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Gisela & Joe Noci's Discussions

MAP calibration algorithms - has any one done this?

Started this discussion. Last reply by Michael Johnston Jun 4, 2013. 1 Reply

Map Calibration using Affine techniques -Has anyone out there done any work on map calibration using 3 or more known points on a paper scanned map, or a Google Earth map? As you know, you cannot use…Continue

Tags: nampilot, calibration, Map

My Blog On Turret Gimbal obliterated by Moderator?????

Started this discussion. Last reply by Michael Pursifull Feb 5, 2013. 1 Reply

I posted a blog on a Ball turret Gimbal design and build It appeared in DIYDrones with only 2 of the nine photos, and the text all garbled in between!! The preview before posting it was fine .Was…Continue

Tags: SurVoyeur, Gimbal, Turret, Nampilot

Optimizing Wing Aerofoil for UAV long range flight ( not for endurance)

Started this discussion. Last reply by Marco Ricci Apr 3, 2013. 37 Replies

Good Day all,I have been spending a lot of time analyzing many different aerofoil profiles to optimize said profiles for a very specific flight regime. If there are any experts with good knowledge on…Continue

Inertial Sensor Temperature compensation and orthogonality offset corrections

Started Nov 4, 2012 0 Replies

Hi Chris, maybe Bill (P) and other sensor boffins..Is there any temperature compensation and sensor orthogonality offset ( measured on a rate table maybe) done on the APM series autopilots? Has any…Continue

 

Gisela & Joe Noci's Page

Latest Activity

Gisela & Joe Noci commented on Gisela & Joe Noci's blog post A large scale Variable Pitch Tilt Quadrotor implementation.
"@Shehryar, Sorry, I made them all myself, using some ready made helicopter tail rotor parts, etc, but mostly my own milled out aluminium parts. Joe"
Jul 17
Gisela & Joe Noci commented on Gisela & Joe Noci's blog post Some more Antennae, and when is a BALUN not a BALUN
"Hello Jeff,  Yes, any piece of copper wire is ok - not too thick so that is difficult to heat and solder without melting the rest of the installation, if not Teflon coax, and not too thin so that is is difficult to keep snag against the feed…"
Jul 17
Jeff commented on Gisela & Joe Noci's blog post Some more Antennae, and when is a BALUN not a BALUN
"Hi Joe, I appreciate your detailed answer to my question. It has been very helpful in my own antenna design. In your example to a alternate for the bazooka balun, you said "a single 1/4wave length of conductor can be substituted in the…"
Jul 16
Shehryar Niazi commented on Gisela & Joe Noci's blog post A large scale Variable Pitch Tilt Quadrotor implementation.
"Hey! Can you please tell me where you ordered the 'variable pitch and rotor tilt assebly from' .. or did you make it by yourself? I just posted a discussion and would really appreciate some…"
Jul 15
Larry commented on Gisela & Joe Noci's blog post A large scale Variable Pitch Tilt Quadrotor implementation.
"Ryan This is the best I've seen for a VP quad. http://curtisyoungblood.com/V2/products/quadcopters/stingray-500 "
Jun 29
Gisela & Joe Noci commented on Gisela & Joe Noci's blog post Laser Rangefinder Flights tests in Namibia
"I believe Andrew is working on just that... "
Jun 28
Ryan Coxey commented on Gisela & Joe Noci's blog post A large scale Variable Pitch Tilt Quadrotor implementation.
"I want to try to build one of these so bad! But, Don't know of a flight controller that could manage the flight patterns of a VP quad. An Apm 2.6 might work, but will need some adjustments to compensate for the signal going to the Servos…"
Jun 27
Gisela & Joe Noci commented on Gisela & Joe Noci's blog post More Laser Rangefinder based landings - from the NamPilot
"Hi Andrew, Thanks for the compliments - I always follow what you do - think that what you do is amazing, and I wonder if most know what the real commitment is from your side to give this all for Love of Passion!  Great work! Andrew, click on…"
Jun 27

Developer
Andrew Tridgell commented on Gisela & Joe Noci's blog post More Laser Rangefinder based landings - from the NamPilot
"Very nice work as always! Do you know what the range of the SF/10A will be? I don't see any range information on the lightware site. I'd guess it will be long range given the price?"
Jun 26
Hein du Plessis commented on Gisela & Joe Noci's blog post More Laser Rangefinder based landings - from the NamPilot
"Brilliant, can't wait for arduplane to support this!"
Jun 26
William Davidson commented on Gisela & Joe Noci's blog post Laser Rangefinder Flights tests in Namibia
"Has this been integrated with APM?"
Jun 26

