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Jack Crossfire's blog
Archive for February, 2008
Posted by Jack Crossfire | Feb 29, 2008 @ 05:09 PM | 4,810 Views
This is the last simulation of orientation feedback in copter frame, because now orientation feedback is in world frame. Have found the feedback in different simulation runs is different every time. The neural network is seeded with random weights before every run & it may actually remember starting information through entire flights.
Posted by Jack Crossfire | Feb 29, 2008 @ 04:17 AM | 4,009 Views
To try to stop rate damping overflows, clamped the raw unsigned gyros to 16384-49152. Now assuming the gyro center & Vref R close to 32768, the PWM feedback can't wrap. Unfortunately, clamping feedback results would require 32 bits for the entire sequence & no-one wants to risk another main shaft on a test.

Happily, she made it to all 3 waypoints on our waypoint program. Altitude hold failed after finishing the program & she could not maintain a hover.

She can turn 90' pretty easily, but forget about 180. Definitely need the target attitude in global frame if we're going to do any turns.

Unfortunately rcgroups images R broken, so you get links instead.

waypoint map

The satisfy radius is 5m, but it's meaningless with a copter. Maximum velocity was 2m/s. The practical limit with such a confined space is probably 1m/s.

altitude plot

Target altitude was 130m & collective was really well trimmed before flight.
Posted by Jack Crossfire | Feb 28, 2008 @ 05:45 AM | 4,169 Views
Had another idea. Increase the integrals in fixed steps & clamp the accumulators. Previously they incremented forever relative to the error & never seemed to catch up or if we increased the gain, overpowered the P term.

Step integrals definitely seemed to improve the attitude test but gains had 2 B much lower. The auto hover was pretty stable, but still does not track long term attitude. Not sure if it's a problem in the simulation.

Did the first waypoint program today. After 2 tries to get the altitude to work & a few minutes of painful creeping, she finally made it to the target & hovered, the first successful waypoint mission.

On one altitude drop, did a full collective, saturated the yaw gyro & got our first software rate overflow. 1 hour of repairs.

FYI, since the flybarless head is rigid, U can't use the foam widget from Align.

Now today's movie.

rocket truck (0 min 44 sec)

Posted by Jack Crossfire | Feb 27, 2008 @ 01:17 PM | 4,195 Views
Well, leaning again towards blowing the $70 + $32 for VicaGlider. Though electronics may still be getting cheaper, rising shipping costs R more than compensating for it. Now suspect a yaw gyro for roll stabilization & a pitch gyro for damping pitch rate may be enough inertial sensing to do the job.
May use our inventory of full size servos for 2 of the control surfaces.
Posted by Jack Crossfire | Feb 27, 2008 @ 03:17 AM | 4,487 Views
Well, flybar loops didn't track attitude very well. The software flybar loop doesn't have enough force to overcome attitude changes. Could swap in bigger constants during autopilot mode, but that road probably includes monster oscillations. All roads lead to a cyclic neural network or less flybar stabilization.

Went back to full autopilot. Came to the conclusion that part of our problem is altitude oscillation. The neural network may also forget its training & relearn every 25 seconds. A few more PID tweeks and she's roughly where the Corona was.

It definitely looks like the Sirf III binary conversion is less precise than the NMEA output.
Posted by Jack Crossfire | Feb 26, 2008 @ 01:22 AM | 4,208 Views
Stepped up the cyclic feedback & got more plots. The dirty secret is attitude hold has always been pretty awful since Corona days & still is. The extra stiffness from software flybar may aggravate the problem.

Next, the RCE-BL35X lost its configuration & went back into throttle mode. 6 hours without a malfunction is pretty good 4 Taiwan. Should program the autopilot to program the ESC.

Attitude hold using straight proportional feedback works for stabilizers like the Copilot & AP2000, but clearly it's not good enough for targeting velocities. Adding integral increases wind resistance to a point, but does not lock it in before it degrades stability. The Gumstix loop isn't fast enough for rate feedback. Its latency is 14ms. The INS latency is 54ms.

That leaves actually going into the PIC & biasing the software flybar to get the desired rotations. Maybe when autopilot is on, cyclic commands become gyro center values & we start over again with gains of 0.001 & plastic blades.

The heading hold algorithm without lowpass filter is bulletproof but now has a definite jitter. Probably need independant bandwidth for yaw & cyclic.
Posted by Jack Crossfire | Feb 25, 2008 @ 02:58 AM | 4,499 Views
If the Futaba S3102 was dog slow on the Corona, it's probably unusable on the T-Rex. The $60 Futaba S9602 looks like the next step.

