Hello All,Like a few others, I've been working on a UAV Helicopter but I've hit a bit of roadblock and I'm looking for some helpful advice from the community. At the moment, when I try to fly the helicopter (so far as high as 4 inches off the ground), it tries as hard as it can to bank left because it thinks it's leaning at >40%. The problem is vibration and I'm looking for any advice I can get on how to overcome this problem.Below is some slightly massaged data from the accelerometers and gyros. The accelerometer data is adjusted back to "mid" so instead of values being from 0 ~ 1024, they're from -512 ~ +512. The gyroscope data has been converted into degrees per second (i.e. (raw gyro value - gyroMid)* magicNumber). The little table of numbers below the graphs is the average variation in accelerometer values I'm seeing..so when the helicopter is completely assembled, each x-axis acclerometer values is on avarege 300 away from the previous value!
I guess there are two roads to resolving this:1) reducing the actual vibration of the helicopter.2) use less vibration sensitive accelerometers and gyroscopesFor #1, I've done some loosening of the ball joints in the main rotor and upgraded some components to Aluminum and Graphite.For #2 I haven't done much. I'm using Sparkfun's 6DOF (http://www.sparkfun.com/commerce/product_info.php?products_id=8454).What do you think? All advice more than welcome.BTW, I've also been following John Mac's and Bill Premerlani work on CCPM Helicopters so I'm thinking of contributing to their efforts instead of going it on my own.
You'll see from the pic above I've incorporated a number of improvements suggested from my previous blog post including:- better mounting of the AVR 128 and sparkfun 6DOF (now on the back)- lipo batteryI've also upgraded the engine to a hyperion Z3025-06 because the previous one simply couldn't get this big-ish ECO8 off the ground. I've also replaced the "mechanical mixing" with "electronic mixing" -- this is ECO8 speak mostly I think but basically it means that now the three servos at the back, work together to control the pitch, roll and elevation of the main blades instead of using the ECO8's manual mixer thingmy which everyone says doesn't work very well.
Are you running a UAV DevBoard or other IMU to stabilize? If so which gyros do you use and do you see any artifacts of vibration?
I did some vibration isolation on the Red Board with foam etc, as well as careful balancing of the Heli....but the gyros were just too sensitive. We decided to move over to the Green Board and move on with development rather than continue vibration isolation on the Red Board. I always planned to come back to this, since the Red Board is available from SparkFun and cheaper than the other boards, but just haven't gone back. If you have made it work, would love to hear about it.
I have quite a bit of rotor experience please look at my pictures and you will see two of my setups. Weather has had me down for a couple of weeks so I have been tweaking a plane of mine. The helicopter is a challenge in any capacity. I think that it has been addressed that you need to be mounting everything on foam sticky tape or velcro so the best thing I can do is to walk you through a setup that I have and see how everything is mounted to reduce vibration. Some issues I see are choice of airframe and trying to make everything look good you have hard mounted everything to the frame.
Choice of Airframe: Ok it is good that you are using a brushless electric setup as a quality setup will be sell machined and balanced. A helicopter is a vibration in its entire structure. The rotor "wing" is basically the attacking rotor is giving your lift and the retreating blade is actually falling. The tourque of the rotor and the lift - no lift scenarion make the helicopter list to one side as it is in a perfect stable hover. It is tilted off the z axis by several degrees. This being said a large underslung weight from the rotor is going to give you the best stability. I would suggest the smallest helicopter for uav work be at least a 50 size helicopter. I would go electric like a t-rex 600. That is what the two systems pictured in my gallery are. The next issue is to make sure that everything is precisely balanced. The main gear is not too warped and that you have an appropriate gear mesh from the motor pinion to the main gear. It is important to always check every bearing as a slight hitch can cause significant harmonics. Even the placement of the control rod supports for the tail servo linkage is important. You do not want to evenly space the supports because you can get vibration down the tail and with harmonic vibration gain exponential resonance. I will try to take close up pictures of my current front camera mount setup. I will remove the autopilot but it will be setup as I would rc. I have had good luck but short flight times with the electrics. The gassers are awesome but will flame out sometime so kind of a toolbox at that point. Hope that I can be of help. Toby
I have quite a bit of rotor experience please look at my pictures and you will see two of my setups. Weather has had me down for a couple of weeks so I have been tweaking a plane of mine. The helicopter is a challenge in any capacity. I think that it has been addressed that you need to be mounting everything on foam sticky tape or velcro so the best thing I can do is to walk you through a setup that I have and see how everything is mounted to reduce vibration. Some issues I see are choice of airframe and trying to make everything look good you have hard mounted everything to the frame.
