variable pitch quad
My question is the APM 2.0 and APM planner capable of controlling my bird? Four servos control the pitch of each rotor and the esc will run in the constant RPM mode.
Right now, I have the servos connected respectively to outputs 1-4 and by moving the throttle channel to mid point my blades move to zero pitch. The servos then respond correctly to any motion of the APM. All fine for testing but not correct.
Problem one, when the throttle channel is moved below a certain point, the servos jump to full negative pitch. I assume this is for "motor off"
Problem two, what to do with the esc leads. My power comes from the BEC so kinda need it connected to the output buss. What I want, is to toggle a channel and command the APM to raise the motor RPM to operating speed slowly and hold it.
Problem three, at least for the near future, I doubt I will ever want more than one or two degree of negative pitch and then, only in roll, pitch or collective. For yaw they must stop at zero.
See my profile for photos.
Tags: collective, quad, variable
Replies to This Discussion
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Permalink Reply by DanRZ on -
Hello. For problem 2, just know that you can "manage" motors rpms without using the APM
just only with your transmitter and "all the motors" leaded to the same "signal".
In a heli model on your transmitter you can use throttle curves ...
In a variable pitch quad, all the motors should run at the same rpm. Only pitch varies on each prop.
Problem 3 : Why do you use "variable pitch" if you don't want to use negative ?
Not sure the complexity of variable pitch props is really "adequate" in this case ...
The only thing it will help is the landing, that can be lot smoother with variable pitch props.
And for the Yaw, the APM wil add/remove pitch to do the same function with normal props.
Everything will just work the same with the "torque" generated ...
And for problem One, if you separate throttle and pitch, the full negative pitch at start is normal.
Your stick is full negative so the pitch is full negative too.
It wont' cause any problem if your throttle is at zero at this point.
Throttle will graduately increase with the pitch depending on your throttle curves.
Then in flight, you can fly inverted if run a throttle curve with non zero value at full negative ( flat curve ).In my opinion, the only real advantage of variable pitch, is to fly inverted
or do some "tricks" in flight, like flips or other 3D heli movements.
;)
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Permalink Reply by Brad Hughey on -
@DanRZ: You're correct that there may be little advantage to variable pitch in a small quad. However, the advantages grow tremendously with scale and efficiency considerations, even if inverted flight is not part of the mission profile. Dan didn't mention how big his quad is, but the parameters of low rotational inertia and low disk loading are in direct conflict with one another. Low inertia is needed for good control response, but a large disk area is essential for endurance. Incorporating variable pitch adds complexity to be sure, but for large multicopters to have flight durations of any utility, it's almost essential.
Some researchers at MIT did some excellent research in this area, and the resulting paper is rather illuminating.
- Attachments:
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GNC11_Cutler_uber.pdf, 3.7 MB
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Permalink Reply by DanRZ on
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Thanks, very interesting pdf ...
The variable pitch system is clearly a good way to "expand the flight envelope".
The conclusion also says it needs a very precise management of voltage(motor rpm) / pitch to be efficient and so throttle should be managed by the controling board depending on flight mode.
For me it should be possible to fly a variable pitch quad with the APM
but it will need some PIDs tuning because of the different behaviour of a servo, for sure
and a very precise management of motor rpms.
Some ESC have now a governor function, to achieve a constant speed, so throttle is "not that much needed" except for take off with a progressive throttle ...
;)
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Permalink Reply by John Githens on -
@Brad Hughey, That is a fascinating paper. Especially when pondering future "Avoid" capabilities, the need for which has been mentioned in a recent book... And I see what you mean by "...the parameters of low rotational inertia and low disk loading are in direct conflict with one another." Thanks for the info and insights.
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DeveloperPermalink Reply by R_Lefebvre on -
Dan, I think you're going to need a custom program to get this to work. What you need will be a blend of TradHeli and Quad code. Do you know how to compile and upload manually? I'm interested in helping you with this project if you are.
I'd have to create a new frame type like:
#define FRAME_CONFIG VP_QUAD and you'd upload that.
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Permalink Reply by Dan Clemens on -
Yes, R_Lefebvre, I do.
I chose the APM platform because I am very familiar with the ATMEL/Arduino platform. I have another project I developed hardware and software to control a green house robot. It is in production now. I am currently using arduino-0022 and code in C/C++. I have looked a bit at the APM code and it looks to be well documented but have not got into the general flow of the code.
Right now my goal it to get my quad in the air and see. IF it flies, how much it will lift and how maneuverable it is. It may need a diet or bigger rotors. So far, everything has been mathematics and they say two to one thrust ratio. We shall see. It has been designed such that is it easy to increase rotor diameter for more thrust if needed. Right now, each rotor is five blades and about 10 inches in diameter and the quad is 26" in diameter. Current flying weight is 2450 grams. Rotors turn at over 6000rpm.
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DeveloperPermalink Reply by R_Lefebvre on -
Ok, if you want to look into this yourself, but I would direct you to the AP_Motors libraries. This is where the nuts and bolts of sending out the attitude controls to the "actuators" happens. I would suggest you will need to create something like an AP_MotorsVPQuad class. That class will need to be a blend of TradHeli features, and quad features. Namely, you'll need servo objects just like the heli, but control mixing like a quad. You can also use the motor speed controller like the TradHeli too.
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Permalink Reply by Dan Clemens on
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Thank you for the suggestions R but hope you did not get the idea that I want to go this on my own, it would take me much longer than someone who is familiar with it. I am not thrilled at the prospect of analyzing all that code that so much work has gone into. Do you know if there is a write up or flow chart available?
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DeveloperPermalink Reply by R_Lefebvre on -
Oh, that is how I read that.
Maybe I'll see if I can put something together for you to test.
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Permalink Reply by Dan Clemens on
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oh cool! that would be fun!
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Permalink Reply by Aerhead on
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A VVP copter is not very easy. First you need four ESCs that have good governors, hard to find. Or your can use one motor to drive all motors. If you have all props turning at the same RPM than you can adjust the servos so 1000 micro seconds gives you neutral pitch and connect the servos to the motor control PWMs. An extra channel with a potentiometer can be use to control motor(s). This type of setup has worked for some.
There is a couple of threads in the OpenPilot forum you could look at.
good luck.
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Permalink Reply by Dan Clemens on
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