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Jan 20, 2017, 06:02 PM
Wisconsin
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Gimbals 101


Gimbals 101




A gimbal is a device used to stabilize a camera in one or more axis. When we fly an aircraft, it is subject to a lot of movement. Winds, thermals, turbulence and the vibration of props and motors can cause video and still images to blur. A gimbal senses the movement of the aircraft and moves the camera, via a brushless motor or servo, to keep it level even though the aircraft has moved. As the aircraft moves, the gimbal keeps the camera level in one or more axis. When the aircraft moves back to level, the gimbal returns the camera so it remains level in the new position of the aircraft. It does this so fast that the camera appears to sit motionless. Brushless gimbals are much more popular and costly than servo gimbals. The brushless motor can move much faster to keep the camera level than can a servo. But a servo gimbal might be just the ticket for still photography because it is lighter.

For the DIY gimbal builder the first requirement, not unlike a flight controller, is to get the camera perfectly balanced on the gimbal. If you roll, tilt or pan the camera it will stay where you leave it if it is balanced. If the gimbal motors have to assist in keeping the camera balanced they may run out of power to keep the camera level when the aircraft is bouncing around. That is why you need to choose a gimbal that has balance adjustments in all axes. You will need to slide the gimbal arms in and out to balance the weight of the camera. Also, gimbal designs include gimbals that hold the camera from one side or both sides. It is better to support the camera from both sides but it adds weight and cost.




Gimbals are a load on an aircraft and every extra gram of weight reduces your flight time. So it is important to match a gimbal carefully to the camera and to keep the weight of the camera and gimbal as low as possible. Larger gimbal motors are heavier. In every respect you need to choose exactly the right gimbal and motors for the camera. Carefully evaluate your needs and make sure you can't do the job with a lighter camera. Do you really need a dSLR or will a mirror-less do? Can you do the job with a GoPro rather than a mirror-less? The difference in weight is huge. A GoPro or clone weighs less than 100g and the gimbal needed for the GoPro is around 200g. The gimbal for a mirror-less is 500g and the camera 400-500g. A dSLR can double those figures. Remember, every 50 grams of extra weight reduces your flight time by about one minute.







The gimbal motors are controlled by a brushless gimbal controller. This electronics board is similar to a flight controller with built in ESCs. It has many of the same sensors to tell the gimbal controller about the movements of the aircraft so it can compensate and keep the camera level. Brushless gimbal controllers commonly are available to stabilize two or three axis and some can be expanded from two to three axis. They consist of the main board, which has the drivers for the motors, the microprocessor and an interface to communicate with the board. There is another little board which is the sensor (IMU - Inertial Measurement Unit) which determines the position and movement of the camera. The sensor must be mounted on the camera support, so it moves with the camera, while the main board is normally mounted on something that doesn't move. The sensor must be mounted in a particular orientation so it is pointed in the right direction, otherwise the measurements it sends to the main controller will be wrong. Good sensors have an image of the x,y and z axis painted on the board to help determine the right orientation.



One of the challenges for the DIY gimbal builder is managing all the cables from the motors and the sensors which must not inhibit the movement of the gimbal but also not be so loose and floppy to be blown around by the props. A good gimbal design, in my opinion, is one that allows the use of hollow shaft gimbal motors so the cables can go through the gimbal motors to allow a cleaner cable management.

Once the gimbal is mounted, the sensors located correctly, and the camera installed and balanced it is time to tune the gimbal. Most modern gimbal controllers have an onboard USB interface but some require the use of an FTDI cable which interfaces the serial output of the gimbal to a USB port on your desktop or laptop. You may have to install drivers in your computer for the particular USB chip used or the FTDI chip. Brushless gimbal controllers come with software that runs on your laptop or desktop computer to connect to your gimbal controller and configure and tune it.




gimbal tuning (0 min 43 sec)


Tuning a gimbal can be a daunting process. It is similar to tuning the PIDs on a flight controller. Procedures differ for each brand of gimbal controller and step-by-step videos are available on youtube for popular gimbal controller boards.

[Tutorial] Storm32 Configuration & Calibration (11 min 34 sec)


Once the gimbal is tuned and working, you need to connect it to your aircraft. The physical mounting can be difficult. It seems most gimbal designers made no attempt to look at popular multirotors and try to make it easy to mount. The industry desperately needs a standard quick release mount that can be removed for packing up the multirotor. Check before you buy and try to think how you can mount your gimbal to your particular multirotor.