Moved the tail damping back off the lowpass filter since it didn't stop the wagging. May need another round of bandwidth constants & strap down tests with the filter.

Again leaning towards putting VicaGlider on a dual PIC arrangement or Gumstix. The necessary kind of trig is not possible on the single PIC. Another method is just flying in 8 directions.

Now convinced attitude sensing is required in fixed wings. Now the question is whether GPS can replace accelerometers for drift elimination. The theory is if U need GPS assisted inertial navigation in a fixed wing, it might as well go all the way.

Dreamed someone converted an F-35 to a passenger plane. It was never tested before first passengers boarded. After spitting out some metal fragments, it took off normally vertically. After transitioning to forward flight, it started spitting out more metal fragments. Then it entered a runaway pitch down, looping inverted like a propeller. It managed to stabilize after converting back to hover mode & made a hard landing, crashing through a building.

Another dream was about a Nazi concentration camp which operated until 1972. Apparently they never found out the war ended & kept mining supplies deep underground. Through the bravery of a heroine nurse who looked like Liv Tyler in 1999, several prisoners escaped in the 50's. The rest finally got all the way to a boat in 1972, which the Nazis shortly turned around by remote control & exploded.

Nothing left but more wide angle excitement.
Posted by Jack Crossfire | Feb 24, 2008 @ 04:03 AM | 4,390 Views
The latest heading hold software goes to full manual control when stick is deflected beyond dead band. Integral feedback is always applied but not incremented in manual mode. Proportional feedback is only applied in automatic mode. The current heading is taken as the target heading in manual mode. This algorithm worked better than any other in a high wind.

Tail servo is now on 6V from the RCE-BL35X. Blade tracking from yesterday came out perfect.

A very slight chance of good weather is predicted on Monday morning with the window definitely closing again on Friday.

Storm movie (0 min 30 sec)

Posted by Jack Crossfire | Feb 23, 2008 @ 04:59 AM | 4,261 Views
Waypoint navigation is pretty useless on VicaCopter because:

Flight time is too short to go anywhere.

Open spaces R too small to go very fast.


Advantages:

Don't have to manually set up every hover.

Just swap batteries & go.

Previously we tried setting waypoints in flight. GPS proved too unreliable to pull that off & the user interface was too confusing. One flight's waypoint was another flight's tree. Now hard coding pragmatic waypoints is the key.

Maximum flight time with flybarless is 11m30s so it added 1 more minute. The kit battery is abysmal.

None of the software heading hold strategies worked. They tended to not allow any yaw control at all & rapidly drift, making for very exciting flying.

Now seeing much less vibration without the flybar. IMU drift with flybarless is too good 2 B true. Unfortunately, the last 3 campaigns had significant oscillations in autopilot & manual cyclic was very unstable. Oscillations R in the 2 sec range, hardly what we expected with flybarless.

Autopilot pushes target attitude all the way over with no response from the cyclic. May be more yet to discover about flybarless dynamics unless it's blade tracking.
Posted by Jack Crossfire | Feb 22, 2008 @ 01:16 PM | 4,203 Views
Squeezed in a daylight hover during day job hours. Had a 15 minute window without rain & wind. She did much better at high altitude than low altitude.

Manual heading hold program was pretty awful. It increments the yaw regardless of current state, causing sudden movements & oscillation. Probably need direct stick -> rudder offset. When the stick is outside its dead band, heading hold is turned off & current heading is taken as the target. There's also staying in heading hold mode but taking current angle + fixed offset as the target.
Posted by Jack Crossfire | Feb 22, 2008 @ 03:47 AM | 5,002 Views
Servo headers started feeling a bit loose, so time to resolder all the headers. U know, if we just spent another $10 on a decent proto board back in 2006, would never have had so many broken solder traces. That would be $20 today.

Rain began in the middle of today's test flight. Suspect wind is higher in the flight area than it is in the pilot area because she didn't do very well. Also having great difficulty calibrating the magnetometer after moving it. Min & max flux R not enough. It also needs a rotation & rotation sign correction.

Finally have software heading hold for manual flying implemented, but the next weather window is now Monday.

Now the weather forecast:
> High wind watch in effect from Saturday afternoon through Sunday morning...
> ...big storm that will pound our region with fierce winds and driving rains...
> ...trees toppling more easily...
> ...longer period of strong winds than on Jan 4th...
> ...downed trees and power outages...
> ...periods of heavy rain...
Posted by Jack Crossfire | Feb 21, 2008 @ 02:50 AM | 5,157 Views
Carbon fiber blades went on, then a training gear test, then full autopilot. No problem with the software flybar, but nervewracking oscillations in autopilot. Also significant tail wagging remains despite gain reduction on the ARM.

