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Old Mar 22, 2012, 11:22 PM
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Orlando, FL
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Trying to understand how Multirotor FCs work

1. Are the gyros always working in HH mode?

2. If the above question is yes, then why the term "Rate Mode" is being used when self-leveling (Acc=0) turned off?
Term "Rate Mode" has always been associated with normal mode gyro operation in traditional helicopters. Why the change here?

3. Is it okay to use Tx trims to fine tune the multi at hover? Or fine tuning should be done by balancing the multi?

Thanks...
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Old Mar 23, 2012, 07:56 AM
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I don't want to believe that so many people fly multis and no one knows (or wonders) the answers to the above questions...

I specifically posted this in RC group because Multi forum is very active here...

I searched the web for days before posting here but couldn't find much info on. If you don't have the answer but recommendation on where I might find it, would you please share it with me?

Thanks
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Old Mar 23, 2012, 08:11 AM
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Quote:
Originally Posted by iskoos View Post
1. Are the gyros always working in HH mode?

2. If the above question is yes, then why the term "Rate Mode" is being used when self-leveling (Acc=0) turned off?
Term "Rate Mode" has always been associated with normal mode gyro operation in traditional helicopters. Why the change here?

3. Is it okay to use Tx trims to fine tune the multi at hover? Or fine tuning should be done by balancing the multi?

Thanks...
1. I think in pretty much all multi-copter flight modes, gyros will be working and used!
2. sorry! not aware of helicopter modes...
3. I'm sure it is OK to use Tx trims
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Old Mar 23, 2012, 08:35 AM
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Quote:
Originally Posted by iskoos View Post
1. Are the gyros always working in HH mode?

2. If the above question is yes, then why the term "Rate Mode" is being used when self-leveling (Acc=0) turned off?
Term "Rate Mode" has always been associated with normal mode gyro operation in traditional helicopters. Why the change here?

3. Is it okay to use Tx trims to fine tune the multi at hover? Or fine tuning should be done by balancing the multi?

Thanks...
1- as Mike said the gyros are always working regardless of mode.
2- I'm not familiar with the term "rate mode" in regards to multi-rotors, I haven't seen this in the MultiWii or KK software. Perhaps if you could clarify where you saw it. The acc tries to push the multi-rotor back to level which helps beginners and and also assists in AP, turning it off is best for sport flying.
3- yes and yes. You always want to make sure a multi is balanced properly after building it, but even if perfectly balanced it'll probably need a little trim. And it can change from flight-to-flight too, it's affected by wind, weather, and motor temp. Every time I launch I start out with a hover to check trim settings before flying.
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Old Mar 23, 2012, 08:40 AM
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Rate mode = more or less hh on heli.
Attitude mode = auto level.
You should never have to trim rate mode - if your sticks are centered and hardware isn't broken, it should stay in place.
Likewise, you should never have to trim attitude mode either.

Im talking about transmitter trim here.
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Old Mar 23, 2012, 09:37 AM
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In rate mode, a gyro will resist external attempts to move the heli from it's original heading. But once that external attempt stops, the gyro stops trying to correct it, even if it's moved far from the original heading. In HH mode, the gyro will attempt to return to the original heading. In other words, rate mode resists changes, HH mode says I don't care how far you turn me, I'm going to try to return to where I was. Of course, on a quad control board, you can modify either of these behaviors.
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Old Mar 23, 2012, 10:04 AM
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Thank you. I am happy that I got some answers...

Mike and Tres, first question isn't asking if gyros working or not. Rather it is asking which mode gyros are working. HH (Heading hold) or Normal...

Balr, you explained something I already know. I wasn't asking how HH mode works against rate mode. I was asking if gyros working on HH mode, then why call it Rate mode since rate mode refers to Normal gyro mode operation...

And about trimming, I am still not clear if Tx trim should BE or NOT BE used.
Some says yes, some says no. Does it depend on which board is in use?