The gimbal controller board will have a power input you need to supply from a BEC that is appropriate for the voltage requirement. Most gimbal controller boards require 12v (which is 3s) so if you have a 4s (16v) to 6s (24v) system you will need a BEC or step down converter to lower the voltage for the gimbal. Next, you will have one to three servo connectors to plug into the flight controller, for tilt, roll and/or pan. You can set up switches or sliders on your transmitter to control the tilt, roll and pan of the gimbal. But you don't have to. The gimbal will do its job of stabilizing the camera but if you want to be able to steer the camera you will need to set the controls on your transmitter to up to three spare channels. If you are using a pwm receiver, you can connect the gimbal directly to those three channels to control the gimbal. If you are using a flight controller that will do that job, you can connect the gimbal to the flight controller on three outputs of the flight controller. One advantage of connecting a gimbal to a flight controller is if the flight controller is capable of auto missions then it can control the gimbal during those missions automatically.

*** Mackay video of Pixhawk controlling Gimbal during mission ***

SToRM32 gimbal with ArduCopter 3.3-dev (2 min 17 sec)


Next, you need to install your camera and control it. You normally will want to have video output from the camera sent back to the ground so you can compose your shots. You will also need to trigger the camera and possibly change from video to still mode. This is where we start to run into problems with not having enough radio channels to control everything. If we wanted to do everything we just outlined we would need two more channels to control camera trigger and video mode in addition to the one to three channels to control tilt, roll and pan. That is five channels and if you have an eight-channel radio you don't have enough channels for all the other things you need on a multirotor. Many newer radios, receivers and flight controllers will support up to 16 channels. One strategy is to limit your control to just tilt and have your camera turn on and start shooting on the ground. Let it film the whole time it is in the air. Most digital cameras and camcorders using larger SD cards can film continuously during short flights eliminating the need for as many channels. Another way to control all the features is to use a newer flight controller that allows you to connect the gimbal controls to the flight controller with a serial connection or a Can bus connection. These single wire connections are similar to using a single wire PPM_sum or S.bus connection from your receiver to your flight controller instead of the 8 or more 3 wire PWM connections. Ii saves output channels from your flight controller to your gimbal but it still requires your RC Transmitter to have enough channels and switches to control everything you want. Some gimbal controllers allow you to feed your s.bus or ppm_sum signal directly from your receiver or as a pass-through from your flight controller.

It is important to note that many modern cameras do not have an AV output to send video back to the ground. They may be designed to use wifi which is a real problem for a multirotor since wifi range is very short and most RC transmitters work on the same frequency causing the wifi to sometimes interfere with RC control. So it is important when choosing a camera to check if it can send video out live while composing not just when filming. Also, what interface does it use? Standard video or HDMI? The typical analog video transmitters we use in RC are standard video and will not work with HDMI without an adapter. Also note where the connections come out of the camera. Will you be able to get cables out without hitting or being blocked by the gimbal arms? How does the camera trigger the shutter? Is it IR/Mechanical/Wifi/USB? Is there an adapter that will convert a servo output from your receiver to control the trigger on your camera? There may not be. Gentles (http://www.gentles.ltd.uk) in the UK, Flytron (http://www.flytron.com/) and VP-Systems (http://vp-systems.eu) make adapters for cameras but it is important to check before you buy a camera for aerial video or stills to see if there is a radio controller that works.



https://www.rcgroups.com/forums/show...tch-for-FPV-AP

You may also want to use a video switch. Your video transmitter can only transmit video from one camera at a time. If you have a flight camera and want to also see the live output of your filming camera then you need a video switch to take the inputs from both your cameras and allow you to control which one is transmitted by the video transmitter. But this requires another channel on your radio transmitter!

There are digital video solutions now that may have other options for transmitting video to the ground. Some systems offer dual controllers--one for controlling just the gimbal used by the videographer and one used by the pilot to control the aircraft. It is hard to pay attention to flying and filming at the same time.

I have built a lot of gimbals and I must say that it is hard to get good stabilization. Tuning a gimbal can be a challenge. The gimbal can do a good job of moving the camera to keep it level with the movements of the aircraft but the gimbal is not good at removing fine, fast vibrations transmitted through the mechanical connection of the gimbal to the aircraft. The aircraft can produce a lot of vibration, especially if the props are not balanced. Hence, most gimbals are mounted to the aircraft with some kind of suspension system to isolate and dampen the multirotor's vibrations so they don't interfere with the gimbal. An important thing to look for in a gimbal is how this attachment is done. The little rubber balls commonly used for this suspension are designed to work in compression not tension. What that means is that the gimbal should hang on the balls not from them. Many gimbals are wrongly designed to hang from the suspension balls rather than attach to a plate above the suspension balls and push down on the them. It therefore makes a big difference where you mount your gimbal. If you try and mount the gimbal way forward, on rails, to clear the view of the propellers from the video, you may expose the gimbal to prop wash causing more "jello" vibration in your videos. Also, mounts that hang from the rubber balls allow the possibility that in a gust of wind your camera might pull down hard enough on the rubber balls to pull them out, causing your camera and gimbal to fall to the ground.

