I'd like to find out how gyros work. Do they use a gimbal rotated by a small electric motor, or are they solid state? How many axies do they have i.e. pitch, roll, yaw? How does the gyro interface with the receiver? Can a gyro be used to control something other than a tail rotor i.e. ailerons, elevator, rudder on a fixed wing craft? I know that gain is used to control the sensativity (control theory), but what else should I know about it?

Maybe there is some online resource where I can read about them, or you can explain it to me here?

Thanks,
-Q.

What is the use of gyros and how do they help?


The gyro is positioned so that it senses yaw. It then feeds small inputs to the tail rotor servo to counter the yaw that it detects. This keeps the helicopter from yawing to the left and right when you don't want it to. Left-right movement of the left stick also supplies input to the tail rotor servo; so you and the gyro are both giving control inputs to the tail. A gyro is a MUST. It's probably not an exaggeration to say that gyro-based stabilization of the tail rotor made R/C heli flying feasible. It is possible to fly an R/C heli without a gyro, and it's also possible to juggle seven balls. It's just darn hard! Furthermore, it's definitely not something you want to try tackling when you're just getting started. Without a gyro, the heli can begin to whip around wildly as soon as the skids leave the ground. The heli will do a 180-degree turn and you're looking at an angry helicopter coming right at you before you know what happened. Definitely not something for a beginner to tackle.

http://www.repairfaq.org/filipg/RC/F_RC3.html#RC_010

Only older gyro's actually use a gyroscope in them. Today they're mostly solid state piezo's. When an electric field is applied to a crystal it vibrates at certain rate depending on the geometry of it. Conversely, if a physical stress is applied to the crystal it will cause the resonant frequency to change which can be measured. Piezo gyro crystals are designed to be most sensitive to motion in a specific direction. You can use them for anything you want to react to changes in the rate of acceleration. Futaba's state of the art ones are SMM gyro's. I don't have a clue how they actually work but they involve actual moving parts built using the same processes as micro processor's so they're insanely tiny. The only downside to gyro's is they can only detect changes in the rate of acceleration not relative motion so they're useless for keeping a plane straight and level over any long period of time because they drift. But that makes them perfect for keeping helli's tails from spinning randomly by canceling out acceleration it detects.

So. To sum up.
A gyro is "solid state" and detects acceleration in one axis.
I guess I'm looking for something that will detect position change. This can be done with acceleration, but the difficult task is going to be to keep track of all accelerations and compute change in position from it i.e. sum of accelerations = velocity, sum of velocities = distance. So, I'm going to need a micro controller chip with a bit of RAM to do it. I guess I don't need to go that far if all I'm looking for is to smooth out small variations in yaw, etc. I hear that some guys use gyros for the rudders of their turbine jets.

Thanks very much for your input.

-Q.

You can't detect position change with a rate gyro, it's not possible. Even the most sophisticated Heading Hold Gyro's (which use micro controllers to do just what you're talking about) still drift to the point where they become useless after time. It's a law of control theory that this type of thing is impossible because of parasitic loses. The only way to do what you're talking about is to create a custom device or purchase an attitude gyro, I have however never found anything that's suitable for RC work in the attitude gyro department, the soul exception being the FMA co-pilot, which successful stabilizes on two axis' using a special sensor that can detect the horizon based on the infrared signature. You may be able to hack apart this product in order to get the raw artificial horizon data you're looking for. But I doubt that's a project for the faint of heart.

Thanks for the info. I was aware of the long term drift and the need for an something else. However, my application is not to fly the plane unaided for an extended period of time. It just need to keep the plane stable for a maximum of 10-20 min. The plane would be controlled to some degree by a pilot, but is intended to keep a plane dosile enought for beginner thumbs and also to prevent certain unwanted scenarios such as stalls. I would say that long term position sensoring would best be done by a GPS device, however the resolution requires that the plane fly high enough that the GPS error does not slam the plane into the ground.

-Q.

Added stability is what gyro's were brought into the hobby for. GPS can't do attitude adjustment (artificial horrizon) You would have to have local attitude sensor for that. GPS is good for location information not useful or reliable for detailed orientation info.

There are two types of gyros we use for model helicopters. Rate damping gyros and rate demand gyros.

Standard non-heading hold gyros are rate damping. What this means is that the gyro will always try to counter movement. Even when you give a rudder command a damping gyro will try to counter it. Your saving grace is that you have more "stick" control than the gyro has damping control. Before heading-hold, the trick technique was to "over-drive" the available control throw. You did this by maximizing servo wheel throw with no regard to whether or not you were binding the pushrod. This allowed you to turn up the gyro gain to get good performance from the gyro. In flight you didn't have to worry about binding up the controls because the gyro was damping, or using up, some of the available control throw. You had a good compromise of tail holding, lots of gyro gain, for backwards flight and stuff and still had max throw for a good piro rate. But even with the best piezo gyros and the fastest servos it was still a compromise and during hard 3D the tail would get away from you.

Then came Heading-Hold gyros. These are rate demand gyros. The premise upon which this gyro operates is the continual force-vector calculations performed in the microprocessor. This calculation is an angular velocity. Every time the gyro moves it moves a certain direction, the angle of movement, at a certain speed, the velocity of the movement. What the gyro does is read this information from the piezo sensor. It will then immediately feed to the servo any and all available throw required to produce the opposite angular velocity to bring the gyro back to the center position. Hence, rate demand. If the movement from the gyro is gentle then the microprocessor will demand gentle corrections. If your flying in a viscious cross wind then the microprocessor will have to demand more aggressive servo action to counter the movement. The higher end the gyro, the more controls you have over the characteristics of this demand. This is a nut-shell explanation as there are alot more factors involved.

