Adding 3rd axis compensation to Picotilt using Magnetometer

[air]

Hi All,
I recently purchased a PicoTilt Unit (www.u-nav.com) to experiment with.
It is a two axis accellerometer based camera stabilisation controller.
Basically the pico tilt unit is mounted in the aircraft and when the pitch or roll of the aircraft changes in one direction, the picotilt moves the camera mounting servo in the other direction, keeping the lens pointing straight down.
I'm not sure how the unit maintains its heading accuracy over time, perhap it works on the assumption that the average position of the airframe is the neutral one or something.

Anyway the 3rd axis which the sensor doesnt control is the pan axis of the camera.
With the camera mounted vertically this axis is the yaw axis of the plane.

If one is attempting to take sequential vertical airphotos along a given flight path, one obviously wants the y axis of the photos to be parallel with the flight path at all times.

What I am proposing is to develop an additonal control element to compensate the camera orientation for when the plane's heading is at an angle to the actual overall flight path due to cross winds or whatever.
If this is achieved the y-axis of the lens will always be parallel with the flight path.

In order to achieve this one needs two pieces of information - the heading of the plane (the direction its facing) and the true flight path heading.
The error between these would be used to drive the compensation unit.

My initial thoughts are that the true flight heading can be taken directly from a GPS while a magnetometer could provide the actual heading.

In terms of hardware I'm thinking that this would comprise a PIC reading an NMEA stream from a GPS and calculating true flight heading (unless a gps that will provide it directly is used) while at the same time reading the actual heading from a magnetometer.

Honeywell has a large range of magnetometers (http://www.ssec.honeywell.com/magnetic/datasheets.html)
including digital ones that give a serial output with the heading, negating the need to process an analogue signal.

The HMR3300 is only 7.5grams and provides 1 degree heading accuracy when level. It has an onboard UART that operats at CMOS logic levels for direct interfacing to a pic.
The chip is $385 from DigiKey, development kit is $499.
Part # 342-1032-ND and 342-1033-ND respectively from Digikey


The third element of the hardware would be the servo pwm which should be pretty straight forward.

Overall I think it shouldnt be too difficult given that it will be basically down to a simple board layout and number crunching on the PIC.

Similar hardware could be used to make a tracking narrow band antenna aiming system for telemetry links.

I'd appreciate any comments on this, in particular I'm totally unfamiliar with magnetometers etc.
How would they perform in the eflight environment (ESC's etc)?

[Mr.RC-CAM]

The only major operational obstacle you may face is that magnetometers, that are not tilt compensated, must remain perfectly level to ensure accuracy. It sounds like you have that covered.

BTW, if you can get by with 2.8 degrees of resolution, then the Hitachi HM55B Compass sensor is a great candidate. It is a new part that will easily interface with a microcontroller (A/D input). It is under $30 at parallax.com. Circuit Cellar Ink Magazine recently had a good construction article for it (you could buy a digital reprint from the CCI online store).

RC-CAM

[air]

Thanks Mr RC, I used to work in Honeywell so a friend might be able to source one of those for me at cost.
I think the HRM3300 is OK with tilt as you mentioned.
I'll get the reprint and the Hitachi anyway.
Thanks for the help!

[Mr.RC-CAM]

I think the HRM3300 is OK with tilt as you mentioned.The spec sheet mentions that it allows up to 60 degrees of tilt, which is excellent. However, since your platform is going to be stabilized by the u-nav, the magnetometer will probably avoid tilt attitudes. In case that turns out to be true in practice, I thought it would be good to mention the very affordable $30 solution. :)

[air]

Agreed, if the u-nav unit is doing it's job the mag sensor should experience little or no tilt / roll.
I've seen a few consumer magnetic compasses being sold very cheaply since, I might buy one or two & tear them up to check out the innards.
Does anyone know whether or not this technique is used for stabilising cameras in full scale survey planes?

[Mr.RC-CAM]

Does anyone know whether or not this technique is used for stabilising cameras in full scale survey planes?I understand that they use traditional electro-mechanical gyro stabilized systems.

[Artemetra]

I understand that they use traditional electro-mechanical gyro stabilized systems.

Are we talking about the infamous Steadicam and its ilk? (Interested to know if you are thinking of anything else?)

[Mr.RC-CAM]

Are we talking about the infamous Steadicam and its ilk?
Have you looked at the Kenyon Gyro Stabilizers? The KS2 would fit on a .60 size model aircraft or heli.

[air]

I was actually questioning in particular whether full scale aircraft use magnetic referencing to correct for crabbing along a flight path.
I havent heard of the Steadicam but I'd certainly be interested in a link.
As regards the Kenyon Gyros, as I understand it they use a large mass spinning in one axis to provide stabilisation against transient movements in that axis. One gyro required per axis.
As the application is still rather than video I'm not too worried about transient vibrations once the mean orientation of the camera is vertical.
Perhaps this is a misconception on my part, I'm not an expert in this area.

[Mr.RC-CAM]

I was actually questioning in particular whether full scale aircraft use magnetic referencing to correct for crabbing along a flight path.Sorry, I misunderstood your app. FWIW, Kenyon gyros are electro-mechanical. They are old-school spinning mass devices. There are a couple of firms selling the KS2 for R/C AP still camera platform stabilization. They weigh over a pound, so the aircraft/heli needs to be on the larger size.