The bottom line is that a gimbal designed by the manufacturer for a specific camera has a big advantage over general purpose gimbals. The camera-specific gimbal can be purpose made to balance perfectly for that camera model with no user adjustments needed. The exact correct cables can be routed internally through the arms so they don't hang up on the movement of the gimbal. Then the gimbal controller can be pre-tuned for the specific camera weight and balance so you can pretty much just install the camera, turn it on and go. The weight can be shaved to the minimum. The video output and trigger functions can be built-in. If you want the best chance of having a gimbal work out of the box, look for gimbals designed specifically for your camera. This does have the disadvantage of locking your gimbal into one camera and as times change you may be stuck with a dedicated gimbal for a camera you no longer use.



The little HMG SJM10 gimbal is designed specifically for the SJCAM M10 camera. Note all the cabling is inside the arms. The gimbal hangs from the pan motor which is attached to the upper plate and pushes down on the rubber balls rather than hanging from them. The video output for live video is built-in. The mount is designed for a standard Tarot two-tube setup found on many multirotors. The hanging arms are actually attached to the lower plate, isolating the gimbal from the multirotor vibrations through the rubber balls. I am not recommending this camera or gimbal but just using it as an example of a purpose-built gimbal. It balances perfectly for this camera and no tuning is required. It can be used “right out of the box” and the total weight of camera and gimbal is 286g.
Last edited by mike_kelly; Sep 13, 2017 at 08:41 AM.
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Sep 24, 2017, 01:24 PM
KD2PBU - Fly No Evil
davidbitton's Avatar
For a custom design, should the roll motor be centered on the camera or centered on the CG of the pitch arm?
Sep 24, 2017, 04:16 PM
Wisconsin
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Quote:
Originally Posted by davidbitton
For a custom design, should the roll motor be centered on the camera or centered on the CG of the pitch arm?
I am not sure what might be ideal but all gimbals I have seen attach directly on the yaw arm and the pitch assembly slides left and right to balance on the roll motor. It definitely is not centered on the camera, which might be good, but instead the pitch motor causes the offset of the camera from center because of it's weight. If you had a dual attachment point gimbal where the pitch platform is attached at both sides of the camera, not just one. Then you can design it to balance with the camera in the center. It would make subtle difference in the pan with the lens not offset from the pivot point but I am not sure anyone would notice in a video. From the gimbals point of view I don't think it cares. It is just a mass that needs to be balanced before it does it's job.
Latest blog entry: UC4H: Gimbal flight test
Sep 24, 2017, 04:53 PM
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davidbitton's Avatar
Mike,
Take a look at this pic. The roll motor is directly behind the motor; offset to one side by using only two screws. It's quite heavy on one side. So what your saying is the roll motor should be at the center of gravity. That would help because in order to keep it level, I have the power setting jacked up to 200.
Sep 24, 2017, 05:26 PM
Wisconsin
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Quote:
Originally Posted by davidbitton
Mike,
Take a look at this pic. The roll motor is directly behind the motor; offset to one side by using only two screws. It's quite heavy on one side. So what your saying is the roll motor should be at the center of gravity. That would help because in order to keep it level, I have the power setting jacked up to 200.
All the loads on the motors need to be at the center of gravity of the motors to balance. So what is attached to your roll motor needs to balance perfectly anywhere you move it to. So if you press down on the side of the camera pushing left or right or down it should stay where it is when you remove your finger, not return to where it was or anywhere else. That is a balanced gimbal.
Latest blog entry: UC4H: Gimbal flight test
Sep 24, 2017, 05:28 PM
Wisconsin
Thread OP
Quote:
Originally Posted by mike_kelly
All the loads on the motors need to be at the center of gravity of the motors to balance. So what is attached to your roll motor needs to balance perfectly anywhere you move it to. So if you press down on the side of the camera pushing left or right or down it should stay where it is when you remove your finger, not return to where it was or anywhere else. That is a balanced gimbal.
The plate attached to the roll motor may not be able to be centered due to the rest of the load. So it is very possible it will not be attached to the roll motor at it's center but offset where needed to balance the rest of the gimbal. Same for all axis.

In fact it is so offset that you are probably not going to be able to balance it as is and you will have to make a new roll plate that allows the pitch plate to be shifted left to be more centered on the roll motor at the balance point.
Latest blog entry: UC4H: Gimbal flight test
Sep 24, 2017, 08:28 PM
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davidbitton's Avatar
Quote:
Originally Posted by mike_kelly
The plate attached to the roll motor may not be able to be centered due to the rest of the load. So it is very possible it will not be attached to the roll motor at it's center but offset where needed to balance the rest of the gimbal. Same for all axis.