Choice of Airframe: Ok it is good that you are using a brushless electric setup as a quality setup will be sell machined and balanced. A helicopter is a vibration in its entire structure. The rotor "wing" is basically the attacking rotor is giving your lift and the retreating blade is actually falling. The tourque of the rotor and the lift - no lift scenarion make the helicopter list to one side as it is in a perfect stable hover. It is tilted off the z axis by several degrees. This being said a large underslung weight from the rotor is going to give you the best stability. I would suggest the smallest helicopter for uav work be at least a 50 size helicopter. I would go electric like a t-rex 600. That is what the two systems pictured in my gallery are. The next issue is to make sure that everything is precisely balanced. The main gear is not too warped and that you have an appropriate gear mesh from the motor pinion to the main gear. It is important to always check every bearing as a slight hitch can cause significant harmonics. Even the placement of the control rod supports for the tail servo linkage is important. You do not want to evenly space the supports because you can get vibration down the tail and with harmonic vibration gain exponential resonance. I will try to take close up pictures of my current front camera mount setup. I will remove the autopilot but it will be setup as I would rc. I have had good luck but short flight times with the electrics. The gassers are awesome but will flame out sometime so kind of a toolbox at that point. Hope that I can be of help. Toby
Just happened to see this thread. As you noted, Bill and I have been working on applying the UAV DevBoard to CCPM helis. There are two threads on this.....
What you describe was seen on the Red UAV Board that has LISY300AL gyros. They completely saturate on a heli. We then went to the Green Board (not sold by SparkFun any longer) that has ADXRS401 gyros. This worked fine. They are considerably less vibration sensitive. The problem was that they saturate at abouut 75 deg/sec. A heli can exceed this if not careful, but can be used for "normal flight". Next Bill added ADI ADXRS610 gyros mounted on a redboard. These are both vibration insensitive and have the 300 degrees/sec that the LISY's have...so best of both worlds. Unfortunately, these were custm made and not available from SparkFun. Bill is working on a new board over the Winter that should incorporate the "good" gyros and a few other upgrades.
thanks for the close up. The cable coming out from green board on the right side photo is doing no good to accelerometer at all. Like Mark and other said you need to tune heli first for the smoothest possible hover, do all you can to reduce the vibrations like everyone have said above.
Hmm , no Rx :) . I did not think of that , I will borrow your idea and java code if I may later for some thing I have in mind. Can you use the trainer cross like mark said? I was under impression the heli wants to do 45 deg side flip/bend pretty fast and so your choice of tie down board. You still will need them once the course vibrations are done with and for further fine tuning..
In any case IMHO getting this heli to fly+tune using normal RC Tx+Rx first would be my first choice.,It almost looks alike that whole setup is just replacement for simple RC Rx. if that is not a option then trying the green board just below battery with thin slice of PUC or any light heat insulating material( LiPo gets hot-very some times) in between green board and battery and use Velcro strap if possible else cable tie may of some relief to green board from vibrations. The point is to play with various placements possible that this sensor board allows. BTW is there any particular reason for two missing screws in green board , a rubber grommet will might help, all I am trying to do is dampen enough just to bring the vibration/noise floor just low enough to get some decent signal quality that can be useful for your project. Hope you know that this small heli might never be silent flyer. BTW Have you thought of any quadracopter or some thing like that for your project? good luck once again.
Tuning a small heli is a mechanical black art, Check RC Groups for tips. Use medium lock-tite on fasteners, also manage the wire harness (tie down with twist ties works for me) if wires shake they will cause airframe resonances, wire chaffing too, be sure cg is under main shaft. I would use training gear (two crossed arrow shafts with ping pong balls held together with rubber bands) helps with hard landings and will absorb some vibrations. Once the heli is dialed in it will behave much better, Hover will be smoother and hands off will be possible for several seconds, at this point the heli is ready for Autopilot control, also it is a lot more fun to fly! (ditch the trainig gear after tuning is satisfactory)
Thanks a lot for the feedback, I'll be putting some/all of these to good use.
Mark, I'll try the balancing. It's definitely gotten better with the latest round of tuning..maybe a few more weekends spent on that will take the bulk of the vibration out.