After all though I am still not clear on my first question. That's the one I dying to find out. Are they working similar to HH gyros. From what I see so far they are not. (I am not talking about self-leveling. That's another issue that involves mostly accelerometers. I am asking this for boards that are equiped with 3-axis gyros only. Like KK board...)
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Old Mar 23, 2012, 10:59 AM
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Quote:
Originally Posted by iskoos View Post
1. Are the gyros always working in HH mode?

2. If the above question is yes, then why the term "Rate Mode" is being used when self-leveling (Acc=0) turned off?
Term "Rate Mode" has always been associated with normal mode gyro operation in traditional helicopters. Why the change here?

3. Is it okay to use Tx trims to fine tune the multi at hover? Or fine tuning should be done by balancing the multi?

Thanks...
1. I would say it depends. The gyro measures angular velocity, and in auto-level mode those thing are just put into the PID-equations.
2. Don't have a proper explanation here.
3. No. If your copter drifts (in zero wind) you should redo the acc-calibration.
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Old Mar 23, 2012, 11:21 AM
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Hi I,
A well-setup multicopter with perfect motors and ESCs properly set throttle ranges and Tx/Rx setup in the flight controllers setup utility for zero trim at the stick neutrals should generally not require more than 5 clicks of trim with the trim steps on the Tx set to 1 to achieve a 20 second hands off stable platform in a no wind flight. Most Tx trim step defaults are actually 4 steps and can be set to "1" for finer trimming which works better for multicopters. Most multicopter FCBs, need to know the sticks neutrals for the accelerometer to work properly. Otherwise any excessive additional trim over the course of the flight time eventually screws up the leveling corrections to the point where Gents will find they need to apply significant stick corrections to keep it level. If I find my multicopters require more than 5 clicks (steps) of trim then I know to look for mechanical motor/prop wear or balance issues or multicopter dynamic neutral stability, i.e. motor tilt, frame and board mount rigidity.

To try to answer your gyro question. All gyros measure angle velocity rate. Good coding uses this data for angle measurement. Some code for multicopters sets the pitch and roll and yaw stick movement to rate control and some coding sets stick movement to angle control. Most sport flyer's prefer rate control for the control "feel". All multicopters use the yaw gyro for holding the heading with the coding to do this as does a heli that is in heading hold mode. A gyro used in helicopters for heading hold similarly uses gyro rate changes and tries to minimize them with tail motor speedup and slow-down commands/voltage/PWM and coding resulting in a perceived heading hold. For this to work properly with flight controllers that speed up and slow down alternate axis motors to achieve yaw control, you need to setup mechanical dynamic neutral stability. See the Project Quads/Hexa/Octos section TIP 4 in the Quadrocopter and Tricopter Info Mega Link Index for more info

Maybe too much detail? A quick answer to your question is a that the yaw gyro coding in a multicopter is always considered "heading hold" and the pitch and roll can either be rate mode or angle mode.
.
Hope that helps
Cheers,
Jim
Quote:
Originally Posted by iskoos View Post
Thank you. I am happy that I got some answers...

Mike and Tres, first question isn't asking if gyros working or not. Rather it is asking which mode gyros are working. HH (Heading hold) or Normal...

Balr, you explained something I already know. I wasn't asking how HH mode works against rate mode. I was asking if gyros working on HH mode, then why call it Rate mode since rate mode refers to Normal gyro mode operation...

And about trimming, I am still not clear if Tx trim should BE or NOT BE used.
Some says yes, some says no. Does it depend on which board is in use?

After all though I am still not clear on my first question. That's the one I dying to find out. Are they working similar to HH gyros. From what I see so far they are not. (I am not talking about self-leveling. That's another issue that involves mostly accelerometers. I am asking this for boards that are equiped with 3-axis gyros only. Like KK board...)
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Old Mar 23, 2012, 11:29 AM
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Quote:
Originally Posted by iskoos View Post
Thank you. I am happy that I got some answers...

Mike and Tres, first question isn't asking if gyros working or not. Rather it is asking which mode gyros are working. HH (Heading hold) or Normal...