Moderator
Gary Mortimer commented on Gisela & Joe Noci's blog post More Laser Rangefinder based landings - from the NamPilot
"Spitfire curved approach then, now that's classy. "
Jun 26
Antonie Kruger commented on Gisela & Joe Noci's blog post More Laser Rangefinder based landings - from the NamPilot
"Good post - thanks for sharing."
Jun 26
Gisela & Joe Noci posted a blog post

More Laser Rangefinder based landings - from the NamPilot

Seeing Andrew Tridgell's post on the SF02 LRF used for landings on APM, I thought I would post some flight logs of our landings using the same rangefinder.NOTE…See More
Jun 26
Gisela & Joe Noci commented on Johann van de Venter's blog post Flying drones, quad-copters in South Africa: legal or illegal?
Jun 20
Vladimir "Lazy" Khudyakov commented on Gisela & Joe Noci's blog post More 433 MHz antenna - the 1/2wave Ring Radiator
"Vladimir, now you have me interested... Why not? :))) Join forces :)))"
Jun 7

Profile Information

About Me:
Software/Hardware Engineer, Aircraft designer.
Please tell us a bit about your UAV interest
UAVs in civilian aerial photography and Anti-Poaching applications
Hometown:
Swakopmund

Gisela & Joe Noci's Blog

More Laser Rangefinder based landings - from the NamPilot

Posted on June 26, 2014 at 6:59am 5 Comments

Seeing Andrew Tridgell's post on the SF02 LRF used for landings on APM, I thought I would post some flight logs of our landings using the same rangefinder.…

Continue

More 433 MHz antenna - the 1/2wave Ring Radiator

Posted on May 30, 2014 at 9:15am 35 Comments

This is a reasonably compact, linearly polarised and well behaved antenna, with a gain , when well made, of around 1dBD    ( 1dB more than a dipole). It is smaller than the Big wheel and the Skew Planar wheel, and I have used it very successfully on 868MHz on our SurVoyeur UAV.

It is derived from a 1/2wave ring, located some distance above a large conducting ground plane. The ring is at a point on its circumference shorted to…

Continue

Dipole style antenna for 433MHz

Posted on May 29, 2014 at 3:06pm 26 Comments

Hugues and a few other folk have been looking at using dipoles on 433MHz, I presume this to be the telemetry or RCS radio frequencies used by them. 

I have been playing with some concepts here to try come up with a repeatable implementation with reasonable performance.…

Continue

Some more Antennae, and when is a BALUN not a BALUN

Posted on May 23, 2014 at 1:28pm 37 Comments



Jeff asked about adding a 'Balun' to a simple dipole, so I thought to disertate a little once more on the subject...

The purpose of a Balun is to allow the connection of a coaxial feed line ( unbalanced feedline) to a balanced antenna , such as a dipole.

Picture at left is a Circularly polarised…

Continue

Comment Wall (24 comments)

At 4:56pm on September 22, 2010,
T3
William Premerlani
said…
Hi Gisela,

We have made good progress with wind estimation. It is now incorporated into MatrixPilot. The theory is described here.

With respect to the cross-coupling of the acceleration into pitch estimation during a launch, we have not addressed the issue yet, it is not high up on my list. Paul Bizard did a lot of simulations, and found values of the PI feedback gains that were a reasonable compromise between rejection of gyro drift, and rejection of acceleration on takeoff. I think the time constant is between 10 and 15 seconds.

Beyond that, there are many things you could do, including turning off the PI roll-pitch compensation during high acceleration. There are many people who are using this approach.

Finally, you might want to take a look at the uavdevboard website, or join its discussion group.

Best regards,
Bill
At 5:05pm on September 22, 2010,
T3
William Premerlani
said…
Hi Gisela,

One more thing I forgot to mention, I don't recall whether it is mentioned in the "DCM document":

You can approximately adjust the accelerometer value along the axis parallel to the fuselage by computing the forward acceleration from the stream of GPS velocity reports. Although this does not help much during high acceleration launches, it will do a reasonable job once the plane is in flight, because the errors caused by the GPS dynamic behavior as the plane accelerates and decelerates tend to cancel each other out.

Best regards,
Bill
At 12:03pm on October 2, 2010,
T3
William Premerlani
said…
Hi Gisela,
You asked:
"(1) ...I assume that in a normal body axis convention (as in ArdIMU as opposed tp MatrixPilot ie X is +ve forward - this would be the fisrt column - rxx, rxy, rxz)?"