Nima, Timing the sampling to coincide with the RPM of the motor is something I never would have thought of - it is possible with the sparkfun IMU to modify the sample frequency although currently I have it pretty much hardcoded to 200 samples per second. I guess instead of changing the sampling speed I could also change the RPM until I find one that works...anyway, a new line of thought.
Morli,
1&2) I've attached a close up of the front and back. As you suspected all the sensors are in the green pcb at the back. I can't actually take it apart sadly. On the top of the sandwich are all the sensors..on the bottom level is an ARM7 processor that does all the ADC conversions and sends them to the main CPU up front.
3) yes, I'll try the staged approach and make sure the heli itself is at least stable.
5) at the risk of exposing my amateurism, i'll tell you there is no receiver! When I want to do manual control I use an esky simulator's receiver shaped joystick. I've got a java program that reads the joysticks and then sends the appropriate commands to the atmega 128 via the xbee.
Ryan, good to hear that confidence. I think I will consider another set of Gyros/accelerometers after I've exhausted the more mechanical solutions.
I don't know much about those gyros but I use the same acceleromter on my airplanes and its fine. I'd say even with a vibe entensive environment my filter would be fine because I lean more on the gyros than anything else. However its quite odd that your gyros have that much noise infulenced from vibration. I keep hearing people talk about this and find it odd. I have had tons of luck with my ADXRS300's. It kind of sucks you are stuck with hardware. I was lucky, I got good hardware on the first shot.
I know some of the comments below have been about mechanical fixes but I think you can only go so far in that regard. I've only briefly had a look at the Sparkfun IMU board but it might be worthwhile investigating if you can change the sampling rate of the A/Ds and the rate at which you read from them. The basic concept is that say if your vibration function is a Sin() wave (which is a good approximation), you can have an incorrect reading if you constantly sample when the Sin function is at the peak, or the trough. So one thing you can try is to measure the RPM of the rotor blades using a hall-effect sensor and sample the A/Ds at double this rate. this way you will take samples at peaks and troughs.
Calculations pending, I think anything other than an even multiplier of the rotor RPM, and you'll develop a beat pattern in your readings and not a true average. I haven't implemented this myself but plan to in the next version of my Autopilot.
Hope this helps, if it does please come back and let me know.
Comments
I'm using the $2 MPU6050 and flying with a CCPM Autopilot of my own design.
Feel around the frame with the motor spinning at various speeds to find the sweet spot. Moongel works best, but good luck mounting to it.
Are you running a UAV DevBoard or other IMU to stabilize? If so which gyros do you use and do you see any artifacts of vibration?
I did some vibration isolation on the Red Board with foam etc, as well as careful balancing of the Heli....but the gyros were just too sensitive. We decided to move over to the Green Board and move on with development rather than continue vibration isolation on the Red Board. I always planned to come back to this, since the Red Board is available from SparkFun and cheaper than the other boards, but just haven't gone back. If you have made it work, would love to hear about it.
Best,
John
Choice of Airframe: Ok it is good that you are using a brushless electric setup as a quality setup will be sell machined and balanced. A helicopter is a vibration in its entire structure. The rotor "wing" is basically the attacking rotor is giving your lift and the retreating blade is actually falling. The tourque of the rotor and the lift - no lift scenarion make the helicopter list to one side as it is in a perfect stable hover. It is tilted off the z axis by several degrees. This being said a large underslung weight from the rotor is going to give you the best stability. I would suggest the smallest helicopter for uav work be at least a 50 size helicopter. I would go electric like a t-rex 600. That is what the two systems pictured in my gallery are. The next issue is to make sure that everything is precisely balanced. The main gear is not too warped and that you have an appropriate gear mesh from the motor pinion to the main gear. It is important to always check every bearing as a slight hitch can cause significant harmonics. Even the placement of the control rod supports for the tail servo linkage is important. You do not want to evenly space the supports because you can get vibration down the tail and with harmonic vibration gain exponential resonance. I will try to take close up pictures of my current front camera mount setup. I will remove the autopilot but it will be setup as I would rc. I have had good luck but short flight times with the electrics. The gassers are awesome but will flame out sometime so kind of a toolbox at that point. Hope that I can be of help. Toby
Choice of Airframe: Ok it is good that you are using a brushless electric setup as a quality setup will be sell machined and balanced. A helicopter is a vibration in its entire structure. The rotor "wing" is basically the attacking rotor is giving your lift and the retreating blade is actually falling. The tourque of the rotor and the lift - no lift scenarion make the helicopter list to one side as it is in a perfect stable hover. It is tilted off the z axis by several degrees. This being said a large underslung weight from the rotor is going to give you the best stability. I would suggest the smallest helicopter for uav work be at least a 50 size helicopter. I would go electric like a t-rex 600. That is what the two systems pictured in my gallery are. The next issue is to make sure that everything is precisely balanced. The main gear is not too warped and that you have an appropriate gear mesh from the motor pinion to the main gear. It is important to always check every bearing as a slight hitch can cause significant harmonics. Even the placement of the control rod supports for the tail servo linkage is important. You do not want to evenly space the supports because you can get vibration down the tail and with harmonic vibration gain exponential resonance. I will try to take close up pictures of my current front camera mount setup. I will remove the autopilot but it will be setup as I would rc. I have had good luck but short flight times with the electrics. The gassers are awesome but will flame out sometime so kind of a toolbox at that point. Hope that I can be of help. Toby
Just happened to see this thread. As you noted, Bill and I have been working on applying the UAV DevBoard to CCPM helis. There are two threads on this.....
