Phase-Universal virtual flybar - My Winter Project
This winter I decided to build something that would help me fly better and crash less. Using a 32-bit Parallax Propeller processor and a 2-axis IDG500 gyro, I built myself a virtual flybar unit.
I call it Phubar, for Phase-Universal Flybar.
It allows for adjusting the phase angle of the command inputs and the gyro stabilizing inputs to the swashplate. It has angular gain and rate gain settings (Proportional and Derivative coefficients, for you control systems people) with angular decay so it mimics how a weighted flybar will "follow the heli" after a second or two. The gain, decay, and phasing parameters can be changed using a serial terminal app on the PC that talks to the PhuBar via USB cable.
The whole unit weighs about 13 grams and is 35x35x18mm. By comparison, the stock flybar plus one pair of weights is about 19 grams. It has four receiver inputs - aileron, elevator, aux(pitch), and a spare - and four outputs for servos. An LED lights to tell you when it will accept input via USB, and blinks for 3 seconds when the gyro is calibrating so you know not to move it.
I have tested it on a HBFP and a FireFox Ep200 so far, and it is working very well. It took a lot of test flights for me to find the right gain settings. Photos below show the flybarless head and the PhuBar installed. I made the FBL head for the HBFP by removing the flybar and making direct swash-to-head links using link ends and balls from a FireFox 200. TX and RX are Spectrum.
The video below shows how the swash moves as the heli is tilted. Here the phasing is set to zero degrees, so movement on only one axis causes only one servo to respond.
Then you see it in flight, with the PhuBar set to 45º phasing so as to mimic the 45º head arrangement found on the Kestrel, Free Spirit, CB100/CB180, etc. It is pretty stable. About 43 seconds into the clip I flick the cyclic stick hard right to show how it recovers from a disturbance. Blink and you will miss it - it recovers that quickly.
I sure am glad to be rid of that P.I.T.A. flybar.
The software now incorporates CCPM mixing and handles up to 3 swashplate servos in any configuration. The placement of each servo can be specified during setup.
If all goes well, I will probably open-source the code and publish /docs/instructions on how to build it and load the software into it. It takes about $110.00 in parts and the ability to solder surface-mount devices (SMDs). The parts list is in the PhuBar.doc file and in a separate .txt file attached below. I had the printed circuit board made by BatchPCB.com. Gerber and drill files suitable for uploading to BatchPCB.com are in a file at the bottom of this blog entry.
Oh, and after tuning this thing, I now understand why helicopters with physical weighted Bell flybars tend to wobble a lot when hit by a disturbance or large control input. It's because a Bell flybar can only provide the Proportional term in the stabilization equation. It cannot provide the Rate(derivative) term which can damp out the wobble. Look at the promotional videos of the CB180Z with it's built-in 3-axis gyro AND Bell flybar. With the gyro turned off, it wobbles. With the gyro turned on, much less wobble, because it can apply rate-based control. So it adds the D to the P to make a PD control loop.
Update: 9/1/2010 Attached new gerber files and schematic. Had to add some pulldown resistors to propplug tx line and AUX signal.
Update 11/6/2010 Began hosting 3-axis PhuBar3 design data and code on Google Code site here: http://code.google.com/p/phubar/
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