Balr, you explained something I already know. I wasn't asking how HH mode works against rate mode. I was asking if gyros working on HH mode, then why call it Rate mode since rate mode refers to Normal gyro mode operation...

And about trimming, I am still not clear if Tx trim should BE or NOT BE used.
Some says yes, some says no. Does it depend on which board is in use?

After all though I am still not clear on my first question. That's the one I dying to find out. Are they working similar to HH gyros. From what I see so far they are not. (I am not talking about self-leveling. That's another issue that involves mostly accelerometers. I am asking this for boards that are equiped with 3-axis gyros only. Like KK board...)
I won't pretend to understand what HH or rate exactly means, but if I have it right, the answer is it depends. It depends on how many degrees of freedom your FC can measure and what kind of control program it uses. A proper control system uses a PID control loop (proportional integral derivative), but there are some programs that do only PI or mostly just P. That is, if there is a change in gyro signals, a proportional response is sent to the motors. This isn't really HH because the response is only there as long as the gyro measures a change in it's reading, I think this is the 'rate' you're referring to. When a control loop has a properly working integral function, it can take the gyro readings over time and integrate them (or make integral approximations) to determine what changes in it's heading has probably happened and then correct for them, this would probably be the heading hold you're looking for.

The fact is that as I understand it, on a properly working FC there should be both 'rate' and 'HH' working simultaneously. But still this is no gurantee that it will actually hold it's heading; IMU's are very peculiar in the fact that they can only measure accelerations (linear and angular) as far as I know and there is a possibility for errors in readings to add up. It's kind of like putting a blind man inside a closed box and asking him to describe how the box is moving from what he feels.

Whether your quad should be trimmed or not depends on if you have accelerometers or not and if you are using them. A gyro only board can't determine it's velocity in any direction so it will drift where ever the hell it wants to, even when trimmed properly it needs serious stick corrections to keep it in place. I don't have experience with boards with accelerometers, so I couldn't really comment on those.
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Old Mar 23, 2012, 11:46 AM
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It's called rate mode because gyros measure the rate of angular change. To answer your first question - in a gyro-only design (like the KK board from HobbyKing) the gyros are not operating like HH mode in a heli gyro.

Quote:
Originally Posted by iskoos View Post
Thank you. I am happy that I got some answers...

Mike and Tres, first question isn't asking if gyros working or not. Rather it is asking which mode gyros are working. HH (Heading hold) or Normal...

Balr, you explained something I already know. I wasn't asking how HH mode works against rate mode. I was asking if gyros working on HH mode, then why call it Rate mode since rate mode refers to Normal gyro mode operation...

And about trimming, I am still not clear if Tx trim should BE or NOT BE used.
Some says yes, some says no. Does it depend on which board is in use?

After all though I am still not clear on my first question. That's the one I dying to find out. Are they working similar to HH gyros. From what I see so far they are not. (I am not talking about self-leveling. That's another issue that involves mostly accelerometers. I am asking this for boards that are equiped with 3-axis gyros only. Like KK board...)
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Old Mar 23, 2012, 09:03 PM
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Thank you so much for the inputs guys. I was out all day today. I couldn't follow up.

Before I start my comment I wanted to say a few things about HH and rate mode since some of the guys here don't have traditional heli background these terms might look unfamiliar to them.

Rate mode = Normal Mode Gyro operation. Gyro tries to correct an unwanted movement by applying opposite input to control. It doesn't try to restore the original state.
HH (= Heading Hold/Lock, AVCS) refers to gyro operation mode where the gyro does everything in the (above) rate mode plus it tries to restore the original orientations. But this is definitely not what we know as self-leveling in Multi FC boards that have accelerometers. In traditional helis gyros would only be used for the tail (this is before the advent of flybarless electronics) and if the gyro would be in HH mode, the helicopter drag in the wind but it would keep its heading all the time. With only the rate mode selected for the tail gyro, the heading wouldn't be maintained but any unwanted yaw movement would be compensated by the gyro.
That's why I was associating the Rate mode with Normal gyro operation...