http://www.diydrones.com/forum/topics/discussion-of-application-of
and
http://www.diydrones.com/forum/topics/progress-report-on-uav?id=705...
What you describe was seen on the Red UAV Board that has LISY300AL gyros. They completely saturate on a heli. We then went to the Green Board (not sold by SparkFun any longer) that has ADXRS401 gyros. This worked fine. They are considerably less vibration sensitive. The problem was that they saturate at abouut 75 deg/sec. A heli can exceed this if not careful, but can be used for "normal flight". Next Bill added ADI ADXRS610 gyros mounted on a redboard. These are both vibration insensitive and have the 300 degrees/sec that the LISY's have...so best of both worlds. Unfortunately, these were custm made and not available from SparkFun. Bill is working on a new board over the Winter that should incorporate the "good" gyros and a few other upgrades.
Check out our threads for more info.
Best,
John
Hmm , no Rx :) . I did not think of that , I will borrow your idea and java code if I may later for some thing I have in mind. Can you use the trainer cross like mark said? I was under impression the heli wants to do 45 deg side flip/bend pretty fast and so your choice of tie down board. You still will need them once the course vibrations are done with and for further fine tuning..
In any case IMHO getting this heli to fly+tune using normal RC Tx+Rx first would be my first choice.,It almost looks alike that whole setup is just replacement for simple RC Rx. if that is not a option then trying the green board just below battery with thin slice of PUC or any light heat insulating material( LiPo gets hot-very some times) in between green board and battery and use Velcro strap if possible else cable tie may of some relief to green board from vibrations. The point is to play with various placements possible that this sensor board allows. BTW is there any particular reason for two missing screws in green board , a rubber grommet will might help, all I am trying to do is dampen enough just to bring the vibration/noise floor just low enough to get some decent signal quality that can be useful for your project. Hope you know that this small heli might never be silent flyer. BTW Have you thought of any quadracopter or some thing like that for your project? good luck once again.
Mark, I'll try the balancing. It's definitely gotten better with the latest round of tuning..maybe a few more weekends spent on that will take the bulk of the vibration out.
Nima, Timing the sampling to coincide with the RPM of the motor is something I never would have thought of - it is possible with the sparkfun IMU to modify the sample frequency although currently I have it pretty much hardcoded to 200 samples per second. I guess instead of changing the sampling speed I could also change the RPM until I find one that works...anyway, a new line of thought.
Morli,
1&2) I've attached a close up of the front and back. As you suspected all the sensors are in the green pcb at the back. I can't actually take it apart sadly. On the top of the sandwich are all the sensors..on the bottom level is an ARM7 processor that does all the ADC conversions and sends them to the main CPU up front.
3) yes, I'll try the staged approach and make sure the heli itself is at least stable.
5) at the risk of exposing my amateurism, i'll tell you there is no receiver! When I want to do manual control I use an esky simulator's receiver shaped joystick. I've got a java program that reads the joysticks and then sends the appropriate commands to the atmega 128 via the xbee.
Ryan, good to hear that confidence. I think I will consider another set of Gyros/accelerometers after I've exhausted the more mechanical solutions.
Calculations pending, I think anything other than an even multiplier of the rotor RPM, and you'll develop a beat pattern in your readings and not a true average. I haven't implemented this myself but plan to in the next version of my Autopilot.
Hope this helps, if it does please come back and let me know.
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