Now then, in the real world, the calculation is not done every now and then. I'm not sure the rate of the calculations. But I think its several hundred times a second and is why the helicopter seems to hold its heading. Once you establish "center" the gyro will perform the calculations, and command the servo to maintain that center, or heading.

However, there are some things that can affect the calculations, If the processor is not a very capable one then you will get rounding errors. The higher quality processors can make the neccessary calculations, at the neccesary speed, to a higher number of significant digits. Rounding the 10th significant digit has much less impact as opposed to rounding the 4th significant digit. This is because the 10th digit number is much more accurate of a measurement than a 4th digit number. Again, I'm not sure the number of digits calculated in the different gyros. What I'm getting at is that after several hundred calculations over several seconds, the error produced from rounding will be much, much greater in the lower quality processor. You'll see this in flight. Its one of the reasons a heading-hold gyro will drift after a while if you don't touch the stick. The caluclations to bring the gyro back to center will slowly degrade and the heading position will slowly drift off. If your constanly throwing the sticks around (flying 3D), of course you'll never see the calculation drift as the heading is never constant for any length of time. But, if your an FAI pilot where you do alot of precision hovering manuvers, you'll notice the difference right away between a high quality and low quality gyro.

Another problem is temperature change. Its no secret that electronics perform differently at different temperatures. The temperature of the components will affect the angular velocity calculation. It'll affect both the speed and ability of the processor and the piezo sensor. Futaba has found a neat way to make temperature much less of a factor in their AVCS (heading hold) gyros than other brands. First off is their processer. Its a very high quality and very temperature stable over a wide range. The biggest advantage though is the piezo sensor. It is mounted on a micro machined "vibrating ring". I'm not going to explain the physics of the vibrating ring because I'm 1) not sure I fully understand it and 2) not sure I could do a good job of it. But I think they are using vibration phasing to effectively cancel out temperature as a factor.

Back to rate demand. Up until now I have talked about gyros in a hovering situation. When you give a command to the gyro in flight, The gyro ceases to function in the classic sense as a gyro.... per se'. At this point it interprets the rudder stick movement as a rate demand. It will feed command to the servo to get the angular velocity you are asking for. Little stick movements generate little angular velocity demand while large stick deflections generate large angular velocity demand. This demand is a function of the ATVs you have set-up. This is why alot of people have a hard time understanding that the rudder stick has no control over servo throw limits in the tradinonal sense. In the higher end gyros, the rudder stick is a rate demand variable trimmer. Servo throw limits are set-up "In the gyro" itself. This is also why you should not have any rudder trim or subtrim in the transmitter. Any rudder command away from center will be interpreted as a rate demand and the gyro will drift around and not hold a heading. Sometimes though a couple of clicks of trim in the morning on a not so temperature stable gyro will "recenter" the gyro to allow it to hold. Usually by the end of the flight or a few minutes in the sun will require you take the trim back out.

Because the microprocessor constanly sees the information from the piezo sensor. It is very easy to set-up the core programming to interpret the information and make the gyro perform in a rate damping sense. That is why most HHold gyros also allow you set-it up as non-heading-hold gyro.

Mike

How well do rate-dampening gyros deal wind changes or thermals in straight constant speed flight? Are these sophiscated gyros comparable to the Copilot?

Gyro's and a copilot are not really comparable. A copilot forces the plane perfectly level regardless of external conditions so long as the sticks are close to neutral. A gyro helps to cancel out buffeting action from the wind or other external forces. Modern heading hold gyros can lock a single axis stable for the better part of a few minutes.

I take it that only the Copilot can bring a plane back to level or deal w/ a downburst when the controls are released. But how well does it work over varied terain or snow?

Nothing can deal with a downburst, not even the copilot. It can keep the plane straight in a downburst but it can't correct for altitude. The Co-pilot doesn't work well, or sometimes not at all with a lot of snow or terrain.

Do trees affect the Copilot?

Do you guys think it's feasible to use some kind of standard RC Gyro to try to measure angular velocity?? I'm thinking on feeding the gyro input with a standard PPM square wave and measure it's diference with the output wave, I'm thinking this diference should be proportional to the angular velocity sensed. What do you guys think of this idea?

Best regards,
Marco

will two gyro work one for the rotor one for the main rotor, will this make it more stable?

a088008,

I find your cruise missile project interesting.

The technology is definitely out there to home build a guided missile relatively cheaply nowadays. HH Gyros, small gas turbines, autopilots, GPS, etc etc...

That is what your planning on doing isn't it?

I must say, i am really impressed just about every day at how knowledgeable people are on this site about tons of things. Jesus Lynx , lol, you rock on sonny. I also must say keep the knowledge rolling, because i am learning so much and applying it to so many different things.
Any jam i get into i am always brought out of it by the members on RCU. w00t...!

A "Cruise Missile" is about the furtherest thing from my mind. I was thinking more of something like a trainer type model aircraft that an 8yr old could learn to fly.

Well, that's a relief!

Wouldn't it be cheaper to teach them on a sim and then just fly a normal trainer?

just read all ur guys posts about gyros. Been researching them for awhile and haven't been able to read anything close to the level of gyro-knowledge you guys seem to have. I like that we're talking about missles, cause I'm up against russian rocket scientist in my quest to stabalizing the head on my camera crane. I'm sure you guys have all seen the ultimate arm, or the first generation russian arm. It's a gyro=stabilized head and crane. Something like 15 gyros throughout. Anyways... I'm trying to build my own. On a smaller level of course. I've worked with the guys for sometime, and I have a relative idea on how it works, but... thats relative. I have the crane guy. I have the dc motor guy. I just need a gyro expert. So I came to a helicopter site in hopes to find him... or her. If anyone has any knowledge on large gyros and how to manage them, I'm interested in talking with you. I'll be checking the site periodically. Let me know. Hope all is well.

rp