Anyways after this quick note, I have to say that I am now very positive that the gyros in FCBs (gyro only boards that do not have accelerometes in other words known as the ones that do not provide self-leveling) must be working in Normal mode or Rate mode as commonly used. The behaviour that a multi-rotor shows is same as what a convetional heli tail shows when its gyro work in rate (normal) mode.

Also Jesolins mentioned that only tail (we should say yaw since there is actually no tail) gyro works in HH mode and others(Pitch and Aileron) works in normal/rate mode. This makes sense so far...

Then as for my 3 questions. Here is how I would answer them right now...

1. Gyros work in Rate/Normal mode (for gyro only boards. When Accs involved, I am sure they work in HH mode [or maybe combination of HH and rate mode] to provide self-leveling)

2. Since the answer to question#1 is NOT yes, then this question invalidates itself. Rate mode really refers to rate/normal mode gyro operation. No problem or confusion...

3. I think that the Tx trims (if needed) should be used for gyro-only boards or for the boards when they work in rate mode.
If auto stabilization (self-leveling) mode is active, then the trimming should be done thru FC software. Trying to use Tx trims here would definitely interfere with stabilization system operation in my opinion since the stabilization system constantly autotrims the pitch/roll to keep the level orientation.
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Old Mar 23, 2012, 09:32 PM
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3. I think that the Tx trims (if needed) should be used for gyro-only boards or for the boards when they work in rate mode.
What would this achieve? Let's say model is level, and sticks are level, and gyro is idling around 0-point with slight jitter which fc corrects right away back into same level/zero position. Model stays in same place, assuming tehre's no wind/other factors.

Now if you take your roll trim and move it say 10 points to the left. Now it will look to the model as if you're constantly trying to roll it left with stick, which means it will try going left all the time and you will need to be counteracting this with right roll stick. You should have no reason to trim in any multi mode really, rate is just that, and accel is trimmed by leveling the board and/or inflight acc trimming.
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Old Mar 23, 2012, 10:00 PM
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Originally Posted by iskoos View Post
1. Are the gyros always working in HH mode?
HH only refers to the Yaw axis in helis and quads for which only one gyro is used. Some quadcopters use HH for yaw. Most use a magnetometer to achieve this. Quads respond slower in yaw than a heli does so a heli HH gyro doesn't work as good for quads. Mags are also prefered for quads since they don't drift, at least not as much as gyros do.

Quote:
Originally Posted by iskoos View Post
2. If the above question is yes, then why the term "Rate Mode" is being used when self-leveling (Acc=0) turned off?
Term "Rate Mode" has always been associated with normal mode gyro operation in traditional helicopters. Why the change here?
Some people refer to the gyro/accel self leveling feature as HH for Pitch and Roll on a quad.

The roll and pitch gyros can either be in pure rate mode or they can be combined with an accelerometer for a "self leveling" feature where only the Roll and Pitch axis constantly tries to keep the quad level, similar to what a HH gyros does for a heli tail. So the gyro/accel self leveling feature is kind of like HH for the Pitch and Roll axis.

Quad boards are basically the same thing as a flybarless system for helis. I don't think flybarless systems refer to a level mode as being HH for Roll and Pitch.

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3. Is it okay to use Tx trims to fine tune the multi at hover? Or fine tuning should be done by balancing the multi?...
Each quad is different but I use TX trimming all the time to correct for inperfections in my quad builds. TX trimming is best if all your gyros are in rate mode. If you use the gyro/accel self leveling feature then you need to read the manual for your quad as TX trimming can sometimes adversely affect the level mode.
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Old Mar 23, 2012, 10:02 PM
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How could tx trimming possibly correct "build imperfections"?
If a motor is spinning too fast/angled too much/pushes too much offcenter, the gyro will receive appropriate feedback, and FC will correct it, keeping the model still in same position. With tx trim you have CONSTANT error introduced into model position, which it will be constantly trying to achieve.

Why do you think hanging a battery / some other heavy weight off one arm of a gyro-only FC and it still flies, and stays level as long as you don't move the sticks?
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