I found that using solder 60% tin 40% lead a lot easier to use and was easy to get on ebay.
a little silver, instead of lead, also helps. not sure which is more toxic to the body. i use nothing or silver (Radio Shack has it).