T Frame Pentacopter

[jem524]

Hi everyone,

I am currently working on a capstone project and we are trying to do a copter design with 4 Motors in the front and one motor in the rear. We are using Pixhawk as our flight controller and in Ardupilot we have set the copter up as a "Y tricopter" with the front four motors connected in parallel and one motor in the rear with a servo mechanism to stabilize yaw. When conducting flight tests of our configuration we are having major roll issues and are now wondering if that is due to our configuration itself.

-Is something like our configuration a "T frame pentacopter" even feasible?

- Is the roll issue something we can account for by tuning such as changing the PID values/gain values or is this not possible due to our configuration?

- Would we have to tell the pixhawk that there are five motors somehow or is it fine that we have two pairs of motors connected together in the front like our configuration?

- Has anyone ever done anything like this that someone could point us to?

Any help would be greatly appreciated!

 

[LitterBug]

You will need to split the front motors in to at least two groups. That will make it behave more like a traditional tricopter and give you the ability to control Roll. The Tail should give you pitch and yaw control.

Pay attention to the rotation of the props. They should move in the typical rotation for a tricopter based on the "standard rotation" for each of the three arms.

Ideally you would have a seperate mix for each motor with mixes weighted based on their location.

LB

 

[jem524]

You will need to split the front motors in to at least two groups. That will make it behave more like a traditional tricopter and give you the ability to control Roll. The Tail should give you pitch and yaw control.

Pay attention to the rotation of the props. They should move in the typical rotation for a tricopter based on the "standard rotation for each of the three arms.

Ideally you would have a seperate mix for each motor with mixes weighted based on their location.

LB

I have seen others do something similar and my intuition tells me that the PIDs should account for the different roll issues in stabilize modes. Is it a problem with the configuration that I have the copter set up in or is it a tuning issue?

 

[LitterBug]

So I can give a better answer, we need a bit of clarification on the current configuration.

First off, do you have any pictures of your build? If so please post them.

What do you mean by "front four motors connected in parallel"? Are all four connected to the same pixhawk output, or is every motor hooked to individual pixhawk outputs?

What direction are each of the individual motors turning? This can have an impact on the ability to control yaw. In theory, with four motors in the front and alternating prop directions, yaw could be controlled by the front motor momentum in addition to the yaw servo. For simplicity's sake, I would not set that mix until after you get a stable hover working first.

In a typical tricopter, the front right and left arms control roll. if you do not split the motors between the right and left arms, the ability to control roll will not exist. It will have thrust, but no roll control.

LB

 

[jem524]

So I can give a better answer, we need a bit of clarification on the current configuration.

First off, do you have any pictures of your build? If so please post them.

What do you mean by "front four motors connected in parallel"? Are all four connected to the same pixhawk output, or is every motor hooked to individual pixhawk outputs?

What direction are each of the individual motors turning? This can have an impact on the ability to control yaw. In theory, with four motors in the front and alternating prop directions, yaw could be controlled by the front motor momentum in addition to the yaw servo. For simplicity's sake, I would not set that mix until after you get a stable hover working first.

In a typical tricopter, the front right and left arms control roll. if you do not split the motors between the right and left arms, the ability to control roll will not exist. It will have thrust, but no roll control.

LB

I think I have not explained very well how we have the copter configured. Here is a photo of something like how the copter is configured. The 2 left front motors are wired together to Channel 6, the 2 front right motors are wired togeth to channel 5, and the single rear motor is wired to channel 8. There is also a tilting servo connected to channel 7 to tilt the rear motor to account for yaw. Because of the rear servo yaw has not been much of an issue but roll has been a significant one. This leads to the question I posted earlier

 

[LitterBug]

yeah, Ok, in that case it is probably PID tuning related. Also, make sure the ESCs have been calibrated so they respond in unison.

FYI, I have about 7 tricopters of various sizes, just never thought to double up the motors on the front.

LB

 

[jem524]

yeah, Ok, in that case it is probably PID tuning related. Also, make sure the ESCs have been calibrated so they respond in unison.


LB

Do you have any procedure or process on how we would go about doing with Mission Planner as our GCS? We do not have a lot of experience with this type of tuning process as this is our first time creating a copter like this. Additionally, how would we or is there any way to check if it was an esc/flight controller issue? We have calibrated them before but are unsure if that is the problem.

 

[LitterBug]

If you have calibrated the ESCs, it is most likely a tuning issue.

Does it hover at all, or does it try to flip over immediately?

LB

 

[jem524]

It tries to pitch and roll almost immediately

 

[LitterBug]

It's possible that the flight controller orientation is not configured correctly.

pictures would be most helpful

 

[jem524]

So you think that if we had the flight controller oriented correctly, the escs all properly calibrated, and the PIDs with this configuration it would theoretically work?

 

[LitterBug]

Yes and weight balance correct too. I would put the majority of the weight forward so the rear motor doesn't have to work overtime.

 

[L Edge]

Does your system in the FC allow for "scaling"? (I used mine with Open Aero and KK2 Fc)
With the addition of 2 extra motors, I see that the roll should be really be no problem (2 motors+ and 2 motors-) so programming that is easy in the normal way.

The problem you have is having 4 plus fan units times unit distance to CG and only 1 fan unit negative times distance to CG. In scaling using my system, the sum of forward(+) and aft(-) should be zero. So pitch control for the FC is way out of control.

So by programming "scaling", it allows you to have a range of P,I,D's to suit your flying skill.

Just looking at it, 4 motors changing against 1 to balance pitch, probably overdrives the single fan past it's settings.

So use the scaling method in your system to bring it back under control. You might even have to re-locate the CG to bring it back into balance.

I hope this helps you in the right direction.