Why no physically separate multicopter controllers and autopilots yet?

There are already many good basic multicopter controller boards on the market. Think of Naze32, KK2, etc. Then there's also great firmware, some of which that support multiple boards and PID controllers (e.g. Cleanflight). Most of these boards and firmware fly great, but lack or have poor navigation support (alt/GPS hold, RTH, etc). 

On the other hand we have Arducopter which is a multicopter controller that also has very feature rich and stable autopilot capabilities.

Wouldn't it be great if users could pick the basic flight controller of their choice and combine it with the autopilot of their choice? I'd like to do that, and I'm convinced that once these modular designs start appearing, that it'll boost both the development of both basic controllers and as well as autopilots.

What I'm thinking of is an autopilot board that acts as a PPM-sum filter between the RC receiver and the basic flight controller (e.g. barebones Naze32 or KK2), and has it's own gyro+accelerometer and I/O connectors for PPM-in, PPM-out, GPS+nav, and that's it. The autopilot need not even know how many motors the flight controller controls. It only has to be told via some configuration tool, what the functions of the various channels are and have configurable navigation PIDs.

If the autopilot/navigation board is designed small enough, then it can be simply stacked above the flight controller board and added at a later stage after the frame has been tuned.

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I don't really understand your point.  You already have Altitude Hold, Vertical Speed Hold and Turn Rate Hold.  It's just that they are controlled with sticks, rather than knobs.  You could certainly slave these functions to knobs though, if you really wanted.

It seems gimbals use very precise inertial units, would it be possible to use those units to navigate from starting point?

Actually, gimbal IMU's underperform ours quite badly.  This is precisely because of the fact they don't employ GPS data fusion.  This is the cause of "horizon drift" that plagues virtually all of them.

Regarding my comment on dials to adjust speeds (V & H), altitude hold, etc, it comes from my experience as an aircraft pilot, it is completely different to fly holding the control yoke or using the autopilot dials. Using the autopilot the plane is much more stable, as direction and speed changes are somewhat limited.

And regarding the inertial units, we used to fly planes just on the inertial system, there was no GPS at the time. Depending on the plane model there was crosscheck with ground radio stations, but many times those were out of range (sure not the same cheap inertial units, just explaining my prior comment).

Example of inertial system from the 60's, 2 mile error on 500 mile long flight:

http://en.wikipedia.org/wiki/LN-3_Inertial_Navigation_System

How much did those gyro systems in the airliners weigh, and what did they cost?

For sure not suitable for a small drone, but since 50 years have passed from that model and now we have such small and cheap units (which I find incredible) I just asked if they would be capable of doing dead reckoning (500m long flights).

It would't surprise me if they could, as their very existence and price is fantastic.

It's not possible at this time.  Even if the chips were accurate enough, these quadcopters vibrate way too much.  The signal-to-noise-ratio is very low.

  I have tried implementing dead reckoning.  The micro sensors used in today's drones are roughly 1000x too noisy and imprecise for significant dead reckoning use.  The accumulation of error due to the double integration of the accelerometer data happens very quickly even under ideal circumstances.

  An accelerometer designed for dead reckoning would also need to output 24 or 32bit numbers (currently 16bit) and have a sample rate of ~10kHz or greater.  These are huge changes from today's sensors.

Beyond the sensors improvements, the following are also required:

  • Flight controller computations must be done with 64bits numbers to minimize "rounding" errors.
  • Flight controllers must have a very precise high speed clock.

These last two items are no real problem today, but many flight controllers do not currently support them.

Thank you for your answers, very clarifying.

And what about Doppler navigation, would it be possible on small drones? I have these questions about dead reckoning because it is not always that I can count on good GPS signal.

Canberra UAV implemented dead reckoning in 2012 in the Ardupilot code, on GPS loss. I have not used it personally, ie switched off GPS, but it is in the code we used for the OBC.

In the OBC it was required to circle for 30 seconds and then fly some 8km to home for a manual landing. The alternative was flight termination on GPS loss.

Dual redundancy can work, if each is limited to 50% authority. With two independent controls the output will be the average of the two. The 50% level is defined as that which results in neutral control conditions, i.e. a position hold in a multicopter. Dual redundancy can be achieved with, for example, a co-axial quad with each controller driving four motors. and each thrust limited so one controller-motor set can only maintain altitude at full power.

Triple Redundancy would result in a better outcome if one control channel is lost or goes to max, as the other two can easily overcome it, in this scenario each channel is limited two 33% authority. By limiting authority no voting is necessary between controllers and outputs are averaged only. In the case of multicopters, averaged thrust per motor set is the vote.

Modluar designs have been around for some time, just not within the price range of the average mortal. The Military has been using modular designs for quite some time. Some upscale UAV companies have been using these products, but again, the average mortal does not have $50K for a UAV and its equipment. Eventually some of this stuff trickles down in to the consumer market. I guess the best thing to say is be patient.

41 years ago when I built my first R/C airplane it was balsa wood and paper. No one could afford plastic molded part planes. There were a few on the market, but for the most part every one built out of balsa wood and paper.

this just popped up on ars technica... takes your idea a bit further

http://arstechnica.com/information-technology/2015/03/for-a-brighte...

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