You are correct.

"(2) You mention that the plane must change attitude for this to work - and then refer to the denominator of Equ6. Now Equ6 computes the estimated airspeed, and if the denominator is zero, this cannot be computed. Is this the reason behind your statement?"

Yes.

" If in fact I have a measure of airspeed - via a pitot - and hence do not need to compute equ6 - can I then be computing regardless of the planes change of (or lacl of chnage) in attitude?"

Yes.

Best regards,
Bill
At 3:25am on October 3, 2010, Gisela & Joe Noci said…
Hi Bill
Thanks for the answers above. I would like to take these questions further, relating specifically to the use of GPS CourseOverGround to determine Yaw Gyro Bias

In the DCM theory document on page 21, you comment that the assumptions made when using GPS CourseOverGround (as opposed to actual Aircraft Heading – the direction in which the aircraft is actually pointing) are violated in the case of strong wind. This I understand.

However, if I am prepared to accept that the value of “yaw” as will be computed from the resulting DCM matrix is in fact a “course over ground” as opposed to an “aircraft heading”, and that the prime purpose here is as accurate a determination of yaw gyro bias as possible, that this is achieved irrespective of wind?

(The advantage of this would be that we would not be contaminating the errorYaw vector with what can only be a best estimate of wind. As this vector gets combined with the errorRollPitch vector to compute the final correction vectors, I assume any inaccuracies in the errorYaw vector will affect all gyro bias values – not only yaw).

Regards,
Gisela
At 4:01pm on October 9, 2010,
T3
William Premerlani
said…
Hi Gisela,

Regarding the effect of the wind on yaw gyro drift correction, the wind is relevant only during turns.

If your flight plan involves mostly long straight paths with turns only once in a while, you do not need to account for the wind if you use GPS course over ground to compensate for yaw drift. The controls will rotate the plane the exact amount needed for the wind.

However, unless you somehow measure and account for the wind in the navigation calculations, there will be a temporary yaw error after a turn, that will gradually dissipate.

An example might help. Suppose the plane is flying along at 10 meters/second airspeed with a cross wind of 5 meters/second. It will be crabbing into the wind with an angle of 30 degrees. Then suppose the navigation controls decide to make a 90 degree turn into the wind. In other words, navigation commands a change in course over ground of 90 degrees. It will rotate the plane by 90 degrees. However, all that is needed to turn directly into the wind and to change the course over ground by 90 degrees, is a 60 degree turn measured by the IMU, so with a 90 degree turn, the plane will wind up flying with a 30 degree error in course over ground. That will show up in the GPS course over ground, and the drift compensation algorithm will respond to it, but it will take 10 or 20 seconds to do so. During those 10 or 20 seconds, there will be a difference between the desired and actual course over ground.

So, if your flights will involve frequent turns, it is best to account for the winds in the navigation calculations. You can either estimate the winds using the method I have suggested, or you can use a magnetometer to be able to distinguish between course over ground and the direction the plane is pointing.

You can also simply ignore the wind if you are willing to tolerate temporary course errors.

Best regards,
Bill
At 4:44pm on October 10, 2010,
T3
William Premerlani
said…
Hi Gisela,


Actually, cross coupling between the earth and body frames of references reduces the drift of the yaw gyro, without any negative effect on roll and pitch. Tests have proven out the theory of an interesting effect. Consider an extreme case, either as a thought experiment, or as a real test, to see what is going on:

1. Start up your IMU close to being level, without any GPS or magnetometer connected for yaw drift. Program it to perform roll-pitch drift compensation only. During initialization, gyros will be approximately zeroed. After some time, the accelerometers will perfectly lock the roll and pitch. Yaw will slowly drift, at a small residual rate, because there is not any yaw feedback.

2. Rotate the IMU by 90 degrees, either by rolling or pitching, so that the axis of the Z gyro is horizontal instead of vertical.

3. Wait a few minutes. During this period of time, the accelerometer information is zeroing the Z gyro! During the same period, one of the other gyros will not receive any feedback, but it will be close to be perfectly zeroed, so its drift will be low.

4. Rotate the IMU back to level. The Z gyro is now zeroed, although the yaw angle is random.

There is another effect that you might be interested in: the bottom row of the direction cosine matrix is independent of yaw, so any yaw drift does not impact it. The reason for this is that the bottom row represents the earth frame Z axis as seen in the body frame. Because the earth frame Z axis does not contain any yaw information, it appears the same in the body frame, no matter what the yaw angle is. In fact, if you are only interested in roll and pitch, you only need to compute the bottom row of the matrix, which can be computed using only the bottom row information, and all three gyro signals. The fact that the Z gyro may have some drift is irrelevant, it all works out ok.