Definitely more photogenic with carbon fiber blades. Software flybar did feel a bit looser on the carbon fiber, but still stiffer than the mechanical flybar. The benchmark is ground effect hovering & backwards figure 8's. Software flybar is the gold standard in both.

Well, she certainly likes oscillating. After the photo session, stepped up the I gains & oscillation except for tail wag virtually disappeared. May also be weather sensitivity. Tail wag may require gain reduction on the PIC.

Well, flight times in previous blog posts were more like 10 minutes. The 13 minutes was including initialization. Have not taken flybarless to exhaustion, but it's getting at least 10 min.

Need another rate cut & liquidity injection into the blade budget. Bernanke can use our printer until he's fired....Continue Reading
Posted by Jack Crossfire | Feb 20, 2008 @ 09:21 PM | 3,909 Views
Instead of sitting in traffic, got some eclipse shots from the unique view of our day job. Too cloudy to match the monster quality from 8/29/07 of course

http://www.rcgroups.com/forums/showthread.php?t=735167
Posted by Jack Crossfire | Feb 20, 2008 @ 03:16 AM | 3,952 Views
The answer is yes. She hovers autonomously without a flybar.

The answer is no. Did not get any pictures. The window between storms is too short to get any meaningful flight times & she drifts around significantly in the light gusts. With light drizzle, good chances of a strong gust & very high repair costs, not confident enough to swap controls for camera.

Had another complete tail rotor failure in which the belt drive autonomously built up slack since installation. Need 2 keep on top of the slack & abort if tail rotor begins drifting. 1 main shaft gone.

The T-Sux 450 is sucking money faster than our boss's subprime mortgage. February so far:


$24.96 2 Belts
$25.28 Ball link tool
$18.36 plastic blades & bolts
$40.18 CF blades
$22.87 1 Belt, main shaft & bolts
--------
$131.65


With such a blood sucking, fragile copter, VicaGlider may be unaffordable.

Put the yaw damping on the lowpass filter and got no difference. Still a bit of a wag. Expanding the flybarless envelope, it's more like attitude hold than full manual. Hovering in ground effect is rock solid. Rapid descents R almost hands off.

Americans think she's a UFO. Indians think she's just a flybarless T-Rex 450 using ADXRS150 gyros & very tight PID loops.

Got the PIC up to 85 IMU updates without radio signal & 90 IMU updates with radio signal. Peculiar loss of clockcycles without radio. No IMU recalibration was necessary.

In the current configuration the PWM duty cycle goes from 4000 - 24000. The maximum rate damping offset is 32767 * 32 / 256 = 4096

If a servo is full over & opposing angular velocity saturates the gyros, she could overflow, but that situation is already lethal. With a human pilot, rate damping always opposes servo deflection instead of adding to it. In a tail rotor failure, overflow could happen, however.

There aren't enough clockcycles to clamp feedback. The tail rotor fails all the time.
Posted by Jack Crossfire | Feb 19, 2008 @ 03:16 AM | 5,222 Views
The software flybar mode started working & it is so stiff, it's like Al Gore. Though using plastic blades, it's far stiffer than the carbon fiber + mechanical flybar ever was. A miracle anyone still sells flybars.

The key was moving cyclic feedback to the lowpass filter. Now we have rudder feedback from raw gyro data & cyclic feedback from lowpassed gyro data. All feedback gains were 32/256. The ideal cyclic ranges ended up the same as a mechanical flybar.

The biggest fear with lowpassed gyros was reappearance of monster exponential oscillation from excessive delays. That didn't happen with cyclic at least.

The 2 causes of oscillation R:

1) alias frequencies from vibration

2) delay in feedback

For the flybar, it was alias frequencies. For the rudder, it was delay.

Software flybar tests (1 min 0 sec)


Current hardware:

3 * ADXRS150 gyros
3 * Hitec HS65MG cyclic servos
1 * Futaba S3102 tail servo
Castle BEC
PIC 40Mhz

Would use the 75 deg/sec gyros if there was a take 2. These servos must have no dead band. Optimized the PIC to hammer software rate damping & reduce attitude sensing cycles. Another round of IMU calibration & attitude hold is now required.

