Can someone provide guidance, please. First post on these forums.
I would like to implement an autopilot in a model RC hovercraft I have built. By hovercraft, I mean cusioncraft - i.e. a vehicle with a skirt that floats above the ground on a cushion of air - I see the term hovercraft seems to be used for flying quadcopters on these forums.
It’s quite a quick vehicle, and controlling its direction takes practice, but I was hoping to implement some sort of active directional stability. The hovercraft uses differential thrust from two thrust motors to manoeuvre.
First implementation would provide some sort of directional stability. I’m more interested in track stability i.e. not just a heading hold, but the hovercraft should maintain a track over the ground to account for cross-wind.
And the more I read, the more interesed I am to implement full autopilot functions.
I assume I’m on the right track with ArduBoat? How does the software handle massive drift angles? For example, I can drive the hovercraft round a corner with about 60degs drift. Can the software cope with that?
Are there any examples of hovercraft implementations?
There have been some use cases for differential thrust boats that have worked with a skid steer configuration. The motors don’t reverse? Tuning may present a challenge but it could be a fun one. Check out the Acro and Steering Modes in the Wiki. Steering Mode may be interested as its controlled by lateral acceleration with steering input.
if the motors really do not reverse, you should find a way to use forward/reverse ESCs.
If you already have reversing ESCs, arduboat/rover should have no problem controlling your hovercraft ( a hovercraft is an air cushion vehicle, I never saw someone call a quadcopter a hovercraft)
I have propeller driven omnirover with four castor wheels. So it is basicly a hovercraft, regarding the driving characteristics.
Driving the rover in manual mode was nearly impossible, but the assisted modes (acro, steering, etc.) worked great after tuning. In the assisted modes ardurover tries to keep the commanded forward speed, so if you yaw the vehicle, the forward speed becomes sideways movement and forward speed gets lower or even zero. Ardurover then throttles up to hold forward speed and pushes the rover in the new direction. I can drive fullspeed circles without a problem with this rover with the help of ardurover.
Good luck with your hovercraft!
A hovercraft with motors in one direction (forward/reverse) can not build up lateral acceleration, because nothing can apply a sideways force.
My rover works in steering mode, but I do not know why.
The motors reverse so the hovercraft can turn on the spot, but in forward motion, only a thrust differential is usually required.
All hovercraft have to drift round corners, so the thrust vector from the engines resolved perpendicularly to the track provides the lateral acceleration. See my pic.
This is why I ask the question, it looks like I can use ‘skid’ steering, but can the software cope with large drift angles? I.e. where the heading of the vehicle is not the same as the track over the ground.
There must be lateral acceleration or it would go straight. In trying to get my head around this your Air Powered Omni vehicle comes to mind. The casters offered a degree of freedom not too unlike Andrews Hovercraft it seems to me. I know, different vehicle altogether with different power vector forces available but interesting anyway…
The motors provide a turning moment around the vehicles vertical axis. The vehicle turns and the motors accelerate it in the new direction. You have to overturn the vehicle, so the motors can eliminate the previous movement direction and accelerate in the new direction. Once the vehicle is moving in the desired direction, you turn its nose to the front again. All acceleration the FC experiences is longitidunal.
Of course there is air resistance and it gets bigger as soon as the side of the vehicle points in the direction of travel, but at slow speeds, it is not doing much to help the vehicle steer.
In hovercraft racing they roll the craft to go around corners, so the bigger area and the thrust from the lift motor/diversion helps to turn quicker.
Some big hovercrafts like the LCAC, have pivoting thrust nozzles at the front to help maneuvering.
My omnirover worked just like his hovercraft, because the “omni” part only works in manual mode.
Great, all that writing and now my smartphone shows @APC_d2180s post and drawing explaining it.
As I wrote in my first post, the hovercraft should work. You will have to use relativly high PID values for steering to make it work and I would use nice and slow speeds in auto mode, but acro mode should be a blast.
Here is a video of my “almost hovercraft” omnirover in acro and RTL mode. These are the old wheels, it is now twice as fast and rolling much smoother. You can hear the motors throttle up in turns as ardurover tries to keep the forward speed: