I have finally put a Tri-Copter together in an attempt to get Tri's up to the same level of performance as the more common quad, Y6, X8, Hex and Octo. This is what I have put together:
based on these Hobbyking parts:
The tricopter has tuned up reasonably well using autotune and I feel happy with the performance so far. I didn't find the initial setup much more complicated compared to normal quad. I did find the first flight much more nerve racking because of the poor yaw performance during take off. I am interested to see if my confidence in the yaw performance improves now I have a reasonable yaw tune.
I am looking for feedback on what issues Tri-Copter pilots are having with Arducopter. Some of the issues I see are:
1. Dynamics of having the rear motor and propeller CG above the pivot point causes the opposite rotation to the thrust.
2. Roll, pitch, yaw and lift coupling potentially causing issues with auto-tune.
3. The frame type needs a stability patch to prioritize motor output.
4. There is no thrust adjustment on the rear motor based on angle.
5. Yaw auto-tune could be made more efficient by removing any D term or filter frequency tuning and going directly to Rate P tuning.
6. There may be improved motor mixing that does a better job at isolating the 4 control axis.
So let me know what you think I should be looking at that is particularly relevant to Tri-Copters.
thanks for looking into the tricopter specific issues and to everyone bringing this so far.
As it happens I am just building one with a Pixhawk lite and 3.3rc9, had a few test flights now; I am quite happy with performance in general ( hovering around with little wind ), but ran into some issues
1. autotune failed. Could someone kindly post a link or explain what to look for in the log to tune a PID properly, either to tune manually or see why it failed in AT?
2. tail is wagging slowly. I am at Stab P 2.5, Rate P 0.4 Rate I 0.04. the lower stab p seems to slow down the oscillation; only with rate p at 0.4 does the servo seem to move enough to be called 'authority' over the tail. I am used to helis and was hoping that a tricopter could be a bit more aggressive on the tail than a quad...?
As for point 4. ( no thrust adjustment ) above, would that not mean that the motor rpm and servo are slowly fighting each other? A trad. heli can start pumping it's tail if the governor and tail servo are not in sync.
Another small issue I came across during setup: how to set up the tail for maximum travel without binding? Is there a mode where I can give direct stick to the servo or make it move to it's endpoints from MP?
Please find some pix to give an idea of the frame, and the latest log , and again thanks for the help
Try 3.2.1 it is the most stable f/w right now and with low enough Stab Yaw value doesn't wag unless you have some play in your yaw assy.
In 3.2.1 you need to adjust min/max pwm values for ch7
In 3.3 it is changed, but thee is a wiki entry explaining.
Also, I have a lot of experience flying a tri with 15" props and 380/400kv motors on 6s lipo - for some reason those never were able to autotune well, though with pwm_comp on SK and dumped light on BLheli I was getting usable PIDs which needed some significant lowering.
the Pix Lite can have issues with older FW ( Gyro health ) , that's why I stayed away from it. But if you say that 3.2.1 will work better on the tail I will give it a go.
Here's one for you, guys. Have you found autotune near impossible with a tri due to drift during alt-hold? I used to think it was only related to breeze but I tried tonight in near absolute calm, and it continually drifted away as if it had a persistent moderate tailwind. It's enough that I don't think autotune would even be possible without hundreds of acres of empty space (quite a premium anywhere near a metro area). I have tried letting it tune then pulling it back any time it nears an obstacle, but I have found the start/stop tune like that leads to poor results.
Has anyone else seen this issue, and is it something I can account for somehow in order to allow for an autotune?
Sounds like you need to do an in-flight level. There are a couple of methods described on the gui.
I have a plan that will automatically stop the drift if you enter Autotune from loiter. But I have not done that yet.
Thanks Leonard. Should I try Auto Trim?
yes, auto trim works perfect, just make sure you do it in 0 wind condition, I do it in an indoor basketball hall :D
0 drift if vibes are good and no wind afterwards...
Leonardt, setting rate_yaw_filt_hz to 20 made the trick! it is much much better than the stock 5hz... but still not like 3.2.1
btw, here is a log of a short flight I had on 3.3rc10 with rate_yaw_filt_hz set to 20
I cannot comprehend the new imu logging. it used to be that +/-3 and -5/-15, now I can't even reach those numbers, but copter still flies ok, also, I do not have any clipping (unless I am touching stuff mid-flight) and VIBES mostly stay in one place, can you explain or give a link with a good explanation of what too look for in the new logs.
p.s. ditch those plastic props for good they vibrate badly due to flex.... here are some really good ones On those 2216 800 multistars they will give you at least 19min of active flight with 5200mah 4s multistar pack as long as you keep your AUW under 1.5kg (the log attached is on CF graupner style so current is about 10amp more total).
I personally use Auto Trim because I can do a quick trim then if I see perfect conditions I can do a better job at any time.
The Vibe log pretty much tells you everything you need to know now. As long as the Vibe X, Y, and Z stay below 20 ish and the Clip logs stay at zero or only go up when you land, then you are fine.
You don't actually have the IMU logging turned on but if it was you would see that it is pretty much the same as it was before. The ACC logging is actually the raw imu data and shows the pre-filtered accelerometer values. The IMU logging that you are used to is post filtered accelerometer values.
Thanks on the pointer for the props. This is really just a development platform so I am not too concerned about optimising performance until I have finished fixing all the low hanging fruit. Once I have done that I will probably need to go through the process of sorting out the props but I will probably just use some APC props as I have plenty of them.
First quick flight in gusty conditions with v3.3rc10 directly after setup with a AUAV-X1 flight controller on a small carbon tricopter. No problems encountered except for a persistent compass variance error which required COMPASS_ORIENT2 to be set to 8 (Roll180), the same as AHRS_ORIENTATION on these boards. Also had a couple of bad AHRS errors initially but they went away. The log file is attached.
Auto Analysis with Mission Planner didn't work for some reason with this error: Could not add reference to assembly mtrand.dll
Yaw Wag can be down to the servo frequency and speed ., but to minimize the load on the servo I keep the CG forward so the tail motor will only stabilize the copter the most of the lift are from the two mains