Rain & wind has returned until March....Continue Reading
Posted by Jack Crossfire | Feb 17, 2008 @ 10:41 PM | 3,838 Views
Suspect it'll be as standard as 2.4Ghz in a year.

http://www.rcgroups.com/forums/showthread.php?t=624290

Trying to avoid fractional multiply operations. On the Taiwannosaurus, rotating the pitch +30 deg requires using only aft & starboard servos. Rotating the roll +30 deg requires making the port servo oppose the other 2 servos.

Back on the test stand, starting to behave like a real copter, but clearly there is too much noise in the cyclic gyros. Probably need to reduce gains to get her to spool up without flipping over. Reduce gains & U can't trim.

Flybarless test 2 with 30 deg phase adjustment (1 min 18 sec)

Posted by Jack Crossfire | Feb 15, 2008 @ 03:03 AM | 5,126 Views
They said flybarless testing was dangerous & flybarless testing didn't dissapoint. 1 cracked blade so far. Carbon fiber is no match for the softest grass in the golf course. May end up spending more on blades than a commercial flybar emulator.

1: blade pitch is 90' offset from swashplate pitch. Swashplate roll causes rotor pitch.

2: if significant oscillation with no cyclic input, shut it down instead of adding cyclic.

3: Even with the CCPM rotated, the swashplate is still about 30 deg off. Software wouldn't compensate for the right angular rates without some serious matrix math.

The next ideas R using the innermost servo holes, getting her to sample the IMU on the ground & hold that orientation in a full throttle test. May also be too much servo jitter. Definitely haven't seen any rate damping.

Low throttle test without flybar (1 min 2 sec)

Posted by Jack Crossfire | Feb 14, 2008 @ 05:54 AM | 3,796 Views
Had a wind advisory & 25mph wind in Rain Ramon, so that grounded autopilot operations. High wind is predicted until Saturday.

The answer is yes. Software tail gyro works. The trick was moving the feedback to the PIC, making the A/D conversion -> PWM route as tight as possible. It didn't need a log curve or lowpass filtering, just a straight P term. The result was a very stiff rate damper without oscillation. Results may actually have been better than the PG-03, but in very high wind of course.

Success with software rate damping means we can start flybarless testing.

Mounting the Canon A560 is going 2 B all but impossible with the flybar. Unfortunately if flybarless is too unstable, will need full time software attitude hold in addition to software rate limiting. Instead of setting absolute angles, cyclic would increment absolute angles by fixed amounts. Unlike manual flight, releasing the stick would freeze attitude.

Unfortunately it means never flying an RC copter manually again. Relearning the skill to bring up a future airframe would be hard.

Since flybarless testing can be devestating, plans for flybar testing have ranged from using the test stand from http://www.rcgroups.com/forums/attac...mentid=1337158 to minimizing collective to resetting throttle back to minimum values. The trick with the test stand is to mount Vicacopter upside down.
Posted by Jack Crossfire | Feb 13, 2008 @ 03:40 AM | 4,065 Views
Well, in the XYZ -> LLH conversion, heading was generated in degrees when it needed 2 B rads. With that finished, autonomous hover began working. The neural network dialed in & she hovered 10 minutes facing South before drifting into the trees. Formidable wind developed after 12am. Attitude change in response to position drift was apparent but pitch integrated down when it needed to integrate up.

Velocities R still much less stable on the Trex than the Corona. Oscillation is still significant. Have found excessive I terms cause more oscillation than the P terms beyond a point. Suspect the stiffness in the blades cause the high accelerations.


Found she was using the dX loop for both dX and dY. Doh! The fact that she could hover at all was a neural network miracle. Unfortunately, time to get back in the car and head back to Silicon Valley....Continue Reading
Posted by Jack Crossfire | Feb 12, 2008 @ 12:28 PM | 3,695 Views
With the poplar IMU + 1/2" padding + doubled PID gain + no flybar weights, she controls attitude like Hillary. Release cyclic & she returns to where she thinks the starting point is. The starting point drifts slightly, but it's roughly what the Corona did when it was hovering autonomously.

P was the key factor, needing to be much higher than it was on the Corona to get the attitude to fight back.

4 U homegamers, the recommended drive belt on Taiwannosauruses is the MX400 drive belt. Intended for a Helimax MX400 but fits the 450.

As for position hold, she definitely holds heading where the Corona could not. Still a disaster with position. Converted the GPS routines from NMEA to Sirf III back in December to try to gain some ms. That involves converting from XYZ to LLH in software. Now GPS seems to report starboard motion when she's really moving port. May need to revert back to NMEA to get something to work, before pushing Sirf III more.