In fact it is so offset that you are probably not going to be able to balance it as is and you will have to make a new roll plate that allows the pitch plate to be shifted left to be more centered on the roll motor at the balance point.
that's the plan; centering. the fact is, the roll does level with a high power setting; hot motor too. back to fusion.
Sep 24, 2017, 08:40 PM
Wisconsin
Thread OP
Quote:
Originally Posted by davidbitton
that's the plan; centering. the fact is, the roll does level with a high power setting; hot motor too. back to fusion.
Sure it will work but the power is all going to just balance it. By balancing it first with the power off the motors have to do so much less work when it is turned on.
Latest blog entry: UC4H: Gimbal flight test
Jan 08, 2018, 09:03 PM
Registered User
Hi there mike, really good thread, I never go to bed without learning something new.

I've got a couple of questions, just a brief introduction: I want to build a quad (that is almost finished) for recording rallies with dual ops. The thing is, the quad flies up to 90kmh and gimbals don't like that.

I have been looking for a rtf gimbal and didn't find any that fits my needs: at least 330 of panning and good performance at high speeds. Gave up, decided to build my own gimbal, designing my own frame using the Storm32 controller (this is an action camera gimbal, not a dslr or compact).

Afaik as I know, key ingredients are: a very smooth multirotor, a stiff gimbal frame and a set of powerful motors for the gimbal.

Do you have any recommendations about motors ? I should aim for a 2212 style motor maybe for a 2804 ? I'm not sure about the pros and cons of both, 2204 is generally used in 3 axis gimbals like the hakrc storm32, but I think that may be a little bit weak for the application I got in mind.

Anything else?

Thanks in advance.
Jan 08, 2018, 10:03 PM
Wisconsin
Thread OP
Quote:
Originally Posted by Dashz
Hi there mike, really good thread, I never go to bed without learning something new.

I've got a couple of questions, just a brief introduction: I want to build a quad (that is almost finished) for recording rallies with dual ops. The thing is, the quad flies up to 90kmh and gimbals don't like that.

I have been looking for a rtf gimbal and didn't find any that fits my needs: at least 330 of panning and good performance at high speeds. Gave up, decided to build my own gimbal, designing my own frame using the Storm32 controller (this is an action camera gimbal, not a dslr or compact).

Afaik as I know, key ingredients are: a very smooth multirotor, a stiff gimbal frame and a set of powerful motors for the gimbal.

Do you have any recommendations about motors ? I should aim for a 2212 style motor maybe for a 2804 ? I'm not sure about the pros and cons of both, 2204 is generally used in 3 axis gimbals like the hakrc storm32, but I think that may be a little bit weak for the application I got in mind.

Anything else?

Thanks in advance.
I don't think I can advise because the speed is out of the range of anything I have done. The wind pressure on the gimbal will not be minor at those speeds so I would "err" on the side of more powerful.
Latest blog entry: UC4H: Gimbal flight test
Jan 08, 2018, 10:56 PM
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davidbitton's Avatar
Quote:
Originally Posted by mike_kelly
wind pressure on the gimbal
it would have to be behind a tinted bubble. you'll loose an F stop or two, but it will allow for small -- lighter -- motors. the weigh of the dome is still less than a motor.
Jan 09, 2018, 06:41 AM
Registered User
Quote:
Originally Posted by mike_kelly
I don't think I can advise because the speed is out of the range of anything I have done. The wind pressure on the gimbal will not be minor at those speeds so I would "err" on the side of more powerful.
Thanks mike, it seems Dji has got a trade secret for their gimbals. I don't know how can they get that supreme stability at 95kmh on the Inspire 2.
Jan 09, 2018, 06:41 AM
Registered User
Quote:
Originally Posted by davidbitton
it would have to be behind a tinted bubble. you'll loose an F stop or two, but it will allow for small -- lighter -- motors. the weigh of the dome is still less than a motor.
good idea
Jan 09, 2018, 07:57 AM
Wisconsin
Thread OP
Quote:
Originally Posted by davidbitton
it would have to be behind a tinted bubble. you'll loose an F stop or two, but it will allow for small -- lighter -- motors. the weigh of the dome is still less than a motor.
I am sure the sperical design of those cameras are one benefit to the DJI system, less drag. Which would not be insignificant given the boxy shape of the Gopros and their clones.
Latest blog entry: UC4H: Gimbal flight test
Jan 09, 2018, 01:53 PM
KD2PBU - Fly No Evil
davidbitton's Avatar
Quote:
Originally Posted by mike_kelly
I am sure the sperical design of those cameras are one benefit to the DJI system, less drag. Which would not be insignificant given the boxy shape of the Gopros and their clones.
What if he used one of those "gimball" designs?


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