Best regards,
Bill
At 11:47am on October 11, 2010,
T3
William Premerlani
said…
Hi Gisela,

I think I see what you are missing, but it is going to take some thought on my part to figure out a way to explain it.

In the meantime, here is another data point for you:

There is a roll-pitch-yaw demo program available along with MatrixPilot. I use it routinely for testing purposes. I run it without any sort of yaw compensation, I set the yaw error to zero. The bottom row of the matrix perfectly tracks roll and pitch, no matter what is going on with yaw. The other elements are more or less ok, except there is a slow yaw drift.

I will see if I can think of another way to explain why things work out so well. In the meantime, here are some factors that may be involved in explaining the discrepancy, perhaps you will be able to explain it:

1. MatrixPilot and the roll-pitch-yaw demo software record the gyro offsets during power up, so once the DCM algorithm gets running, the residual drifts are rather small.

2. If the board is level, yaw drift has no impact on roll and pitch. In other words, if roll and pitch are zero and roll and pitch rate are zero, there is no way for yaw to change roll and pitch.

3. If the board is not level, there is enough information from the accelerometers to compensate for yaw drift, so there will be yaw lock.

4. If GPS is available to achieve a strong yaw lock, there may be a phase angle error in yaw, but the yaw rate error will be zero.

5. It is true that all three gyros are involved in computing the bottom row of the matrix. However, there are linear combinations of drifts that will not impact roll and pitch.

I think what may be going on in your simulations is that possibly you are not including the effect of roll-pitch compensation and/or you are introducing arbitrary yaw gyro errors?

As long as the yaw gyro error is a small offset (a few degrees per minute), everything should work out ok.

I have run out of room in this message, I will send you another later.

Best regards,
Bill
At 2:39pm on October 11, 2010,
T3
William Premerlani
said…
Hi Gisela,
Perhaps the amount of Z gyro offset that you were using was too large. That will result in numerical errors due to neglect of second order terms, followed by an effect I call "coning" that is caused by in interaction of the second order terms with normalization, followed by a failure to achieve roll-pitch lock.
Otherwise, the theory says that yaw compensation is orthogonal to roll-pitch compensation, as long as roll-pitch lock has been achieved. Here is why:
The roll-pitch rotation error vector is computed by taking the cross product of the last row of the matrix, with the gravity vector measured by the accelerometers. Since gravity is vertical in the earth frame, the computed roll-pitch error vector is in the horizontal plane of the earth frame.
The yaw rotation error vector is computed by taking the cross product of two vectors that are both in the horizontal plane of the earth frame. The result is parallel to the earth frame vertical.
Therefore, the yaw rotation error vector and the roll-pitch rotation error vector are orthogonal in the earth frame. Therefore they are orthogonal in the body frame. Therefore, an error in the yaw compensation does not impact the accuracy of the roll-pitch compensation.
Best regards,
Bill
At 2:51pm on October 11, 2010,
T3
William Premerlani
said…
Hi Gisela,

Another thought...in case you have not seen them, you might be interested in Robert Mahony's papers. He goes into more mathematical detail.

The point I was trying to make in my previous comment is that:

1. When the IMU is not level, the drift compensations compute rotation corrections that are applied to all three physical gyros, because each gyro receives a weighted sum of the three elements in each rotation error vector.

2. When roll-pitch lock has been achieved, the roll-pitch rotation compensation error vector is orthogonal to the yaw rotation compensation error vector, in any frame of reference. So, when the two rotation error vectors are transformed from the earth frame into the body frame and mapped onto the physical gyros, they should not interfere with each other.

I think I understand your question, which I think boils down to, "Do errors in the yaw rate gyro create errors in roll-pitch values". If that is the question, my answer is that, provided the rate errors are not so large as to break lock, the roll-pitch values should not be impacted by yaw error.

Best regards,
Bill
At 2:27pm on October 12, 2010,
T3
William Premerlani
said…
Hi Gisela,

How much discrepancy in roll-pitch are you seeing? There are a number of known sources of small errors, including:

1. Forward acceleration.
2. Linearization of the non-linear update equation.
3. GPS filtering.
4. Side slip introduces errors in centrifugal compensation.

Are you running simulations?

Best regards,
Bill

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